Category Archives: Atlas of sediments & sedimentary structures

The mineralogy of sandstones: Quartz grains

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This post is part of the How To… series – quartz mineralogy in sandstones

Classification of terrigenous sandstones depends on the identification of two main components: framework grains and matrix. Frameworks are represented by a QFL triad – quartz, feldspar and lithic fragments, where the proportion of each grain type is determined from thin section.  Most classification schemes aggregate all types of quartz, feldspar and lithics into each end-member. This approach is sensible and easy to use.

But simply naming a sandstone (or any rock type for that matter) is not enough. We also want to know about its provenance, the sediment source or sources – was it a stable continent or active mountain belt, volcanic arc or ocean basin, perhaps a far-travelled terrane or tectonic sliver for which the only evidence is the collection of grains that have survived multiple cycles of attrition.

Teasing this information from the rocks requires us to delve into the mineralogy in greater detail. The simplest and cheapest way to do this is with thin sections and a polarizing microscope. We begin with the most common terrigenous component – quartz. Continue reading

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Grain size of clastic rocks and sediments

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grain size of beach sediment

This post is part of the How To… series

You have collected a sandstone that has been described as coarse-grained.

  • What does ‘coarse’ refer to?
  • What do we mean by ‘grain size’? and
  • What parameters do we measure to make such a determination?

Grains, whether they are silt, sand or gravel, assume myriad shapes. Grain shape is determined by several factors, perhaps the most important being:

  • Crystal properties such as habit and cleavage in the case of common minerals like quartz, feldspar or mica,
  • Original texture or fabric in the parent rock from which the grains are derived,
  • The mechanical and chemical stability of grains during transport, and
  • Post-depositional changes like compaction (important in soft lithic fragments), size reduction by mineral dissolution or size enlargement by precipitation.

The problem of defining grain size boils down to one of defining the most useful shape. The image of a beach deposit at the top of the page serves to illustrate this.  Clasts here include those that approximate spheres, are elongate, rod-like, blocky cubes, and those that are more flattened. To determine their size, do we measure maximum, intermediate or minimum diameters? Do we calculate their surface areas? Or do we assume they all approximate spheres and use a diameter that best fits each grain?

The answer – all of the above. Years of experience and experimentation have shown show that the most useful grain size measures depend on what it is we want to study (see the generalized list of analytical methods appended to the USGS Chart, below).

For example, if we are interested in how the sedimentary particles behave hydraulically (e.g. in channel flow or across a wind-blown sand dune) then we could choose a measure that reflects resistance to flow or drag. If we want a more descriptive measure for loose or disaggregated sand and gravel, then we might apply a sieve diameter (the minimum diameter that will pass through a particular sieve aperture). Measurement of very fine particles (clays) that uses pipettes or settling columns, relies on particle attributes such as settling velocity through a column of water, or with the advent of new laser technology, the light-scattering properties of clay-sized particles in a dispersion (with water) or solid state (laser diffraction and laser particle sizer).

In 1922 Chester Wentworth devised a grain size scale based on a geometric progression – this scale is still the most popular (The Journal of Geology, Vol. 30, No. 5 1922, pp. 377-392). Wentworth followed the reasoning advanced earlier by J. Udden, where a geometric progression based on ‘2’ (rather than a linear scale) is sensible because:

  1. It allows a useful subdivision of grain sizes in silts and sands (the most common clastic rock types),
  2. The grain size in samples that represent single grain populations tend to plot as straight lines on log and semilog graphs.
  3. In addition, we now know that small changes in the grain size of sands and silts have hydraulic significance and hence it is very useful to incorporate these size classes into our general classification. In comparison, small changes in the size of cobbles and boulders have little hydraulic significance.

For the Wentworth scale, each successive size class is twice the size in inches of the previous class – hence the geometric progression 1/16, 1/8, ¼, ½, 1, 2, 4 and so on (these days we work in millimeters).

Note that ‘clay’ in this context refers to a size class, and not a mineral class.

A modification of this scale was devised by Krumbein – the well-known Phi Scale (ϕ), that is calculated using the expression

Φ = -Log2 of the grain size in millimeters

The Phi scale simplifies size measurement (sieve mesh sizes usually quote a Phi interval), the graphical representation of grain size populations, and calculation of statistical measures like mean and sorting. The handy USGS chart above shows a comparison of grain size classes in relation to sieve sizes, settling velocities, and threshold velocities for initiation of grain movement.

We now have the tools to describe the grain size of clastic rocks where size classes like coarse-grained sand(stone) have corresponding size ranges expressed in millimeters and Phi values.

