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Quarrying operations at Winnfield salt dome in the North Louisiana basin provide access to calcite, gypsum, and anhydrite cap-rock zones. Sulfide laminae in the anhydrite zone are comprised dominantly of pyrhotite with lesser amounts of sphalerite, galena, pyrite, and marcasite. Sulfides cement euhedral anhydrite grains and represent the products of the episodic introduction of metalliferous waters along the salt-anhydrite interface during halite dissolution and residual anhydrite accumulation. Thus, sulfide laminae provide a chronological record of anhydrite cap-rock accretion.
Two hundred oriented samples were collected in stratigraphic sequence, covering much of the exposed anhydrite section. Alternating field demagnetization readily revealed the magnetic polarity of most samples. Using only reversely magnetized samples with a well-defined stable magnetization (N = 50, ^agr95 = 6.2°) yields a pole position at 71.4°N, 125.7°E, which implies that the sampled cap-rock sequence formed in the Late Jurassic. This age is consistent with geologic evidence indicating that cap-rock formation began in the late Jurassic and was most intense during the Early Cretaceous.
A densely sampled 45-ft stratigraphic interval contains a sequence of normal and reverse polarity zones. Assuming a constant formation rate, these zones can be compared with the M-anomaly sequence. A growth rate of about 67 ft/m.y. (20 m/m.y.) is indicated. This value is about 30 times less than estimates of salt dome growth rates. Because a 10 to 50-fold decrease in volume is associated with halite dissolution and anhydrite accretion, the paleomagnetically determined value for cap-rock formation rate is reasonable.
This investigation is the first such study undertaken. The results are encouraging and offer a unique approach for investigating the timing of various geologic processes related to salt dome formation.
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