Category Archives: The (really) Ancient Earth

Which sequence stratigraphic model is that?

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Postglacial rebound has resulted in forced regression of the shoreline along these small fan deltas. The numbers indicate approximate poisition of shorelines, 1 being the oldest. Each successive shoreline and associated fan delta deposits downstep towards the modern coast. Helicopter lower right. This is Emma Fiord, Ellesmere Island in 1987.

Postglacial rebound has resulted in forced regression of the shoreline along these small fan deltas. The numbers indicate approximate positions of shorelines, 1 being the oldest. Each successive shoreline and associated fan delta deposits downstep towards the modern coast. Helicopter lower right. This is Emma Fiord, Ellesmere Island in 1987.

Assessing different sequence stratigraphic models.

The concept of unconformity-bound sequences was first proposed by Sloss et al. (1949, Sloss (1963) for continent-wide Phanerozoic successions. The unconformities were formed during prolonged subaerial exposure and erosion along basin margins. However, subaerial unconformities form landward of any shoreline and, except where shallow marine and non-marine strata interfinger, the Sloss model cannot be used for coeval marine successions. All this changed in 1977 when the AAPG published Memoir 26; Seismic Stratigraphy: Applications to Hydrocarbon Exploration. Here, sequence stratigraphy was introduced as a new method of stratigraphic analysis.  The method recognises that the sedimentary record is organized into discrete, but genetically related stratal packages bound by key stratigraphic surfaces, surfaces that repeat through time and are dynamically controlled by changes in baselevel: the surfaces include subaerial unconformities, maximum regressive, maximum flooding, forced regression, and ravinement surfaces, plus their correlative conformities.

The new method divided stratigraphic successions into depositional sequences, each sequence bound by subaerial unconformities and their correlative conformities. Each sequence consists of systems tracts, each systems tract contains several depositional systems, and each depositional system contains lithofacies and lithofacies associations.

The challenge to traditional lithostratigraphy (formations, members etc.) was a watershed in the science of stratigraphy and the way we think about sedimentary basins. Fundamental differences between the two methods of analysis include:

  • Lithostratigraphy is deemed an objective approach to stratigraphic analysis, where interpretation plays no role in defining a formal stratigraphic unit; the starting point for sequence stratigraphy is the basic (objective) description of sedimentary rocks and sedimentary facies, but beyond this the analysis becomes more interpretive, or genetic. For example, identification of a particular depositional system relies on the interpretation of process-response characteristics of sedimentary facies.
  • The fundamental lithostratigraphic units are formations which are inherently diachronous; the key stratigraphic surfaces in sequence stratigraphy have chronostratigraphic significance – the case for this was derived from the analysis of seismic reflections.
  • The early proponents of sequence stratigraphy (1977 to 1988) proposed that sequences and their systems tracts were controlled primarily by eustatic changes in baselevel – hence the designation of highstand, falling stage, lowstand, transgressive, and regressive systems tracts. This proposal met with a great deal of criticism, primarily because it tended to filter out the variable effects of tectonism, basin subsidence, sediment supply, and the influence of autogenic processes on stratigraphic architecture.

Today, there is less emphasis on eustatic drivers, and more on how sequences develop during a complete cycle of changes in sediment accommodation, baselevel, and sediment supply.

[Note: In addition to AAPG Memoir 26, an excellent collection of papers in SEPM Special Publication 42 (1988) discuss important modifications and challenges to the original sequence stratigraphic model.]

 

The evolution of sequence stratigraphic models

Any scientific model of worth will be tested – and tested again. Models frequently undergo modification to account for any deficiencies or are replaced by new models. Sequence stratigraphy is no different; the original model (Vail and others) has gone through several iterations, including two proposals for alternative models. Most of the changes are based on different interpretations of the timing of events that produce key stratigraphic surfaces and systems tracts in relation to fluctuating baselevels, as well as the preservation potential for surfaces like the basal surface of forced regression. One problem of note is the contentious issue of identification and timing of correlative surfaces, or correlative conformities. For example, a subaerial unconformity develops during baselevel fall and the ensuing regression; this surface forms down depositional dip as the shoreline shifts basinward; the hiatus is a minimum at the youngest shoreline. At any time during this basinward shift there will be a correlative conformity in the marine parts of the basin – shelf, slope, and beyond. So, which correlative conformity does one choose – the one at the beginning of regression, the one at the end of regression, or somewhere in between? Identification of a correlative conformity in the stratigraphic record poses an additional problem; tracking seismic reflections across a basin is reasonably successful but putting one’s finger on a particular bedding plane in outcrop, core, or wireline logs is far less so, particularly in deposits that accumulate basinward of storm wave-base.

The diagram below (modified from Catuneanu et al, 2010) shows some of the important steps in this iterative process.

Summary of the 6 sequence stratigraphic models, their systems tracts, sequence boundaries, and relationship between their bounding surfaces and stages of baselevel change. Modified from Catuneanu et al. 2010, Figure 2. Systems tracts abbreviations are: HST = highstand, LST = lowstand, TST = transgressive, FSST = Falling stage, and RST = regressive, mfs = maximum flooding surface, mrs = maximum regressive surface. On the sea level curve, NR = normal regression, TR = transgression

Summary of the 6 sequence stratigraphic models, their systems tracts, sequence boundaries, and the relationship between their bounding surfaces and stages of baselevel change. Modified from Catuneanu et al. 2010, Figure 2. Systems tracts abbreviations are: HST = highstand, LST = lowstand, TST = transgressive, FSST = Falling stage, and RST = regressive, mfs = maximum flooding surface, mrs = maximum regressive surface. On the sea level curve, NR = normal regression, TR = transgression

Depositional sequence I is the original model proposed by Vail, Mitchum and others in AAPG Memoir 26 (1977), based on seismic stratigraphy. The sequence boundary is a subaerial unconformity and its correlative conformity, that begin to form at the onset of baselevel fall.

Model II (e.g. Posamentier et al. 1988) maintains the sequence boundary of Model I and is divided into four systems tracts; the HST, LST, TST, and a shelf margin systems tract. There was also an attempt to clarify issues with the correlative conformity at the base of the lowstand fan. At this juncture, the Exxon group proposed two types of sequence based on the degree of erosion beneath a subaerial unconformity and its down-dip extent: Type 1 sequences are bound by a subaerial unconformity that extends to the edge of the shelf, the associated facies also transiting seawards. The subaerial unconformity bounding a Type 2 sequence is confined to the basin margin. Type 2 sequences also contain the shelf margin systems tract (SMST) which overlies the sequence boundary and is overlain by a transgressive surface. Thus, the SMST accumulates during baselevel fall. Amidst the confusion that followed, several problems quickly became apparent with this scheme (Embry, 2002; Catuneanu, 2006):

  • Distinguishing between Type 1 and Type 2 unconformities was quite subjective.
  • The shelf margin and lowstand systems tracts are basically the same thing; they both form from sediment shed off the basin margin and shelf during baselevel fall. One problem here is that if a Type 2 sequence was identified, then a shelf margin systems tract must follow. In other words, the model was determining whether a shelf margin or lowstand systems tract was present, rather than the nature of the rocks themselves.
  • The correlative conformity basinward of the subaerial unconformity occurs within the lowstand deposits, and
  • In many parts of a marine basin, the correlative conformity has “little objective expression” (quoted in Embry, 2002); in other words, there may be little or no stratigraphic and sedimentologic criteria upon which one could identify such a boundary.

Depositional model III attempts to rectify these problems by combining sequence Types 1 and 2 and moving the sequence boundary to the end of baselevel fall (i.e. the beginning of baselevel rise) (e.g. Van Wagoner et al, 1988). The shelf margin systems tract was also discarded. In this version, the former lowstand fan deposits become part of the late highstand systems tract. However, the beginning of baselevel rise does not coincide with the end of regression – regression continues for some time because there is plenty of sediment available in shallow marine environments. Thus, there may be no specific stratigraphic record of any surface that indicates the start of sea level rise.

 

 

Comparison of the Exxon systematics corresponding to Model III, and the changes wrought by Hunt and Tucker (1992) in Model IV. Note the different placement of the correlative conformity (sequence boundary), the inclusion of a forced regressive systems tract that includes the down=stepping shoreface wedges and the correlative, deeper water fan deposits, and the lowstand prograding wedge in the normal regressive portion of early baselevel rise. Modified from Hunt and Tucker, 1992.

Comparison of the Exxon systematics corresponding to Model III, and the changes wrought by Hunt and Tucker (1992) in Model IV. Note the different placements of the correlative conformity (sequence boundary), the inclusion of a forced regressive systems tract that includes the down-stepping shoreface wedges and the correlative fan deposits, and the lowstand prograding wedge in the normal regressive portion of early baselevel rise. Modified from Hunt and Tucker, 1992.

Depositional model IV is, perhaps, the first significant departure from the original Exxon story. It was based on the recognition that forced regression occurs when sediment supply is less than the rate of baselevel fall (progradation and aggradation require sediment supply to exceed baselevel fall). To account for this dynamic in the sequence stratigraphic model a falling stage systems tract (FSST) was added by Plint (1988) and Hunt and Tucker (1992). The lower boundary is the basal surface of marine erosion. A subaerial unconformity occurs at the top of the FSST, stepping downward in concert with the shoreline trajectory; the correlative conformity is placed at the base of the redefined lowstand systems tract. Here, the LST begins with the final stages of baselevel fall and continues during normal regression in the early stage of baselevel rise; it overlies the basinward part of the FSST. Of all the depositional sequence model iterations, the Hunt and Tucker scheme is the probably the most accepted.

 

Two alternative models:

Genetic sequences (Galloway, 1989) are bound by maximum flooding surfaces (MFS). Galloway’s model relies on Frazier’s (1974) definition of depositional episodes, that represent time-bound units of progradation, aggradation and retrogradation, and form in response to transgression and regression. A Genetic sequence is the stratigraphic record of a depositional episode.  Although a Genetic sequence records changes in baselevel, it is not tied to eustatic cycles – in fact, Galloway’s model was partly an attempt to extricate sequence stratigraphy from the idea that eustasy was the driving force. It was also a kind of plea for stratigraphy to return to objective lithofacies analysis, rather than being model driven.

The MFS was chosen as the Genetic sequence boundary because:

  • It is the surface that signals the end of transgression and landward shoreline excursion, and the beginning of regression (note that the end of transgression does not coincide with the end of baselevel rise).
  • It has low diachroneity.
  • It is an easily recognizable and mappable surface. Maximum flooding surfaces commonly overlie condensed sections or omission surfaces; they signal an abrupt lithological change from coarse-grained or cemented lithologies, to fine-grained beds that indicate the beginning of regression.
  • The MFS can be traced into coeval non-marine deposits where they interfinger with marine strata; Galloway maintains that the surface can also be traced to deeper parts of the basin.
  • The MFS is the only surface that can, in principle, be used across all parts of a basin.

