Tertiary gravels and prairie evolution
Tertiary Gravels and Evolution of the Western Canadian Prairie Landscape Beginning in the
Oligocene to Miocene
Dale A. Leckie* (Canadian Occidental Petroleum Ltd., Calgary, Alberta)
Jan Bednarski (Geological Survey of Canada, Terrain Sciences Division, Calgary, Alberta)
Harvey Young (Brandon University, Brandon, Manitoba)
Our Premise. The prairie landscape of western Canada began to evolve during the Oligocene with erosion
of a basin wide unconformity followed by deposition of an extensive braidplain that was subsequently
incised.
Background. Remnants of unconformity-based Tertiary gravels extend east-west across the southern
Canadian prairies for more than 700 km, which is perpendicular to paleo-depositional dip. Four main
deposits are considered here. The Cypress Hills Formation, which extends east-west across southwest
Saskatchewan and southeast Alberta for 310 km, capping much of the Cypress Hills and Swift Current
uplands. The Wood Mountain Formation, extends east-west across southernmost central Saskatchewan
for 200 km. The Flaxville Gravel, is up to 37 m thick and extends across northern Montana for 155 km.
Lastly, the Souris River Gravel, which occurs in southwest Manitoba. These gravels contain wellconstrained vertebrate assemblages, ranging from Eocene to Miocene in age, covering a period of ~44 to
~14 Ma.
All four stratigraphic units consist of gravel and sand with lesser amounts of clay. The formations were
largely deposited as laterally continuous sheets of braided river gravels, with some occurrences of
meandering-river sedimentation. Paleocurrent data for the Cypress Hills and Wood Mountain formations
indicate that regional paleoslope dipped towards the north-northeast. The climate, based on
paleontological (of other workers) and sedimentological data, was semi-arid and warmer than today, with
only a few days of winter frost.
A regional cross section across southern Saskatchewan shows at least three major topographic levels that
correspond to the different gravel distributions. Highest is the dissected topography of the Cypress Hills
Plateau that lies greater than 1370 m above sea level. Second is the Wood Mountain Plateau, east of
Cypress Hills, lying 320 m lower at ~1040 m. The Wood Mountain Plateau decreases towards the third
level which lies at as elevation of 440 m in Manitoba.
Why are the gravels there? These Tertiary gravels were deposited during and following an episode of
emplacement of intrusive stocks, laccoliths, sills and dikes associated with central-Montana Alkalic
Province. The main igneous activity in the Highwood and Bearpaw Mountains south of the gravels was
from 69 to 60 Ma and 54 to 50 Ma, which precedes deposition of the Cypress Hills Formation. The
intrusions are related to regional uplift in Montana that corresponds to the cessation of compressional
tectonics in the Rocky Mountains and the beginning of extensional tectonics.
The general evolution of the western Canadian prairie landscape is recorded by a sequence of events
shown by erosional surfaces and deposition of the Tertiary gravels. Following deposition of the
nonmarine Ravenscrag Formation in the Paleocene, regional uplift and igneous intrusive activity took
place in Montana. A regional erosional pediment surface developed dipping north-northeast. The boulder
conglomerate of the Cypress Hills Formation was deposited over a period that lasted 25 to 28 million
years during the Eocene and Oligocene. With continued uplift, which appears to have been greatest in the
west, over the Sweetgrass Arch, the fluvial system shifted eastwards but still flowed north. This resulted
in erosion and formation of a pediment on which the Wood Mountain Formation was deposited.
Provenance and paleocurrent data indicate that the source of the sediment is from southern and western
Montana. The Wood Mountain Formation was not derived from the eroded Cypress Hills Formation, nor
Tertiary gravels and prairie evolution
is the Cypress Hills Formation derived from the Cordillera to the west, as has been suggested by others.
Ongoing uplift of the Cypress Hills and Wood Mountain plateaus resulted in the dissection of the Wood
Mountain Plateau and further eastward shifting of the drainage system to deposit the Souris River Gravels
in Manitoba. The Flaxville Gravels in northern Montana are an intermediate phase between the Wood
Mountain Formation and Souris River Gravels. We propose that the Cypress Hills and Wood Mountain
formations and the Flaxville and Souris River gravels are deposits of the ancestral Missouri and
Yellowstone rivers, that flowed north into Canada as early as the Eocene and deposited sediments as old
as the Oligocene Cypress Hills Formation. Work by others suggests that the Missouri and Yellowstone
rivers flowed into southern Manitoba as recently as the "penultimate" glaciation.
The preglacial drainage systems were disrupted when continental ice sheets advanced from the northeast
and blocked the northward drainage. Broad preglacial valleys were filled with glacial drift and lesser
glacial landforms where superimposed on the broader bedrock physiography. Where previous erosion
removed the Tertiary gravel cover, flat-lying Cretaceous beds were readily thrust and deformed by
successive glacial advances and the overall topography was smoothed. During deglaciation, meltwaters
were routed along the ice front cutting across uplands and previous drainage divides. Much Tertiary
sediment was extensively reworked during these times. Deglaciated lowlands were inundated by icedammed glacial lakes, which deposited thick sediments and further smoothed the landscape.
The advancing ice sheets were deflected around the Cypress Hills and Wood Mountain Upland. The ice
was relatively thin and parts of the uplands escaped glaciation all together. Moraine belts surrounding the
uplands mark the former marginal positions of successive glacial advances. Prominent channels were cut
into the flanks of the uplands, as meltwaters were re-routed south into the present Missouri system.
Several short-lived glacial lakes were impounded between the ice front and the flanks of the uplands. In
the early postglacial, steep sides of former meltwater channels were subjected to slumping and many
slopes were mantled by colluvium. Recent erosion is limited to slumping along current river courses and
gullies where recessive bedrock is exposed.
When did the prairie landscape evolve? The Cypress Hills Formation was deposited by the end of the
Oligocene (~25 Ma). Down cutting started thereafter but before deposition of the Wood Mountain
Formation. Thus, it can be said that the erosion of the western Canadian prairie landscape began by about
24 Ma. Radiometric dates and paleomagnetic data collected by other workers indicate that the erosion of
the Cypress Hills had started by at least 10 Ma, with the present relief in place by 9.3 to 8.3 Ma. It is
likely that the landscape would have largely attained its broad morphology by ~10 million years ago. The
Late Tertiary drainage systems remained relatively stable until the major disruptions caused by
continental glaciation starting about upper Middle Pleistocene time (~ 0.4 Ma). Radiocarbon dates
indicate that the last glaciation in the area occurred from ~23 ka to ~12 ka BP. Fluvial aggradation on the
prairies may have been enhanced by earlier Quaternary glaciations in the Rocky Mountains where there is
evidence of at least six Cordilleran glaciations going back to the Late Pliocene.
Dale Leckie is Chief Geologist at the Canadian Division of Canadian Occidental Petroleum Ltd. He is
interested in developing and applying solutions to geological problems. He has worked in the Western
Canada Sedimentary Basin and internationally.
References:
Fulton R.J., Fenton M.M. and Rutter N.W. 1986. Summary of Quaternary stratigraphy and history,
western Canada. Quaternary Science Reviews v. 5: 229-241.
Klassen R.W. 1994. Late Wisconsinan and Holocene history of southwestern Saskatchewan. Canadian
Journal of Earth Sciences v.31: 1822-1837.
Tertiary gravels and prairie evolution
Distribution of Tertiary gravels in the southern Canadian Prairies and potential source areas. (Derived
from several sources). Paleocurrent summaries for the Cypress Hills and Wood Mountain formations
shown.