22-8 Evaporite Sedimentology of the First Core of the Permo- Triassic Spearfish Formation, North Dakota: Evidence for Lacustrine Deposition

Abstract:

The recently recovered EERC Halite #1 core from McKenzie County, North Dakota represents the first core through the Spearfish Formation, contains the first halite samples from this formation, and provides a unique opportunity for a comprehensive sedimentological investigation. The depositional environment remains unclear, as previous studies have relied largely on well logs and incomplete outcrop exposures that preserve only portions of the formation. Here, we use a combination of detailed core description and thin-section petrography to describe the sedimentary features that provide new insights on the depositional setting of the Spearfish Formation.

The Spearfish Formation is comprised mainly of evaporites and red beds, which appear lithologically similar to the underlying Permian Opeche Shale. Stratigraphically between the Opeche Shale and the Spearfish Formation is the Permian Minnekahta Limestone. Both the Opeche Shale and Minnekahta Limestone have been interpreted as saline lake deposits. The lithologies of the Spearfish Formation in the core, listed from most abundant to least abundant, are red mudstone, bedded halite, displacive halite, bedded gypsum, red sandstone, and pale gray mudstone. The red mudstones contain discontinuous lamina, mudcracks, and dewatering structures. Bedded halite is composed of chevron and cornet crystals, cumulate crystals, and some thin microcrystalline salt efflorescent crusts. Some bedded halite appears to have recrystallized crystals. The displacive halite lithology consists of randomly oriented, clear halite crystals in a matrix of red-brown mudstone. Bedded gypsum units are composed of halite pseudomorphs after bottom-growth, swallow-tail gypsum crystals. Thin lamina of red quartz arenite has bimodal grain size distribution and low-angle cross-bedding. Rare lamina of gray mudstone is massive. We interpret the bedded halite and bedded gypsum to have formed in ephemeral salt lakes, the displacive halite to have formed in saline mudflats, and the siliciclastics to have formed in associated environments, including saline and dry mudflats and eolian sandflats. This new data about the Spearfish Formation adds to our growing knowledge of Pangean environments.

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