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Wilderness Canoe Tripping
Yukon Territory - North/Black/Liard River Canoe Trip, River Geology
Quick links to other geology related pages: | Mining | Setting the sceneThe North/Black/Liard River trip is in the Omineca morphogeological belt. Of course, the river we chose to paddle this year happens to be in the most complex and varied belt in the whole Territory. The land is described as variably metamorphosed sediments and granites located in large mountain ranges. The area of the belt that interests us was heavily glaciated during the last ice age, which exposed craggy bedrock higher up, and deposited river aluvium and lake sediments in the lower country. Probably the single biggest geological feature of the North/Black/Liard River trip is that, for the Black and Liard River portions, we paddled along the Tintina Fault, a long straight topographical feature going from southeast to northwest. In fact, we probably paddled over, or maybe even stepped on, the fault several times. The North River section isn't exactly on the fault, but it does parallel it only a few miles/km away. The fault happens to be the boundry between the 'old' and 'new' Yukon. The 'old Yukon,' most of the land north and east of the fault, is the original continental land mass, much of which has existed for at least the last one to two billion or more years. It consists mostly of Paleozoic ocean sediments (roughly 250 million to 600 million year old) on top of crystalline granite basement rocks. The 'new Yukon,' southwest of the fault, consists of several mostly newer, smaller, geologically differing land masses called 'terranes' that were added to the North American coastline starting a little less than 200 million years ago. The rock units are older than that, but may not have been part of the North American continent when they were formed. For the most part, these terranes were thought to have been 'rafted' in from various unknown parts of the world on sea floor plates during tectonic crustal movement. The ocean bedding they moved on has disappeared, subducted below the then existing continental shelf, while the terrane was scraped off and accreted, or cemented, to the old, ever seaward extending BC and Alaska coastline.
Some of the RocksThe larger part of the terranes we paddled over and through are found to the southwest of the fault. We spent all of our time paddling along the edge of these terranes. One narrow band of the oldest terrane is attached to the continental (NE) side of the fault, and in that narrow band is where we started our trip. The strata in this terrane are mostly sedimentary, igneous, and metamorphic rocks from 20 to 350 million years old. The terranes were placed between 120 and 190 million years ago. Remember, the rocks in the terranes were formed before attachment to the coastline, and probably jumbled a bit in the process. Very recent geologic findings indicate that parts of the terranes we were in may have just been moved up from farther south in Canada (or the US?), possibly as well as some coming from exotic overseas destinations. This is still unsure as of 1996, the way I read the literature.The Terrane in which the North and Black trip starts is the Yukon-Tanana Terrane, the oldest of those landmasses that moved here from somewhere else. Most of the Yukon-Tanana is southwest of the Tintina Fault and has slid towards and even some into Alaska through Tintina Fault action (a transform fault -- right-lateral, slip-strike). The very small part of the terrane northeast of the fault where we started our trip includes all the North and Black River part of our run, though I'm sure we also saw small pieces of the Slide Mountain Terrane somewhere in there. The Yukon-Tanana Terrane consists largely of quartzites, quartz mica schist and marble at least 400 million years old, some maybe close to a billion years. Most of the Yukon-Tanana rock was originally sediments that have undergone extreme metamorphosis, high heat and pressure conversion. Metomorphosis was variable, of course. Another somewhat younger (Mississipian age - 320-360 million years old) subassembly also includes a lot of carbon which makes some of the rock formations feel graphitic. These are probably most evident in the lower Black River where whole cliffs are of a very dark, black, slippery feeling rock. There are also thin exposures of low grade coal about. The Slide Mountain rock formations are upthrusted old sea floor volcanics mixed with other overlaying marine sediments like chert, limestones, and shales. Another terrane we see is the Cassiar Terrane, on our right as we paddle down the Liard River. The river valley here is referred to as the the Liard Plains. It is so wide and covered with glacial till that we do not see any of the bedrock. It is supposedly granite (felsic granitic plutons), probably