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Development of the Eocene Elko Basin, Northeastern Nevada : implications for paleogeography and regional tectonism Haynes, Simon Richard

Abstract

Middle to late Eocene sedimentary and volcanic rocks in northeastern Nevada document the formation of broad lakes, two periods of crustal extension, and provide compelling evidence that the Carlin trend was a topographic high during a major phase of gold formation. The Eocene Elko Formation consists of alluvial-lacustrine rocks that were deposited into a broad, extensional basin between the present-day Ruby Mountains-East Humboldt Range metamorphic core complex and the Tuscarora Mountains. The rocks are divided into the lacustrine-dominated, longer-lived, eastern Elko Basin, and the alluvial braidplain facies of the shorter-lived western Elko Basin. The base of the Eocene sedimentary rocks in the eastern basin is marked by a coarse boulder (>1 m clasts) conglomerate, overlain by a -150 m sandy pebble conglomerate with paleocurrent indicators indicating that rivers flowed to the northwest. U-Pb (zircon) dating of an air-fall tuff near the base of the pebble conglomerate constrains timing of initial basin fill to have begun by 46.1 ± 0.2 Ma. The coarser clastic rocks at the base are overlain by nearly 600 m of lacustrine limestone, siltstone, shale, and oil shale that interfinger with overlying air-fall tuff, which mark the end of clastic sedimentation at 38.9 ± 0.1 Ma (U-Pb, zircon). The eastern Elko Basin is the regional lacustrine depocentre, which originally formed as a half-graben in the hanging wall of a future core complex. The western Elko Basin fill is composed dominantly of cobble to pebble conglomerate, less than 200 m thick (fine-grained lacustrine rocks are largely absent), and sedimentation began by ~42 Ma. This area contains thick successions of ash-flow tuffs that began erupting by 40.5 Ma. Steeply dipping, south to southwest-striking normal faults subsequently cut strata of the Elko Formation and overlying air-fall tuff into a series of blocks on the order of hundreds of metres wide, and tilted them to the east and southeast. Andesite-dacite lava flows overlie the tilted strata and constrain the initiation of a second period of extensional deformation to between 39.5 and 38.5 Ma in the western basin, and ~38 Ma in the east. Paleocurrent measurements indicate that clastic sediments were shed away from the Carlin trend, to the north and the west during the Eocene. Likewise, the distribution of sedimentary and volcanic rocks demonstrates a general thinning towards the Carlin trend, and there is no evidence to suggest that the Elko Formation was deposited over this area. The Carlin trend was a topographic high during the Eocene, contemporaneous with gold formation.

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