Generalized Geologic Column
Silver City Area (San Lorenzo to the Burro Mountains)

Quaternary Alluvium Unconsolidated gravel, sand, and silt on valley floors and along streams.


Gila Conglomerate Poorly to well cemented deposits ranging from sand to gravels. Occasional interbedded basalt flows. Deposited in faulted grabens. Rare mammal fossils (mostly teeth). 0-1000 feet thick.
Volcanic Rocks Ash-fall deposits and lava flows with a wide-range of composition. In some cases interbedded volcanic sandstone and gravels. 0-3000 feet thick.
Tertiary (Paleocene) Intrusive igneous rocks. Igneous rocks that intruded and metamorphosed the surrounding sedimentary rocks. Stocks carried significant mineralization.
Cretaceous Colorado Shale Dark green, brown, and black shale. Some sandstone layers. Ammonite fossils may be found. Up to 1000 feet thick.
Cretaceous Beartooth Quartzite White to grey sandstone. Occasional beds of conglomerate. Cross-beds and ripple marks common. 150 feet thick.
~~~~~~~~~~ UNCONFORMITY ~~~~~~~~
Permian Abo Formation Red shale and mudstone. Absent in most of the Silver City Area, it is present only northeast of the Central mining district.
Pennsylvanian Magdalena Limestone Gray, crinoidal limestone and mudstone with chert bands. In the mining district the Magdalena Formation is subdivided into the Syrena and Oswaldo Formations. Fusilinids are abundant. The Syrena is absent in Silver City and the Oswaldo is 175 feet thick. Fusilinids are abundant.
Mississippian Lake Valley Limestone Gray limestone, crinoidal, upper part is massive, lower part includes some shale beds. Chert is present throughout. In addition to crinoids, fossils include brachiopods, corals and bryozoa. 300-400 feet thick.
Devonian Percha Shale Shale, the lower part is black and contains very few fossils. The upper part is gray and has abundant limestone nodules and thin beds of limestone. Fossils in the upper Percha include abundant brachiopods, and occasional horn corals and snails.
Silurian Fusselman Dolomite Gray dolomite. Vugs and chert very common. Where present fossils include: Brachiopods, horn corals and gastropods. 100 feet thick.
Ordovician Montoya Formation Light gray to gray, finely crystalline dolomite. Banded chert in the middle part. The lowermost 20-30 feet is sandstone. Rare fossils. 300-350 feet thick.
Ordovician El Paso Formation Medium gray to gray brown dolomite and sandy dolomite, containing chert. 350-500 feet thick. Gastropods, cephalopods and less abundant brachiopods, sponges, and trilobites. 300 - 500 feet thick.
Cambrian Bliss Formation Red brown and gray sandstone, conglomeratic and arkosic in places. Cross-beds abundant. Rare brachiopod fossils. 150 - 200 feet thick.
~~~~~~~~~~ UNCONFORMITY ~~~~~~~~
Precambrian Burro Mountain Granite Medium to coarse-grained granite. Includes fragments of Precambrian metamorphic rocks in the Silver City Range.
Precambrian Metasedimentary Rocks. Greenstone at San Lorenzo. In Silver City and Burro Mountains includes metamorphosed sedimentary and metamorphic rocks.

Cross-Section of Silver City Area


Cunningham, J. E., 1974, Geologic maps and sections of Silver City Quadrangle, New Mexico:
         NM Bureau of Mines and Mineral Resources, GM-30.

Jones, W. R., Hernon, R. M., and Moore, S. L., 1967, General geology of Santa Rita Quadrangle:
         USGS Professional Paper 555, 144 p.

Guidebooks of the New Mexico Geologic Society.