186-1 Revisiting the Fort Hays Limestone and Smoky Hill Chalk members of the Niobrara Chalk, Western Kansas, USA: Updates to Hattin’s (1982) High-resolution Stratigraphy and Biostratigraphy

Abstract:

The Coniacian–Santonian Niobrara Chalk of western Kansas, comprising the Fort Hays Limestone and Smoky Hill Chalk members, records deposition in the Western Interior Seaway (WIS) from approximately 89.5 to 81.5 Ma during a period of widespread anoxia or dysoxia that coincided with Oceanic Anoxic Event 3 (OAE3) and the preservation of abundant marine vertebrate remains. Donald E. Hattin’s (1982) landmark monograph, Kansas Geological Survey Bulletin 225, established a high-resolution stratigraphic framework for these units, using 23 numbered marker units defined by distinctive bentonite beds, limonite stringers, and prominent calcareous horizons to enable centimeter-scale correlation across tens of miles of poorly exposed outcrop. As part of a Kansas Geological Survey mapping project in Logan County and vicinity, the authors revisited Hattin’s measured sections in Logan, Gove, and Trego Counties to integrate his work into a modern context of taxonomy, biostratigraphy, geochronology, and subsurface log data.

Field work focused on relocating Hattin’s sections, photographing outcrops, remeasuring beds, and tying them to his marker units, and attempting to reestablish his correlations. Although access to several key localities was limited—including the only section that Hattin measured in the Fort Hays Limestone—an alternate vertical exposure at Cedar Bluff State Park matched Hattin’s section bed-for-bed, confirming both his measurement precision and the lateral continuity of the chalk beds. We also examined Hattin’s invertebrate collections at the University of Kansas, University of Indiana, and the Fort Hays State University Sternberg Museum, reviewed his unpublished field notes, maps, and additional sections. Macroinvertebrates—dominated by inoceramid bivalves (Cremnoceramus, Platyceramus, Volviceramus, Cladoceramus, Magadiceramus, Cordiceramus, and Cataceramus), Pseudoperna oysters, and rare aberrant ammonites—were rephotographed at high quality and identified using updated taxonomy.

We present photo-based cross sections with marker units identified at the relocated section sites, an updated invertebrate biostratigraphy (including inoceramid species not previously identified in the Niobrara of Kansas), and refined correlations to coeval sections elsewhere in the WIS and to European reference sections. We also establish ties between the invertebrate and vertebrate biostratigraphies. Finally, we document salt-tectonic structural complications affecting Hattin’s correlation of marker beds from reference sections, and tie Hattin’s sections to the extensive subsurface record in Kansas to provide a field-ready bridge between Hattin’s classical framework and a modern Niobrara stratigraphy.

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