Tectonic framework of Albuquerque country
— Lee A. Woodward

There have been at least five major episodes of deformation in the area of this field conference, including the Precambrian, late Paleozoic, Laramide (Late Cretaceous—early Tertiary), middle Tertiary, and late Cenozoic. Precambrian tectonics are not considered here because of numerous unresolved problems concerning reliability of radiometric dates, lack of data on directions of stratigraphic younging, complications from folding and faulting, and general difficulty in correlations of rock units (Fulp and Woodward, 1981; Armstrong and Holcombe; Cavin and others; and Grambling, this guidebook).

From Cambrian through Devonian time this region was mildly positive, being located on the south flank of the transcontinental arch; as a result, there are no lower Paleozoic strata here. During the Missis-sippian a thin sequence of shelf carbonates accumulated (Armstrong, 1967; Armstrong and Mamet, 1979). High-angle faulting and epeiro-genic uplift resulted in removal of most of the Mississippian strata prior to deposition of Pennsylvanian rocks.

In the late Paleozoic a structurally and topographically high area, the Pefiasco axis (Read and Wood, 1947), developed along the present Nacimiento uplift. This positive area shed granitic elastic debris into adjacent marine basins during Pennsylvanian time and persisted as a mildly positive area into the Permian (Baars, this guidebook). Mesozoic strata were deposited as relatively uniform blankets across the region. Epeirogenic uplift and tilting resulted in regional, low-angle unconformities between Permian and Triassic, and between Jurassic and Cretaceous strata, with large parts of the Triassic and Jurassic systems being absent.

The major elements of the Colorado Plateau and Rocky Mountains, the Lucero uplift, Acoma sag, Nacimiento uplift, and San Juan Basin, attained their present structural outlines during Laramide time. Nu-merous dikes, sills, stocks, and laccoliths were emplaced in the Ortiz porphyry belt during middle-Tertiary time. These igneous bodies, mainly porphyritic laccoliths, form the Ortiz porphyry belt.

Late Cenozoic crustal extension was superimposed on the older struc-tures and resulted in development of the Rio Grande rift, a series of en echelon grabens and half grabens filled with elastic sediments. Contemporaneous volcanism occurred in the Mount Taylor and Jemez areas as well as within the rift.

The present physiography of the region is due mainly to late Cenozoic deformation with subordinate topographic expression of Laramide and middle-Tertiary structures.