Sedimentologic and Stratigraphic Analysis of Units Defining the Basal Sauk Supersequence Across the Craton Margin Hinge Zone, Southeastern California

In the Death Valley and Mojave Desert regions of southeastern California, the contact separating the lower and middle members of the Wood Canyon Formation (WCF) is currently interpreted as a regional scale unconformity coincident with the base of the Sauk Sequence. Regional mapping of this surface, however, reveals a nonconformable contact with underlying crystalline basement in cratonic settings, and a relatively conformable contact atop a northwest thickening wedge of miogeoclinal strata that is capped by the lower member of the WCF. Consistent with an unconformity, the progressive loss of three carbonate units within the lower member of the WCF has been attributed to incision by the base of the middle member WCF. However, fossil evidence and correlation based on carbon isotope compositions of each lower member WCF dolostone units rejects top-down erosion to describe their loss and overall cratonward thinning. Results from multiple detailed, measured, stratigraphic sections of a conglomerate found at the base or low in the middle member WCF also do not support a top-down erosion model because the conglomerate has variable stratigraphic position and absence in some locations. Middle member WCF conglomerate clasts also reveal variation in composition and grain size across the regions. Sequence stratigraphic architecture indicates that filling of available accommodation space and short period normal regression, as opposed to a forced regression, are the causal mechanisms for formation of the basal middle member WCF unconformity, and that the base of the Sauk Sequence rests lower in the stratigraphic section.