SEQUENCE STRATIGRAPHY OF A FLUVIAL-LACUSTRINE SUCCESSION IN THE TRIASSIC LOWER CHINLE FORMATION, CENTRAL UTAH, USA

The Shinarump, Monitor Butte, and Moss Back Members of the Upper Triassic Chinle Formation, exposed near White Canyon, UT, represent an incised-valley fill succession in a continental depositional setting. Although incised valleys, and the sequence stratigraphic architecture of their sedimentary fill, have been documented in marginal-marine settings, valley-fill successions from continental settings, updip from any base-level influence of sea level, have not been evaluated from a sequence stratigraphic perspective. These three members of the Chinle Formation fill paleotopographic valleys cut into the underlying Lower Triassic Moenkopi Formation. The basal valley surface is a sequence-bounding unconformity. It is marked by truncation of underlying Moenkopi units and extensive pedogenic modification, especially in interfluve areas. The Shinarump Member fills the lowest portions (incised channels) of the valleys. It is a trough-crossbedded conglomeratic sandstone, interpreted to have been deposited as bars within amalgamated fluvial channels. The top of the Shinarump is an extensive lacustrine flooding surface, and therefore the fluvial succession between the basal sequence boundary and this surface is a lowstand systems tract. Greenish-gray mudstones and brown, rippled sandstones of the Monitor Butte Member overlie the lacustrine flooding surface. The Monitor Butte is interpreted as a highstand systems tract, deposited as a prograding lacustrine delta complex that was fed by a rapidly aggrading fluvial system. The top of the Monitor Butte is a sequence boundary, marked either by an erosional surface at the base of the discontinuous Moss Back Member sandstone where it is present, or a well-developed paleosol characterized by distinctive red coloration and carbonate nodules. The Moss Back Member is a trough crossbedded sandstone characterized by large-scale lateral accretion beds. It represents a smaller, inset incised valley cut into the underlying Monitor Butte lacustrine valley fill. The tripartite fluvial-lacustrine-fluvial succession in the lower Chinle valley fill is reminiscent of other documented lacustrine basin fills (rift basins, etc.). However, this valley fill setting is unique and may be a model for other ancient lacustrine deposits not associated with active tectonics.