Monthly Archives: February 2019

Atlas of modern coral reefs

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honeycomb cowfish

This is a companion post to the Atlas of Cool-water carbonates

Modern coral reefs and carbonate platforms provide the key ingredients of process and product to interpret ancient carbonate deposits. Here we can observe directly the relationship among all those processes – biological, physical, chemical – that contribute to the construction of solid rock.

 

Contributors

The inaugural collection of images for modern coral reefs has been generously donated by Charlie Kerans, the Department Chair and Robert K. Goldhammer Chair in Carbonate Geology in the Department of Geological Sciences, Jackson School of Geosciences, The Univeristy of Texas at Austin.  Charlie is a carbonate specialist. I first met Charlie  at Carleton Univeristy in Ottawa, where we shared an office whilst both of us were undertaking Doctoral research. Both of us worked on Precambrian rocks; Charlie on carbonates. He hasn’t stopped looking at carbonates.

The collections here are from Palancar Caves, and Columbia reef, Cozumel, off the Caribbean coast of Yucatan Peninsula. They are popular diving spots, for good reason as the images will attest. The water is clear and there is great diversity of reef and off-reef fauna and flora.  It is a fantastic location to look at analogues for ancient reef systems – hard corals, soft fan corals, algae (particularly Halimeda), sponges, bryozoa, fish.

I’d be grateful for any corrections and clarifications to the species identifications.

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).  Copyright of images is retained by the owner, as indicated. Contact Charlie for further information (link above).

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 Atlas categories.

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

The images: Palancar Cave, Cozumel

palancar cave reef Porites coral cactus coral palancar deep sea fan palancar cave reef structure palancar cave reef porositypalancar cave reef palancar cave reef corals porites coral moray eel palancar cave reef lettuce coral Angel fish palancar cave reeffrench grunts palancar cave reef porites and sponge palancar cave reef structure eagle ray drum fish honeycomb cowfishporites coral community tube sponges cactus coral vase sponge dead coral tube sponges palancar cave reefbroken staghorn coral

 

Columbia Reef, Cozumel

This reef, part of the reef system along the west coast of Cozumel, is located a little south of the Palancar reefs.

 

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The Rule of Vs in geological mapping

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Deciphering the outcrop expression of beds and topography

This post is part of the How To… series

Geology maps and topography maps go hand in hand. Try drafting a map of surface geology without reference to the local topography and you will end up in a heap of trouble. The rules inherent in topography maps are pretty basic: contours represent horizontality, contours always close (except on the edge of a map), contours never cross although they may become very close on steep terrain or cliffs, closely spaced contours represent steepness, and the geometry of contours changes in predictable ways between valleys and ridges. The latter is called the Rule of Vs, where:

  • contours V up-gradient ( upstream) in valleys, and
  • down gradient on adjacent ridges.

Geologists have borrowed the Rule of Vs to describe the geometry of mapped strata. The Rule also serves to predict the surface location of strata in areas of poor exposure. The Rule applies to the simplest case where bedding is a flat plane, dipping at any angle. Keep in mind that the topographical expression of bedding represents erosional remnants of rock layers that once extended above the present surface; a lot of rock has been removed to produce the topography you see today.

To get a sense of this in three dimensions, take the example where bedding dips upstream (up gradient). Imagine these beds extending above the present surface. In your mind’s eye, whittle away at the strata until your imaginary surface coincides with the present surface. The mapped expression of the beds will show them V-ing upstream in the valley and down gradient on the adjacent ridges.

Map expressions for six different bedding orientations are shown in a short animation, in 3D block views and their corresponding map views. I have taken the liberty of modifying the examples shown in Donal M. Ragan’s excellent text Structural Geology: An introduction to geometrical techniques. Pause each frame if you need to spend more time looking at it. The animation shows, in sequence, beds that dip: horizontally, upstream, vertically, downstream dip greater than stream gradient, downstream dip equal to stream gradient, and downstream dip less than stream gradient. The second part of the animation is presented at a faster frame rate to give a more ‘animated’ sense of changing map expressions.

