Quaternary stratigraphy and geology of the Rocky Mountain Foothills, southwestern Alberta

Abstract

Surficial geology mapping at a scale of 1:50 000, cosmogenic dating, and detailed stratigraphic and sedimentological logging of 86 cliff-bank exposures and more than 3000 roadcuts and hand-driven auger borings have resolved the century-old controversy as to the number and timing of continental glaciations in the Rocky Mountain Foothills of southwestern Alberta. The stratigraphy shows a consistent succession repeated throughout much of the area: one to several montane tills are always topped by either one continental till or a complex of continental tills, or a gravel unit containing Canadian Shield lithologies where a continental till has been eroded. Exceptions only occur along the Rocky Mountain Front, at the mouths of major valleys where no continental till is present. There is no evidence of subaerial erosion or soil-forming processes that would represent long hiatuses between glacial advances responsible for continental till deposition. The same holds true for montane tills, with the exception of those found in the subsurface of Cloudy Ridge and the sediments capping Mokowan Butte. The drift units underlying paleosols at these two sites clearly predate the last interglaciation. The upper elevation limits of glacial erratics from the Canadian Shield and their abrupt western limit indicate that montane glaciers prevented penetration of continental glacial ice up major Rocky Mountain Foothills valleys, hence continental and montane glaciers coalesced during the culmination of the maximum continental ice advance. Paleomagnetic sampling of the oldest tills infilling buried valley systems shows that all deposition occurred during the Brunhes chron (<0.78 Ma). Cosmogenic 36Cl exposure dating of the foothills erratics train and the former limits of the continental ice-sheet cover constrains the single continental glaciation to the Late Wisconsinan. The area was deglaciated by ca. 11 200 radiocarbon years before present.

Glacial kettles as archives of early human settlement along the Northern Rocky Mountain Front

“…At the peak of the late Pleistocene (Wisconsinan) glaciation, the Laurentide Ice Sheet coalesced with the Cordilleran Ice Sheet and montane and piedmont glaciers (Pinedale glaciation in Montana) over much of the Northern Front, with tracts of unglaciated land separating them on the Porcupine Hills and just south of the Alberta-Montana border (Bobrowsky and Rutter, 1992;Cannon, 1996;Fullerton et al, 2004;Jackson et al, 2008;Dalton et al, 2020). Ice sheets and glaciers left a dramatic mark on the landscape, with deep U-shaped valleys in the mountains and thick glacial deposits on the foothills and the plains (Fenton et al, 2013;Atkinson et al, 2020).…”

Beryllium-10 dating of the Foothills Erratics Train in Alberta, Canada, indicates detachment of the Laurentide Ice Sheet from the Rocky Mountains at ~15 ka

“…2 and 3h and i). The samples lie in between two generations of large ice-marginal meltwater channels oriented in a north–south direction (Jackson et al, 2008; Atkinson et al, 2014a; note that Rains et al [2002] interpreted these channels as subglacial tunnel channels; Fig. 2b).…”

Sedimentary architecture of a glaciolacustrine braidplain delta: proxy evidence of a pre‐Middle Wisconsinan glaciation (Grimshaw gravels, Interior Plains, Canada)

“…Major cities and topographic features are underlain by a digital elevation model (GMTED2010; data available from the U.S. Geological Survey). Numbered localities are: 1 = Charlie Lake map area (Mathews 1978;Hartman 2005;Hartman & Clague 2008); 2 = Dawson Creek map area (Hickin et al 2016); 3 = Watino (Westgate et al 1971(Westgate et al , 1972Liverman et al 1989); 4 = Simonette (Liverman et al 1989); 5 = Buffalo Head Hills (Andriashek & Barendregt 2017); 6 = Athabasca oilsands area (Andriashek 2003); 7 = Sand River map area (Andriashek & Fenton 1989); 8 = Edmonton (Catto 1984;Young et al 1994); 9 = Elk Island National Park (Jenner 1984); 10 = Hand Hills (Younget al 1999); 11 = Dinosaur Provincial Park (Evans & Campbell 1992); 12 = Medicine Hat (Proudfoot 1985); 13 = Taber (Jackson et al 2011);14 = Kipp (Jackson et al 2008, 2011; 15 = Lethbridge (Jackson et al 2008(Jackson et al , 2011; 16 = Brocket (Jackson et al 2008(Jackson et al , 2011. [Colour figure can be viewed at www.boreas.dk] Shaftesbury bedrock valley; Figs 1, 2), indicating that the deposit is pre-Middle Wisconsinan in age.…”