The Permian Minnekahta Limestone: a Saline Lake Gypsum Replaced by Calcite

Abstract

The late Permian Minnekahta Limestone of the Black Hills of South Dakota is not a typical limestone. Sandwiched stratigraphically between continental redbed siliciclastics, it is distinct in its gray-pink appearance and carbonate mineralogy and is considered a local and regional marker bed. Based on its calcite composition and stromatolites, it has previously been interpreted as a tidal flat deposit, leading to speculation of a regional late Permian marine transgression. Here, we show observations that question the original composition, reexamine and refine the sedimentary and stratigraphic characteristics, and suggest an alternate depositional environment. Petrography and x-ray diffraction indicate that the Minnekahta Limestone is dominated by an interlocking crystal mosaic of calcite and gypsum. We have documented pseudomorphs after bottom-growth gypsum crystals, alternating laminae of mm-scale bottom growth gypsum crystal shapes and mud drapes, as well as mudcracks, raindrop imprints, and stromatolites. No carbonate grains, fossils, or tidal rhythmites were noted. These observations suggest that the carbonate phase is a replacement of gypsum. In addition to the sedimentology and petrography, close stratigraphic association with overlying and underlying continental redbeds indicates that this was likely a shallow saline-lake deposit. Other Permo-Triassic carbonate strata in continental redbed sequences throughout the midcontinent of North America, such as those in the Goose Egg, Chugwater, Nippewalla, and Spearfish strata, may have similar continental origin. This study serves as a lesson that paired petrographic observations and mineral identifications are important data necessary for the best interpretation of depositional environments and diagenetic histories.

Gypsum lakes, sandflats and soils revealed from the Triassic Red Peak Formation of the Chugwater Group, north‐central Wyoming

“…The Red Peak Formation is similar to other Permo‐Triassic red bed and evaporite deposits from Pangea in that all are composed of fine‐grained red bed siliciclastics and bedded evaporites that formed in continental saline settings. Carbonate minerals are rare in these rocks, and when present, are a replacement phase of gypsum (Benison et al., 2018). Signs of life are uncommon and include root traces and rare suspect insect burrows (Zambito et al., 2012).…”

“…The Red Peak Formation contains an abundance of gypsum and iron oxides and a paucity of carbonates, which together are characteristic of acid brine systems (Benison & Goldstein, 2002). Where there are isolated thin carbonate units in Permo‐Triassic red bed and evaporite successions, they have remnant gypsum and textural characteristics of neomorphism, suggesting that the carbonate phase is a diagenetic replacement phase after gypsum (Benison et al., 2018). For these reasons, the Red Peak Formation may have been an acid saline lake and groundwater system.…”

Alluvial Fans, Loess Plains, Lakes, and Distributive Fluvial Systems: Depositional Systems of the Permian-Triassic Red Beds and Evaporites of Wyoming (USA)

“…The replacement of gypsum by calcite can occur as a syndepositional process or later diagenesis ( Ulmer et al 1990;Anadón et al 1992;El Tabakh and Schreiber 1994;Sanz-Montero et al 2006;Benison et al 2018). Circumstances and characteristics of carbonate replacement of gypsum and anhydrite vary widely, with examples of isolated replacement of specific beds (Kirkland and Evans 1976;Pierre and Rouchy 1988;Anadón et al 1992), regional replacement of an entire unit (Bell 1989;Benison et al 2018), and localized alteration of specific features (Speed and Clayton 1975;Sanz-Montero et al 2006). Three mechanisms have been proposed for the replacement of gypsum or anhydride by carbonate: (1) the post-burial action of sulfur-reducing bacteria consuming migrating oil or another organic source in either a closed or open system, (2) syndepositional replacement of gypsum at the surface, mediated by sulfur-reducing bacteria, and…”

“…The Minnekahta being a saline lake deposit with evaporites replaced by carbonate (Benison et al 2018). The Triassic Lockatong Formation's limestone beds of the Newark Basin of New Jersey (El Tabakh and Schreiber 1994) are another example of thin limestone units associated with red beds.…”