ONSET OF EXTENSION IN THE BASIN AND RANGE: BASIN ANALYSIS OF THE EOCENE ELKO FORMATION, NE NEVADA

The history of early Tertiary extension in the Basin and Range province is commonly obscured by younger events of varying intensity. In NE Nevada, the oldest Tertiary extension is recorded in the fluvial-lacustrine late Eocene Elko Formation and correlative rocks. These rocks were deposited unconformably over late Paleozoic and Triassic sedimentary rocks, and currently lie in the hanging wall of the Miocene Ruby Mountains-East Humboldt Range (RMEH) metamorphic core complex and west to the Tuscarora Mountains. Massive boulder conglomerates, with unsorted clasts up to 1.7 m in diameter, mark the base of the Elko Formation. The conglomerate unit is a proximal alluvial fan deposit derived from nearby fault scarps, and restricted to the eastern outcrops. The basal conglomerate is overlain by fine to medium-grained sandstone and chert pebble conglomerate, with paleocurrent data suggesting a southeast to northwest flow direction. A 46.1 ± 0.1 Ma (U-Pb; zircon) tuff is interbedded near the base of the sandstone unit. Overlying the lower clastic units are 600 m of nearshore to open lacustrine limestone, siltstone and oil shale, indicating a change to a tectonically more stable environment. Swamps lay along the margins of the lakes. The uppermost shales interfinger with late Eocene rhyodacitic tuffs of Indian Well Formation (38.9 ± 0.3 Ma; U-Pb; zircon). Further west, in the Tuscarora and Independence Mountains, Eocene cobble conglomerate, sandstone and tuff less than 50 m thick form proximal alluvial deposits. Thick lacustrine shales found in the Elko Formation to the east are lacking in the western parts of the basin. Tuff near the base of the cobble conglomerate yield U-Pb ages of 41.9 ± 0.1 Ma (U-Pb; zircon) and the entire succession is overlain by felsic ash flow tuffs that yield U-Pb ages of 39.8 ± 0.4 Ma. Development of the Eocene basin(s) was apparently time-transgressive, with upper crustal extension having started several million years earlier in the east than in the west. The overall basin configuration is a N to NE-striking asymmetric half graben that lies west of the RMEH metamorphic core complex. Sedimentation and basin development ceased with the onset of felsic magmatism during the late Eocene.