Tertiary Lake Sedimentation in the Elko Formation, Nevada — The Evolution of a Lake System in an Extensional Setting, and the Changing Location of Deep-Lake Source Rock Facies

Abstract

The Tertiary Elko Formation in eastern Nevada consists of basal sandstones and conglomerates that are overlain by brownish silt- and mudstones with root traces, carbonates, black shales, whitish ashes, and reworked volcaniclastics of the same color. The succession records the development of a fining-upward sequence from basal coarse-grained siliciclastics into overlying carbonates and brownish silt- and mudstones that grade into black shales, and an overlying coarsening recorded in the ashes and volcaniclastics. The succession, however, seems to vary from south to north: in the northern study area, this succession is recorded only once, and is overlain by the conglomerates of the Indian Wells Formation. In the south, in contrast, the basal part of the succession is marked exclusively by ashes and reworked volcaniclastics, whereas only the upper portion of the succession shows brownish silt- and mudstones as well as black shales. The succession is here interpreted to represent one large lake system with significant lateral changes in the development of accommodation space and resulting facies patterns. Sandstones and conglomerates are most likely alluvial deposits, carbonates and the brownish silt- and mudstones reflect marginal lacustrine sediments, black shales signify deep lake deposits, and the ashes and reworked volcaniclastics record infilling of a shallow lake during high influx of volcanic fallout. Based on this interpretation, northern outcrops, envisioned to be significantly older than the southern ones, record the early onset of lake sedimentation overlying basal alluvial conglomerates. There, marginal lacustrine deposition was abruptly substituted by black shale sedimentation, reflecting a significant deepening of this northern part of the lake. During this time, the southern portion of the lake accumulated volcaniclastic debris and carbonates, and was likely characterized by low subsidence. Over time, subsidence in the north must have diminished, and this lacustrine system was filled in with ashes. In the north, the alluvial fans of the Indian Wells Formation prograded over the infilled lake. During a second pulse, however, the southern part of the lake deepened, with black shales being deposited exclusively there, before the lake was filled again with ashes. Black shale source rocks therefore occur in the north and south of this lacustrine system, yet they represent rocks of different ages not correlatable throughout the Elko Formation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015