AGE AND STRUCTURE OF THE CHICOMUSELO FOLD BELT, CHIAPAS, MEXICO

The Chicomuselo Fold Belt (CFB) is a little known large scale orogen distributed along a thin fringe between southern Mexico, Guatemala and Belize. It is formed by a succession of sedimentary and metasedimentary folded Paleozoic rocks including the Santa Rosa Inferior-SRI, Santa Rosa Superior-SRS, Grupera-G and Paso Hondo-PH formations, which include black phyllite and metapsamites, slate and sandstone, shale and sandstone interbedded with limestone, as well as limestone and dolomite in massive strata, respectively. The age of the SRI and PH formations was constrained between the Mississippian and Permian. These rocks were folded and foliated, and are unconformably covered by middle Jurassic redbeds, which suggests that shortening in the CFB occurred between the Late Permian and the Early Jurassic. The first slate cleavage (S1) is pervasive along and across the CFB, it dominates the fine-grained units and has a NW-SE trend. S1 is locally affected by kink bands, and seems to be folded on a kilometer-scale. Open to closed folds of hundreds of meters are common in the PH Fm.

About 70 samples of phyllite, slate and shale were collected across the belt, and the illite crystallinity index (ICI) analyzed in them. The results suggest that the deformation in the CFB occurred between high anchizone and high diagenetic conditions, with a gradient that slowly increases from the SRI to the PH formations. Samples for illite dating were also collected in these two units. In the first sample, mica fishes were observed under the scanning electron microscope, where a second generation of hairy illite overgrows such fabric. In the second sample only one generation of micrometric size illite was detected. However, the characterization and dating of several clay size fractions (0.2-0.5, 0.5-1 and 1-2µm, in addition to 2-4 µm in the first sample) indicate that both samples contain essentially two generations of illite, one related to the event of penetrative deformation occurring at 246±6 and 253±8 Ma, and another less intense event affected these rocks between 170±6 and 178±6 Ma, respectively. U-Pb calcite ages from veins emplaced in the PH Fm. also support these results. These data suggest that the CFB was formed about the Permo-Triassic threshold, and then affected by middle Jurassic faults, presumably associated to the opening of the Gulf of Mexico. The SRI and SRS formations include thousands of meters of black shale deformed at low anchizone-high diagenesis. This brings up the question: Were these units oil source rocks in the Gulf of Mexico reservoirs at some point?