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Geochemical Drivers in Carbonate Shoreline Settings: Examples From Permian of West Texas and New Mexico

Abstract

Carbonate shoreline and tidal flat deposits often display sedimentary features associated with early cementation such as beach rock, tepee structures, and pisoids. These features serve as shoreline indicators in the rock record and can be used to track changes in accommodation. However, changes in the character of shoreline-associated cements and fabrics may also reflect global or regional changes in water chemistry that affect carbonate saturation state. Along with the abundance of precipitates, the type and chemistry of marine cements may provide a geochemical record of these changes if the original signal is sufficiently preserved. Shoreline and tidal flat deposits offer an opportunity to understand how oceanographic conditions combine with other drivers such as accommodation to influence facies architecture. This study examines Guadalupian (Permian) shelf crest tepee/pisolite shorelines in the Delaware Basin. Previous work in the Guadalupe Mountains provides a sequence stratigraphic framework for the analysis of trace element, isotope, and field data. Measured sections collected in this project focus on the critically understudied Seven Rivers Formation, which is equivalent to the Lower Capitan. Shelf crest facies in the lower Seven Rivers differ from their counterparts in the younger Yates and Tansill Formations in that they have 1) smaller, more widely spaced tepee structures, 2) clear cycle stacking patterns that are rarely amalgamated, and 3) a relative scarcity of cm-scale botryoidal marine cements. We investigate the evolution of shelf crest facies from early Seven Rivers to late Yates/Tansill time by testing two different drivers. Analyses of cycle stacking patterns and progradation to aggradation (P/A) ratios offer insight into the role of accommodation change and shoreline stability (represented by P/A ratios) for tepee growth. Comparison of this data with similar studies in Yates/Tansill strata suggests that while some changes in shelf crest stacking patterns coincide with changes in accommodation, other co-drivers must be considered. Isotope and trace element geochemistry are explored as a possible record of changing basin water chemistry throughout the late Guadalupian. We predict that the trend toward more amalgamated shelf crest cycles and larger tepee structures in younger strata is coincident with higher δ18O values associated with the closing of the Delaware Basin and/or an increase in aridification towards the end of the Guadalupian.