A major criticism of Galloway’s scheme is that subaerial unconformities occur within a sequence, which means that it can no longer be considered as a relatively conformable stratigraphic unit.  And unless the correlative conformity coincides with a condensed section or omission surface, its identification in deep water strata suffers from the same kinds of problems that the other models contend with.

 

Transgressive-regressive (T-R) sequences

Sequence stratigraphic models according to Embry (2002), including Genetic sequences (GS) and T-R sequences. The boundaries of depositional sequence (DS) models II and III are also indicated. SU-R = subaerial unconformity - ravinement surface, RSME = regressive surface of marine erosion at the base of each forced regressive wedge.

Sequence stratigraphic models according to Embry (2002), including Genetic sequences (GS) and T-R sequences. The boundaries of depositional sequence (DS) models II and III are also indicated. SU-R = subaerial unconformity – ravinement surface, RSME = regressive surface of marine erosion at the base of each forced regressive wedge.

The general plan of attack for T-R sequences was formulated by Embry and Johannessen (1993). T-R sequences are bound by subaerial unconformities and their marine equivalents, maximum regressive surfaces (MRS); the subaerial unconformity may be partly or wholly replaced by a ravinement surface. This model avoids the complications of correlative conformities in shallow marine strata and maintains the general mantra of relatively conformable successions (unlike Genetic Sequences). The relative ease of identification of the bounding surfaces in outcrop and seismic profiles gives this model some advantage over its predecessors. T-R sequences contain only regressive and transgressive systems tracts; the boundary at the top of the TST is the maximum flooding surface.

However, like depositional sequences, there is a common problem of identifying the correlative conformity in deep water deposits including the deep shelf seaward of storm wave-base, and slope-submarine fan deposits that are dominated by sediment gravity flows and mass transport deposits (slumps, slides etc.). An additional problem is the claim that the MRS has low diachroneity which, as Catuneanu (2006) and others have noted, may be true along depositional dip, but not along depositional strike because of significant variations in sedimentation rates from one part of the basin margin to another.

The regressive component of T-R sequences assembles the highstand, falling stage, and lowstand systems tracts of depositional and Genetic sequences into a single stratigraphic entity (RSTs). This has the advantage of simplicity. But there is also an advantage in knowing which components derived from normal and forced regression – this information helps us understand basin dynamics. This can still be done by identifying the relevant depositional systems within the RST, but there is no explicit inclusion of the corresponding systems tracts within the T-R model.

 

Which model is best?

If the geological literature is anything to go by, one might assume that the Exxon-type depositional sequence schemas are the only models worth considering; Genetic and T-R sequence models barely get a mention. And yet there are as many advantages and disadvantages to depositional sequence schemes as there are for the two alternatives. Perhaps it’s a function of shouting loudest. To compound the issue, it is not always clear in many publications which depositional sequence version is being used.

A group of stratigraphers has grappled with these problems, publishing their deliberations and recommendations in several papers (e.g. Catuneanu et al. 2010, and  2011). The group acknowledges the historical significance of the depositional sequence models introduced by Vail and his colleagues, but not their primacy. Several conclusions stand out:

  • All the models have value, they are all sensible and logical; each has advantages and disadvantages.
  • No single model solves all stratigraphic problems.
  • The fundamental basis for stratigraphic interpretation is sound lithofacies analysis.
  • Choose the model that best suits the geological environment and the available data: interpretation of the stratigraphy should be based on geology and not driven by a model.
  • Identify this model in your reporting.

This is sound advice!

 

This post is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

 

Other posts in this series on Stratigraphy and Sequence Stratigraphy

Stratigraphic surfaces in outcrop – baselevel fall

Stratigraphic surfaces in outcrop – baselevel rise

A timeline of stratigraphic principles; 15th to 18th C

A timeline of stratigraphic principles; 19th C to 1950

A timeline of stratigraphic principles; 1950-1977

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Parasequences

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic lapout geometry

Stratigraphic condensation – condensed sections

Depositional systems and systems tracts

 

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Stratigraphic condensation – condensed sections

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An idealised cross-section of a condensed section at an omission surface. Condensation is a function of sediment removal by tidal currents, a sea floor starved of sediment, or sediment bypass to deeper waters. The original soft sediment biota (trace fossils, invertebrates) changes to a different set of fauna and flora as the hardground develops. Palimpsest shells become encrusted with barnacles and bryozoa. The hardground biota is characterised by boring and encrusting organisms.

An idealised cross-section of a condensed section at an omission surface. Condensation is a function of sediment removal by tidal currents, a sea floor starved of sediment, or sediment bypass to deeper waters. The original soft sediment biota (trace fossils, invertebrates) changes to a different set of fauna and flora as the hardground develops. Palimpsest shells become encrusted with barnacles and bryozoa. The hardground biota is characterised by boring and encrusting organisms.

Condensed sections record multifarious interactions of physical, biological, and geochemical processes played out over long time spans, all condensed into a few very thin beds.

 

The stratigraphic record is anything but continuous. We know that unconformities and disconformities represent missing geological time, significant gaps where the rock record either never developed or was removed by erosion. But what about the rest of the stratigraphic record? How complete is it?

Andrew Miall (2014) refers to the “emptiness of the stratigraphic record”, echoing Ager’s (1973) earlier comment that the “sedimentary record is more gap than record “. Take your average shallow marine, turbidite, or fluvial succession. At the most basic, bedding planes represent breaks in sedimentation, breaks that might be measured in minutes or centuries. Likewise, fluvial channel deposits that are replete with cross-cutting bedforms might represent 10s to 100s of years accumulation and yet we are only witnessing the final acts of preservation, wherein the record of all previous channel bedforms exists only in the facies models we derive. Miall’s assessment of the Late Cretaceous Mesaverde Group indicates that the actual record of deposition exposed in Book Cliffs may be as little as 10% of its 4.86 million-year duration.

Condensed sections are a special kind of ‘stratigraphic incompleteness’. Kidwell (1993), and a more recent review by Föllmi (2016 – who also provides a list of well-known examples), define a condensed section as:

  • Very thin units (decimetres) relative to
  • The span of time they represent (105 to 107 years). For example, the fossiliferous Late Oligocene Kokoamu Greensand in southern New Zealand, is commonly 1-5 m thick but represents a time span of about 4 million years.
  • Containing a biota representative of the span of time, meaning that over a period of 106 years, several faunal or floral biozones may be present. However, the paleontological record may be complicated by postmortem mixing, commonly by bioturbation.
  • Containing internal discordances and hiatuses caused by erosion or non-deposition. Bedload transport and erosion of sediment on the sea floor is typical of shoreface dynamics on siliciclastic shelves and carbonate platforms. Non-deposition may be due to sediment starvation (other than fine particles in suspension), for example during transgression when river supply is thwarted by rising baselevels; or situations where sediment bypasses a shelf or platform on its way to deeper waters.
  • Containing abundant authigenic minerals (i.e. precipitated at or close to the sea floor) such as carbonate, phosphate, silica (chert), glaucony (glauconite green sands), manganese and iron oxides.
  • Hardgrounds are common sites of condensation. Examples include carbonate hardgrounds on shallow carbonate platforms, that also provide domicile to boring and entrusting organisms; iron-manganese hardgrounds and nodules at much greater bathyal to abyssal depths, and phosphate nodules or hardgrounds in regions of nutrient upwelling.
Manganese nodules, 2 - 10 cm across, litter the deep sea floor off Cook Islands, South Pacific. They precipitate in-situ on the sea floor, under conditions of extremely low sedimentation

Manganese nodules, 2 – 10 cm across, litter the deep sea floor off Cook Islands, South Pacific. They precipitate in-situ on the sea floor, under conditions of extremely low sedimentation  Image credit: USGS, https://www.usgs.gov/media/images/cook-islands-manganese-nodules

The most likely place you will see a condensed section is in outcrop or core. The common array of wireline logging tools may also help identify these units, based on elevated gamma response (for example, responding to the elevated potassium in glauconite), or abrupt changes in density in sonic and density logs across hardgrounds.

 

Omission surfaces

Stratigraphic surfaces swept bare by erosion or starved of sediment are commonly referred to as omission surfaces. Omission surfaces are important components of condensed stratigraphic sections. Although not a necessary condition, the term is commonly reserved for surfaces that have been modified by benthic organisms; examples include encrusting molluscs such as oysters, bryozoa, corals, barnacles, and calcareous algae (e.g. Lithothamnion), plus critters that are capable of boring through firm or cemented sediment.

A condensed section in the Middle Jurassic Carmel Formation, Utah. The cross-section through a carbonate hardground shows encrusting oysters (upper half of image – black arrow) and cemented sands and muds bored by bivalves. The yellow arrow points to a possible geopetal structure within an oyster shell. Image credit: Wilson44691 2008, https://commons.wikimedia.org/wiki/File:CarmelHardgroundSection.jpg

Omission surfaces commonly develop on hardgrounds and firmgrounds, that also harbour specialized trace fossil assemblages, particularly those that can bore through cemented sediment. More specifically they are called ichnological omission surfaces. MacEachen et al (2012) listed several diagnostic criteria to help identify these surface (note that some of the criteria taken individually are common to many non-omission trace fossil suites, but taken collectively are convincing):

  • Trace fossils cut across sedimentary structures and other trace fossils; borings may also transect earlier-deposited or cemented shells.
  • If the boring organism is a shelly mollusc, such as a pholad bivalve, then preservation of the organism within its home is possible (unlike most other trace fossils). The bored hole will also fill be filled by sediment from the exposed surface.
  • Traces are unlined but may show wall markings such as scratches.
  • Hardgrounds are less likely to encourage critters that prefer soft substrates.
  • Burrows in hardgrounds are less likely to compact during sediment burial.
Modern Pholad bivalves have bored into indurated sandstone. The shells of some remain in the holes. Some abandoned bore holes have been preferentially cemented by calcite and are resistant to subsequent erosion.es

Modern Pholad bivalves have bored into indurated sandstone. The shells of some remain in the holes. Some abandoned bore holes have been preferentially cemented by calcite and are resistant to subsequent erosion (arrows).

 

A condensed section from the Middle Jurassic Bowser Basin

 

A middle Jurassic condensed section represents the transgressive component of a shelf parasequence. MRS = maximum regressive surface; MFS = maximum flooding surface. Arrow top left points to an ammonite. The section is 50 cm thick; the nderlying regressive component is about 10 m thick.

A middle Jurassic condensed section represents the transgressive component of a shelf parasequence. MRS = maximum regressive surface; MFS = maximum flooding surface. Arrow top left points to an ammonite. The section is 50 cm thick and records a passage of time similar to the underlying 10 m thick regressive component.

