These are the front ranges of the Canadian Rockies, part of a vast sea of mountains that
stretches from western Alberta across to the Pacific coastline of British Columbia. The
Rockies consist almost exclusively of marine sedimentary strata that were originally deposited
below sea level on the stable continental shelf of ancient North America. Over the course of
hundreds of millions of years the sediments were progressively buried and lithified. But about
150 million years ago they began to be thrust upward and eastward as a result of complex
collisions between the North American plate and a collage of oceanic crustal blocks that
greeted onto the continent's western edge. In front of these relatively young mountains
are the foothills with their much more subdued topography. And beyond the foothills the prairies
that stretch uninterrupted as far as the eye can see. Beneath these foothills and the prairies is
one of the world's largest sedimentary basins. Its origin is intimately related to the mountain
building. As the mountains were forced upward they were continually eroded and sculpted by
the action of water and ice. As a result they shed large amounts of sediments eastward into
the depression in front of the rising mountains. The preserved sedimentary rocks that infilled
that forlorn depression make up the Alberta Foreland Basin.
The Alberta Foreland Basin is one of the best explored by geologists. It hosts tremendous
fossil fuel resources of coal, natural gas and oil. Recent estimates indicate that the
basin contains about 6 trillion cubic meters of natural gas and on the order of 3 billion
cubic meters of recoverable oil. In addition there are literally hundreds of billions of
cubic meters of bitumen trapped in oil sands, the largest petroleum deposits in the world.
The energy industry is one of the linchpins of the economy of Western Canada.
The term Foreland Basin has been used for decades by geologists to describe an elongate depression
adjacent to an actively building mountain chain. Around the world there are quite a
number of sedimentary basins that fit this mold. One of the largest is the Indus Ganga
Basin in front of the Himalayas. The Alpine Foreland Basin of Switzerland and southern
Germany is another example. As Africa collided with Europe, squeezing up the Alps, large
amounts of sedimentary material accumulated to the north in what is called the Molas Basin.
The Pyrenees have two associated foreland basins, one to the north in France and the
other to the south in Spain. These basins can all be shown to have either subtly or
profoundly different plate tectonic settings. The theme that is common to all of them is
that they are associated with convergent plate margins and resultant compressional
tectonic regimes. The Himalayan Foreland Basin clearly resulted from the convergence
of two large masses of continental crust, Asia and the Indian subcontinent. The Rocky
Mountain Basin of the United States is attributed to subduction of the oceanic Pacific plate
beneath the continental margin of North America. When the Canadian Rockies were formed, the
crustal depression to the east was developed over a very broad area in Alberta, Saskatchewan
and Manitoba. It's one of the widest foreland basins in the world. The foreland depression
forms in response to the load imposed upon the crust by the weight of the stacked thrust
sheets in the mountains. As long as the thrusting continues, the basement underneath gets bowed
down. But the basement in front of the mountains also subsides, forming a moat that traps sediments
eroded from the mountains. With each episode of mountain building, two things happen. First,
the basin undergoes a pulse of increased subsidence because of the added load of the mountains.
And second, there is increased sediment input into the basin because the mountains are higher
and are more actively eroded. The weight of the sediment further depresses the basin.
Eventually, the sea retreats and the whole basin fills with sediment.
One analogy is that of the bent beam. If we imagine this plank representing the top of
the continental crust with the fixed end somewhere here in Manitoba stretching some 1,300 kilometers
across to where the mountains were forming in Alberta and British Columbia, then the
loading of the crust by the thrusting and thickening can be very simplistically represented
by placing this rock on the end of the beam. The wedge-shaped depression that forms in
front of the mountains ultimately gets filled with Portland Basin sediments. And the whole
process repeats itself with the next episode of thrusting.
The horizontal line we've used as our reference may coincide with sea level. And there's lots
of sediments in the basin that were deposited below sea level. But others were deposited
above the line. They were laid down by river systems building coastal plains out from the
mountains. Of course, in the real world, the internal geometries of the thrust belt and
the Portland Basin are very much more complicated than this. But even so, a cross-section through
the Alberta Portland Basin shows this characteristic tapering shape with the thickest basin fill
near the mountain source area thinning towards the cretonic interior.
