DEPOSITIONAL SETTING OF METALLIFEROUS, ORGANIC-RICH BLACK MUDROCK IN THE HEATH FORMATION (UPPER MISSISSIPPIAN), CENTRAL MONTANA

The Upper Mississippian Heath Fm., which accumulated in the Big Snowy Trough of central Montana, has been known for 30 yrs to contain metalliferous black mudrock and oil source rocks and is recognized as a prospective tight oil play. New petrographic, geochemical, and carbon and sulfur isotope data from seven drill cores spanning up to 146 m of immature to marginally mature strata indicate that the Heath Fm. consists of five 3^rd-order transgressive-regressive cycles (C1-5) deposited during a fluctuating climatic regime.

Cycles comprise thinly interbedded gray to black mudrock and carbonate strata capped by coal (humid climate: C1, 3, 4) or gypsum (arid climate: C2). Maximum flooding surfaces in C1, 2, 4, and 5 lie within black mudrock beds ~0.5–3 m thick with 10-28% TOC, type I and type II kerogen, up to 7340 ppm Zn, 1980 ppm V, 1590 ppm Mo, and 509 ppm Ni, relatively low values of δ¹³C_TOC and δ³⁴S_pyrite, and a limited-diversity fauna of locally abundant, thin-shelled pelecypods (Dunbarella? sp.). C3 has more gray mudrock and normal marine carbonate beds than the other cycles, ≤4.2% TOC, lower metal content, and mainly type III kerogen. Mid-Chesterian foraminifers in C3, combined with previously published fossil data, suggest that 3^rd order cycles recorded in the Heath Fm. were ~1-2 myr in duration.

Factors leading to high metal values and high TOC in Heath Fm. black mudrock include a low sedimentation rate, resulting in minimal dilution by clastic detritus, and high primary productivity. On a millennial time scale, a balance was achieved between primary productivity (high enough that oxygen was consumed by sinking organic matter) and seawater inflow (strong enough to maintain a supply of metals without compromising anoxia). On a shorter time scale, the Big Snowy Trough experienced episodic anoxia linked to seasonal and/or longer-duration climate shifts. Lithologic variation within the Heath Fm. was shaped by climate fluctuations at various scales, from <0.1 myr to >1 myr, interacting with changes in relative sea level and sediment input that were driven by glacial eustacy and local tectonic factors. Heath Fm. black mudrock shares intriguing lithologic and geochemical similarities with mudrocks in other Paleozoic units such as the Devonian-Mississippian Bakken Formation and Pennsylvanian cyclothems (e.g., Excello Shale).