Magnetostratigraphy of mid-Cretaceous limestones from the Sierra Madre of northeastern Mexico

We present the results of a palaeomagnetic study of four mid-Cretaceous lim estone sections exposed in northeastern Mexico, The limestones are weakly m agnetized and exhibit two- to three-component magnetizations. These magneti zation components appear to be carried by both a sulphide mineral and a mag netite-titanomagnetite mineral, The sulphide mineral carries a reverse pola rity overprint that often makes it difficult to isolate definitively the hi gher-unblocking-temperature component. The high-unblocking-temperature comp onent is well defined in the upper portion of the Santa Rosa Canyon section and in the Cienega del Toro section and passes the fold test, The characte ristic remanent magnetization (ChRM) inclinations agree well with predicted mid-Cretaceous inclinations for these sites, although the declinations dif fer by more than 100 degrees. The relative rotation between these two sites probably occurred as the thrust sheets were emplaced during Laramide defor mation. At two of the sections, namely Cienega del Toro and the overturned Los Chorros sections, only normal polarity directions are observed, The La Boca Canyon and Santa Rosa Canyon sections exhibit zones of both normal and reverse polarity magnetization, Correlation of these polarity zones with t he geomagnetic polarity timescale provides a time framework for lithostrati graphic and palaeoceanographic studies of these sections.