COSMOGENIC DATING OF THE FOOTHILLS ERRATICS TRAIN

The chlorine-36 method for surface exposure dating relies on the accumulation of the isotope chlorine-36 produced by reactions of cosmic rays with the nuclei of K, Ca, and Cl atoms. It is one of several methods based on the accumulation of cosmogenic nuclides. The surface one to two metres of rock shield underlying rock from most types of cosmic radiation. When rock is plucked by a glacier from below this depth range and subsequently deposited on the surface, it begins to accumulate chlorine-36 produced by the cosmic radiation.

The rate of chlorine-36 production by cosmic rays (thermal and fast neutrons) depends on the concentrations of potassium (K), Calcium (Ca) and chlorine (Cl), the elevation of the rock, surface orientation, and geomagnetic latitude. The rate of accumulation of chlorine-36 depends on the balance between the production rate of chlorine-36 and on the rate of erosion of the rock surface. The surface erosion rate is usually poorly constrained; thus an apparent exposure age is usually calculated for a plausible range of erosion rates. The systematics of chlorine-36 production and its application to surface exposure dating are described by Zedra and Phillips (2000).

The exposure history of the erratics is generally well constrained: the large size and thickness (many >2 m) of these blocks make it unlikely that the present surface of an erratic was exposed to cosmic radiation prior to falling from a mountain side onto passing ice. If one side of a block had been exposed in the cliff face, then the chances range between about 0.17 and 0.50 (for blocky and equant erratics and tabular erratics, respectively) that the pre-exposed side would be the present upper surface. As is indicated in Table 1, some erratics did receive some pre-exposure but the resulting cosmogenic ages significantly post date the penultimate glaciation of the region. Their large heights and the generally low relief and precipitation of the windy and treeless eastern Foothills also make it unlikely that they were initially buried and subsequently exhumed and minimize the degree to which snow or vegetation has shielded the erratics from cosmic rays. The low relief and gentle slopes of the region also make it unlikely that the erratic boulders have rolled over, except at the time of initial emplacement as ice melted beneath them. The quartzite that composes the erratics is extremely tough and resistant with little evidence of spalling along bedding planes. An erosion rate of less that 1 mm/1000 years is a reasonable assumption. Other variables that cannot be known in detail are the amount and over what period that the erratic changed its orientation with relation to the sky due to compaction of underlying sediments following deposition and the amount of exposure that the sampled surface received during transport from the Rocky Mountains to the site of deposition. The latter is considered to be minor—measured in hundreds of years. Glacial flow of 1 km/year would transport an individual erratic the 600 km length of the erratics train in 600 years.

FIELD AND LABORATORY METHODS

Samples were collected from nine erratics over a distance of about 200 km (Table 1). Samples were taken from upper 4 cm of the rock as fragments from hammer and chisel, from thin beds that were pried loose, or closely spaced diamond drill cores. Laboratory methods followed those described by Zedra and Phillips (2000). In brief, the samples were dissolved in a mixture of HF and HNO3 and the Cl was extracted as AgCl. Isotopically enriched 35Cl was added during dissolution and isotope dilution mass spectrometry (during the accelerator mass spectrometric analysis of the chlorine-36/Cl ratio) was used to determine the Cl content. Complete major element analyses and analyses of B, Gd, U, and Th were used to compute thermal neutron profiles and chlorine-36 production from radiogenic neutrons which are produced from radioactive decay within the rock. The chlorine-36/Cl ratio was measured by accelerator mass spectrometry at PRIME Lab, Purdue University. The rate of production of chlorine-36 over time is as a function of latitude and elevation were scaled according to Table 2 in Lal (1991). Specific details can be found in Jackson et al. (1997). Results are presented in Table 1. For each sample the apparent age was calculated as a function of erosion rate for rock erosion rates varying from 0 to 5 mm per thousand years. Uncertainties related to analytical factors and parameter values are probably on the order of 10%.

RESULTS

Table 1 below gives the results of cosmogenic dating of eight of the Foothills erratics.

Two are from a single very large erratic (B) and these are statistically distinguishable because their error values overlap. Six of the ages clearly fall within the age range of the Late Wisconsinan Glaciation (ca. 25 000 to 10 000 years ago).

TABLE 1 Cosmogenic chlorine-36 ages of Foothills Erratics
Erratic designation	Latitude Longitude	Elevation (m)	0 erosion age	5mm/1000 year erosion age
H	49° 57’ 20” 113° 50’ 10”	1280	15 800±400	15 500±400
G	49 ° 00’ 52” 112° 02’ 50”	1059	30 300±1160	26 300±930*
E	50° 31’ 55” 114 °08’ 18”	1198	13 500±500	12 400±450
D	50° 31’ 55” 114° 08’ 18”	1198	14 200±400	14 200±600
C	49° 26’ 12” 113° 25’ 38”	1041	12 000±600	11 000±600
B-1	49° 24’ 53” 113° 27’ 00”	1076	17 600±450	16 000±390
B-2			14 200±430	13 200±380
A	49° 22’ 52” 113° 19’ 25”	1082	53 300±1500	47 200±1400

*3.3mm/ky (from work completed in 2003)

The range in ages likely reflect events during the history of each rock that could affect its exposure to cosmic radiation such as rotation of the rock due to settlement which could have changed its orientation with the sky. The two erratics that yielded ages in excess of the Late Wisconsinan have ages that fall within a period during which there was no glaciation known as the Middle Wisconsinan. These ages likely reflect previous exposure of the rock to cosmic radiation. The Foothills erratics originated as one or more rock avalanches on to a valley glacier in the Rocky Mountains. These rocks were likely cliff faces or at shallow enough depths before cliff collapse that they received some cosmic ray exposure that caused these ages to predate the last glaciation. The penultimate glaciation of the Interior Plains, the Illinoian glaciation predates the last interglaciation ca. 130 000 years ago which predates the age of the oldest erratic by about 80 000 years. The deposition of these erratics clearly occurred during the last glaciation of the region.

Learn more about cosmogenic dating from PRIME Lab at Purdue University

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