In this coarsening-upward parasequence, shoreface sandstones are terminated by a surface of maximum regression (MRS) above which are thin transgressive deposits. The preserved record of transgression is little more than 0.5 metre of thick, between the MRS and a flooding surface (MFS), but probably represents a passage of time the same order-of-magnitude as the underlying regressive portion of the parasequence. Thus, the record of transgression is condensed; it consists of two main components:

  • A fossiliferous, pebbly, sandy mudstone that overlies the MRS, and represents the reworking of coarse sediment, bivalves and ammonites left on the sea floor at the end of regression. Little new sediment was introduced to the sea floor at this time, other than some mud from suspension, i.e. the sea floor was essentially starved of new sediment. Some shells are encrusted with bryozoa that may have grown on the reworked debris.
  • Sediment starvation continued and calcite precipitated within the uppermost mud and silt. This component of the condensed section is overlain by the maximum flooding surface (MFS), that in turn is overlain by several metres of mud, silt and fine sand that herald the return to regression.

 

Condensed sections are fascinating in and of themselves because…

They are stratigraphically skinny, and yet they record the multifarious interactions of physical, biological, and geochemical processes, played out over long periods. They also contribute to stratigraphic analysis:

  • They indicate dramatic changes in sediment flux.
  • They frequently contain evidence for dramatic changes in biota, for example the shifting demographics of organisms as soft substrates transition to hardgrounds.
  • They help define key stratigraphic surfaces such as subaerial unconformities, the maximum regressive surface, and transgressive flooding surfaces and, therefore, changes in baselevel.
  • They help define clinoform lapout, for example along surfaces of (transgressive) onlap and (regressive) downlap.
  • They are useful stratigraphic markers within basins, and in some cases between sedimentary basins.

 

This post is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

 

Other posts in this series on Stratigraphy and Sequence Stratigraphy

Stratigraphic surfaces in outcrop – baselevel fall

Stratigraphic surfaces in outcrop – baselevel rise

A timeline of stratigraphic principles; 15th to 18th C

A timeline of stratigraphic principles; 19th C to 1950

A timeline of stratigraphic principles; 1950-1977

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Parasequences

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic lapout geometry

Depositional systems and systems tracts

Which sequence stratigraphic model is that?

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Parasequences

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Paleocene Highstand, Falling Stage systems tracts, Axel Heiberg I.

Forced regression across a Mid-Late Paleocene delta, Axel Heiberg Island, Canadian Arctic.

 

Parasequences are the building blocks of shallow marine systems tracts.

The stratigraphy of ancient, shallow marine deposits is commonly presented in outcrop,  core, and wire-line logs as cyclic repetitions of shallowing-upward facies, an observation that dates back almost two centuries. Each cycle contains facies transitions from some part of the shelf or platform, to shallower conditions (for example shoreface through shoreline). The ubiquity of these 4th – and 5th -order cycles in the rock record, and the recognition that collectively they comprise larger 3rd -order cycles, led early proponents of sequence stratigraphy to introduce the concept of parasequence. For Van Wagoner et al. (1988), parasequences are the building blocks of stratigraphic sequences. They define parasequences as (op cit, p. 39):

A relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces.”

This definition has survived, reasonably intact, the vagaries of geological debate – witness the attempts to find a common sense of purpose in sequence stratigraphic terminology by Catuneanu et al. (2010, 2011); a group of co-authors all at the forefront of sequence stratigraphic analysis.

The definition contains several implied and explicitly stated conditions:

  1. Parasequences are fully marine.
  2. In the original definition, parasequences are bound by marine flooding surfaces and their correlative surfaces; later definitions have removed reference to the correlative Flooding surfaces represent abrupt changes in water depth during transgression; the change in facies across the flooding surface is also abrupt, from shallow to deep. This means that fluvial successions, and at the other end of the water depth scale, basin-floor fans, are excluded. This does not mean that 4th and 5th order cycles do not occur in fluvial and submarine fans, but that their equivalence in time and space is difficult to demonstrate.
  3. A relatively conformable succession – Brief diastemic breaks are common, for example storm scour surfaces across the shoreface, but there are no unconformities or disconformities.
  4. Genetically related beds – This phrase refers to two conditions: that successive facies are associated in time and space. For example, subtidal-intertidal-supratidal deposits are linked by the dynamics of sedimentation (tidal flux, currents, waves), biotas, and for carbonates and evaporites, seawater chemistry. This relationship, together with the requirement for a comfortable succession, also means that Walther’s Law applies – ‘‘. . . only those facies and facies-areas can be superimposed primarily which can be observed beside each other at the present time’’ (Walther 1894).

Parasequences represent relatively short-lived periods of progradation that are superimposed on, or punctuate 3rd order regressive or transgressive trends. As such, the most common stratigraphic facies trends shallow upward.  Depending on the relative rates of sediment supply and sea level change, the stacking of parasequences will result in 3rd order:

  • Progradation (possibly with a component of aggradation), where the normal regressive (seaward) shoreline trajectory of successive parasequences is approximately horizontal,
  • Progradation during forced regression where successive shorelines have a down-stepping trajectory, and
  • Progradation during transgression where the step-like shoreline trajectory of successive parasequences is retrograde, or landward.

 

Three examples of parasequences

For each example, refer to the schematic depositional trends depicted in the sea level curve below.

Stages of regression and transgression in relation to relative sea level, in a sequence stratigraphy context

Stages of regression and transgression in relation to relative sea level, in a sequence stratigraphy context

Bowser Basin, British Columbia

Middle Jurassic shelf deposits in Bowser Basin have well developed cyclicity represented by coarsening upward mudstone through sandstone successions, 5m to 20m thick. A typical example is shown below (refer to the diagram):

Shelf parasequence representing progradation during normal regression. Bowser Basin

An example of a coarsening-shallowing upward parasequence from the Middle Jurassic of Bowser Basin, (Mt. Tsatia). The outcrop version is shown below.

  • Grey mudstone interbedded with thin siltstone and sandstone at the base, gives way to gradually thicker and more frequent sandstone beds. These facies represent parts of the shelf that are deeper than storm wave-base.
Coarsening-upward shelf parasequence, Bowser Basin. The cycle top and base are indicated by arrows. Hummocky crossbedding and the transgressive, condensed, calcareous mudstone are shown in separate images.

Coarsening-upward shelf parasequence, Bowser Basin. The cycle top and base are indicated by arrows; it is about 7m thick. Hummocky crossbedding and the transgressive, condensed, calcareous mudstone are shown in separate images. MRS = maximum regressive surface (or transgressive surface); FS = marine flooding surface.

 

  • Hummocky crossbeds are common in the lower part of the resistant sandstone beds; these structures denote a relative position on the shelf corresponding to storm wave- base.
Hummocky crossbedding developed at storm wave base. The hammer lies along a basal pebble layer that was deposited by a bottom-hugging density current during collapse of a storm surge. Tsatia Mt. Bowser Basin.

Hummocky crossbedding developed at storm wave base. The hammer lies just below a basal pebble layer that was deposited by a bottom-hugging density current during collapse of a storm surge. Tsatia Mt. Bowser Basin.

  • Sandstones in the upper 2-3m contain planar crossbeds, and a few pebble-lined scours that probably indicate the passage of storm waves across the upper shoreface.
  • The sandstones are capped by a fossiliferous, pebbly, calcareous mudstone. The base of the mudstone is slightly erosional and is interpreted as a maximum regressive surface (MRS) or transgressive surface (TS), although the erosional contact may indicate ravinement during initial transgression. The top of the pebbly mudstone is a marine flooding surface (FS) and represents maximum transgression (or close it).
The transgressive part of a typical shelf parasequence; Bivalves and bryozoa abound. The unit is highly calcareous. The underlying sandstones are crossbedded. MRS = maximum regressive surface (or transgressive surface); FS = marine flooding surface. Lens cap bottom centre.

The transgressive part of a typical shelf parasequence; Bivalves and bryozoa abound. The unit is highly calcareous. The underlying sandstones are crossbedded. MRS = maximum regressive surface (or transgressive surface); FS = marine flooding surface. Lens cap bottom centre.

Overall, the parasequence represents progradation at high sedimentation rates during normal regression, followed by transgression.

 

Sverdrup Basin, Canadian Arctic

Parasequences in the Strand Bay Formation (Middle to Late Paleocene) represent forced regression following a period of (Early Paleocene) highstand progradation-aggradation. The example shows:

Schematic representation of a forced regressive sandstone wedge, Middle-Late Paleocene, Axel Heiberg Island. RSME = Regressive Surface of Marine Erosion; HST = underlying Highstand deposits.

Schematic representation of a sharp-based, forced regressive sandstone wedge, Middle-Late Paleocene, Axel Heiberg Island. RSME = Regressive Surface of Marine Erosion; FS = Marine flooding surface.

  • A resistant sandstone unit that has an abrupt, erosional base (usually with pebble lags and coalified wood fragments), and an equally abrupt top. The bulk of the sandstone contains abundant planar and trough crossbeds that represent deposition across a high-energy shoreface.
  • The sandstone is sandwiched between mudstones and siltstones that represent transgressive, outer-shelf deposits.
  • The abrupt contact at the top of the sandstone is a flooding surface, but its preservation was complicated by shoreface ravinement during the early stage of transgression.
A sharp-based sandstone wedge deposited during forced-regression. This exposure at Expedition Fiord, Axel Heiberg Island. RSME = Regressive surface of marine erosion; FS = marine flooding surface, HST = top of underlying Highstand succession.

A sharp-based sandstone wedge deposited during forced-regression. This exposure at Expedition Fiord, Axel Heiberg Island. RSME = Regressive surface of marine erosion; FS = marine flooding surface, HST = top of underlying Highstand succession.

Belcher Islands, Hudson Bay

The Paleoproterozoic Fairweather Formation (Belcher Islands) contains mixed siliciclastic-carbonate facies organized into shallowing upward cycles, 2-5m thick. In a typical cycle we see:

Schematic representation of mixed siliciclastic-carbonate tidal cycles in the Paleoproterozoic Fairweather Formation, Belcher Islands, Hudson Bay.

Schematic representation of mixed siliciclastic-carbonate tidal cycles in the Paleoproterozoic Fairweather Formation, Belcher Islands, Hudson Bay.

  • Basal siliciclastic sandstone and dolomitic grainstone, interbedded with dolomitic mudstone; planar and herringbone crossbeds, lenticular ripple bedding, and reactivation surfaces indicate shallow subtidal to intertidal conditions.
  • Sandstones higher in the succession have the same composition, but the sedimentary structures indicate progressively shallower conditions as desiccation cracks become more frequent.
Mixed, crossbedded siliciclastic-grainstone facies of intertidal persuasion, overlain by pisolitic dolostone. The contact appears abrupt but in fact is diffuse over several centimeters, and represents a ‘weathering-diagenetic front’. Hammer at center.

