Abstract

Detailed lithosome analysis of the continental Upper Triassic Chinle Formation in southern Utah and northern Arizona documents a complex origin by alternating fluvial and lacustrine depositional systems. Chinle sedimentation occurred in an enclosed continental basin on the cratonic edge adjacent to an evolving back-arc basin. Six depositional phases are responsible for three fining-upwards cycles. We define phase as a characteristic depositional system that was dominant during a non-quantitative period of time. 1) The lower cycle comprises the lower fluvial phase and lower paludal phase; 2) the middle cycle comprises the middle fluvial phase, the middle paludal phase; and the upper fluvial-lacustrine phase; 3) the upper cycle comprises the upper fluvial phase.

The lower fluvial phase which consists of the Shinarump Member was deposited by braided streams in confined paleovalleys and on open alluvial plains. The lower paludal phase comprises the Monitor Butte Member and was deposited in a paludal complex with braided and meandering streams, lakes, and swamps under the influence of large influxes of volcanic ash. The middle fluvial phase is represented by the type Moss Back Member and was deposited by braided streams in a broad open paleovalley that trended northwest across southeastern Utah. The middle paludal phase includes the Petrified Forest Member and the lower part of the Kane Springs strata of the Moss Back Member. Conditions similar to those of the Monitor Butte Member prevailed over most of the area. Near Moab, Utah tectonics associated with the unstable Salt Anticline region resulted in complex fluvial deposition. The upper fluvial-lacustrine phase which consists of the Owl Rock Member and upper portion of the Kane Springs strata was deposited by widespread cyclic lacustrine systems. Streams in the Salt Anticline region built fan-deltas into the Owl Rock lakes. The Church Rock Member was deposited during the upper fluvial phase when a variety of streams deposited sand and silt during a period of increasing aridity. Dunes of the overlying Wingate Sandstone encroached on the region from the northwest and eventually buried deposits of the Chinle Formation.