Sedimentary Structures

Introduction In this laboratory exercise, we will learn the four classes of sedimentary structures: erosional, depositional, deformational, and biogenic. Erosional structures are formed during the initially high shear stress before deposition of the bed. Depositional structures form during the waning of flow and the decreasing shear stress of a depositional event. Deformational structures form by the deformation of a previously deposited layer of sediment and may occur as the sediment is being deposited or afterwards. Biogenic structures include a variety of tracks, trails, burrows, and borings made by organisms.

Erosional Structures During the early stages of a depositional event, high shear stresses typically cause erosion of the sediment surface. Often, but not always, this erosion scours down into firm and cohesive mud, which can be sculpted and eroded by the flow, yet retain its shape. As shear stress begins to decrease, this scoured surface is filled with sand. As a result, the bottom of a sandy bed forms a cast of the erosional features; these casts project from the bottom of the sandy bed and are called sole marks. We will learn eight common erosional structures; the first four are produced by fluid scour, the last four are produced by bedload.

1) Flute casts are elongate ridges with a bulbous nose at one end and a flared shallow end at the other. Flutes range from 5 to 50 cm long, 1 to 20 cm wide, and up to 10 cm deep. Flutes are formed by turbulent fluid scour generated by flow detachment at an initially low point on the bed, such as at an impact mark or burrow. The nose of the flute points upstream, and the flared shallow end points downstream.

2) Longitudinal scours are a series of closely spaced ridges and furrows that may locally coalesce. The ridges are spaced 0.5 – 1 cm apart, and are a fraction of a centimeter deep. The ridges are sharp, and the erosional furrows between them are broad and rounded. Longitudinal scours are generally preserved as casts on the bottom of sandstone beds. Like flute casts, longitudinal scours are produced by fluid scour. Longitudinal scours are aligned with the direction of flow, although it is generally not possible to tell which of the two ends is upstream. Image from Bridge and Demicco, 2008.

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Sedimentary Structures

3) Obstacle scours are horseshoe-shaped or crescent-shaped grooves that are eroded by fluid flowing around an obstacle, such as a pebble or shell. They can be centimeters to tens of centimeters wide and long. Obstacle scours are useful paleocurrent indicators because the two “tails” point downstream. In some cases, can remove all surrounding sediment except for a ridge of sand on the lee side of the object. Image from Allen, 1982.

4) Rill marks are small-scale dendritic channels formed by the erosion of non-cohesive sand. Unlike most erosional structures, they are typically found on the tops of beds of sand, not as casts on the sole of sand beds. Rill marks are a few centimeters to tens of centimeters wide, and they commonly form on beaches and riverbanks as water drains across a gently sloping surface. Rills coalesce in the direction of slope.

5) Prod marks are part of a broad category of tool marks, all of which form by the impact of an object on the sediment surface. Prod marks usually have sharply defined edges, often with one end deeper than the other. Prod marks range from millimeters to centimeters wide, and from centimeters to decimeters long. They are formed by an object striking the bottom, then lifting off the bed, with the indentation being subsequently cast on the bottom of a sandstone bed. The deep end of a prod mark points downstream, opposite of what is seen in flute marks. Prod marks are distinguished from flute marks by their sharply defined edges, their small size, and their typically parallel, non-flaring sides. Prod marks made by currents have their deep ends facing in the same direction, and are said to be unipolar. Bipolar tool marks have their deep ends facing in opposite directions and are formed by waves.

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Sedimentary Structures

6) Gutter casts are isolated elongate ridges on the soles of sandstone and limestone beds. They are symmetrically or asymmetrically U or V-shaped in cross-section. Gutters are often covered with prod marks and groove casts that run parallel to the gutter. The bottoms of gutters are often littered with skeletal fragments. Gutters are usually about 10 cm wide and deep and can extend for several meters. The formation of gutters is not fully understood, but they appear to be cut by the repeated impact of objects such as shells, and the presence of bipolar tool marks suggests that waves are necessary for their formation. The long axis is inferred to be parallel to the direction of flow. Image from Bridge and Demicco, 2008.

