Early Coarse Clastic Deposition in the Western Elko Basin, Piñon Range, Northeastern Nevada: Implications for Basin Evolution and Petroleum Potential

Abstract

The Eocene Elko Formation of northeast Nevada consists of highly variable siliciclastic, carbonate, and volcaniclastic rocks, which are commonly interpreted to have accumulated in a half graben-lacustrine setting resulting from early extension of the Ruby Mountains metamorphic core complex. Black shales in this succession are potential petroleum sources. Despite recent interest in Elko Basin hydrocarbons, controversy remains about the basin’s age, shape, and extent. Recent mapping and U/Pb detrital zircon dating of conglomerates around the Piñon Range reveal that many units mapped as Mississippian-Permian Antler foreland and overlap sequences contain Eocene zircons (youngest age populations of ~45.3-43.7 Ma; one 38.6 Ma) and are part of the lowermost Elko Basin. One Eocene conglomerate in the western Piñon Range extends the basin beyond its previously interpreted southwestern limits. The oldest Eocene conglomerates rest almost exclusively on Upper Paleozoic clastic strata, whereas the youngest rests on Devonian carbonate. However, bedding in these conglomerates is mostly parallel to bedding in underlying units. These data indicate varying degrees of pre-Eocene folding, uplift, and erosion to ultimately form Eocene topographic highs that bound and underlie the developing Elko Basin.

45-44 Ma zircons probably derive from reworked tephra from northern sources as magmatism in and near the Piñon Range was exclusively ~38 Ma. However, 3-4 cm, subrounded, porphyritic igneous clasts from one sample were hand-picked and dated separately from matrix material. Results reveal Eocene (45.2 Ma) and Jurassic (158.2 Ma) zircon populations that likely represent contributions from two distinct ages of felsic igneous clasts. Jurassic clasts almost certainly were sourced from the Frenchie Creek Volcanics exposed 20km to the west in the Cortez Mountains, but nearby sources for the 45 Ma clasts are unknown.

Conglomerate dominates the Elko Formation rocks in our refined western basin. Thick shale is restricted to one area in the northern Piñon Range, although published thicknesses differ significantly and drilling has encountered major shale sequences just 25 km to the east. The rock type distributions suggest the basin was partly fed by drainages from the west and the western part of the basin has lesser volumes of potential source rocks and thus lesser petroleum potential than areas farther east.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018