Page 29 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

28 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
shales. In a few places a thin bed of gray earthy limestone has been noted, which according to Darton occurs near the base of the Abo, and which has yielded fossils indicating Permian age. At the base of the Abo in many exposures is a limestone conglom- erate, the pebbles of which were derived from the underlying rocks of Magdalena age. The thickness of the formation varies from 400 to 800 feet.
The Chupadera formation, as de- scribed by Darton, 15 includes the rocks of Permian age above the Abo sandstone which were formerly known as the Yeso forma- tion and the San Andres limestone. In places in Sierra County and elsewhere in the State it is not possible to separate these rocks into the two formations. The lower division (Yeso) of the Chupadera formation consists mainly of gypsum, gypsiferous shale, limestone and soft sandstone, which are colored red, pink, yellow and gray. In places a yellow massive sandstone is the top member. Some of the gypsum beds are massive and attain a thickness of as much as 140 feet. The Yeso is from 500 to 1,000
feet thick.
The San Andres limestone, the upper division of the Chupa-
dera formation, consists for the most part of gray sandy lime- stone in which some Permian fossils are found. The thickness is from 300 to 500 feet. In places gypsum and sandstone beds are intercalated with the limestone. This limestone has a character- istic appearance wherever noted, the surface of many beds within it being cracked, pitted, and roughened, as though a sandy, limy ooze had been laid down in extremely shallow water, and before complete consolidation it had been repeatedly expoSed to the atmosphere with attendant development of a confused mass of shrinkage cracks, ripple marks, rain drop impressions, etc., none of which are now separately distinguishable.
Chupadera strata outcrop along the eastern slopes of the Fra Cristobal, Caballos, and Cuchillo ranges, and they possibly overlie the Abo sandstone along the east front of the Black Range.
CRETACEOUS SYSTEM
So far as known, Triassic, Jurassic and Lower Cretaceous rocks are absent in the county, and Upper Cretaceous rocks directly overlie the Chupadera formation. This hiatus may be due to a long period of non-deposition from Permian to Upper Cretaceous time owing to the elevation of the area above sea level, or to the removal by erosion of sediments that were depos- ited. The Upper Cretaceous formations consist of the Dakota sandstone, Mancos shale and Mesaverde formation.
Chupadera Formation.
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Outcrops of Dakota sandstone occur in the low foothills along the east slope of the Sierra Caballos
15 Dayton, N. H., "Red Beds" and associated formations in New Mexico: U. S. Geo!, Survey Bull. 794, pp. 21, 22, 1928.
Dakota Sandstone.
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and in the southeast flank of the Fra Cristobal Range. The