Paleomagnetism and counterclockwise tectonic rotation of the Upper Oligocene Sooke Formation, southern Vancouver Island, British Columbia

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 35° ± 12°. 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 rotations 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 molluscan fauna, and also a number of unique marine mammals found in the same reversed magnetozone in Washington and Oregon.

Discovery of an Active Forearc Fault in an Urban Region: Holocene Rupture on the XEOLXELEK‐Elk Lake Fault, Victoria, British Columbia, Canada

“…In northern Cascadia, the forearc region above the subducting slab experiences both interseismic elastic strain induced by partial coupling of the subduction zone interface (e.g., Dragert & Hyndman, 1995; Miller et al., 2001; Savage & Lisowski, 1991), and permanent strain resulting from subduction zone coupling and far‐field tectonic forces (e.g., Delano et al., 2017; Finley et al., 2019; Nelson et al., 2017; Sherrod et al., 2013; Wang et al., 2003; Wells et al., 1998). Permanent forearc deformation is evidenced by instrumental crustal seismicity (e.g., Balfour et al., 2011; Bostock et al., 2019; Brocher et al., 2017; Savard et al., 2018; G. Li et al., 2018), paleoseismic studies of Quaternary‐active forearc faults (Barrie & Greene, 2018; Bennett et al., 2017; Blais‐Stevens et al., 2011; Blakely et al., 2009; Duckworth et al., 2021; Greene & Barrie, 2022; Harrichhausen et al., 2021; Kelsey et al., 2012; Morell et al., 2017, 2018; Nelson et al., 2017; Personius et al., 2014; Schermer et al., 2021), and paleomagnetic, geologic, and geodetic data that show counterclockwise rotation of the Cascadia forearc north of, and margin‐parallel shortening south of, the Strait of JdF (Finley et al., 2019; Mazzotti et al., 2002, 2003; McCaffrey et al., 2007; Miller et al., 2001; Prothero et al., 2008; Wells & McCaffrey, 2013).…”

“…Permanent forearc deformation is evidenced by instrumental crustal seismicity (e.g., Balfour et al, 2011;Bostock et al, 2019;Brocher et al, 2017;Savard et al, 2018;G. Li et al, 2018), paleoseismic studies of Quaternary-active forearc faults (Barrie & Greene, 2018;Bennett et al, 2017;Blais-Stevens et al, 2011;Blakely et al, 2009;Duckworth et al, 2021;Greene & Barrie, 2022;Harrichhausen et al, 2021;Kelsey et al, 2012;Morell et al, 2017Morell et al, , 2018Nelson et al, 2017;Personius et al, 2014;Schermer et al, 2021), and paleomagnetic, geologic, and geodetic data that show counterclockwise rotation of the Cascadia forearc north of, and margin-parallel shortening south of, the Strait of JdF (Finley et al, 2019;Mazzotti et al, 2002Mazzotti et al, , 2003McCaffrey et al, 2007;Miller et al, 2001;Prothero et al, 2008;Wells & McCaffrey, 2013).…”

A new member of the family Plotopteridae (Aves) from the late Oligocene of British Columbia, Canada

“…Shallow marine facies of the Sooke Formation at southwestern Vancouver Island are correlated with a reversed magnetozone and Chron C6Cr (latest Oligocene ages, 24.1-24.8 Ma) (Prothero et al, 2008). The Sooke Formation rocks are temporally comparable to the Pysht Formation (23.7-30.5 Ma) in western Washington (e.g., Muller et al, 1981;Brandon et al, 1998;Prothero et al, 2001Prothero et al, , 2008 Jewett Sand (24-25 Ma) in California (Scheirer and Magoon, 2007).…”

“…Shallow marine facies of the Sooke Formation at southwestern Vancouver Island are correlated with a reversed magnetozone and Chron C6Cr (latest Oligocene ages, 24.1-24.8 Ma) (Prothero et al, 2008). The Sooke Formation rocks are temporally comparable to the Pysht Formation (23.7-30.5 Ma) in western Washington (e.g., Muller et al, 1981;Brandon et al, 1998;Prothero et al, 2001Prothero et al, , 2008 Jewett Sand (24-25 Ma) in California (Scheirer and Magoon, 2007). These and other Cenozoic sediments were deposited in a fore-arc basin as the Farallon plate (Juan de Fuca plate) subducted beneath the North America plate (e.g., Hyndman et al, 1990;Snavely and Wells, 1996;Brandon et al, 1998).…”

New insights on the most primitive desmostylian from a partial skeleton ofBehemotops(Desmostylia, Mammalia) from Vancouver Island, British Columbia

“…The age of the Sooke Formation at a locality to the southeast of the provenance of this specimen has been estimated to be Chron C6Cr age, 24.1-24.8 Ma, based on recent paleomagnetic studies (Prothero et al, 2008). If this is correct, then the age of this specimen is late Zemorrian (D late Chattian), contemporaneous with other material of Behemotops proteus from the Pysht Formation of Washington State (Domning et al, 1986;Ray et al, 1994) and Oregon (Domning et al, 1986), as well as Behemotops katsuiei and Ashoroa laticosta from Hokkaido, Japan (Saito et al, 1988;Inuzuka, 2000), and Cornwallius sookensis from Unalaska Island, Alaska (Beatty, 2006a;Jacobs et al, 2007), Vancouver Island, British Columbia (Cornwall, 1922;Hay, 1923;Beatty, 2006b), Oregon (Reinhart, 1982;Beatty, 2009), andBaja California Sur, Mexico (VanderHoof, 1942;Applegate, 1986;Beatty et al, 2001).…”