Tag Archives: geology field work

Measuring and representing paleocurrents

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Large sand ripples exposed on bedding allow measurement of paleocurrents

This post is part of the How To… series

Identifying sedimentary structures that indicate paleocurrent directions is an important task in any study of sedimentary rocks. Knowing the direction of sediment transport will help you decipher paleoenvironments and sedimentary facies,  paleoslope dip directions, possible sources of sediment,  and the location of sediment sinks.

It all begins with a humble crossbed, flute cast, or current aligned object.  Identifying any of these is reasonably straight forward; knowing what to measure can be a bit tricky.

Asymmetric current ripple and dune bedforms exposed on bedding planes can be measured by noting the facing direction of lee slopes (face down current – see the image above). In such cases, measured bearings for individual bedforms provides a unique sense of flow.

However, it is more common to find crossbed cross-sections in 2-dimensional exposures like cliffs and road-cuts. In situations like this, the crossbed foresets are more likely to present an apparent dip direction, rather than true direction of flow. The trick here is to look for nooks, crannies, joint or fracture faces that present a degree of three-dimensionality to the outcrop.

In the example below, crossbed foresets are exposed in two rock faces of a joint block, presenting us with two apparent dips. Measure both plunge and bearings, and find the true dip using stereographic projection.  In most cases, the direction of maximum foreset dip will be close to the paleoflow direction. BUT! You must be certain that the foresets belong to the SAME CROSSBED SET. This stipulation is important in situations where multiple crossbed sets cut one another – a feature of sandy fluvial and shallow marine deposits.

Measuring trough crossbeds in 3D exposures to calculate paleocurrent flow direction

In exposures where crossbeds have been eroded parallel or slightly oblique to bedding, crossbed laminae are outlined in sinuous and festoon patterns. Trough crossbeds, and various 3D ripples exposed in this way (e.g. lunate ripples) provide an opportunity to take multiple paleocurrent measurements.  In the example of festooned crossbeds shown here the concave aspect of each trough set faces downstream.

Flow directions can easily be measured in festooned crossbeds exposed on or slightly oblique to bedding

Of all the sole structures, flute casts are the most useful, providing (relatively) unambiguous paleoflow; flow is parallel to the length of the flute, from the deeper spoon-shape scour to the thin feather edge. However, paleoflow determined from groove casts is ambiguous, the two possible directions 180o apart. Data from sole structures is improved if flutes and grooves occur together.

Flute casts provide unambiguous paleocurrent flow directions

Graphical representation of paleoflow

How you treat the data graphically and statistically depends on the number of measurements at any one locality, and the geographic – stratigraphic distribution of data. A few questions you need to ask are:

  • Does the number of data points at each locality warrant separate treatment for each locality, or should the data be lumped into a single point of analysis?
  • Is the data distributed over a narrow stratigraphic interval (e.g. 1 or 2 beds, or a single coarsening upward sequence of beds), or a more extensive stratigraphic interval?
  • If data from multiple localities or stratigraphic intervals is aggregated, will important variations in paleocurrent trend be represented. For example, if there are local bimodal trends representing tidal ebb and flood currents, will these be ‘lost’ if all coastal data is analysed as a single block of data?
  • If mean flow direction is calculated, how useful is this measure of central tendency in the context of the overall spread of paleoflow directions?
  • Are corrections needed to account for structural dip?

 

Rose diagrams provide the simplest way of representing data in diagrammatic form. Data is plotted as a circular histogram through 360o.  Several software programs are available to do these plots, but it is also a simple task to do it by hand. The inset shows you how to do this.

  • With the data in hand, choose a bearing interval (the example here is 20o intervals)
  • Organize the data in the intervals and calculate the percentage of measurements for each interval.
  • Plot each interval so that the length of each sector of the rose is proportional to the number of measurements for that interval. The example here uses intervals of 20%.

Tabulation of directional data and plotting a rose diagram for paleocurrent flow

The distribution is clearly unimodal. We could have chosen a 10o or 15o bearing interval for the plot which would probably show some finer detail about the paleocurrents.

