Eustatic Controls on Stratigraphy, Chemostratigraphy and Water Mass Evolution Preserved in an Upper Wolfcamp Mudrock Succession, Delaware Basin, West Texas, USA

Abstract

Upper Wolfcamp Series (Wolfcampian, Lower Permian) mudrock successions in Delaware Basin vary between silicate- and carbonate (dominated by Fe-dolomite)-rich facies, largely marking depositional responses to sea-level lowstands and highstands, respectively. In a 310-ft (94.5 m) core from Energy Resources No. 1 East Vermejo well (Ward Co. TX), Upper Wolfcamp cycles comprise siliceous-calcareous mudrock couplets. Cyclicity was interpreted from mineralogical calculations using energy-dispersive x-ray fluorescence (XRF) measurements of elemental content. Elemental abundances (Ca, Mg, Si, K) enabled predicting mineralogy based on stoichiometric relationships between elements and dominant minerals (dolomite, calcite, quartz), and from average values of Si and K in published analyses of illite. Core sample spacing for XRF measurements was 2 in (5.1 cm). Six lithofacies were defined on the basis of carbonate/illite/quartz values. Redox-indicative trace metal (Mo, Ni, V, Cr, U) distributions document likely variations in basin circulation of marine water. Measurements and mineralogical interpretations provide: 1) information regarding stratigraphic organization of lithofacies, 2) a stratigraphic framework within which to interpret sea-level variations, 3) insights into ocean-chemistry variations attendant with deposition and organic carbon distribution, and 3) sampling for petrographic and organic matter analyses (TOC, δ13CTOC, δ15N) that will address marine chemical processes responsible for organic carbon preservation. Redox-sensitive element concentrations are higher in siliciclastic-dominated intervals and show close relationships with clay-mineral content. These data indicate that anoxia and increased organic carbon preservation were greatest in clay-rich sediments that accumulated during sea-level lowstands when normal-marine water flowing east from the Panthalassa Ocean was inhibited through narrow inter-basin channels or over sills.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015