A new strategy for analyzing the chronometry of constructed rock features in deserts

Abstract

The western Great Basin contains thousands of constructed rock features, including rock rings, cairns, and alignments. Unlike subtractive geoglyphs, such as the Nasca Lines of Peru, that remove desert pavement, these surface features alter the location and positioning of cobble-to bouldersized rocks. The chronology of surface rock features has remained unconstrained by numerical ages because no prior chronometric approach has been able to yield age control. We propose a new strategy for studying these features by analyzing anthropogenic modifications to rock coatings, an approach that permits the use of several dating methods, two of which are assessed here: radiocarbon dating of pedogenic carbonate and rock-varnish microlaminations. Initial results from Searles Valley, eastern California, suggest that constructed rock features may be as old as early Holocene and terminal Pleistocene. Archaeological surveys of desert areas would be greatly enhanced if they noted altered positions of rock coatings.

Rock varnish microlamination dating of late Quaternary geomorphic features in the drylands of western USA

“…Although future progress in high-resolution radiometric dating of paleoclimatic records will unavoidably lead to some refinement and modification of the VML age scale used in this study, the generalized late Quaternary varnish layering sequence should basically remain the same and can be used independently as a regional correlation and mapping tool. As demonstrated in this and other studies (Liu and Dorn, 1996;Bell et al, 1998;Friend et al, 2000;Liu, 2003;Cerveny et al, 2006;Liu and Broecker, 2007; also see VML Dating Lab's website at www. vmldatinglab.com for more examples), application of the VML dating will surely open new research avenues in studies of desert geomorphology, geoarchaeology, and neotectonics in the western USA drylands.…”

“…Once radiometrically calibrated and climatically correlated, varnish microstratigraphy can be used as a correlative dating tool to estimate numerical ages of geomorphic features in deserts (Liu and Dorn, 1996;Bell et al, 1998;Friend et al, 2000;Liu, 2003;Douglass et al, 2005;Cerveny et al, 2006;Liu and Broecker, 2007). In the following sections, we present several case studies to demonstrate the potential use of VML dating in studying late Quaternary alluvial fans, desert pavements, and colluvial boulder deposits in the study region.…”

Spatial, temporal and geographic considerations of the problem of rock varnish diagenesis

“…Garvie et al (2008) analyzed only three samples. For these samples, they (1) do not specify the type of rock varnishes they collected, and (2) do not explain whether or not their sample came from a geomorphic position that was formerly a rock fracture (Coudé-Gaussen et al, 1984;Villa et al, 1995;Dorn, 1998;Cerveny et al, 2006).…”

“…Since Garvie et al analyzed samples with "crosscutting Mn-stringers" and "Mn oxides filling cracks", Krinsley et al (1990) and Krinsley (1998: Figure 4) established previously that these stringers form in conduits for water flow and reprecipitation of Mn-Fe oxides -thus disequilibrium through greater intravarnish water flow would be expected. Scenario 3: if Garvie et al (2008) collected varnish that originated in a rock joint, exposed by spalling (Coudé-Gaussen et al, 1984;Villa et al, 1995;Dorn, 1998;Cerveny et al, 2006), then disequilibrium through an adjustment to the subaerial environment would be expected. Liu and colleagues are explicit in that they do not interpret paleoenvironmental changes from varnishes in these contexts Zhou et al, 2000;Liu, 2003;Broecker, 2007, 2008a,b;Liu, 2010).…”

“…The photograph of varnish shown in Garvie et al (2009: Figure A) shows ground-line band varnish, a unique type of varnish recognized by Engel and Sharp (1958); such a sample is completely unsuitable for paleoenvironmental analysis because it is heavily influenced by soil effects. Upon visitation to their sample sites 2 and 3, without knowing the exact spot of sampling, we can only state that both sites have varnishes with a large abundance of microcolonial fungi (Dragovich, 1993) and a high abundance of varnishes that originated in rock fractures (Coudé-Gaussen et al, 1984;Villa et al, 1995;Dorn, 1998;Cerveny et al, 2006).…”