 

D.M. Ragan, 1968. Structural Geology: An introduction to geometrical techniques. John Wiley & Sons Inc. 166 p. This book is into its 4th Edition.

Some other useful posts in this series:

Measuring dip and strike

Solving the three-point problem

Folded rock; some terminology

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Measuring a stratigraphic section

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measuring a stratigraphic section

Some ideas on field measurement of stratigraphic sections

This post is part of the How To… series

Next to measuring dips and strikes, this is probably the most important task you will undertake as a field geologist (and I include well-site stratigraphy in the general term “field work”). Stratigraphic sections provide you with:

  • the boundaries of rock units (formations, stratigraphic sequences, sedimentary facies),
  • The age of rock units (using fossils and other datable rock or mineral components),
  • The presence of hiatuses such as unconformities and disconformities,
  • The vertical extent of depositional units and if adjacent sections are available, the lateral extent of these units,
  • The juxtaposition of depositional units based on bed thickness, composition, sedimentary structures, fossils,
  • Stratigraphic trends such as fining upward successions,
  • Nearly all sedimentary rocks occur within much larger and grander sedimentary basins; your stratigraphic section is a sample of that basin. Stratigraphic sections added piece by piece, like a jigsaw puzzle, will give you a more complete picture of the basin.

 

“What is your weapon of choice, Master Baggins?”
For this task you will need (in addition to all the other stuff, like boots and lunch):

  • Jacob’s Staff; 1.5 m is a good length, marked off in 10 cm or finer increments. Make sure it has a decent inclinometer and bubble level attached with a lock-nut so that it doesn’t fall off (if it does fall off it is bound to drop into a crevice or down a cliff). The inclinometer is set at the local dip (that you have already measured).  This is one of the best pieces of equipment ever invented – it dates to the 14th century where it was used for geographic and astronomical measurements.  It allows you to measure true bed thickness directly. Check the dip regularly.

using a Jacobs Staff

  • A tape measure (if you cannot find or make a Jacob’s Staff); useful for measuring the thickness of thin beds and crossbeds. You will need to correct each measurement for dip.
  • A compass for strikes, dips and paleocurrent measurements, The compass inclinometer can also be used on the Jacob’s Staff.
  • Water-proof field note book and more than one pencil,
  • Camera. Take lots of photos.

Getting started:

  • If possible, decide beforehand where the section traverse should go to take advantage of the best, most continuous exposure, noting areas that may be difficult or even dangerous to work.
  • Locate the section (top and bottom) on a topography map (grid references), aerial photo or image. There’s no point measuring the section if you cannot later locate it geographically. Add the date and other relevant information.
  • At the outcrop determine stratigraphic way-up. We usually measure a section from oldest to youngest (borehole stratigraphy is measured the other way).
  • Decide how you will measure and record the section. Are you interested in detail and hence measure bed by bed, or do you have one day only to record a two kilometre-thick section, in which case you will probably measure packages of strata? The choice is a bit arbitrary and depends on the nature of the project and its desired outcome.
  • Decide how you will record your observations: notes, sketches, stratigraphic columns, photos, samples. My own preference is to draw a stick-like stratigraphic column as I work through the section, adding side notes and symbols for sedimentary structures, fossils, samples etc. I like this method because I can begin to visualize any stratigraphic variations such as coarsening or fining upward trends. The scale for this column will depend on the level of detail you require. There are also Apps and programs that will automate this process, if you’re using digital devices.  BUT
    keep in mind that you are not just recording the stratigraphic section – you are trying to see through it. Automated field tasks are fine for speeding up the recording process, but nothing can replace manual, pencil-driven sketching to help you understand the problem in front of you. Don’t use the excuse “but I can’t draw”. Field sketches are not meant to be works of art. They are supposed to be quick renderings of the outcrop, hillside or whatever, capturing the ingredients that are essential to geology and landscapes. You do not need to reproduce the landscape with Andrew Wyeth precision.

