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Geology.Gsapubs.Org on January 30, 2012 Downloaded from geology.gsapubs.org on January 30, 2012 Geology Pleistocene reversal of the Fraser River, British Columbia Graham D.M. Andrews, James K. Russell, Sarah R. Brown and Randolph J. Enkin Geology 2012;40;111-114 doi: 10.1130/G32488.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2012 Geological Society of America Downloaded from geology.gsapubs.org on January 30, 2012 P leistocene reversal of the Fraser River, British Columbia Graham D.M. Andrews1,2*, James K. Russell2, Sarah R. Brown1, and Randolph J. Enkin3 1Earth Research Institute, University of California–Santa Barbara, Santa Barbara, California 93106, USA 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada 3Geological Survey of Canada, PO Box 6000, Sidney, British Columbia V8L 4B2, Canada ABSTRACT The Fraser River in British Columbia, Canada, is the longest non-dammed river on the west merging in the interior plateaus region (Fig. 1), coast of North America and supplies 20 × 106 t/yr of sediment to the Pacifi c Ocean. Abundant which composes ~70% of the area of the Fra- geomorphological evidence indicates that the Fraser River reversed its course to southward ser Basin (Fig. 1 inset) at a mean elevation of fl ow in the recent geological past. Investigation of two volcanic dams at Dog Creek demon- ~1100 m above sea level (asl). It then drains strates northward fl ow of the Fraser until at least 1.06 Ma, before reversal and erosion of the southward through the Fraser Canyon before 270-km-long Fraser Canyon. We propose that the submarine Nitinat Fan off the coast of Brit- entering the sea at Vancouver. ish Columbia records the reversal and sudden input of coarse continental-derived sediment Since the Miocene, evolution of the Fraser ca. 0.76 Ma. This study confi rms reversal of the Fraser River and places a fi rm constraint River has been accompanied by regional and on the maximum age of that reversal. Reversal likely followed stream capture in response to global change, including uplift of the Coast enhanced glaciofl uvial erosion and uplift of the Coast Mountains. Mountains (Farley et al., 2001), multiple gla- ciations (Jennings et al., 2007), and enhanced INTRODUCTION 128°W 124°W 120°W Modifi cation of major rivers and their drain- 3000 PEACE Fraser basin outline 56°N age basins is a poorly understood process asso- RIVER 2000 NECHAKO 56°N Major Fraser River tributary ciated with active tectonics, glaciation, and RIVER 1000 Drainage direction climatically infl uenced erosion (Bishop, 1995). Rocky Mountains 3 Elevation (m asl) Fraser Basin present / past (Ma) Such events strongly infl uence paleogeography, 0 0 50 100 sediment budgets, and provenance. In this paper % area above elevation Quesnel Location mentioned in text InteriorInn plateaus we identify and date a volcanic succession in tet Pleistocene intraplate eri volcanoes central British Columbia, Canada, that records or p laa two episodes of damming of the ancestral Fra- tete 0 km 100 auuus ser River; reconstruction of the paleodrainage s leads us to the inevitable conclusion that the 54°N 54°N Fraser River reversed course from a northward NECNNECHAKOECHAKO N to southward fl ow. RIVER Prince George Several studies have used high-precision FRASERFR RIVER Cariboo Mountains R ages of volcanic dams to date Pleistocene fl u- InteriorInn plateausNazNazkoko AS tet vial systems in North America (e.g., Huscroft et ri Cone or ER p al., 2004; Karlstrom et al., 2007). Here we fi rst la QUESNEL te R RIVER au outline geomorphic evidence for reversal of the IV s Fraser River and then describe the lithostratigra- ER CHILCOTIN Wells C 52°N phy and Ar/Ar geochronology of two prerever- RIVER Fig. 2 OL Grey RIVERUMBIA sal volcanic dams. Although the exact causes of 52°N Coast volcanicvoo cac ic 3 field drainage reversal remain unresolved, dating of 13 Dog the succession tightly constrains the maximum Creek 1 age of drainage reversal. Furthermore, we are 9 able to demonstrate geomorphic and sedimen- Mountains 8 tological consequences of reversal in the Fraser Fraser CanyonCanyoCa THOMPSONTHOHO Basin and adjacent Pacifi c Ocean. MPSP ONN RIVER Innterior plateaus Canyon tet 50°N Fraser River