Lower Sappington Formation palynofacies in Montana confirm upper famennian black shale paleoenvironments and sequences across western North America

Abstract

For the first time, Late Devonian palynofacies analyses of the Lower Sappington shale units U1A─D was carried out at Peak 9559 (Sacajawea) and Ainger Lake in the Bridger Range of Montana. Diagnostic spore species Apiculiretusispora verrucosa, Diducites mucronatus and D. versabilis allowed the correlation of our U1A-D with the European Late Famennian A. verrucosa-V. hystricosus Palynozone. The first appearance of Gorgonisphaeridium winslowiae in U1 is the oldest record before the inception of Retispora lepidophyta. Four palynofacies correlative with U1A-D and new findings of invertebrates and microfossils allow the interpretation of paleoenvironmental changes. Amorphous organic matter, marine phytoplankton and pyrite in black shales of U1A-B indicate anoxic bottom conditions occurred in offshore marine environments. An erosional fossiliferous phosphatic lag above these units confirms a regional SB. U1C black shales composed by AOM, marine and terrestrial phytoplankton, land–derived remains and pyrite reveal shallower, dysoxic-anoxic, brackish water environments. A thin layer at the base of Unit 1D yielded AOM and marine phytoplankton and low terrestrial input indicating dysoxic-anoxic conditions were maintained. The lack of organic matter and the presence of invertebrates and microfossils in a thin green fossiliferous mudstone supports the establishment of normal, oxygenated marine conditions maintained in the basal Middle Sappington Member (U2). These two thin units are not part of the underlying anoxic black shale (U1A-B) sequence as commonly was over-simplified. Instead, they are part of a transgressive interval with the basal Middle Sappington. A correlation of the U1 shale interval is established with the global multiphase Dasberg Event.