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Sequence Stratigraphy of Tide-Influenced Deltas of the Torrey Member of the Early Triassic Moenkopi Formation, Capitol Reef National Park, Utah

Hall, Bethiah 1; Kamola, Diane 1
1 Department of Geology, University of Kansas, Lawrence, KS.

The Torrey Member of the Early Triassic Moenkopi Formation is identified as a mixed tide and fluvial deltaic deposit. Recent field studies examined sedimentologic and stratigraphic characteristics of the Torrey Member to define facies and depositional environments and place them in a stratigraphic context. The Torrey Member is interpreted as deposits of stacked interdistributary channel and delta front environments. The delta front facies consist of upward-coarsening siltstones with very-fine wave and current rippled sandstones. Fine-lower sandstones comprise the interdistributary channel fills which truncate delta front deposits. The channels have scour bases and the fill contains low-angle cross stratification, planar stratification and climbing ripples. The system was very muddy and sediment source was very fine grained with a maximum grain size of fine-lower sand in channel fills. Tidal evidence is found throughout every facies and includes lenticular, wavy, and flaser bedding, mica-rich mud drapes, reverse flow indicators, inclined heterolithic strata, and compound complex stratification.

Progradational delta front deposits occur within 11 parasequences that range in thickness up to a maximum of 16 meters. All occur within highstand systems tracts. These 11 parasequences are separated by 3 sequence boundaries, and grouped into 4 depositional sequences. Sequence 1, the lowermost sequence, is stratigraphically the simplest depositional sequence, and contains 4 progradationally stacked parasequences, all of which are comprised of delta front and channel successions. The remaining depositional sequences are more complex and contain incised valley fills. Sequence 2 contains parasequences 5 and 6, parts of which are locally eroded and replaced by incised valley fill. Sequence 3 consists of an incised valley fill, overlain by four parasequences (#7-10) with delta front and interdistributary channel strata. Sequence 4 also consists of an incised valley fill overlain by the final parasequence (#11) at the top of the member. The three incised valley fills have an exposed maximum fill thickness of 20 meters. Incised valley fills include nested channels with inclined heterolithic strata. Intraformational conglomerates, consisting of clasts eroded from younger delta front strata, are found along the basal surfaces of the valley fill deposits. The incised valley fills are interpreted as late lowstand and transgressive systems tracts events.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009