Tag Archives: Bowser Basin

Stratigraphic cycles: What are they?

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The cyclic repetition of conglomerate beds (resistant units) and interbedded (recessive) shales resulted in a 700+ m-thick stack of fan deltas, where sediment accommodation was generated in the footwall of an active reverse fault ( Jurassic, northern British Columbia). The near vertical dips resulted from later thrust faulting and folding. Stratigraphic top to the right.

The cyclic repetition of conglomerate beds (resistant units) and interbedded (recessive) shales resulted in a 700+ m-thick stack of fan deltas, where sediment accommodation was generated in the foot wall of an active reverse fault ( Jurassic, northern British Columbia). The near vertical dips resulted from later thrust faulting and folding. Stratigraphic top to the right.

Stratigraphic repetition, stratigraphic cycles, and stratigraphic trends 

We know with reasonable certainty that the sun will rise tomorrow.  We know this because it has taken place every day for the last 4.6 billion years. The repetition of this event is comforting; it adds symmetry to our lives – it is predictable.

The universe is full of repeatable events and processes, from the birth and death of stars to the spin of an electron. Some events occur in a more or less haphazard way, with little sense of regularity, or periodicity. Volcanic eruptions are notoriously difficult to predict – any periodicity, if it exists, has alluded our concerted efforts to decipher it. Other events are remarkably regular in their occurrence: night follows day, the ebb and flood of tides, sunspot maxima to minima.

We refer to the regular repetition of events as cycles. Cycles have periodicity – the time it takes to complete one revolution (completion of Earth’s orbit around the sun), or the passage of a wave.  The period of sea or lake surface waves is the time it takes one wave crest  (or trough) to the next crest to pass a reference point; surface wave periods are measured in seconds. Milankovitch (astronomical) orbital cycles have periodicities measured in 104 to 105 years. One of geology’s grandest cycles is the Wilson Cycle that describes the birth and death of ocean basins over 180-200 million years.

We also recognise cycles in stratigraphy, recorded in the repetition of sedimentary facies, fauna and flora associations,  sediment chemistry,  and hiatuses or discordant surfaces, that all represent changing depositional environments over time, fluctuations in relative sea level and sediment accommodation,  migrating shorelines, and the changing conditions of sediment storage and release. Stratigraphic cycles are known from almost all depositional environments on Earth – terrestrial to deep marine; some are 10s of metres thick, others a few millimetres. All these cycles have periodicities, although measuring them continues to be a problem for stratigraphers, particularly those of durations less than the resolution of biostratigraphic or radiometric measures of age.

An example from the Jurassic of northern British Columbia

Packages, or cycles of shale through sandstone, each a few metres thick, are repeated 5 times over about 35m in this outcrop view. Arrows indicate the base and top of each cycle. The example is from Bowser Basin, northern British Columbia.

Packages, or cycles of shale through sandstone, each a few metres thick, are repeated 5 times over about 35m in this outcrop view. Arrows indicate the base and top of each cycle. The example is from Bowser Basin, northern British Columbia.

The outcrop image shows the repetition of shale and sandstone beds over a stratigraphic thickness of about 35m. Working through the outcrop, we observe the following sedimentary and stratigraphic associations:

  • Each package of grey, laminated mudstone, siltstone and thin, ripple-bedded sandstones (2-3m thick), abruptly overlies thick sandstones. Fossils include ammonites, trigonid bivalves, and Buchia, all indicative of an open marine environment.
  • For each package, working our way upward, the proportion and thickness of sandstone beds increases; the upper 1-2m are predominately sandstone.
  • With the increase in the proportion of sandstone beds is a concomitant increase in grain size, from fine-grained to coarse-grained sandstone at the top.
  • The size and frequency of crossbeds increases upwards.

The pattern of facies, from mudstone to sandstone, is repeated five times in this outcrop view, and in fact is repeated dozens of times through the remainder of the exposure. We recognize this stratigraphic repetition as cyclical. The cycles are 5-20m thick. In each cycle there is a reasonably consistent change in grain size from bottom to top – we refer to this stratigraphic trend as coarsening-upward (a descriptive term).

For each cycle, the vertical transition in facies is interpreted as a change from relatively low energy, outer shelf (the mudstone facies), to higher energy inner shelf where wave and current action move sediment (crossbedded sandstone facies). Based on the interpretations, we refer to this kind of stratigraphic trend as shallowing upward (a genetic term).

We designate the base of each cycle at the abrupt contact between mudstone-shale and the underlying sandstone; the rationale for this is:

  1. It is easily identified and is mappable,
  2. It is consistent from one cycle to the next, and
  3. It signals an abrupt change in depositional environment – from relatively shallow shelf (shoreface), to deeper outer shelf. The contact thus represents a significant change in relative sea level (in this case a transgression).