drawing a stratigraphic column

  • Chose a convenient scale for the stick column – one that allows plenty of notes. I find it necessary to vary the scale from time to time, if I want more or less detail in parts of the section. Any changes in scale can be accounted when redrawing the column to a common scale.
  • Most field geos use some form of shorthand for recording their observations, particularly when there is a lot of repetition. This comment also applies to use of symbols, for example different kinds of crossbed, trace fossil, or bed contact. Some institutions will have their own set of symbols and abbreviations. What ever system you adopt, make sure it is used consistently. Keep in mind that the field notes are primarily for YOU. You can redraw stratigraphic columns and sketches to make them legible for others, or for publication.

sketching in the field

  • Decide ahead of time the kinds of samples you need to collect – macro/microfossils, petrographic, radiometric dating. Will you need materials to wrap delicate fossils, or tough bags for sharp-edge rocks? Make certain that sample labels are waterproof and won’t rub off after a day of rattling around in your backpack. It is imperative that samples are keyed to the correct stratigraphic unit.
  • Take lots of photos. Link these images to your columns, notes and sketches. There is no such thing as ‘too many images’. Take close-up photos before you hack away at the outcrop, structure or fossil. No one will be interested in a photo of a pile of rubble. Backup your images when back at camp.
  • It is unlikely your section will traverse a nice straight line; you may need to move laterally to find good exposure if the outcrop is covered by scree, snow etc. It is important when shifting the line of section that you work from the same stratigraphic horizon. Mark the progress of the traverse on a map or aerial image.
  • If the line of traverse shifts sideways a significant distance, then it should be considered a new section and located-annotated accordingly. There is no set rule for this. Whatever shifts in the line of section are deemed necessary, they can be illustrated on a cross-section that shows the relative positions of the stratigraphic columns.
  • At the end of the section or back at camp, take photos of your notes, columns and sketches – as backup. Then backup all your images.

Before launching at the outcrop, it is a good idea to stand back for a more expansive view of your intended traverse. Things to look for include potential hazards, ease of access, changes in the quality of exposure, and any large-scale stratigraphic or structural features that can be checked when you are face to face with the rocks. When you have finished measuring the section it pays to look back again, from a distance, checking for a stratigraphic trend or structure you didn’t observe the first time.

At the end of your field day, back at camp with a cup of tea or gin-and-tonic, it is important to review your notes, images and samples. Field notes and sketches tend to be messy, crossings out, smudged with dirt and rain drops. If your field notes are immaculate then you have probably spent too much time on the outcrop or section. When back at camp, redraw the stratigraphic column, adding any additional notes or questions about some aspect of the geology that needs checking. At this stage you can spell out or explain the abbreviations and add lists of symbols. This process may seem redundant, but it is redundant only if you learn nothing from it.

Plotting the stratigraphic section and annotating it with all the observations (and questions) is very satisfying. Here you have the basis for some initial interpretation of paleoenvironments, sedimentary facies and paleocurrents. However, your interpretations will only be as good as the quality of the data collected. You can never collect too much data!

With a bit of luck, the adjacent section you measure the following day will make the interpretations more complicated (and therefore more interesting).

Some more useful posts in this series:

Determining stratigraphic tops

Identifying paleocurrent indicators

Measuring and representing paleocurrents

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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The hydraulics of sedimentation; Flow Regime

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This post is part of the How To… series

We introduce some basic hydraulics of sediment movement, bedforms and the concept of Flow Regime.

