This EarthCache is a rock monument located on the grounds of the Calgary Branch of the Geological Survey of Canada and consists of a sample piece of a 375 million years old Devonian reef extracted near the town of Canmore. It is an example of the much larger formations lying under the Alberta plains and holding vast deposits of oil and gas.
Fig. 1 - Devonian Reef EarthCache
About 375 million years ago the Rocky Mountains were still not uplifted and the future North America continent was bordered by a shallow sea covering much of the future province of Alberta. In these marine conditions, vast tracts of carbonates accumulated in the clear, warm, shallow seas. Myriad marine organisms extracted calcium carbonate from the sea water in order to build their shells and construct broad carbonate banks. In areas where circulation patterns and water clarity permitted, algae, corals, brachiopods and creatures called stromatoporoids – extinct organisms related to sponges – built chains of organic reefs on top of carbonate banks.
Fig. 2 - 375 million years Devonian reef model created by the Royal Tyrrell Museum
In these shallow waters, the sea floor was continuously receiving sediments from the continent brought by rivers, as well as abundant organic matter derived from decomposed marine organisms. These new deposits accumulated beside and over these reefs creating black, limy shales, full of organic matter. As the reefs were buried the calcite in the centres of stromatoporoids’ shells underwent a chemical change and became dolomite, creating interconnected holes inside the now buried reef. With progressively deeper burial, the added pressure and heat caused the organic material in the surrounding shale to change into liquid and gaseous petroleum that, in turn, was squeezed out of the limy shales and into the holes of the reefs, where it became trapped.
Fig. 3 - Drawing representing a Devonian Stromatoporoid Reef 375 million years ago
375 million years later these reefs are buried some 1,500 to 2,500 metres below the surface of the Alberta plains. When the Rocky Mountains were uplifted between 80 to 55 million years ago some parts of these reefs were exposed and are now visible. The rock in this EarthCache is a piece of a Devonian limestone reef collected from rocks in the Front Ranges of the Rockies, more precisely from the White Man Gap, overlooking the town of Canmore, Alberta. But deep below the surface of Alberta, the interconnected pores of much larger reefs are filled with oil and gas acting as petroleum and gas reservoirs. Oil fields such as Leduc, Redwater, Swan Hills, Virginia Hills, Judy Creek and many others produce petroleum from such reefs.
Fig. 4 - Current location of Devonian Reefs in Alberta
Sources:
- Geological Highway Map of Alberta, published by The Canadian Society of Petroleum Geologists, 2000.
- How Old Is That Mountain?, Chris Yorath, published by Habour Publishing, 2006.
To log this EarthCache, cachers are asked to e-mail the answers to the following questions to me through my geocaching.com profile. Cache loggers that do not send me answers within 2 weeks will have their logs deleted.
1. E-mail (do not post). What is the estimated volume of the rock in cubic metres?
2. E-mail (do not post). If you get closer to the monument you will notice the rock is full of holes. These are the holes that once harboured shells of stromatoporoids some 375 million years ago. Estimate the percentage of the rock's volume occupied by holes.
3. E-mail (do not post). Based on the volume of the rock from answer 1 and the percentage of volume occupied by holes from answer 2, What volume of oil and gas in litres (1 cubic metre = 1,000 litres) this single rock could hold in its holes? And how many oil barrels (1 Oil barrel = aprox. 159 L)?
4. E-mail (do not post). 15 metres SW of the rock there is another monument - a Cairn. What is this cairn covered by and where does the covering material comes from?
5. Optional: Post a photo of yourself with GPS in hand or if you are alone, a photo of your GPS at the EarthCache site.
Warnings:
This EarthCache is located on the grounds of the Geological Survey of Canada and therefore on private land. Cachers are authorized to enter the grounds, park on the visitor parking spots during daylight hours and visit the cache site. The monument is located outside the main building entrance so there is no need to enter the building. The GSC has a great library with thousands of publications on geosciences. You can sign in and visit if you like. This is a federal building and holds a research laboratory, therefore the whole complex is monitored and under security. Please do not take samples of the monument rock or any other part of the property.