Another feature of Portland Basins is illustrated here at Mount Yamnaska. The enormous cliffs
of old Cambrian limestones are thrust over much younger Portland Basin shales and silt
stones. These sediments were deposited when the main mountain building was going on well
to the west of here. You see, the front of thrusting started in the west and moved progressively
eastward with time. Some of the original Portland Basin rocks were ultimately caught up in the
tectonic activity themselves and became deformed. This detailed cross-section clearly shows
how the Portland Basin rocks in yellows and greens are deformed by the thrusting of Paleozoic
slabs in blue in the front ranges of the mountains. Further out in the foothills as well, Portland
rocks are heavily thrusted. In the general area of the foothills, the preserved thickness
of Portland Basin fill is at its greatest, on the order of 4,000 meters. And because
of thrusting, various levels in the succession are exposed at the surface. East of the foothills,
where the sediments of the basin are undeformed, the wedge ultimately tapers to zero, some
thousand kilometres away in the plains of Saskatchewan and Manitoba, where thousands
of boreholes give us access to the rocks. So, we've got a pretty good idea of the sedimentary
strata inside this Portland Basin. This is a schematic log of the stratigraphic succession
in the thickest part of the basin. The finest chales are shown as narrow columns, with silts,
sands and conglomerates progressively wider. There are two features of importance. The
first is that there are a number of recognisable coarsening upward sequences, developed on
various scales. These exhibit a transition from fine-grain sediments deposited in deep water
to coarser, sandy material deposited in shallow shoreline environments and to coarser, mixed
sediments deposited above sea level in continental settings. The second important feature is
that the tops of these coarsening upward sequences are commonly cut off sharply and abruptly
overlaid by the fine, deep water shales at the start of the next coarsening upward sequence.
So, while the retreat of the sea and the sedimentary filling of the depression is well recorded
in the coarsening upward sequences, the invasion of the sea is commonly quite rapid and erosive,
leaving little evidence of its passing. We'll begin our analysis of the Portland Basin fill
by examining the first major coarsening upward sequence. Just inside the boundary of Banff
National Park, by the side of the Trans-Canada Highway, some of the oldest Jurassic rocks
of the Portland Basin are beautifully exposed. Here, we're at the base of the first coarsening
upward sequence. These dark shales indicate a deep water, marine environment, grading
upward into a series of thin turbidites, about 30% interbedded with the shale. These beds,
originally deposited in the foreland, are now right in the mountains, and they've been
folded and turned on end, slightly overturned in fact. Each sandstone bed is coarse-grained
at the base and slightly finer-grained near what was originally the top, indicating that
it was deposited by a bottom-hugging density current. These sand ribs were probably laid
down in just a few minutes, whereas the shales in between may have taken a thousand years
of slow sedimentation to form. In places, you can see soul marks on the bases of the
beds, indicating the direction in which the density flow was moving. Sandstones like these
were deposited in a couple of hundred metres of water, below even the influence of storm
waves. Moving slightly higher up this Jurassic sequence, the sandstones become thicker and
more dominant. In these, the individual laminae in the sandstone appear to curve and bend,
and there are low-angle truncations. These are called Hamaki cross-strata. This subtle
but important sedimentological feature suggests that the depositional environment was affected
by storm waves, but was below the influence of fair weather waves. Certainly the water
depth here was less than 200 metres. Higher up, there are increasing signs of the water
becoming more and more shallow. Fossil trees are preserved. This one was a bit like a palm
tree, which floated out near the shore where it became waterlogged and sank. Near the top,
the sequence is virtually all sandstone, laid down in a shallow shore-face environment.
Even higher, above the thick sandstones, you find thin seams of coal, which tell us that
we've reached an emergent environment, a coastal plain swamp. So we've passed up section from
deep marine muds through shallow marine silts and sands to shore-face and shoreline sediments,
a coarsening upward sequence. In fact, though it's not exposed at this locality, the sequence
continues up through 1500 metres of coal-bearing sediments and shales deposited in river channels
and associated floodplain settings. At the top of the cycle, there is an abrupt cut-off,
marking where the seas transgressed rapidly over the land, plunging the whole area back
into a deep marine environment. So about halfway up the Foreland Basin succession, you find
deep water muds again. This 100-metre section records the transition from marine to non-marine
strata in the second major sequence of the Foreland Basin. Even from a distance, you
can recognise the coarsening upward nature of the cycle, recessive shales to start, grading
up to more and more massive sandstone. This section near the town of Lundbrek is one of
the best-developed coarsening upward sequences in the Foreland Basin. It takes us up out
of a fairly long-standing seaway through a shoreline zone and into continental sediments
above. At this level in the succession, nothing but burrowed muds were being deposited in
very quiet water with ribs of sand between that represent density current deposits. The
thin sandstones were deposited in water depths beyond the influence of wave action. In the
thicker-bedded sandstones higher up, we see hummocky cross strata, bedforms moulded on
the sandy bottom by the action of high-amplitude storm waves. Further evidence of the action
of waves is the presence of symmetrical ripples produced by oscillatory movement of the water
just above the sediment. Remarkably precise evidence of depositional environment and even
water depth is also provided by trace fossils, like these burrows. In this case, they indicate
passage up into the shore face zone. We've progressed upwards from almost pure shale
at the base to almost pure sand. Near the top, these sands indicate deposition in upper
shore face and beach environments. There are non-marine beds above. This succession records
the passing of a shoreline. The foreland basin was subsiding, but sediments were being shed
from the rising mountains fast enough that the whole shoreline could move east through
this area. Thus we have good sandy shoreline and continental deposits, overlying sediments
deposited in much deeper water. The coarsening upward brings us to the shoreline, but the
story continues well beyond, for there are commonly thick continental deposits above.
