Structural Geology of the Mountains

Transcription

1 Structural Geology of the Mountains Clinton R. Tippett Shell Canada Limited, Calgary, Alberta INTRODUCTION The Southern Rocky Mountains of Canada (Figure 1) are made up of several northwest-southeast-trending belts, namely (from east to west) the Foothills, Front Ranges, Eastern Main Ranges, Western Main Ranges and Western Ranges. The differentiation of this mountain chain into various ranges reflects differences in the rocks that form them, both in terms of ages and of composition or lithology. Rocky Mountains 1 The Rockies are one of the classic structural provinces of the world and demonstrate almost a pure end member of what is called a thin-skinned style of deformation. This implies that the rocks that form the belt have been compressed, broken into thrust sheets and carried to the east above a deep, virtually flat detachment or failure surface near the base of the sedimentary section. In contrast, thick-skinned belts have faults that cut much deeper. In order to understand the means by which this has occurred it is essential to first have a good mental image of these strata. Then one is able to visualize how they have been broken up and carried far from their place of deposition. This brief

2 outline considers the mountain belt primarily as it is exposed along the transect formed by the Bow River Valley. STRATIGRAPHY This is the word used to describe the various layers of rock that form the mountains. At the most simplified level there are only three packages of rock. Clastic Wedge Underlying Basement Passive Margin 2 From youngest to oldest (Figure 2) they are: 1. Jurassic, Cretaceous and oldest Tertiary rocks (Clastic Wedge). These are primarily sandstones and shales. Their internal sequences record the interaction of rising mountains, erosion, transport and deposition under conditions of varying sea level and basin subsidence. Classic exposures along the Bow Valley include the thinly layered deep water turbidites near Banff that mark the beginning of mountain uplift (Late Jurassic), the coal measures of the Canmore region (latest Jurassic) and the lower cliffs of Mount Yamnuska (Late Cretaceous). 2. Cambrian, Ordovician, Devonian, Carboniferous, Permian and Triassic rocks (Passive Margin). These are mainly limestones and dolostones with some shaly sections. On this transect excellent exposures occur in: a. the upper cliffs of Mount Yamnuska (Middle Cambrian carbonates), b. the quarries to the east of Canmore (Carboniferous), c. the highly porous dolostones of Grassi Lakes (Devonian), d. the black shales of Jura Creek (basal Carboniferous), e. the massive cliffs of Mount Rundle and Cascade Mountain (Devonian and Carboniferous), f. the equally impressive cliffs of Castle Mountain (Cambrian) and, eventually,

3 g. the steep cliffs of both carbonate (Figure 3) and shale in the vicinity of the world famous trilobite-bearing Burgess Shale near Field, B.C. (Cambrian). Eastern Main Ranges 3 3. Precambrian metamorphic and igneous rocks of the underlying basement. These are not exposed along this transect as they are not involved in thrusting but are known to exist from seismic data. STRUCTURAL GEOLOGY Beginning approximately 165 million years ago, events along the western margin of North America began to be unsettled by its collision with huge microcontinentsized blocks of the Earth s crust. These terranes began colliding with the wedge of sedimentary rocks that had up until that time been gradually building out towards the west. The impact of these features caused great structural damage to the older sediments. To the west some parts of the sedimentary wedge were dragged down to great depths, heated, metamorphosed and partially melted. To the east, the sediments broke into great slabs that were piled on top of each other and folded even as they were uplifted and eroded. A number of rules governed how these thrust sheets evolved and can be used to understand the erosional remains of these deformed panels. Most of the sheets are bounded on both edges by sharp northwest-southeast-trending faults or thrusts. In cross-section these faults are known to alternatively climb up through the sediments over ramps and glide along individual layers on flats.

4 Northward disappearance of the Rundle Thrust Sheet 4 As a general rule, the longer the thrust is in a north-south direction, the larger its maximum offset. This is the bow and arrow rule. The implication is that faults decrease in displacement towards their ends and eventually disappear as other faults increase in displacement. Folds 5

5 For example, to the north of Cascade Mountain, the Rundle Thrust dies out into shaly Jurassic strata (Figure 4). Faults grow or propagate by first forming a fold and then breaking through it. Such folds can often be seen at the surface at the ends of thrusts, for example at Mount Kidd in Kananaskis Country where the Lewis Thrust dies out within the Rundle Thrust Sheet (Figure 5). Significant folds can also occur within thrust sheets as they warped during transport or as new folds struggled to grow within them. Within the Foothills this sort of draping effect over deeper structures is important in forming traps for oil and gas. In some places the presence of shaly equivalents to the thick carbonates has led to a very pronounced fold-dominate style even within the Paleozoic section. It is generally difficult to imagine how solid rocks are capable of moving in the manners described. This process is assisted by the presence of numerous cracks and fractures in the sediments that allow the rock masses to almost flow during movement. HYDROCARBON RESOURCES The eastern portion of the Rocky Mountains, for the most part the Foothills Belt east of the McConnell Fault at Mount Yamnuska have been known to be rich in natural gas since the 1914 discovery of the Turner Valley Field (Figure 6). Complex thrusting and, to a lesser extent, folding have led to the formation of numerous traps in which porous rocks such as sandstones and dolostones are sealed by tight rocks such as shales. Considerable wealth has been generated through the production of natural gas, condensate, sulphur and crude oil. 6 Turner Valley Field

6 CONCLUSIONS The Southern Rocky Mountains began as a passive continental margin that started to get mixed up in collisions with exotic terranes along the eastern edge of the Pacific Ocean about 165 million years ago. As the sediments were deformed and uplifted, erosion and re-deposition to the east was followed by the gradual migration of thrusting and folding in that same direction and to a process of cannibalization. The deposition of thick piles of sediments and the buildup of stacks of thrust sheets led to the heating of organic-rich rocks in the section and to the formation, migration and entrapment of oil and gas which now form a cornerstone of Alberta s economy. FIGURES 1. Geological map of the Southern Canadian Cordillera showing the position of the Rocky Mountains along its eastern edge. From the Geological Survey of Canada. 2. Stratigraphic wedge diagram with datum on the base of the Triassic. Shell Canada Limited. 3. Thick Cambrian carbonate strata in the Eastern Main Ranges near Mount Assiniboine. C. Tippett photo. 4. North plunge of the Rundle Thrust Sheet just north of Cascade Mountain. Mount Rundle in the foreground. C. Tippett photo. 5. Folded Paleozoic carbonates at Mount Kidd, Kananaskis Country. C. Tippett photo. 6. Oil pumpjack on the western flank of the Turner Valley Oil Field. C. Tippett photo. RECOMMENDED READING Yorath, C.J Where Terranes Collide. Orca Book Publishers, Victoria, B.C., 234 p.