Three-Dimensional Reconstruction of an Evolving Fluvial System: Chinle Formation, Northeastern Arizona

Marshall W. Deacon, Larry T. Middleton

Exceptional three-dimensional (3-D) exposures of the Triassic Chinle Formation in northeastern Arizona allow detailed architectural analyses of a complex fluvial system. Lateral profiling, element analysis, and hierarchy of bounding surfaces demonstrate deposition in both low- and high-sinuosity channel systems.

Two conglomerate bodies, the Shinarump and the Sonsela, were deposited in low-sinuosity, valley-confined systems as indicated by internal sequences of sedimentary structures and geometry of the macroforms. Stacked channel sequences consist of in-channel sandy bed forms separated from large, in-channel, downstream-accreting sandy macroforms (up to 5 m thick by 50 m long) bounded by fourth-order surfaces. Within localized channels, conglomerate bar complexes (up to 10 m thick by 150 m long) record downstream and oblique accreting macroforms bound both laterally and vertically by in-channel elements.

Sandstones between the basal Shinarump and the Sonsela contain numerous downstream and oblique accreting macroforms up to 5 m thick. Each element is the product of episodic and rapidly fluctuating flow conditions. These features are dominated by low-angle, third-order surfaces separating discrete depositional events. Most deposition occurred rapidly during short-lived events. Sand bodies above the Sonsela likewise consist of laterally accreting macroforms but exhibit an array of bounding surfaces and internal facies associations that document longer periods of more sustained flow in higher sinuosity systems. Plan-view exposures indicate numerous sinuous, mutually truncating channels with well-developed ridge and swale topography.

Architectural features in the Chinle reflect intrabasinal as well as extrabasinal controls on sedimentation. Major changes in stream sinuosity are largely controlled by gradient changes associated with increased subsidence rates with time.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.