The Sisters Creek Formation: Pleistocene sediments representing a nonglacial interval in southwestern British Columbia at about 18 kal Stephen R. Hicock and Olav B. Lian Abstract: Sisters Creek Formation is formally defined, stratotypes are established for it, and the time-space chart is updated for the Fraser Lowland, southwestern British Columbia. The Sisters Creek is a Pleistocene formation comprising in situ and reworked organic-rich sediments, and nonorganic silt, sand, and gravel. The formation was deposited during the Port Moody interstade (within the Late Wisconsinan Fraser Glaciation; 1 3 ~ ~ 0 stage 2) between the Coquitlam stade (early Fraser Glaciation) and the main Vashon stadial maximum that occurred about 14.5 ka. The Sisters Creek Formation represents a glacial recession in southwestern British Columbia that generally coincided with the timing of the last global glacial maximum. The new time-space chart implies that, in Fraser Lowland, the Fraser Glaciation represents the rapid advances and retreats of glacial lobes issuing from surrounding mountains, which remained ice-covered during interstades. Resum6 : La Formation de Sisters Creek est dtfinie formellement, ses stratotypes sont ktablis et le diagramme temps-espace est mis 2 jour pur les basses terres du Fraser, dans le sud-ouest de la Colombie-Britannique. Le Sisters Creek est une formation plCistockne, incluant des sdiments remanits riches en matiere organique dkposb in situ, et du gravier, sable et silt non organiques. Les matkriaux de cette formation ont t t t dtposks durant l'interstade de Port Moody (2 l'intkrieur de la Glaciation Fraser wisconsinieme tardif; ttage 2 6180), entre le stade de Coquitlam (Glaciation Fraser prkoce) et le stadiaire culminant principal de Vashon qui est apparu vers 14,5 ka. La Formation de Sisters Creek reprksente une phase de retrait glaciaire dans le sud-ouest de la Colombie-Britannique, laquelle co'incidait en gknkral avec l'kpisode du dernier maximum glaciaire global. Le nouveau diagramme temps-espace implique que, dans les basses terres de Fraser, la Glaciation de Fraser represente des rtavanckes et des retraits rapides de lobes glaciaires issus des montagnes avoisinantes qui demeuraient recouvertes par les glaces durant les interstades. [Traduit par la rhction] Introduction Fifteen years ago, a seemingly short interval of glacial retreat from the western Fraser Lowland (Fig. la) was recognized that coincided with the last global glacial maxi- mum, considered to be about 18 ka (Clague et al. 1980). This retreat represents a lag before the Late Wisconsinan maxi- mum that occurred about 14.5 ka on the west coast (Hicock et al. 1982a, 1982b), and was informally named the Port Moody interstade by Hicock and Armstrong (1985, p. 755). In Received October 15, 1994. Accepted February 2, 1995. S.R. HicockZ and O.B. liar^.^ Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada. ' Dedicated to the memory of Dr. John E. (Jack) Armstrong, who built a solid foundation for modern Quaternary studies in southwestern British Columbia. Corresponding author (e-mail: shicock@julian.uwo.ca). Present address: Department of Geography, Simon Fraser University, Burnaby, BC V5A 1S6, Canada (e-mail: olian@sfu.ca) . the 1980's it was known to have occurred in the Coquitlam - Port Moody area (about 50 km2). In the 1990's organic beds of the same age have been discovered in mountain valleys 10 km (Lian 1991; Lian and Hickin 1993) and 20 km (this paper) farther west of Port Moody, indicating that the Port Moody interstade was a regional event in the Fraser Lowland. Consequently, the time has come to define and for- malize sediments deposited during the Port Moody interstade as a new lithostratigraphic unit in the Fraser Lowland: the Sisters Creek Formation. Sisters Creek Formation is not to be confused with the obsolete name "Sisters varved clay" of Armstrong (1956, p. 6), which he later equated to pre- Wisconsinan Westlynn drift (Armstrong 1984, p. 8). The position of the Sisters Creek Formation within the lithostratigraphic framework of southwestern British Colum- bia is shown in Fig. 2. Deposits of the Sisters Creek Forma- tion were formerly included within post-Coquitlarn Quadra I Sand at Port Moody and Mary Hill (Hicock and Armstrong 198 I), and within Vashon advance glaciolacustrine sedi- I ments in Seymour valley (Lian 1991 ; Lian and Hickin 1993). In this paper, we define the formation, establish stratotypes for it, present new evidence for and summarize the paleo- Can. J. Earth Sci. 32: 758-767 (1995). Printed in Canada / Imprimi au Canada