Coarsening-shallowing upward cycles are common in marine successions that are subjected to fluctuating baselevels, sediment accommodation and the seaward or landward migration of shorelines. The opposite trend, fining upward cycles are also common, particularly in fluvial systems; the classic meandering (high sinuosity) channel and point bar cycle contains trough crossbedded sandstone at its base, fining upward to mud-silt-coal facies deposited on  floodplains. In this case, the cycles represent changes in channel baselevel, depositional slope, and storage or release of sediment.

 

Cycle hierarchies

Five hundred years and more of stratigraphic analysis has shown us that there is a hierarchy of cycles that we tend to equate with changes in baselevel,

typically relative sea level. We presently recognize:

1st order cycles – about 50-100 Ma; Continent-wide sequences that represent tectonic plate reorganization such as the break-up of Pangea and Gondwana, and protracted events like the opening of Atlantic Ocean. Commonly cited examples include the Absaroka, Tippecanoe, and Sauk Sequences of North America.

2nd  order cycles – about 5-50 Ma; Superimposed on 1st-order cycles, they may represent changes in mid-ocean ridge spreading rates. 1st and 2nd-order sea level fluctuations are controlled by tectonic, isostatic, and thermal processes (e.g. thermal cooling following a rift episode), as well as glacio-eustatic and climate variations.

3rd order cycles – about 0.2-5 Ma;  There is no single mechanism that controls these brief excursions in relative sea level, but probably includes some short-lived plate reorganizations, the isostatic response to tectonic loading and unloading (particularly at convergent plate boundaries), and glacio-eustatic episodes. Sea level fluctuations are measured in 10s of metres. Note that 3rd-order cycles generally coincide with 3rd-order stratigraphic sequences in the original Vail-Exxon Sequence Stratigraphy model.

4th order cycles – about 100-200 thousand years (ka); Sea level excursions are measured in metres to perhaps 20m (see notes below)

5th order cycles – 10 years -100 ka; As for 4th-order cycles, sea level excursions are measured in metres to perhaps 20m. Although local tectonics may play some role in these short-duration cycles, the primary controls on sea level are thought to be caused by thermal (steric) and glaciation-deglaciation changes to ocean volumes. These short-term fluctuations have periodicities coincident with changes in solar insolation (a measure of the incident radiation reaching Earth), that varies according to Milankovitch orbital cycles: Eccentricity cycles (about 100 ka), Obliquity cycles (41 ka), and Precession cycles (22 ka). Milankovitch orbital cycles are commonly invoked as controls on high order stratigraphic cycles – an hypothesis worthy of debate. These controls on baselevel (sea level) are external to most 4th-5th order depositional systems (i.e. they are allogenic). We now know that autogenic processes can produce cycles at these periodicities. Thus, it is important when analysing 4th and 5th order cycles to take into account the relative contributions of autogenic sedimentation dynamics.

Each cycle order does not occur on its own, independent of the other cycles. Think of them as nested, for example where 2nd-order cycles are superimposed on 1st-order, 3rd-order on 2nd-order, and so on. A useful analogy to help visualize these interactions is the interference of surface waves propagated in different direction; the interference may be destructive or constructive. Thus, a 4th-order sea-level rise that is superimposed on a 3rd-order sea-level fall, will tend to decrease the rate of fall, or even create a temporary rise in the 3rd-order trend. One of the first geologists to consider quantitatively the interactions among the cycle hierarchy was Joseph Barrell (1917).

 

The Jurassic example again

A mountain of 4th-5th order cycles that make up the larger 3rd order outer shelf to fluvial cycle. 'X' marks the approximate location of the cycle outcrop image above. The thickness of the 3rd order cycle here is about 1100m (yellow line). The shelf succession is separated from slope shales by a string of conglomerate-filled shelf-edge gullies.

A mountain of 4th-5th order cycles that make up the larger 3rd order outer shelf to fluvial cycle. ‘X’ marks the approximate location of the cycle outcrop image above. The thickness of the 3rd order cycle here is about 1100m (yellow line). The shelf succession is separated from slope shales by a string of conglomerate-filled shelf-edge gullies.

The Jurassic cycles described above are 4th or 5th order cycles. They constitute part of a much larger stratigraphic succession, shown in the panoramic view. There are dozens of these high-order cycles over the nearly 1100m stratigraphic thickness of this Jurassic shelf. The entire section is interpreted as a 3rd-order cycle, upon which the 4th-5th order cycles were superimposed. The 3rd order cycle contains a stack of 4th-5th order cycles that at the base are each interpreted as representing the transition from outer shelf to inner shelf, and at the top (brown hues – ridge line) represent shallow marine (beach) and coarse-grained fluvial environments. Thus, the 3rd order cycle displays overall a shallowing upward trend, mimicking the much thinner 4th and 5th order cycles.

