Elsevier

Sedimentary Geology

Volumes 269–270, 15 August 2012, Pages 28-36
Sedimentary Geology

Geophysical surveys of a pluvial lake barrier deposit, Beatty Junction, Death Valley, California, USA

https://doi.org/10.1016/j.sedgeo.2012.05.016Get rights and content

Abstract

We used ground penetrating radar (GPR) and seismic refraction to image the internal stratigraphy of a beach barrier deposit at Beatty Junction, Northern Death Valley, to better understand its depositional environment in the context of variations in the level of the former Lake Manly. The deposit is a gravelly bar ~ 500 m long with 4 m of surface relief that formed at a shoreline of Lake Manly during the end of the last Pleistocene ice age. The GPR profiles provide subsurface images that we interpret as progradational foreset beachface strata in the uppermost 2 m and the surface of an earlier bar at depths of 2 to 6 m. We conclude that the crest of the bar migrated in a landward direction during the construction of the uppermost 4 m of the bar as lake level rose. The seismic survey indicates a sharp velocity increase from 760 m/s to 1510 m/s at the base of the bar, which we interpret as the boundary between well-sorted gravelly beach deposits, and underlying older fan deposits. The depth of the base of the bar varies between 5 m and 10 m. The elevation of the bar is comparable to that of other shoreline features in Death Valley that formed during the MIS 6/5e (186–120 ka) highstand. Measurements of fault slip on the nearby Northern Death Valley fault have documented only strike–slip motion. In absence of any evidence for significant vertical uplift in the area during the late Pleistocene and Holocene, we conclude that the bar probably formed during MIS 6/5e. This conclusion is subject to uncertainty due to discrepancies in age dates reported for the deposit.

Highlights

► Near-surface geophysical survey of Late Pleistocene barrier. ► Sedimentary structures imaged using ground penetrating radar. ► Base of bar imaged using seismic refraction. ► Prograding shoreface strata during highstands. ► Model for barrier formation during rising lake level.

Introduction

Many of the enclosed basins in the now arid to semi-arid American Southwest contained pluvial lakes during the late Pleistocene ice ages (Benson, 1999). Variations in the extent of these paleolakes over time are indicated in many cases by relict shoreline features, both erosional and depositional (Warnke and Ibbeken, 2005). The development of shoreline features at pluvial lakes was discussed by Adams and Wesnousky (1998) in a study of Lake Lahontan. The development of barrier islands in coastal settings in response to sea level changes was described by Reineck and Singh (1980, pp. 339–345) and McCubbin (1982). In this study, we investigated shoreline deposits in Death Valley associated with a highstand of Lake Manly (Fig. 1a), which was one of a series of lakes in the American Southwest that alternately filled and desiccated during the Pleistocene and Holocene, and that were sometimes connected. Death Valley is a pull-apart basin bounded by two right-lateral fault systems, the Northern Death Valley and Southern Death Valley fault zones, and one normal fault system, the Black Mountains fault zone. The older and inactive fault zone that extends beyond the southeastern end of the Northern Death Valley fault zone is called the Furnace Creek fault zone (Machette et al., 2001b). Death Valley is floored by a salt pan with an average elevation of approximately 75 m below mean sea level (− 75 m), and contains Badwater Basin, which with an elevation of − 85.5 m is the lowest point in North America.