Some other useful links

Describing sedimentary rocks; some basics

Analysis of sediment grain size distributions

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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Describing sedimentary rocks – some basics

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This post is part of the How To… series

The formation of sediment and sedimentary rock involves many physical, chemical and biological processes, sometimes operating separately but more commonly in concert. The journey from loose sediment to hammer-ringing rock is one of the marvels of the geological world. Deciphering this journey requires us to delve into the rock record. To do this we need a starting point – we need to know what it is we’re looking at and be able to communicate this information to others.

Asking how one describes sedimentary rocks is a bit like questions about bits of string; there is a bewildering array of them, each deserving a rational description. Fortunately, the guidelines for this task are well established. We have a century and a half of sedimentology icons to thank for this: Sorby, Wentworth, Twenhofel, Folk, Pettijohn, Potter, and Bathurst, to name a few.

Decades of collective experience allows us to place sedimentary rocks into four broad categories:

  1. Siliciclastic (or terrigenous) deposits contain fragments derived by erosion or weathering of older rocks. The fragments range in size from house-sized blocks to micron-sized clays – conglomerate and breccia (gravel in an unconsolidated state), sandstone-siltstone (sand and silt), and mudrocks (mixtures of silt, and clay-sized fragments or crystals). They represent the recycling of material across Earth’s surface. Some sediment components can survive more than one cycle of deposition, burial and induration (hardening), uplift, and erosion – quartz grains fit this category well, as do grains of zircon because of their chemical stability and hardness that enables them to withstand the rigours of erosion and deposition.
  2. Sediments derived primarily by biogenic processes (biological and biochemical) include carbonates (limestones and dolostones), peat, coal, and other accumulations of organic matter such as oil shale, and phosphates. These deposits may also contain fragmental debris, such as bioclastic fragments in limestones (e.g. coral, molluscs), but unlike multicycle siliciclastics the primary constituents are directly associated with organic processes; they are mostly first-cycle deposits.
  3. Chemical deposits include those formed directly by precipitation from saline water. The most common deposits in this category include evaporites (halite, gypsum), iron formations (particularly those in the Precambrian), and siliceous deposits that form chert (in Precambrian iron formations iron oxide precipitates and chert are commonly found together). As a general rule the environments of precipitation are low energy where the influx of siliciclastic sediment is low.
  4. Volcaniclastics represent fragmental debris derived from volcanic eruptions. This includes air-fall ash (tephras) and pyroclastic flow deposits (e.g. hot and cold ignimbrites) that are derived directly from eruption events, and material that is redeposited by lahars or subaqueous sediment gravity flows (turbidites, debris flows).

Like most classification schemes, particularly one as all-encompassing as this, there are exceptions – in fact exceptions are quite common. Mixed siliciclastic-carbonate deposits are common in stratigraphic intervals that are transitional between carbonate-dominated and siliciclastic-dominated environments.  Precambrian iron formations, like the Gunflint Formation in southern Ontario, may contain coarse-grained fragmental deposits and stromatolites that indicate high energy shoreline or shallow marine environments. Volcaniclastic debris that is redeposited may incorporate siliciclastic or biogenic carbonate material. We could deal with all these exceptions with a bit of fine tuning, subdividing each category into any number of variants, but I’ll leave that for others to do (I’m more a lumper than a splitter).

The posts that follow deal with sedimentary rock description at the hand-sample – outcrop scale. The descriptors were invented not just for the sake of description, but to provide fundamental links to depositional processes such as hydraulics, and the mechanical winnowing or sorting of grains based on shape and density. Our descriptions make use the following physical sediment properties:

  • Colour. This may seem pretty basic but colour provides valuable information about environmental conditions such as REDOX chemistry (i.e. the reduction-oxidation potential of chemical constituents), particularly the oxidation state of iron. For example, iron is in a reduced 2+ state in green mudrocks, and an oxidized state (3+) in terrestrial red beds.
  • Texture. This is a broad term that includes qualities like clast framework, and quantities like grain size and shape, and grain size distributions from which we can estimate statistical measures of central tendency, and standard deviations that sedimentologists refer to as sorting. Note that the term ‘grain’ is used here to mean any kind and size of clast, such as siliciclastic sand and gravel, or carbonate bioclasts and oolites.
  • Fabric, is a textural property that describes the orientation of clasts (including fossils). Common fabrics include parting lineation that develops from the alignment of individual sand grains, and imbrication of platy clasts in gravels. Sedimentary fabrics like these record paleocurrent flow directions.
  • Porosity is a measure of the void, or pore space between adjacent grains. At the point of deposition, all sediments have porosity. Weakly indurated rocks tend to retain some of their original porosity, but this decreases as compaction and cements develop.

