Abstract

The Galisteo-Tonque area lies on the east margin of the Rio Grande Depression, in north-central New Mexico. It extends from the north end of the Sandia Mountains to the south end of the Sangre de Cristo Mountains.

The Eocene Galisteo formation, deposited by streams flowing southwest from the Sangre de Cristo Mountains, was buried by volcanic detritus deposited in great alluvial fans peripheral to contemporary eruptive centers in the Ortiz Mountains and Cerrillos Hills. Surface volcanism was accompanied and followed by emplacement of stocks, laccoliths, sills, and dikes, the larger bodies localized in a north-south line, marked by the present Cerrillos Hills, Ortiz, San Pedro, and South Mountains. Igneous activity heralded and accompanied regional deformation, during which broad, northward-plunging, faulted folds were formed in a block north of a northeast-trending fault system.

Oligocene (?) deformation and igneous activity were followed by a period of erosion, interrupted locally by extrusion of limburgite and basalt flows (Cieneguilla limburgite). Subsequent broad warping in the early (?) Miocene led to deposition of alluvial and lacustrine beds in a sedimentary basin centering northwest of the Galisteo-Tonque area, in the Rio Grande Depression. Detritus was contributed locally to the basin from highlands eroded in the areas of Oligocene uplift, and tuffaceous materials in a part of the basin probably represent the distal end of the Abiquiu tuff fan, spread from contemporary centers of eruption in northernmost New Mexico.

Deposition of the Santa Fe formation in the same basin continued during the Miocene and Pliocene. The Abiquiu tuff fan was buried by arkosic fans built southwestward from the Sangre de Cristo Mountains. During the late Pliocene, movement began on some of the normal faults bounding the present Rio Grande Depression. The boundary faults outline a structural depression coincident with the earlier basin of deposition in a general way, but differing considerably in detail. The diastrophic movements were reflected first by lithologic changes in the Santa Fe formation, and eventually by the change from deposition to erosion in the Rio Grande Depression.

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