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this exercise you will learn to recognize and identify sedimentary structures. F
this exercise you will learn to recognize and identify sedimentary structures. For this exercise, read through the following descriptions of sedimentary structures and then complete the assignment on Sedimentary structures. Download assignment on Sedimentary structures.Submit assignment to Dropbox.
Primary sedimentary structures are those which form during (or shortly after) deposition of the sediment. Some sedimentary structures are created by the water or wind which moves the sediment. Primary sedimentary structures can provide information about the environmental conditions under which the sediment was deposited; certain structures form in quiet water under low energy conditions, whereas others form in moving water or high energy conditions.
Secondary sedimentary structures form after deposition – such as footprints, worm trails, or mudcracks.
TYPES OF PRIMARY SEDIMENTARY STRUCTURES
I. INORGANIC SEDIMENTARY STRUCTURES
A. BED FORMS AND SURFACE MARKINGS
Bed forms are features which form on the surface of a bed of sediment. At the time of formation, the “surface of a bed” is equivalent to the sea floor, or the bottom of a lake or river, for example. In a sequence of sedimentary rock, bed forms and surface markings are found on bedding planes.
1. Ripples are undulations of the sediment surface produced as wind or water moves across sand. Ripples which form in unidirectional currents (such as in streams or rivers) tend to be asymmetrical. Crests of asymmetrical ripples may be straight, sinuous, or lobe-like (lingoid ripples), depending on water velocity. Asymmetrical ripples have a gentle slope on the upstream side, and a steep slope on the downstream side. Because of this unique geometry, asymmetrical ripples in the rock record may be used to determine ancient current directions or paleocurrent directions. In waves or oscillating water, symmetrical ripples are produced. Crests of symmetrical ripples tend to be relatively straight, but may bifurcate (or fork).
Asymmetrical Ripples
Asymmetrical ripples and cross bedding
Symmetrical Ripples
Symmetrical ripples
Interference Ripples
Interactions between waves and currents may produce a more complex pattern of interference ripples.
Interference ripples
2. Mudcracks are a polygonal pattern of cracks produced on the surface of mud as it dries (occur at tops of beds, paleo-up indicator). The mud polygons between the cracks may be broken up later by water movement, and redeposited as intraclasts (particularly in lime muds).
Mudcracks
3. Raindrop prints are circular pits on the sediment surface produced by the impact of raindrops on soft mud.
Raindrop prints
Raindrop prints with ripples and bird tracks
B. INTERNAL BEDDING STRUCTURES
These are sedimentary structures which are best seen looking at a side view of a sedimentary rock or sequence of sedimentary rocks.
1. Stratification (or layering) is the most obvious feature of sedimentary rocks. The layers (or strata) are visible because of differences in the color or texture of adjacent beds. Strata thicker than 1 cm are commonly referred to as beds. Thinner layers are called laminations or laminae. The upper and lower surfaces of these layers are called bedding planes.
Laminations and laminations on a beach, St. Simons Island, Georgia
Stratification (also called bedding) in Paleozoic rocks in the Red Mountain road cut, Birmingham, Alabama.
Varves are a special type of lamination which forms in glacial lakes. Varves represent deposition over one year, and their formation is related to seasonal influences. Varves are generally graded, with the coarser material at the bottom (silt or sand) representing the spring and summer meltwater runoff, and the finer material at the top representing slow settling of clays and organic matter from suspension during the winter months when the lake is covered with ice. Counting of varves in the geologic record has been used to measure the ages of some sedimentary deposits.
2. Graded bedding (turbidites) results when a sediment-laden current (such as a turbidity current) begins to slow down. The grain size within a graded bed ranges from coarser at the bottom to finer at the top. Hence, graded beds may be used as “up indicators”.
Graded bedding in a bottle, resulting from the settling of sediment
Graded bedding and graded bedding in New Jersey
3. Cross-stratification (cross bedding) is a general term for the internal bedding structure produced in sand by moving wind or water. If the individual inclined layers are thicker than 1 cm, the cross-stratification may be referred to as cross-bedding. Thinner inclined layering is called cross-lamination.