Sediment framework and matrix. The literature on sedimentology is replete with references to attributes like “clast-supported framework”. What does this mean? Frameworks in this context refer to the essential ingredients that make something a sandstone, a conglomerate, a coquina – in other words the sand grains, pebbles cobbles and boulders, and bioclasts, respectively. Sediment framework is basically all the large bits.

Matrix refers to the fine-grained sediment, commonly clay, silt and fine sand sized material that occurs between or separates framework clasts. Matrix is a depositional product; it accumulates with the framework. Matrix clays are prone to alteration during burial diagenesis.

          

These definitions may seem a bit vague. We tend to make the distinction between framework and matrix only in ‘coarse-grained’ deposits. Trying to decide what is framework in mudrocks is next to impossible – they are mostly matrix.  Given this seemingly arbitrary distinction, why do we bother? The main reason is that identifying these textural properties helps us decipher depositional processes. Consider the two conglomerates shown above. The one on the left consists of clasts that are in direct contact such that the framework is clast-supported; the interstitial matrix here is a small amount of mud and sand. The second example also contains pebbles and cobbles but in this case they are separated by muddy sediment; they appear to ‘float’ in the matrix – this is a matrix-supported framework. In each case, the hydraulic conditions associated with their deposition were very different. On the left, flowing water was energetic and continuous enough to move gravel and remove most of the finer sediment.  On the right, the flow was energetic enough to move the gravel but not continuous enough to remove the matrix. The clast-supported conglomerate was deposited by a braided river where sediment was washed and reworked by strong currents, and the matrix-supported conglomerate by a submarine debris flow in which pebbles and cobbles were thoroughly mixed and transported as a muddy torrent – in this case there was no mechanism to remove the matrix.

Keep in mind that all the above properties can change during sediment burial. Diagenetic features are usually determined by examining thin sections with a polarizing microscope. Physical compaction and chemical diagenesis can:

  • Change the organization of grains one to the other (i.e. their framework),
  • Change the shape of softer grains,
  • remove some grains by dissolution (e.g. fossil fragments, or feldspars) – this is one mechanism that creates secondary porosity,
  • result in an increase in grain size as a result of precipitation; common examples of this are quartz overgrowths on quartz grains, and
  • produce cements that replace matrix and in some cases framework clasts.

Some other useful posts in this series:

Grain size of clastic rocks and sediments

Analysis of grain size distributions

Some controls on grain size distributions

Measuring a stratigraphic section

Crossbedding – some common terminology

The hydraulics of sedimentation: Flow Regime

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Using S and Z folds to decipher large-scale structures

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parasitic folds in Precambrian mudrocks

This post is part of the How to… series

A problem frequently encountered when mapping structurally deformed rocks is deciding whether the fold you see in outcrop is part of a larger structure – an anticline or syncline, overturned or recumbent, plunging? The problem is exacerbated if stratigraphic facing (younging) criteria are absent or ambiguous. Fortunately, there is a solution to the problem based on fold asymmetry.

Large folds commonly have smaller-scale folds in their limbs and crest. They are usually referred to as higher-order (2nd, 3rd order etc.) or parasitic folds. They form during flexure of layered rock where slip occurs between rock layers – a mechanism called flexural slip. Structural geology teachers frequently use a soft-cover book to demonstrate this mechanism; bend the book into a fold and watch the slip between adjacent pages that is required to accommodate extension on the outer arc of the fold and shortening on the inner arc.

 

Parasitic folds are common in deformed sedimentary rocks where slip takes place along bedding planes or between layers with contrasting strength, such as mudstone and indurated sandstone (mudrocks have abundant clays and micas that are prone to slip and shear). The example below shows an anticline (1st-order structure) and smaller 2nd-order folds developed in a relatively weak layer. The 2nd-order folds have asymmetries related to the sense of slip on each fold limb and are called S- and Z-folds. Note that the 2nd-order M-folds in the hinge are symmetrical and in this example, upright.

description of s and z folds

The difference between S- and Z-folds lies in their sense of rotation, or vergence. The long limbs of S-folds are connected by a shorter limb that implies counter-clockwise rotation or sense of displacement; the opposite applies to Z-folds. Thus, the vergence of parasitic folds is towards the hinge line (or zone)

parasitic folds in banded iron formation parasitic folds in Dalradian psammites

S- and Z-folds are three dimensional structures and will have hinge lines (or fold axes if we consider them to be cylindrical folds) and axial surfaces that can be measured. Another important property of parasitic folds is that their hinge lines (or fold axes) are parallel (or approximately so) to the hinge line of the 1st-order fold.

