Abstract

Paleomagnetic analysis of specimens from the Middle Devonian Sulphur Point Formation in four wells in the Rainbow South field, Alberta, Canada, showed magnetization that correlated with the different lithological fractions and facies observed within the wells. In limestone, the fine-grained mudstones contain a Late Devonian magnetization with the paleopole at 36.3°N, 99.4°E (dm = 6.3°, dp = 11.7°) while the coarser-grained grainstones and packstones contain a Tertiary or present-day magnetization with a paleopole at 81.2°N, 210.6°E (dm = 4.7°, dp = 4.5°). Of five previously identified dolomitic fractions or phases, three were isolated in sufficient quantity to allow standard paleomagnetic analysis and age determination: the fine-crystalline dolomite had a paleopole of 75.2°N, 143.0°E (dm = 12.1°, dp = 14.1°) in the Middle to Upper Jurassic; the medium-crystalline dolomite had a paleopole of 69.9°N, 190°E (dm = 10.5°, dp = 11.2°) in the Cretaceous; and the saddle dolomite paleopole at 74.9°N, 203.2°E (dm= 5.6°, dp = 5.8°) was of Paleocene or Eocene age. Insufficient samples of the earliest dolomitic phase, dolomicrite, were available for statistical analysis. However, the paleoinclination suggests a minimum age of Early Jurassic. Paleomagnetic dates thus confirm the relative order of dolomite formation as identified in previous studies.

The presence of the Paleocene–Eocene magnetization is one of the most significant findings in this study. This magnetization is present in specimens containing vug-filling, replacement or veined saddle dolomite, and is carried by magnetite. Thus, in the Rainbow South Field, the timing of saddle dolomite is unambiguous, and its precipitation must be due to the Laramide Orogeny.

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