MICROBIALITES ASSOCIATED WITH THE DEVONIAN-CARBONIFEROUS BOUNDARY HANGENBERG EVENT IN THE WESTERN UNITED STATES

Proliferation of microbialites has been documented following severe mass extinctions. Across the Devonian-Carboniferous boundary, regional expansion of microbialites has been reported in several places including China, Australia, and Russia. In the western U.S.A., microbialites occur in strata bearing a latest Devonian conodont fauna of the pre-Hangenberg Bi. ultimus zone (previously Late expansa-Middle praesulcata zones). These microbialites are predominantly oncolites, with some microdigitate growths and clotted structures of probable thrombolites. Due to their ubiquity in the Sappington Fm. (MT), the Leatham Fm. (UT), and the Pilot Shale (NV and UT), the microbialites have been referred to as an “oncolite bank” and are used as a stratigraphic marker for correlation of latest Devonian strata.

However, these oncolitic beds assigned to the pre-Hangenberg age Bi. ultimus zone also contain macrofauna from the basal Mississippian Pr. kockeli conodont zone (previously Upper praesulcata-sulcata zones) including a Syringothyris brachiopod faunal assemblage, Acutimitoceras (Stockumites) ammonoids, and Pudoproetus trilobites, leading to a reinterpreted Hangenberg crisis age. Carbon isotope analyses of the oncolite beds at Sacagawea Peak (MT), Leatham Hollow (UT), and Little Mile-and-a-Half Canyon (UT) have found positive δ¹³C values that likely belong to the Hangenberg δ¹³C excursion. To further test the relationship between the microbialite occurrence and the Hangenberg δ¹³C excursion, we investigated four sections across the D-C transition in the southern Great Basin. In the South Burbank Hills (UT), a 39-m-thick succession of strata with basal thrombolitic and oncolitic beds has δ¹³C values of 3.2–5.0‰. Oncolitic and oolitic packstones at Mountain Home (UT) exhibit a δ¹³C excursion of +4.5‰ (V-PDB). At Fox Mountain (NV), a +3.3‰ δ¹³C excursion is present in beds containing oncolites with microdigitate overgrowths. The presence of elevated δ¹³C values in all microbialite-bearing strata demonstrates the synchronous occurrence of microbialites and the Hangenberg δ¹³C excursion in the southern Great Basin and western U.S.A.