The 4th-5th order cycles correspond to parasequences in Sequence Stratigraphic terminology. Patterns of 4th-5th order cycle, or parasequence stacking, like those illustrated here, are critical elements in the recognition and interpretation of Stratigraphic Sequences – this is the topic of another post.

 

This 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?

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?

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Atlas of Sequence stratigraphy

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Sequence stratigraphy

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below.

Brian Ricketts –  www.geological-digressions.com

This category of Atlas images, is not intended as a comprehensive outline or set of definitions of sequence stratigraphy, but rather field examples of strata, stratigraphic trends (an essential component of systems tracts), and stratigraphic surfaces.  There are many excellent journal papers, text books, and conference short courses devoted to sequence stratigraphy, so consult them if needs be.

Short articles on important sequence stratigraphic themes are listed here

Some key sequence stratigraphic components illustrated here include (abbreviations on the images):

HST        Highstand Systems Tract (progradational-aggradational) overlies the MFS and underlies the erosion surface formed during the FSST)

FSST       Falling Stage Systems Tract (forced regression and progradation during relative sea-level fall)

LST         Lowstand systems tract (now restricted to end of relative sea-level fall and beginning of sea-level rise)

TST        Transgressive Systems Tract (above the transgressive surface; Retrogradational onlapping during rising relative sea-level and low sedimentation rates. Condensed stratigraphy).

MSF        Maximum Flood Surface (at the transition to sedimentation rates greater than creation of accommodation space. Also the base of the overlying HST).

Each example has a pair of images, one annotated, the other without annotation.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

 

The images:

Sequence stratigraphic framework for the Late Cretaceous - Paleogene Eureka Sound Group, Canadian Arctic. The numbered sequences are referred to for each outcrop image.

Sequence stratigraphic framework for the Late Cretaceous – Paleogene Eureka Sound Group, Canadian Arctic. The numbered sequences are referred to for each outcrop image.

Late Cretaceous to Middle Eocene 3rd-order stratigraphic sequences in the Eureka Sound Group, Canadian Arctic Islands (mostly Ellesmere and Axel Heiberg islands). Sequences 1 and 2, mainly wave-dominated deltas, and along the eastern basin margin, estuarine – shelf. Sequences 3 and 4: river-dominated deltas, sandy inner shelf, muddy outer shelf; Sequence 5 basin inversion and fragmentation into small, thrust-related, syntectonic basins. Sequence 5 represents the acme of the Eurekan Orogeny.

 

                 

Sequence 1, wave-dominated delta parasequences, Strand Fiord, Axel Heiberg Island. There is about 150m of stratigraphy in this view.

 

                   

Detail of Sequence 1 coarsening upward, wave-dominated delta parasequence and MFS. Axel Heiberg Island.

 

                    

Sequence 1, Major subaerial unconformity between Ordovician limestone and onlapping Lower Paleocene estuarine, sandy shelf-bar-sand spit. Combined  karstification and erosion of the limestones produced significant paleotopography. Mount Moore, eastern Ellesmere Island.

 

                    

Sequence 1 estuarine, sand spit, and shallow shelf bars, onlapping karsted Ordovian limestones, Mount Moore, eastern Ellesmere Island. In the foreground, crossbedded sandstone is in direct contact with paleotopography.

 

Reconstructed Early Paleocene paleotopography and Sequence 1 facies.  For details see: Ricketts, B.D.  1991: Lower Paleocene drowned valley and barred estuaries, Canadian Arctic Islands: aspects of their geomorphological and sedimentological evolution; in Clastic Tidal Sedimentology, Rahmani, R.A., Smith, D.G., Reinson, G.E., and Zaitlin, B.A. (ed.); Canadian Society of Petroleum Geologists, Memoir 16, p. 91-106.

 

                     

Sequence 3. River-dominated prodelta – delta front parasequences.  About 300m of stratigraphy here. Abrupt, mappable parasequence tops coincide with the MFS. Axel Heiberg Island.

 

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

Sequence 3: FSST with forced regressive wedge – sharp-based shoreline sandstones formed by wave erosion as sea-level falls.  The subsequent TST and HST extends to the right of the image. Axel Heiberg Island.

 

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

 

Sequence 3, as in the above image pair, focusing on the FSST and forced regressive sandstone wedges. Axel Heiberg Island.

 

Paleocene Falling Stage systems tract, Axel Heiberg I.                       

 

The basal part of the FSST in Sequence 3 (as above), featuring a sharp-based forced regressive shoreline wedge.

 

                       

Sequence 3. Downlap surface with basinward progradation of prodelta mudstone fine-grained sandstone.

 

                         

Sequence 3: Strongly aggradational HST, shelf parasequences, South Bay, Ellesmere Island.

 

                         

Sequence 3: Shelf parasequences, mostly HST, thin TST, and MFS that corresponds with the resistant top of each cycle. For an overview see the next images above. South Bay

 

                   

Sequence 3: A great example of a higher-order subaerial sequence boundary (SB) (top of coal seam), thin TST muddy sandstone, MFS, and HST. The orange blobs are mineralized tree roots. It’s also a much younger me. Strathcona Fiord, Ellesmere Island.

