Paper No. 18
Presentation Time: 9:00 AM-6:00 PM
A SEDIMENTOLOGICAL ANALYSIS OF THE MIDDLE THUMB CONGLOMERATE OF THE HORSE SPRING FORMATION, LAKE MEAD AREA, NV
JOHNSON, Grace, Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, MIDDLETON, Larry T., Department of Geology, Northern Arizona University, Box 4099, Flagstaff, AZ 86011 and UMHOEFER, Paul J., School of Earth Sciences & Environmental Sustainability, Northern Arizona University, 625 Knoles Drive, Box 4099, Flagstaff, AZ 86011, gj92@nau.edu
The tectonic setting and structural mechanisms that produced Basin and Range extension are subjects of continuous debate. The Miocene Lake Mead basin presents a rare and well-exposed imprint of Basin and Range extension on syn-extensional deposits. A thick conglomeratic unit within these syn-extensional deposits provides an opportunity to analyze the sedimentologic processes that operated during deposition and to analyze the structural events that were occurring. The overall coarsening-upward trend of the medial conglomerate of the Thumb Member of the Horse Spring Formation, along with abrupt facies transitions, suggests that deposition was controlled by basin margin tectonism or changes in climate. An understanding of the sedimentary changes that occurred within the depositional systems helps in reconstructing the history of the Lake Mead basin and thus the tectonic evolution of the area as well.
Sedimentologic and stratigraphic analyses have documented two facies associations, which closely matches the facies associations within the fandelta depositional model developed by Horton and Schmitt (1996). The first facies association includes finer-grained, laminated siltstone and massive sandstone, interpreted as lacustrine deposits. The second facies association comprises matrix-supported conglomerate, clast-supported conglomerate, and horizontally stratified sandstone and reflects deposition by subaerial and subaqueous gravity flows. The fandelta is interpreted to be progradational, due to its overall coarsening-upward trend. The extrabasinal controls on fandelta sedimentation include regional tectonics and climate. The tectonic effects on basin margin gradients affect sedimentation rates (i.e. higher gradients resulted in higher sedimentation rates) and thus caused a transition from a low-energy lacustrine environment into a fandelta depositional setting dominated by debris and mass flow processes. Although climatic drivers cannot be discounted, it is more likely that syn-depositional structural adjustments were the primary control on fandelta sedimentation.