Mixed, crossbedded siliciclastic-grainstone facies of intertidal persuasion, overlain by pisolitic dolostone. The contact appears abrupt but in fact is diffuse over several centimeters, and represents a ‘weathering-diagenetic front’. Hammer at center.

  • The overlying carbonates (all dolomitized) consist of highly distinctive, vadose pisolitic dolostone (e.g. caliche). The evidence for this is:
  • Close-fitted packing of pisoids,
  • Some elongation of individual pisoids ( gravity induced),
  • Discordances between pisoids caused by multiple episodes of dissolution and precipitation, and,
  • The irregular, diffuse contact with underlying sandstones that developed during soil-caliche weathering and fluctuating watertables.
Successive vadose pisolite beds formed by multiple periods of in-situ precipitation and dissolution attendant on fluctuating watertables and the occasional marine flooding of supratidal flats by storm and spring tides.

Successive vadose pisolite beds formed by multiple periods of in-situ precipitation and dissolution attendant on fluctuating watertables and the occasional marine flooding of supratidal flats by storm and spring tides.

This post is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

 

Other posts in this series on Stratigraphy and Sequence Stratigraphy

Stratigraphic surfaces in outcrop – baselevel fall

Stratigraphic surfaces in outcrop – baselevel rise

A timeline of stratigraphic principles; 15th to 18th C

A timeline of stratigraphic principles; 19th C to 1950

A timeline of stratigraphic principles; 1950-1977

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic lapout

Stratigraphic condensation – condensed sections

Depositional systems and systems tracts

Which sequence stratigraphic model is that?

 

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A timeline of stratigraphic principles; 15th-18th C

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A timeline of stratigraphic principles; 15th-18th C

A timeline of stratigraphic principles; 15th-18th C

This and the following two posts look briefly at the development of stratigraphic concepts and the characters responsible for them.

This is part of the How To…series  on Stratigraphy and Sequence Stratigraphy

Preamble: My intent was to write a single post on this topic, as an introduction to Stratigraphy in general, and Sequence Stratigraphy in particular. But I kept adding names to the timeline, and got carried away with some of the characters on it. So, the number of posts was expanded. This post deals with five icons spanning the 15th through 18th centuries.

Modern stratigraphy requires us to observe strata, to decipher their origin, and evaluate the time relationships between one bed and the next, from one location to the next. Modern stratigraphy is concerned with the dynamics of Earth processes in a historical context. The stratigraphic history of Earth is an epic tale, told with maps and images of its sedimentology, paleontology, volcanology, geochemistry, geochronology, geophysics, and all their subdisciplines.

It has not always been thus. Sequence Stratigraphy, the current paradigm, is only 43 years old. But inquisitive people have been recording and unraveling stratigraphic problems for at least 500 years. The evolution of stratigraphic principle is a fascinating story in itself, told by folk who had to battle religious and political repression (sometimes at great personal cost), the prejudice of class, and the egos of fellow scientists.

I have picked a few names out of the historical hat; some are well known, others less so. The list is not exhaustive – I apologise in advance if I’ve left out your favourite historical personality. Not everyone on the list contributed memorable pronouncements or axioms. Some we remember as much for their role in breaking down social barriers as for their science.

I begin with Leonardo da Vinci, not because of any particular discovery, but because he was a quintessential Renaissance polymath. His notes on strata and fossils, compiled about 1508, reveal a remarkable ability to reason beyond dogma, while being fully aware that in doing so he would annoy the church hierarchy. Da Vinci’s notes are all written in mirror image, and it is tempting to imagine him doing this to avoid discovery by zealous theologians, but the jury is still out on that debate.

The timeline ends with Peter Vail and Robert Mitchum. Their 1977 publications on sequence stratigraphy set in motion a new and exciting way to look at strata, ancient environments, and geological time. Since then sequence stratigraphy has been tweaked, poked and prodded, but its underlying structure has remained reasonably intact.

 

Leonardo da Vinci (1452-1519)

Leonardo da Vinci depiction of stratified rock, 1508-1511

DaVinci’s notes on fossils were discovered in 1690, but were relatively unknown until 1717 when they were purchased by the Earl of Leicester – hence the name Codex Leicester. This, and other codices were never published during his life. It is unlikely that Steno or Hutton knew of its existence (or for that matter Newton or Galileo). However, Charles Lyell quotes a short paragraph of da Vinci’s explanation concerning the fossilisation of marine shells, so knowledge of the Codex had infiltrated the world of British and European naturalists, perhaps during the 19th century (Lyell, 1872, Volume 1, p.31). The Codex also contains many notes on flowing water and hydraulics.

Stephen J. Gould’s essay The Upwardly Mobile Fossils of Leonardo’s Living Earth, provides an erudite account of the Renaissance context of da Vinci’s foray into geology. The prevailing 15th and16th century explanation for fossil clams found on mountain tops was that they had grown, by some mechanism of mineralization, within the rocks. The word fossil was used to describe these structures, but it had no biological or organic connotation – fossils grew by mineral emanations. Da Vinci’s observations and inferences about fossil clams were astute. By observing living molluscs, he surmised that his mountain clams were originally living forms on the sea floor, that became filled with mud. They now reside at high elevations because sea levels were once higher. The interpretation has, at its core, a kind of pre-Huttonian version of Uniformitarianism. His arguments specified that:

  1. If they were mineral formations, why are they not forming now?
  2. Why were complete, articulated clams mixed with broken fragments – growing as broken fragments did not make a lot of sense.
  3. He recognised that certain shell layers occurred on both sides of a valley. They must have been, at some time, part of the same continuous layer. This conclusion is even more fascinating because it presaged Steno’s axiom of lateral continuity.
  4. Clams occurred in successive layers – these could not have been deposited during a single deluge.
  5. He identified growth rings on the outer surfaces of shells. If they had grown in the rock, what did they eat?
  6. Da Vinci was well versed in the action of water with sediment (there are many sketches of flowing water attached to his ideas on hydraulics). He noted that living clams were associated with water-borne sediment, and that the fossil versions showed similar characteristics.
  7. He observed that living clams are associated with worm traces, and that this association was also present in fossil form – his must have been some of the earliest descriptions of trace fossils as organic structures.

 

Nicolaus Steno (1638-1686)

Nicolaus Steno was a Danish scientist, a pioneer in both anatomy and geology who became a Catholic bishop in his later years. He, like da Vinci, was one of the early proponents of paleontology, recognizing fossils as remnants of ancient life, rather than structures that had grown in the rock. Unlike Da Vinci, Steno published his observations and interpretations.

 

Nicolaus Steno the Bishop, and the title page to his opus on stratigraphy, publication 1669

Steno the Bishop, and the title page to his opus on stratigraphy

Steno is best known for his three stratigraphic principles which were published in de Solido… in 1669, but were largely forgotten until rediscovered by Alexander von Humboldt 154 years later – apparently von Humboldt let Charles Lyell know of the book’s existence (Hansen, 2009). However, Steno also contributed important philosophical narratives on scientific method, emphasizing its separation from religious arguments, reducing problems to their simplest terms while keeping an eye on the whole picture, and making inferences based on experiment, analogy and logic. His 17th century version of scientific method is remarkably modern.

The law of superposition is an axiom that is fundamental to geology, archaeology, and other fields dealing with geological stratigraphy. In plain language, it states that in undeformed stratigraphic sequences, the oldest beds will be at the bottom of the sequence.

The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity.

The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. This concept is central to geological mapping and correlation of beds or successions of beds.

 

Antoine Lavoisier (1743-1794)

Lavoisier was one of those enlightenment personalities who dabbled in everything. He is best remembered for his discovery of the elements oxygen and sulphur, and the process of burning.

His contribution to stratigraphy (1789) was also significant in the following ways:

  • He recognised that the grain size of sediment on the sea floor changed from beach to offshore, and that this property could be used in the stratigraphic record where beds having different character occur in succession. This concept is central to facies analysis and paleoenvironmental reconstructions.
  • Importantly, he associated these stratigraphic changes with rises and falls in sea level, and consequently migrating shorelines. His published diagram clearly illustrates the progress of sedimentation during transgression and regression – concepts that form the basis of modern stratigraphic analysis. The formulation of migrating shorelines is commonly attributed to Walter Grabau, 1909 (Carozzi, 1965), but Lavoisier may have beaten him to it.

Lavoisier's depiction of transgression and regression

Carozzi’s paper contains his translation of the original Lavoisier memoir “General Observations on the Recent Marine Horizontal Beds and on Their Significance for the History of the Earth”. In his reconstruction, Chalk (2) and near shore littoral strata (including chalk-chert nodule conglomerate – 3) onlap bedrock (1) during sea level rise. They are overlain by deep water, fine-grained pelagic deposits (4), the depositional edge of which also migrates landward. The pattern reverses during regression, where littoral deposits (5) and the shoreline, migrate seaward – in Lavoisier’s own words “I shall call these deposits littoral beds formed in receding sea in order to distinguish them from similar ones formed in rising sea” (Carozzi, 1965, p. 79 – the emphases are Lavoisier’s).

Lavoisier’s inquisitiveness and passion extended to finance and politics. He must have rubbed some folk the wrong way during the French Revolution, tragically losing his head, a victim of the unbridled violence of the time.

 

James Hutton (1726-1797) and  The Principle of Uniformity

Hutton unconformity at Lochranza, Arran

The prevailing dogma during most of the 17th and 18th centuries was that earth was no older than about 6000 years according to calculations based on scripture by Bishop Ussher. The processes that form landscapes, volcanoes and mountain belts were thought of as Catastrophic, a concept that has its roots in the Noachian deluge. James Hutton, regarded by most as the founder of modern geology, presented in 1785 to the Royal Society of Edinburgh, his contrary thoughts on how the Earth formed. Hutton understood that the Earth was really old, a conclusion based on lengthy observation of natural processes. He could find “… no vestige of a beginning, no prospect of an end”. This is a profound statement. For Hutton geological time was immense. It had to be immense because the processes he observed (that produce landscapes) worked inexorably slowly from a human perspective. His ideas were the complete antithesis of Catastrophism.

One of the most important of Hutton’s corollaries was that the natural processes resulting in landscapes, mountain belts and oceans progressed with the same intensity and as uniformly in the distant past as they do today. This statement has had a profound influence ever since. This is the concept of Uniformity; the expression uniformitarianism was coined by William Whewell in 1832. The other iconic expression – the present is key to the past – was invented by geologist Archibald Geikie in 1905.

