Castle Dome Mountains, Yuma County, Arizona, USAi

Regional Level Types
Castle Dome Mountains	Mountain Range
Yuma County	County
Arizona	State
USA	Country

These mountains are comprised of a basement of schist, gneiss, granite, and weakly metamorphosed sedimentary rocks, all intruded by dikes of diorite porphyry and overlain by a thick series of lavas cut by dikes of rhyolite porphyry.

The oldest rocks exposed are schists that comprise two areaas totaling several square miles in the southern portion of the range. Thes outcrops form narrow, rolling surfaces alternating with sharp ridges that generally mark the location of steeply-dipping dikes. These schists have been regarded as Precambrian, but definate evidence of their age is lacking. In places they contain gold-quartz veins.

Associated with the schist in the southeastern portion of the range are considerable areas of gneiss. The rock is typically coarse-grained, with phenocrysts of quartz and feldspar as much as 1/8 or ½ inch diameter. It is rudely laminated by prominent bands of scaly biotite, but on weathered slopes this lamination is not apparent, and the rock resembles granite. Fiekld relations indicate that the gneiss was originally a granite that intruded the schist. It contains a few small quartz veins.

Medium-grained biotite granite crops out in a small area 2 miles northeast of Thumb Butte zand as a small mass at the Sheep prospect, in the southeastern portion of the range. This granite, which unconformably underlies the Cretaceous (?) sedimentary rocks and intrudes the schist and gneiss, may be either Precambrian, Mesozoic, or early Tertiary in age. Only a few gold-quartz veins are known to occur within it.

Chloritized, epidotized syenite of unknown age crops out in a small area southeast of the Big Eye Mine, on the east slope of the range.

A thick series of well-bedded sedimentary rocks, for the most part weakly metamorphosed, rests unconformably upon the granite. These rocks make up an irregular area 9 miles long by 8 miles wide in the southern portion of the range. Almost every exposure of these rocks contains numerous dikes of diorite porphyry and rhyolite porphyry.

These sedimentary rocks consist predominently of greenish-gray, thick-bedded shale and impure cherty limestone, locally with maroon shale and fairly pure limestone. Gray to brown arkosic sandstone, quartzite, and conglomerate are abundant in places. The gray shale contains considerable alumina and some magnesia, but very little lime carbonate. These beds clearly exceed 1,000 feet in total thickness.

Volcanic rocks of probable late Cretaceous or Tertiary age make up the major portion of these mountains. These include rhyolite, andesite, tuff, and obsidian, with a total thickness of more than 2,000 feet. In places, they are covered by several hundred fet of basalt, probably of Quaternary age. Locally, the volcanic rocks are cut by dikes of and large irregular masses of rhyolite porphyry.

The Cretaceous (?) sedimentary rocks predominently strike northwesterly and dip 30º to 70ºSW, but many local differences occur. As a rule, the dikes dip steeply and strike parallel to the principal lamination, jointing, or bedding of the formation that they cut. The volcanic rocks prevailingly strike subparallel to the trend of the range, but they show many local departures. The basalts are essentially flat or dip at angles of a few degrees.

Localities in this Region

Other Regions, Features and Areas containing this locality

References