--> Sedimentologic and Stratigraphic Investigation of Carboniferous Formations in Northern Utah and Central Montana: A Record of Late Paleozoic Climate Change

AAPG ACE 2018

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Sedimentologic and Stratigraphic Investigation of Carboniferous Formations in Northern Utah and Central Montana: A Record of Late Paleozoic Climate Change

Abstract

Late Mississippian cyclothems in Montana (Heath Fm.) and Utah (Manning Canyon Fm.) record a paleotropical response to the onset of the Late Paleozoic Ice Age (LPIA). Core and outcrop analysis of these units and enclosing formations permits an appraisal of late Paleozoic paleotropical climate change. Salient features of formations from both study areas are summarized from oldest (Visean) to youngest (Moscovian).

The upper Madison Gp. contains silicified and brecciated limestone overprinted by karst that is overlain by the Humbug (NE Utah) and Kibbey and Otter (Montana) formations. The Humbug preserves cross-bedded sandstone and silicified microbiolith alternations. The Kibbey contains red channel-form clastics and brecciated limestone that grade into micrite, oolites, stromatoliths, and silicified paleosols of the Otter.

The Heath and Manning Canyon contain cyclic packages of mudstone, sandstone, and limestone preserved in the bathymetric divisions of a muddy homoclinal carbonate ramp. Paleosol and coal are coastal plain facies present in the upper Manning Canyon and lower Heath. Microbioliths and anhydrite are peritidal facies exclusive to the upper Heath.

Regional scale sandstone beds of the Tyler Fm. incise into the Heath and contain intervening coaly shale, siderite-bearing paleosols, and minor ironstone that grade into sequences of limestone, red siltstone and silicified microbiolith of the Alaska Bench Fm. In Utah the Manning Canyon is overlain by limestone-shale cycles of the Round Valley Fm. A regional scale paleosol marks the transition to limestone, carbonate-bearing paleosol, and clastic cycles of the Morgan Fm., which grade into the large-scale cross-bedded sandstone of the Weber Fm.

A Serpukhovian onset of cyclothems in the Heath and Manning Canyon records several 3rd to 4th order transgressive-regressive cycles, entailing sea-level excursions ≤10s of meters in magnitude that coincide with the eustatic signal of the LPIA. The upward transition from paleosol-coal to limestone-anhydrite associations in the Heath records a Serpukhovian paleotropical humid to arid shift absent in NE Utah until the Moscovian. This variation is explained by a 20° difference in paleolatitude of the study areas, resulting in a relative temporal lag in their northward passages into arid climate belts. Humid climate indicators are coincident with known Gondwanan glacial intervals and provide insight into the interplay of ice growth and paleotropical climate evolution.