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Perkins, Timothy E.1, Diane L. Kamola1 
(1) University of Kansas, Lawrence, KS

ABSTRACT: Incised Valleys within Tidally Influenced Deltaic Strata: Torrey Member of the Lower Triassic, Moenkopi Formation

Tidally influenced, deltaic deposits within the Torrey Member of the Lower Triassic Moenkopi Formation in southern Utah consist of inter-laminated sandstone and siltstone of varying ratios, often punctuated by thin to medium bedded sandstone. These sediments were deposited in inter-distributary mudflats and tidal flats. Some intervals display upward coarsening successions, typical of prograding deltas. However, in some sections thick (up to 8m), nested, channelized sandstones truncate these deltaic deposits, forming the base of sequences. 
The lowermost channels eroded underlying deltaic sediments and incorporated large rip-up clasts (up to 1m in size) of siltstone, heterolithic strata, and sandstone and deposited them just above sequence boundaries. A series of nested channels overlie these erosional surfaces, the result of a relative rise in sea level. Some channel sediments form sheet sands deposited by braided streams. These channel bases contain large rip-up clasts and are overlain by trough cross-beds and planar laminations. Upsection, the channel fills are dominated by alternating planar laminations and climbing ripples, indicating waning flow caused by discharge fluctuations in a high energy channel. Other channels have lateral accretion units with heterolithic strata, indicating deposition by meandering streams. In some channels inclined heterolithic strata have tidal indicators and include features like double mud drapes, reverse flow indicators, reactivation surfaces, wavy and flaser bedding, sigmoidal cross-bedding, and evidence for spring-neap cyclicity. These nested channels are interpreted to represent incised valley fills, deposited in fluvial to estuarine environments.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.