Stratal Architecture of the Middle Part of the Morrison Formation, Utah: Implications for Downstream Base-Level Fluctuations

Ali-Adeeb, Riyad A.¹, Timothy M. Demko¹ (1) University of Minnesota Duluth, Duluth, MN

Changes in sand to mud ratios in the fluvial deposits of the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation in south-central Utah have been attributed to the evolution in sedimentation from a sandy, amalgamated, low-sinuosity, channel system to a muddy, non-amalgamated, moderate- to high-sinuosity channel system. This change in depositional style is interpreted to be a response to downstream base-level fluctuations whose origins are not well understood. Sequence stratigraphic interpretations are implemented to establish a high-resolution chronostratigraphic framework for the Mid-Morrison strata in order to better correlate changes in lithology to

changing downstream base levels.

A sequence-bounding regional unconformity separates the two members of the Morrison Formation and is characterized by overbank deposits and well-developed paleosols that suggest extended subaerial exposure, pedogenesis, and/or possible erosion in between deposition of the two units. Along specific localities are unique fluvial channel-fill conglomerates that have incised into uppermost Salt Wash deposits and basal Brushy Basin deposits. These lithologically and morphologically distinct units are characterized by a small, discontinuous and narrow lateral extent, and by red and green chert pebble clasts. These units are interpreted to have been deposited in small, incised paleovalleys after deposition of the lower Salt Wash Member and prior to or concomitant with the early stages of deposition of the Brushy Basin Member. Their unique lithology, stratigraphic position, and limited lateral continuity suggest a previously overlooked lowstand depositional cycle in between deposition of the Salt Wash and Brushy Basin Members.