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Miocene tectonics and climate in the Lake Mead region recorded by Horse Spring Formation carbonates

Thomas A. Hickson, Anders J. Ness and Melissa A. Lamb
Miocene tectonics and climate in the Lake Mead region recorded by Horse Spring Formation carbonates (in Miocene tectonics of the Lake Mead region, central Basin and Range, Paul J. Umhoefer (editor), L. Sue Beard (editor) and Melissa A. Lamb (editor))
Special Paper - Geological Society of America (June 2010) 463: 121-145


The Oligocene-Miocene Horse Spring Formation consists of sedimentary strata that record the onset and evolution of Miocene extensional tectonics in the Lake Mead region. The sedimentary basins of this formation hold critical clues for evaluating and testing competing models that attempt to explain the tectonic evolution of this important part of the Basin and Range. Detailed sedimentology, stratigraphy, isotope geochemistry, and new geochronology of carbonates of the Horse Spring Formation shed light on the details of middle Miocene depositional systems and provide important paleoclimatic and paleotopographic data that further our understanding of the geological evolution of this area. We investigated four carbonate sections in detail, two from the Bitter Ridge Limestone Member (Slot Canyon section, near the Gale Hills, and the West Longwell section, at the Bitter Ridge), one from the Thumb Member (East Longwell section, near the Bitter Ridge), and one from the Rainbow Gardens Member (Rainbow Gardens Recreation Area section), to understand the evolution of carbonate lake systems, to extricate paleoclimatic from tectonic signals in the sedimentary record, and to develop a more clear picture of the evolution of Horse Spring sedimentary basins. New (super 40) Ar/ (super 39) Ar dates from the Bitter Ridge Limestone, combined with dates in the published literature, suggest that the Bitter Ridge Lake may have evolved time-transgressively from the White Basin area in the east to the Rainbow Gardens area in the west. Possibly contemporaneous with this evolution, the lake gradually shifted from an open to a closed lake system, most likely due to tectonic partitioning of the basin or the creation of a tectonic sill that cut off the overflow for the lake. Stable isotope and lithofacies analyses provide one of the first detailed proxy records of paleoclimate for the Miocene of the Basin and Range and show strong evidence for an orbitally forced climate signal that represents changes in the precipitation/evaporation ratio for the Bitter Ridge Lake system. Because we can effectively show a climatic signal in the Bitter Ridge Limestone units over 100 k.y. and, likely, 40 k.y. time cycles, longer time-scale shifts in isotopic ratios are more likely due to tectonic processes. Based on a strong negative shift in oxygen isotopic ratios, previous researchers have suggested that the Lake Mead region experienced an increase in paleoelevation during Horse Spring time, while the remainder of the central Basin and Range to the north experienced a decrease in elevation for the same time period. Our data, when compared with data from the Pliocene Hualapai Limestone and those presented by previous researchers, appear to constrain the timing of this isotopic shift to between 15 and 13 Ma, coincident with the timing of the onset of rapid extension in this part of the Basin and Range. We hypothesize that this isotopic shift was due not to a change in paleoelevation due to magmatic activity alone, but was influenced by either (1) longer travel distances of air masses and the development of increased topographic corrugation as the Lake Mead region experienced accelerated rates of extension or (2) drainage reorganization of the early Colorado Plateau and the infusion of isotopically lighter waters from this emergent source.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 463
Title: Miocene tectonics and climate in the Lake Mead region recorded by Horse Spring Formation carbonates
Title: Miocene tectonics of the Lake Mead region, central Basin and Range
Author(s): Hickson, Thomas A.Ness, Anders J.Lamb, Melissa A.
Author(s): Umhoefer, Paul J.editor
Author(s): Beard, L. Sueeditor
Author(s): Lamb, Melissa A.editor
Affiliation: University of Saint Thomas, Geology Department, St. Paul, MN, United States
Affiliation: Northern Arizona University, Department of Geology, Flagstaff, AZ, United States
Pages: 121-145
Published: 201006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 45
Accession Number: 2010-088817
Categories: StratigraphyStructural geologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. charts, 3 tables, geol. sketch maps
N36°04'60" - N36°30'00", W115°00'00" - W114°25'00"
Secondary Affiliation: U. S. Geological Survey, USA, United StatesUniversity of Saint Thomas, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201047
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