7) Groove casts are narrow, elongate, nearly straight ridges on the bases of sandstone beds. Although they can occur in isolation, they are more typically found in groups. Groove casts are usually only a few millimeters wide or less, but may be centimeters to meters long. Rarely, groove casts can be as large as gutter casts, but are distinguished by the presence of numerous minute grooves along their surface, instead of the prod marks that are more typical of gutter casts. Grooves are cut by objects dragged along the bottom by flow, and are subsequently cast on the bottom of sandstone beds. The axis of a groove casts is parallel to the direction of flow.

8) Chevron marks are a linear strip of stacked V-shaped or chevron-shaped marks. Chevron marks are typically 2 – 3 cm wide, and less than 5 mm deep. They are formed by an object being dragged along the bottom, and the deformation of weak but cohesive mud. Chevron marks are typically found as casts on the bottom of sandstone beds. The apex of the chevrons points downstream.

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Sedimentary Structures

Depositional Structures Depositional structures include normal and inverse grading, as well as bedforms and the stratification that they produce. We will cover bedforms and stratification in lecture, and the table below summarizes the main types.

Other depositional structures that we will cover in class include normal grading, seaward-inclined laminae, flaser bedding, wavy bedding, lenticular bedding, tidal bundling, herringbone cross-stratification, and compound dunes. Your lecture notes will include figures depicting these.

Deformational Structures As a bed of sediment is being deposited, but more commonly afterwards, it can be deformed by a variety of processes. Some of this deformation is at the exposed sediment surface, such as desiccation cracks, synaeresis cracks, and raindrop impressions. Other types of deformation affect some thickness of sediment, and most of these arise from gravitational instability of the layers, owing to differences in sediment density, viscosity, and fluid content. Disturbance of these layers, such as through rapid deposition or earthquakes, can cause them to deform. We will learn nine of the most common deformational structures.

1) Desiccation cracks are produced by the subaerial drying of a clay-rich layer of sediment and the formation of polygonal, vertically tapering cracks, which may range from a few millimeters to a few centimeters wide. Desiccation polygons are commonly five, six, or seven-sided and may be several centimeters to up to a meter across. Desiccation cracks are often filled with coarser sediment.

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Bedform Stratification

2D current ripples tabular (planar) cross-lamination

3D current ripples trough cross-lamination

2D dunes tabular cross-bedding

3D dunes trough cross-bedding

plane bed planar lamination

hummocky cross-stratification hummocky cross-stratification

vortex ripples wave-ripple lamination

post-vortex ripples post-vortex ripple lamination

Sedimentary Structures

2) Synaeresis cracks are doubly-tapering prismatic cracks that are several millimeters to several centimeters long. Although some synaeresis cracks may intersect, they do not form polygonal networks like desiccation cracks. Synaeresis cracks form by the subaqueous dewatering and shrinkage of clay-rich sediment, usually in environments in which salinity varies. For example, if a layer of mud is laid down in the presence of fresh water, and is later covered with salt water, the fresh water within the mud will be drawn out into the overlying salt water, causing the mud layer to shrink and crack.

3) Raindrop impressions are formed by rain falling onto a cohesive sediment surface. Raindrop impressions are characterized by a shallow pit surrounded by a slightly elevated rim, and are a few millimeters in diameter. Raindrop impressions are typically found on the upper surface of beds. Raindrop impressions are easily eroded and are therefore not common. Image from Reineck and Singh, 1980.

4) Wrinkle structures are a series of millimeter-scale ridges and troughs, sometimes taking on the appearance of minute ripple marks. They are not bedforms, as they are formed by the deformation of microbial mats. Wrinkle marks are typically found on the upper surface of sandstone beds.

