Chemostratigraphy of the Woodford Shale, McAlister Cemetery Quarry, Ardmore Basin, Oklahoma

Abstract

Chemostratigraphic analysis is capable of objectively measuring subtle, yet potentially significant, variations within mudrock successions. An approximately 325ft thick exposure of the Woodford Shale is present in the McAlister Quarry in the Ardmore Basin, OK. Samples were collected every stratigraphic foot and analyzed for 23 elements using hand-held XRF (HHXRF). At the same scale, a gamma ray (GR) profile was obtained from a GR scintillometer to correlate this outcrop to subsurface well logs. These data were plotted as chemostratigraphic profiles and placed within a sequence stratigraphic framework in order to evaluate local trends in the depositional environment through geologic time. Detrital sedimentation is inferred from titanium (Ti), zirconium (Zr), aluminum (Al), and potassium (K) with Al and K representing the clay component. These proxies show a general decreasing trend throughout the lower 275ft of the Woodford at this location. This indicates retrogradation as a result of long scale transgression in this portion of the Ardmore Basin. Above 275ft, the detrital component concentrations gradually begin to increase, suggesting progradation and regression of stratigraphic base-level. The amount of quartz, both biogenic and detrital, is derived from the Si/Al ratio. The Si/Al profile shows sudden spikes in concentration at 75ft, 155ft, and a broad zone of elevated levels from 260-280ft. These elevated successions are associated with the appearance of radiolarian rich “white beds”. The zone at 260-280ft corresponds to the transition from transgression to regression in the detrital proxies. This zone is interpreted as the condensed section (CS) within a sequence stratigraphic context. Bottom water circulation is interpreted from the concentration of vanadium (V) and molybdenum (Mo). These elements will precipitate in anoxic and euxinic conditions, respectively. These elements show the highest degree of basin restriction in the lower portions of the Woodford Shale, during the onset of transgression, with increasingly well circulated bottom waters occurring upsection. This observation is consistent with transgression. Results from chemostratigraphic analysis reflect an environment with changes in sediment inputs, base-level, and bottom-water circulation. The lower 275ft of the section is best characterized as a part of the transgressive systems tract (TST) while the uppermost 50ft is interpreted as the highstand systems tract (HST).

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016