TECTONIC AND CLIMATIC CONTROL ON CONTINENTAL PALEOVALLEY FILL ARCHITECTURE: EVIDENCE FROM THE UPPER TRIASSIC, LOWER CHINLE FORMATION, SOUTH CENTRAL UTAH, USA

Erosional unconformities and pedogenically modified horizons found within the Late Triassic Chinle Formation in central Utah indicate a depositional history involving alternation of landscape degradation and aggradation. Field and well log data suggest the lower portion of the Chinle (Shinarump, Monitor Butte, Temple Mountain, and Moss Back Members) can be divided into three periods of incision and subsequent valley fill. The initial period of degradation is marked by interfluve paleosols and truncation of the underlying Moenkopi Formation creating a paleovalley which constrains the deposition of these four members. The first paleovalley fill, represented by the Shinarump Member, is interpreted as a confined sandy low-sinuosity river system. A second period of incision is marked by truncation of the Shinarump and correlative paleosols and pedogenically modified strata. The Monitor Butte and correlative Temple Mountain Members overlie this unconformity and consist of mudstones and sandstones representing fluvio-lacustrine deposition and vertisols and interbedded mudstones and sandstones deposited in a high-sinuosity river system. A final cut and fill within the pre-Shinarump paleovalley is filled by the high- and low-sinuosity fluvial deposits of the Moss Back Member. Truncation of the Monitor Butte and Temple Mountain Members and the Moenkopi and interfluve pedogenesis mark the surface of degradation. The tectonic setting of the Chinle basin within a dynamically subsiding back-arc basin may provide a mechanism for uplift and erosion of Lower Triassic strata creating the master paleovalley surface. The cut-and-fill character of the valley fill are more likely related to climatic controls on sediment flux and discharge.