structural geology of the mountains

7/27/2019 Structural Geology of the Mountains

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Structural Geology of the Mountains

Clinton R. TippettShell Canada Limited, Calgary, Alberta

[email protected]

INTRODUCTION

The Southern Rocky Mountains of Canada (Figure 1) are made up of severalnorthwest-southeast-trending belts, namely (from east to west) the Foothills,Front Ranges, Eastern Main Ranges, Western Main Ranges and WesternRanges. The differentiation of this mountain chain into various ranges reflectsdifferences in the rocks that form them, both in terms of ages and of compositionor lithology.

The Rockies are one of the classic structural provinces of the world anddemonstrate 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 beencompressed, broken into thrust sheets and carried to the east above a deep,virtually flat detachment or failure surface near the base of the sedimentarysection. 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 firsthave a good mental image of these strata. Then one is able to visualize how theyhave been broken up and carried far from their place of deposition. This brief

RockyMountains

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outline considers the mountain belt primarily as it is exposed along the transectformed by the Bow River Valley.

STRATIGRAPHY

This is the word used to describe the various layers of rock that form themountains. At the most simplified level there are only three packages of rock.

From youngest to oldest (Figure 2) they are:

1. Jurassic, Cretaceous and oldest Tertiary rocks (Clastic Wedge). These areprimarily sandstones and shales. Their internal sequences record the interactionof rising mountains, erosion, transport and deposition under conditions of varyingsea level and basin subsidence. Classic exposures along the Bow Valley includethe thinly layered deep water turbidites near Banff that mark the beginning ofmountain uplift (Late Jurassic), the coal measures of the Canmore region (latestJurassic) 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 shalysections. 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 andCarboniferous),f. the equally impressive cliffs of Castle Mountain (Cambrian) and, eventually,

Clastic Wedge

Passive Margin

UnderlyingBasement

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g. the steep cliffs of both carbonate (Figure 3) and shale in the vicinity of theworld famous trilobite-bearing Burgess Shale near Field, B.C. (Cambrian).

3. Precambrian metamorphic and igneous rocks of the underlying basement.These are not exposed along this transect as they are not involved in thrustingbut are known to exist from seismic data.

STRUCTURAL GEOLOGY

Beginning approximately 165 million years ago, events along the western marginof North America began to be unsettled by its collision with huge microcontinent-sized blocks of the Earths crust. These terranes began colliding with thewedge of sedimentary rocks that had up until that time been gradually buildingout towards the west. The impact of these features caused great structuraldamage to the older sediments. To the west some parts of the sedimentarywedge were dragged down to great depths, heated, metamorphosed andpartially melted. To the east, the sediments broke into great slabs that were piledon 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 usedto understand the erosional remains of these deformed panels. Most of thesheets are bounded on both edges by sharp northwest-southeast-trending faultsor thrusts. In cross-section these faults are known to alternatively climb upthrough the sediments over ramps and glide along individual layers on flats.

Eastern Main Ranges

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As a general rule, the longer the thrust is in a north-south direction, the larger itsmaximum offset. This is the bow and arrow rule. The implication is that faultsdecrease in displacement towards their ends and eventually disappear as otherfaults increase in displacement.

Northward disappearanceof the Rundle Thrust Sheet

Folds

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For example, to the north of Cascade Mountain, the Rundle Thrust dies out intoshaly Jurassic strata (Figure 4). Faults grow or propagate by first forming a foldand then breaking through it. Such folds can often be seen at the surface at theends of thrusts, for example at Mount Kidd in Kananaskis Country where theLewis 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 newfolds struggled to grow within them. Within the Foothills this sort of draping effectover deeper structures is important in forming traps for oil and gas.

In some places the presence of shaly equivalents to the thick carbonates has ledto 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 themanners described. This process is assisted by the presence of numerouscracks and fractures in the sediments that allow the rock masses to almost flowduring movement.

HYDROCARBON RESOURCES

The eastern portion of the Rocky Mountains, for the most part the Foothills Belteast of the McConnell Fault at Mount Yamnuska have been known to be rich innatural 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 ofnumerous traps in which porous rocks such as sandstones and dolostones aresealed by tight rocks such as shales. Considerable wealth has been generatedthrough the production of natural gas, condensate, sulphur and crude oil.

Turner Valley Field

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CONCLUSIONS

The Southern Rocky Mountains began as a passive continental margin thatstarted 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 weredeformed and uplifted, erosion and re-deposition to the east was followed by thegradual migration of thrusting and folding in that same direction and to a processof cannibalization. The deposition of thick piles of sediments and the buildup ofstacks of thrust sheets led to the heating of organic-rich rocks in the section andto the formation, migration and entrapment of oil and gas which now form acornerstone of Albertas economy.

FIGURES

1. Geological map of the Southern Canadian Cordillera showing the position ofthe Rocky Mountains along its eastern edge. From the Geological Survey ofCanada.

2. Stratigraphic wedge diagram with datum on the base of the Triassic. ShellCanada Limited.

3. Thick Cambrian carbonate strata in the Eastern Main Ranges near MountAssiniboine. 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. 1990. Where Terranes Collide. Orca Book Publishers, Victoria,B.C., 234 p.

structural geology of the mountains

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