SEDIMENTOLOGIC AND STRATIGRAPHIC INVESTIGATION OF THE HEATH FORMATION, CENTRAL MONTANA AND MANNING CANYON FORMATION, UTAH: IMPLICATIONS FOR LATE PALEOZOIC CLIMATE CHANGE

Late Mississippian cyclothemic successions in central Montana (Heath Fm.) and in northern Utah (Manning Canyon Fm.) record the paleotropical response to the onset of the Late Paleozoic Ice Age. Re-evaluation of these units facilitates an appraisal of late Paleozoic paleoenvironmental change.

In core, the Heath Fm. contains stratigraphic cycles that preserve the bathymetric divisions of a muddy homoclinal carbonate ramp. Variably fossiliferous laminated black mudstone and normally graded calcareous siltstone are outer ramp deposits. Alternating calc-clastic siltstone, fine sandstone, and wackestone-packstone are mid ramp deposits. Blocky, root penetrated calcareous siltstone and coal, exclusive to the lower Heath, are coastal plain and mire deposits. Algal boundstone with variable proportions of anhydrite, exclusive to the upper Heath, are subtidal to peritidal deposits.

In core, the Manning Canyon Fm. preserves cyclic stratigraphic packages consisting from base to top, of 1) Planar to inclined marine packstone and wackestone. 2) Interlaminated calcareous siltstone and fine, rippled sandstone. 3) Laminated siltstone with copious organic debris and minor bivalves. 4) Blocky, root penetrated calcareous siltstone. 5) Coal. In outcrop, tabular cross-bedded sandstone and rippled heterolith are preserved as single units and progradationally stacked packages in the apparent absence of coal.

The lateral continuity of cyclic marine and nonmarine facies in these cyclothems permits their interpretation as depositional sequences bounded by sequence boundaries: represented by paleosols in the lower Heath and the Manning Canyon, and by evaporites in the upper Heath. Both formations record several 3^rd to 4^th order transgressive-regressive cycles, with variation in sea level ≤10s of meters in magnitude that are likely a paleotropical, eustatic response to initial Gondwanan ice growth-decay cycles.

The upward transition from paleosol-coal to limestone-anhydrite associations in the Heath records a Serpukhovian paleotropical humid to arid climate shift that is absent in the Manning Canyon. This variation is explained by a ~20° difference in paleolatitude of the study areas, which caused a temporal offset in their respective passages from humid to arid climate belts forced by long-term N-NW plate migration.