A note of caution; the sense of fold rotation-displacement will change if a fold is viewed from the opposite direction (i.e. S-folds will appear as Z-folds). Hence it is necessary to indicate the direction in which observations are made. Where possible, folds should be viewed down-plunge.

The geometric disposition of S- and Z-folds is extremely useful for deciphering large-scale folds, particularly when exposure is incomplete (as is commonly the case). The diagram below shows a scenario, where small folds are exposed in two outcrops.

deciphering large scale folds

Our view indicates the left outcrop is a Z-fold; the one on the right an S-fold. We can also determine the general attitude of the 1st-order fold limbs from dips and strikes on associated beds. If 1st-order fold-closure is beneath the surface, then it is a syncline (or synform if we don’t know facing direction); if above, an anticline or antiform. Both parasitic folds indicate vergence above the outcrops. If the structure was a syncline then the vergence should be in the opposite direction. The structure is therefore an antiform. If we have good facing direction data we could confirm the 1st-order structure is an overturned anticline (stratigraphy in the right outcrop is overturned).

Some other posts in this series:

Measuring dip and strike

Stereographic projection – the basics

Stereographic projection – unfolding folds

Stereographic projection – poles to planes

 

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Folded rock – some terminology

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plunging anticlines Belcher Islands

This post is part of the How to… series

Folds invoke a sense of awe, of some gargantuan, earth-bound push-and-shove. A hand bending and buckling rock with plasticine ease, like some Wallace and Grommet joke. Folds the size of your finger or entire mountains. They are windows into the forces that shape our world.

An analysis of folds is central to unravelling the structural complexities of solid Earth, the history of mountain belts, the formation and emplacement of Earth resources. And like any scientific analysis we need some terminology. Fold terminology is pragmatic; it is based primarily on what one commonly observes at the rock face or hillside. Its utility, combined with simple geometric rules, enables us to project structures beyond the immediately observable, to learn that small outcrop-scale folds formed sympathetically with much larger structures, or decipher the relative movement of fault blocks.

fold terminology

Although there are a myriad fold shapes and sizes, most can be described as variations of two basic forms: synforms which are concave upward folds, and antiforms which are convex upward. If we know the stratigraphic order of folded layers then we can use the more common names: anticline for folds that are convex in the direction of younging, and syncline for those that are concave in the direction of younging. If we do not know the younging or facing direction, a common problem in metamorphic rocks, then we must use the synform-antiform terminology.

The diagram above also shows some basic fold orientations and the inter-limb angles that determine the tightness of a fold. Have a look at the images at the bottom of this page for examples.

fold axes and axial planes

All folds have hinge points – the point (or narrow zone) of maximum curvature on a folded surface. Points of maximum curvature connected along a fold surface will define a hinge line; the hinge line may be straight or curved. However, to define the orientation of the fold uniquely, we need one other measure – the axial surface. The axial surface is defined by connecting all the hinge lines for each layer in the fold. The axial surface may be curved or planar – if it is the latter, we call it an axial plane. The orientation of an axial plane is determined by its strike and dip. Note that hinge lines also lie on the axial surface/plane.

If a hinge line is straight, it is called a fold axis (that must also lie on the axial plane). A fold axis is an imaginary straight, or nearly straight line that, when moved parallel to itself, will recreate the fold. This property distinguishes fold axes from hinge lines. The brief animation here shows diagrammatically the generation of a fold about the locus of a fold axis.

 

cylindrical folds

We can use the geometric identity of a fold axis to define two other classes of folds: cylindrical and non-cylindrical folds. All cylindrical folds have fold axes; all cylindrical folds have axial planes (non-cylindrical folds have axial surfaces). An ideal cylindrical fold is analogous to a soup can with both ends removed. Of course, real folds rarely look like cylinders – most have some degree of hinge-line curvature. But most folds can be divided into straight-line or nearly-straight line segments, such that aspects of cylindricity can be identified in each segment (as in the diagram).

This may sound a bit contrived – actually it is – but it provides us with a sensible approach to fold analysis, particularly using stereographic projections. In the simplest case, a dip and strike measurement on each limb of a cylindrical fold will plot on a stereonet as two great circles, the intersection of which is a point that represents the trend and plunge of the hinge line, or fold axis. We can begin to make sense of more complex geometries in large-scale folds if we analyse each cylindrical fold segment in this way. The stereographic projection method of fold analysis will be described in another post.

