Abstract

Limestone and dolomite samples obtained from four unoriented Swan Hills Formation cores from the Caroline gas field were examined with the goal of dating both dolomitization and emplacement of solid hydrocarbon. Detailed analysis of 110 specimens showed no difference in the magnetic components in limestone, dolomite, and bitumen-bearing rocks. The first-removed component (A) appears to have been acquired during sampling and is not considered further. The second component (B) is interpreted as a record of the present-day Earth's magnetic field. The third component (C) is a steeply dipping vector acquired during a period in the past when the Earth's magnetic field was reversed, and is recovered from almost all specimens. Declination values of component C were resolved by using component B as a "fossil compass needle" in cases where these two occurred together. This yielded an in-situ direction of declination/inclination = 155.7 degrees /-73.3 degrees , k = 26.4, alpha 95 (the cone of 95% confidence) = 7.1 degrees for the C component. The corresponding paleopole (74.7N, 192.3E, with 95% confidence limits of dp = 11.4, dm = 12.7) plots directly on the apparent polar wander path for (APWP) stable North America yielding a Late Cretaceous-early Tertiary age for the C component. Because the C component is not unique to the dolomite or bitumen-bearing rocks, but is shared by limestone far removed from the dolomite reservoir, we are hesitant to assign this Late Cretaceous-early Tertiary age to the dolomitization or bitumen formation; however, this record does testify to a fluid pulse at the end of Laramide orogeny. Such pulses currently are believed to be common events proximal to orogenic fronts and may participate in the migration of petroleum. If we accept the flow of fluids through the reservoir at Caroline as being responsible for the remagnetization, then this suggests that the gas charge was not in place (or completely in place) until after this event. This constrains the time of charging to during or after the Late Cretaceous-early Tertiary.

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