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Sedimentary Structures

5) Load casts are irregular, rounded lobes of sand projecting from the bottom of sand bed into an underlying bed of shale. Laminae within the sand bed are deformed upward at the edges of the casts. Load casts can range in size from a few millimeters wide up to several meters wide. Load casts are formed by the sinking of dense sand into an underlying fluid mud; this gravitational instability is often triggered by earthquakes, breaking waves, or rapid deposition of the sand layer.

6) Flame structures are upward pointing fingers of mud or shale that project between downward hanging lobes of sand, often load casts. The tips of flame structures are commonly bent over in the same direction, indicating the direction of downslope slippage of the overlying bed. Flame structures are usually a few millimeters to a couple centimeters tall, and they are most easily observed on the side of a bed.

7) Ball and pillow structures are hemispherical to kidney-shaped masses of sand encased within mud or shale. Laminations within ball-and-pillow structures are usually contorted and folded upwards at the edges of the sand pillows. Ball and pillow structures are usually several centimeters to several meters wide. Unlike load casts, ball and pillow structure commonly results in lobes of sand detached entirely from the original layer of sediment in which they were deposited.

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Sedimentary Structures

8) Convolute lamination is characterized by pervasive and often complex folding of laminae bounded above and below by undisturbed laminae. Convolute lamination is produced by rapid rates of deformation or by shocking of the sediment, such as by earthquakes. In either case, mechanically weak layers fold and deform, whereas mechanically strong layers are undisturbed.

8) Synsedimentary folding is similar to convolute lamination but deforms multiple beds and occurs on much larger scales, often deforming layers 1–50 m thick. If folding is caused by downslope sliding, the axes of the synsedimentary folds are largely perpendicular to the direction of motion. Image from Bridge and Demicco, 2008.

9) Dish structures are thin, dark colored, concave-upward clay-rich laminae within siltstone and sandstone units. Individual dish structures range from 1–50 cm across. Pillar structures may also be present (one is at the right) and consist of dark, near-vertical concentrations of heavy grains that mark the position of pipes of vertically-escaping fluid. Dish structures record partial liquefaction of sediment. Image from Bridge and Demicco, 2008.

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Sedimentary Structures

Biogenic structures Biogenic structures are tracks, trails, burrows, and borings and are therefore records of the activity of organisms. Trace fossils can occur on the top of the bed, inside the bed, or on the bottom. Particularly common are burrow casts, known as convex hyporeliefs (see figure at right). Such casts can occur simply by filling of a groove on the sediment surface, by the filling of an organism burrowing at the interface between a lower muddy layer and an upper sandy layer, or by the erosional exhumation of a burrow and its subsequent filling with sand.

Trace fossils are given dual names, much like body fossils or living organisms, but these names do not refer to particular animals, but to distinctive types of behavior. There are many hundreds of named trace fossils, called ichnogenera and ichnospecies, and we will cover only ten of the most common ichnogenera that all geologists should know. Remember that ichnogenera should be italicized.

1) Skolithos is a simple unbranched vertical burrow, which may be lined or unlined. Skolithos commonly occur in great densities, creating a structure in sandstone called “Pipe rock”. The burrows are usually a few millimeters wide, but may be 10 to 20 centimeters long. Image from Basan, 1978.

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2) Thalassinoides is a large branching burrow and tunnel system. The burrows range from 1 to 7 cm in diameter, much larger than most burrow systems. The burrow system is highly complex, with the burrows branching and intersecting in many places. The burrow system is commonly sub-horizontal and can penetrate to depths of over 1 m. Image from Basan, 1978.

3) Ophiomorpha is a distinctive large burrow characterized by a thick lining that is nodular to pelleted on the outside, and smooth on the inside. Ophiomorpha forms large burrow systems very similar in geometry to Thalassinoides (they are made by same organism, the ghost shrimp), but Ophiomorpha has thick burrow linings, Thalassinoides is unlined. Image from Basan, 1978.

