Physiography and Classification

Physiography (mapping/description of natural features of the landscape) and Classification

Vuntut National Park and the Old Crow Flats Special Management Area are located at the northern end of the Cordilleran Orogen geological region (orogen refers to mountain building, cordillera is a Spanish word meaning a series of mountain ridges). The Cordilleran region is the mountain chain that extends all the way from South and Central America, north through Mexico, the U.S.A. and British Columbia, into the Yukon where, in Ivvavik National Park, it meets the Arctic Continental Shelf geological region on the arctic coastal plain (Gabrielse and Yorath 1992). The Cordillera continues westward through Alaska and into northeastern Russia, forming part of the circum-Pacific mountain system.

The Canadian Cordillera includes four main landscape elements: mountains, plateaus, lowlands, and valleys. Local physiographic units (physiography is the science of mapping and description of natural features) consisting of one or more of these landscape elements are grouped into three major systems that extend the length of British Columbia and the Yukon: the Western System, Interior System, and Eastern System (Clague 1989). The Western System includes the great mountain ranges of southwestern Yukon and western B.C., coastal lowlands, valleys and fiords between the mountains, and the continental seafloor of B.C. The Interior System is dominated by the large plateaus of interior B.C. and Yukon, and the bordering mountain ranges. The Eastern System is a region of rugged mountains, plateaus, lowlands, and valleys bordering the Interior Plains on the east. Its main elements are the Arctic, Mackenzie, Franklin, and Rocky mountains, and the Peel Plateau. The Old Crow Basin links the Eastern and Interior Systems of the Cordillera in the northern Yukon and is found within the Arctic Mountains physiographic unit.

The Arctic Mountains composite physiographic unit in the northern Yukon is broken down into eight physiographic regions; British Mountains, Barn Mountains, Richardson Mountains, Dave Lord Range, Keele Range, Old Crow Range, Old Crow Pediplain, and Old Crow Flats (Norris 1997). The Old Crow Basin is composed of parts of most of these regions.

In terms of geological structure, the Old Crow Basin is part of the Northern Yukon Fold Complex, one of eleven geological divisions that are found in the Yukon. The Northern Yukon Fold Complex is composed of relatively undeformed sedimentary basins, plus areas of arches, uplifts and folding of rock layers, and areas where igneous (molten) rocks have intruded from below.

The ages of rocks are described by geologists using a geologic time scale or chart, which is divided into four major "Eras"; Precambrian (the oldest, with few fossils, over 540 million years ago), Paleozoic (old, many fossils, 540 to 251 million years ago), Mesozoic (middle, the time of dinosaurs, 251 to 65 million years ago), and Cenozoic (most recent, modern mammals, 65 million years ago to the present). These eras are subdivided into "periods", e.g., Devonian, Carboniferous, Permian, Triassic, Jurassic, and Cretaceous periods, in an attempt to classify geologic time. The Cenozoic Era is divided into two periods (Tertiary and Quaternary), which are further subdivided into seven "Epochs" e.g., Paleocene, Miocene, and Pleistocene.

Geological Periods are defined on the basis of life forms preserved as fossils. The boundaries of the periods are defined by important changes in these life forms, usually extinction events. The more significant the extinction, the more important is the boundary. Thus, the beginning of the Cambrian marks the first occurrence of abundant fossils, believed to correpond to the first widespread development of hard-shelled life forms. Similarly, the ends of both the Paleozoic Era (the Permian-Triassic or P-T boundary) and the Mesozoic Era (Cretaceous-Tertiary or K-T boundary) reflect extreme extinction events. This time scale, built up over two centuries of world-wide scientific study, is called a relative time scale because it is independent of any absolute measure of time. In the past, Igneous rocks were also dated using a relative time scale because the intrusion age is bracketed by the age of the youngest rocks intruded and the oldest overlying rocks. Since the 1960s several systems using isotops have been available for providing absolute ages for igneous rocks. This has allowed approximate correlations between the relative and absolute time scales. For sedimentary rocks, the age given refers to the time when the layers of sediments were deposited. For igneous rocks, the age refers to the time of intrusion.

The area of the northern Yukon has been divided into seven geographic blocks for ease in assessment of the mineral potential (GSC 1981). These areas correspond to Norris (1981) and Dixon (1998) and are used as a framework to describe geological formations. Vuntut National Park and the Old Crow Flats Special Management Area (SMA) are encompassed by one block named the "Old Crow-Babbage Depression". This block is bounded by four other blocks; on the north by the Romanzof Uplift, on the south by the Keele Range and the Dave Lord fold and thrust belt, on the east by the Barn Uplift, and on the west by the Mount Ammerman granite stock (Figure 3.3).

Geologic Provinces (and areas or blocks) are broken down into what geologists call "formations". Formations are assemblages of rocks in a series of layers or strata which have some common characteristic. They refer to individual rock layers or units of sufficient size and with sufficiently distinct boundaries that a geologist can plot their distribution on a map. Formations in turn can be subdivided into "members" which are distinguishable portions of the complex structure or they can be linked together as "groups". Formations are often named after the geographical location where they were first described. The science of determining the age and origin of sedimentary rocks by studying the layers in relation to other features is called stratigraphy. The stratigraphy of the Old Crow-Babbage Depression features rock formations from at least seven geologic periods in the four major geologic eras (Precambrian to Cenozoic).

In the following section of this chapter, the geological formations occurring in the Old Crow Basin (Vuntut National Park and the Old Crow Flats Special Management Area), and the geological processes by which these rocks are thought to have formed, will be described in sequence, under the four geologic eras, from oldest to most recent. With each formation will be given, where it is possible, the rock type, age, location, stratigraphy, fossil content, and notes on the significance or history of the name. The geological map for the Old Crow Basin (Dixon 1996) shows the distribution of the surface rocks of the geological formations in colour (Figure 3.5). The use of the geological resources and their economic importance will be covered in section following.