MIALL, ANDREW D.
Department of Geology, University of Toronto, Toronto, Ontario

Abstract: Cryptic Sequence Boundaries in Braided Fluvial Successions

Where nonmarine sequence boundaries juxtapose contrasting facies the sequence boundaries are not difficult to recognize. However, in proximal successions composed of coarse, braided-fluvial deposits it may be difficult to distinguish a sequence boundary from any other channel-scour surface. Examples of three such cryptic sequence boundaries are described.

Near Moab, Utah, in the Paradox Basin, active salt anticlines disrupted the Triassic Chinle alluvial plain spreading westward from the Uncompahgre Uplift. Syndepositional angular unconformities are locally exposed and may be traced laterally to areas lacking structural disturbance, where they appear to be conformable channel scours.

The Jurassic Kayenta Formation east of Moab was also disrupted by salt tectonics, which led to major shifts in fluvial dispersal patterns and to exposure and early cementation of abandoned channel and bar deposits. The sequence boundary occurs at a major change in paleocurrent patterns and is characterized by distinctive early cementation textures. Steep cut-and-fill relief on this surface indicates that it was unusually resistant to channel scour.

The Lower Castlegate Sandstone (Upper Cretaceous) of the Book Cliffs, north of Moab, has been shown by regional mapping to consist of at least two superimposed successions. Missing at the sequence boundary west of Green River is the marine Buck Tongue, a shale unit that was presumably removed by uplift and erosion during Castlegate sedimentation. The sequence boundary may be mapped updip into a continuous braided-fluvial sandstone succession only by plotting the position of subtle changes in detrital petrographic composition that reflect slow changes in provenance as the Sevier highlands to the west were being uplifted and unroofed.

It is suspected that many fluvial successions consisting of tens to hundreds of metres of unvarying facies may contain similar cryptic sequence boundaries that indicate substantial periods of "missing" time and unsuspected episodes of tectonism or base-level change.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas