The Basic Geology of Manitoba

The oldest and most abundant rocks in Manitoba are Precambrian in age (rocks between 4.5 billion and 570 million years old). Precambrian rocks represent the initial formation of the earth’s crust when lighter rock material was separating from denser mantle material deeper in the earth. This upward separation of lighter rock material initially resulted in volcanic extrusions of basalt onto the sea floor as evidenced by some pillow lavas in various areas in Manitoba. The accumulation of basaltic material on the ocean floor resulted in the eventual rise above sea-level of volcanic islands. Subsequent subaerial (open air) erosion of these the extrusive volcanic rocks resulted in the accumulation of greywackes (coarse grained sandstones derived from the erosion of the volcanic rocks) which were deposited on the flanks of the volcanoes.

Precambrian rocks in Manitoba are divided into two provinces. The Superior Province is composed of rocks between 3 to 2.5 billion years old, and the Churchill Province is composed of rocks between 2.8 and 1.7 billion years old. A mountain building episode related to the tectonic convergence of the earth’s first crustal plates, two of which are represented by the Superior and Churchill Provinces, accounts for the deformation (folding and faulting) of the volcanics and their related sediments, as well as, intrusions of granitic plutons into the volcanics. Heat and pressure related regional metamorphism, resulting from burial of the rocks during mountain building, altered the volcanics to greenstones (named for the presence of green metamorphic minerals such as chlorite, actinolite and epidote) and the sediments to schists and quartzites. A boundary zone, known as the "Churchill Superior Boundary Zone (CSBZ), occurs between the two Precambrian Provinces in Manitoba. The CSBZ consists of concentrated igneous intrusions and, consequently, the rich base (Ni, Cu, Zn) and precious (Au, Ag) metal deposits associated with plutonic hydrothermal activity (the Thompson Nickel Belt).

Manitoba’s Paleozoic Era rocks (rocks between 570 and 245 million years old) were deposited as sediments (limestones, dolomites, shales, sandstones) on top of the Precambrian rocks between 570 and 235 million years ago. Paleozoic rocks overly the Precambrian Shield in the Hudson Bay Lowlands and in southwestern Manitoba. These sedimentary rocks formed in topographically low areas of the crust known as sedimentary basins such as the Hudson Bay Basin in the northeast portion of the province and the Williston Basin in the southwest portion of the province.

After the Paleozoic Era (about 235 million years ago) a major marine regression occurred that exposed the Paleozoic Era rocks to subaerial erosion. This resulted in the development of hills and valley’s, sinkholes and caves in the limestone Paleozoic strata. Later during the Mesozoic Era (rocks between 245 and 65 million years old) a marine transgression episode deposited sandstones and shales over the Paleozoic strata. Marine vertebrate fossils of mosasaurs and plesiosaurs (marine amphibians) are found in the Mesozoic rocks of Manitoba. Occasional beds of volcanic ash (most likely from volcanic eruptions in Western North America) are found inter-bedded with the Mesozoic strata.

Minor occurrences of Cenozoic Era rocks (rocks between 65 and 1.6 million years old) occur in the Turtle Mountain area of Manitoba. These sediments consist of sandy to silty shales.

During the last 1.6 million years (the Pleistocene Epoch) Manitoba has been subjected to repeated glacial advances and retreats. Glacial ice advances resulted in erosive scouring of the Precambrian, Paleozoic, Mesozoic and Cenozoic rocks. The eroded material picked-up by the ice was later deposited as till (a mixture of rock and mineral fragments scoured from rocks over which the glacial ice moved), outwash deposits (as water flowed from the melting glacier) and fine grained lake deposits (in pro-glacial lakes; Lake Agassiz) as the glacial ice receded.

Background Information

The Relative Geological Time Scale (page 272, text)

-based on the superposition of the fossil record (index fossils: narrow age range, yet wide geographic distribution) pieced together from stratigraphic records from different areas.

Time Scale Terminology (page 274, text): eon, eras, periods and epochs (decreasing magnitude of time).

Precambrian (rocks that are 4.5 billion to 570 million years old): refers to rocks older than Cambrian Period rocks (570 to 505 million years in age)(named for Cambria, the ancient name for Wales) in which there appear the first abundance of fossils.

Paleozoic Era: (Primary Era)(characterized by the initial abundance of life on earth)

Cambrian Period: (see above)

Ordovician Period (named after the Celtic tribe Ordovicies; age of marine invertebrates) (505 to 438 million years old).

Silurian Period (named after the Celtic tribe Silures; age of first marine vertebrates) (438 to 408 million years old).

Devonian Period (named after Devonshire, England; age of fishes)(408 to 360 million years old).

Carboniferous Period (sometimes referred to as the Mississippian and Pennsylvanian Periods)(age of land plants giving rise to coal (carbon) deposits) (360 to 286 million years old).

Permian Period (named after the Perm province in Russia; the age of amphibians)(286 to 245 million years old).

Mesozoic Era: (Secondary Era) characterized by the evolution and abundance of reptiles and gymnosperms (plants with seeds in cones)

Triassic Period: (named for it’s three fold division (Tri-) of strata; age of first reptiles)(245 to 208 million years old).

Jurassic Period: (named after the Jura Mountains between France and Switzerland; age of cycads (a gymnosperm)(208 to 144 million years old).

Cretaceous Period: (named for the Latin word for the abundance of chalk creta that was deposited during this period)(114 to 65 million years old).

Cenozoic Era: characterized by the evolution and abundance of mammals

Tertiary and Quaternary Periods: (65 to 1.6 million years old).(Tertiary; age of mammals, Quaternary; age of humans)