COMBINING SEDIMENTOLOGY AND ICHNOLOGY TO REFINE PALEOENVIRONMENAL INTERPRETATIONS IN FLUVIAL-DELTAIC-SHALLOW-MARINE CLASTIC SYSTEMS
We provide examples from both outcrop and core investigations that highlight the value of combining ichnology and sedimentology in clastic sedimentologic studies. Examples include mixed fluvial-flood and tidally influenced systems of the Western Cretaceous Interior Seaway (Loyd, Sego, and Neslen fms), a Cretaceous delta of Arctic Alaska (Schrader Bluff Fm), Permo-Triassic fluvial and deltaic systems of Antarctica (Mackellar, Fairchild, Buckley, and Fremouw fms), and Jurassic floodplains of Mississippi (Cotton Valley).
In these examples trace-fossil and lithofacies associations are used to: 1) identify nearshore, shallow-marine environments including tidal deltas, tidal flats, bayhead deltas, and subaqueous channels and discriminate between continental and marine strata; 2) differentiate between muddy interdistributary bays and floodbasins, 3) identify a fluvial-flood-dominated delta that prograded into a brackish water basin; 4) provide evidence for lake marginal forests; 5) ascertain the distribution of low- vs. high-latitude ichnological signatures of continental fluvial and lacustrine deposits in a Greenhouse world; and 6) recognize floodplain continental crevasse-splay and floodplain environments in otherwise enigmatic core intervals. The studies also provide examples of the trace-fossil identification of significant surfaces, otherwise nearly undetected, within both sandbodies and mudstones. This is important because missing such surfaces has implications to the reconstructing environments, sea-level curves, hydrology, and climate. We suggest that ignoring even subtle trace-fossil data can be detrimental to interpreting paleoenvironments, which can lead to erroneous assumptions about paleoenvironmental evolution and sandbody-shale geometries.