TYPES OF PRIMARY SEDIMENTARY STRUCTURES
I. INORGANIC SEDIMENTARY STRUCTURES
I. INORGANIC SEDIMENTARY STRUCTURES
- Bed forms and surface markingsTheseare features which form on the surface of a bed of sediment. Atthe timeof formation, the "surface of a bed" is equivalent to the seafloor, orthe bottom of a lake or river, for example
- Ripples are undulations of the sediment surface produced as wind or water moves across sand.
Asymmetrical ripples and cross-bedding
Symmetrical wave ripples and wave ripple cross-stratification
Symmetrical wave ripples
and wave-ripple cross-stratification in
Triassic lakebeds from the Culpeper Basin, Virginia
Asymmetrical ripples on a beach in Australia
Interferenceripples on the Georgia coast; south end of Jekyll Island. These ripplesare produced by the interaction of waves and currents in the ridge andrunnel system at the beach.
Interference ripples in Paleozoic rocks in the Appalachian Valley and RidgeProvince of northwestern Georgia
- Mudcracks area polygonal pattern of cracks produced on the surface of mud as itdries. The mud polygons between the cracks may be broken up later bywater movement, and redeposited as intraclasts (particularly in limemuds).MUDCRACKS
Recent mudcracks in a quarry near Frederick, Maryland
Triassic mudcracks in a quarry in Culpeper, Virginia
- Raindrop prints are circular pits on the sediment surface produced by the impact of raindrops on soft mud.
RAINDROP PRINTS
- Internal bedding structures
Theseare sedimentary structures which are best seen looking at a side viewof a sedimentary rock or sequence of sedimentary rocks. - Stratification(or layering) is the most obvious feature of sedimentary rocks. Thelayers (or strata) are visible because of differences in the color ortexture of adjacent beds. Strata thicker than 1 cm are commonlyreferred to as beds. Thinner layers are called laminations or laminae . The upper and lower surfaces of these layers are called bedding planes.
Laminations
Laminations on a beach,
St. Simons Island, Georgia
Stratification (also called bedding) in Paleozoic rocks
in the Red Mountain roadcut, Birmingham, Alabama
Varvesare a special type of lamination which forms in glacial lakes. Varvesrepresent deposition over one year, and their formation is related toseasonal influences. Varves are generally graded, with the coarsermaterial at the bottom (silt or sand) representing the spring andsummer meltwater runoff, and the finer material at the top representingslow settling of clays and organic matter from suspension during thewinter months when the lake is covered with ice. Counting of varves inthe geologic record has been used to measure the ages of somesedimentary deposits. - Graded bedding results when asediment-laden current (such as a turbidity current) begins to slowdown. The grain size within a graded bed ranges from coarser at thebottom to finer at the top. Hence, graded beds may be used as "upindicators".
Graded bedding - Cross-stratificationis a general term for the internal bedding structure produced in sandby moving wind or water. If the individual inclined layers are thickerthan 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 isinclined at an angle to the horizontal, dipping downward in thedowncurrent direction. Hence, cross-beds may be used as paleocurrentindicators, or indicators of ancient current flow directions.Cross-beds usually curve at the bottom edge, becoming tangent to thelower bed surface. The upper edge of individual inclined cross-beds isusually at a steep angle to the overlying bedding plane. Hence,cross-beds may also be used as "up indicators".
Asymmetrical ripples and cross-bedding
Cross-stratification in a beach cut, Jekyll Island, Georgia
Cross-stratification in an outcrop of upper Paleozoic rocks in Birmingham, Alabama
Large scale cross-bedding in Triassic dune sands,
Bay of Fundy, Nova Scotia, Canada - Sole marks
Sole marksare bedding plane structures preserved on the bottom surfaces of beds.They generally result from the filling in of impressions made into thesurface of soft mud by the scouring action of the current, or by theimpacts of objects carried by the current. If sand is deposited laterover the mud, filling in these structures, they will be preserved inrelief on the bottom of the sandstone bed. (These structures are notusually seen on the surfaces of shale beds because they tend to weatheraway.)
- Tool marks are produced as"tools" (objects such as sticks, shells, bones, or pebbles) carried bya current bounce, skip, roll, or drag along the sedimentsurface. They are commonly preserved on the lower surfaces ofsandstone beds as thin ridges. Tool marks are generally alignedparallel to the direction of current movement.
TOOL MARKS
- Flute marks areproduced by erosion or scouring of muddy sediment, forming"scoop-shaped" depressions. They are commonly preserved asbulbous or mammilary natural casts on the bottoms of sandstonebeds. Because of their geometry, flute marks (also called flutecasts) can be used to determine paleocurrent directions.