Indeed, in the region near the mountain source area, a majority of the stratigraphic column
is continental in origin, made up of coastal plain fluvial or river lane sediments and
other types of continental deposits. One surefire indication of continental deposition is the
presence of coal seams, and there are a number of coal mines in the foothills. Thick seams
such as this in the Adanac mine yield coking quality coal. These strata have been subjected
to temperatures high enough to achieve coking rank as a result of being deeply buried before
being thrust and brought to the surface. Just above the coal seam at Adanac is a massive
chert pebble conglomerate. It was laid down by a braided river system relatively close
to the mountain front, in a flow regime that was capable of transporting and depositing
pebble-sized material. Most of the continental deposits of the Forland Basin are fine-grained
floodplain sediments, laid down in association with meandering streams. Sands were deposited
in the streams themselves and appear as rather discontinuous lenses, separated by floodplain
muds. Each channel sandstone has a relatively sharp erosional base, overlaid by trough cross-bedded
sandstone showing downstream sediment transport. The sloping internal bedding within the sandstone
is called accretion bedding, a feature unique to meandering rivers. Deposits such as these
were laid down on coastal plains, perhaps hundreds of kilometres inland from the shoreline.
So the Forland Basin reveals a history of coarsening upward cycles, capped by thick
continental deposits. These are called regressive cycles, and they represent progradation of
shorelines seaward from the sediment source areas in the mountains, filling the basin
first with marine deposits and then continental deposits. The record of regression, or seaward
building of sedimentary wedges, is commonly well-preserved, whereas the transgressive
events where the sea invades and drowns the land are not well represented. With a new
episode of mountain building, the processes of shoreline progradation and basin infilling
are initiated again. What are the master controls on the thickness and geographic extent of
individual cycles? In the rock record of the Alberta Forland Basin, there are observable
trends and patterns, but there are also quite a few possible explanations of how various
large-scale tectonic and sedimentological factors may have interacted to produce any
given succession that we can see in the field. Many questions remain, and the answers are
difficult and controversial. I went with Geological Survey of Canada geologist Dale Leckie to
the cardium section near the Horseshoe Dam on the Bow River. Around here, the succession
is dominated by black shales, but amidst the shales are sheets of sandstone and conglomerate
that are apparently anomalous. How do we explain the dramatic shift in energy conditions that
would be required to transport coarse-grained cardium sediment out into the basin? Both
before and after this influx of sand and conglomerate, there was nothing but very quiet water sedimentation.
So these are pretty good marine muds here, Dale, and yet we've got this sandstone sequence
right in the middle of it. What are our options with regards to explaining how that might
have gotten in there?
Grant, I think we've got three basic options to consider here. We've got ecstatic sea level
fluctuations, where we get worldwide rises and falls of sea level. We know that there
was a major sea level drop that occurred at the time when these rocks were deposited.
Sea level drop and rose. That would be one way to flush this coarse material out into
the basin. A second possible control that we have is a tectonic activity. We know that
at the time that these rocks were being deposited, there were major collisional events further
to the west. And with those collisional events, we get thrusting. With the thrusting, we get
stacking of major thrust plates.
And down-warp in the Foreland Basin now.
That's right. And with that thrusting and stacking, we've got a gradient. The gradient can transport
large gravels, coarse sands. So we've got a gradient, as well as we've got a source
for this coarser material to bring it out into the basin.
You could flush it out this far.
That's right.
And then the third possibility, perhaps, is perhaps the local switching of delta lobes.
Does that seem likely here at all?
