Magnetostratigraphy and geochronology of the upper Balsas Group, Guerrero, Mexico

The Balsas Group in the Guerrero-Morelos platform consists of continental redbeds, of fluvial, alluvial, and lacustrine origin that post-date Laramide deformation in southern Mexico. Although this unit is widespread in the region, its stratigraphy, thickness and facies variations, and tectonic significance are largely unknown. It lacks fossils, but it is sandwiched between the radiometrically dated Tetelcingo Formation (ca., 65 Ma) and the Tilzapotla caldera products (ca. 33 Ma.). Red sandstone, siltstone, and mudstone, intercalated with abundant gypsum beds, outcrop along Federal Highway-Autopista del Sol, north of the Balsas River. These are interpreted as part of the Balsas Group, but its position within the sequence is uncertain. They appear to overlie typical limestone boulder conglomerate alluvial facies and underlie buff-colored lacustrine deposits. The section is approximately 110 m in thickness. A pink-colored ignimbrite occurs within the sequence and has been dated using Ar-Ar techniques. The ignimbrite is about 4 m thick and is associated with an altered rhyolitic flow. The ignimbrite includes lithic fragments, green altered pumice, as well as sanidine and biotite crystals in an aphanitic matrix. 40Ar-39Ar age determinations provide relatively imprecise ages and spectra do not provide strict plateaus; interpreted pseudo-plateau ages of 35.7+/-0.5 Ma (biotite) and 31.3+/-1.3 Ma (sanidine), indicate a late Eocene-early Oligocene age. Samples collected for magnetostratigraphy from this section and a section exposed within the Amacuzac river valley, north of Atenango del Río, are consistent with deposition during the chron sequence C17-C12, within the resolution allowed by the sampling interval. This result shows that this part of the Balsas Group partly contemporaneous with the main ignimbrite emplacement episode in the region. Paleomagnetic directions are concordant, suggesting that previously determined post-Laramide tectonic rotation had ceased by Early Oligocene time.