Resumo

ARVIZU-GUTIERREZ, Irving Rafael; GONZALEZ-NARANJO, Gildardo Alonso; STANLEY MOLINA-GARZA, Roberto  e  CHAVEZ-CABELLO, Gabriel. Magnetostratigraphy of the Lower Cretaceous San Marcos Formation, Coahuila, Mexico: tectonic and paleogeographic implications. Bol. Soc. Geol. Mex [online]. 2011, vol.63, n.2, pp.157-170. ISSN 1405-3322.

We report the results of a magnetostratigraphic study of the San Marcos Formation in central Coahuila, at two localities along the San Marcos Fault. These localities are characterized by outcrops of a sequence of conglomerates, immature sandstone and red mudstone, cemented by hematite; the sequence was deposited in a continental environment, and has been assigned to the Early Cretaceous. All specimens, from a total of 35 paleomagnetic sites, were subjected to alternating field and thermal demagnetization, the latter being the most effective in isolating the (high temperature) characteristic magnetization. The characteristic component is unblocked between 625° and 660 °C. Both normal and reverse polarity magnetizations are represented, and the overall mean is of Dec = 352.7 °, Inc = 55.5 °, N = 3 localities (18 sites), k = 27.07, α₉₅ = 6.8 °. This result suggests a small clockwise rotation with respect to the expected reference direction for the Cretaceous, estimated from the apparent polar wandering curve for cratonic North America. However, the data suggest that there was relative rotation between localities, and the observed rotations are more likely explained as local vertical-axis rotations. The magnetic polarity zonation in the San Marcos Formation suggests that this unit represents the Early Barremian through Early Aptian. The correlation to global magnetic polarity timescale suggests that this unit is correlative with Cupido Formation, and that the transgressive littoral facies of the Las Uvas Formation, as well as flooding of Coahuila paleo-island at Potrero Colorado, occurred in the Early Aptian. The best correlation with the global magnetic polarity time scale corresponds to the interval from M5n (normal) to M0r (inverse) approximately between 131-125 Ma. The sedimentation rate indicated by this correlation, assuming a constant rate, is relatively low (~30-50 m/Ma) for an active tectonic environment and is more consistent with a region where accommodation is achieved by isostatic adjustment between crustal blocks (Coahuila Block and Sabinas Basin).

Palavras-chave : Cretaceous; Coahuila; Sabinas Basin; magnetostratigraphy.