Cross-stratification forms beneath ripples and dunes. The layering is inclined at an angle to the horizontal, dipping downward in the down-current direction. Hence, cross-beds may be used as paleocurrent indicators, or indicators of ancient current flow directions. Cross-beds usually curve at the bottom edge, becoming tangent to the lower bed surface. The upper edge of individual inclined cross-beds is usually at a steep angle to the overlying bedding plane. Hence, cross-beds may also be used as “paleo-up indicators”.
Flow direction in Asymmetrical ripples and cross bedding
Cross-stratification at various beaches and rock outcroppings
C. SOLE MARKS
Sole marks are bedding plane structures preserved on the bottom surfaces of beds. They generally result from the filling in of impressions made into the surface of soft mud by the scouring action of the current, or by the impacts of objects carried by the current. If sand is deposited later over the mud, filling in these structures, they will be preserved in relief on the bottom of the sandstone bed. (These structures are not usually seen on the surfaces of shale beds because they tend to weather away.)
1. Tool marks are produced as “tools” (objects such as sticks, shells, bones, or pebbles) carried by a current bounce, skip, roll, or drag along the sediment surface. They are commonly preserved on the lower surfaces of sandstone beds as thin ridges. Tool marks are generally aligned parallel to the direction of current movement.
Tool marks, flow direction and tool marks in shale, Kentucky
2. Flute marks are produced by erosion or scouring of muddy sediment, forming “scoop-shaped” depressions. They are commonly preserved as bulbous or mammillary natural casts on the bottoms of sandstone beds. Because of their geometry, flute marks (also called flute casts) can be used to determine paleocurrent directions.
Flute marks with current direction
II. ORGANIC OR BIOGENIC SEDIMENTARY STRUCTURES
Organic or biogenic sedimentary structures are those which are formed by living organisms interacting with the sediment. The organisms may be animals which walk on or burrow into the sediment, or they may be plants with roots which penetrate the sediment, or they may be bacterial colonies which trap and bind the sediment to produce layered structures.
A. Trace fossils or ichnofossils
Trace fossils or ichnofossils include tracks, trails, burrows, borings, and other marks made in the sediment by organisms. They are bioturbation structures formed as the activities of organisms disrupt the sediment. As organisms tunnel through sediment, they destroy primary sedimentary structures (such as laminations) and produce burrow marks. Bioturbation continuing over a long period of time will thoroughly mix and homogenize the sediment. Through this process, laminated sediment can be altered to a massive, homogeneous sediment with no readily discernable layering or other sedimentary structures.
1. Tracks or footprints are impressions on the surface of a bed of sediment produced by the feet of animals. Examples include dinosaur footprints or bird tracks. In some cases, tracks are found as sole marks on the bottoms of beds, where sediment has infilled the tracks, and preserved them as casts.
A trackway is a line of tracks showing the path along which an animal walked (as opposed to an isolated footprint).
Dinosaur tracks, Dinosaur State Park, Rocky Hill, Connecticut
Modern raccoon trackway, North Carolina
2. Trails are groove-like impressions on the surface of a bed of sediment produced by an organism which crawls or drags part of its body. Trails may be straight or curved.
Trails. Climactichnites, 505 million years old, Late Cambrian, New York and Trails, Triassic, Culpeper Basin, VA
3. Burrows are excavations made by animals into soft sediment. Burrows may be used by organisms for dwellings, or may be produced as a subterranean organism moves through the soil or sediment in search of food. Burrows are commonly filled in by sediment of a different color or texture than the surrounding sediment, and in some cases, the burrows may have an internally laminated backfilling. Burrow fillings may become cemented and hard, weathering out of the rock in rope-like patterns.
Several types of burrows, including branched, U-shaped, and vertical. Burrows surrounded by pellets, Georgia coast. Burrows in Triassic rocks, Deep River Basin, North Carolina, Left = Zoophycos burrows in limestone, Kentucky. Right = worm burrows in quartzite. Cambrian Weverton Quartzite, Harper Formation, or Antietam Formation. Cross-stratification and laminations about 1 cm are present in some of the samples. Stream cobbles found in Henson Creek, Prince Georges County, Maryland. Scale in centimeters and inches.
