Abstract
Oligocene sandstones (Frio Formation) of South Texas, deposited by the ancestral Rio Grande, contain a heavy mineral assemblage that is markedly altered, even in the shallowest sample examined (approximately 1 km depth). The weight fraction of detrital heavy minerals in the modern Rio Grande is approximately 1% whereas subsurface Frio sandstones contain as little as 0.1% (or less) heavy minerals. Modern Rio Grande sand contains a diverse and unstable heavy mineral assemblage dominated by pyroxenes, amphiboles, and an assortment of iron-titanium oxides; minor components include apatite, garnet, sphene, epidote, and zircon. Heavy minerals in subsurface Oligocene sandstones include abundant apatite, zircon, and titanium oxide among detrital components; minor iron-rich garnet also survives in the subsurface to depths of about 3 km. Other very stable, but also very minor, heavy minerals include tourmaline, chromite, spinel, and monazite. Locally, the heavy mineral fraction in the Frio is dominated by authigenic components such as siderite, pyrite, or barite. Authigenic sphalerite is also observed among the heavy minerals. Feldspars and rock fragments suggest that sediment deposited by the ancestral Rio Grande was more strongly dominated by volcanic detritus than is the modern river. It is reasonable to infer that the initial heavy mineral assemblage of the Frio Formation was likewise more unstable than that which appears today in the Rio Grande. Thus, heavy mineral loss determined by comparing subsurface Frio sandstones with the sand in the modern Rio Grande is conservative. Comparison of Sr and Nd concentrations in modern Rio Grande sands and subsurface Frio sandstones suggests that heavy mineral destruction in the subsurface does not cause major perturbations of whole rock isotopic systems involving these elements.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.