Delta switching is a major control in certain parts of the world. The Mississippi River
Delta, for instance, covers a big area. But I don't think that we should use that as one
of our controls for the cyclicity that we see here.
Elsewhere in the Foreland Basin, though, we see manifestations of that, don't we?
We do. We really do. One of the reasons that it doesn't really fit with these cycles, or
as an explanation for the cyclicity that we see here, is that these cycles can be traced
for hundreds of kilometers throughout the Foreland Basin. And if it was delta switching,
we would get lenses of sediment, which were not laterally extensed.
And that's true of this particular sequence here, isn't it? It's really widespread.
It really is.
But here, then, the controversy that still goes on, as far as I can see, really relates
to how you get that sand into such a muddy sequence. Perhaps, perhaps the dropping of
sea level is one real possibility here?
It is, Grant, because this particular sequence of rocks right here was deposited at a time
that we know there was a major worldwide sea level drop and sea level, and then subsequent
rise.
Yeah.
And that would be one very good explanation of how this wedge of coarser material was
in the middle of this thick sequence. Came out so far. That's right. But at the same
time, we've got tectonic controls, and we can't eliminate one or the other. There is
a signature where perhaps the tectonic controls are over-printed on top of the eustatic controls.
So we just have to try to sort through more definitive criteria to separate some of these
things. More and more detailed work in this basin will help it off the rock.
Big cycles swamping little ones. That's right.
Far out in the Foreland Basin, there are quite a number of coarsening upward regressive cycles
that are most logically explained by delta lobe switching. Detailed examination of such
cycles is possible in borehole cores.
Here we are in the Core Research Centre in Calgary, where the cores from every well drilled
in Alberta are curated and made available for public examination. We've come here to
have a look at a core through the Cold Lake oil sands. Now these materials take a little
bit of getting used to. The sands are jet black because they're saturated throughout
with bitumen. The shales, on the other hand, appear as these brownish bands. Now the core
here is laid out in such a way that up is in this direction, and this surface fits onto
here and so on throughout the core. We have about 30 metres of section here in the Cretaceous
laid out. At the bottom, we have nothing but burrowed muds laid down in the delta front
or pro delta area. But as we track upwards, the sand takes over more and more through
delta front deposits and ultimately up into the distributary mouth bar at the top. Then
the delta must have been abandoned, and the next thing we see are the muds at the base
of the next cycle.
Cretaceous oil sands, like Cold Lake, are dominant in the eastern part of Alberta, but in fact
there are scores of conventional hydrocarbon fields throughout the remainder of Western
Canada. Practically all of the fields contain both oil and natural gas. Many of these reservoirs
are Mesozoic in age and were formed in the Forland Basin, but others lie in older Paleozoic
strata beneath the Forland Basin wedge. At the thin end of the wedge, the reservoirs
are so close to the surface that the oils have been degraded to bitumen as a result
of freshwater washing and bacterial action. It is further west that conditions were ideal
for crude oil generation. There were plenty of marine shales to serve as source rocks.
Mud shales, rich in marine microbes, are ideal for oil formation. In addition, the rocks
were buried to depths of only 2,000 to 3,000 metres, just nicely raising their temperature
to the optimum for crude oil, about 100 degrees Celsius.
Near the foothills, the hydrocarbon reservoirs are mostly natural gas. This is partly because
the source rocks have a much higher proportion of land-derived plant material in them. Terrestrial
organic matter favours gas generation. The other reason that the thick end of the wedge
is gas-prone is simply that the sediments have been more deeply buried, cooked at hotter
temperatures, a condition that favours natural gas generation in preference to liquid hydrocarbons.
In the foothills themselves, the hydrocarbon traps are structural in nature. The strata
have been extensively deformed by compression to yield complex traps capped in folds or
closed off at thrust faults. They are difficult traps to find, but in fact the first successful
wells in western Canada were drilled into these foothill structures.
Since the end of the Second World War, there has been intense exploration and development
in western Canada, with tremendous economic growth. What makes cities like Calgary the
modern and bustling centres they are, is the reality of the energy industry.
The city of Calgary is set in the heart of the Alberta Foreland Basin. It's built upon
a thick succession of basin-filled sedimentary rocks. Much of the economy of the region relates
to the resources of the basin, oil and gas, coal and bitumen. Because of the exploration
data that's available to us, we've learned quite a bit about the geology out in the basin,
and we've come to recognise more and more that sedimentation patterns in the basin relate
very closely to tectonic events in the mountains as the Rockies were being built.
Thanks for watching!