Saturday, 15 December 2018


In the early 1980s, Tim Tozer, Geological Survey of Canada, looked at the distribution of marine invertebrate fauna in the Triassic of North America.

Tozer's interest in our marine invert friends was their distribution and what those occurrences could tell us. How and when did certain species migrate, cluster, evolve — and for those that were prolific, how could their occurrence — and therefore significance — aide in an assessment of plate and terrane movements that would help us to determine paleolatitudinal significance.

In the western terranes of the Cordillera, marine faunas from southern Alaska and Yukon to Mexico are known from the parts that are obviously allochthonous with regard to the North American plates. Lower and upper Triadic faunas of these areas, as well as some that are today up to 63 ° North, have the characteristics of the lower paleo latitudes. As far as is known, Middle Triadic faunas in these zones do not provide any significant data. In the western Cordillera, the faunas of the lower paleo latitudes can be found up to 3000 km north of their counterparts on the American plate. This indicates a tectonic shift of this magnitude.

There are marine triads on the North American plate over 46 latitudes from California to Ellesmere Island. For some periods, two to three different fauna provinces can be distinguished from one another. The differences in fauna are obviously linked to the paleolatitude. They are called LPL, MPL, HPL (lower, middle, higher paleolatitude). Nevada provides the diagnostic features of the lower; northeastern British Columbia that of the middle and Sverdrup Basin that of the higher paleolatitude. A distinction between the provinces of the middle and the higher paleo-situations can not be made for the lower Triassic and lower Middle Triassic (anise). However, all three provinces can be seen in the deposits of Ladin, Kam and Nor.

Diatoms / Microalgae dominant components of phytoplankton
If one looks at the fauna and the type of sediment, the paleogeography of the Triassic can be interpreted as follows: a tectonically calm west coast of the North American plate that bordered on an open sea; in the area far from the coast, a series of volcanic archipelagos delivered sediment to the adjacent basins. Some were lined or temporarily covered with coral wadding and carbonate banks.

Deeper pools were in between. The islands were probably within 30 degrees of the triadic equator. They moved away from the coast up to about 5000 km from the forerunner of the East Pacific Ridge. The geographical situation west of the back was probably similar.

Jurassic and later generations of the crust from near the back have brought some of the islands to the North American plate; some likely to South America; others have drifted west, to Asia. There are indications that New Guinea, New Caledonia and New Zealand were at a northern latitude of 30 ° or more during the Triassic period. The terranes that now form the western Cordillera were probably welded together and reached the North American plate before the end of the Jurassic period.

Tozer, ET (Tim): Marine Triassic faunas of North America: Their significance for assessing plate and terrane movements. Geol Rundsch 71, 1077-1104 (1982).

Danner, W. (Ted): Limestone resources of southwestern British Columbia. Montana Bur. Mines & Geol., Special publ. 74: 171-185, 1976.

Davis, G., Monger, JWH & Burchfiel, BC: Mesozoic construction of the Cordilleran “collage”, central British Columbia to central California. Pacific Coast Paleography symposium 2, Soc. Economic Paleontologists and Mineralogists, Los Angeles: 1-32, 1978.

Gibson, DW: Triassic rocks of the Rocky Mountain foothills and front ranges of northeastern British Columbia and west-central Alberta. Geol. Surv. Canada Bull. 247, 1975.