4. Borings are holes made by animals into hard material, such as wood, shells, rock, or hard sediment. Borings are usually circular in cross-section. Some snails are predators and produce borings or “drill holes” into other molluscs, such as clams, to eat them. Another mollusc, known as the “shipworm”, drills holes into wood. Sponges also produce borings, commonly riddling shells with numerous small holes.
Boring in Arca bivalve shell, produced by carnivorous moon snail, Lunatia or Polynices. Note the conical, tapering shape of the hole, like a countersunk hole for a screw. Borings in bivalve shells, St. Augustine, FL. Borings in fossil giant oyster produced by Clionid sponge.
5. Root marks are the traces left by the roots of plants in ancient soil zones (called paleosols). Rootmarks typically branch downward in a pattern resembling an upside-down tree. Root marks are sometimes gray or greenish, penetrating reddish-brown paleosols. This contrast in color can make them easy to see and identify.
Rootmarks in the Triassic Deep River Basin
B. Biostratification structures
Biostratification structures are sedimentary layers produced through the activities of organisms. Stromatolites are the only type of biostratification structure we will study.
1. Stromatolites are mound-like structures formed by colonies in sediment- trapping cyanobacteria (commonly called blue-green algae). These organisms inhabit some carbonate tidal flats, and produce dome-like laminations in lime mud (fine-grained limestone or micrite). Stromatolites prefer warm, tropical hypersaline marine waters. Here is a link with more information about stromatolites from the Bush Heritage, Australia page.
Stromatolites are “organo-sedimentary structures”, and not fossils because they contain no recognizable anatomical features.
Stromatolites form today in only a few places in the world, primarily in hypersaline environments (such as Shark Bay, Australia), and a few freshwater carbonate- precipitating lakes. In the geologic record, most stromatolites are found in Precambrian and lower Paleozoic limestones. The cyanobacteria which formed these stromatolites were photosynthetic, and they are therefore responsible for changing the character of the Earth’s atmosphere from one dominated by carbon dioxide to one with significant quantities of free oxygen.
Stromatolites, Ordovician, western Maryland. Digitate (finger-like) stromatolites, Ordovician, western Maryland. Stromatolite
DETERMINING “PALEO-UP DIRECTION”
When you examine a sequence of beds which has been tectonically deformed and possibly overturned, it is necessary to determine the “up direction”. This is done by studying the sedimentary structures for clues.
Sedimentary structures such as graded beds, cross beds, mudcracks, flute marks, symmetrical (but not asymmetrical) ripples, stromatolites, burrows, tracks, and other structures can be used to establish the original orientation of the beds. (Fossils can also be used to establish up direction, if they are present in the rock in “life position”.)
Carefully examine the sedimentary structures in any dipping sedimentary sequence, because the rocks can be overturned by tectonic forces, and what initially appears to be younger because it is on top, may in fact turn out to be at the bottom of the section!
Illustration of overturned beds. Left column = right-side-up; Right column = upside-down
Summary
The following list is a summary of the sedimentary structures mentioned in this lab:
I. Inorganic sedimentary structures
A. Bedforms and surface markings
1. Ripples
Asymmetrical ripples (including lingoid and rhomboid ripples)
Symmetrical ripples
Interference ripples
2. Mudcracks
3. Raindropprints
B. Internal bedding structures
1. Stratification(strata)
Beds
Laminations or laminae
Varves
2. Graded bedding
3. Cross-stratification
Cross-bedding (cross-beds)
Cross-lamination
C. Sole marks
1. Toolmarks
2. Flutemarks
II. Organic or biogenic sedimentary structures
A. Trace fossils or ichnofossils
Tracks
Trackways
Trails
Burrows
Bioturbation
Borings
Rootmarks
B. Biostratification structures
1. Stromatolites
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