 

 

 

 

Sequence 3, a different perspective, as immediately above.

 

 

 

 

 

Jurassic mid-outer shelf parasequence, Bowser Basin. The MFS immediately overlies the resistant ledge, above which is a thick coarsening  upward HST. Tsatia Mountain, northern British Columbia.

 

                       

Mid-shelf parasequences with well defined MFS, transgressive surfaces, and TSTs. Tsatia Mountain, Bowser Basin.

 

                       

Closer view of a mid-inner shelf parasequence, Tsatia Mountain, Bowser Basin. The coarsening upward HST is capped by a pebbly, fossiliferous TST and MSF. The transgressive surface is one of marine erosion during changing wave-base.

 

                    

Detail of the top HST, transgressive surface (of erosion), the fossiliferous, pebbly TST (ammonites and bivalves), a calcareous mudstone that is part of the condensed TST stratigraphy when terrigenous sediment input was at its lowest, the MFS, and succeeding HST. Tsatia Mountain, Bowser Basin.

 

                      

The Tsatia Mountain section contains some shelf parasequences that are truncated by lowstand, channelized fluvial sandstone – the LST. The TST is a thin, pebbly mudstone similar to that in the image immediately above. Bowser Basin.

 

                       

The abrupt contact between the erosional base of the TST fluvial channel, and the preceding shelf parasequence. Tsatia Mountain, Bowser Basin.

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Atlas of fan deltas

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Fan deltas at several locations along Tanquary Fiord, Ellesmere Island

Fan deltas, their deposits and structural associations

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do).  I

Fan deltas are like alluvial fans except they dip their toes in lakes and shallow seas. So, in addition to the alluvial component, there is subaqueous deposition down a relatively steep, angle-of-repose slope. Sedimentation along the delta front, or slope, commonly produces large, basinward-dipping foresets, one of the defining characteristics of fan deltas.

Fan delta deposits are generally coarse-grained; there is much sand and gravel. Distributary systems tend to be braided. Sediment is supplied to the delta front from where it avalanches down-slope or transforms to debris flows. Gravitational instability may also influence depositional mechanisms.

Fan deltas tend to accumulate where there is a decent supply of sediment; close to steep uplands, active faults, mountain fronts, thrust fronts, glacial lakes and fiords, and pull-apart basins.    Deposition outboard of active extension faults can produce spectacular fan delta stacks on the hanging-wall block. Fan deltas associated with thrust faults may accumulate as basinward overlapping packages in the footwall, that are subsequently overthrust. In pull-apart basins, the locus of fan delta stacking parallels strike-slip displacement; often likened to a horizontal stack of dominoes – the Devonian Hornelen Basin (Norway) and Late Miocene Ridge Basin (California) are classic examples.

Here’s a paper on Bowser Basin fan deltas: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

 

The images:

A sizeable fan delta encroaching into Tanquary Fiord, Ellesmere Island. Arrow points to a Geological Survey of Canada base camp in 1988. The gravel delta top and foreslope are derived from Paleozoic rocks.

 

 

 

The head of Strand Fiord, Axel Heiberg Island, contains a braid-plain fan delta (center), the outwash drainage from Strand Glacier (distant right). A smaller, ‘radial’ fan delta is growing along the south (right) fiord shore.  See image below for a different perspective of this fan delta.

 

 

Looking west along Strand Fiord (Axel Heiberg Island); several small fan deltas drain the bordering ridges. In the foreground is the fan delta shown in the preceding image, fed by a braided river.

 

 

 

 

                     

Typical Arctic fan deltas: Left Slidre Fiord. Braided stream supply to the delta front is clear, with the active channels regularly moving across the delta top. A gravel beach ridge formed along the inactive delta front, has become detached.  Right: Small, steep sloped fan delta along Emma Fiord.

 

                          

This small, very recent, dissected fan delta accumulated on the beach face at Kariotahi, south Auckland (Tasman Sea coast). Storm drainage through the weakly indurated Pleistocene dune-beach sands behind, deposited sand during high tide. The small delta built across the beach, and as the tide ebbed, the stream eroded into its delta. The overall concave (down) top surface is evident in both images.

Cross-section through the Kariotahi mini fan delta. Mostly Laminated and rippled sand and a few mud stringers, with a layer of disrupted sand-mud at the red arrow.

 

 

 

 

Pleistocene Gilbert delta exposed in the Bradner Road pit, Fraser Valler, Vancouver. The dipping foresets have a clear topset sand unit (laminated and small crossbeds).  Foresets show numerous pinchouts and local discordances, probably reflecting changing stream flow and sediment supply, and possibly local slumping down the foreslope. The delta is at least 6m thick.  It accumulated in a glacial outwash lake. The overlying grey deposit is a diamictite.

