FACIES STACKING PATTERN WITHIN THE ORDOVICIAN RED RIVER FORMATION, WILLISTON BASIN: A CASE STUDY FROM EASTERN NORTH DAKOTA
The Red River Formation in the study area is characterized by burrow-mottled texture, with Thalassinoides-like burrow fills within a finely crystalline matrix. Major carbonate lithofacies include: (1) skeletal dolomudstone and dolowackestone (deeper subtidal); (2) skeletal dolowackepackstone (shallow subtidal); (3) crinoid dolopackstone (high-energy shoal); and (4) thrombolite (upper subtidal-lower intertidal). Major skeletal components include disarticulated crinoid stems and fragments of brachiopods and trilobites. The studied succession exhibits an overall shallowing-upward trend, with predominantly deeper subtidal facies in the lower part, and shallow subtidal to lower intertidal in the upper part. The lithofacies are characteristically stacked into meter-scale cycles (parasequences). Muddy parasequences (dolomudstone-dolowackestone) are 1-20 m thick and make up the lower part of the formation. Grainy parasequences (dolowackepackstone and dolopackstone; thickness 0.3-5 m) predominate in the upper part of the sequence where they commonly are capped by thrombolite.
Dolomite crystal size varies between 6 μm and 826 μm (mean 55 μm). Two types of dolomite can be identified based on crystal size and shape: (1) coarse-grained euhedral (planar-e) dolomite with well-developed crystal faces and sharp boundaries, and (2) fine-to-medium-grained anhedral and non-planar dolomite with irregular crystalline boundaries. Coarse euhedral dolomite fills burrows; intercrystalline pores are well connected and porosity is high. Anhedral and non-planar dolomite is more abundant and it makes up the matrix, where tightly interlocking crystals result in very low to no porosity.