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1 a Geologic and Oceanographic Journey Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 A GEOLOGIC AND OCEANOGRAPHIC JOURNEY ACROSS THE SALISH SEA (SOUTHWESTERN CANADA AND NORTHWESTERN USA) Alice S. Chang and Arun Kumar The Salish Sea is also an active waterway for fisheries, industry, transportation, and recreation….. The year 2020 marks the tenth anniversary of the official naming of the Salish Sea….. The rocks of Wrangellia are mainly represented by the Karmutsen Formation, a 6,000- meter thick pile of flood basalts (flows and pillow lavas) and volcanic tuffs that were part of an oceanic plateau some 230 million years ago (late Triassic)….. Several hypotheses have been proposed to explain the origin of the Georgia Depression. These include Cretaceous–Tertiary (66 million-year-old) downwarping or down faulting of the crust, eastward tilting of the Vancouver Island block, and crustal weakening between the Insular and Coast belts. The Strait of Georgia, the Strait of Juan de Fuca, Puget Sound, and the surrounding waterways of southwestern British Columbia (BC), Canada, and northwestern Washington State, USA, are collectively called the Salish Sea (Figure 1). As one of the world’s most productive ecosystems, the Salish Sea is also an active waterway for fisheries, industry, transportation, and recreation. Scenic port cities (Vancouver and Seattle) and provincial and state capitals (Victoria and Olympia, respectively) call the shores of the Salish Sea home, as do 70% of BC’s and 60% of Washington’s populations. We decided to write this popular science article because as earth scientists, we have a long association with this region through recreation, as well as research at the universities of BC (Vancouver) and Victoria (Vancouver Island), and at Carleton University (Ottawa, Canada), where one or both of us were involved. We have crossed the Salish Sea many times, whether aboard passenger ferries (e.g., BC Ferries, MV Coho, Victoria Clipper) or research vessels (e.g., Canadian Coast Guard Ship (CCGS) John P. Tully, CCGS Vector, MV Strickland), and have gained an appreciation and respect for the natural beauty of this area. With this article, we also hope that readers will learn more about the Salish Sea as a cohesive geographical region. The year 2020 marks the tenth anniversary of the official naming of the Salish Sea. Yet in May 2019, the Sea Doc Society and Oregon State University surveyed 2,405 participants and found that only 9% of Washingtonians and 15% of British Columbians could identify the Salish Sea when shown a map of its boundaries (Trimbach and Gaydos, 2019). While 1 Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 scientists have accepted the new concept, these results show that further steps are required to educate the general public about the shared histories—both cultural and natural—of this place. We offer this article as a small step toward familiarizing our audience with the Salish Sea through the lens of geology and oceanography. We first discuss how the Salish Sea name came to be. Then we describe the geological processes, from plate tectonics to ice ages, that shaped the land and modern waterways. Throughout the article, we use our ferry trips across the Salish Sea as platforms for observation, and also describe excursions to points of interest on land. A BRIEF HISTORY OF NAMES The Strait of Juan de Fuca was named after a Greek pilot, Apostolos Valerianos—better known as Juan de Fuca—who sailed with the Spanish navy and was the first European to enter the strait in 1592. The Strait of Georgia was incorrectly named “Gulf of Georgia” in 1792 by Captain George Vancouver for King George III of England, even though the strait was named a year earlier by the Spanish naval officer Francisco de Eliza. The Spanish name, “Gran Canal de Neustra Señora del Rosario la Marinera,” did not stick. What did stick was the name “Gulf Islands” in the Strait of Georgia from when the strait was called a gulf. Also in 1792, Captain Vancouver entered Puget Sound and named it after one of his lieutenants, Peter Puget. The name “Salish Sea” was first proposed in 1989 by Bert Webber, a Canadian-born professor of marine biology at Western Washington University (Bellingham). He recognized the shared oceanography and ecology of these waterways and proposed the name to honour the Coast Salish peoples who have lived in the area for millennia. In 2008, the Stz’uminus (Chemainus) First Nations proposed to rename the Strait of Georgia as the Salish Sea and the idea gained momentum. By 2009, renaming the three waterways collectively as the Salish Sea was endorsed by the government of British Columbia, the Geographic Names Board of Canada, the Washington State Geographical Names Board, and the US Board on Geographic Names. Although the name was officially adopted in 2010, the original names of the individual waterways are still retained. GEOLOGIC HISTORY In order to understand the natural history of the Salish Sea, we must first step back and take a look at the geology of the larger picture. Exotic Lands The Canadian Cordillera can be divided into five northwest to southeast trending zones based on bedrock geology. The two westernmost zones, the Insular and Coast belts, are of interest in this article (Figure 2, inset). 