 

An impressive stack of Upper Jurassic fan deltas in Bowser Basin, northern British Columbia. Each delta package is separated by recessive, interfan turbidites and mudstone. The stack accumulated during active faulting close to the basin margin. Icebox Canyon.

 

 

 

A different perspective of the Icebox Canyon fan delta stack: fan foresets are dipping towards the viewer (top to the left). Some fan packages coalesce, others are separated by thin turbiditic sandstone and mudstone.

 

 

 

Closer view of delta packages, shows foresets, and thin bedded interfan deposits. Icebox Canyon, Bowser Basin

 

 

 

Foreset geometry is clearly expressed in this view of the Icebox Canyon fan delta stack

 

 

 

 

Interfan turbidites, mostly Tb-d components of Bouma cycles. Top to the right.

 

 

 

 

Gravel ripples developed along some fan delta foresets, indicating some down-slope bedload movement of sand and gravel. Icebox Canyon, Bowser Basin.

 

 

 

 

                         

Clear discordances between foreset conglomerate beds, and topset conglomerates in fan deltas at Mt. Cartmel (left), and Tsargoss Lake (right). Topset beds at Mt. Cartmel contain planar and trough crossbedded, clast-supported conglomerate that is interpreted as the briaded, alluvial portion of the fan delta. Bowser Basin.

Some fan Deltas in Bowser Basin, migrated to the shelf-slope break, and were probably instrumental in supplying gravel to the deeper basin submarine gullies, canyons, and submarine fans. Here, foreset toes interfinger with slope shale and thin sandstone. West of Tsatia Mt.

 

 

Admittedly a bit dark, but look closely and you will see fan delta foreset toes interfingering with slope mudrocks, and overlying the delta, coarsening-upward shelf deposits. West of Tsatia Mt, Bowser Basin.

 

 

 

                                 

Non-cohesive – greater degree of clast-support (left) and cohesive-muddy (right) debris flow conglomerate composing some fan delta foresets at the Mt Cartmel delta.

Reconstruction of fan delta-shelf and shelf-break gullies, outboard of active Late Jurassic thrusting, Bowser Basin, BC.  For details, see: Ricketts, B.D., and Evenchick, C.A. 2007. Evidence of different contractional styles along foredeep margins provided by Gilbert deltas; examples from Bowser Basin, British Columbia, Canada: Bulletin of the Canadian Petroleum Geologists, v. 55, p. 243-261.

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Atlas of slope, shelfbreak gullies, and submarine canyons

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do). 

Marine slopes are bona fide geological settings in themselves, but from a geotectonic perspective they are the region where continental crust is transitional to oceanic crust, and where sediment bypasses the shelf as it heads towards the deep ocean floor – typically as submarine fans.  Slopes, as their name suggests, have significantly greater dip than an adjacent shelf; the break between the shelf and slope is  defined by this break in sea floor gradient.  Slopes are frequently cut by gullies and submarine canyons; the gullies tend to be localized across the shelf-slope break, whereas canyons extend across the shelf (sometimes coming within a few 100m of the shore), to the full depth of the slope.  Gullies and canyons focus sediment transfer to the ocean deep. The Black’s Beach and Point Lobos canyons were visited on an AAPG trip with Tor Nilsen; the Bowser Basin examples I worked on in the late 1980s – early 90s.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.  There, you will also find links to the other categories.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

The images:

                     

The iconic, Eocene Pt. Lobos submarine canyon, California, where canyon-fill conglomerate (brown hues) is in abrupt contact with Salinian granodiorite (white weathering) –  an example of a steep canyon wall.

Looking south, along the Pt. Lobos canyon axis. Conglomerate at the base, overlain by turbidites.

 

 

 

 

Layered Pt Lobos canyon-fill conglomerate against the blocky weathering granodiorite bedrock. California. The canyon wall is indicated by arrows.

 

 

 

 

Discordant packages of conglomerate canyon-fill, Eocene Pt. Lobos submarine canyon, California.

 

 

 

 

Interbedded canyon-fill conglomerate and turbidites, Eocene Pt. Lobos submarine canyon, California. Some of the conglomerate beds have debris flow characteristics, others may be down-canyon traction current deposits.

 

 

 

Local slope facies between channelised, canyon-fill conglomerate, presenting delicately laminated siltstone-mudstone, starved ripples with mud-drapes, thin graded beds (looking more like distal turbidites),soft-sediment load structures, and a few sand-filled burrows. Eocene Pt. Lobos submarine canyon, California.

