Stacked Late Devonian lowstand shorelines and their relation to tectonic subsidence at the Cordilleran Hingeline, western Utah

Abstract

ABSTRACT The Upper Devonian (lower Famennian) Cove Fort Quartzite and associated strata of western Utah comprise three stratigraphic sequences, each containing a thick (as much as 65 m) lowstand systems tract. Coarse-grained shoreface quartz arenite and lagoonal dolomicrite of the lowstand systems tracts display the following features characteristic of forced regression: (1) they contain coarse-grained proximal shoreline deposits that overlie an erosional surface (sequence-bounding unconformity) cut into fine-grained distal-marine carbonate facies; (2) they lie seaward of a zone of sediment bypass, and are spatially detached and separated from the previous highstand shoreface; (3) they are confined geographically to a relatively narrow zone less than 30 km wide; (4) they are onlapped and overlain by transgressive systems tract deposits consisting of deep subtidal carbonates (West Range Limestone); and (5) landward of the lowstand shoreline pinch-out, the sequence-bounding unconformity beneath them merges with a transgressive surface of erosion above them. A thick wedge of siliciclastic forced-regression deposits is present at the base of each depositional sequence. The stratigraphic position of each wedge above the basal sequence boundary was controlled by eustatic sea-level fall. The forced-regression deposits are geographically restricted to a narrow strike belt along the eastern margin of the Pilot basin, which coincides with the trace of the Cordilleran Hingeline, a zone of crustal discontinuity. The geographic confinement and unusual stratal thickness of barrier-bar complexes in the lowstand systems tract, as well as the stratigraphic stacking of the forced-regression deposits, were controlled by flexurally induced high subsidence rates focused at the Cordilleran Hingeline.