SEDIMENTARY GEOCHEMISTRY USED TO DETERMINE PROVENANCE AND TECTONIC SETTING OF SANDSTONES IN THE LA POPA BASIN, MEXICO

Whole-rock geochemical analyses of Late Cretaceous to Early Tertiary sandstones in the La Popa basin in northeast Mexico provide constraints on the tectonic setting of their source areas. The five units examined by this study in stratigraphic order are: Muerto Fm., Delgado Sandstone Tongue, Upper Sandstone Member (both of the Potrerillos Fm)., Viento Fm., and Carroza Fm. Potential source areas for La Popa sandstones include: Triassic plutonic and metamorphic rocks of the Coahuila platform, Triassic-Jurassic island arc rocks of the accreted Guerrero terrane, accreted ocean island basalts of the Arperos terrane, recycled sedimentary rocks from the Sierra Madre fold and thrust belt, and the coeval Alisitos continental arc.

Preliminary data show a wide range of compositions in La Popa sandstones: SiO₂=50.9-80.5 wt%; SiO₂/Al₂O₃=5.3-9.3; Sr=155-590 ppm; Rb=23-59 ppm; Zr=89-328 ppm; Nb=4-10; and Y=11-18 ppm. These data refine provenance models. For example, the high percentage of volcanic lithic fragments in Muerto sandstones suggests a source in either the Arperos or Guerrero terrane. Volcanic arcs have low Nb, Y, and Zr and high Sr and Y/Nb ratios; conversely, ocean island basalts have high Nb and Y accompanied by low Zr and Y/Nb ratios. The Muerto Formation has low Al₂O₃ (6.4 wt%), Y (11 ppm), Nb (4 ppm), and Zr (89 ppm) and high Sr (591 ppm), suggesting that these sands originated in either the Guerrero island arc or the later Alisitos arc. Similarly, the Upper Sandstone Member consists of basement sands and recycled sedimentary detritus containing high Zr (328 ppm), Rb (26 ppm), Y (17 ppm), and Nb (18 ppm) with low Sr (130 ppm). The high Rb and low Sr values are consistent with K-feldspars and the high SiO₂ concentration (80.5 wt%) is consistent with recycled sedimentary sands. These data are consistent with an origin from the granitic Coahuila block and its sedimentary cover.