Paleocurrent distributions in sedimentary basins generally fall into 3 or 4 categories: Unimodal (one primary direction), bimodal bipolar (2 directions 180o apart), bimodal oblique 2 directions at different angles), and polymodal (widely distributed).  Vector means for unimodal distributions are useful for comparing paleoflow among locations and assessing regional patterns of flow. However, the mean directions for strongly bimodal or polymodal distributions may have little real-world value in this context.

The Mean paleoflow vector can also be calculated, but the usual arithmetic methods DO NOT APPLY to azimuthal data. Calculation of the mean for our unimodal distribution is shown below.

Calculating the mean from a set of paleocurrent azimuths

Note: All the bearings are in the SW quadrant and therefore Sine and Cosine values are all negative, and Tangent values are positive. Other distributions may have a mix of positive and negative values – make certain you use the correct sign.

 

Here are a couple of free Rose plot programs (there are lots of commercial programs available):

GeoRose (free) for Windows and Mac

GEOrient (free for academic users)

 

Additional posts in this series:

Measuring a stratigraphic section

Identifying paleocurrent indicators

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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Measuring dip and strike

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How to measure dip and strike

This post is part of the How To… series

If we had to designate one set of measurements that is fundamental to all geology, it would have to be Dip and Strike. These simple measures define uniquely the orientation (compass bearings and angles) of a planar surface – any plane: bedding, faults, axial planes, mineralized veins, dykes and sills. Armed with dips and strikes, a geologist can project planes and the rocks they encompass across valleys, through mountains and deep beneath the Earth’s surface.  They are fundamental to deciphering Earth structures.

Strike: The compass bearing of an imagined horizontal line across a plane. If the plane is flat there is an infinite number of strike lines, all having the same dip (zero) but different bearings. If the plane is curved (e.g. a plunging fold) the bearing may change systematically over the fold.

Azimuth, or compass bearing is recorded as either (for example) 035o or N35E, or its counterpart 215o and S35W.

Visual description of dip and strike on bedding.

Dip: Dip is the angle of inclination measured from a horizontal line at right angles to strike. The angle is measured by placing a compass on the line of dip and rotating the inclinometer to the point where a spirit level indicates horizontal. The direction of dip need not be measured (it can be calculated directly from the strike bearing), but an approximate direction should always be recorded to avoid ambiguity, as in 48oNW.

The inclination measured at right angles to strike is the true dip. Inclinations measured at other angles on the plane will always be less than true dip – these are called apparent dips.

The animation below was made from still images: use the pause and play buttons as you work through the exercise.

Dip and strike indicate the orientation of a plane at a specific location.  Dips and strikes of folded strata will tend to show systematic changes at different locations. In the example below the fold axis is horizontal and axial plane vertical. Strikes at any point on the fold limbs will all have the same azimuth, but dips will change progressively from one limb to the other.  Dips and strikes recorded on geological maps can be used to reconstruct the 2- and 3-dimensional structure of deformed strata.

Systematic change in dip through an folded plane; in this example the strike remains the same throughout.

Some other useful posts in this series:

Solving the three-point problem

The Rule of Vs in geological mapping

Plotting a structural contour map

Stereographic projection – the basics

Stereographic projection – unfolding folds

Folded rock; some terminology

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In the field: Beaches, sand dunes, and shellfish for lunch

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Aupouri Peninsula and a view north along Great Exhibition Bay, NZ

Final exams over, a moment of tomfoolery, and I found myself disconsolate in a hospital bed, recovering from an operation on a dislocated knee (no such thing as key-hole surgery back then).  I had completed my BSc (November,1972) and was contemplating doing a Masters on something sedimentological. A visit from one of my professors, a period of impatient recovery, then loaded the aging Morris Minor and headed north almost as far as it is possible to drive in New Zealand. This was to be my first bona fide field project.

