AAPG Bulletin, V. 83 (1999), No. 8 (August 1999), P. 1223-1235.

Late Cretaceous-Early Tertiary Remagnetization of the Devonian Swan Hills Formation Recorded in Carbonate Cores from the Caroline Gas Field, Alberta, Canada¹

K. P. Gillen,² R. Van der Voo,³ and J. H. Thiessen⁴

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received July 18, 1997; revised manuscript received July 20, 1998; final acceptance January 29, 1999.
²Vox Terrae International, Suite 1430, 700 4th Ave. SW, Calgary, Alberta T2P 3J4, Canada; e-mail: [email protected]
³Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063.
⁴Beau Canada Exploration Ltd., Floor 47, 150 6th Ave SW, Calgary, Alberta, Canada.

We thank Shell Canada Ltd. for allowing this work to be published. The paper benefited significantly from reviewer comments, especially those of Neil Hurley, Mark Longman, and Doug Elmore. 

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°/-73.3°, k = 26.4, a₉₅ (the cone of 95% confidence) = 7.1° 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.