Ripples and dunes form when a fluid (usually water or air) flows across a sediment surface. Structures formed by air flow are called subaerial ripples or dunes; those in water have the qualifier subaqueous. These structures are given the general name bedform. The construction of bedforms requires certain conditions:
–    The sand must be cohesionless (i.e. grains do not stick together).
–    Flow across the sediment surface must overcome the forces of gravity and friction, and
–    There is a critical flow velocity at which grain movement will begin; this also depends on the mass of individual grains, and to some extent their shape.

bedforms, grain size and flow velocity

Ripples and dunes form under a relatively limited range of flow conditions. We can illustrate this in a graph of flow velocity against grain size, plotting areas on the graph that correspond to bedform growth. Most of the data for plots like this are derived from flumes where experimental flow conditions can be monitored closely. The plotted distribution shows that bedforms can be categorized according to flow and sediment conditions. This partitioning of bedforms was used to construct the Flow Regime hydraulic model, first published in the now classic 1965 paper by J.C. Harms & R.K. Fahnestock and used widely ever since.

flow regime

The Flow Regime model considers three fundamentally different states of flow:

  • No bed movement where there is too little energy in the system to initiate and maintain sand grain movement,
  • A Lower Flow regime in which all common bedforms develop. Here, plane bed (basically parallel, planar laminae with no ripples) represents the lowest velocity, or energy conditions where sediment movement is initiated. It has been observed in flumes and in natural channels that the size of bedforms increases from ripples to large subaqueous dunes in concert with flow velocity. Dune type also changes from two dimensional structures (straight crests and planar crossbed bounding surfaces), to three dimensional structures that have sinuous, arcuate and lunate outlines and spoon or scour-shaped bounding surfaces (commonly seen as trough crossbeds).
  • An Upper Flow Regime where the power of stream flow washes out ripples and dunes, replacing them with plane bed (this kind of plane bed commonly has parting lineations), plus antidunes, and erosional chutes and pools.
  • As stream flow increases the transition from Lower to Upper flow regime produces one of the more interesting bedforms – antidunes. They are mostly found in shallow channels (e.g. fluvial and tidal channels). You can recognize that this transition has taken place when you see standing surface waves – watch closely and you will see the waves migrate upstream. Antidunes are the bedforms that develop immediately below standing waves (the two are in-phase). If high flow is maintained, the antidunes will also migrate upstream. However, once flow slackens they tend to wash out; the preservation potential of antidunes is low.

The example above shows standing waves in a tidal channel on an out-going tide. Tidal flow is to the left; standing wave migration is to the right (Mangawhai Heads, North Auckland).

  • Hydraulic jumps: The transition from Lower to Upper flow regime passes with a change in bedform, in particular washing out of subaqueous dunes, but there is no sudden break in surface flow – the transition is reasonably smooth. This is not the case for an Upper to Lower flow regime transition that is marked by an abrupt increase in water level and turbulence – a hydraulic jump. Hydraulic jumps can be thought of as standing waves. They are caused by a reduction in Upper Flow Regime velocity, a change in stream-bed gradient or water depth, or combinations of all three.

making a hydraulic jump

We can use the Flow Regime concept in the field as a quantitative indicator of changes in paleoflow in time (i.e stratigraphically) and space (laterally).  For example, a stratigraphic sequence that shows a layer of ripples overlain by a layer of trough crossbeds indicates that flow velocities, and hence stream power increased abruptly. What kind of paleoenvironment might this have occurred in? This is one of the central questions for any sedimentological analysis.

Here are a couple of important references:

Ashley, G.M. 1990. Classification of large-scale subaqueous bedforms: A new look at an old problem. Journal of Sedimentary Petrology, v.60, p. 160-172.

Harms, J.C. and Fahnestock, R.K. 1965. Stratification, bed forms, and flow phenomena (with an example from the Rio Grande). S.E.P.M. Special Publication 12, p. 84-115.

 

Some more useful posts in this series:

Identifying paleocurrent indicators

Measuring and representing paleocurrents

Crossbedding – some common terminology

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Crossbedding – some common terminology

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This post is part of the How To… series

tabular crossbedsEocene fluvial tabular crossbeds exposed along MacKenzie River, northern Canada

Crossbeds are ubiquitous in sedimentary rocks. They can be found on the deep ocean floor, the driest desert, and pretty well any depositional environment in between. They are most common in sandy deposits. They are less common, but no less important in gravels (e.g. low sinuosity – braided rivers). Crossbeds form where air and water flow across a bed of loose sediment, so long as the individual sediment grains are cohesionless (non-sticky). Mud crossbeds are rare because individual clay particles tend to bind to one another (a result of residual electric charges).

