Temporal Evolution of Fluvial Style within a Compound Incised Valley, Ferron Notom Delta, Henry Mountains Region, Utah
Nonmarine sequence stratigraphic models hypothesize systematic changes in fluvial architecture and style within individual sequences and across sequence boundaries. These models, however, are largely assumed rather than documented.
Major fluvial bodies are well exposed along depositional strike near the top of the Ferron Notom Delta in the Henry Mountains region, Utah. Field correlation and mapping show that these fluvial facies are contained within a wide compound incised valley. The valley is partitioned into two unconformity-bounded sequences, each of which comprises a multistorey fill that reaches a maxiumum thickness of about 30 m. The two major erosional surfaces extend laterally for several kilometers and shows erosional relief up to 10 m. Each erosion surface shows a marked basinward shift in faces.
Detailed bedding architectural analysis establishes the fluvial style within each valley fill. Within the younger valley, there is a systematic evolution of fluvial style from braided, to braided with meandering reaches, and finally to a low sinuosity river systems with channels occupied by dune fields. In contrast, formative rivers within the older valley are always meandering. A major change in valley sedimentology is recognized as shown by the changes in regional flow direction, over all grain size, channel geometry, and fluvial style. Paleocurrent data shows that there is a 30° eastward shift in main flow as the compound incised valley evolves from the older to the younger system. A clear change in fluvial style from meandering into braided streams across the basal erosional surface of the younger valley is documented. The river deposits also show a distinct increase in overall grain size and greater preserved dune height across the boundary. Paleohydraulics calculations suggest an increase in river dimension and discharge across the boundary.
Changes in fluvial style within the younger valley are interpreted as the result of loss of slope and/or discharge during the gradual filling of the valley. The change in fluvial style across the sequence boudary is interpreted, however, as the result of a possible high-frequency climate change driven by Milankovitch cycles, even though short-term and low-amplitude eustasy is also the driving force for the development of the incised valleys.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009