PETROLOGIC AND KINEMATIC ANALYSIS OF DEFORMATION BANDS IN THE LATE CRETACEOUS SIXMILE CANYON FORMATION, CENTRAL UTAH

Deformation bands are the most common form of strain localization in porous sandstones and sediments (Fossen et al. 2007). In central Utah, several Cretaceous units exhibit pronounced deformation bands. These porous rocks contain diagenetic paleoflow fronts that have been influenced by the presence of deformation bands. The study area in central Utah is located in the Basin and Range/Colorado Plateau Transition Zone, where Mesozoic and Cenozoic strata in this region have been affected by multiple stages of deformation. Late Jurassic-Early Paleogene structures of the Sevier fold-thrust belt have been overprinted by both pre- and regular Basin and Range extension.

This study focuses on the Late Cretaceous (Campanian) Sixmile Canyon Formation (Indianola Group), located on the Wasatch Plateau. The Sixmile Canyon Formation is dominated by grey, medium- to coarse-grained sandstones and pebbly sandstones, interbedded with mudstones and rare coal beds. The unit is inferred to be marine/marginal marine at its base, grading into terrestrial alluvial plain (Weiss 1994).

Preliminary petrographic analysis of the Sixmile Canyon Formation reveals three types of kinematic deformation bands: compaction, shear, and dilation. Compaction bands contain a cataclastic core. In the core, there is grain microfracturing and both grain-size and pore space reduction when compared to the host rock. Shear bands show both right-lateral and left-lateral shear, with displacements up to 10 mm. Dilation bands show an increased matrix/grain ratio responsible for an increase in porosity across the band. These kinematic classifications of deformation bands are further enhanced by Schmidt Hammer measurements taken in the field. This data (R-value) shows noteworthy strength differences between the host rock and the band core for various band types. Field observations and measurements included band morphology, trace length, width, and attitude. Stereonet analysis reveals distinct sets of deformation bands in the unit. The petrographic data, porosity estimates, and stereonet analysis together help determine the deformation mechanisms and tectonic stress regimes (compressional and tensional) responsible for formation of deformation bands in the Cretaceous Sixmile Canyon Formation.