PALEOENVIRONMENTS OF THE LATE MISSISSIPPIAN HEATH FORMATION (BIG SNOWY TROUGH, MONTANA) AND THEIR RELEVANCE TO UNDERSTANDING LATE PALEOZOIC PALEOENVIRONMENTAL CHANGE

In the Big Snowy Trough of central Montana, the late Mississippian cyclothemic succession records the paleotropical response to the onset of the Late Paleozoic Ice Age. This re-evaluation of these units facilitates an appraisal of late Paleozoic paleoenvironmental change.

Field and core observations disclose two recurrent stratigraphic motifs in the Heath Formation, each composed of repetitive successions of facies. The stratigraphic motif in the lower Heath consists (base to top) of: fossiliferous black laminated siltstone to shale, variably fossiliferous calcareous siltstone, calcareous siltstone with plant fragments, slickensides, and pedogenic fabrics, and coal (overlain by the next black shale). The stratigraphic motif in the upper Heath consists (similarly) of: fossiliferous dark grey laminated siltstone, variably fossiliferous, calcareous siltstone with irregularly dispersed very fine-grained sandstone, biolaminated limestone and dolomite, anhydrite, and a cap of fossiliferous dark grey laminated siltstone.

Fossiliferous black muds settled from suspension in marine waters. Fossiliferous, calcareous siltstones are low-energy marine deposits. Biolaminated limestone and dolomite are intertidal to subtidal deposits, and anhydrite layers are evaporitic. Calcareous siltstones with plant fragments, slickensides and a pedogenic fabric, and coals are coastal plain deposits.

Alternations between marine and nonmarine facies in these cyclothems, and their apparent lateral continuity, allow them to be interpreted as depositional sequences bounded by sequence boundaries (represented mainly by paleosols, and later by evaporites). Cycles record sea level excursions up to 10s of meters in magnitude that are most likely a paleotropical, eustatic response to initial Gondwanan ice growth and decay cycles. Deposition of marine facies over coals and anhydrites in repetitive cycles as little as ~2 m thick suggests these units were formed in a low accommodation and low sediment supply setting.

The upward transition from pedogenically modified mudrocks and coals to limestone-anhydrite associations in the Heath Formation records a known paleotropical humid to arid climate shift, but we here identify it as Serpukhovian in age, earlier than suggested by previous studies.