499 Paleomagnetism and counterclockwise tectonic rotation of the Upper Oligocene Sooke Formation, southern Vancouver Island, British Columbia Donald R. Prothero, Elizabeth Draus, Thomas C. Cockburn, and Elizabeth A. Nesbitt Abstract: The age of the Sooke Formation on the southern coast of Vancouver Island, British Columbia, Canada, has long been controversial. Prior paleomagnetic studies have produced a puzzling counterclockwise tectonic rotation on the underlying Eocene volcanic basement rocks, and no conclusive results on the Sooke Formation itself. We took 21 samples in four sites in the fossiliferous portion of the Sooke Formation west of Sooke Bay from the mouth of Muir Creek to the mouth of Sandcut Creek. After both alternating field (AF) and thermal demagnetization, the Sooke Formation produces a single-component remanence, held largely in magnetite, which is entirely reversed and rotated counterclockwise by 358 ± 128. This is consistent with earlier results and shows that the rotation is real and not due to tectonic tilting, since the Sooke Formation in this region has almost no dip. This rotational signature is also consistent with counterclockwise rota- tions obtained from the northeast tip of the Olympic Peninsula in the Port Townsend volcanics and the Eocene–Oligocene sediments of the Quimper Peninsula. The reversed magnetozone of the Sooke sections sampled is best correlated with Chron C6Cr (24.1–24.8 Ma) or latest Oligocene in age, based on the most recent work on the Liracassis apta Zone mol- luscan fauna, and also a number of unique marine mammals found in the same reversed magnetozone in Washington and Oregon. Re´sume´ : L’aˆge de la Formation de Sooke sur la coˆte sud de l’ıˆle de Vancouver, Colombie-Britannique, Canada, a long- temps e´te´ controverse´. Des e´tudes pale´omagne´tiques ante´rieures ont donne´ une rotation perplexe dans le sens inverse des aiguilles d’une montre sur les roches volcaniques du socle (E´ oce`ne) et aucun re´sultat concluant sur la Formation de Sooke elle-meˆme. Nous avons pre´leve´ 21 e´chantillons de quatre sites dans la portion fossilife`re de la Formation de Sooke a` l’ouest de la baie de Sooke, de l’embouchure du ruisseau Muir a` l’embouchure du ruisseau Sandcut. Apre`s une de´saiman- tation par champs alternatifs (CA) et par traitements thermiques, la Formation de Sooke a produit une aimantation re´ma- nente, surtout dans la magne´tite, qui est comple`tement renverse´e et tourne´e dans le sens contraire des aiguilles d’une montre de 35 8 ±128. Cela concorde avec des re´sultats ante´rieurs et montre que la rotation est re´elle et non pas cause´e par un basculement tectonique, e´tant donne´ que la Formation de Sooke dans cette re´gion n’a pratiquement pas de pendage. Cette signature de rotation concorde aussi avec les rotations dans le sens inverse des aiguilles d’une montre obtenues de l’extre´mite´ nord-est de la pe´ninsule Olympic dans les roches volcaniques de Port Townsend et les se´diments, datant de l’E´ oce`ne–Oligoce`ne, de la pe´ninsule de Quimper. En se basant sur les plus re´cents travaux sur la zone de faune mollusque a` Liracassis apta et d’un certain nombre de mammife`res marins uniques trouve´s dans la meˆme magne´tozone de polarite´ inverse dans les e´tats de Washington et d’Oregon, la magne´tozone de polarite´ inverse des sections e´chantillonne´es de Sooke est le mieux corre´le´ avec la chronozone C6Cr (24,1–24,8 Ma), soit un aˆge Oligoce`ne terminal. [Traduit par la Re´daction] Introduction Vancouver Island have also been called Sooke Formation (Clapp and Cooke 1917; Jeletzky 1973; Cameron 1980). The Sooke Formation is widely exposed along the First described by Richardson (1876–1877), the rich fossil beaches and creeks of southern Vancouver Island west of deposits of the Sooke Formation were originally collected Sooke Bay (Fig. 1) and includes beach and seacliff outcrops in the 1890s, and described by Merriam (1896, 1897, 1899). from southeastern side of Sooke Bay to San Juan Point Clapp and Cooke (1917) made the first detailed maps of the (southwest of Port Renfrew). Outcrops at Carmanah Point region, and Clark and Arnold (1923) conducted the first and on Nootka Island, further north on the western side of complete description of the fossils. On Nootka Island, Received 6 September 2007. Accepted 4 March 2008. Published on the NRC Research Press Web site at cjes.nrc.ca on 29 May 2008. Paper handled by Associate Editor F. Cook. D.R. Prothero1 and E. Draus. Department of Geology, Occidental College, Los Angeles, CA 90041, USA. T.C. Cockburn. 