Stable carbon isotope chemostratigraphy and tectonic setting of the Pennsylvanian Ely-Bird Spring basin, Nevada and Utah: interpreting three-dimensional basin evolution using multiple stratigraphic techniques
AuthorSturmer, Daniel Murray
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The Pennsylvanian was a tectonically active time throughout Laurasia. In western Laurasia, the Ely-Bird Spring basin (EBSB) formed during early to middle Pennsylvanian time as one of a series of tectonically-generated, stacked, late Paleozoic basins. Timing and extent of these basins is well constrained; but the tectonic framework that resulted in basin formation is poorly known. Farther east, Ancestral Rocky Mountain basins and uplifts formed synchronously with the EBSB, as a far-field response to collision between Laurasia and Gondwana.This dissertation was undertaken in order to develop a method that combines whole-rock carbon isotope chemostratigraphy with lithostratigraphy and biostratigraphy. These data were used to elucidate the three-dimensional evolution and tectonic setting of the EBSB. To do this, six nearly complete sections located throughout Nevada and western Utah were densely sampled for carbon isotope stratigraphy. The results of this work are presented in three chapters as outlined below.Chapter 1 tests the hypotheses that carbon isotope shifts mirror cyclothemic lithologic shifts and that these systematic isotopic shifts can be used as a correlation tool. Three sections from the north, central, and southern portions of the EBSB were densely sampled for carbon isotope stratigraphy. Generally, the isotopes tracked changes in rock type, with shifts to more positive carbon isotope values toward the tops of shallowing-upward lithologic cycles. Therefore, pattern matching of whole-rock isotope shifts, tied to lower resolution biostratigraphy, is a feasible method for high-resolution correlation within the basin. Chapter 2 presents new fine-scale carbon isotope stratigraphy from six sections around the basin, combined with existing biostratigraphy, to elucidate the three- dimensional evolution of the basin. Correlations between sections were investigated using Match-2.3 (Lisiecki and Lisiecki, 2002), a dynamic programming algorithm developed for stratigraphic signal correlation. Based on these correlations, the highest initial relative sediment accumulation rates occurred in the eastern part of the basin. However, for most of the life of the basin, the highest relative accumulation rates were in the northwestern part of the basin. High rates of sediment accumulation in the northwestern part of the basin may be a response to renewed tectonism to the west. This corroborates previous work which shows that tectonism culminated in angular unconformities and overturned folding that deformed the northern basin strata during middle Pennsylvanian time, but left southern basin strata relatively unaffected. Chapter 3 compares the three-dimensional evolution of the EBSB to broadly coeval Ancestral Rocky Mountains basins. The purpose was to test whether the Ancestral Rocky Mountains and EBS basins were related tectonically. Geohistory analyses for the EBSB and four Ancestral Rocky Mountains basins indicate that the basins were in different tectonic settings. Three-dimensional subsidence analysis of the EBSB suggests it is more compatible with a foreland basin subsidence model whereas most of the Ancestral Rocky Mountains basins are consistent with subsidence models for strike-slip basins.