Crossbeds in the rock record are visible in bed cross-sections, or as exhumed 3D ripples and dunes on exposed bedding planes. The term crossbed refers to their internal structure; i.e. laminations that are usually inclined in the down-flow, or down-stream direction.

The laminae are called foresets. In a 2D cross-section view, a single crossbed consists of any number of foresets bound above and below by flat or curved boundaries. The geometrical arrangement of foresets, their bounding surfaces and their size or amplitude gives us the information needed to decipher:

  • the kind of crossbed,
  • the hydraulic conditions under which the crossbed formed, and to some extent
  • the paleoenvironment in which they formed; keep in mind that most crossbeds can be found in a range of paleoenvironments but used in conjunction with other criteria such as body and trace fossils, sediment composition and stratigraphic trends (e.g. fining upward) will help pin-point specific depositional settings.

Our interpretations can be advanced further if we are lucky enough to see exhumed structures on bedding, such that we can define:

  • the shape of the ripple or dune crest line (is it straight or sinuous?)
  • the wavelength between successive ripple or dunes, and

a relatively unambiguous measure of ripple-dune migration across the bed (i.e. paleocurrents).

Most of our knowledge about ripples and dunes (collectively referred to as bedforms) and how they form has been garnered from studies of modern environments.  Afterall, if on your walks across a tidal flat or subaerial dune field you see ripples that look identical to those preserved in rocks, it is quite reasonable to predict that the ancient bedforms developed in ways similar to the modern analogues (this is the Uniformitarian Principle at work).

In fact, they have also been videoed forming in real time on Mars.

ripple and crossbed terminology

This terminology has evolved from an original 1953 description by McKee and Weir (see references at the end of the post). An SEPM workshop in 1987 (Ashley,1990) sought to incorporate in a revised terminology, the 3-dimensional aspects of bedforms larger than common ripples and their inherent hydraulic properties. They recommended that the term dune be used, with the basic distinction between subaerial and subaqueous dunes, of all sizes. Subaqueous dunes can be further separated into:

  • 2 dimensional subaqueous dunes having relatively straight crest lines and planar foreset contacts; they correspond to tabular crossbeds (in the above diagram), and
  • 3 dimensional subaqueous dunes having sinuous crest lines and spoon- or scour-shaped foreset contacts. These correspond to the classic trough crossbeds.

Trough crossbeds are most common in channelized, or confined flow (rivers, tidal inlets and channels, rip currents). Three dimensional subaqueous dunes tend to form at higher current velocities than their 2D counterparts.

The SEPM nomenclature is widely used, but deeply entrenched terms like trough and tabular crossbed are still popular.

two dimensional dunes           three dimensional dunes

Here are some classic older texts on the topic (and just because they are older than 10 years doesn’t mean they are irrelevant!)

Allen, JRL. 1963. The classification of cross-stratified units. With notes on their origin. Sedimentology, v. 2, p.93-114

Ashley, G.M. 1990. Classification of large-scale subaqueous bedforms: A new look at an old problem. Journal of Sedimentary Petrology, v.60, p. 160-172.

McKee, E.D. and Weir, G.W. 1953. Terminology for stratification and cross-stratification. Geological Society of America Bulletin v. 64, p. 381-390.