An important part of Hutton’s expression of deep time is unconformities. The example from Lochranza (Arran, west Scotland) illustrates beautifully the contortions of rock units that must have contorted Hutton’s mind when confronted with it. That he was able to unravel the sequence of geological events at this and other well-known sites attests to the depth of his inquiry and creativity. We now know that the time gap at the unconformity is about 240 million years.

 

Mary Anning (1799-1847)

Portrait of Mary Anning pointing at an ammonite, by B. J. Donne from 1847

Portrait of Mary Anning pointing at an ammonite, by B. J. Donne from 1847,

At a time when science was undertaken primarily by ‘gentlemen’ of sufficient social standing, Mary Anning’s rise to prominence raised more than a few eyebrows. The thought of a woman clambering along cliffed beaches, hems muddied, was too much for such gentlefolk. She began her naturalist leanings by collecting fossils, specifically Jurassic fossils from the Lyme Regis coast (southwest England). Many specimens were sold to help make ends meet. She became so proficient at collecting and identifying molluscs (particularly ammonites and belemnites) and other fossil groups, that luminaries like Louis Agassiz, Henry de la Beche, and William Buckland  requested her assistance in the field.  She discovered the first complete Ichthyosaur, plus a relatively intact Plesiosaurus that William Conybeare obtained and presented to scientific circles, but omitted mention of Anning. Mary was the first to identify coprolites for what they really are – fossil excreta.

Despite her reputation and skills (intellectual and physical) she was spurned by learned societies (as were all women at the time). Even the gentlemen who used her contributions in their own publications often refused to acknowledge her. Agassiz on the other hand did acknowledge her in naming a couple of species of fish.

 

Additional links in this series:

A timeline of stratigraphic principles; 19th C – 1950

A timeline of stratigraphic principles; 1950-77

All the stratigraphies

Baselevel, Base-level, and Base level

Sediment accommodation and supply

Facies and facies models

How to read a sea level curve

Autogenic or allogenic dynamics in stratigraphy?

Stratigraphic cycles: What are they?

Sequence stratigraphic surfaces

Parasequences

Shorelines and shoreline trajectories

Stratigraphic trends and stacking patterns

Clinoforms and clinothems

Stratigraphic lapout

Stratigraphic condensation – condensed sections

Depositional systems and systems tracts

Which sequence stratigraphic model is that?

 

References

A.V. Carozzi, 1965. Lavoisier’s fundamental contribution to stratigraphy. Ohio Journal of Science, v.65.

S.J. Gould, 1998. The Upwardly Mobile Fossils of Leonardo’s Living Earth. In: Leonardo’s Mountain of Clams and the Diet of Worms. Harvard University Press, p. 17-44.

J.M. Hansen, 2009. On the origin of natural history: Steno’s modern, but forgotten philosophy of science. Geological Society of America, Memoir 203, p. 159-178.

G.V. Middleton, 1973. Johannes Walther’s law of the Correlation of Facies. Geological Society of America Bulletin, v. 84, p. 979-988.

A.E. Popham, 1965. The Drawings of Leonardo Da Vinci. The Reprint Society London

S. Turner, C.V. Burek, R.T.J. Moody, 2010. Forgotten women in an extinct saurian (man’s) world. In, Moody, R. T. J., Buffetaut, E., Naish, D. & Martill, D. M. (eds) Dinosaurs and Other Extinct Saurians: A Historical Perspective. Geological Society, London, Special Publications, 343, 111–153.

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Mineralogy of carbonates: Stromatolite reefs

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Large, elongate stromatolite mounds - a Precambrian reef complex

Large, elongate stromatolite mounds – a Precambrian reef complex

An example of a Paleoproterozoic stromatolite reef

This is part of the How To…series  on carbonate rocks

Reefs are the largest biological constructions on Earth. They are ecological powerhouses, domicile to a multitude of invertebrates and vertebrates, algae and phytoplankton, all playing their part in acts of communion, competition, and symbiosis.

Reefs are also geological constructions:

  • Modern coral reefs, and most reefs since the beginning of the Cambrian (541 million years ago) have rigid frameworks of corals, stromatoporids or rudists.
  • Reefs fringe islands, platforms and continents.
  • They range in size from small, isolated pinnacles and oceanic atolls, to extraordinarily large barrier systems; Great Barrier Reef extends about 2300km along the eastern coast of Australia.
  • Reefs act as baffles to ocean waves. Waves support reef life by constantly refreshing oxygen and nutrients, but they can interrupt progress during damaging storms.
  • Reefs have a partnership with lagoons.

These are the reefs we are most familiar with. They even evoke romantic notions, the splendours of nature; diving amongst brilliantly coloured corals and fish, home to apex predators, ships dashed upon… etc.

Earth’s earliest reefs looked quite different. For almost 3 billion years, Precambrian reefs consisted of stromatolites, constructed by micro-organisms; primarily cyanobacteria.  They lacked rigid frameworks – most were susceptible to erosion, although sea floor cementation might have rendered them crusty. There were no grazing invertebrates – the ecological web must have been much simpler. Despite these fundamental differences, Precambrian reefs occupied and responded to similar oceanographic conditions as their Phanerozoic cousins:

  • Cyanobacteria are photosynthetic prokaryotes which means the buildups accumulated within the photic zone, responding to changes in filtered light and seawater temperature.
  • Stromatolite reefs defined platform margins, sometimes extending across entire platforms and shelves.
  • Lagoons, tidal flats and sabkhas were partnered with the offshore buildups.
  • Reefs that developed on platforms or shelves were usually associated with deeper water slopes that deposited hemipelagic material plus any coarse-grained shallow water sediment that bypassed the reefs. Slope facies may include mass transport deposits (slumps and slides, turbidites and debris flows).
  • Stromatolite reefs and associated platform facies responded to fluctuating sea levels.
  • Extensive buildups acted as baffles to waves and currents, and in turn were moulded by these forces.
  • They were occasionally battered by storms.
  • They were long-lived constructions. Large reefs probably represent hundreds of thousands and even millions of years accumulation.

 

Stromatolite reefs of the Mavor Formation

Landsat image of Belcher Islands, Hudson Bay.

LANDSAT’s view of Belcher Islands (north at top). The aerial view (above image) is from Tukarak Island. Image credit: NASA, Jesse Allen, University of Maryland.

The Paleoproterozoic succession on Belcher Islands (Hudson Bay, Canada) contains carbonate platforms replete with stromatolites and cryptalgal laminates. One stratigraphic unit, the Mavor Formation, contains spectacular mounded buildups. One of the best exposures is shown below; here one can walk through a continuous succession, from beach to platform margin. The margin itself is delineated by an abrupt change in sedimentology, from large, high synoptic-relief stromatolitic buildups, to thin bedded, carbonate-rich rhythmites and shale deposited on the seaward slope.  Slump packages and turbidites do occur but are not common. A few small, isolated stromatolites domes grew on the upper slope (i.e. outboard of the platform margin) – this probably reflects the maximum limit of the photic zone. The platform margin is a mappable boundary; slope deposits belong to the Costello Formation. The stratigraphic thickness of the reef package in the aerial view is 244m.

One of the better aerial views of the stromatolite reefs and platform margin (dashed line), east Tukarak Island. The horizontal distance across the buildups (left to right) is about 800m; beds dip right (east) 10o-15o. Buildup stratigraphy is 244m thick. Small subtidal mounds (left) coalesce towards the outer margin into larger buildups. Locations (a) through (d) refer to the 3D reconstructions shown below.

One of the better aerial views of the stromatolite reefs and platform margin (dashed line), east Tukarak Island. The horizontal distance across the buildups (left to right) is about 800m; beds dip right (east) 10o-15o. Buildup stratigraphy is 244m thick. Small subtidal mounds (left) coalesce towards the outer margin into larger buildups. Locations (a) through (d) refer to the 3D reconstructions shown below.

Shallow water facies

Depending on location, the landward part of the platform changes from sandy beach to intertidal-shallow subtidal flats. Facies indicative of beach settings include crossbedded grainstone, edgewise conglomerate pavements, and beachrock. Typical tidal flat structures are lenticular and flaser cross bedding, plus indications of prolonged exposure with mud cracks and gypsum crystals (replaced by dolomite). Local clusters of branched stromatolites and laminates show frequent scouring and reworking by storms.

Cross-section of tabular beds and lenses of edgewise conglomerate, interbedded with lenticular crossbedded grainstone. Conglomerate slabs consist of early cemented carbonate mud and cryptalgal laminates, probably ripped up during storm surges. Bedding views of the conglomerate show them to have formed extensive pavements.

Cross-section of tabular beds and lenses of edgewise conglomerate, interbedded with lenticular crossbedded grainstone. Conglomerate slabs consist of early cemented carbonate mud and cryptalgal laminates, probably ripped up during storm surges. Bedding views of the conglomerate show them to have formed extensive pavements.

Reef transitions across the platform

Three-dimensional reconstruction of Mavor platform mounds, from shallow subtidal laminates at the base, to high relief mounds at the platform margin. The stratigraphic section corresponds to that shown in the aerial view. Total stratigraphic thickness here is 244m.

Three-dimensional reconstruction of Mavor platform mounds, from shallow subtidal laminates at the base, to high relief mounds at the platform margin. The stratigraphic section corresponds to that shown in the aerial view. Total stratigraphic thickness here is 244m.

In the image above, the four panels reconstructed from field sketches and photos, show the stratigraphic changes in buildup organization. Beyond the beach, shallow subtidal mounds are relatively small, have synoptic reliefs of a few centimetres, and consist of laterally linked hemispheroidal domes and columns. Mounds are elongate, parallel to the overall trend seen in the aerial view; elongation direction is normal to the paleoshoreline, the strike of which was established from bedform azimuths.

Shallow subtidal, relatively simple, laterally linked and bridged mounds with synoptic relief of a few centimetres.

Shallow subtidal, relatively simple, laterally linked and bridged mounds with synoptic relief of a few centimetres.

Moving across the platform, corresponding to higher stratigraphic levels, mounds coalesce into fewer but larger buildups, with concomitant increases in synoptic relief. Synoptic relief is a maximum 2m to 3m at the platform margin. Here, buildups consist almost entirely of wavy, undulating and crinkly laminates; digitate branching is far less common. In addition, smaller scale elongate mounds are superposed on the larger structures. Thin beds of mud and laminate rip-ups, floored by shallow scours, indicate the passage of storms. Disruption by storms is seen in all the buildups including those near the platform margin. This implies that the depth of the margin itself was close to storm wave-base.

The exposure in places allows one to walk out individual beds from mound crest through the adjacent trough to the opposite mound; the only thing that’s missing is the water over your head.

High synoptic relief reef mounds viewed towards the platform margin, slightly oblique to elongation axes. In places, individual beds can be traced across mounds. These large structures were constructed entirely of laminated cryptalgal laminates. Eastern Tukarak Island.