Well-preserved sequences of shoreline features are found in Death Valley at Mormon Point and Shoreline Butte (Hooke, 1972, Hooke, 1999, Meek, 1997, Ku et al., 1998). Shoreline features at these and other locations in Death Valley have been dated using radionuclide methods (Ku et al., 1998, Lowenstein et al., 1999, Lowenstein, 2002) and cosmogenic nuclide methods (Machette et al., 2008, Owen et al., 2011). The majority of the better-preserved shoreline features have been found at elevations between approximately +30 to +90 m, and formed during a highstand of former Lake Manly during Marine Isotope Stage (MIS) 5e− 6, from 120,000 to 186,000 years ago (120 ka to 186 ka). Knott et al. (2002) argued that only the highest bench at Mormon Point (+90 m) is a shoreline, and that the benches and risers at lower elevations described by others as shorelines are in fact fault scarps (Hooke, 2004, Knott et al., 2004). The maximum lake level during MIS 2 (10 ka to 35 ka) was much lower than the MIS 5e− 6 highstand, and almost certainly did not exceed sea level (Ku et al., 1998, Knott et al., 2002, Hooke, 2004). The water depth history of the former Lake Manly during the past 200 ka was determined based on U-series dating (Ku et al., 1998) and sediments and microfossils (Forester, 2005) from drill core of the 186-m deep well DV93-1 at Badwater Basin. Ku et al. (1998) also dated tufa deposits associated with paleoshorelines at several locations in central Death Valley between Badwater and Mormon Point. Tufa deposits of MIS 5e− 6 age were found at elevations from +55 to +90 m and associated with a deep lake, while tufas of MIS 2 age were found only at much lower elevations (− 22 m and − 30 m) and associated with a shallow, perennial, lake. The paleoenvironment of the MIS 2 lake was described in detail by Li et al., 1996, Li et al., 1997 based on mineralogy and sedimentary structures of evaporites in drill core DV93-1. Using cosmogenic 36Cl dating, Machette et al. (2008) measured the age of the Hanaupah shoreline deposit, located on the western side of Death Valley, as 130 ka (late MIS 6).

Ground penetrating radar (GPR) has been successfully used elsewhere in modern and ancient coastal sedimentary environments to image shoreline deposits, e.g., by Jol et al. (2002) in a study of a coastal barrier spit in Washington State, Smith et al. (2003) in a study of shoreline deposits of Pleistocene Lake Bonneville, Lindhorst et al. (2008) to measure the migration of a Holocene barrier spit in the German Bight, and Clemmensen and Nielsen (2010) to map a beach ridge system in Anholt, Denmark for the determination of paleo-sea level history.

Seismic refraction methods have been used extensively for near-surface geophysical surveying in a wide variety of geologic settings. Traditional data processing schemes, including delay time methods and the generalized reciprocal method (GRM), are suited for lithologic sequences with a small number of layers, and sharp velocity contrasts at the layer interfaces (Kearey et al., 2002). Tomographic methods, namely traveltime inversion, provide gridded solutions that are better able to accommodate gradually varying velocity fields or lithologic sequences with many layers, with small contrasts at layer boundaries (Shearer, 2009).

In this study, we used GPR and seismic refraction to image the internal structure of a relict barrier bar in the Beatty Junction bar complex in order to understand the physical processes that formed it and to provide a basis for comparison with similar deposits in Death Valley and elsewhere. We describe the bar in the context of the lake level history of Lake Manly and discuss the evidence for its age relative to shoreline features at other locations in Death Valley.

The Beatty Junction bar complex, located in northern Death Valley 2.9 km north of Beatty Junction on Beatty Cutoff road, consists of a large and relatively intact bar, the subject of this study, and the remnants of three smaller bars (Fig. 1b and c). The main bar, which we refer to as Beatty bar, corresponds to Klinger's (2001) Spit B. The bars mark late highstand lake levels at the northernmost extent of Lake Manly during the Pleistocene or Holocene, thus providing a record of previous climatic conditions here. Klinger (2001) proposed that the bars were formed from material from the adjoining hill, and that the material was transported to the north and east and deposited as spits on an alluvial platform. In the regional geologic compilation by Workman et al. (2002) the bar complex is mapped as lacustrine deposits of late to middle Pleistocene age, the adjoining hill as Pliocene to Miocene sedimentary rocks, and the alluvial apron to the northeast as ranging in age from early Pleistocene to Holocene. According to Klinger (2001) the hill adjoining the bar complex may be as young as middle Pleistocene. The hill, bar complex, and alluvial apron all contain abundant clasts of crystalline rock derived from Proterozoic rocks of the Funeral Range located 7 km to the northeast. Hunt and Mabey (1966) described Beatty bar as having a shingled, cross-bedded texture and better sorting than the older fan gravels upon which it rests. Orme and Orme (1991) surveyed a 500 m long elevation profile along the full length of the bar and prepared two cross sections of the bar 90 m wide that show a maximum relief of about 4 m. They described the sedimentary texture and structures of the bar as similar to those of a coarse clastic ocean beach environment, and as having developed under the influence of storm waves driven by strong winds from the SSE with a fetch extending the 80 km length of central Death Valley.

