The Jabal Dalfa quadrangle (sheet 21/43 C) is part of the Najd province in west-central Saudi Arabia. The quadrangle is mostly a plain, tilted gently northeastward, but local inselbergs and two areas of dissected uplands rise as much as 200 m above the plain. Wadi Bishah and Wadi Ranyah terminate in the quadrangle.
The quadrangle is underlain by Precambrian metavolcanic, metasedimentary, and plutonic rocks. The gneiss outcrops in the northeast and east-central parts of the quadrangle are apparently the oldest rocks. After they were emplaced, a wide variety of metavolcanic and metasedimentary rocks were deposited at Jabal Dalfa and Umm Shat, and in the northeast part of the quadrangle as the Arfan formation. Subsequently, granite gneiss was emplaced in the west part of the quadrangle and intruded by gabbro. Metabasalt and meta-andesite were extruded in a wide north-trending belt through the middle of the quadrangle and at Jabal Silli. Intrusion of small bodies of granitic rocks and Najd faulting conclude the Precambrian history of the area.
Surficial deposits include sand and gravel covering the plains, alluvial fans, and voluminous dune sands.
In the southeast part of the quadrangle, the layered rocks strike north and dip steeply. They are oriented parallel to the Nabitah fault zone. In the northeast and east-central parts of the quadrangle, layered rocks and gneiss are sheared into slices by the southernmost faults of the major Najd fault zone. Bedding and foliation in these slices strike northwest, parallel to the faults. Gneiss in the west part of the quadrangle also strikes northwest, and dips steeply to vertically; layered rocks underlying Jabal Silli strike northeast.
Layered metamorphic rocks in the Jabal Dalfa quadrangle are mostly in the greenschist facies.
Projection of data from other quadrangles suggests that the oldest gneiss is about 780 Ma old and the Arfan formation, Umm Shat, and Jabal Dalfa layered rocks are about 775 to 745 Ma old. The gneiss of Shaib Hadhaq is probably about 720 Ma old, the gabbro is 640 to 625 Ma old, and the granite is about 600 Ma old.
The airborne magnetometer survey suggests that considerable additional gabbro underlies the west part of the quadrangle. Concealed serpentinite bodies, particularly in the Nabitah fault zone, are also suggested. Elongate anomalies and linear gradients indicate that the northeast part of the quadrangle is probably part of the Najd fault zone.
A number of gold-bearing quartz veins in the quadrangle were mined in ancient times, and on, at Jabal Umm Matirah has been drilled and found to be uneconomic. A nickel-copper prospect has proved uneconomic. geochemical survey for additional deposits of metals was completed in 1982.
Additional Publication Details
USGS Numbered Series
Reconnaissance geology of the Jabal Dalfa Quadrangle, sheet 21/43 C, Kingdom of Saudi Arabia