Biogenic Sedimentary StructuresGEOL 342 Sedimentation and StratigraphyLecture 5: Biogenic sedimentary structures14 February 2006Assoc. Prof. A. Jay KaufmanBiogenic Sedimentary StructuresSedimentary structures formed by organisms are known as trace fossils or ichnofossils.These give useful clues to the depositional environment, in particular to water depth andchemistry. What do these trace fossils represent?The environmental subdivisions are often known as ichnofacies. While these traces areoftentimes given Linnaean names, but they are not true organisms.IchnofaciesSkolithos – shallow marine environment above wave base with high sediment input 1Cruziana – shallow marine waters below normal wave base, but above storm wave baseZoophycos – associated with lower oxygen levels and higher organic matter in quiet bottomwatersNereites – deep ocean associated with turbidites on the abyssal plainSome trace fossils are of great importance as they are recognized as index markers forkey boundaries in Earth history. One example is Phycodes pedum, the first appearance of whichin a homogeneous and continuous succession of rock demarcates the Precambrian-Cambrianboundary.StromatolitesAn organo-sedimentary structure produced by sediment trapping, binding, and orprecipitation as a result of the growth and metabolic activity of micro-organisms, principallycyanophytes (blue-green algae). These have a variety of gross forms, from nearly horizontal tomarkedly columnar, domal or subspherical. These typically form in carbonate-dominatedenvironments lacking metazoans, like Shark Bay, Western Australia. 2Soft-bodied organismsThe earliest known animals that evolved on Earth (ca. 575 Ma) were the Ediacarananimals. These enigmatic organisms occurred in shallow marine environments, and are oftenassociated with thick microbial mats. The Ediacaran Period was recently ratified, and defined asthe interval immediately above glacial deposits of Marinoan age (ca. 635 Ma) and thePrecambrian-Cambrian boundary (ca. 543 Ma).The traces of these organisms typically occur on bedding surfaces as their imprints in themat are often filled with coarser sediment producing a cast. Known examples were buried undervolcaniclastic material following a nearby eruption, or within turbidites.The high surface-to-volume of these sessile organisms suggests that they absorbednutrients through seawater, and likely required higher oxygen concentrations to survive. 3Swartpuntia Ernietta
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