Section snippets

Ground penetrating radar

The GPR method utilizes radar waves to image geologic media. In reflection profiling, waves are transmitted and received using a pair of surface antennas that are moved along the ground. As is the case in seismic reflection, downgoing waves are reflected from horizontal geologic interfaces at the midpoint beneath each source–receiver pair. However, GPR fundamentally differs from seismic reflection in that it utilizes electromagnetic rather than elastic waves, and a GPR reflection at a geologic

Ground penetrating radar

As is the case for other geophysical imaging methods, GPR resolution and penetration depth depend on source frequency. Penetration in this study was typically 4 m to 6 m at frequencies of 50 MHz, 2 m at 225 MHz, 1 m at 450 MHz, and 0.5 m at 900 MHz. The 50 MHz cross-barrier profiles (Fig. 4, Fig. 5) appear to image buried bar crests on the south (lakeward) side of the crest exposed at the surface today. On lines 05–02 and 05–03 (Fig. 4) about 40 m south of the surface crest, a subsurface bar appears to be

Lake level history

The two principal highstands of Lake Manly in the past 200 ka that produced shoreline deposits with elevations similar to those of the Beatty Junction barrier complex correspond to MIS 5e− 6 and MIS 2 (Ku et al., 1998, Lowenstein et al., 1999, Forester, 2005). Shoreline deposits in central Death Valley formed during MIS 5e− 6 at Mormon Point and Shoreline Butte with elevations ranging from +57 m to +90 m (Ku et al., 1998, Hooke, 1999, Lowenstein, 2002). Machette et al. (2008) used the cosmogenic

Discussion and conclusions

We interpret the GPR profiles recorded in this study as showing progradational beachface strata, overwash fan strata, and erosional berm‐crest surfaces. The lower-frequency GPR profiles appear to have imaged an older erosional berm surface at depths of 2–6 m, while higher-frequency profiles resolved foreset laminations in the uppermost 2 m. The resolution and maximum depth of penetration of GPR may be limited at this site due to the presence of caliche, as noted at other sites by Jol and Bristow

Acknowledgments

We thank the National Park Service for permission to conduct Study DEVA-00118. Students from the California State University, East Bay, Spring 2005 Geology 3200 Regional Field Methods course and the University of Wisconsin Eau-Claire, Spring 2007 Geography 401 Field Seminar helped to record the data.

References (38)

  • G.S. Baker et al.

    An Introduction to Ground Penetrating Radar (GPR)

  • L. Benson

    Records of millennial-scale climate change from the Great Basin of the western United States

    AGU Monograph

    (1999)
  • R.M. Forester

    An ostracode based paleolimnologic and paleohydrologic history of Death Valley; 200 to 0 ka

    GSA Bulletin

    (2005)
  • K.L. Frankel et al.

    Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

    Journal of Geophysical Research

    (2007)
  • Geometrics, Inc.

    SeisImager/2D Manual, Version 3.3., San Jose, CA

    (2009)
  • R. LeB Hooke

    Geomorphic evidence for late-Wisconsin and Holocene tectonic deformation, Death Valley, California

    Geological Society of America Bulletin

    (1972)
  • C.B. Hunt et al.

    Stratigraphy and Structure, Death Valley, California

    U.S. Geological Survey Professional Paper 494-A

    (1966)
  • H.M. Jol et al.

    GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide

  • 1

    Deceased 24 October 2008.

    View full text