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Late Ordovician-Early Silurian Sequence Framework and C-Isotope Stratigraphy of the Williston Basin


Although the Williston Basin has been a major hydrocarbon exploration region since the 1950s, the precise age and sequence stratigraphic framework of the Upper Ordovician-Lower Silurian succession has remained uncertain. This study aims at establishing a detailed sequence stratigraphy for that time interval, and then use C-isotope stratigraphy correlate to it regionally. The focus is on the Red River, Stony Mountain, Stonewall, and lower Interlake Formations in the subsurface of North Dakota, based on continuous core data, coupled with wireline-logs, thin section analysis, and stable C-isotope data from the bulk carbonate matrix; samples for stable isotopes were collected at 90 cm intervals. The facies identified include, from deepest to shallowest: argillaceous skeletal mudstone and wackestone with cm-thick skeletal packstone-grainstone interbeds (deep subtidal around storm-weather wave base), argillaceous skeletal wacke-packstone (rare; deep subtidal, sub fair-weather wave base), burrowed skeletal dolomudstone (shallow euphotic subtidal below fair-weather wave base), barren dolomudstone (restricted euhaline to mesohaline shallow subtidal), flat-pebble breccia (rare; lag deposit); (dolo)thrombolite (shallow mesohaline subtidal), laminated dolomite (shallow mesohaline subtidal to intertidal), and nodular and laminated anhydrite (shallow penesaline subaqueous setting, sabkha). Facies are characteristically stacked into m-scale cycles (parasequences) that are associated with open-marine to mesohaline and penesaline settings; these include mud- and grain-dominated cycles, and laminated dolomite-, breccia-, and anhydrite-capped cycles. Based on the characteristic stacking pattern of cycles, the studied Upper Ordovician-Lower Silurian succession of the Williston Basin comprises 11 transgressive-regressive depositional sequences, majority of which are bounded by anhydrites. The Williston Basin C-isotope stratigraphy enables regional (and intercontinental) correlation, with prominent excursions in the Red River (Whitewater/Moe excursion) and Stony Mountain Formations (Elkhorn/Paroveja excursion). The identification of the Hirnantian (HICE) and lower Silurian excursions is pending analysis of conodont biostratigraphy.