 

 

Slump discordant packages of interchannel, thin-graded fine-grained sandstone, Eocene Pt. Lobos submarine canyon, California

 

 

 

 

A muddy debris flow consisting almost entirely of slope facies mudstone rip-ups, plus a few pebbles, overlain by clast-supported, canyon-fill conglomerate. Eocene Pt. Lobos submarine canyon, California

 

 

 

Black’s Beach, iconic coastal cliffs that reveal sediment gravity flow deposits (mainly turbidites and debris flows), and the remnants of an Eocene submarine canyon. This view is north of Scripps Pier, California.

 

 

 

                         

Pebble-lined canyon floor at Black’s Beach, cutting into estuarine and other paralic facies (root structures and burrows are common). Eocene, California

Basal conglomerate filling the canyon floor, Black’s Beach, California

 

 

 

 

 

                       

Typical channel conglomerates eroding into thick (proximal) turbidites and thinner channel overbank facies, Black’s Beach submarine canyon. Signs at the beach entrance warn of rock falls,  house collapses, and other exposures.

Discordant canyon-filling conglomerate and thick proximal turbidites, Black’s Beach, California.

 

 

 

 

A shelfbreak gully, incised into slope deposits, overlain by cyclothemic, and progressively shallowing shelf facies. Gully fill is mostly conglomerate. It is thickest at the waterfall (about 40m). Initiation of gullies was by fluvial erosion during sea level lowstands, aided by slumping in inherently unstable slope deposits. The gullies delivered gravel and sand to the basin beyond the slope.  Upper Jurassic, Tsatia Mt, Bowser Basin, British Columbia.

A paper on this topic: Ricketts, B.D. and Evenchick, C.A. 1999.  Shelfbreak gullies; Products of sea-level lowstand and sediment failure: Examples from Bowser Basin, Northern British Columbia.  Journal of Sedimentary Research, v. 69, p. 1232-1240.

The base of ‘waterfall’ shelfbreak gully, overlying slope mudrock and thin turbidites.  Bowser Basin, British Columbia.

 

 

 

 

A closer view of the ‘waterfall’ gully margin (Tsatia Mt), showing numerous discordant contact within the slope mudrock facies, and minor slumping of the gully fill. At least two major episodes of fill are recorded here. Bowser Basin, British Columbia.

 

 

 

Shelfbreak gullies extend down slope. Here, two packages of channelized conglomerate (along the ridge line) occur entirely within slope facies.  The small lenses of conglomerate below are thought to represent channel spillover lobes. Joan Lake, Bowser Basin, British Columbia.

 

 

A large slump block of gully-fill conglomerate, embedded in slope mudrocks, shows the inherent instability of the gullies and associate slope deposits. Bedding within the block are also disrupted. The block is located just below the right margin of the ‘waterfall’ gully, shown in the above images.  Tsatia Mt, Bowser Basin, British Columbia.

 

 

                         

Slope facies, here consisting of relatively undisturbed thin, graded, very-fine grained sandstone-mudstone (thin turbidites), and a few small starved ripples in the laminated mudstone-shale. Bowser Basin, British Columbia.

                                            

Thin graded sandstone beds, starved ripples, laminated sandy mudstone, small slump folds, syn-sedimentary pull-aparts or boudinage, and microfaults, all features that are  typical of slope facies mudrocks. Bowser Basin, British Columbia.

                      

Left: laminated mudstone-siltstone and a few thin graded sandstone beds.  Slope facies, Bowser Basin, British Columbia. Right: stratigraphic discordances occur at all scales in the Bowser Basin slope deposits. Many are caused by slumping, but discordant mudrock packages also arose from flows spilling over the channel-gully margins.

Slump-induced, listric-style fault in slope mudrocks, Tsatia Mt.  The fault flattens out along a thin turbidite bed; displacement decreases towards the fault tip at top right, where overlying beds are continuous. Bowser Basin, British Columbia.

 

 

 

 Slope mudrock and thin sandstone beds are truncated by a synsedimentary fault (just above the lens cap). Bowser Basin, British Columbia.

 

 

 

 

Upper Jurassic Submarine canyon complex of stacked channels, within a slope assemblage, Todagin Mt, Bowser Basin, British Columbia. Like the shelfbreak gullies, although on a grander and more prolonged scale, the canyon delivered mud, sand and gravel to the deeper Bowser Basin This view taken from Tsatia Mt.

 

 

View from the center of Todagin canyon-fill, showing the step-like stacking of successive channels. Maximum thickness of the conglomerate-fill exceeds 300m.  On this ridge, an equivalent thickness of slope mudrock overlies the canyon. Bowser Basin, British Columbia.

 

 

 

                            

Two views of the Todagin canyon base, and bedded conglomerate-fill, most of which was deposited by debris flows, sometimes separated by thin turbidites. Each view shows about 25m of section. Bowser Basin, British Columbia.

Stacked channel conglomerate; the channel margin is almost vertical through about 20m thickness. The steep margin may be synsedimentary fault controlled – the overlying beds are not displaced. Opposite the margin are typical slope mudrocks and thin turbidites. Bowser Basin, British Columbia.

