Luncheon – March 15, 2002

Title: The Middle Jurassic Elko Orogeny — A Major Tectonic Event in Nevada-Utah
Speaker: Charles H. Thorman, U.S. Geological Survey, Emeritus
Date: March 15, 2002
Publication: The Outcrop, March 2002, p. 5-6

The Elko orogeny is a widespread Middle Jurassic event that extended from central Nevada to central Utah and involved miogeoclinal and eugeoclinal rocks. The region has undergone a continuum of eastward-progressive deformational pulses from the middle Paleozoic to early Tertiary. Each successive orogenic event resulted in the superposition of structures and has made it difficult to impossible to determine the age of many of them because of their similarity in style, unless crosscutting relationships exist. These deformational events include the Antler (late Devonian-middle Mississippian), Humboldt (late Pennsylvanian), Sonoma (late Permian-early Triassic), Elko, and Sevier (middle Early Cretaceous-early Tertiary) orogenies.

The Elko appears to have occupied the same area as the Sevier in Nevada, but did not extend as far to the east as the Sevier. Many structures in eastern Nevada-western Utah attributed to the Sevier are more likely Elko in age.

A Middle Jurassic Elko foredeep in western Utah along the leading eastern edge of the orogenic belt is inferred from Middle Jurassic strata that thicken westward and abruptly terminate at the edge of the Colorado Plateau. The depocenter of the westward expanding sedimentary sequence was in western Utah, and possibly easternmost Nevada, but was uplifted and eroded during the Sevier orogeny. The existence of a Middle Jurassic Elko foredeep subsequently uplifted and eroded during the Sevier orogeny is compatible with the orogenic belt to the west and explains the missing westward thickening wedge.

Miogeoclinal and eugeoclinal rocks ranging in age from Proterozoic to early Middle Jurassic were deformed during the Elko orogeny. Late- to post-tectonic late Middle Jurassic plutons (155 to 165 Ma) cut structures and regionally metamorphosed rocks throughout the region. Major features include eastward-directed older-over-younger thrust faults and bedding parallel ‘thrust’ faults. Another important type of structure includes E- to ESE-trending strike-slip or tear faults. Some strike-slip faults are deep-penetrating structures, whereas others are restricted to allochthonous thrust sheets as tears and/or lateral ramps. The orientation of the larger strike-slip faults appears to have been inherited from Precambrian basement and strongly influenced Sevier and Basin-and-Range faulting and emplacement of plutons of all ages. Large-scale crustal shortening of Middle Jurassic age has yet to be demonstrated, though many Elko structures indicate crustal shortening occurred. In addition, extensional faulting has been documented in numerous ranges, a style of deformation atypical of the Sevier.

The role of the Elko orogeny in the formation and accumulation of hydrocarbons in eastern Nevada and western Utah is not well understood at the present time. Burial depths, and subsequent maturation, of Paleozoic and early Mesozoic strata prior to and during the Elko for the majority of the region undoubtedly differed little, if any, from that calculated for Sevier time. No major mineral deposits of Elko age are documented in eastern Nevada-western Utah. Mineralization related to the plutons formed predominately small to moderate polymetallic base-metal deposits with subordinate gold. It is of major importance, however, to note that Elko-age mineralization was strongly controlled by the E- to ESE-trending faults and related structures. This being the case, migration of hydrocarbons at the Elko was most certainly influenced by these same structures. Thus, Mesozoic migration of hydrocarbons controlled by structures most likely began, at a minimum, in Middle Jurassic time. The superposition of Sevier deformation ~30-50 Ma later probably disrupted traps resulting in the loss of the trapped hydrocarbons or their migration to other sites.