Applications of Ichnology to High-Resolution Genetic Stratigraphic Paradigms

Abstract Trace fossils and trace fossil suites can be employed to aid in the recognition of various discontinuity types and to assist in their genetic interpretation. Ichnology can be employed to resolve surfaces of stratigraphic significance in two main ways: 1) through the recognition of discontinuities using omission suites reflecting palimpsest softground ichnofacies and substrate-controlled ichnofacies (i.e., Glossifungites, Trypanites, and Teredolites ichnofacies), and 2) through careful analysis of vertical softground (penecontemporaneous) ichnologic successions (analogous to facies successions). Integrating the data derived from omission suites with paleoecological data from vertically and laterally juxtaposed softground ichnological suites greatly enhances the recognition and interpretation of a wide variety of stratigraphically significant surfaces. This, coupled with conventional sedimentary facies analysis and sequence stratigraphy, constitutes a powerful approach to the interpretation of the rock record. Continued analysis of the utility of omission suites has shown that autocyclically generated breaks are common to the terrestrial realm, inshore, intertidal settings, and to some slope settings. Autocyclicity is less of an issue in shelf and shallow marine settings, where erosion of the substrate is typically associated with deposition, closing the colonization window. In the case of cohesive substrates, autocyclic breaks are associated with less indurated “stiffgrounds” that demonstrate smaller, less penetrative and commonly somewhat compacted structures compared to true firmground counterparts that characterize allocyclic discontinuities. Allocyclic discontinuities vary in character spatially, depending upon the lithologic character of the exhumed substrate, the degree of coherence of the exhumed substrate, the energy regime at the time of colonization, and the paleoenvironment that prevailed during colonization. Modern and ancient studies demonstrate that a single discontinuity may host omission suites that span the entire range from palimpsest softground, through firmground, woodground to hardground. Further, within the firmground suites, recurring proximal to distal variability has been documented, recording depositional conditions that prevailed during colonization. Proximal expressions tend to be characterized by vertical structures of inferred suspension-feeding and passive-carnivore infauna. Distal expressions, in contrast, are wholly dominated by the horizontal dwellings of inferred deposit feeders. Surfaces of sequence stratigraphic importance such as regressive surfaces of erosion (RSE) and subaqueous extensions of sequence boundaries (SB) locally host omission suites. In particular, the bases of forced regressive and lowstand shorefaces and deltas may contain palimpsest softground and firmground trace suites. Such deposits pass seaward into correlative conformities (CC) that lack omission suites, though the preservation potential of the CC is exceedingly low in forced regressive settings. Transgressive surfaces widely host omission suites, particularly where they are erosional, or where they onlap earlier sequence boundaries. Non-erosional transgressive surfaces include marine flooding surfaces (MFS) and bay-margin flooding surfaces (BFS). In shelf and offshore settings, MFS dominate and commonly demarcate parasequences boundaries, and are locally associated oxygen-restricted ichnocoenoses (ORI) and condensed sections. Sediment starvation coupled with oceanic bottom currents, which may prevail on slope environments, may lead to firmground and stiffground omission surfaces that correlate with the MFS. In the inshore settings, embayment and estuarine incised valleys may contain BFS within their successions that correlate with MFS seaward. In shallower or higher-energy positions, the BFS may onlap an older sequence boundary during coastal retreat, and permit omission suite development. Higher energy conditions lead to wave or tidal-scour ravinement, which exhumes older deposits and/or incises through or modifies earlier discontinuities. This is particularly common in the coastal-margin realm, where transgressive surfaces of erosion (TSE) cut through older sequence boundaries (FS/SB). Coastal embayments, and estuarine incised valleys are particularly prone to such amalgamation of discontinuities. Continued transgression results in flooding over embayment and valley margins and transgressive ravinement of the interfluve areas. If transgression is incremental or shows pauses in the rate of deepening, short-lived periods of shoreline progradation can occur over the transgressive surface. This results in a distinctive transgressive facies relationship. In distal positions, the discontinuity is overlain by offshore to shelf deposits that accumulated below fair-weather wave base, but overlie an erosional discontinuity that was cut above fair-weather wave base. Firmground omission suites associated with the discontinuity highlight its erosional character and permit its differentiation from the correlative conformity of the lowstand shoreface. Resumed transgression may lead to the development of condensed section accumulation and associated ORI. These relationships demonstrate that the integration of ichnology with facies analysis and sequence stratigraphy is essential for high-resolution reconstructions of paleogeography, paleoenvironment, and depositional architecture.

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