rir oro 50°N r The 1375-km-long Fraser River of British yon plp 3 n la Columbia discharges 3475 m /s of water and 20 Vancouver atat eaea 6 us × 10 t/yr of sediment (Swain and Holms, 1985) Ca nada Strait of Georgia from the 233,000 km2 Fraser Basin into the Pacific Ocean Vancouver Hope Island Strait of Georgia and the Pacifi c Ocean (Fig. 1), USA CANCACANADAA ADAA and sustains the largest alluvial delta on the west USAA coast of North America. Currently, the upper 124°W 120°W Fraser River and its many tributaries drain the Figure 1. Physiography of Fraser Basin showing present-day (white arrows) southward fl ow Coast Mountains and Rocky Mountains before of Fraser River through Fraser Canyon to the Strait of Georgia at Vancouver (British Co- lumbia), and northward (black arrows) paleocurrent directions (Read, 1988; Andrews et al., *E-mail: [email protected]. 2011). Insets show location and hypsometric curve of the basin (asl—above sea level). © 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2012; 2012 v. 40; no. 2; p. 111–114; doi:10.1130/G32488.1; 4 fi gures; Data Repository item 2012035. 111 Downloaded from geology.gsapubs.org on January 30, 2012 erosion rates (Shuster et al., 2005; Ehlers et al., deltas separated by the glaciosedimentary Dog 2006). Presumably the origins and history of the Creek Formation (Fig. 2B; Mathews and Rouse, Fraser River were infl uenced by one or more of A 122°W 1986). The dams are asymmetric; subaqueous Chilcotin these processes. 0.812 Ma Paleo-dow lithofacies only occur south of the dam axis. River ±0.0175 N Alkali Creek The lavas are part of the much larger Chilcotin S u EVIDENCE OF DRAINAGE REVERSAL baer 7.2 Ma Group, a basaltic volcanic province that under- nstream i Several elements of the geomorphology of ±0.6 al lavas lies much of the Fraser River basin (Mathews, the Fraser Basin are typical of reversed drain- 1989). ages: (1) regional slopes that are not parallel 1.107 Ma The lower portion of this volcanic succes- 1.05101.0588 MaM ±0.050 Dam axes to the channel, (2) barbed drainage patterns, ±0.013 sion was emplaced onto the ancestral Fraser Paleo-upst (3) bedrock canyons, and (4) elevated terraces HPB delt River valley fl oor at the mouth of the ances- Dog and hanging paleovalleys. These elements are Creek tral Dog Creek tributary (Fig. 2A). Dog Creek described in the following. a was then an ice-free, although recently glaci- 2.79 Ma r The northeastern fl ank of the Coast Moun- ±0.30 eam ated (buried striated surfaces), U-shaped val- tains drains into the Fraser River via the north- ley. The Dog Creek paleovalley is now fi lled Canoe ward-fl owing Chilcotin and eastward-fl owing Creek by an 80-m-thick subaerial lava pile dominated Nechako Rivers (Fig. 1). Northward regional <0.1 MMa by thin (≤5 m thick) pahoehoe and intercalated drainage existed across the southern interior pla- autobreccias. Imbricated stretched vesicles indi- teaus from the Paleocene until at least the Mio- 9.03 Ma cate fl ow along the Dog Creek paleovalley to the cene (Tribe, 2005). Currently, the Fraser River ±0.06 west, into the Fraser River valley. 11.2 Ma fl ows to the south. ±0.4 Mapped vesicle stretching directions diverge A pronounced barbed drainage pattern is within the ancestral Fraser River valley at the formed by the 130° bend of the Fraser River mouth of Dog Creek, indicating north and . (Lay, 1940), where the upper Fraser changes CrC south divergent fl ow of the lava (Fig. 2A), and Bar g Bar Cr from northwestward to southward fl ow north Bii we infer this to be the lava-dam axis. Subaerial FrFraser River of Prince George (Fig. 1) and at the confl uence asa lavas occur northward on the terrace beyond with the Quesnel River (160° bend). Lay (1940) er Alkali Creek (~30 km) where they taper out; used these observations to fi rst propose reversal R stretched vesicles consistently show northward iv of the Fraser River, albeit in the Eocene. eer fl ow. South of Dog Creek, subaerial lavas form a The lower Fraser River is confi ned by the thin (15–20 m), laterally extensive subhorizontal 51 ≤ °N 270-km-long and 1000-m-deep bedrock Fraser sheet extending into Canoe Creek (~15 km), and Canyon between Macalister and Hope (Fig.
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