STABLE ISOTOPE PALEOHYDROLOGY OF THE LOWER CRETACEOUS (APTIAN) POISON STRIP SANDSTONE MEMBER OF THE CEDAR MOUNTAIN FORMATION, EASTERN UTAH

The Poison Strip Sandstone Member is a unique unit in the early Cretaceous Cedar Mountain Formation of eastern Utah. A previous chemostratigraphic study of the Poison Strip Sandstone at the Ruby Ranch Road (RRR) section east of Green River, Utah produced a noteworthy dataset with interesting δ¹⁸O results (Ludvigson et al., 2015, Cret. Res. 56:1-24). The Poison Strip at the type section of the Ruby Ranch Member contains poikilotopic calcite cements that yield δ¹⁸O values that range between -16 and -13.5‰ VPDB (ibid.). These isotopic values represent a major departure from the typical δ¹⁸O values of about -8‰ VPDB documented during earlier studies of the Cedar Mountain Formation and have major paleoclimatic or later diagenetic implications for the unit. To determine the most likely explanation for these extremely depleted δ¹⁸O values, this study performed detailed petrographic, diagenetic, and stable isotopic analysis of hand samples collected from the Poison Strip Sandstone from both outcrop and core. Optical cold-cathode cathodoluminescence (CL) imaging was utilized to allow for distinction of cement types and growth zones, as well a deciphering the sequential stages of diagenesis. Imaging was also used to develop maps to guide carbonate microsampling for stable isotope analyses. Interpretations of the patterns in C & O isotope space of the δ values produced from calcite cements in the Poison Strip Sandstone hinge on the recognition of distinctive diagenetic trends that are characteristic of individual carbonate components. A plot of the resulting δ¹⁸O and δ¹³C values characteristic of a later-stage brightly luminescent cement zone are arrayed in the horizontal pattern of highly variable δ¹⁸O values characteristic of burial diagenesis (Hasiuk et al. 2016, JSR:86:1163-1178). Burial cements typically display widely-ranging ¹⁸O in comparison to older authigenic cement generations. These changes reflect increasing burial temperatures. Results from this project are significant in that they eliminate the possibility that these anomalous δ¹⁸O values resulted from a major terrestrial paleoclimatic event in the Western Interior Basin, and shed light on basinal fluid migration related to an important natural gas reservoir in the Unita Basin.