Abstract of paper presented at CSEG & CSPG Joint National Convention, Exploration Update ‘94, May 9-12, 1994, Calgary, Alberta

Paleomagnetic dating of orogenic fluid-migration events in carbonates from the Western Canada Basin and Northern Alaska

D. R. Van Alstine and J. E. Butterworth, Applied Paleomagnetics, Inc.

It is becoming increasingly apparent that long-distance brine migrations are associated with major orogenies and that migrating "orogenic fluids" leave major mineralogic changes in their wake. As the chemical reaction front propagates through sedimentary sequences, profound diagenetic changes can occur in response to the changing reduction-oxidation (redox) environment. Paleomagnetism can play a key role in monitoring redox conditions, by detecting and age-dating a variety of authigenic magnetic minerals: iron oxides (magnetite, hematite, and maghemite), iron oxyhydroxides (goethite), and iron sulfides (pyrrhotite and greigite). Paleomagnetism can also help determine the timing of fluid migration relative to development of structural traps and fluid conduits, such as faults and fractures.

Although monitoring redox conditions is crucial in exploring for uranium "roll front" deposits, the significance of redox changes associated with migrating hydrocarbons is only beginning to be appreciated. Considering the enormous amount of iron, sulfur, oxygen, and hydrocarbons remobilized by "orogenic fluids," migration of the chemical remagnetization front would probably be associated with major diagenetic consequences affecting porosity and permeability.

After a decade of paleomagnetic work on subsurface drillcores from the world's major petroleum provinces, we are impressed by the enormous scale and pervasiveness of the orogenic-fluid remagnetization phenomenon. In many cases, chemical remagnetization extends to depths in excess of 4 km and extends laterally up to 600 km ahead of the mountain front.

An important principle underscored by paleomagnetic studies in orogenic belts is that orogenies are commonly time-transgressive, both along strike and perpendicular to strike. In the Appalachian foreland, remagnetization ages associated with the Alleghenian orogeny get younger from Alabama to New York. In northern Alaska, remagnetization ages associated with the Brooks Range orogeny change from Cretaceous (causing normal-polarity remagnetization in Lisburne Limestone from the western to central Brooks Range) to Paleocene (causing reversed-polarity remagnetization in the northeastern Brooks Range and Prudhoe Bay). Orogenic-fluid remagnetization during the Brooks Range orogeny in northern Alaska and during the Sevier/Laramide orogeny in southern Nevada to Alberta exhibits an important similarity: time transgression of both orogenies, parallel and perpendicular to strike, can be monitored relative to a distinctive change in "polarity bias" near the Cretaceous/Tertiary boundary.

In Paleozoic carbonates from northern Alaska, Cretaceous/Early Tertiary orogenic-fluid remagnetization obliterated pre-existing magnetizations. In some Paleozoic carbonates from the Western Canada Basin, especially in the Main Ranges, a pre-Laramide but still "secondary" magnetization is preserved, dating from the Late Permian or Early Triassic. This older remagnetization in the Western Canada Basin appears to be coeval with the Sonoma orogeny, which also appears to have remagnetized carbonates in Nevada, Utah, Wyoming, and Montana.