Gowganda Tillite: Evidence for Early Proterozoic Continental Glaciation

 By Steven Wade Veatch

Gowganda tillite (figure 1) is lithified glacial till (sediment deposited by glacial ice) from a Precambrian glaciation event that occurred over two billion years ago. Glacial till was buried, and over time, it lithified or turned into rock (tillite). 

Figure 1. The Gowganda tillite is composed of sediment that was deposited by a glacier and later cemented to form a rock. Its pink clasts make it distinctive. Gowganda tillite is among the oldest rocks on Earth—about 2.3 billion years old. This specimen is from the outer limits of the city of Gaylord in Otsego County, Michigan. From the collection of S. W. Veatch. Photo by S. W. Veatch. 

Billions of years later, glaciers from the last Ice Age moved fragments of Gowganda tillite from their source location in Canada and dropped them onto the Michigan landscape as the climate warmed and the ice melted. Their pink clasts make them distinctive. This is only one of several Precambrian tillites found in the glacial drift of Michigan. The Gowganda and other tillite deposits in North America provide a rock record of the continental glaciation that occurred during the Early Proterozoic (Lindsey, 1969; Young and Nesbit, 1985, Crowell, 1999).

Geologists are now convinced that widespread glaciation occurred throughout the Early Proterozoic Era, based on at least 300 Precambrian sites, including Finland, South Africa, India, and Australia, that have tillite or deposits that resemble tillite (Wicander and Monroe, 2016). Five more major periods of widespread glaciation followed the Early Proterozoic (see table 1).

Table. 1. When Glaciers Covered Parts of the Earth (Crowell, 1999)
1	Late Cenozoic glaciation: began 33.9 million years ago at the Eocene-Oligocene Boundary and is ongoing
2	Late Paleozoic ice ages 338 to 256 million years ago
3	Late Devonian-Early Carboniferous ice ages, two short episodes between 353 to 363 million years ago
4	Ordovician-Silurian ice ages 429 million years ago to 445 million years ago
5	Late Proterozoic ice ages ~ 520 million years ago to 950 million years ago
6	Early Proterozoic ice ages ~ 2.2 to 2.4 billion years ago
7	Archean glaciation ~ 2.91 to 2.99 billion years ago

The source of Gowganda tillite is the Gowganda Formation which forms part of the Huronian Supergroup of Precambrian sedimentary rocks exposed in central Ontario, stretching from Lake Superior to Quebec (Lindsay, 1969; Elyes and Young, 1994). Radiometric dating places the age of the Huronian Supergroup from 2.1 to 2.5 billion years old (Van Schmus, 1965, p. 769).

Geologists have interpreted rock fragments in the Gowganda tillite to be the outwash associated with the Early Proterozoic Gowganda glaciers centered southwest of Hudson Bay. Melting ice rafts (calved from ice sheets) dropped these pink granite fragments—from tiny particles to boulder-sized debris—into open water. These pink pebbles fell through the water and settled into fine-grained sediments (Kurtz, 1980). Those sediments likely originated around a glacier's margins (Kesler, 2019). Over time, these sediments were lithified or turned to rock.

After these sediments were lithified, they were carried away, at least two billion years later, by Pleistocene glaciers. After the ice of these last Ice Age glaciers melted, the ancient conglomerates were released onto the landscape, later to be found on beaches and in farmers’ fields in Michigan. Scientists have also found Gowganda tillites in Wyoming and Quebec, Canada.

Gowganda tillite (figure 2) is a conglomerate composed of well-rounded to sub-angular, to angular, poorly sorted clasts (granitic and gneissic pebbles dominate) scattered in a tough, massive matrix of coarse to very fine sand and chloritic[1] material ( Kurtz 1980). Rounding suggests some history of water transport prior to incorporation into the tillite. 

Figure 2. A sawed section of Gowganda tillite shows dropstones of various sizes. Sorting is completely lacking in most Gowganda tillite. From the collection of S. W. Veatch Photo by S. W. Veatch.

 The Gowganda tillite is one of the most well-known ancient glaciogenic deposits in the world because of its characteristic pink, granite clasts (pebbles) held in a fine-grained gray matrix (Kesler, 2019). Today, the Gowganda tillite—among some of the oldest rocks on Earth—continues to be studied by geoscientists. Samples are also sought after by rock and mineral collectors for their unique combination of unsorted pink pebbles, age, and interesting formation.

References and further reading:

Crowell, J. C., 1999, Pre-Mesozoic ice ages: their bearing on understanding the climate system (Memoir 192). Geological Society of America.

Eyles, N. and G. M. Young, 1994, Geodynamic controls on glaciation in Earth history, in, The Earth’s Glacial Record, eds. M. Deynoux, et al, eds: Cambridge, Cambridge University Press, p. 1-28.

Kesler, S. E., 2019, Great Lakes Rocks: 4 Billion Years of Geologic History in the Great lakes Region: Ann Arbor, University of Michigan Press.

Kurtz, D. D., 1980, Stratigraphy and Genesis of Early Proterozoic Diamictites: North America: PhD Thesis, Huston, TX, Rice University.

Lindsey, D. A., 1969, Glacial sedimentology of the Precambrian Gowganda Formation, Ontario, Canada: Geol. Soc. America Bull., v. 80, p. 1685-1702.

Young, G. M., and H. W. Nesbitt, 1985, The Gowganda Formation in the southern part of the Huronian outcrop belt, Ontario, Canada: Stratigraphy, depositional environments and regional tectonic significance: Precambrian Research, v. 29, p. 265-301.

Van Schmus, R., 1965, The geochronology of the Blind River-Bruce Mines area, Ontario, Canada: Jour. Geology, v. 73, no. 5, p. 755-780.

Wicander, R. and J S. Monroe, 2016, Historical Geology: Evolution of Earth and Life Through Time: Boston, Cengage Learning.

 

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[1]Chlorite is a group of silicate clay minerals occurring in both macroscopic and clay particle sizes; they are hydrous aluminum silicates, usually of magnesium and iron. Chlorites have a silicate layer structure similar to that in micas. Source: Britannica.