COLORADO AND NEW MEXICO ROCKIES 



399 



of depressions between bordering ranges and plateaus or platforms. The 

 basins are known collectively as the Rio Grande depression. See Fig. 25.11 

 for names and locations of the principal physiographic features. The 

 ranges are the result of Laramide deformation first, and later graben 

 faulting which has resulted in the general depression. This north-south 

 belt is generally recognized as the southward continuation of the Rockies 

 from Colorado, as well as a graben or rift belt of late Cenozoic 

 age. The Colorado Plateau lies on the west and the Great Plains on the 

 east. 



The basins of the Rio Grande depression have an approximate en 

 echelon arrangement, and the principal ones are as follows beginning on 

 the north in southern Colorado: 1, San Luis; 2, Espafiola; 3, Belen-Al- 

 buquerque; 4, San Marcial; 5, Engle; and 6, Palomas. The northern end 

 of the Belen-Albuquerque basin is called the Santo Domino basin. The 

 Estancia basin is separated from the Glorieta Mesa by normal faults, and 

 although shallow and irregularly alluviated, it is probably part of the 

 rift belt. The Jornado del Muerto basin, however, is not part of the rift 

 belt, but mainly a Laramide downwarp between two Laramide uplifts. 

 The Tularosa Valley is a downfaulted basin, and although not part of 

 the Rio Grande depression, is associated with it tectonically, and is part 

 of the general rift belt. 



Major Laramide Structures 



If the map of Fig. 25.11, and particularly the Geologic Map of New 

 Mexico ( 1928 ) are studied, a major anticlinal uplift is suggested by the 

 geology of the Sacramento Mountains front, the Oscura Mountains, and 

 the San Andres Mountains. The Tularosa Valley, which is mostly a down- 

 faulted basin, appears to have formed essentially in the core of the large 

 uplift. The uplift evidently had folds within it because several small 

 islands of Pennsylvanian strata appear in the alluvium well out in the 

 basin, and therefore, one cannot assume that erosion had stripped the 

 central part of the broad uplift everywhere to the Precambrian before 

 the graben faulting occurred. See cross sections S and T of Fig. 25.16. 

 The postulated uplift is labeled the San Andres in Fig. 25.12. 



It has been assumed by some geologists that the westward tilted strata 



Fig. 25.12. Laramide uplifts and late Cenozoic belts of rifting around the Colorado Plateau. 

 The uplifts with extensively exposed Precambrian cores are shown by double line; those with 

 Paleozoic core principally, by single line. Refer to Fig. 25.8 for New Mexico, and Fig. 25.2 for 

 Colorado. Rift valleys stippled. 



