Upper Cambrian-lowermost Ordovician stratigraphy and microbialites of the Great Basin, United States.

摘要

Upper Cambrian-lowermost Ordovician shelf strata were correlated across the Great Basin through lithostratigraphy and microbialite morphostratigraphy. This interval was the time of the “stromatolite resurgence” in which global, low-latitude shallow marine ecosystems were dominated by microbialite buildups.; The study area is 250 kilometers wide and 750 kilometers long. The interval studied is up to one kilometer thick and represents 10 million years of deposition (∼502 to 492 Ma). Eleven columnar sections were compiled, and other locations reconnoitered, from eastern California to northern Utah. Petrographic thin-sections were made of critical rock and microbialite units. Biostratigraphically useful fossils, such as trilobites and conodonts, were rare over much of the studied interval, with the exception of the molluscs Matthevia and Matherella.; Four main lithologic successions were identified and correlated across the Great Basin (in ascending stratigraphic order): (Succession I) cylical dolomite or silty limestone; (Succession II) silty/cherty limestone, grainstones, and shale; (Succession III) [dolomitized] microbial boundstone; and (Succession IV) heterogeneous deposits of silty/cherty limestone and dolomite and encrinite.; Twelve main forms (=morphotypes) of microbialites were differentiated based on macro- and mesostructural attributes (the samples were too dolomitized for microstructural analysis): Form A—cylindrical columnar stromatolite; Form B—domical hemispherical stromatolite; Form C—‘rind’-type stratiform stromatolite; Form D—‘biostromal’-type stratiform stromatolite; Form E—macroclotted cylindrical columnar thrombolite; Form F—mesoclotted large columnar branched thrombolite; Form G—mesoclotted small columnar branched thrombolite; Form H—mesoclotted domical hemispherical thrombolite; Form I—domical hemispherical dendrolite; Form J—columnar dendrolite; and Form K—stratiform (both biostromal and rind-type) dendrolite.; Based on the twelve morphotypes, six microbialite morphozones and one morphosubzone were established. All of the zones can be correlated across the Great Basin with the exception of the Form E Subzone which can not be found in the southern Nevada sections. The morphostratigraphy proposed in this dissertation differs from previously established pre-Phanerozoic stromatolite biostratigraphy in that the morphotypes are distinguished on meso- and macrostructural attributes, not microstructure and that the interval of time represented by each zone is probably one or two million years long.