The Manitoba Escarpment - vestiges of an ancient ocean

In the southwestern corner of the province lies the Manitoba Escarpment: a wedge of Cretaceous shales that was deposited in an inland sea that expanded across the centre of North America approximately 80 million years ago. These rocks contain economically important deposits, as well as the remains of prehistoric marine reptiles and fish that inhabited the inland sea.

The Manitoba Escarpment

In mid-April, a few geologists from the Manitoba Geological Survey and several people from the Canadian Fossil Discovery Centre went on a two-day field trip to study Cretaceous shale in the Morden-Miami area of Manitoba.

We stopped at several localities to examine exposures of different rock units and to look at some reclaimed and active quarries. Among the localities we visited included a paleontologically significant site that is currently being excavated by the Canadian Fossil Discovery Centre. There were great exposures of the Pembina Member of the Pierre Shale at this site.


The Pembina Member is composed of black shale interbedded with layers of cream-coloured bentonite beds.

The Pembina Member is composed of black shale that is interbedded with seams of bentonite. Bentonite was initially deposited as volcanic ash at the bottom of the sea by volcanoes that were most likely located in Montana. Through interactions with ancient sea water, that volcanic ash has been altered into clays. Historically, bentonite was an important industrial mineral and was quarried in the Morden-Miami area. Bentonite is also used to preform age-dating, allowing geologists to pinpoint the exact age of the rocks (give or take a few thousand years!). The Pembina Member also contains numerous fossils of Cretaceous vertebrates including plesiosaurs, giant turtles, sharks, fish, and the mighty mosasaur. The Canadian Fossil Discovery Centre has many fossils and reconstructed skeletons of these amazing creatures on display.




The Boyne Member, underlying the Pembina Member;
thin, darker-brown beds are bentonite seams.

Underlying the Pembina Member is the Boyne Member, which is a buff to grey unit that contains few brown-coloured bentonite seams. The upper part of the Boyne is composed of an upper chalky unit, pictured right, and a lower calcareous shale unit.

Historically, the Boyne Member was mined for natural cement rock. Occasionally well preserved fossils, such as shark teeth, can be found from the Boyne. Many road cuts along the southern escarpment provide excellent exposure of the Boyne, as well as other members.




The Babock beds are resistive beds located immediately above the lower part
 of this outcrop that is covered by vegetation.



At another outcrop of the Boyne we were able to observe the upper and lower units. The units are roughly separated by the Babcock bed, which is a layer of resistive, calcareous shale. Above the Babcock bed is the buff-coloured chalky unit, and below is the grey-coloured calcareous shale unit, that is partially covered in slumped material and vegetation.

 

Brick Production

Beehive kiln of the old brick plant. Please note this site is on private
property and cannot be accessed without permission of the landowner.

We travelled up the Roseisle Creek valley to explore more important sites. We stopped at the old Leary's brick plant that produced bricks from 1900-1907 and 1947-1952. There used to be several brick plants across southern Manitoba, using a variety of shales and clays to create bricks in a wide range of colours. Bricks are no longer produced in the province, but if you're lucky you might spy some Manitoba-made bricks on older buildings.


 

Shale from the Morden Member was extracted from the banks of the Roseisle Creek (pictured on the left), and produced bricks with a rich red colour. It was carried across the creek and fired in the beehive kiln (pictured on the right).

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The Manitoba Escarpment provides wonderful scenery and a variety of localities that are great for observing Cretaceous sedimentary rocks. Road cuts and rivers provide opportunities to work up or down section to observe how the rocks changed through time. Changes in geology represent changes in the environment the rocks were deposited in. Fossils of the animals that lived in this seaway provide important information about paleoenvironments, and the evolution and adaptation of these creatures.

Written by Kathryn Lapenskie