My field area incorporated a 22km stretch of glorious coastline that, at its northern extent, culminated in a sandspit (Kokota Spit) and Parengarenga Harbour. The task was to decipher the sand deposits, from the very recent to maybe a million or so years old. This narrow strip of land is part of the much larger Aupouri Peninsula, founded on sandy bars and spits that connect small islands and headlands of harder, older igneous and sedimentary rock. Tasman Sea washes the west flank of Aupouri, and the Pacific the east; the two seas meet at Cape Reinga, the place where Maori spirits begin their new journey. Continue reading

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The crosses I bear

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I have had occasion recently to bear the brunt of criticism about some of the decisions one makes in life.  These comments, made during a discussion on climate change, were a polite but thinly veiled criticism of some of my choices about where and for whom I had worked over the past 4 decades.  I was given to understand that working for oil companies (mainly offshore Taranaki) and mining companies (NZ, Chile) was a blight on my character; how could I talk sensibly about climate change science when I was one of the main contributors to the world’s problems.  That I had assuaged my conscience by also working on groundwater and geothermal projects (renewable), CO2 sequestration,  teaching and research seemed irrelevant.  I had given in to the dark side.

I’ve had discussions like this on several occasions and as with them, little was resolved during this particular discourse. But I did feel annoyed.  The phrase “but you had choices” and something about selling of souls had been trotted out two or three times during the chat, and it was this insistence that grated most.  Choice, it seems is a convenient crutch on which to foist an opinion.

There are plenty of everyday choices which are obvious: what to have for dinner, who not to vote for.  These kinds of choices are explicit and for the most part reasoned.  But there are also choices that, while not necessarily unconscious are hidden, or if not hidden then conveniently tucked away lest they become uncomfortable.  Discussions about renewable resources, climate change and environmental issues are good examples where our efforts to be responsible citizens (and scientists) can begin to unravel if we take too much notice of these hidden choices.

Consider the following – you are going to a conference.  Here is a list of a few inconvenient but inevitable choices you will make:

  • You will need to travel (car, plane, train). Every step of this journey will require the use of fuels, plastics, metals, food, clothing, communications (the list goes on). Hydrocarbons will be front and centre of virtually everything that gets you from A to B.  Would you choose not to attend the conference because of the carbon footprint?  Most will feel some guilt but attend anyway, either ignoring the issue or designing some arcane explanation involving necessity.
  • Accommodation (same kind of list)
  • Most people at the conference will have at least one form of communication – phone, tablet, laptop (plastics, metals, including rare earths). Tweeting directly from a conference is now common place. But will any thought be given to the atrocious working conditions under which some foreign governments and mining companies extract the rare earth metals used in these devices?

There may of course be arguments that the improvements to society and science that someone makes by going to the conference (or to the office, the lab, the field, home) is worth the sacrifice of a few dark choices. Or that some choices are worse than others. But this is a pretty self-serving and unnecessary position to take. I doubt there would be many who would begrudge the scientist traveling to Antarctica to collect data that improves the veracity of climate models.

I now feel vaguely vindicated. My soul is largely intact. I have played my part in providing the wherewithal for conference goers and field trippers. In the end whatever tasks I might have undertaken for the dark side, were no better or worse than the crosses that our conference goer and field scientist have to bear.  As Jane Austin’s Mr Bennett said “I will get over it and probably more quickly than I should.”

I also have faith in science’s and society’s ability to find solutions to many of these unfortunate choices.  I expect it will be a gradual process.  Yes, we can learn to recycle, locally, our old cell phones, and purchase electric vehicles when they are reasonably priced and when there is sufficient infrastructure.  But in the meantime most of us will still need to fill the petrol tank or hop on that plane.  If climate change is a reality (and there seems to be a consensus that it is), then so too is the length of time it will take to make the necessary structural changes to the way we live.  In the end, moral indignation at these darker circumstances seems neither fair or useful.

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A 40 Million Year Old Forest, Looking Like it Formed Yesterday

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The Stunning Preservation of an Arctic Fossil Forest

Location map for Geodetic Hills, Canadian Arctic In 1985, my field assistant and I were examining sedimentary rocks on central Axel Heiberg I. in the Canadian Arctic.  The project was part of a broader science program being run by the Geological Survey of Canada. I had surmised, from some of my earlier work that the deposits here had formed in response to tectonic upheaval in the region about 40 to 45 million years ago (a geological time called the middle Eocene); we were on the look-out for additional information to assess this hypothesis.  Our helicopter had dropped us off at the base of a gentle ridge, known as Geodetic Hills. Continue reading

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