The importance of sedimentary structures in fine-grained deposits in re-interpreting the Facies mosaic of Lower Triassic tide-influenced deltas

Despite considerable progress in their description and characterization, tide-dominated deltas remain the least understood of the three main delta classes. Fine-grained sediments of the deltaic subenvironments remain particularly understudied, especially via ultra-thin section analysis. Such analysis can aid in identifying subenvironments typically missed in paleodelta analysis (e.g., delta plains). Combined with other methods, they can prove valuable in evaluating depositional and sequence-stratigraphic models and exploring the interplay between sediment supply and channel morphologies.

This study focuses on the Lower Triassic Torrey Member of the Moenkopi Formation of southern Utah. Previous interpretations posit that the Torrey Member represents a tide-dominated delta deposited in three depositional sequences. Integrating mudstone thin section analysis helped reveal that the Torrey Member contains ten facies associations, with the facies architecture indicating deposition by tide-dominated or tide-influenced deltas over a single depositional sequence. The progradational phase is notable for its decreased channel density and concurrent increase in subaerial exposure indicators. This study posits that hyperycnal flows, including bedload transport, dominated deposition in the prodelta and delta front of the Torrey Member, with no direct evidence of tidal processes.

This study broadly demonstrates the importance of integrating mudstone thin-section analysis in paleodelta studies. This paper particularly draws particular attention to delta plain identification in semi-arid and arid settings and suggests that thin-section analysis may reveal examples previously missed in similar paleodeltas.