The Structural Geology and Volcanic Petrology of the Rattlesnake Hills, Wyoming
Abstract The Rattlesnake Hills, covering 150 square miles in central Wyoming, are the eroded remnants of a northwest plunging Laramide anticline on the north flank of the Sweetwater Arch. The topography has been modified by Middle and Late Eocene intrusive and extrusive activity with associated local faulting, and by Miocene or Pliocene normal faulting. At least 42 discrete intrusive bodies, radiometrically dated as Late Eocene, are present in the map area. These intrusives are divisible into seven rock types: rhyolite, quartz latite, latite, trachyte, soda trachyte, alkali mela-trachyte, and phonolite. The alkalic intrusives are clustered in a 20 square mile area, with the quartz latite and rhyolite intrusives occurring in 2 groups to the northeast and southwest of the central alkalic group. The pyroclastic debris in the Middle and Late Eocene Wagon Bed Formation appears to be primarily from the quartz latite eruptive centers. The alkalic centers were primarily laccoliths and volcanic domes, with little extrusive activity. The petrogenesis of the intrusive rocks is thought to involve the generation of two separate magmas, quartz latite and phonolite, by partial fusion in the lower crust, followed by occasional fractional crystallization and/or mixing of the two magmas to produce the observed suite of undersaturated, saturated, and over saturated rocks. The structural development of the Rattlesnake Hills is related to the known structural history of Central Wyoming given by Love (1970) and Keefer (1970). After the end of the Laramide orogeny in the earliest Eocene, the Sweetwater Arch continued to rise and resulted in the formation of a horst at the end of Early Eocene time. The development of this horst may have lowered the pressure in the lower crust, favoring the formation of magma for the volcanic activity in the Rattlesnake Hills during the Middle and Late Eocene. The Oligocene was a time of crustal stability in the Rattlesnake Hills, when a major erosion surface was cut on the Precambrian hornblende schist and the Middle and Late Eocene Wagon Bed Formation. Boulder channel deposits of northeast flowing Oligocene streams now form linear ridges resulting from topographic inversion on the north flank of the Rattlesnake Hills. A detailed study of the faulting in the Rattlesnake Hills indicates three main periods of faulting: 1) faulting during the Laramide orogeny, 2) faulting associated with the intrusive activity of the Middle and Late Eocene volcanic episode, and 3) faulting in conjunction with the subsidence of the Granite Mountains after Middle Miocene time. A new estimate of 1000 −1200 feet of post Middle Miocene offset on the North Granite Mountains fault system is in agreement with earlier estimates by Love (1970).