 

Some more useful posts in this series:

Determining stratigraphic tops

Identifying paleocurrent indicators

Measuring and representing paleocurrents

The hydraulics of sedimentation: Flow Regime

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Measuring and representing paleocurrents

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large sand ripples

This post is part of the How To… series

Identifying sedimentary structures that indicate paleocurrent directions LINK 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 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.

festooned crossbeds

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 cast paleocurrent indicators

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%.

plotting a rose diagram

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.

mean vector of paleocurrent

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)

 

Some more useful 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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Identifying paleocurrent indicators

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Sedimentary structures that indicate paleoflow. Measuring and plotting paleocurrent indicators are treated in a separate post.

subaqueous dunes and ripples

This post is part of the How To… series

Sediment that is moved along a substrate (e.g. the sea floor, river bed, submarine channel, wind-blown surface) will commonly generate structures that record its passing.  Sedimentary structures that preserve directionality (paleoflow) are indispensable for deciphering whence the sediment came and where it went; for interpreting sedimentary facies (local scale) and sedimentary basins (regional scale). Paleocurrents are a measure of these ancient flows.

A single structure, such as a ripple will give a unique measure of paleoflow at a certain point in space and time. An important question for this single piece of data is – how relevant is it to the bigger picture of sediment dispersal? To get a sense of regional flow and sediment transport patterns, we need many measurements so that we can tease the overall pattern of flow from whatever local variations might exist.

We can illustrate this central problem by looking at flow in a fluvial meander belt with depositional settings like the main channel (arrows), point-bars and adjacent flood plain. This snapshot in time shows clearly the huge variation in local flow directions. We also need to account for other ‘snapshots’ in time, because even at a local scale (e.g. one meander bend and point-bar), the directions of flow and sediment transport will vary from flood to low water stage. We can try to circumvent this problem if we measure a large number of flow directions over an equally large area of the river and floodplain.  In modern drainage basins this is straight forward but for the rock record, exposure is likely to be discontinuous and even structurally disjointed.

Landsat of Marmore meandering river in Bolivia

Structures indicating unique flow directions

Subaqueous dunes and ripples: These bedforms are built by 2-dimensional (straight-crested) dunes and ripples. Hence, the boundaries between adjacent crossbed sets tend to be planar (cf. trough crossbeds). Flow direction is approximately at right angles to dune or ripple crests.

  Precambrian subaqueous dunes              interference ripples

 

Trough crossbed, or 3D subaqueous dunes Spoon-shaped troughs filled by migrating, sinuous dunes produce trough crossbedding. This kind of crossbed is common in confined, channelized flow (e.g. fluvial and tidal channels). The mean flow direction is along the axis of the trough.

              

Left: Festooned trough crossbeds exposed approximately parallel to bedding. Paleoflow is the direction of the hammer handle Proterozoic Loaf Fm.).  Right: Cross-section view of multiple trough crossbeds – only apparent flow directions can be surmised from outcrop (Eocene Buchanan Lake Fm.).

A caution about wave-formed ripples; This bedform does not arise from bedload transport in flowing currents, but from wave orbitals. Wave ripples are not paleocurrent indicators. However, wave ripple crests will be oriented approximately parallel to the strike of the ancient shoreline.

 

Imbrication  Flat and platy clasts are commonly oriented by strong currents, such that the ‘plates’ dip upstream. These fabrics are common in gravelly fluvial deposits.

pebble imbricationImbricated platy cobbles and pebbles in a modern stream. Flow is to the right.

 

Flute casts  Flutes originate from erosion of a soft, commonly muddy substrate and are filled with sand – they are part of the overlying bed and are usually seen as casts on the sole of the overlying bed. Flow direction is towards the open, shallow end of the flute.

large flute castsLarge flute casts on a turbidite bed sole (Omarolluk Fm, Belcher Islands). Flow was from top left to bottom right

 

Structures indicating ambiguous flow directions:

Groove casts  Objects dragged across a soft substrate by strong currents (e.g. bottom currents, turbidity currents) will scour linear grooves that become filled by the overlying sedimentary layer. Like flutes, they are usually seen as casts on the soles of beds. In the absence of other indicators, the two possible paleoflow directions are 180o apart.

groove castsGroove casts on a bed sole, indicate flow in either direction. other criteria, like flute casts, are need to specify unambiguous flow directions.

