The Las Vegas Formation

Abstract

The Las Vegas FormationCover. Buff-colored deposits sit against the backdrop of the Las Vegas Range in the northern Las Vegas Valley. Once thought to be remnants of a large pluvial lake, these sediments actually record the presence of extensive paleowetlands that supported a diverse flora and fauna. Throughout the Quaternary, the wetlands expanded and contracted in response to abrupt changes in climate. These dynamic ecosystems are preserved in the geologic record as the Las Vegas Formation. (Photograph by Eric Scott, Cogstone Resource Management, Inc., used with permission) For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov.

Desert wetlands record hydrologic variability within the Younger Dryas chronozone, Mojave Desert, USA

“…In the southwestern United States, paleowetland deposits dating to the YD chronozone are common and have been identified at sites throughout the southern Great Basin (Haynes, 1967; Quade, 1986; Quade and Pratt, 1989, Quade et al, 1995, 1998, 2003; Springer et al, 2015, 2018), Mojave Desert (Pigati et al, 2011), and Sonoran Desert (Haynes, 2007; Ballenger et al, 2011). At most of these locations, the YD-age deposits are a few centimeters to a few tens of centimeters thick, and are often bioturbated, which limits their potential temporal resolution.…”

“…In the Mojave Desert and southern Great Basin, there are discrepancies between different hydrologic systems regarding the nature of the YD cold event as a whole, and essentially no speculation as to the drivers of intra-YD variability. Lake records along the Mojave River system, for example, seem to suggest a reduction in winter precipitation and an overall return to drier conditions during the YD (Kirby et al, 2015; Glover, et al, 2017), whereas a speleothem record from Leviathan Cave, Nevada, indicates a shift to colder temperatures (Lachniet et al, 2014) and wetland systems throughout the region clearly show that wet conditions prevailed at this time (Haynes, 1967; Quade, 1986; Quade and Pratt, 1989; Quade et al, 1995, 1998, 2003; Pigati et al, 2011; Springer et al, 2015, 2018). In our view, it is premature to speculate about potential drivers of intra-YD hydrologic variability in the southwestern United States (and probably worldwide) until more high-resolution records are developed.…”

“…Charcoal and organic material are recognized as suitable material for 14 C dating paleowetland deposits (Quade et al, 1998; Haynes, 2008; Jull and Burr, 2015) and were treated using the standard acid-base-acid (ABA) procedure. Previous work has shown the Succineidae shells also yield reliable 14 C ages regardless of the local habitat, environmental conditions, and geologic substrate (Pigati et al, 2010; 2013), including desert wetland ecosystems in the southwestern U.S. (Pigati et al, 2009, 2011; Springer et al, 2015, 2018). Clean, dry gastropod shells selected for 14 C dating were broken and examined under a dissecting microscope to ensure that the interior whorls were free of secondary carbonate and detritus.…”

“…Subsequent studies have documented evidence of millennial-scale hydrologic changes in paleowetland systems at multiple localities in the deserts of the American Southwest (Quade, 1986; Quade and Pratt, 1989; Quade et al, 1995, 1998, 2003; Haynes, 2007, 2008; Pigati et al, 2009, 2011). Recent work in the Las Vegas Valley of southern Nevada has shown that desert wetlands are more dynamic than previously observed, having responded to abrupt climate change events on submillennial timescales (Springer et al, 2015, 2018). Here, we show that under the right conditions, the sensitivity of desert wetlands can rival even the most highly resolved proxy systems, as paleowetland deposits at Dove Springs Wash, Mesquite Springs, and Little Dixie Wash in southern California (Fig.…”

“…1), for example, bed E 2a of the Las Vegas Formation consists of 1–2 m of massive, olive-green silts and sands (Fig. 2a) and is interpreted as representing an ancient marsh (Springer et al, 2018). This unit contains charcoal and terrestrial gastropod shells (Succineidae) that previously yielded calibrated 14 C ages between 12.90 and 12.35 ka (Springer et al, 2015), placing it squarely within the YD chronozone.

Figure 2(color online) Paleowetland deposits dating to the Younger Dryas cold event and the associated basal dates of the unit at (a) Las Vegas, Nevada; (b) Valley Wells, California; (c) Piute Valley, California; and (d) Fenner Wash, California.

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