FACIES ARCHITECTURAL STUDY OF A STEPPED, FORCED REGRESSIVE COMPOUND INCISED VALLEY SYSTEM IN THE FERRON NOTOM DELTA, SOUTHERN UTAH

Outcrop exposures of a compound incised valley system in the Turonian Ferron Notom Delta in Southern Utah allow a detailed bed-scale facies architectural study within a recently developed high-resolution sequence stratigraphic framework. This allows linking the internal facies variability of the valley fill to relative sea level change, as well as evaluating the diachroneity of the basal composite erosional discontinuity.

In a recent regional sequence stratigraphic study of the Notom Delta, the incised valley system is correlated for 30km in both depositional dip and strike directions. The erosional base of the incised valley is interpreted as a regional composite sequence boundary (SB1). The oldest incised valley 1 is filled completely with fine-grained tidal dominated estuarine deposits, which suggests that deposition occurred close to the ancient shoreline, possibly during a subsequent fast transgression. Younger incised valley 2 is dominantly filled with medium-grained fluvial deposits with minor tidal influenced deposits (channel 4, about 1m thick) at the upper part of the valley. It is inferred that the amalgamated coarser-grained fluvial deposits which filled the lower part of incised valley 2 were deposited further landward relative to the shoreline than the tidal deposits in incised valley 1. The youngest incised valley 3 is filled with purely fluvial deposits without distinctive marine signature. The lateral and upward amalgamated stacking pattern of the channel storeys in incised valley 3, however, is interpreted to be the result of fluvial aggradation in response to a relative sea level rise, even though the valley was never flooded before complete filling. The composite valley fill thus shows a complex cutting and filling history. Older terraces are more marine-influenced, whereas younger valley fills record increasingly proximal fluvial facies, which correlates with a longer-term, stepped relative sea level fall, punctuated by decreasingly vigorous transgressions. The major composite erosional discontinuity underlying the incised valley system was consequently not formed instantaneously but is highly dischronous.