 

Some other posts in this series:

Measuring dip and strike

Solving the three-point problem

Stereographic projection – the basics

Stereographic projection – unfolding folds

Stereographic projection – poles to planes

The Rule of Vs in geological mapping

 

Some older, but incredibly useful texts on folds and structural geology:

G.H. Davis and S.J. Reynolds. 1996 Structural geology of rocks and regions. John Wiley & Sons, Inc. New York, 776 p.

B.E. Hobbs, W.D. Means, and P.F. Williams, 1976. An outline of structural geology. John Wiley & Sons, Inc. New York, 571 p.

J.G. Ramsey, 1967. Folding and fracturing of rocks. McGraw-Hill Book Co., New York, 560 p.

J.G. Ramsey and M.I. Huber. 1987. The techniques of modern structural geology, v.2. Folds and fractures. Academic Press, London, 381 p.

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Atlas of cool-water carbonate petrology

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This is the companion Atlas to the Cool-water carbonates – outcrop images.

New Zealand cool-water carbonates are predominantly bioclastic, consisting of fauna like bivalves, calcareous bryozoa, barnacles, echinoderms, and flora such as rhodoliths (calcareous algae that encrust rock fragments and shells).  This is particularly the case on shelves with little terrigenous sediment input, such that there is a diverse epifauna. Good examples of this setting occur around New Zealand. There is no evidence in any of the Oligocene through Pleistocene stratigraphy for aragonite-producing algae like Halimeda and Penicillus.

However there is a range of bioclast compositions, ranging from low- to high magnesian calcites and aragonite, The bioclast compositional variation has a significant impact on cement types (micrite and rhomohedral calcite envelopes) calcite spar, and neomorphic replacement by calcite of original bioclast aragonite. Like cements in more tropical realms, the cement paragenesis in cool-water carbonates reflects complex histories of fluid flow and evolving fluid chemistry through sea floor cementation, burial, uplift and ingress of meteoric water. Some useful references describing the paragenesis of Pliocene cool-water carbonates from the east coast of North Island (Te Aute Group) are given below.

 

Contributors:

Vincent Caron is a lecturer in geology and researcher in carbonates based at the Université de Picardie Jules Verne in Amiens, France. He is also a member of the Basins Resources Reservoirs research group. Most of the images presented here on Te Aute limestones formed part of his PhD research at Waikato University. A short list of his publications on the Te Aute is shown below.

Cam Nelson, is one of the original adherents of the Cool Water Carbonate paradigm, who set the scene with his studies of the Oligocene Te Kuiti Group. Cam is an Emeritus Professor at Waikato University.

CS Nelson, PR Winefield, SD Hood, V. Caron, A Pallentine, and PJJ Kamp. 2003. Pliocene Te Aute limestones: Expanding concepts for cool-water shelf carbonates. New Zealand Journal of Geology and Geophysics, 46, 407-424.

V Caron, CS Nelson, and PJJ Kamp. 2004. Contrasting carbonate depositional systems for Pliocene cool-water limestones cropping out in central Hawke’s Bay, New Zealand. New Zealand Journal of Geology and Geophysics, 47, 697-717.

B.D. Ricketts , V Caron & C.S. Campbell 2004. A fluid flow perspective on the diagenesis of Te
Aute limestones. New Zealand Journal of Geology and Geophysics, 47:4, 823-838

V Caron and CS Nelson. 2009. Diversity of neomorphic fabrics in New Zealand Plio-Pleistocene limestones: Insights into aragonite alteration pathways and controls. Journal of Sedimentary Research, v. 79, p. 226-246.

 

The images:

Pliocene Te Aute Group, Hawkes Bay, New Zealand

neomorphic calcite cement neomorphic calcite cement

partial neomorphism neomorphic calcite calcite replacing aragonite calcite filling aragonite holes calcite cementscalcite cements micrite and spar cements coarse spar cement

aragonite biomoldsacicular calciteneedle cacliteacicular calcite

 

Pleistocene Pukenui Limestone, southern North Island, in plain polarized light and cathodoluminescence.

cathodoluminescence zoned cement cathodoluminescence zoned cement

 

Oligocene Potikohua Limestone near Greymouth, South Island

bryozoan limestone

 

Oligocene Orahiri and Otorohanga limestones, Te Kuiti Group. Courtesy of Cam Nelson

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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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