Detrital zircon geochronology of the Upper Miocene Boleo Formation in the Santa Rosalia, Baja California Sur, Mexico
No Thumbnail Available
Authors
Meeting name
Sponsors
Date
Journal Title
Format
Thesis
Subject
Abstract
The Santa Rosalía Basin (SRB), Baja California Sur, is a transtentional to rift basin that formed during the opening of the Gulf of California. The Boleo Formation is the oldest and dominant sedimentary fill of the SRB, but its age is poorly constrained. We carried out a U-Pb detrital zircon study of the Boleo Formation to constrain its maximum depositional age. The Boleo Formation has basal limestone and gypsum sections that may be time transgressive. These are overlain by a much thicker clastic sequence, with coarsening upward cycles of mudstone, sandstone, and conglomerate. The formation also includes bedded Cu-Zn-Co-Mn ore deposits (known as mantos) that cap the conglomerate on each cycle, numbered 1, 2, 3 and 4 (from top to bottom of section). Sandstone samples were collected for U-Pb detrital zircon geochronology below each manto and in a clastic layer within the gypsum. A field wash table was constructed as a first step to concentrate heavy minerals and further processed in the lab using standard zircon separation methods yielding roughly 1,000 zircons per sample. We analyzed 315 zircons per sample by LA-ICPMS. Detrital zircon U-Pb ages from the Boleo Formation range dominantly from Late Miocene through Early Cretaceous, with minor Paleozoic and Precambrian ages. Only 5 to 22 grains per sample are less than ~10 Ma, and of those, all stratigraphic levels are dominated mostly by ~9 Ma zircons, except for the stratigraphically highest sample. Zircons from the top section of the Boleo Formation form a coherent group of 3 with a TuffZirc age of 5.5-5.7 Ma and an age of 8.9 (+0.04/-0.10) Ma for the base. The 8.9 Ma below the lowest copper bed and limestone in the Boleo Formation marks the maximum age of the inception of the Santa Rosalia basin which is also constrained by underling rift transition volcanic rocks. These zircon-based age results indicate the existence of a seaway in the gulf region after 8.9 Ma and certainly by 5.5-5.7 Ma paralleling Late Miocene crustal extension in the Gulf of California and development of the offshore Guaymas Basin.
Table of Contents
Introduction -- Background -- Methodology -- Results -- Discussion -- Conclusion -- Appendix A. U-Th-Pb decay system -- Appendix B. Isochron dates from Busby et al., 2018 -- Appendix C. Stratigraphic columns from sample collection outcrops within the Boleo Fm -- Appendix D. Final weights of samples after field processing -- Appendix E. Elemental mapped sample images -- Appendix F. Intensities for 206/238, after Ludwig (2012) -- Appendix G. Concordia diagrams for all six samples (<20 Ma) -- Appendix H. Histograms on ages of all six samples -- Appendix I. Data for detrital all six samples -- Appendix J. Map with sample location of this study and Busby et al. (2018, in revision)
DOI
PubMed ID
Degree
M.S. (Master of Science)