4) Diplocraterion is a U-shaped burrow with spreite (lines that mark former positions of the burrow) usually between the uprights of the active burrow, but sometimes below the bottom of the active burrow. Diplocraterion range from 3 – 15 cm wide. Arenicolites is a similar to Diplocraterion, but it lacks spreite. Rhizocorallium is also similar to Diplocraterion, but the burrow turns, such that most of the U-shaped burrow is oriented horizontally. Image from Basan, 1978.

5) Rusophycus is the resting trace of a trilobite and consists of a bilobed cast of the impression excavated by the two sets of legs of the trilobite. Rusophycus is commonly about 2–3 cm long, but can be as long as 20 cm. Rusophycus is most common in the lower Paleozoic and is rare afterwards, reflecting the mid-Paleozoic drop in trilobite diversity.

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6) Chondrites is characterized by a downward branching system of tubes that looks superficially like plant roots. The tubes are most commonly about 1 mm in diameter, but can be up to 15 mm in diameter. Vertical cross-sections through Chondrites may display obvious branching or simply a cluster of burrows. Sometimes, the burrows of Chondrites fan out along a particular bedding plane. Image from Basan, 1978.

7) Planolites is a straight to sinuous, horizontal, unbranched burrow that can range from 0.5 to 20 mm in diameter, but is usually 3–7 mm in diameter. Planolites is an unlined burrow and has a fill that differs from the surrounding sediment. Image from Basan, 1978.

8) Palaeophycus is very similar to Planolites in size and shape, but Palaeophycus is a lined burrow and is filled with sediment that is similar to the surrounding sediment.

9) Zoophycos is characterized by one or more broad, spreite-filled loops, and most of this trace fossil consists of spreite, not the active burrow. The successive spreite often give this trace a distinctive “rooster-tailed” appearance. The individual loops may range from 10 – 100 cm across. Image from Basan, 1978.

10) Nereites is a tightly meandering horizontal burrow that never intersects itself. The Nereites burrow has sharp bends that recurve and parallel other portions of the burrow. This pattern is repeated such that the entire sediment surface can be covered by this tightly spiraling and meandering trace. Image from Häntzschel, 1975.

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Sedimentary Structures

References Allen, John R.L., 1982. Sedimentary Structures: Their Character and Physical Basis, Volume II. Elsevier Scientific Publishing: Amsterdam, 663 p.

Basan, Paul, 1978. Trace Fossil Concepts. SEPM Short Course No. 5, Oklahoma City, 181 p.

Bridge, John S., and Robert V. Demicco, 2008. Earth Surface Processes, Landforms and Sediment Deposits. Cambridge University Press: Cambridge, UK, 815 p.

Collinson, J.D., and D.B. Thompson, 1989. Sedimentary Structures, 2nd edition. Unwin Hyman, London, UK, 207 p.

Häntzschel, W., 1975. Treatise on Invertebrate Paleontology. Part W: Miscellanea, Supplement 1, Trace Fossils and Problematica, 2nd edition. Geological Society of America, Boulder, Colorado, 269 p.

Leeder, Mike, 1999. Sedimentology and Sedimentary Basins: From Turbulence to Tectonics. Blackwell Science, Oxford, UK, 592 p.

Reineck, H. E. and I. B. Singh, 1980. Depositional Sedimentary Environments, 2nd edition. Berlin, Springer-Verlag, 551 p.

What to turn in This is a two-week lab exercise. We will not have completed the cross-bedding lectures by the time of the first week’s lab period, so you will not be able to work on those samples. The samples are therefore divided into groups for week one and week two of the lab.

The writeup for each week’s samples will be turned in at the beginning of the lab period in the following week, as usual. In other words, even though it is one lab, you will turn your work in two phases, each corresponding to one week’s samples.

For each sample, answer the questions posed. Your write-up should be typed; any drawings that are asked for can be attached on a separate sheet, scanned and placed into your write-up, or cut and pasted onto your write-up.

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