2 Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 Figure 1. The Salish Sea, with location names and ferry routes discussed in this article. Inset map shows the entire Salish Sea boundary; yellow box shows the location of the larger image. Satellite images courtesy of Microsoft. 3 Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 British Columbia and Washington State are mostly composed of distinct strips of the earth’s crust, called terranes, each with its own geological history (see reviews in Johnston, 2008; Gibson and Monger, 2014; Hildebrand, 2015). These terranes, composed of either volcanic island chains (island arcs), or metamorphosed rocks, or pieces of sea floor with ancient tropical fossils, were formed far away from where they would eventually end up. Starting in the mid Jurassic (175 million years ago), through the processes of plate tectonics and continental margin subduction, the terranes began accreting one after another to the western edge of Laurentia—the proto-North American continent—which moved westward following the earlier break-up of the supercontinent Pangaea. Around this time, the Cascadia Subduction Zone was born: extending offshore from northern Vancouver Island to northern California, the ancient Farallon plate and today’s Juan de Fuca plate (both oceanic) plunge beneath the continental North American plate. Melting of the subducting plates is responsible for forming the Coast Plutonic Complex, which is the largest granite outcrop in North America, and related volcanoes (e.g., Mt. Baker) (Figure 2). Crustal faulting and large earthquakes also result from subduction. By Cretaceous time (100 million years ago), the accretion of major terranes to western Laurentia was complete, although accretion of minor terranes continued into the Eocene epoch (40 million years ago). Table 1. Physiography and Oceanography of the Main Waterways in the Salish Sea. Strait of Georgia Strait of Juan de Fuca Puget Sound Physiography Length (km) 222 160 161 Width (km) 28 – 58 18 – 40 16 (maximum) Surface area (km2) 6,800 4,068 2,632 Mean water depth (m) 156 63 Max. water depth (m) 447 (south of Texada Is.) 250 (mouth of strait) 286 (Central Basin) Oceanography Flow type estuarine estuarine estuarine Min. surface salinity 15 (freshet) – 29.5 (spring) 26 (spring, east) 27 (summer, South Basin) Max. bottom salinity 31 (winter) 33.5 (winter, west) 31 (winter, Admiralty) Tides semidurnal diurnal/semidurnal semidurnal Tidal range (m) 2.3 – 3.35 1.85 – 2.45 2.4 – 4.6 Max. current speed (cm/s) >50 (southern) 250 (Port Angeles) 340 (Deception Pass) Units: km: kilometers, km2: square kilometers, m: meters, cm/s: centimeters per second Sources: Gustafson et al. (2000), Holbrook et al. (1980), Thomson (1981) 4 Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 Figure 2. Geologic terranes and units discussed in this article. Inset map shows the main geological belts. Unshaded map areas represent geologic units not discussed in this article. CPC: Coast Plutonic Complex. Based on Brown et al. (2007), Gibson and Monger (2014), Wells et al. (2014), Wheeler et al. (1991), and Yorath and Nasmith (1995). Satellite images courtesy of Microsoft. 5 Open Access e-Journal Earth Science India- www.earthscienceindia.info Popular Issue, 12 (IV), October, 2019, 1-31 Main Terranes and Geologic Units We observed several terranes and geological units during our travels of the Salish Sea. Wrangellia (part of the Insular Superterrane) makes up most of the bedrock for Vancouver Island (Figure 2), and extends north to parts of the Coast Mountains of southwestern BC, the islands of Haida Gwaii (formerly called the Queen Charlotte Islands), and southeastern Alaska (Wrangell Mountains). The bedrock in the Crescent Terrane (part of the larger Siletzia Terrane) is found on the southern tip of Vancouver Island, and on the Olympic Peninsula and around Puget Sound in Washington State (Figure 2). Sandwiched between Wrangellia and the Crescent Terrane on Vancouver Island is the Pacific Rim Terrane, bounded by two north-dipping faults. The rocks of Wrangellia are mainly represented by the Karmutsen Formation, a 6,000- meter thick pile of flood basalts (flows and pillow lavas) and volcanic tuffs that were part of an oceanic plateau some 230 million years ago (late Triassic).
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