Using Ichnofossils to Reconstruct the Depositional History of Sedimentary Successions in Alluvial, Coastal-Plain, and Deltaic Settings

Ichnofossils and paleosols are powerful tools for reconstructing paleoenvironments and depositional histories of sediments in the coastal transition zone, where physicochemical factors of the continental and marine realms meet. Distinguishing alluvial, coastal-plain, deltaic, and estuarine environments in this zone is a major issue in outcrop and core. Alluvial environments contain terrestrial and freshwater ichnofossils and paleosols, although deposits in paralic settings may record tidal influence as evidenced by mud drapes, reactivation surfaces, and bidirectional crossbeds. Deltaic, estuarine, and coastal-plain environments contain a variety of marine, terrestrial, and freshwater ichnofossils and paleosols, depending on the salinity, sedimentation rate, groundwater profile, water oxygenation, and depositional-energy flux from fluvial, wave, and/or storm action. Delta-plain environments commonly contain weakly to moderately developed, poorly aerated, and waterlogged paleosols formed in proximal-to-distal settings. Ichnofossils and palynomorphs in the lower delta plain, including the distributary-channel, interdistributary-floodbasin, and associated lacustrine deposits, may reflect fluctuating salinity from freshwater to brackish water. Interdistributary-bay deposits commonly resemble deposits of estuarine settings, recording freshwater through brackish to open-marine water salinities. More marine conditions favor an increase in marine ichnofossil diversity and abundance. Environmental stress resulting from the input of freshwater, fine-grained sediments, and/or terrigenous organic material are recorded by lower ichnofossil diversity and/or greater abundance; freshwater deposits record few and simple ichnofossils not indicative of a particular animal or environment. Subsequent subaerial exposure of these settings by sea-level fall or strandline progradation produce a vertical sequence containing marine ichnofossils overlain by traces of terrestrial plants and animals commonly associated with paleosols. In many cases, continental ichnofossils and other pedogenic features overprint marine ichnofossils and facies. The juxtaposition, tiering, and crosscutting relations among ichnofossils and paleosols in the coastal transition zone, therefore, differentiate sediments deposited in alluvial, coastal-plain, estuarine, and deltaic environments.