 

 

The upper section of Todagin canyon, showing back-stepping channel stacking. Bowser Basin, British Columbia.

 

 

 

 

Near the top of the canyon succession, two cycles of thinning- upward turbidites, that may have formed as the active channel moved across the canyon floor, away from this site of deposition. Note the slump discordance in the lower cycle. Bowser Basin, British Columbia.

 

 

Turbidites overlie the main Todagin canyon-fill conglomerate, about 10m thick, capped by a smaller channel. Note the slump discordance in the lower cycle.  Bowser Basin, British Columbia.

 

 

 

 

The top of the main canyon-fill conglomerate here is overlain by slope mudrock, cf. the image above.  Turbidites, at the location in the image above, have thinned significantly or pinched out completely in this exposure. The overlying conglomerate forms a smaller, more isolated channel, pinching out to the right. The overall influence of the submarine canyon is waning at this stage.  Bowser Basin, British Columbia.

 

Mudstone rafts, captured by debris flows, near the base of the Todagin canyon succession. Bowser Basin, British Columbia.

 

 

 

 

 

                         

Contrasting debris flow textures. Left: mud-supported clasts in a more plastic debris flow. Right: clast-supported frameworks that probably formed in a more fluid, sheared debris flow. Both types are common in the Todagin canyon succession, and in the gullies. Bowser Basin, British Columbia.

Well-developed layering in this debris flow, probably formed during prolonged, quasi-continuous surging flow of grit to cobble sized clasts. The whole unit is about 8m thick. Hammer bottom left. Todagin canyon.

 

 

 

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Atlas of shelf deposits

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The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source (it is the polite, and professional thing to do).

The term ‘shelf’ is used here loosely – it covers a range of submarine settings, mostly shallower than about 300m, from the upper slope to shoreline, the shoreface, fairweather and storm wave-base.  There is some overlap with the ‘Paralic’ category, but the context of the shallowest examples (like beach, shallow subtidal) is in their relationship to their deeper counterparts.  The separation of the ‘Shelf’ and ‘Paralic’ categories is a bit artificial, and one of convenience.

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.

Click on the image for an expanded view, then ‘back page’ arrow to return to the Atlas.

The images:

Coarsening- and bed-thickening upwards shelf (about mid shelf) to shoreface cycle, Jurassic Bowser Basin, northern British Columbia.  The coarser facies contains hummocky crossbeds (HCS) at storm wave-base, and subaqueous dune-ripples above fairweather wave-base.  There are numerous trace fossils indicative of high energy,

such as Ophiomorpha, Rosellia, and Thalassinoides.

 

Coarsening=upward cycle at about outer- to mid-shelf – some HCS at the top of the sandstone. This is a more seaward cycle to that shown above.   Jurassic Bowser Basin, northern British Columbia.

 

 

 

 

This shale to thinly bedded sandstone cycle occurs close to the shelf edge, at the transition to slope deposits.  There are a few bottom current ripples, but no HCS or larger dune structures. Jurassic Bowser Basin, northern British Columbia.

 

 

 

The chert-pebble conglomerate accumulated in a shelfbreak gully.  The uninterrupted transition from shale-dominated slope to shelf is located immediately to the right of the gully margin.  Jurassic Bowser Basin, northern British Columbia. Details of the gullies have been published here: Shelfbreak gullies; Products of sea-level lowstand and sediment failure: Examples from Bowser Basin, northern British Columbia. 1999,  Journal of Sedimentary Research 69(6):1232-1240

 

Hummock cross stratification (HCS) in a typical lower shoreface shelf cycle (storm wave-base),  Jurassic Bowser Basin, northern British Columbia. Hammer rests on a thin pebbly debris flow that immediately underlies the HCS unit.  It is generally thought that HCS forms during storms, from the combination of a unidirectional flowing bottom current, possibly as a sediment gravity flow, that is simultaneously moulded by the oscillatory motion of large storm waves.

Possible swaley bedding, formed in much the same way as HCS, but where the hummocks have been eroded leaving the concave-upward swales. Jurassic Bowser Basin, northern British Columbia.

 

 

 

 

Storm rip-ups of shelf muds in a mid-shelf cycle.  Jurassic Bowser Basin, northern British Columbia.

 

 

 

 

Many shelf cycles in the Bowser Basin succession, terminate abruptly and are overlain by a bed of fossiliferous (ammonites, trigoniids and other molluscs), pebbly, mudstone.  This marks the transition form a highstand (HST) to succeeding transgression; the mudstone is the TRansgressive Systems Tract (TST).

 

 

Transition from a sandy HST, to fossiliferous mudstone (small ammonite near the lens cap) of the TST. The top of the TST corresponds to a maximum flooding surface (MFS) – the stratigraphic record of maximum transgression.  Jurassic Bowser Basin, northern British Columbia.

