STRATAL ARCHITECTURE OF THE MIDDLE PART OF THE MORRISON FORMATION, UTAH: IMPLICATIONS FOR DOWNSTREAM BASE-LEVEL FLUCTUATIONS

Changes in lithology and stratal architectures between 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-sinousity channel system. It has been suggested that this change was a reflection of downstream base-level fluctuations that may or may not have been related to tectonism within the Morrison Basin. Magnetostratigraphic and sequence stratigraphic analyses have been implemented to establish a high-resolution chronostratigraphic framework for these units in order to tightly correlate the strata and to better interpret how these fluvial systems evolved.

A sequence-bounding regional unconformity separates the aggradational and degradational fluvial and alluvial complexes of the two members. This surface is characterized by truncation of Salt Wash deposits by Brushy Basin channel sandstones in places, and by overbank deposits and well-developed unconformity paleosols in interfluve areas that indicate extended subaerial exposure, pedogenesis, and/or erosion in between deposition of the two units. Along this unconformity is a specific fluvial channel-fill conglomerate that is incised into basal Brushy Basin and uppermost Salt Wash deposits. This lithologically and morphologically distinct unit is characterized by red and green chert pebbles, and a narrow lateral extent and discontinuous nature. This unit was deposited in a small, incised paleovalley in between deposition of the Salt Wash and Brushy Basin Members or during earliest Brushy Basin times. Its unique lithology, stratigraphic position, and limited lateral continuity may suggest a lowstand depositional cycle that has previously been overlooked in between deposition of the Salt Wash and Brushy Basin Members.

Preliminary magnetostratigraphic analyses of the proximal facies of the uppermost Salt Wash and basal Brushy Basin deposits allow a tight correlation of stratal units within the proximal facies, and also to the distal facies in western Colorado and northwestern New Mexico. This will enhance our understanding of how these fluvial systems evolved and provide unique insights into the conditions responsible for their evolution.