High synoptic relief reef mounds viewed towards the platform margin, slightly oblique to elongation axes. In places, individual beds can be traced across mounds. These large structures were constructed entirely of laminated cryptalgal laminates. Eastern Tukarak Island.

 

Small mounds commonly adorned the crests and flanks of the high relief structures. Their elongation directions parallel the trend of the larger mounds. Jacob’s Staff (centre) is 1.5m long. Eastern Tukarak Island.

Small mounds commonly adorned the crests and flanks of the high relief structures. Their elongation directions parallel the trend of the larger mounds. Jacob’s Staff (centre) is 1.5m long. Eastern Tukarak Island.

 

Smaller-scale domes superposed on the large reef structures, are simple, laterally linked laminates, having synoptic relief in the range 20-30 cm. (hammer centre right). Eastern Tukarak

Smaller-scale domes superposed on the large reef structures, are simple, laterally linked laminates, having synoptic relief in the range 20-30 cm. (hammer centre right). Eastern Tukarak Island.

 

Wavy cryptalgal laminates from an outer platform buildup, disrupted by a shallow scour surface and overlain by rip-up clasts. Disruption by storms was common.

Wavy cryptalgal laminates from an outer platform buildup, disrupted by a shallow scour surface and overlain by rip-up clasts. Disruption by storms was common. Also some nice stylolites.

 

Wavy and crinkly laminates make up the bulk of the high-relief buildups that grew on the middle and outer platform.

Wavy and crinkly laminates make up the bulk of the high-relief buildups that grew on the middle and outer platform.

The slope

The transition from reef buildups to slope shale and carbonate rhythmites is abrupt, although a few small, isolated stromatolite domes occur immediately outboard of the platform margin, within slope mudrocks; these occurrences are inferred to represent the photic zone depth limit of stromatolite growth . The rhythmites are particularly striking, with alternating intervals of red and white dolomitized carbonate mudstones and calcilutites. There are a few graded beds containing more sandy sediment (mudstone fragments, ooids) and climbing ripples, deposited by turbidity currents. Clearly, some coarse-grained, shallow water sediment was moved onto the platform, perhaps through troughs between the mounds, eventually bypassing the margin.  Slump structures also occur but like the turbidites, are not common.

Slope rhythmites composed of calcilutite and dololutite. The ‘lumpy’ appearance is mainly due to incomplete replacement of calcitic mud to dolomite; this is one of the few units in the Belcher stratigraphy that still contains some calcite. Left: A relatively continuous succession of rhythmites, cut by numerous small extension faults. Top right: Bouma c (climbing ripples) and thin d (laminated mud) intervals of a turbidity current. Sandy material in the c interval was derived from the inshore deposits. Bottom right: Dololutite with some terrigenous mud (red hues), interbedded with white calcilutite rhythmites.

Slope rhythmites composed of calcilutite and dololutite. The ‘lumpy’ appearance is mainly due to incomplete replacement of calcitic mud to dolomite; this is one of the few units in the Belcher stratigraphy that still contains some calcite. Left: A relatively continuous succession of rhythmites, cut by numerous small extension faults. Top right: Bouma c (climbing ripples) and thin d (laminated mud) intervals of a turbidity current. Sandy material in the c interval was derived from the inshore deposits. Bottom right: Dololutite with some terrigenous mud (red hues), interbedded with white calcilutite rhythmites.

The Mavor platform was dominated by muddy carbonate sediment. It was a large carbonate factory where the production line consisted entirely of cyanobacteria.  Reef and slope stratigraphy indicate long-term transgression, conditions conducive to the viability of factory production. There are no chronostratigraphic controls on this part of the succession, but I speculate the cycle duration is roughly equivalent to a 3rd-order Sequence.

 

Links to other posts in this series:

Mineralogy of carbonates; skeletal grains

Mineralogy of carbonates; non-skeletal grains

Mineralogy of carbonates; lime mud

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; cements

Mineralogy of carbonates; sea floor diagenesis

Mineralogy of carbonates; Beachrock

Mineralogy of carbonates; deep sea diagenesis

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates; Karst

Mineralogy of carbonates; Burial diagenesis

Mineralogy of carbonates; Neomorphism

Mineralogy of carbonates; Pressure solution

Stromatolites in outcrop

 

References

Bosak, A.H. Knoll, and A.P. Petroff, 2013. The Meaning of Stromatolites. Annual Reviews of Earth & Planetary Science, v. 41, p. 21-44. Free Access. Lots of great references.

T.E. Playton, X. Janson and C. Kerans. 2010. Carbonate slopes and margins. In N.P.James and R Dalrmple (Editors), Geological Association of Canada, Facies Models 4, Chapter 18. P. 449-476.  Available for download

E.P. Suosaari, R.P. Reid, and M.S. Andres, 2019. Stromatolites, so what?! A tribute to Robert N Ginsberg. Depositional Record, v. 5, p. 486-497. Open Access. Evaluates some of the main controversies with stromatolites and microbialites. Lots of great references.

B.D. Ricketts and J.A. Donaldson, 1989. Stromatolite reef development on a mud-dominated platform in the Middle Precambrian Belcher Group of Hudson Bay. In, H. Geldsetzer, N.P. James and G. Tebbutt (Eds.), Reefs, Canada and adjacent areas. Canadian Society of Petroleum Geologists, Memoir 13.

H.D. Williams and others, 2011. Investigating carbonate platform types: Multiple controls and a continuum of geometries. Journal of Sedimentary Research, v. 81, p.18-37. Good account of factors influencing platform and ramp geometries using 2D modelling. Available for download

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Sedimentary structures: Stromatolites

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Exhumed stromatolite domes

A look at stromatolites and cryptalgal laminates in outcrop

This is part of the How To…series  on describing sedimentary rocks

Two billion years ago, life was in full swing. Rocks in the Belcher Islands (Hudson Bay, Canada) record some of these ancient life forms, the microorganisms, dominated by prokaryotic cyanobacteria (sometimes called blue-green algae). Careful teasing of the rock record reveals their story, where we see a wonderful array of delicately laminated mats (or cryptalgal laminates), and stromatolites, those architectural delights are preserved as simple domes, complexly branched columns, and even vast, reef-like build-ups. The environments in which they grew ranged from the most landward extent of ocean tides (supratidal) to deeper shelf and platform. Prokaryotic cyanobacteria are mostly photosynthetic organisms meaning that stromatolite growth took place within the photic zone; the water depth limit of modern marine photic zones is about 200m.

All the images here are from Belcher Islands. Al the examples occur in dolostones, where the original carbonate (aragonite, calcite, high magnesium calcite, protodolomite) was replaced by dolomite during early through late burial diagenesis.

The Atlas of Stromatolites and Cryptalgal Laminates contains many more excellent examples.

 

What is a stromatolite?

Most definitions of usually include the following attributes (Walter, 1972; Bosak et al. 2013, Suosaari et al. 2019):

  • They are organosedimentary structures, which means that biological and sedimentary processes are involved in their growth
  • Their biological foundations are microbes; prokaryotic cyanobacteria during the Precambrian, with increasingly important contributions from eukaryotes like green and red algae,  and phytoplankton such as diatoms during the Phanerozoic.
  • They tend to accrete from the sediment-water or substrate interface via trapping of fine sediment and precipitation of carbonate.
  • Accretion may be laminar (laminated stromatolites abound in Precambrian rocks), or as diffuse, non-structured or clotted forms known as thrombolites.

Some studies in the 1950s-60s, particularly by Russian geologists (or at that time Soviet geologists), advocated biological controls on stromatolite morphology, conferring biostratigraphic value to Precambrian genera like Inzeria, Conophyton, and Tungussia (an example is shown below). This would indeed have been a happy circumstance, given that most of the Precambrian lacks any other biostratigraphic markers. Unfortunately, with burgeoning interest in stromatolites from the 1970s and on, it became apparent that stromatolite morphology was largely a function of environmental conditions – this is particularly true for high energy environments (e.g. waves and currents in intertidal and shallow subtidal environments), although biological controls may gain in importance in low energy conditions (e.g. deeper subtidal).

 

Something to keep in mind as you work through these structures

Stromatolites in outcrop commonly appear as columns or domes, in some cases extending vertically several metres. But their sea-floor profiles, or synoptic relief during growth were low. We can visualize this when tracing individual laminae or sets of laminae (ie. the original mat surface) from one column to the next. Your average intertidal, shallow shelf or platform stromatolite extended no more than a few millimeters or centimeters above the sea floor. Some large mounds, or reef-like structures had a few metres relief; but nothing like more recent coral reefs. This also means that the environmental conditions for incremental growth must have been stable for long periods of time (decades to perhaps 1000s of years). This needs to be kept in mind when looking at cryptalgal structures in outcrop; their apparent size can be misleading.

Illustration of synoptic relief on the growing surfaces of domal and bulbous stromatolites. The dashed line traces a single set of laminae across several structures.

Illustration of synoptic relief on the growing surfaces of domal and bulbous stromatolites. The dashed line traces a single set of laminae across several structures.

Stromatolite morphology

The chart shown here is one of M.R. Walter’s (1972) early categorizations of stromatolite shapes, laminae structure, branching, and ornamentation. There have been a few iterations, but the basic descriptive attributes have not changed.

Chart showing the morphological description of stromatolites

 

Associated sedimentary structures

The list includes some of the more common structures in sedimentary facies associated with stromatolitic buildups. Typical environmental indicators are also noted. Phanerozoic stromatolites may be accompanied by benthic, infaunal and epifaunal invertebrates.

 

Other useful links in this series

Mineralogy of carbonates: Stromatolite reefs

Sedimentary structures: coarse-grained fluvial

Sedimentary structures: fine-grained fluvial

Sedimentary structures: Mass Transport Deposits

Sedimentary structures: Turbidites

Sedimentary structures: Shallow marine

Describing sedimentary rocks; some basics

Measuring a stratigraphic section

 

The images

A nice polished surface showing anastomosing and coalescing domal stromatolites, and oncoids that became stabilized, forming the foundations for larger domes. Subtidal, washed by currents and subjected to occasional storms. McLeary Formation.

A nice polished surface showing anastomosing and coalescing domal stromatolites, and oncoids that became stabilized, forming the foundations for larger domes. Subtidal, washed by currents and subjected to occasional storms. McLeary Formation.

 

Large stromatolite domes with low synoptic relief and bridging laminae. Probably deeper subtidal. Mavor Formation.

Large stromatolite domes with low synoptic relief and bridging laminae. Probably deeper subtidal. Mavor Formation.

 

Exhumed, elongate stromatolite domes on bedding. Stromatolite elongation and orientation is useful for paleocurrent analysis. The intermound muds have been preferentially removed by erosion. Synoptic reliefs here commonly 5-10 cm. Shallow subtidal subjected to wave and current wash. McLeary Formation.

