STRUCTURAL COMPARISON OF THE DEFORMATION STYLES IN THE ARAPIEN FORMATION (TWELVEMILE CANYON MEMBER: HARDY UNITS B AND C), NINEMILE RESERVOIR, CENTRAL UTAH

The Middle Jurassic Arapien Formation was deposited in a restricted epeiric sea and later subjected to Sevier east-west contractional deformation. The unit is characterized by mechanically weak lithologies, and in places, by contorted bedding and tectonic thickening. In central Utah, the Arapien is the core of the Sanpete-Sevier Valley Anticline (SSVA), a prominent fold in the frontal triangle zone of active, oil-producing areas. Historically, deformation in the Arapien has been interpreted to be either from tectonism or salt diapirism. The objectives of this study are to compare the deformation in two units of the Arapien (B, C) and to assign deformation mechanisms to the exposures at Ninemile Reservoir, where the Arapien exposes the western overturned limb of the SSVA.

Older Unit B is composed of red and gray mudstone and shale with interbedded gypsum. Data collected includes a 75 m stratigraphic column and related petrographic samples. In addition, bedding attitudes were taken at 65 sites, which were mapped with DGPS and later with ArcMap. Results show that Unit B’s bedding attitudes abruptly change orientation, often exhibiting small-scale folding and contortion in the larger SSVA limb.

Younger Unit C is dominated by gray to beige calcareous mudstone and thinly-bedded shale, with minor siltstone to sandstone fins that exhibit ripples and flaser/lenticular bedding. A 50 m stratigraphic column of the basal part of the unit, petrographic samples, and a gamma-ray profile (K/U/Th) permit correlation to subsurface well logs in the area. Bedding attitudes were taken, and results show a more consistent deformation response with less small-scale folding in the SSVA limb. Several systematic joint sets were measured, and these correlate to known paleostress fields in the region.

Drone imagery details the outcrop pattern of Units B and C along Ninemile Reservoir. This research focuses on comparing the deformation between the two units. The mechanically weak, bedded gypsum in Unit B is distinct from Unit C’s cyclic interbedded mudstones and minor siltstones/sandstones. Both units comprise the western limb of the SSVA, which was a result of tectonism due to Sevier contraction. However, contorted strata in Unit B suggest that tectonism initiated diapiric activity in portions of the Arapien as it was folded into the SSVA.