Geology of the Jackson Mountains, Humbolt County, Nevada

Field: Earth and Planetary Sciences (Physical Sciences)

The Jackson Mountains, a prominent range near the center of Humboldt County, Nevada, are of interest because the Cretaceous rocks in the range record the effects of a late Cretaceous to early Tertiary orogeny. Such an orogeny has been assumed to have effected all of the Great Basin, but the rock record is sufficiently complete to provide positive dating in only a few areas such as the Jackson Mountains.The oldest rocks in the range are the Permian and older(?) volcanic rocks of the Happy Creek volcanic series which make up most of the northern half of the range. In a few places the Happy Creek volcanic series grades upward into undivided Permian and Triassic rocks, which consist of interbedded elastic sedimentary rocks and basic volcanic rocks, with some shaly and siliceous limestone. The Happy Creek volcanic series is also overlain by an unnamed predominantly limestone unit of Triassic age. A phyllite and slate unit of probable Triassic ass is in fault contact with the Permian and Triassic undivided rocks. At several other localities the Happy Creek volcanic rocks are overlain by the early Cretaceous King Lear formation or by the Cretaceous or Tertiary Pansy Lee conglomerate, which are the two units of chief importance in dating the Cretaceous and early Tertiary orogenic events.The King Lear formation consists of locally derived pebble and boulder conglomerate and interbedded siltstone and greywacke, and lenses of limestone.The Pansy Lee conglomerate is a pebble conglomerate with considerable interbedded coarse-grained sandstone. The pebbles consist of chart and quartzite completely unlike rocks pow exposed in the Jackson mountains.Dioritic rocks were intruded both before and after the King Lear formation was deposited. Granodioritic intrusive bodies in the range cut rocks no younger than Triassic but the granodiorite is believed to be of late Cretaceous or early Tertiary age.Tertiary intrusive and extrusive volcanic rocks and sedimentary rocks are widely distributed along the east side and south end of the range.The most extensive tectonic feature of the Jackson Mountains is the Deer Creek thrust, which is discontinuously exposed from Rattlesnake Canyon northeastward to the north side of Deer Creek Peak. The thrust has brought the Happy Creek volcanic series over the King Lear and Pansy Lee formations, and thus it is of late Cretaceous or early Tertiary age.An earlier period of Cretaceous deformation is shown by a northeastward-plunging syncline in the King Lear formation on the southeast side of King Lear Peak.Pre-Cretaceous deformation is shown by a tight fold in lime/atom of the undivided Permian and Triassic unit beneath the King Lear formation at the mouth of Rattlesnake Canyon.The late Tertiary deformation vas almost exclusively a response to vertically directed stresses, which generally produced high angle faults rather than folds. The range has probably been uplifted principally by displacement on faults that are buried beneath the alluvium some distance to the east and vest of the range.Ore deposits in the range include tome small but high-grade iron deposits, some low-grade quicksilver deposits and some small copper prospects.The iron occurs in veins that cut the Happy Creek volcanic series or as replacement bodies near the contact between diorite and the Happy Creek volcanics. Bleached volcanic rocks that are cut by numerous closely spaced joints with a film of hematite in the volcanic rock on either aids of the joints suggest that the iron of the iron deposits has been derived from the Happy Creek volcanic rocks. The diorite intrusives may have provided heat and solutions to mobilize the iron of the volcanic rocks.