COLORADO AND NEW MEXICO ROCKIES 



405 



form. The Santa Fe has been assigned to ages that range from late Miocene 

 to Pleistocene. In its typical development it is an alluvial-fan deposit of a 

 characteristic pinkish or light-tan color. Although it is locally grayish, it 

 i generally stands in fairly marked contrast to the somber brown, purplish- 

 brown, or grayish-white of the middle Tertiary sediments upon which it often 

 rests. The Santa Fe is typically a relatively non-volcanic sediment, but in 

 imany places, especially along the west side of the depression, its coarse frag- 

 iments may be almost exclusively volcanic, but even in these places the 

 characteristic pinkish color is evident in the clay and sand beds. The Santa Fe 

 i in large part reflects the rocks which were at the surface in the adjoining 

 uplifts, and the superposition of its local members commonly roughtly reflects, 

 iin reverse order, the stratigraphic superposition of the adjoining areas. In 

 many places where the adjoining uplift consisted of carbonate rocks such as 

 the Magdalena, San Andres, or lower Paleozoic formations, the adjacent Santa 

 Fe is largely a calcirudite fanglomerate. Elsewhere playa and lake deposits 

 form a large part of the Santa Fe. Pyroclastic breccia and tuff may be abundant 

 'in the Santa Fe, and this is especially true around the Jemez uplift. Basaltic 

 flows are almost a characteristic of the Santa Fe, and are intercalated sparingly 

 throughout the section. 

 j 



CENTRAL NEW MEXICO PORPHYRY BELT 



A zone of Laramide intrusions extends from Rlack Mountain in the 

 Sangre de Cristo Range southward through central New Mexico into 

 Mexico. It coincides with the belt of faulting and uplifts in the central 

 part of New Mexico. The intrusions take the form of stocks, laccoliths, 

 dikes, and sheets. The largest stocks are in the Sandia and Ortiz Moun- 

 tains just southwest of Santa Fe, and in the Sierra Blanca, Capitan, and 

 Gallinas Mountains, just north of the Sacramento uplift. Also, the Organ 

 Mountains northwest of El Paso are mostly of intrusive rock. These plu- 

 jtons are clearly intrusive into the Paleozoic strata, and in places into the 

 Upper Cretaceous, and all are believed to be Laramide. Like the uplifts, 

 most of them have not been accurately dated because the youngest rocks 

 intruded are commonly Paleozoic. See Chapter 36 for further discussion 

 of the igneous rocks. 



GUADALUPE AND MARATHON UPLIFTS 



An arm of the Sacramento uplift extends southeastward to the Texas 

 boundary, where the Guadalupe Mountains compose themselves and ex- 



tend south-southeastward about 80 miles to the Davis Mountains volcanic- 

 area. The Guadalupe Mountains separate the salt basin on the west from 

 the Delaware basin on the east. See cross sections S and T of Fig. 25.16. 

 Like the other uplifts, they are an asymmetrical structure, an eastward- 

 tilted block with a complex zone of high-angle faults along the west side. 

 Examine the Tectonic Map of the United States. 



The Davis Mountain volcanic area is in effect a large structural basin. 

 Along its northeast border is a belt of long, fairly gentle anticlines and 

 synclines that lead to the Marathon uplift, in whose core the late Paleozoic 

 compressional structures known as the Marathon Mountains are exposed. 



The Marathon uplift is dome-shaped and surrounded by Cretaceous 

 beds. The exposed core is not centrally located in the Cretaceous dome; it 

 is mostly in the western half. According to King ( 1937 ) , the base of the 

 Lower Cretaceous strata 4 miles north and south is 5500 feet below the 

 base in the Glass Mountains that form the north rim. The dip of the Cre- 

 taceous beds to the east is about 100 feet per mile. 



The western margin of the Marathon dome is formed by the Del Norte 

 and Santiago mountains, which are eroded out of a sharp monocline or 

 anticline. The fold is overturned toward the west and broken in most 

 places by an eastward-dipping thrust that has raised Paleozoic and Lower 

 Cretaceous rocks on the east against younger Cretaceous beds on the west 

 (King, 1937). This narrow belt of compressional deformation seems to 

 be alone in the belt of domes and basins of central New Mexico and the 

 Trans-Pecos region of Texas. About 75 miles to the southeast, the Sierra 

 Madre Oriental of Coahuila, Mexico, which is a system of folded and 

 thrust structures, continues in the strike of the Santiago thrust, and may be 

 a continuation of it. The Sierra Madre Oriental, however, seems more 

 related to the Laramide folding and thrust belt of southern Arizona, 

 southwestern New Mexico, and the Quitman Mountains region of Texas- 

 Mexican border, southeast of El Paso and northwest of the Davis Moun- 

 tains volcanic area. See the map of the Laramide orogenic belts. Fig. 19.1. 



The Marathon dome is younger than the Lower and Upper Cretaceous 

 beds, but only part of the elevatory movement preceded the effusion of 

 the Davis Mountain volcanics, which are Eocene and Oligocene in age. 

 In one place, the lavas overlie Upper Cretaceous strata, and in another 



