COLORADO AND NEW MEXICO ROCKIES 



401 



in the San Andres Mountains and the eastward tilted strata in the Sac- 

 ramento front are due to the block faulting, and hence that the ranges 

 are entirely late Cenozoic or Rasin and Range in age. If late Cenozoic 

 sediments are found to rest on Mesozoic beds on the floor of the graben, 

 then this is the proper interpretation, but if the basin fill rests on Pale- 

 ozoic and Precambrian beds, which seems to be the case, then the 

 uplift is much older than the faulting, and would be considered Lara- 

 mide. 



Another fairly evident large uplift is indicated by the geology of the 

 Ladron and Lucero Mountains, Puerco Platform, and Nacimiento Moun- 

 tains on the west, and the Los Pinos, Manzano, Manzanita, and Sandia 

 Mountains on the east. Precambrian rock appears to have been ex- 

 tensively exposed in the core before graben faulting of the Relen-Al- 

 ,buquerque basin. The uplift is called the Sandia, in Fig. 25.12 after the 

 imposing Sandia Mountains. 



, A thrust fault of Laramide age runs along the west side of the Naci- 

 miento mountains, and it has resulted in the Precambrian crystallines 

 .resting on the Cretaceous (Renick, 1931; Wood and Northrop, 1946). Ex- 

 famine cross sections H and I of Fig. 25.13. The thrust is of fairly high 

 ^ angle along most of its length, but at the north end it has several im- 

 bricate slices that dip at low angles. The maximum stratigraphic throw 

 ; ( is 3500 feet (Wright, 1946). 



A thrust that flanks the east side of the south end of the Sierra Naci- 

 miento is shown by Renick ( 1931 ) but not by Wood and Northrop ( 1946 ) . 

 See section I, Fig. 25.13. The elevated block between the two opposing 

 thrusts contains Paleozoic strata that are somewhat folded and faulted. 

 Northward, the general structure of the range is an asymmetrical faulted 

 uplift, the west flank being composed of steeply upturned and overthrust 

 beds. 



A broad, faulted monocline with downthrow on the east (see Fig. 25.14) 

 leads southward from the Sierra Nacimiento about 45 miles to the Lucero 

 uplift, where again thrusting has been recorded. The thrust on the east 

 front of the Lucero uplift extends from the Ladron Mountains northward 

 30 miles to Carrizo Arroyo, where it dies out in a tear fault (Kelley and 



Fig. 25.14. Faulting of the monocline between the Sierra Nacimiento and the Sierra Lucero. 

 After Hunt, 1938. Ku, Cretaceous shale and standstone; Jm, Jurassic standstone. 



Wood, 1946). The thrust dips westward at angles ranging from high to 

 low with the west side, the Lucero uplift, overthrust eastward. The 

 stratigraphic displacement ranges from 1000 to 4000 feet. 



The connecting monocline and its faults may have come into existence 

 later than the Nacimiento and Lucero thrusts (Wright, 1946), but defi- 

 nitely before Miocene time (pre-Santa Fe formation, Upper Miocene). 



Disconformities between the Ojo Alamo sandstone and the Torrejon 

 formation, and between the Torrejon and the Wasatch, point to the be- 

 ginnings of uplift in latest Cretaceous and early Paleocene time ( Reeside, 

 1924). Probablv the main uplift and thrusting occurred at the beginning 

 of Eocene time, preceding the deposition of the Eocene Wasatch, and 

 continued for some time during its deposition. 



From the above it is evident that the postulated large Sandia uplift was 

 not a simple anticline, but that it had small thrust structures within it, 

 such as the Lucero, and possibly folds. Also the development probably 

 proceeded in phases from latest Cretaceous into the Tertiary. 



