SUBTLE EXPRESSIONS OF A REGIONALLY EXTENSIVE SEQUENCE BOUNDARY: INTEGRATING SEDIMENTOLOGY AND ICHNOLOGY IN THE TURONIAN FERRON SANDSTONE

The recognition of sequence boundaries in surface or subsurface stratigraphic sections is critical for sequence stratigraphic analysis. Major surfaces are typically identified by some combination of erosional relief, paleocurrent variation, and facies change. Some such boundaries are, nonetheless, subtle and easily missed even in well-exposed terrains. In this work, we integrate sedimentology with ichnology to aid in interpretation of facies changes across a sequence boundary. Within the Turonian Ferron Sandstone Member of the Mancos Shale in the Henry Mountains of south-central Utah, mapping along depositional strike (67 km north-south) and dip (27 km east-west) has allowed definition of a regionally extensive sequence boundary. Within the present study area (4.0 Km²) the sequence boundary is marked by an incised channel body up to 5.5 m thick and 150 m wide that erodes into medial delta front deposits. The main channel body displays well-defined pinch-outs and inclined heterolithic strata interpreted as lateral accretion sets formed in a coastal, meandering fluvial system. Up depositional dip, the channel preserves three stacked channel stories of well-sorted, medium-grained sandstone separated by thin, bioturbated siltstone intervals. The channel fill passes downdip and laterally into bioturbated, tabular, fine-grained sandstone bodies less than 1.2 m thick that are the distal expression of terminal distributary channels. In such distal locations, the sequence boundary locally juxtaposes lithologically similar heterolithic facies. The trace fossil assemblage of the marine heterolith allows it to be distinguished from its nonmarine counterpart. A second sequence boundary is locally evident <1 m above the main channel body succession and is locally overlain near the channel margins by an organic rich, plant fossil-bearing siltstone abruptly overlying marine trace-bearing facies. The delta plain interval is capped by a thoroughly bioturbated, fine-grained sandstone, recognized as a transgressive surface. Our work documents the paleogeographic variations of a terminal distributary channel network within an ancient lower delta plain environment and illustrates the variable and subtle expression of a major sequence boundary near its distal termination.