Paleomagnetic study of Jurassic and Cretaceous rocks north of San Marcos fault, central Coahuila, México

Abstract

Upper Jurassic and Lower Cretaceous continental strata at Potrero Colorado, in the core of Sierra La Fragua anticline in central Coahuila, carry dual–polarity characteristic magnetizations that we interpret as near primary chemical remanent magnetizations. Upper Jurassic fluvial and eolian sandstones of the Colorado beds yield a tilt–corrected mean of D=10.2° I=28.2° (n=18 sites, k=22.1, α₉₅=7.5°). A steeper inclination magnetization observed in five sites of the Colorado beds is interpreted as a secondary, post–Laramide folding, magnetization. Lower Cretaceous fluvial sandstones of the San Marcos Formation yield a combined mean of D=358.1°, I=46.4° (n=13 sites, k=22.7, α₉₅=8.9°), but the stratigraphically lower sites on the eastern side of the anticline have south–southwest directed and moderately steep negative magnetizations (mean of D=191.7° I=–54.9°; k=38.7, α₉₅=9.8°, n=7 sites) which are statistically distinct from northwest directed magnetizations (mean of D=333.6° I=58.0°; k=28.3, α₉₅=12.8°, n=6 sites) observed in stratigraphically higher sites in the west–central part of the anticline. We interpret the difference in declinations in the San Marcos Formation as the result of rotation during deposition of this unit. The characteristic shallow–inclination magnetization in Jurassic strata is also discordant with respect to the Late Jurassic reference direction. Both, the lower San Marcos Formation and the Colorado beds indicate clockwise rotations of 38.5°±8.3° and 30.0°±2.3°, respectively. The sampling sites are located within a clastic wedge that records activity of the San Marcos fault during Late Jurassic and Early Cretaceous time. These data indicate that during deposition of the Colorado beds and San Marcos Formation, the fault behaved as a normal fault with a small right–lateral component. The area of Potrero Colorado is interpreted as a zone of relay in the normal San Marcos fault, which accommodated clockwise rotation recorded by Upper Jurassic and Lower Cretaceous strata.