NEW INSIGHTS INTO THE COMPOSITION AND TEXTURE OF LIMESTONE – MARLSTONE COUPLETS OF THE BRIDGE CREEK LIMESTONE MEMBER, COLORADO

Originally identified within the Bridge Creek Limestone Member of the Greenhorn Formation in the late 1800’s, limestone – marlstone couplets have since been documented in basins around the world throughout the Phanerozoic. While deposition of such interbedded lithologies represent cyclic processes such as climate change and eustasy, resolving the major drivers remains a challenge. This is due to the lack of detailed lithologic observations regarding the primary components and matrix material comprising both limestones and marlstones in the varied depositional environments in which they are deposited.

In this study, we evaluated limestone – marlstone couplets of the Bridge Creek Limestone in the USGS #1 Portland Core in central Colorado. Methods include detailed sedimentologic observation at the core and thin section scale and geochemical analyses, including total organic and inorganic carbon (TOC, TIC) and major and trace element concentrations. Results from this study demonstrate that limestones (> 75 % CaCO3) are composed dominantly of carbonate cement, with little observed component grains, organic matter, or bedding. Conversely, marlstones (25 – 75 % CaCO3) exhibit a high degree of heterogeneity in both their composition and structure. Marlstone lithologies range in composition from calcareous to argillaceous with TOC concentrations ranging between 0.2 and 7 wt. %. The composition of these lithologies represent varied component and matrix materials including detrital quartz, argillaceous clasts, foraminiferal tests, shell fragments, fish scales and bones, coccolithophore-rich pellets, organic matter, dispersed argillaceous material, and carbonate and silica cement. Additionally, bedding is also highly variable in terms of continuity, thickness, and texture. Marlstones are variably bedded, ranging from homogenized to normally graded beds of varied thickness and continuity. Where preserved, beds range in thickness from < 5 mm - > 5 cm and contain wave and current ripple laminae along erosive bases and bioturbated tops. The heterogeneity in composition and structure observed in the limestone – marlstone couplets in this study suggest that multiple sedimentary processes were associated with their deposition and that no singular mechanism is responsible for their accumulation.