Sedimentology and Structure of Growth Faults at the Base of the Ferron Sandstone Member Along Muddy Creek, Utah

Abstract This paper describes normal growth faults at the base of the Ferron Sandstone exposed along the highly accessible walls of Muddy Creek Canyon in central Utah. Although there have been several studies of growth faults in outcrops this is the first that integrates detailed sedimentological measured sections with fault kinematics and section restorations. We measured 20 sedimentological sections and interpreted a photomosaic covering approximately 200 m (550 ft) lateral distance. The outcrop is oriented parallel to depositional dip and perpendicular to the general strike of the faults. Distinctive pre-growth, growth, and post-growth strata indicate a highly river-dominated crevasse delta, that prograded northwest into a large embayment of the Ferron shoreline. The growth section comprises medium- to large-scale cross stratified sandstones deposited as upstream and downstream accreting mouth bars in the proximal delta front. Deposition of mouth bar sands initiates faults. Because depositional loci rapidly shift, there is no systematic landward or bayward migration of fault patterns. During later evolution of the delta, foundering of fault blocks creates an uneven sea-floor topography that is smoothed over by the last stage of deltaic progradation. Faults occur within less than 10 m (30 ft) water depths in soft, wet sediment. Detailed examination of the fault zones shows that deformation was largely by soft-sediment mechanisms, such as grain rolling and by lubrication of liquefied muds, causing shale smears. Mechanical attenuation of thin beds occurs by displacement across multiple closely spaced small throw faults. Analogous river-dominated deltaic subsurface reservoirs may be compartmentalized by growth faults, even in shallow-water, intracratonic, or shelf-perched highstand deltas. Reservoir compartmentalization would occur where thicker homogenous growth sandstones are placed against the muddy pre-growth strata and where faults are shale-smeared, and thus potentially sealing.