55-11 Potential Late Cretaceous displacement on the Little Grand Fault during Mancos Shale deposition: Evidence from Hatch Mesa, Grand County, Utah

Session: Sequence Stratigraphic, Geochemical, and Geochronologic Correlation of the Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) in the Cretaceous Western Interior Seaway (KWIS) and the Gulf Coast

Presenting Author:

Authors:

Abstract:

            Structural field mapping in Grand County, Utah was conducted to determine possible structural controls on the accumulation of sandy sediment gravity flow deposits preserved in the upper parts of the Upper Cretaceous Mancos Shale.  These deposits, which outcrop south of Hatch Mesa, have been interpreted as lobate turbidity current deposits. Paleocurrent indicators taken from sole marks/ripple cross lamination indicate an east-northeast direction of sediment transport, an orientation similar to the trend of the Little Grand Fault, a south-dipping normal fault that cuts the Mancos Shale to the north of the Hatch Mesa sediment gravity flow outcrops. Detailed mapping of the Little Grand Fault in the vicinity of Hatch Mesa was conducted to identify any sediment gravity flows correlative to the hanging wall deposits north of the fault.  In the process, the Little Grand Fault Zone was mapped in detail for a distance of 5 km to the west-northwest of Hatch Mesa.  Mapping revealed that deformation associated with the Little Grand Fault occupies an ~100-200 m wide zone that contains both antithetic and synthetic normal faults with dips at angles ranging from 60°- 80°.  Observed faults display abundant slickensided calcite and selenite veins that indicate dip slip.  Strike and dip data collected from the Mancos Shale in both the hanging wall and footwall allowed construction of cross sections that indicate that the stratigraphic throw on the fault decreases from ~100 m in the west to ~50 m in the vicinity of Hatch Mesa.  During our mapping, we could not, with confidence, identify footwall sediment-gravity flow deposits with bedding/thickness characteristics similar to those in the hanging wall.   In addition, the stratigraphic thickness between marker beds in the Mancos Shale display slight thickness increases in the hanging wall relative to the footwall. These observations suggest that the Little Grand Fault may have been active during deposition of the Mancos Shale during Late Cretaceous time.  Displacement on the fault may have influenced sea floor topography so as to influence both the flow direction and accumulation of sediment during Western Interior Seaway turbidity currents sourced from deltaic environments to the west.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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