SEQUENCE STRATIGRAPHIC FRAMEWORK AND CARBON-ISOTOPE STRATIGRAPHY OF THE UPPER ORDOVICIAN LOWER RED RIVER FM., EASTERNMOST WILLISTON BASIN

The Red River Formation in North Dakota is the second most important conventional play of the Williston Basin, with total reserves exceeded only by the Mississippian Madison Formation Play. This study is focused on documenting the facies and high resolution sequence stratigraphy of the lower part of Red River Formation based on core data from eastern North Dakota, and evaluating the isotopic signature preserved in the carbonates. The Red River Formation in the subsurface of eastern North Dakota is characterized by burrow-mottled texture, with Thalassinoides-like burrow fills within a finely crystalline matrix. Major carbonate lithofacies include, from deepest to shallowest: skeletal dolomudstone-wackestone (deeper subtidal); skeletal dolowackepackstone (shallow subtidal); crinoid dolopackstone (high-energy shoal); and thrombolite (upper subtidal-lower intertidal). These facies are stacked into predominantly subtidal parasequences, typically of skeletal wackemudstone capped by skeletal wackepackstone. Lack of tidal flat facies caps to parasequences indicates incomplete shallowing to intertidal depths, and suggests that some of the parasequences are amalgamated. The Red River parasequences are arranged in six 50-140 ft thick depositional sequences bounded by grainy intevals (skelelal (dolo)wackepackstone to packstone); each sequence is composed of 3-12 parasequences. Sequences are subtle in the core, not delimited by significant bounding disconformities, and lack clear maximum flooding surfaces. The basal three sequences are dominated by parasequences consisting of skeletal wackemudstone capped by skeletal wackepackstone. The uppermost part of sequence 3, and the remaining sequences 4, 5 and 6 are predominated by grainy lithofacies, with parasequences consisting of skeletal wackepackstone grading up into skeletal packstone that is locally capped by thrombolite. The carbon-isotope values vary from -1.6 to +1.1‰ VPDB (average -0.14‰ VPDB), and show two significant positive isotope excursions that can be correlated regionally.