DOI: 10.5066/p14tggx4
title: Cosmogenic Nuclide Burial Isochron Data for the Central City Gravel, Malta Gravel, and Lava Creek B Ash Locations: Classic Locations of Early to Middle Pleistocene Deposits, South Platte and Arkansas Rivers, Central Colorado, USA

description: This data release includes Al-26/Be-10 cosmogenic nuclide concentrations and burial isochron ages for three locations in central Colorado, USA, with preferred burial ages ranging from ~2 Ma to 600 ka, that provide constraints on geologic mapping and chronostratigraphic correlations of Pleistocene Colorado Rocky Mountain (CRM)-Great Plains deposits. This new dataset helps to validate original identification and characterization of the deposits by renowned work of early to late 20th century geologists. In addition, this dataset is critical for any models of geomorphic development of modern Colorado Rocky Mountain (CRM)-Great Plains depositional systems, including analyses of regional relationships involving the Malta Gravel, Nussbaum Alluvium, Rocky Flats Alluvium, Verdos Alluvium, and Slocum Alluvium (Scott, 1960; Tweto, 1961; Epis and others, 1976, 1980). Paired with cosmogenic analyses for the Nussbaum Alluvium at Baculite Mesa (1.30 ± 0.08 Ma) (Odom and others, 2024) and a glacial advance of the Laurentide icesheet of Balco and Rovey (2010) at 1.31 ± 0.13 Ma, this dataset provides the chronologic constraints for onset of Pleistocene Rocky Mountain river incision and Great Plains river entrenchment for the South Platte and Arkansas Rivers, and the establishment of the current river positions. Three classic localities of previously mapped and described deposits were sampled in the field for cosmogenic nuclide analysis.

A summary of the characteristics of these sites is as follows:
(1) The Central City gravel is a 160 meter-thick (525 feet), matrix-supported, pebble to boulder gravel with a silty sand matrix, nonstratified to crude stratification, and no distinct clast imbrication (Kellogg and others, 2008) lying within the CRM Front Range foothills along Clear Creek at an elevation of approximately 2,430 m/7,960 ft asl. It was originally identified as the first Pleistocene glacial deposit emanating from ice caps developed on the Continental Divide across the CRM Front Range (Spurr and others, 1908; Atwood and Atwood, 1938). More recent work has placed this deposit as a Neogene/Miocene gravel associated with the Ogalalla Formation on the Great Plains (Kellogg and others, 2008), and disregarded its glacial origin and designation as till by the initial investigators. Seven samples of cobbles and clayey, silty sand matrix were measured for an Al-26/Be-10 burial isochron. Two samples, 17-CO-047-CEN and 18-CO-002-CEN, featured depressed Al-26/Be-10 ratios indicative of prior burial and were excluded from further calculations. The final burial isochron for this site yielded a burial age of 1.44 ± 0.26 Ma (1σ analytical uncertainty).

(2) The Pliocene-early Pleistocene, Malta Gravel (Tweto, 1961; McCalpin and others, 2012; Ruleman and others, 2018) is a reddish-brown to yellow-red, angular to subangular, pebble to cobble gravel with a silty sand matrix exposed in a railroad cut ~5 km/3.1 mi west of Leadville, Colorado at the headwaters of the Arkansas River (McCalpin and others, 2012). It was originally defined as an early-middle Pleistocene glaciofluvial deposit (Tweto, 1961), but was more recently identified as non-glacial in origin and possibly within the uppermost section of the Neogene Dry Union Formation (McCalpin and others, 2012; Kellogg and others, 2017). Twelve samples of sand and cobbles were collected for analyses approximately 5 meters (16.4 feet) below the deposit surface and below a heavily oxidized pedogenic horizon overlain by pre-Bull Lake glaciofluvial deposits of early or middle Pleistocene age. One sample, 17-CO-039-LV-Ac, featured a depressed Al-26/Be-10 ratio and was excluded from further calculations. The final burial isochron for this site yielded a burial age of 1.97 ± 0.04 Ma (1σ analytical uncertainty). 

(3) Lava Creek B ash location in the central upper Arkansas River valley, north of Buena Vista, CO is a heavily oxidized, fluvially reworked tephra within a faulted and fractured outcrop of clayey, silty sand deposits, overlain by pre-Bull Lake glacial deposits of middle Pleistocene age (Scott, 1975; Kellogg and others, 2017). The ash lies within the uppermost Dry Union Formation (Kellogg and others, 2013). Probable seismogenic sand dikes are pervasive throughout the exposure. Twelve samples for analyses were collected approximately 9 meters (29.5 feet) below the surface and 40 centimeters (15.8 inches below the basal Lava Creek B ash layer within a silty sand and pebble gravel. One sample, 17-CO-032-BV-A, featured a depressed Al-26/Be-10 ratio indicative of prior burial and was excluded from further calculations. All other samples had nearly identical mean Be-10 concentrations clustering between 601,000-651,000 atoms/gram (at/g) and Al-26 concentrations clustering between 2,913,000-3,429,000 at/g. These overlapping concentrations precluded a burial isochron calculation and necessitated minimum/maximum burial dating. The mean minimum burial age was ~570 ka and mean maximum burial age was ~690 ka, overlapping with the preferred sanidine 40Ar/39Ar isochron date of 631.3 ± 4.3 ka for the Lava Creek B ash (Matthews and others, 2015).

These new ages provide a middle Pleistocene age for the minimum age of the Dry Union Formation, and chronostratigraphic correlation to the uppermost Dry Union Formation with the uppermost Santa Fe Group within the San Luis Valley (Ruleman and others, 2021).

References:

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