SEDIMENTOLOGY AND PALEOMAGNETISM OF THE ZICAPA FORMATION, GUERRERO: AN APPROACH TO UNDERSTANDING OF THE EARLY CRETACEOUS ACTIVE MARGIN IN WESTERN OAXAQUIA PREVIOUS THE INSTAURATION OF THE GUERRERO-MORELOS PLATFORM

In this work we report the preliminary results of a study that includes geological mapping, stratigraphy, sedimentology, paleomagnetism and detrital geocronology of the Sierra Madre del Sur in the states of Guerrero and Puebla, southern Mexico. The Lower Cretaceous Zicapa Formation is characterized by the presence of clastic rocks interbeded with layers of limestones and calcareous sandstones, deposited in continental to marine transitional environment, with an important input of primary volcanic material contemporaneous with de depositation. This volcanism has been dated near Chiautla with a maximum deposition U-Pb age of 139 My in one volcanoclastic sandstone. Data presented here is for the San Juan de las Joyas locality, in the Mixteca terrain, east of the limit of this terrane and the Guerrero terrain, east of the Papalutla fault. In this locality the Zicapa Formation covers nonconformably Paleozoic metavolcanic rocks of the Acatlán Complex, and underlies platform limestones of the Morelos Formation. Zicapa then shows a transition from an active margin with clastic sedimentation to a passive margin in the Early Cretaceous. The paleomagnetic data from 17 sites selected in the San Juan de las Joyas locality, in one continuous segment of 110 m of fine to coarse lithic sandstones, siltstones and local layers of conglomerates. The sites are in red to purple sandstones and siltstones, where the characteristic magnetizations reside primarily in hematite. The accepted sites yield a tilt corrected mean of declination Dec= 273°, inclination Inc= 33.1° (α95=11.7 y k=18). The mean directions are discordant with respect to expected North America reference for Early Cretaceous indicating a counterclockwise rotation of about 50°. This rotation may be caused drag of the Tierra Colorada fault system to the south, which also causes the reorientation of the laramidic folds in the area.