Master of Science (MS)


Geology and Geophysics

Document Type



Volcanic clasts at the base of the Tertiary Santa Fe Group of the San Luis Basin (SLB), a major extensional feature of the northern Rio Grande Rift, provide evidence for the direction of sediment transport and timing of regional tectonic events. A combination of clast whole rock geochemistry (major and trace element), mineral chemistry (amphibole, biotite, pyroxene and feldspar) and geochronology (40Ar/39Ar of amphibole and biotite and U-Pb of zircons) is used to constrain the possible source scenarios. Several potential sources with requisite geochemical and geochronological information exist for the Santa Fe Group volcanic clasts including rocks from Spanish Peaks (SP) and Mount Mestas to the east, the San Juan volcanic field (SJVF) to the west and the Thirtynine Mile volcanic field to the north of the basin. Petrographic analysis and whole rock geochemistry establishes that the Santa Fe Group contains volcanic clasts of a wide compositional range (trachybasalt to rhyolite). Trace element data show a strong overlap of SLB volcanic clasts with rocks of the SJVF and Thirtynine Mile volcanic field, while significant differences exist between the SLB and Mount Mestas rocks. There are differences in Na and Ti contents of amphiboles between SLB and SP rocks, but no significant differences occur between SLB and SJVF amphiboles. Geochronology of the SLB clasts indicates an age range of 35-29 Ma, similar to Thirtynine Mile volcanic field and to the Conejos Formation of SJVF, but too old for SP (~26-21 Ma) and Mount Mestas (~25 Ma). Based on this data, the Santa Fe Group volcanic clasts in the SLB are interpreted to have a western and northern provenance. This implies that sediment was likely sourced from the west and north, but not from the east. The Culebra Range, a part of the Sangre de Cristo Mountains that currently bound the eastern SLB, was likely at a position and elevation to hinder the transport of eastern sediments to the basin during deposition of the Santa Fe Group.



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Committee Chair

Barbara L Dutrow