slid up from northern British Columbia via transform fault action. There were periods of volcanism about 100 million, and then again about 55 million years ago, which placed new igneous rock in the Tintina trench. The fault is a crack, an easy place for magma to reach the earth's surface. The literature also mentions some small, localized conglomerates that were placed in areas along the fault, but I haven't located any details on them yet. The volcanism caused more metamorphism of surrounding rocks and placed mineral concentrations which are slowly being discovered today. Gold and silver were obviously the primary targets of miners in the past, of which the Klondike gold strike near Dawson is the most famous. That heyday is over, but there is still some limited, but productive mining done in that area. Other, smaller, more recent gold discoveries have been made along the Tintina Fault. Ores of several other metals have also been discovered. The mining towns of Faro and Mayo are on, or close to, the Tintina Fault. A copper, zinc, lead, cobalt, and low grade gold ore 'strike' was made in the headwaters of the North River system in 1995-96. The major site exploration there is being done about 10 km (6 miles) from the fault, and the closest big town for this activity is Watson Lake. It has been known since 1960 that there was potential in the area, but how good it was had not been determined until this time. This strike has reportedly triggered the biggest mine claim staking rush in recent Yukon history (mining is Yukon's largest industry, 30 percent of its economic base). More GeomorphologyAside from possibly an occasional northwestward slip of terranes southwest of the Tintina Fault, by about 20 million years ago, the southern Yukon was geologically pretty stable. The terranes had all docked and the topography had weathered down to mostly smooth, rounded hills and valleys in the plateau regions. The southern Yukon rivers all flowed pretty much south, through gradually decending river valleys, and had no high mountain ranges to cut through. The ground was there, but the St. Elias Mountains had not risen yet. They started uplifting about this time, though, from renewed tectonic action. The plateau area to the north and east rose also, but much slower than the coastal mountains. By 12 million years ago the St. Elias Range was high enough for alpine glaciers to form, and ice was certainly there by 8 million years ago. River systems probably changed a little, but most of the larger rivers kept up with the mountain building activity and managed to maintain their courses. There were some deep canyons carved by rivers in southwest Yukon during this time. This uplift is continuing even today, and still affects the rivers we paddle on. GlaciationAnd then, of course, came the Pliestocene glacial period, which reworked landscape topography significantly. There is evidence for glacial events as old as a billion years, but interpreting data of that age is very difficult. The glaciation that started less than 2 million years ago is somewhat easier, but by no means clear. There were six major events that affected the Yukon in the last 1.5 to 2 million years, warm periods like the one we are now enjoying separated them. Most geologists and climatologists think we are in a short warming period between glacial events and another will be starting soon. Geologically speaking? Maybe not! The ice started as alpine glaciers, and worked from the higher mountains down into the valleys. Since central Yukon was low when the glaciation started, the ice moved north from BC, and the St. Elias and other coastal mountains, not south from the Yukon into BC as is thought by most laymen. In fact, much of northern Yukon remained ice free through most, if not all, of the Pleistocene glacial period -- probably because of lack of precipitation, rather than any warm temperatures. Because the ice flowed north, it gouged the valleys, lowered passes, and changed river courses so that many rivers changed from south flowing to north flowing. Land slides, ice dams, and the weight of the ice depressing the land beneath it contributed in no small part to this effort. Moraines, eskers, glacial till, lake sediments, and loess deposits (wind driven dust from glacier scoured rocks) are common throughout the whole North/Black/Liard run. According to one map, there are even some very recent (1250 year old) volcanic ash deposits in the area, though they will be difficult to find. The North and Black Rivers were at about the fringe of noticeable deposition. These deposits can be easily seen closer to Whitehorse. You can't see the Tintina Fault very easily, if at all, but the fault formed a feature that is easily picked out on most Yukon Maps. Here, you can read about the Tintina Trench. Bibliography: (so far)
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