 

 

The upper portion of this coarsening upward shelf cycle, the highstand systems tract, contains low-angle planar lamination and some hummocky cross-stratification (HCS). The base of the transgressive unit (TST) is an erosional surface. Jurassic Bowser Basin, northern British Columbia.

 

 

 

                          

Two views of a lenticular, trough crossbedded pebbly sandstone that has cut into the top of a shelf cycle. This has been interpreted as a lowstand fluvial channel, that traversed and eroded the shelf as it was exposed during falling sea level.  This was one mechanism for transporting gravel and sand to the slope and deeper basin, via shelfbreak gullies (like the one pictured above).  Jurassic Bowser Basin, northern British Columbia.

The same fluvial, lowstand channel shown in the images above. The channel is about 2m thick.  Jurassic Bowser Basin, northern British Columbia.

 

 

 

 

Panorama of a slope-shelfbreak gully-shelf-to fluvial transition, beautifully exposed at Mt Tsatia, Jurassic Bowser Basin, northern British Columbia. Conglomerate on the immediate right are equivalent to the rusty beds near the opposite summit. The shelfbreak is located at the top of the wedge-shaped gully (corresponds to the top of the waterfall) – below the gully are slope deposits. The thickness of strata in this view is more than a kilometre.

A really nice (folded) succession of coarsening upward shelf cycles, Eocene Eureka Sound Group, South Bay, Ellesmere Island. The Eocene shelf was laterally equivalent to river-dominated deltas (Iceberg Bay Fm.) to the north and east.

 

 

 

                         

Coarsening upward mid-shelf – shoreface cycles at South Bay, Ellesmere Island (same location as image above). Small subaqueous dunes, ripples and HCS are common.

 

                         

Coarsening upward muddy shelf cycles, mostly below storm wave-base, but the occasional cycle extending into lower shoreface (some HCS).  Eocene, Eureka Sound Group, Ellesmere Island

Downlap of muddy outer shelf siltstone and mudstone, Eocene Strand Bay Fm, Ellesmere Island

 

 

 

 

 

                        

Sandy, Paleocene shelf dunes forming part of large sandwave complexes. Most of the crossbeds are the planar, or 2D type. The right image shows detail of crossbed foresets, with some reactivation surfaces (probably tidally induced); crossbed is about 40cm thick.  There is some indication here of tidal (flood-ebb) couplets.  Expedition Fm, Eureka Sound Group, Ellesmere Island.

Sandwave complex on a Paleocene sandy shelf, made up of multiple dunes. Eureka Sound Group, Ellesmere Island.

 

 

 

 

 

                        

The abrupt, corrugated surface here is a Late Pleistocene wave-cut platform, eroded across Pliocene mudstones (Tangahoe Fm). The wave-cut platform and overlying estuarine-dune sands are part of the Rapanui Formation, near Hawera, New Zealand.  The eroded corrugations and channels contain wood, shells and pebbles.

                                           

Late Miocene – Early Pliocene coarsening upward shelf cycles, from outer-mid shelf siltstone-sandstone, to shoreface, tidally induced sandy coquina sandwaves (left image).  The 3 images show part of the highstand systems tract. The carbonate facies are part of the classic, cool-temperate water limestones of Wanganui Basin, New Zealand.  Matemateaonga Fm, Blackhill.

Thick HST calcareous sandstone – limestone, Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

 

 

 

 

Large planar crossbeds in shelf sandwaves (HST), overlain by a pebbly shellbed deposited during the next transgressions (TST).  Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

 

 

 

 

Typical transgressive systems tract (TST) shellbed, Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

 

 

 

 

Detail of shelf dune foresets with backflow ripples climbing up foreset dip. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

 

 

 

 

Subtidal sandstone with lenticular and wavy bedding deposited during ebb-flood tides. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

 

 

 

 

Large planar crossbedded calcareous sandstone, formed either as shelf sandwaves or platform of a tidal inlet flood delta. Late Miocene – Early Pliocene Matemateaonga Fm, Blackhill.

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In the field: Mountain storms and surprise encounters, northern British Columbia

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Joan Lake, northern British Columbia

The interior of northern British Columbia is rugged, mountainous country. Roads, that tend to be quite rough were frequently opened to provide access to mines and small settlements. It is an isolated part of the world, beautiful, even majestic, but also unforgiving.  East of the Coast Mountains and about 200km south of Yukon, is a huge swath of sedimentary rocks, referred to collectively as Bowser Basin.  The rocks are Jurassic to Cretaceous, recording a history of about 70 million years duration. Humungous volumes of sediment were eroded from older rocks to the north, that were uplifted and deformed as tectonic plates, or terranes, collided with the ancient margin of North America. Gravel, sand and mud were carried by braided rivers, supplying coarse sand and gravel to the coast and beyond, and to large deltas that supported lush forests (later converted to coal). Continue reading

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