Abstract
Beach ridges constructed by pluvial Lake Clover in Elko County, Nevada during the Late Pleistocene were investigated with ground-penetrating radar (GPR). The primary objective was to document the internal architecture of these shorelines and to evaluate whether they were constructed during lake rise or fall. GPR data were collected with a ground-coupled 400-Mhz antenna and SIR-3000 controller. To constrain the morphology of the ridges, detailed topographic surveys were collected with a Topcon GTS-235W total station referenced to a second class 0 vertical survey point. GPR transects crossed the beach ridge built by Lake Clover at its highstand of 1725 m, along with seven other ridges down to the lowest beach at 1712 m. An average dielectric permittivity of 5.0, typical for dry sand and gravel, was calculated from GPR surveys in the vicinity of hand-excavations that encountered prominent stratigraphic discontinuities at known depths. Assuming this value, consistent radar signals were returned to a depth of ~3 m. Beach ridges are resolvable as ~90 to 150-cm thick stratified packages of gravelly sand overlying a prominent lakeward-dipping reflector, interpreted as the pre-lake land surface. Many ridges contain a package of sediment resembling a buried berm at their core, typically offset in a landward direction from the geomorphic crest of the beach ridge. Sequences of lakeward-dipping reflectors are resolvable beneath the beach face of all ridges. No evidence was observed to indicate that beach ridges were submerged by higher water levels after their formation. Instead, the GPR data are consistent with a model of sequential ridge formation during a monotonic lake regression.
Highlights
A large area of southwestern North America, known as the Great Basin, is internally drained, offering no route to the ocean for surface water [1]
“pluvial” climates of the Pleistocene, numerous large lakes occupied local low-points within this overall endorheic topography [3,4]. The former presence of extensive surface water is challenging to visualize in such an arid landscape, the pluvial lakes are represented by indisputable geomorphic evidence in the form of beach berms, spits, bars, and wave-cut terraces, many of which have been recognized for over a century [5,6,7]
At the core of many ridges, a buried berm is resolvable that represents the first beach ridge constructed when the lake level stabilized at a given elevation
Summary
A large area of southwestern North America, known as the Great Basin, is internally drained, offering no route to the ocean for surface water [1]. During wetter “pluvial” climates of the Pleistocene, numerous large lakes occupied local low-points within this overall endorheic topography [3,4] The former presence of extensive surface water is challenging to visualize in such an arid landscape, the pluvial lakes are represented by indisputable geomorphic evidence in the form of beach berms, spits, bars, and wave-cut terraces, many of which have been recognized for over a century [5,6,7]. The pluvial lakes are, the most iconic evidence for the profound hydroclimate variability that accompanied glacial-interglacial cycles in this part of North America Despite their significance as indicators of paleoclimate conditions, pluvial lakes are challenging topics of study. Their sediments are typically devoid of organic materials suitable for radiocarbon dating, and where present, shells and other remains of aquatic organisms may be vulnerable to Quaternary 2020, 3, 9; doi:10.3390/quat3010009 www.mdpi.com/journal/quaternary
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