ARCHITECTURE OF BASAL EDIACARAN DEPOSITS (NOONDAY GROUP) IN SOUTHWEST LAURENTIA

The late Precambrian successions of southwest Laurentia are among the most complete Cryogenian-Ediacaran successions on the globe. The most scrutinized of these successions occurs in the eastern Death Valley region. With the advent of the snowball Earth hypothesis, the Noonday Dolomite has received particular attention because it is a potential correlative of the Marinoan cap carbonate sequence. However, understanding the sequence framework and global context of these strata have been hampered by uncertainties in correlations at both local and regional scales. Based on new geological mapping and stratigraphy from the central Panamint Range to the west of Death Valley we herein elevate the Noonday to group status, and propose a stratigraphic framework and nomenclature that ties together the well known but generally incomplete platform facies that dominate the eastern Death Valley region (PF) with more complete basinal sections best preserved in the Panamints (BF), but also locally present in the eastern Death Valley region. Deposition begins with the tube-bearing, vuggy cap dolostone of the Sentinel Formation, represented in BF as a 2-10 m marker horizon variably overlain by siliceous carbonates, and in PF as 100-200 m of algal dolostone. This cap represents the first of three major marine highstands. Each shoals upward into sand sheets with erosive bases, thereby defining sequence boundaries E0, E2 and E4 (flooding surfaces) and E1, E3 and E5 (bases of sand sheets). The sand sheets mature upward from arkosic wacke in the first sequence (Ibex Formation) to orthoquartzite in the third (Quartzite Member of the Johnnie Formation). We define the Radcliff Formation to include strata between E2 and E4, and the Mahogany Flats Formation between E4 and E5, and assign strata above E5 to the Johnnie Formation. In this nomenclature, PF sections (e.g., in the southern Nopah Range) include the Sentinel (Noonday 1) overlain unconformably by sub-E3 Radcliff (intermound fill), all truncated by E5 and overlain by basal Johnnie dolomitic sandstone (Noonday 2 or “Transitional Member” of Johnnie Formation). This new stratigraphic understanding resolves a variety of problems in Death Valley geology and enables better comparison of the Noonday Group to Marinoan cap carbonate sequences worldwide.