Exhumed, elongate stromatolite domes on bedding. Stromatolite elongation and orientation is useful for paleocurrent analysis. The intermound muds have been preferentially removed by erosion. Synoptic reliefs here commonly 5-10 cm. Shallow subtidal subjected to wave and current wash. McLeary Formation.

 

Closely spaced, parallel branched stromatolite columns. Good wall structure, and some ornamentation on the columns. Shallow subtidal, washed by waves. McLeary Formation

Closely spaced, parallel branched stromatolite columns. Good wall structure, and some ornamentation on the columns. Shallow subtidal, washed by waves. McLeary Formation

 

Bedding showing a plan view of densely packed stromatolite columns. This is the same bed as the image of parallel-sided columns shown above. Shallow subtidal, washed by waves. Hammer for scale (circled)

Bedding showing a plan view of densely packed stromatolite columns. This is the same bed as the image of parallel-sided columns shown above. Shallow subtidal, washed by waves. Hammer for scale (circled)

 

Highly divergent branching typical of the form Tungussia. I have made an attempt to trace the columns and laminae (inset). Mavor Formation. Dolomite recrystallization in this unit has been relatively intense; this tends to obscure the finer structural details

Highly divergent branching typical of the form Tungussia. I have made an attempt to trace the columns and laminae (inset). Mavor Formation. Dolomite recrystallization in this unit has been relatively intense; this tends to obscure the finer structural details

 

Some really nice microdigitate and wavy-undulating cryptalgal laminates. Scouring interrupted growth at several stages, and also produced mud-mat rip-ups. Mostly intertidal. McLeary Formation

Some really nice microdigitate and wavy-undulating cryptalgal laminates. Scouring interrupted growth at several stages, and also produced mud-mat rip-ups. Mostly intertidal. McLeary Formation

 

Changing mat morphology, from undulating to wavy, pustulose, and ultimately microdigitate structures, reflecting progressive deepening from supratidal, through intertidal. possibly shallowest subtidal. McLeary Formation. Scale is 3 cm long.

Changing mat morphology, from undulating to wavy, pustulose, and ultimately microdigitate structures, reflecting progressive deepening from supratidal, through intertidal. possibly shallowest subtidal. McLeary Formation. Scale is 3 cm long.

Right image shows traces of some of the growth and erosion surfaces. The sequence begins with simple domes that contain many internal discordances resulting from desiccation and erosion by storms. These morph into digitate columns that grew atop the domes and along their sloping margins. The brown coloured columns have been silicified (resistant); the remainder are dolomite (all in grey hues). Domes and columns are truncated by a major event that left a skinny deposit of cryptalgal mat rip-ups and oncoids in various stages of growth. The topmost layer consists of relatively simple cryptalgal-microbial laminates, but these too contain local erosional discordances. Coin is 19 mm diameter.

Right image shows traces of some of the growth and erosion surfaces. The sequence begins with simple domes that contain many internal discordances resulting from desiccation and erosion by storms. These morph into digitate columns that grew atop the domes and along their sloping margins. The brown coloured columns have been silicified (resistant); the remainder are dolomite (all in grey hues). Domes and columns are truncated by a major event that left a skinny deposit of cryptalgal mat rip-ups and oncoids in various stages of growth. The topmost layer consists of relatively simple cryptalgal-microbial laminates, but these too contain local erosional discordances. All these events contained within a sequence 38 cm thick. Coin is 19 mm diameter. Kasegalik Fm. Belcher Islands.

 

Lots going on in this 40 cm thick interval. In the lower section intertidal-supratidal microbial mats and carbonate mud layers have been disrupted by desiccation and moved around during storm surges. In the middle section, eroded carbonate hardground slabs are stacked edgewise (aka edgewise conglomerate) that in bedding views can be mapped as discontinuous pavements; modern analogues form on relatively high energy beaches. The pavements were later colonised by thin cryptalgal laminates (mats) that contain delicately preserved microdigitate structures - the laminate carbonate here has been partly replaced by silica (chert). Top section is a mix of mud rip-ups, oncolites (concentric layered cryptalgal structures) and carbonate sand that may also be the product of storm surges. 2Ga Belcher I.

Lots going on in this 40 cm thick interval. In the lower section intertidal-supratidal microbial mats and carbonate mud layers have been disrupted by desiccation and moved around during storm surges. In the middle section, eroded carbonate hardground slabs are stacked edgewise (aka edgewise conglomerate) that in bedding views can be mapped as discontinuous pavements; modern analogues form on relatively high energy beaches. The pavements were later colonised by thin cryptalgal laminates (mats) that contain delicately preserved microdigitate structures – the laminate carbonate here has been partly replaced by silica (chert). Top section is a mix of mud rip-ups, oncoids (concentric layered cryptalgal structures that rolled around the sea floor) and carbonate sand that may also be the product of storm surges. 2Ga Belcher I.

 

References

 T. Bosak, A.H. Knoll, and A.P. Petroff, 2013. The Meaning of Stromatolites. Annual Reviews of Earth & Planetary Science, v. 41, p. 21-44. Free Access. Lots of great references.

E.P. Suosaari, R.P. Reid, and M.S. Andres, 2019. Stromatolites, so what?! A tribute to Robert N Ginsberg. Depositional Record, v. 5, p. 486-497. Open Access. Evaluates some of the main controversies with stromatolites and microbialites. Lots of great references.

M.R. Walter, 1972. Stromatolites and the biostratigraphy of the Australian Precambrian and Cambrian. The Palaeontological Association London, Special Papers in Palaeontology 11. Free Access

M.R. Walter, 1976 (Editor). Stromatolites. Developments in Sedimentology 20, Elsevier. Contains papers on all aspects of cryptalgal laminate, including discussions on biostratigraphic utility (or lack of).

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Mineralogy of evaporites: saline lake brines

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Gypsum books, several cm across, precipitated from supersaturated brines within fine-grained sediment and exumed by wind ablation. Altiplano, Chile

Gypsum books, several cm across, precipitated from supersaturated brines within fine-grained sediment and exhumed by wind ablation. Altiplano, Chile

The evolution of brines that produce saline lake evaporites.

This is part of the How To…series  on evaporites

 

Seawater is generally considered to be isochemical because its composition varies little from sea to sea. There are some changes in saturation levels of calcite and aragonite in deep ocean waters (below their compensation depths), and salinity in basins having more restricted seawater inflow (e.g. Mediterranean Sea, Arabian Gulf), but the range of composition variation is limited. This means that evaporation of seawater follows a predictable chemical and thermodynamic path.

Not so terrestrial waters (rivers, groundwater) whose starting compositions are remarkably varied. Most natural waters contain Na in abundance plus an array of cations and anions like K+, Ca2+, Mg2+, Cl, SO42-, HCO3, CO32-, and SiO2. But the starting concentrations of these ions varies greatly meaning that the resulting evaporite mineralogy is also varied. Enter Lawrence Hardie and Hans Eugster whose chemical modelling in the late 1960s and 70s showed us how to make sense of this natural variability and complexity. Their publications are still cited 50 years later (a few are listed below); they are compulsory reading for any student who is interested in evaporites and geochemistry.

Hardie and Eugster (1970) calculated evaporation paths and mineral products for natural waters having different compositions at atmospheric pressure and 25oC. The waters are assumed to be in equilibrium with CO2 (pCO2 is constant) – this is important for CO32- and HCO3 and calculation of pH.  Some of the relevant calculations have been described in a previous post on carbonate geochemistrythe same principles apply to evaporites.

  • Equilibrium constants for each mineral (common evaporite minerals from saline lakes are listed below),
  • Ion activities and activity coefficients,
  • Ion activity products (IAP),
  • Degree of mineral saturation,
  • Ionic strength, low values in dilute water, increasing with evaporation,
  • Note that complex ion pairs are not included in the calculations.

As water evaporates, the activities of cations and anions increases. If the ion activity product equals the equilibrium constant for a mineral, then the solution has become saturated and that mineral will precipitate; at this point the IAP remains constant. The important results in this process are, using calcite as an example:

  • During precipitation, both Ca2+ and CO32- are removed from the original water in equal proportions,
  • Calcite will continue to precipitate until one or both Ca2+ and CO32- have been depleted.
  • It is likely that the original concentrations of Ca2+ and CO32- are not equal. If, in this example Ca2+ > CO32-, then calcite will precipitate until the CO32- is used up, leaving excess Ca2+ available for some other mineral like gypsum.
  • If on the other hand Ca2+ < CO32-, then the Ca2+ will be depleted first leaving none for gypsum.
  • Water is continually removed during evaporation and its concentration decreases; if the process proceeds to desiccation then the concentration of water is zero.

As evaporation proceeds the model predicts which minerals are likely to precipitate and which will not, depending on the starting composition of the water. The succession of mineral precipitates defines the evaporation path for that particular water.

Two reasonably typical examples are shown below (from Hardie and Eugster, 1970, Figure 2). Here, the apices of each triangle plot the changes in ion concentrations as evaporation proceeds. For the example of calcite precipitation, the path moves away from the Ca and CO32- – HCO3 apex towards the Cl apex.

Two examples of ternary plots of water-brine evolution showing brine evolution pathways

Note that the evaporation paths turn abruptly at the point where precipitation depletes the relevant ions.

Example (a): Sulphate rich water that evolves to a Cl brine via precipitation of gypsum.

Example (b): Carbonate rich – sulphate poor water; no gypsum precipitates and the system evolves to a CO3–Cl brine. If calcite precipitates, the brine will likely become rich in Na and K. Halite saturation will be reached if evaporation continues.

In their model, brine evolution curves are complicated by SiO2 and precipitation of sepiolite (illustrated in the flow-path diagrams below). Sepiolite is a hydrated Mg-silicate; its precipitation will influence the Mg path. Precipitation of sepiolite also releases H+ and is therefore an important part of pH buffering (along with the carbonate equilibria). As pH is lowered, CO32- will decrease. Therefore, there is potential for reversal of the CO32- enrichment trends.

