OSL investigations at Hardisty, Alberta, Canada

This report is concerned with optically stimulated luminescence (OSL) dating investigations of sediments associated with, and enclosing artefacts of First Nations historic significance in the Battle River Valley area, near Hardisty, east central Alberta. The OSL ages reported here provide chronological control to the archaeological investigations at this site, led by Rob Wondrasek, which have identified thousands of historical artefacts, including projectile points and lithic fragments indicative of occupation. The investigations were commissioned by Enbridge, ahead of the construction of the Edmonton-Hardisty Pipeline, and form one part of a historic resources impact assessment study, to characterise the archaeological site, and evaluate/mitigate the impact of the pipeline related excavations. This report describes the background to the investigation, sampling, and luminescence analysis undertaken to generate sediment chronologies for the Hardisty sediment stratigraphies. Ken Munyikwa visited the archaeological investigations at Hardisty in June 2014 to sample key stratigraphic units within the sediment stratigraphies for OSL dating. Samples were collected from two profiles: from strata encompassing the artefact-bearing horizon, and from strata immediately beneath and overlying this horizon, thus providing terminus post quem (TPQ) and terminus ante quem (TAQ) on the age of this unit. Samples were submitted to the luminescence laboratories at SUERC for dating in August 2014. All samples were subjected to laboratory preparation of sand-sized quartz, and purity checked by scanning electron microscopy. Dose rates for the bulk sediment were evaluated using analyses of the uranium, thorium and potassium concentrations obtained by high resolution gamma spectrometry coupled with beta dose rate measurement using thick source beta counting. Equivalent doses were determined by OSL from 64 aliquots of quartz per sample using the quartz single-aliquot-regenerative (SAR) procedure. The material exhibited good OSL sensitivity and produced acceptable SAR internal quality control performance. Dose distributions from the aliquots were examined using radial plotting methods. All samples revealed some heterogeneity in their equivalent dose distribution, reflecting variable bleaching at deposition and indicating that each sample enclosed mixed-age materials. Age estimates were based on the weighted mean estimate of the stored dose, which weights the stored dose estimate towards the lowest population of equivalent doses, potentially representing the better bleached (at deposition) component. The quartz OSL ages reported herein for the sand sequences at Hardisty-1 and Hardisty-2, have provided the first means to assess the temporal distribution of artefacts within the Hardisty profiles, and furthermore provide TPQ and TAQ for the inferred occupational phases. The sediment chronologies established for each profile are internally coherent, spanning at HD-01 from 7.8 ± 0.7 ka (SUTL2692) to 11.7 ± 0.5 ka (SUTL2694), and at HD-02 from 4.5 ± 0.2 ka (SUTL2695) to 8.7 ± 0.5 ka (SUTL2697; Table 4-1). TPQ for the occupation of the Hardisty site is provided by SUTL2697 at 8.7 ± 0.5 ka. TAQ for the occupation of the Hardisty site is provided by SUTL2695 at 4.5 ± 0.2 ka