Lithostratigraphy and paleomagnetism of pre-Fraser glacial deposits in south-central British Columbia 1 Olav B. Lian, R.W. Barendregt, and R.J. Enkin Abstract: Lithostratigraphic records spanning considerable parts of the Pleistocene were studied at three sites in south- central British Columbia. A sedimentary succession near Pavilion includes three distinctly different till units. While the surface till can be associated with the last glaciation (the Fraser Glaciation; δ 18 O stage 2), the ages of the two older till units are presently unknown. However, optical dating of outwash silt resting on the oldest till indicates that this outwash unit and all the overlying units are younger than -160 ka. In Big Bar Creek valley, about 50 km north of Pavilion, an aggradational sequence of indurated glaciofluvial sand, gravel, and till is exposed. A silt unit and a sand lens within an overlying till bed near the top of the section have reversed magnetization, indicating deposition prior to 780 ka, probably during the Matuyama chron. The Big Bar Creek sequence also includes glacio(?)fluvial sediments near the base that are normally magnetized, suggesting that they were deposited, at the latest, during the Jaramillo subchron (-1.0 Ma), but probably during the Gauss chron, before 2.6 Ma. Reversely magnetized glacio(?)fluvial gravel and sand also occur along Jesmond Road between the Marble Range and Edge Hills. These units give support for the development of at least one Cordilleran ice sheet in the Early Pleistocene or Late Pliocene. Résumé : Les registres lithostratigraphiques, couvrant des portions considérables de sédiments du Pléistocène, ont été étudiés pour trois localités de la région sud-centrale de la Colombie-Britannique. Une succession sédimentaire exposée près de Pavilion inclut trois unités de till nettement différentes. L’unité supérieure de till peut être associée à la dernière glaciation (la Glaciation de Fraser; δ 18 O stade 2), cependant, les âges des deux autres unités de till plus anciennes ne sont pas encore connus. D’autre part, la datation optique du silt d’épandage fluvio-glaciaire qui repose sur le till le plus ancien révèle que l’unité d’épandage fluvio-glaciaire, ainsi que les unités sus-jacentes, sont plus jeunes que -160 ka. Dans la vallée de Big Bar Creek, à environ 50 km au nord de Pavilion, est exposée une séquence alluvionnaire indurée de sable fluvio-glaciaire, gravier, et till. Une unité de silt et des lentilles de sable dans une couche de till sus-jacente près du sommet de la coupe sont caractérisées par une aimantation inverse, révélant que ce dépôt est antérieur à 780 ka, et qu’il fut formé probablement durant le chron de Matuyama. La séquence de Big Bar Creek expose également près de la base des sédiments glacio(?)fluviatiles, lesquels ont une aimantation normale, ce qui suggère qu’ils ont été déposés au plus tard durant le souschron de Jaramillo (-1,0 Ma), mais peut-être avant 2,6 Ma durant le chron de Gauss. On observe également des dépôts de gravier et de sable glacio(?)fluviatiles, à aimantation inverse, le long du Jesmond Road entre Marble Range et Edge Hills. Ces unités appuient la thèse de l’existence d’au moins une calotte glaciaire cordillérienne durant le Pléistocène précoce ou le Pilocène tardif. [Traduit par la Rédaction] Lian et al. 1370 Introduction The interior of British Columbia was the centre of the last, and probably several earlier, Cordilleran ice sheets. During each glaciation, ice advanced from the Coast and Co- lumbia mountain systems (Fig. 1), depositing thick sedimen- tary sequences within the relatively low-lying parts of the interior. During interglacial–glacial cycles, many basins were repeatedly incised and filled with sediments. However, despite an abundance of natural exposures, the Pleistocene history of the interior of British Columbia is relatively poorly understood. This is in part due to the fact that strati- graphic units are, in most places, laterally discontinuous and (or) are complicated by the ubiquitous presence of natural slumps and slides (e.g., Ryder 1976, 1981). Furthermore, or- ganic material for radiocarbon dating has not been preserved in the semiarid valleys, and it is in these places where the best stratigraphic sequences are exposed. On the relatively moist plateaux, fossil organic material is commonly covered with younger deposits (e.g., Clague et al. 1990) or is too old to be dated by radiocarbon analysis. The geoclimatic and stratigraphic units that have been defined for the interior of British Columbia are shown in Fig. 2. In this paper we present lithostratigraphic and chronologi- cal information from three previously unstudied sections lo- cated in south-central British Columbia between Pavilion Can. J. Earth Sci. 36: 1357–1370 (1999) © 1999 NRC Canada 1357 Received March 15, 1998. Accepted March 11, 1999. O.B. Lian. 2 Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada. R.W. Barendregt. Department of Geography, The University of Lethbridge, Lethbridge, AB T1K 3M4, Canada. R.J. Enkin. Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC V8L 4B2, Canada. 1 Geological Survey of Canada Contribution 1999172. 2 Corresponding author. Present address: Department of Physics, Simon Fraser University, Burnaby, BC V5A 1S6, Canada (e-mail: olian@sfu.ca).