SEDIMENTARY STRUCTURES: BEDDING PLANE STRUCTURES SECONDARY MECHANICAL SEDIMENTARY STRUCTURES SECONDARY BIOGENIC SEDIMENTARY STRUCTURES 1
BEDDING PLANE STRUCTURES Generated on bedding surfaces by both erosion and deposition Cohesive sediments (silts, clays) Sole Marks 2
FLUTE CASTS Elongated wells/ridges with a bulbous nose that flares out Occur in swarms, aligned with current direction Created by scour behind an object (ie., stone) 3
TOOL MARKS Small gouges Skipping, bouncing, rolling objects GROOVE MARKS 4
DESICCATION FEATURES (mudcracks and raindrop craters) mudcracks, Precambrian,. N. Montana mudcracks, in Death Valley http://www.geosci.unc.edu/faculty/glazner/images/sedrocks/sedstructures.html http://darkwing.uoregon.edu/%7emillerm/crax.html SECONDARY MECHANICAL SEDIMENTARY STRUCTURES Due to soft-sediment deformation Sediment deposited so rapidly that the beds are liquid and unstable, and heavier sand beds tend to sink downward into soupy muds below due to gravity settling DO NOT mistake for structural deformation!!! 5
CONVOLUTE BEDDING Complex folding, crumpling of beds or laminae into small scale anticlines/synclines Triassic Helsby Sandstone Formation, East Irish Sea Basin Common in fine or silty sands turbidites, intertidal flats, flood-plains, point-bars Plastic deformation of liquefied sediment LOAD CASTS Deformation of uncompacted, watersaturated mud - uneven loading of overlying sand layers (rapid deposition) Irregular shapes Not true casts 6
LOAD CASTS Submarine fan, Pescadero, CA LOAD CAST 7
FLAME STRUCTURES Wavy, flame shaped tounges of mud that project into overlying layer sediment loading of water saturated mud-layers (less dense than overlying sand-layers) Mud layer Sand layer crests 8
Flame Structures and Graded Beds, Permian, Inyo County, California SECONDARY BIOGENIC SEDIMENTARY STRUCTURES Trace fossils, ichnofossils, or lebensspuren ( living traces in German) Anything produced by organisms in sediment Often best clue to sedimentary environment Also can be good clue about behavior of extinct organisms 9
ICHNOFOSSILS: PROBLEMS Traces are NOT body fossils, same organism can produce multiple traces, or different organisms can produce same trace Yet tradition is to give them taxonomic names (ichnogenera and ichnospecies) with the full recognition that they are not organisms, but fossilized behavior Names can sometimes be correlated to an organism, but in many cases tracemaker is unknown ICHNOFACIES 10
SKOLITHOS ICHNOFACIES Vertical tubes (length >> width) Very shallow water, nearshore burrowing, suspension feeders (horseshoe worms) Phoronid SKOLITHOS PIPEROCK Vertical cylindrical tubes 1-5 mm in diameter, up to 30 cm long dense clusters resemble organ pipes Common in shallow marine Cambrian sandstones, rare afterwards 11
OPHIOMORPHA Cylindrical vertical or branching burrows wider than Skolithos (1-3 cm in diameter) usually lined by bumps from fecal pellets of calianassid ghost shrimp Thalassinoides Branched burrows (smooth walls) Parallel to bedding plane crustaceans Paleozoic to present Upper Cretaceous Bad Heart Formation, Alberta Cretaceous, Recife, Brazil 12
ZOOPHYCOS feeding burrow - U-shape swath, swath repeated adjacent to the previous swath - clockwise or counterclockwise direction variety of water depths low oxygen, abundant organic matter ZOOPHYCOS Pennsylvanian Oquirrh formation, Utah http://www.geol.umd.edu/~tholtz/g331/331ichno.htm 13
Chondrites Small branching burrows common in low oxygen Nematodes Cambrian to present Late Paleocene, ODP Site 1259 Pennsylvanian Oquirrh formation, Utah http://www.geol.umd.edu/~tholtz/g331/331ichno.htm ICHNOFACIES Environmental indicators (ichnofacies): Scoyenia ichnofacies: nonmarine. Trypanites ichnofacies: lithified marine surface. Glossifungites ichnofacies: firm, unlithified, submarine surface. Skolithos ichnofacies: intertidal, soft substratum. Cruziana ichnofacies: shallow subtidal. Zoophycos ichnofacies: marine, below wave base. Chondrites ichnofacies: bathyl and abyssal environments. 14
DIPLOCRATERION U-shaped, vertical burrows High energy, shallow marine Burrowing, suspension feeder Scoyenia isp. horizontal to vertically oriented, simple, straight to slightly curved burrow with a "ropelike" architecture; individual burrows comonly cross one another. feeding burrow made by an arthropod (possibly a larval beetle) typically associated with nonmarine environments, originally formed in moist soils. 15