Optically stimulated luminescence ages from the Lake Agassiz basin in Manitoba

Abstract

Geomorphic analysis and optically stimulated luminescence (OSL) ages from undated Lake Agassiz beaches and adjacent fluvial sediments on Riding Mountain in Manitoba provide insight into their early history. New OSL ages of 14.5±2.4 and 13.4±0.7 ka on the oldest (Herman to Norcross) beaches of Lake Agassiz near the Canada-U.S. border indicate that the Laurentide Ice Sheet (LIS) retreated from that part of the Agassiz basin by ~14.5 ka. To the north along Riding Mountain, the Herman strandlines are absent, and OSL ages on the oldest beach there average 12.9 ka, which links it to the younger Norcross-Tintah strandlines. In adjacent Riding Mountain, OSL ages and geomorphological relationships of a large abandoned glacial spillway >200 m above the oldest beaches of Lake Agassiz indicate that this channel predates retreat of the LIS and formation of beaches in this part of the Agassiz basin, with ice remaining in this area until after 14.5 ka. OSL ages on the Gimli beach 170 km to the east are >3000 yr older than conventional assignments, suggesting that it formed during the Moorhead low-water phase 12.8–10.6 ka. Luminescence ages support the conclusion that the Campbell beach formed ~10.9 ka near the end of the Moorhead low-water phase.

Age of Gimli beach of Lake Agassiz based on new OSL dating

“…In addition, and possibly supporting the partial bleaching age interpretation, OSL ages provided to us by Goble on Campbell beach in this region were centuries older than the age established by radiocarbon and OSL dating elsewhere in the basin (e.g. Lepper et al, 2013;Teller et al, 2018a). Data presented in Teller et al (2018a) and in our study support the suggestion that there is some partial bleaching component, although the overdispersion of D E values is rather moderate; the 'leading age model' in Table 3 accounts for partial bleaching issues.…”

“…Another interpretation for the anomalously old OSL ages in the Gimli beach ridge is that they may be related to incomplete bleaching of the quartz sand. As noted by Teller et al (2018a), the bedrock source of quartz in the Gimli beach region is different from that of OSL-dated Lake Agassiz beaches along the western side of the basin, where ages fit reasonably well with the established history of Lake Agassiz. Furthermore, an OSL age on sands in the wave-washed zone of the modern beach of Lake Winnipeg, only a few kilometers to the east, was reported by Teller et al (2018a) as 1.3 ± 0.1 ka; even their leading-edge model gave an age of 0.7 ± 0.1 ka (Table 2).…”

“…As noted by Teller et al (2018a), the bedrock source of quartz in the Gimli beach region is different from that of OSL-dated Lake Agassiz beaches along the western side of the basin, where ages fit reasonably well with the established history of Lake Agassiz. Furthermore, an OSL age on sands in the wave-washed zone of the modern beach of Lake Winnipeg, only a few kilometers to the east, was reported by Teller et al (2018a) as 1.3 ± 0.1 ka; even their leading-edge model gave an age of 0.7 ± 0.1 ka (Table 2). Interestingly, another (unpublished) anomalously old OSL age from Burnside beach on the eastern side of the Agassiz basin (49.4665°N, 96.7882°W), which lies between the 10.6-ka Campbell beach and Gimli beach (Table 1), gave an anomalously old age of 11.7 ± 0.4 ka (R. Goble, University of Nebraska, pers.…”

“…As part of another project, Teller et al (2018a) attempted to date Gimli beach in Manitoba by OSL, and two sites were chosen in the ridge (GB-A and GB-B, Fig. 1).…”

“…As presented in Table 2, their OSL ages were 13.4 ± 1.2 ka [incorrectly stated in their text as 13.4 ± 0.7 ka] and 11.9 ± 1.0 ka [incorrectly stated in their text and in the site GB-B description as 12.1 ± 1.0 ka], both considerably older than the modeled age of 8.7 ka by Teller and Leverington (2004) or the 8.9-9.3 ka age estimates of others. Teller et al (2018a) chose to use 'mean' equivalent dose (D E ) for the ages (see their table 1). Table 1.…”