Earth and Planetary Sciences ETDs

Publication Date

12-1979

Abstract

The present study undertook the geologic mapping and investigation of the stratigraphy, structural geology, igneous petrology and mineral resources of the Cerro del Grant area, a 75- (190 ) region near Coyote, Rio Arriba County, north-central New Mexico. The area is situated at the junction of three geologic provinces: the Chama platform, the Española Basin of the Rio Grande rift, and Jemez volcanic field.

Sedimentary rocks include 2,500 ft (830 m) of sandstone, shale, conglomerate and limestone of Late Triassic to Quaternary age. These strata are unconformably overlain by several hundred feet of mafic, intermediate and rhyolitic lavas which make up the northern part of the Jemez volcanic field. The entire section dips gently eastward, toward the Rio Grande rift, and has been broken by high-angle normal faults reflecting structural deformation during Late Cretaceous and late Cenozoic time.

Two distinct episodes of tectonic activity in the Cerro del Grant area can be seen in high-angle normal faults of weak to locally moderate intensity. A system of north-trending faults, which displace pre-Tertiary strata and have an average vertical separation of 350 ft (115 m), indicates movement during the Laramide, in Late Cretaceous to Eocene time. Several faults of this system appear to have been reactivated during the Tertiary. A superimposed zone of younger, northeast-oriented curvilinear faults, with an average throw of 150 ft (50 m), are late Cenozoic in age and form the western structural margin of the Española Basin. The curvilinear trace of these faults suggests the possibility of listric faulting with diminishing dips at depth. Small-scale displacements in the Bandelier Tuff suggest Quaternary movement. Structural analysis indicates mainly vertical crustal adjustment along high-angle normal faults during the Laramide. Subsequent late Cenozoic tectonic movement was also vertical; however, there may be considerable extensional dip-slip motion associated with the curvilinear faults.

Volcanism in the study area was concurrent with the latter stages of rift development and occurred over a 10-m.y. period, beginning in the mid-Pliocene. Extrusive centers appear to have been controlled by faults which bound the Española Basin. Volcanic rocks include basalts, andesites and dacites of the Lobato and Tschicoma Formations which are unconformably overlain by the Bandelier Tuff, a rhyolitic ash-flow unit. Petrographic study of the volcanics was augmented by nine chemical analyses obtained. Olivine basalt (49.2% Si) of the Lobato Formation, and a picrate basalt dike (44.9% Si) were classified as high-alumina, high-silica alkali basalts. Calc-alkaline andesites (56.4-59.5% Si) of the Lobato Formation are olivine bearing, generally with both ortho- and clinopyroxene present. Chemical trends and progressive phenocryst development in successively younger flows suggest magmatic differentiation in the Lobato andesites exposed at Encino Point. Unconformably overlying the Lobato Formation are coarsely porphrritic calc-alkaline andesite flows and extrusive centers of the Tschicoma Formation, containing about 61% Si. These are in part capped by hornblende and horn blende-biotite dacites (65.1% Si) which locally form small domes and were mapped as the upper member of the Tschicoma. Remnant outliers of the Bandelier Tuff (76.3% Si) unconformably overlie most lower units.

Intermediate volcanic rocks become more differentiated upsection as well as laterally toward the east in the study area. Progressive phenocryst development and probable cognate inclusions are interpreted as evidence of crystal fractionation. Calc-alkaline andesite eruptions appear to have been contemporaneous with minor alkali basalt volcanism. Earliest volcanic activity is recorded by the presence of picrate basalt bombs in a conglomerate of upper Santa Fe sandstone.

There is little evidence indicating metallic mineralization in the study area, although non-metallic resources, such as gypsum, sand and gravel, and pumice, occur in abundance. Uranium exploration is currently in progress, mainly in the Morrison Formation. No thermal waters are known in the area and hydrothermal activity noted at the surface is confined to volcanic centers, inactive for about 4 m.y.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Lee A. Woodward

Second Committee Member

J. Paul Fitzsimmons

Third Committee Member

Albert Masakiyo Kudo

Fourth Committee Member

Jonathan Ferris Callender

Language

English

Document Type

Thesis

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