An Alluvial Surface Chronology Based on Cosmogenic³⁶Cl Dating, Ajo Mountains (Organ Pipe Cactus National Monument), Southern Arizona

Abstract

A chronology of alluvial surfaces on piedmont slopes below the western Ajo Mountains, southern Arizona, has been obtained using cosmogenic 36Cl accumulation and AMS radiocarbon dating. The apparent 36Cl ages of individual boulders range from 520,000 to 13,000 yr, and the 14C ages of organic material in the two young terraces are 2750–2350 and 17,800 cal yr B.P. The sequence of36Cl ages is consistent with the apparent stratigraphic order, but groupings of similar ages for different surfaces appear to result from repeated reworking of older surfaces associated with the deposition of younger ones. The youngest surface gave a distribution of 36Cl ages about 30,000 yr older than the 14C and soil ages; however, this distribution had36Cl ages that overlapped with 36Cl ages from active channels and hillslopes. We attribute the older-than-expected exposure ages of sampled boulders to inheritance of 36Cl while residing near the surface during very slow erosion on the mountain front. Our results show that although cosmogenic nuclide accumulation can help establish chronologies for surfaces in piedmont settings, care must be used in evaluating the effects of complex exposure histories.

Correlation and dating of Quaternary alluvial-fan surfaces using scarp diffusion

“…Squares denote mean value, lines denote one standard deviation from the mean, and bars denote an age range. Using j = 1 m 2 / ky, the photometric method agrees with the Simpson (1991) age estimates while the ground-survey method agrees with Liu et al (1996). Vegetation is thought to be the main discrepancy between the photo and survey methods.…”

“…Simpson (1991) used soil-carbonate morphology to place AJ1 at early to middle Pleistocene (1.8 Ma to 900 ka) and AJ2 at latest Pleistocene to middle Holocene (300 to 50 ka). The cosmogenic 36 Cl study of Liu et al (1996) concluded that AJ1 stabilized at 440 ka and was possibly reactivated between 230 and 330 ka. AJ2 was dated to between 100 and 180 ka.…”

“…A comparison of surface ages on the Ajo Mountain piedmont, AZ, US. Simpson (1991) used soil profile analysis and Liu et al (1996) used 36 Cl dating. Squares denote mean value, lines denote one standard deviation from the mean, and bars denote an age range.…”

“…To tackle the difficult task of fan-surface dating, many methods have been used, including 230 Th/ 234 U analysis of pedogenic carbonate (Ku et al, 1979), qualitative rock-varnish development (Bull, 1991), desert-pavement development (Bull, 1991), 14 C of soil carbonate (Wang et al, 1996), soil-profile analysis (Simpson, 1991), cosmogenic nuclides (Liu et al, 1996), tephrachronology (Reheis et al, 1996), paleomagnetism (Pope, 2000), satellite imagery (Beratan and Anderson, 1998), infrared stimulated luminescence (Clarke, 1994), granulometric studies (Ibbeken et al, 1998), rock weathering (White et al, 1998), and multiparameter relative-age analysis (McFadden et al, 1989). Watchman and Twidale (2002) provide a thorough review of stratigraphic and surface-exposure dating methods and their limitations.…”

Abandonment ages of alluvial landforms in the hyperarid Negev determined by luminescence dating

“…While promising, these methods provide model ages that rely on parameters that are partially known: erosion rates, nuclide inheritance and the complex history of the measured clasts (e.g., Clapp et al, 2000;Hancock et al, 1999;Liu et al, 1996;Matmon et al, 2003;Putkonen and Swanson, 2003;Repka et al, 1997;Watchman and Twidale, 2002).…”