 

Parting lineation  These are subtle structures 2 or 3 grains thick, that are visible only on exposed laminated bedding. The word ‘Parting’ refers to rock breakage along planar laminations. Parting lineation is attributed to high flow velocities where the long axes of sand grains become aligned (in Flow Regime terminology this corresponds to Upper Plane Bed conditions). Paleocurrents are measured parallel to the long direction of parting, but like groove casts, are ambiguous.

parting lineationPaleoflow indicated in this parting lineation was either to the left or right.

 

Current alignment  of elongate fossils, rod-shaped clasts, or bits of wood can generally be treated like groove casts in terms of their paleocurrent value. There are exceptions; for example turreted gastropods may be aligned with their apices pointing downstream.  The example shown here shows fairly consistent alignment of Permian Fusulinid foraminifera parallel to the prevailing flow (but the actual flow direction is ambiguous).

The classic text that deals with paleocurrent analysis is – Potter, P.E. and Pettijohn, F.J. (1977) Paleocurrents and Basin Analysis. 2nd Edition, Springer-Verlag, New York, 425 p. 

Some more useful posts in this series:

Measuring a stratigraphic section

Measuring and representing paleocurrents

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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Determining stratigraphic tops

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a.k.a. Stratigraphic way up, or younging direction

This post is part of the How To… series

Stratigraphy is all about succession in the rock record – which events preceded other events; which is older, which younger. Nicolas Steno (1638-1686) surmised, and four centuries of geologists since have confirmed that in an uninterrupted succession of strata, the youngest layer is at the top.  However, tectonic hiccups and upheavals have frequently turned successions of strata sideways or on their head. In this case knowing which way is ‘up’ will confirm which strata were overturned. Continue reading

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Stereographic projection – unfolding folds

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Restoring paleocurrent trends to their pre-deformation orientations

This post is part of the How To… series

Sedimentary strata are commonly deformed. If we assume that the original depositional dip was close to horizontal (this is a reasonable assumption, although there are exceptions) then we need to account for structural dip. This is important if our sedimentary analysis includes measurement of paleocurrents, depositional dip and strike (e.g. ancient shorelines, rivers), or any paleogeographic determination.

Two methods are outlined here: the standard stereographic projection method that is usually done back in the lab unless you are lucky enough to have access to the internet in the field; and a quick field method using a field note book, pencil and compass; this method can only be used in strata folded by a single phase of deformation.

Stereographic method

In the simplest case (diagrams below), strata containing flute casts have been folded into a syncline with a horizontal fold axis; we need to correct the for dip in the fold limb to find the original orientation of the flute cast.   Bedding strikes 080o (N80E), dip 40oN; the flute cast plunges 38o at 315o (N45W).

On a transparent overlay, plot the plane (strike and dip) and the line (orientation and plunge) representing the flute cast (it will be a point on the great circle) on the stereonet. To return the plane to its original pre-fold orientation, it must be rotated about its strike; the next step is to move the great circle back to its original plotting position. When the plane is rotated to horizontal all points on the great circle will move along small circles to the stereonet perimeter. The point representing the flute cast is now in its original orientation – here 332o, or N28W. The difference between the uncorrected and corrected bearing is 14o.

 

Direct field method

You can use this method when sedimentary structures (or any linear trend) are exposed on bedding planes.  It basically performs the same task as a stereonet correction.

Place a hard, flat surface on bedding – hard-cover note book or board are good – with the long edge oriented along strike. Place a pencil or similar straight-edge on the surface such that it is aligned with the structure of interest (flute long axis, ripple trend etc). Rotate the book about its long edge (i.e. about the strike) until it is horizontal – make sure the straight-edge doesn’t move. Holding the book and straight-edge in place, measure the bearing.

If you are not convinced about the accuracy of the field method, check it against the stereonet correction. You will find with practice, and not a little care, that direct field measurement provides excellent results.

