FACIES, DEPOSITIONAL ENVIRONMENTS AND SEA-LEVEL TRENDS WITHIN THE UPPER ORDOVICIAN STONY MOUNTAIN FORMATION, WESTERN NORTH DAKOTA

The Upper Ordovician Stony Mountain Formation in subsurface North Dakota was deposited in the epicratonic Williston Basin during transitional climatic conditions preceding the end-Ordovician Gondwana glaciation. The formation is subdivided into the Stoughton and Gunton Members in ascending order. This study, based on continuous core data, coupled with wireline-logs, thin-section analysis and structure/isopach maps, evaluates how the stacking of depositional facies relate to the latest Ordovician glacio-eustasy and transitional climates.

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). Skeletal grains are abundant in the Stoughton Mbr. and include crinoids, bryozoans, brachiopods, trilobites and corals; bioclasts are rare in the overlying Gunton Mbr, and occur only in its lower part. The facies are stacked into several-meters-thick cycles (=parasequences) (3-12 m), most of which are subtidal, asymmetric, and shallowing upward.

A change from more humid (Stoughton) to semi-arid (Gunton) conditions is suggested based on an upward trend from argillaceous limestone to finely crystalline (early?) dolomite and anhydrite, and capped by a thin arenaceous dolomudstone. Contrary to the thicker subtidal parasequences of the Stoughton Mbr, the peritidal parasequences in the upper Gunton Mbr. lack any deeper subtidal facies. They also lack any evidence for prolonged subaerial exposure and are likely indicative of small amplitude, high-frequency sea-level oscillations that coupled with semi-arid climatic conditions favored early diagenetic dolomitization.