Hardie and Eugster (1970) identified 4 important groups of natural waters; Eugster and Hardie (1978) added a fifth group. Note (1) not all waters will fit neatly into one of these groups, and (2) the ions that define each group are dominant –subordinate cation and anion species will also be present:

  • Na–CO3-Cl
  • Na–CO3-SO4–Cl (e.g. Lake Magadi, Kenya)
  • Na–(Ca)–SO4-Cl (e.g. Great Salt Lake, Utah)
  • Mg–Na–(Ca)–SO4-Cl (Poison Lake, Washington)
  • Ca–Mg–Na–(K)–Cl (Bristol Dry Lake, Mohave Desert, California; Dead Sea)
Table of common evaporite minerals in saline lakes; click on the image to enlarge

Table of common evaporite minerals in saline lakes; click on the image to enlarge

In all waters, alkali metal carbonates are the first to precipitate, particularly calcite or aragonite depending on the Mg/Ca ratio. This is a critical first stage in evaporation because it determines the subsequent precipitation sequence of minerals as Ca2+ and CO32- are removed from solution. This is the Calcite Divide, separating waters that become HCO3 rich or  HCO3 poor (keep in mind that continued evaporation increases the overall ionic strength). Depending on the SO4 concentration, excess Ca results in gypsum precipitation that, in turn, creates the next geochemical divide – the gypsum divide that determines whether the brines evolve as SO4 rich – Ca poor, or SO4 poor – Ca rich. One of the last minerals to precipitate is halite from Cl-rich brines.

Brine evolution is shown diagrammatically below (modified from Figures 7 & 8 in Hardie and Eugster, 1970; Warren, 2016, Fig. 2). The two pathways represent waters that include sepiolite precipitation, and those lacking significant aqueous SiO2. For the case where sepiolite precipitates, there is an additional path towards gypsum saturation because of the pH effect on CO3-HCO3 (noted above), such that the brine becomes enriched in Ca.

Schema for brine evolution pathways, showing the calcite and gypsum divides

You should also check the flow diagram in Warren (2016) Figure 2, that shows in greater detail complications such as changes in Mg/Ca ratios, variations in HCO3, in addition to sepiolite production.

In this post, I have focused on the work of Hardie and Eugster because it has been pivotal in guiding more recent research. Of course, there have been modifications and improvements to their models, and you can access these recent advances in the literature cited below.

Links to related topics

Mineralogy of evaporites: The rise of diapirs

Mineralogy of evaporites: salt tectonics

Mineralogy of evaporites: Saline lakes

Mineralogy of evaporites: Death Valley hydrology

Mineralogy of evaporites: Marine basins

Mineralogy of carbonates; classification

Mineralogy of carbonates; carbonate factories

Mineralogy of carbonates; basic geochemistry

Mineralogy of carbonates; meteoric hydrogeology

Mineralogy of carbonates: sabkhas

 

References

M. Babel and B.C. Schreiber, 2014. Geochemistry of evaporites and evolution of seawater. Treatise on Geochemistry. Elsevier, p. 483-560. Deals with marine evaporites but many aspects relevant to non-marine. Encyclopedic with extensive list of references

D.M. Deocampo and B.F. Jones. 2014. Geochemistry of Saline Lakes. Treatise on Geochemistry. Elsevier, p. 437-469. Encyclopedic with extensive list of references

H.P.Eugster, 1980. Geochemistry of evaporitic lacustrine deposits. Annual Review of Earth & Planetary Sciences, v. 8, p. 35-63.

L.A. Hardie and H.P.  Eugster, 1970. The evolution of closed basin brines. Mineralogical Society of America, Special Paper v. 3, p. 273-290. 50 years old but still an iconic paper and a must-read.

J. Warren, 2016. Evaporites. In W.M. White (Ed.) Encyclopedia of Geochemistry. Springer International, p. 1-8. Concise summary of saline lake and marine brine.

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Mineralogy of carbonates; Beachrock

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Beachrock on a Rarotonga shore

 

Early lithification of beach sand.  This is part of the How To…series  on carbonate rocks

One of the most recognizable products of seafloor cementation is beachrock; lithified beach sand. Modern beachrock is common on tropical coasts, those that are wave-washed and quieter shores sheltered by reefs and island barriers. It is less likely to be found bordering cooler seas. Beachrock forms in clean carbonate and siliciclastic-volcaniclastic sands. Lithification occurs so rapidly (months, years)  that one can find all sorts of interesting relics entombed – shells, fish skeletons, coconuts, the flotsam and jetsam of wars, boats that have come to grief, or the refuse strewn by centuries of ocean travelers.

Beachrock affords a domicile for algae and invertebrates that would not normally enjoy living on a soft sand beach; thus, its formation may change existing biotas. It also provides a protective carapace to a beach, reducing the impact of waves. Formation of beachrock potentially changes the beach dynamics. Continue reading

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The provenance of detrital zircon

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detrital zircon from Pleistocene dune sands, northern NZ. Note relatively sharp crystal faces

This post is part of the How To…series – using zircon geochronology to decipher provenance

Zircon is a common accessory mineral in igneous and metamorphic rocks so it’s not surprising that it is also a common constituent of sedimentary heavy mineral suites. Detrital zircon has assumed a remarkable popularity over the last 2-3 decades as a provenance indicator because:

  • crystals contain measurable amounts of uranium (U), lead (Pb) and thorium (Th) isotopes and can therefore be dated radiometrically,
  • zircon is resistant to chemical and mechanical change – crystals can survive multiple sedimentary cycles (i.e. episodes of erosion from source rocks, deposition, burial and uplift, whereupon the whole process begins anew), and
  • they commonly contain multiple stages of crystal growth that record magmatic, metamorphic and depositional episodes.

Continue reading

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Bits of North America that were left behind

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Precambrian Lewisian gneiss exposed at Durness, Scottish Hebrides.

How bits of ancient North America (Laurentia) were left behind in the Scottish Hebrides.

The jigsaw puzzle of continents and oceans, the ground beneath you, the seas beyond, even the weather you enjoy or endure, are governed largely by plate tectonics. This grand mechanism creates plates along mid-ocean volcanic ridges, then proceeds to push them down the throat of subduction zones. Plates collide, tearing at each other’s crust. Volcanic hiccups, earthquakes, and crustal dismemberment are all part of a tectonic plate’s stressful life. And occasionally in this mad nihilist rush (after all, millimeters per year is pretty quick), bits are left behind.

The landscapes of north Scotland and northwest Ireland are underpinned by rocks that once belonged to North America, or at least an ancient version of it. As geological puzzles go, they are iconic; here James Hutton unraveled the problems of deep time, and Peach and Horne sliced the ancient crust into moveable slabs. The rocks are part of the Caledonian Orogen, a mountain chain that formed from tectonic plate collisions more than 400 million years ago, stretching from Scandinavia to Scotland-Ireland, east Greenland, and the Appalachians of eastern USA and Canada.

The choice of a starting point for a story like this is a bit arbitrary because continental and oceanic plates, and the plate tectonic mechanisms that propel them across the globe, date back at least one billion years, possibly earlier. For convenience, this tale begins on the ancient continent of Laurentia about a billion years ago; Laurentia was an amalgam of North America, Greenland, and (what would become) north Scotland and northwest Ireland tucked along its eastern margin [The first four figures here are modified from an excellent technical summary of this important period in Earth’s history, by David Chew and Rob Strachan, their Figure 1, in Geological Society of London, Special Paper 390, pages 45-91, 2014].

Unconformity between Lewisian Gneiss and Proterozoic Torridonian sandstone

Three groups of rock that underpin the Scottish Highlands, originally formed along the eastern Laurentian margin. Lewisian gneisses. Some as old as 2.7 billion years, were part of the basement foundations of Laurentia (Panel 1 above). Two major groups of sedimentary rock were also deposited along the eastern margin – the Moine group of rocks, that beneath the Northern Highlands we now see as metamorphic rocks, originally formed as sediment shed from the ancient continent about 1000 to 870 million years ago. Dalradian metamorphic rocks that now form the Grampian Highlands also originated as sediments and volcanics from about 800 to 510 million years – metamorphism occurred much later.

For the next few million years Laurentia moved south (south of the Equator!) towards, it is hypothesized, a volcanic arc, similar perhaps to modern Ring of Fire volcanic arcs that rim the Pacific Ocean (Panel 2). Collision between Laurentia and the Grampian Arc initiated the first phase of Caledonian mountain building 475-465 million years ago (Panel 3).

Several other events were also taking place at this time. Laurentia itself was rotating anticlockwise. Two smaller continental plates appeared on the scene: Baltica (that would later become Scandinavia and north Europe), and Avalonia (whence the rest of England, Wales and south Ireland resided), both were migrating north towards Laurentia. The intervening ocean, the Iapetus, was gradually shrinking as its crust was devoured down at least three subduction zones (Panel 3).

The Iapetus eventually closed; some slivers of oceanic crust (called ophiolites) were scraped off and incorporated into the Caledonian mountain complex, but most of this once-grand ocean basin was consumed in Earth’s grand recycling depot.

Baltica and Laurentia were involved in head-on collision around 435-425 million years (Panel 4). The Moine thrust, one of the defining ‘moments’ of tectonic dislocation and metamorphism in the Caledonian, developed during this interval. In contrast Avalonia’s approach was more oblique and it appears this smallish continental fragment slid past Laurentia. Avalonia’s legacy is that south England, Wales and south Ireland were now stitched firmly to their northern cousins. This plate tectonic assemblage has withstood tempests, bolides, and glaciations for the last 400 million years; 2000 years of geopolitical ructions are insignificant in comparison.

Moine duplex at Loch Eriboll, thrusting during the Caledonian Orogeny

The amalgamation of Laurentia, Baltica and Avalonia eventually became part of a much larger continental mass, a super continent called Pangea that included Africa, South America, Antarctica, Australia and Asia (and of course, New Zealand). This amalgamation was well underway 335 million years ago. Pangea began to break apart about 175 million years ago, a separation that over the next 175 million years would give us our most recent plate tectonic configuration of ocean basins and continents (Plate 5).  Break up of Pangea took place in several stages, but the event that is of interest here took place about 75-80 million years ago. [Chris Scotese has created an excellent animation of these events, set to nice music].

Plate reconstruction for 94 Ma

Atlantic Ocean had its beginnings during the early stages of Pangea break up, 175 million years ago. Atlantic Ocean’s expansion is centered along a submarine spreading ridge of volcanism (that today stretches from Iceland almost to Antarctica). The spreading ridge migrated northwards, which means that new ocean floor was also being created incrementally northwards. During the early stages of North Atlantic Ocean expansion, the British Isles were still firmly attached to the old Laurentia margin.  But by 80 million years the locus of spreading had moved west of Britain and Ireland (Plate 5), and it was at this point that the ancient roots of north Scotland and Ireland became divorced from North America and Greenland – a decree absolute.

The period of Caledonian mountain building is one of the most studied in the geological community (at least two centuries worth, and 100s of 1000s of scientific papers), much of it undertaken before plate tectonics was discovered in the mid-1960s. Nevertheless, plate tectonics theory has provided a more global context, and a more rational, mechanistic approach to solving the myriad geological complexities.

I recently visited some of these rocks in the Scottish Hebrides and Connemara – and yes, there is complexity at every level of observation. The story I have presented is simplified – perhaps woefully so. But even a simple rendition can promote understanding. I’d like to think so.

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