 

Some other posts in this series:

Measuring dip and strike

Solving the three-point problem

Stereographic projection – the basics

The Rule of Vs in geological mapping

Folded rock; some terminology

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Stereographic projection – the basics

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Plotting the orientation of a plane on a stereonet; finding strike and dip from two apparent dips

This post is part of the How To… series

My first exposure to stereographic projection methods of structural analysis was one of my more baffling moments of undergraduate geological learning. Looking around the class I could see I wasn’t the only one. Our tutor, a master at pointing out the obvious, suggested we try thinking in three dimensions. In retrospect it was probably some of the best advice I ever received. After all, geology is nothing if not an exercise in visualizing the 3- and 4-dimensional world around us.

Visualize a sphere; cut the top half off – we don’t need it. If you look directly down the vertical axis of the hemisphere it will appear to be two-dimensional. When you do this, you are basically projecting the 3-D hemisphere onto a 2-D circle. Once this projection has become embedded in your mind’s eye, everything else will fall into place.

Stereographic projection is all about representing planes (e.g. bedding, foliation, faults, crystal faces) and lines (e.g. dip and plunge directions, fold axes, lineations) onto the 2-D circle. In geology, we overlay the 2-D projection with a grid of meridians, or great circles (analogous to longitudes), and parallels or small circles (analogous to latitudes). Thus, all compass points are represented. The net-like grid is called a Wulff Net, or stereonet. It is an equal angle grid where meridians and parallels intersect at right angles.

Note the two extreme projection conditions: the circumference is actually the great circle of a horizontal plane; the opposite is a straight line passing through the centre that is the projection of a vertically dipping plane.  Thus, shallowing dips tend toward the circumference. Try to visualize this by imagining the great circle change as the plane rotates from vertical to horizontal dip in the 3-D view.

In practice we use a transparent overlay pinned to the centre of the stereonet; we rotate the overlay according to the orientation of lines and planes.

Follow the next exercise with the short animation. It was made from still images: use the pause and play buttons as you work through the exercise.

Imagine a plane striking 060o (N60E), dipping 60o SE; the plane passes through the centre of the sphere. In our vertical 2-D view the intersection of the plane with the hemisphere appears as a curved line, or great circle. Rotate the overlay counter-clockwise 60o; mark this point on the circumference. The great circle representing the angle of dip is found by counting along a line at right angles to the strike, i.e. along the E-W axis, counting 60o from the horizontal (the circumference).

Note that in the 2-D view, the great circle meets the circumference at 060o and 240o that are two points on a horizontal line across the plane; the great circle is therefore the strike of the plane.

If a plane projects onto the stereonet as a great circle, then a line or lineation on the plane will plot as a point on the same great circle. To illustrate, we will find the true dip and strike of a plane for which we have measured two apparent dips: (1) 30o at 315o (N45W) and (2) 32o at 014o (N14E). On a transparent overlay, mark the positions of the N-S and E-W axes, and the stereonet centre. For (1), rotate the overlay 45o clockwise, mark the position at the N pole, then count 30o from N along the N-S axis. This point is the projection of apparent dip (1). Likewise, repeat for apparent dip (2). Rotate the overlay until the two apparent dip projections lie on the same great circle – read the bearing from N and the true dip along the E-W axis.

Follow this exercise with the short animation. It was made from still images: use the pause and play buttons as you work through the exercise.

 

Stereographic projection is a powerful method, not just to solve relatively simple (but important) problems of dip and strike, but as an analytical tool for more complex structural geology. There are several good software programs and Apps to automate projections for large data sets. But before you dive into these digital tools, try the simple overlay first – there’s nothing like a hands-on approach to help galvanise an understanding of basic methodology.

Some other useful posts in this series:

Measuring dip and strike

Solving the three-point problem

Stereographic projection – unfolding folds

Stereographic projection – poles to planes

The Rule of Vs in geological mapping

Folded rock; some terminology

Here are some good website links:

Visible Geology

Rick Allmendinger’s Stereo 10 – there’s also a mobile App version

Innstereo (open source)

Links to several programs in The Structural Geology Page

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