SENSITIVE ICHNOLOGICAL RESPONSE TO ALONG-STRIKE CHANGES IN DEPOSITION IN A MIXED-INFLUENCED ASYMMETRIC LOBE IN THE FERRON “NOTOM DELTA”, CAPITAL REEF, UTAH (USA)
An integrated sedimentological and ichnological data set was collected through 34 geological sections to document the 3D architecture of a lobe (parasequence 6, Ps 6) in the wedge. The data set shows delta asymmetry, with an along-strike facies transition from sandier shoreface facies on the updrift north side, southward into heterolithic, river-dominated delta-front facies close to the river mouth, and farther southward into wave-reworked delta-front facies in the downdrift side.
Along with the transition, ichnogenera correspondingly show distinct along-strike changes. On the updrift side, the shoreface facies are overall extensively burrowed and trace fossil suites show a robust and healthy expression of the archetypal Cruziana and Skolithos Ichnofacies. Southard to the river mouth, horizontal, morphologically simple, and facies-crossing structures are most common, with a paucity of dwelling structures of inferred suspension feeding organisms. Overall the river-dominated successions are largely unburrowed, but with sporadic highly burrowed spikes. Further southward on the downdrift side, ichnogenera abundance and diversity increase and in some cases suites can reflect the archetypal ichnofacies.
The diverse and robust trace fossil suite observed on the updrift side of the asymmetric delta is a direct response to sufficient nutrients and oxygen at the water-sediment interface, a stable substrate, and generally ambient environmental conditions, introduced by persistent wave agitations. In contrast, bioturbation at or close to the river mouths is a result of heightened sedimentation rates, elevated water turbidity, and increased substrate instability. High sedimentation and soupy substrate close to the river mouth are also responsible for the common occurrence of “mantle and swirl” structures, fugichnia, and equilibrichnia.
The study shows that ichnogenera are sensitive to changes in deposition and physical-chemical conditions. Identifying depositional elements and systems should, thus, involve ichnological analysis.