The Ichnological Record of Slope Deposits

Slope settings are influenced by numerous factors, including sediment bypass, mass wasting, incision by canyons and gullies, and/or widespread suspension settling of fine-grained particulate debris. Significant deposition of coarse-grained detritus may occur on slopes, particularly associated with ponding in channels and minibasins. Ichnology has been under-utilized in assessing slope strata, mainly owing to erroneous perceptions that slope trace fossil suites are: (1) relatively homogeneous and exclusively assigned to the Zoophycos Ichnofacies; or (2) largely indistinguishable from basin-floor assemblages. Organisms residing on the slope respond to varied parameters such as current energy, sedimentation rate, slope instability, substrate consistency, oxygen and food availability, and water turbidity, which in turn, are controlled by basin circulation, water stratification, shelf width, and sediment supply (e.g., point versus line source). Depending upon the subenvironment, slope trace fossil suites can be assigned to the Zoophycos, Cruziana, Skolithos, Glossifungites or Nereites ichnofacies.

Bioturbation in ambient slope deposits, dominated by deposit-feeding and grazing structures, can be intense and uniformly distributed, with progressively reduced bioturbation where event beds are more abundant. Conduits cross-cutting the slope are generally exposed to elevated and replenished oxygen and food resources. Resulting suites are sporadically distributed, due to gravity flow events that recur at frequencies at least an order of magnitude slower than infaunal recolonization. Erosion commonly eradicates the record of biogenic reworking, particularly in axial channel or canyon settings. Trace fossil suites, where present, are commonly impoverished and record opportunistic behaviors, attributable to the Skolithos Ichnofacies. Erosion associated with submarine conduits also exposes semi-consolidated substrates that are colonized with palimpsest firmground suites of the Glossifungites Ichnofacies.

Channel-levee complexes are characterized by relatively high trace fossil diversities. Suites are commonly assigned to ichnofacies typical of shallow water deposits (e.g., Cruziana Ichnofacies), although the inclusion of graphoglyptids marks a significant departure in its character. In comparison to basin-floor deposits, however, graphoglyptid-makers are generally outcompeted on the slope as a result of unstable environmental conditions and abundant food resources.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California