FLUTE MARKS
Organic or biogenic sedimentary structures are those which are formedby living organisms interacting with the sediment. Theorganisms may be animals which walk on or burrow into thesediment, or they may be plants with roots which penetrate thesediment, or they may be bacterial colonies which trap and bindthe sediment to produce layered structures. - Trace fossils or ichnofossils
Tracefossils or ichnofossils include tracks, trails, burrows, borings, andother marks made in the sediment by organisms. They are bioturbation structuresformed as the activities of organisms disrupt the sediment. Asorganisms tunnel through sediment, they destroy primary sedimentarystructures (such as laminations) and produce burrow marks. Bioturbationcontinuing over a long period of time will thoroughly mix andhomogenize the sediment. Through this process, a laminated sediment canbe altered to a massive, homogeneous sediment with no readilydiscernable layering or other sedimentary structures.
- Tracksor footprints are impressions on the surface of a bed of sedimentproduced by the feet of animals. Examples include dinosaurfootprints or bird tracks. In some cases, tracks are found as solemarks on the bottoms of beds, where sediment has infilled thetracks, and preserved them as casts.
A line of tracks showing the path along which an animal walked (as opposed to an isolated footprint) is called a trackway.
Dinosaur tracks,
Dinosaur State Park,
Rocky Hill, Connecticut
Modern racoon trackway, North Carolina - Trails aregroove-like impressions on the surface of a bed of sediment producedby an organism which crawls or drags part of its body. Trails maybe straight or curved.
Trails
Climactichnites,
505 million years old, Late Cambrian,
New York
- Burrows areexcavations made by animals into soft sediment. Burrows may beused by organisms for dwellings, or may be produced as a subterraneanorganism moves through the soil or sediment in search offood. Burrows are commonly filled in by sediment of a differentcolor or texture than the surrounding sediment, and in some cases,the burrows may have an internally laminated backfilling. Burrowfillings may become cemented and hard, weathering out of the rock inrope-like patterns.
Burrows in Triassic rocks,
Deep River Basin, North Carolina
Several types of burrows, including
branching, U-shaped, and vertical
Skolithos worm burrows in quartzite. Cambrian Weverton Quartzite, Harpers Formation, or Antietam Formation.
Cross-stratificationand laminations about 1 cm thick are present in some of thesamples.Stream cobbles found in Henson Creek, Prince Georges County,Maryland.
Scale in centimeters and inches. Image courtesy of A. O'Neil.
- Borings areholes made by animals into hard material, such as wood, shells, rock,or hard sediment. Borings are ususlly 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 alsoproduce borings, commonly riddling shells with numerous small holes.
- Root marksare the traces left by the roots of plants in ancient soil zones(called paleosols). Rootmarks typically branch downward in apattern resembling an upside-down tree. Root marks are sometimesgray or greenish, penetrating reddish-brown paleosols. Thiscontrast in color can make them easy to see and identify.
Rootmarks in the Triassic Deep River Basin, North Carolina - Biostratification structures
Biostratificationstructures are sedimentary layering produced through the activities oforganisms. Stromatolites are the only type of biostratificationstructure we will study.
- Stromatolitesare mound-like structures formed by colonies of sediment-trappingcyanobacteria (commonly called blue-green algae). Theseorganisms inhabit some carbonate tidal flats, and produce dome-likelaminations in lime mud (fine-grained limestone or micrite). Stromatolites are "organo-sedimentary structures", and not fossilsbecause they contain no recognizable anatomical features. Stromatolites form today in only a few places in the world, primarilyin hypersaline environments (such as Shark Bay, Australia), and afew freshwater carbonate- precipitating lakes. In the geologicrecord, most stromatolites are found in Precambrian and lower Paleozoiclimestones. The cyanobacteria which formed these stromatoliteswere photosynthetic, and they are therefore responsible forchanging the character of the Earth's atmosphere from one dominatedby carbon dioxide to one with significant quantities of freeoxygen.
Stromatolites,
Ordovician, western Maryland
Summary - Bedforms and surface markings
- Ripples
- Asymmetrical ripples
- Symmetrical ripples
- Interference ripples
- Mudcracks
- Raindrop prints
- Internal bedding structures
- Stratification (strata)
- Beds
- Laminations or laminae
- Varves
- Graded bedding
- Cross-stratification
- Cross-bedding (cross-beds)
- Cross-lamination
- Sole marks
- Tool marks
- Flute marks
- Trace fossils or ichnofossils
- Tracks
- Trackways
- Trails
- Burrows
- Bioturbation
- Borings
- Rootmarks
- Biostratification structures
- Stromatolites المصدر::
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