BENTONITE ASH BED AS CHRONOSTRATIGRAPHIC DATUM FOR PERMIAN DELAWARE BASIN AND BEYOND

Abstract

Altered volcanic tephra, preserved as discrete bentonite clay beds within sedimentary depositional units, has previously been described in outcropping Guadalupian-aged deposits in the greater Permian Basin of western Texas and southeastern New Mexico. These bentonite beds have facilitated correlation of stratigraphy, linking depositional regimes across the Late Paleozoic strata in and around Guadalupe Mountains National Park. More recently, <1cm-thick weathered ash bentonite beds were identified and described in 2 conventional hydrocarbon industry cores taken from the Cisuralian-aged Third Bone Spring formation in the Delaware Basin of western Texas. Despite surface wellbore locations being ~36km apart, the lithostratigraphic correlation between the two cores was very straightforward when the ash was used as a datum. Geochemical analyses were conducted to further test the idea that the two bentonite ash beds were correlative. Despite several stages of diagenesis since seafloor deposition of the ash bed around 270 Ma, similar geochemical trends between elemental subsets, particularly the light and heavy rare earth elements, suggest that these two ashes came from the same volcanic arc system. Encouraged by these results, the area of investigation was expanded to include previously described cores in the Delaware Basin that covered the same stratigraphic unit. A bentonite ash of similar thickness and within the same stratigraphic interval was identified in 9 different cores, including multiple cores from wells located south of a prominent mid-basin fault-bounded ridge. This ubiquitous bentonite ash bed now facilitates correlation across a major topographic feature of the Delaware basin, improving the regional model of turbidite and hybrid event bed deposition in Cisuralian (Leonardian) time, as it provides a reliable, basin-wide chronostratigraphic datum. This discovery highlights an outstanding question of whether or not that regional ash marker exists in other cores within the Delaware Basin and perhaps beyond, e.g. in the Midland Basin, enabling a regional, interbasinal understanding of sediment deposition and associated controls like climate and tectonic regimes in the Late Mesozoic midcontinent.