SEQUENCE-STRATIGRAPHY OF THE LOWER SHALE MEMBER OF THE CAMBRIAN LINCOLN PEAK FORMATION

The infilling of a sediment-starved, distal portion of a tectonically controlled basin is represented by the Middle Cambrian Lower Shale Member of the Lincoln Peak Formation of the northern Schell Creek Range, eastern Nevada. Biostratigraphy constrains the deposition of the member between the latest Wuliuan and early Guzhangian Stages—a period of some 6 million years. The Lower Shale Member is interpreted to represent three, third-order sequences. Of these, the first sequence’s sequence boundary is located below the base of the unit, in the Pole Canyon Limestone. The sequences observed in the Lower Shale Member mirror previously described sequences in the coeval Wheeler Shale and overlying Marjum Formation of Utah at a considerably lower resolution. The lower Shale Member is condensed, with 200 m of the Lower Shale Member equivalent to the 400-600 m of the Wheeler Shale and Marjum Formation. This condensation is most evident at the base of the Lower Shale Member, which is almost entirely composed of a highly fissile “paper shale.” This paper shale directly overlies the subtidal deposits of the Pole Canyon Limestone (interpreted as the lowstand systems tract). Upsection, the paper shale deposits are increasingly interbedded with turbidites, and the turbidites become increasingly dominant up section, and are eventually overlain by storm reworked strata. These, in turn, are overlain by subtidal ribbon limestone beds. The Lower Shale Member is capped by peritidal and subtidal beds of the Middle Limestone Member, which mark the start of a sequence that stretches into the overlying Upper Shale Member.

This succession is interpreted to represent the development of a tectonically induced drowning of the local carbonate platform and the development of a hemipelagic basin. This basin was sediment starved with a dysoxic seafloor. The deposition of turbidite deposits from a carbonate ramp then caused a forced regression, which largely filled the basin by the start of the Guzhangian Stage. This lower resolution and extreme condensation are attributed to severely reduced sediment supply to present-day eastern Nevada, associated with the rapid rise in local sea level.