Transgressive-Regressive Cycles in the Highly Metalliferous, Oil Shale-Bearing Heath Formation (Upper Mississippian), Central Montana

Abstract

The Heath Formation (Upper Mississippian) is a heterogeneous unit that is a prospective shale-oil target in central Montana. Petrographic, geochemical, and carbon isotopic studies of seven continuous drill cores through 146 m (maximum thickness) of Heath strata in the western Big Snowy Trough indicate that five 4th-order transgressive-regressive cycles (C1–5) were deposited during a fluctuating climatic regime. C1 is transitional to the underlying Otter Formation and C5 (incomplete) includes the lower part of the Bear Gulch Limestone. C1, 3, and 4 are capped by coal and paralic green mudstone and formed in humid conditions; C2 culminates in evaporite (Loco Ridge gypsum) and was deposited in an arid setting. Maximum flooding surfaces in C1, 2, 4, and 5 are marked by black shale ~0.5–3 m thick with >10% TOC, elevated DOP-T (degree of pyritization of total Fe) and metal contents, and a locally abundant, limited-diversity fauna of thin-shelled pelecypods. In C2, this shale (Cox Ranch oil shale), has up to 28% TOC, 1590 ppm Mo, 509 ppm Ni, 1910 ppm V, and 5140 ppm Zn; highest metal contents in shales of C1, 4, and 5 are 72–859 ppm Mo, 221–378 ppm Ni, 446–1980 ppm V, and 1030–7340 ppm Zn. In contrast, C3 is normal-marine crinoid mudstone to packstone underlain and overlain by (dolo)mudstone with fenestral fabric, peloids, intraclasts and a stenohaline fauna; this cycle has rare elevated metal contents and maximum TOC of 4.2%. Mid-Chesterian foraminifers in C3, combined with previously published fossil data, suggest that cycles in the Heath Formation were ≤1 m.y. in duration and formed during a time of global cooling linked to Gondwanan glaciation. Tectonic, climatic, and paleogeographic factors shaped the cycles of the Heath Formation. Conditions most favorable to marine life were present during C3. Molybdenum contents >100 ppm suggest persistently euxinic conditions during deposition of the black shales in C2, 4, and 5 (Scott & Lyons, 2012) but the presence of shell beds (1 mm-6 cm thick) within these shales denotes bottom oxygenation sufficient to support life, at least periodically. δ13C values for carbonates throughout the section are lower than normal marine, indicating the presence of oxidized organic carbon in the water column. Whether the low-δ13C component reflects decomposition of sinking organic matter, upwelling, or continental runoff has yet to be determined.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015