Holocene rock varnish microstratigraphy and its chronometric application in the drylands of western USA

Abstract

Analyses of hundreds of rock varnish samples from latest Pleistocene and Holocene geomorphic features in the drylands of western USA reveal a regionally replicable Holocene microlamination sequence. This sequence consists of 12 approximately evenly spaced weak dark layers intercalated with 13 orange/yellow layers. Preliminary radiometric age calibration indicates that six dark layers in the upper portion of the sequence were deposited during the last 6000 yr, diagnostic of the Little Ice Age and late Holocene wet events; five dark layers in the lower portion of the sequence were deposited after the termination of the Pleistocene but slightly before 7000 YBP, indicative of the early Holocene wet events; and one dark layer in the middle portion of the sequence was deposited around 6500 YBP, suggestive of the middle Holocene wet phase. Our age calibration further indicates that the Holocene wet events represented by the dark layers largely correlate in time with the millennial-scale Holocene cooling events in the North Atlantic region. This radiometrically calibrated and climatically correlated Holocene microlamination sequence was then used as a unique correlative dating tool to determine surface exposure ages of geomorphic and geoarchaeological features in western USA deserts. The varnish microlamination (VML) dating of debris flow fan deposits in Death Valley, California, yields minimum ages of 12,500, 12,500–11,100, 11,100, 10,300, 9400, and 2800 YBP for six debris flow fan building events, suggesting that such events were more likely to have occurred during relatively wet periods of the Holocene. The VML dating of a prehistoric grinding stone from Chili of northern New Mexico yields a minimum age of 900–1100 YBP for the abandonment of this occupation site by the Anasazi Indians. The VML dating of a prehistoric flaked stone (a primary core) from Ocotillo, southern California, yields a minimum age of 12,500 YBP for the flaking of this stone artifact, suggesting at least a Paleo-Indian human occupation at Ocotillo during the terminal Pleistocene. These results indicate that, when properly applied, the VML dating technique has the great potential to yield numerical age assignments for surface stone tools, petroglyphs, and geoglyphs of prehistoric age in the drylands of western USA.