Contrasting Siliciclastic-Evaporite Strata In Subsurface and Outcrop: An Example From the Permian Nippewalla Group of Kansas, U.S.A

Abstract

Geologists tend to expect that rocks as they appear in outcrop are a very good approximation of the same strata in the subsurface. But this is not the case in mixed siliciclastic-evaporite units, strata that contain both a siliciclastic and an evaporite mineral component. Distinct differences in lithologic and stratigraphic descriptions of ancient siliciclastic-evaporite strata exist between outcrop and core. These differences present challenges in stratigraphic nomenclature, lithologic correlation, age determinations, and interpretations about past depositional environments, diagenetic history, and paleoclimate. An example of siliciclastic-evaporite strata is the middle Permian to early Triassic gypsum-and/or anhydrite-(hereby referred to as gypsum/anhydrite) and halite-bearing red beds throughout the midcontinent of North America and equivalent strata from other Pangean deposits. Here, we use core and field observations of the middle Permian Nippewalla Group in Kansas to show the range of similarities and differences in lithologies and thicknesses of specific lithofacies in outcrop and at various depths. For example, the displacive halite lithology, consisting of red mudstones dominated by randomly oriented halite crystals, is abundant in cores and interpreted as saline mudflat deposits. The outcrop counterpart is less common, friable, massive red mudstones with some gypsum/anhydrite pseudomorphs after displacive halite crystals. We interpret these lithologic differences of this saline mudflat lithofacies as the result of excellent preservation of buried displacive halite and late-stage dissolution of near-surface halite. Additionally, bedded halite and intergranular halite cements in siltstones and sandstones are also vulnerable to late-stage dissolution near the surface, further modifying lithologies and thicknesses. Thickness of the Nippewalla Group in the Amoco Rebecca K. Bounds core of western Kansas is 931 feet (, 284 meters) thick, but it is estimated to be only half as thick in the outcrop belt of south-central Kansas. To estimate amount of rock lost to late-stage dissolution, we conducted dissolution experiments on displacive halite units from cores, which resulted in 60-95% loss of rock thickness and mass due to dissolution of halite. The unconsolidated red sediment resulting from these dissolution experiments compares favorably with much of the fine massive red siliciclastics that make up the greater part of the Nippewalla Group outcrops. We propose that major lithologic differences resulting from near-surface diagenetic processes must be expected in any evaporite-bearing siliciclastic unit. Besides guiding correlation attempts between surface and subsurface, this knowledge is vital for making accurate interpretations of depositional and diagenetic history of siliciclastic-evaporite units.

Using sedimentology to address the marine or continental origin of the Permian Hutchinson Salt Member of Kansas

“…It is important to note that the Leonardian–Guadalupian rocks of Kansas, the Sumner, Nippewalla and Quartermaster Groups, pose some challenges in their study. Because they contain evaporites, including halite, thicknesses and sedimentological descriptions are best represented in the subsurface; in outcrops, halite has been dissolved and only casts, moulds and pseudomorphs attest to depositional halite (Benison et al ., ). However, high recovery cores of evaporites are rare.…”

Acid-saline-lake Systems of the Triassic Mercia Mudstone Group, County Antrim, Northern Ireland

“…In contrast, outcrops and shallow cores of these rocks are likely to be highly altered by halite dissolution by late-stage meteoric groundwaters (Gallois 2001). For example, a study of the Permian Nippewalla Group in outcrops and cores in Kansas showed that displacive halite (early diagenetic halite hosted by red mudstone) was the most common lithology in the subsurface, but its outcrop counterpart consisted of poorly cemented red mudstone (Benison et al 2015). The Carnduff-2 core represents the high preservation potential of the moderate subsurface depth.…”

Report on ICDP Deep Dust workshops: probing continental climate of the late Paleozoic icehouse–greenhouse transition and beyond

“…A list of questions associated with the major and supplementary science topics appears in a data supplement table; overviews appear below. Coring is essential to (1) access buried upland records, (2) achieve continuous records in basinal regions, and (3) recover a pristine section suitable for application of various sedimentologic, geochemical, fluid-inclusion, rock-magnetic, magnetostratigraphic, paleontologic, and geochronologic approaches (i.e., Soreghan et al, 2015b;Benison et al, 2015). Additionally, during the course of both workshops, participants suggested considering expansion to a staged project that could incorporate coring in (1) multiple basins of western Europe and (2) in midpaleolatitude sites (e.g., China).…”

“…Intact and complete continental stratal records from the mid-Permian through Triassic of Pangaea are rare (in both outcrop and standard cores). Within the Anadarko Basin, outcrops are stratigraphically and lithologically incomplete due to intercalated halite in parts of the section, which dissolves at and near the surface (Benison et al, 2015). Furthermore, the gentle dip across a region with minimal relief creates limited outcrop opportunities.…”