7683 Colin Place, Saanichton, BC V8M 1N6, Canada. E.A. Nesbitt. Burke Museum, Box 353010, University of Washington, Seattle, WA 98195, USA. 1Corresponding author (e-mail: [email protected]). Can. J. Earth Sci. 45: 499–507 (2008) doi:10.1139/E08-012 # 2008 NRC Canada 500 Can. J. Earth Sci. Vol. 45, 2008 Fig. 1. Geology and sample sites for the Sooke Formation on the southern tip of Vancouver Island. Geology based on Geological Survey of Canada Map No.1553A by J. E. Muller. Q, Quaternary; S, Sooke Fm; M1, Sooke Gabbro; M2, Metchosin Volcanics; M3, Catface intru- sions. Sample sites this report, 1, 2, 3, 4; Sooke Fm sample sites of Irving and Massey (1990), S1 and S2. Inset map: Location of main map area. LRF, Leech River Fault; SJF, San Juan Fault. the Sooke outcrops overlie the Hesquiat Formation, and (1923) correlated the Sooke invertebrate faunas with those Cameron (1980) used foraminiferal biostratigraphy to place of the Blakeley Formation, on Bainbridge Island, across these rocks in the late Zemorrian Stage (latest Oligocene). Puget Sound from Seattle, Washington. However, the corre- Muller et al. (1981) considered the foraminiferal assem- lation used by Clark and Arnold (1923) was based on fossils blage from the Sooke Formation to be facies restricted and from the Carmanah Point area of Vancouver Island that is not stratigraphically useful. Narayan et al. (2005) devel- outside of the Sooke formation between Sooke Bay and San oped a benthic foraminiferal biozonation from wells drilled Juan Point. Durham (1944) noted that the Sooke molluscan in the Tofino Basin, offshore southern Vancouver Island. fauna was characteristic of outcrops the Blakeley Formation Although there is no outer neritic to bathyal sediment around Lake Sammamish (east of Seattle). This region is the onshore, this offshore scheme indicate that the Sooke For- inner neritic to bayshore facies of the deeper water type mation belongs in the Rectuvigerina branneri Zone (formally Blakeley Formation on Bainbridge Island and has recently called the Siphogenerina branneri Zone) of Kleinpell et al. been mapped as this formation (Booth et al. in review). In (1980) and Finger (1992), latest Oligocene – earliest Mio- the historic Weaver Commission report (Weaver et al. cene in age. 1944), the Sooke was correlated with the upper Twin Rivers The molluscan fauna of the Sooke Formation is particu- Formation, which was then considered early Miocene. larly diverse, with 28 species of gastropods and 35 species Jeletzky (1973), Muller et al. (1981), and Cockburn et al. of bivalves. In addition, two species of brachipods and bar- (1999) assigned the Sooke Formation in the Nootka Sound nacles; single chiton, sand dollar, and coral species; and a area and between Sooke Bay and San Juan Point to the serpulid worm tube have been described (Clark and Arnold molluscan Liracassis (= Echinophoria) apta Zone of Dur- 1923; Cornwall 1927). Despite its rich faunal assemblage, ham (1944; Moore 1984), which spans the late Oligocene – the age of the Sooke Formation has long been controversial. early Miocene interval between 23 and 28.5 Ma (Prothero Merriam (1896) thought that it was upper Miocene or lower 2001). Pliocene in age. Arnold (1906, 1909) placed it in the upper This wide range of opinions of the age of the Sooke mol- Miocene. Weaver (1912) correlated it with other lower Mio- lusks is largely due to the fact that the assemblage comes cene deposits of Washington State. Arnold and Hannibal from intertidal and shallow-subtidal deposits, with relatively (1913) collected additional fossils and placed it in the few of the mid-neritic mollusks that are used to zone the middle Oligocene. Clapp and Cooke (1917), on the other marine beds of the Pacific Northwest elsewhere. This is par- hand, placed it in the lower Miocene. Clark and Arnold ticularly evident in the abundant rocky intertidal forms, such # 2008 NRC Canada Prothero et al. 501 Table 1. Taxa from the Sooke Formation indicative of intertidal to estuarine habitats (from Clark and Arnold 1923; Cockburn et al. 1999; Burke Museum unpublished data). Intertidal rocky shore fauna Chiton Oligochiton lioplax Limpets Five species from the family Lottidae Key-hole limpets Megathura vancouverensis, Puncturella sp. Turban shell Homalopoma vancouverensis Periwinkles Littorina sookensis Cap-shells Calyptraea vancouverensis, Trochita sookensis Slipper limpets Crepidula sookensis Rock-boring piddocks Zirfaea sp. Mussels Four species of the genus Mytilus Oysters Crassostrea sookensis Barnacles Solidobalanus sookensis Intertidal sandy sediment fauna Surf clams Three species of the family Mactridae Razor clams Solen clallamensis Cockles Clinocardium sookensis, Glycymeris vancouverensis Sand dollar Scutella newcombei Geoduck Panope generosa Soft-shell clam Cryptomya quadrata Brackish water – estuarine fauna Mud snails Cerithiopsis newcombei Goniobasis sookensis Estuarine clam Corbicula sookensis as the chiton, plus a diversity of limpets, algal-grazing only known from the Oligocene–Miocene Clallam Forma- gastropods, rock-boring clams, mussels, oysters and bar- tion of Washington and the upper Oligocene Nye Mudstone nacles (Table 1). In addition the fauna includes a sand dollar near Newport, Oregon (Tedford et al.