Ichnofossil Assemblage Research Articles

Ichnofossils of the alluvial Willwood Formation (lower Eocene), Bighorn Basin, northwest Wyoming, U.S.A

The ichnofossil assemblage of the lower Eocene Willwood Formation consists of at least nine distinct endichnia that are preserved in full relief. Four forms (three ichnogenera and four ichnospecies) are new and represent fodinichnia and domichnia of oligochaete worms, an insect or spider, an unknown vertebrate (probably a mammal), and domichnia of an unidentified organism. Other potential trace makers of the ichnofauna include insects, mollusks, and decapods. In contrast to an Egyptian Oligocene fluvial ichnofauna produced largely by animals that burrowed in stream channel deposits, the Willwood assemblage is principally of flood-plain origin. Though the ichnofauna occurs in a variety of paleosol types, most of the fossils are restricted in distribution to specific sediment and soil types and, within paleosols, to specific identifiable horizons. This attribute will make them valuable indiced of paleoenvironment once they are better known in other ancient alluvial sequences. The environment suggested by the Willwood trace fossils (damp, but not wet soils with fluctuating water tables) is consistent with the warm temperate to subtropical (possibly monsoonal) conditions that are interpreted for the Willwood Formation by independent evidence of body fossils and paleopedology.

Storm-Surge-Ebb Origin of Hummocky Cross-Stratified Units of the Andrews Mountain Member, Campito Formation (Lower Cambrian), White-Inyo Mountains, Eastern California

The Andrews Mountain Member of the Campito Formation consists of complexly interbedded and regionally metamorphosed feldspathic quartzites, siltstones, and mudstones of latest Precambrian and Early Cambrian age. Extensive outcrops of the member occur in the White-Inyo Mountains of eastern California. The Andrews Mountain Member accumulated in a shallow water, offshore depositional system within the broadly subsiding Cordilleran miogeocline. The sedimentology of the member suggests that both storm and fairweather processes were involved in its formation. The following idealized sequence of sedimentary structures is recognized within individual beds: 1) an erosional base with localized flute and load casts; 2) fine-grained sands containing parallel and low-angle cross-lamination, hummocky cross-stratification, rare penecontemporaneous deformation, and internal channels and discontinuities; 3) these free sands grade into silts and very fine sands containing climbing ripples, wave-current ripples, flaser bedding, and locally abundant bioturbation; and 4) the uppermost portions of the bed grade into bioturbated muds with Cruziana ichnofossil assemblages and local occurrences of linguoid, symmetrical, and interference ripples. The sedimentary structures in these sequences reflect deposition by high-energy storm events. The flows generated by these events are interpreted to be the product of the ebb of large storm surges. An inferred scenario of storm deposition can be reconstructed for the Andrews Mountain Member shelf: 1) initial erosion and suspension of fairweather sediments in response to long-period storm waves; 2) as a low pressure system moves onshore and dissipates, a sediment-charged, return bottom flow is generated that flows across the shelf and further erodes and entrains material; 3) fine sands are deposited as parallel-laminated and hummocky cross-stratified units as the competence and capacity of the flow decreases; 4) attenuation of the surge leads to low-flow-regime deposition of climbing ripples and, where deposition is within storm wave base, wave-current ripples and flaser bedding; 5) with the resumption of fairweather conditions, softbodied, benthic communities recolonize muddy substrates which are locally reworked by tidal and oceanic currents. Rather than depositing sand sheets over extensive areas, multiple storm surges are inferred to have moved sand incrementally across the Andrews Mountain Member shelf. Thinning/fining-upward and thickening/ coarsening-upward sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source.

Commotion in the ocean: the growth of continents and ocean basins : Dietz Robert S., 1964. In: Studies on Oceanography dedicated to Professor Hidaka in Commemoration of his Sixtieth Birthday, 465–478

A calcareous sandstone sequence that forms part of the Eze-Aku Formation (Reyment) features an assemblage of ichnofossils in a section exposed on a quarry face. The traces encountered include horizontal burrows, some of which are preserved as casts, lined with organic matter and belonging to the ichnogenera Gyrolithes, Pholeus and Arthrophycus. Three other types of trace fossils are described as horizontal crawling trails, flat impressions and cylindrical shafts without formal names. Gyrolithes are by far the dominant traces in this sequence. The degree of bioturbation is high in every horizon. Body fossils are very rare except for scattered occurrence of fragments of calcitic pelecypod shells.Ichnological and lithological considerations suggest that the sediments were deposited in an aerated shallow shelf environment which supported an assemblage of decapods, worms and other shallow water marine benthos. Deposition was generally below wave base under a continuous but relatively slow rate of sedimentation.