REFINED ICHNOLOGICAL ASSESSMENT OF THE UPPER CRETACEOUS GREENHORN FORMATION

This study presents a refined ichnological assessment of the Bridge Creek Limestone Member (BCLM) and the Lincoln Limestone Member (LLM) of the Greenhorn Formation (GF). The GF consists of the BCLM overlying the Heartland Shale Member and basal LLM. In western Kansas, the BCLM and LLM are dominated by marlstone with decimeter-scale limestone beds. The marlshale and marlstone are laminated with little to no bioturbation, whereas the limestone is highly bioturbated. The marlstone and limestone of the BCLM and LLM contain distinctly different ichnocoenoses. Marlstone often has an Ichnofabric Index (ii) of 1–2, whereas the more bioturbated limestone commonly has an ii of 4–6. Five distinct ichnocoenoses were identified within the GF, each ichnocoenosis is strongly associated with the lithology within which it is found. Ichnocoenoses 1 and 2 are found in marlshale to marlstone and contain rare small (~0.5–2 mm) traces. Ichnocoenosis 3 is found in marlstone to calcareous marlstone facies with sparse (2–4 mm) traces. Ichnocoenoses 4 and 5 are associated with the limestone facies, and are well to highly bioturbated with many large (≥1 cm) traces. The marlstone facies are consistent with the Nereites ichnofacies, while the limestone facies are assigned to the Zoophycosichnofacies, based on the ichnocoenoses.

The GF exhibits two types of biogenically modified permeability—weakly defined textural heterogeneity in the limestone, and cryptic bioturbation in the marlstone. Within the marlstone facies, the permeability ranges between 0.56–0.95 mD, while the limestones are between 1.3–1.4 mD. Thalassinoides burrows in the limestones increase permeability up to 4 mD. Within limestone facies, multiple trace tiers were present with piping within limestone beds and into the underlying marlstone. Trace fossil abundance and distribution, ichnocenoces, and ii were used to interpret paleobenthic oxygenation (PBO). Interpreted PBO indicated that the LLM and upper BCLM record lower levels of benthic O₂, while the lower BCLM exhibited a higher benthic O₂. Both members record PBO fluctuations, but the lower BCLM has both more and greater PBO fluctuations. This refined interpretation of the GF has identified 12 ichnogenera, 5 ichnocoenoses, interpreted high-resolution PBO fluctuations, and tied those to permeability and TOC data.