I. Glacigenic and related strata of the Neoproterozoic Kingston Peak Formation in the Panamint Range, Death Valley region, California. II. The Basal Ediacaran Noonday Formation, easternudCalifornia, and implications for Laurentian equivalents. III. Rifting of southwest Laurentia during the Sturtian Marinoan Interglacial: the Argenta Orogeny

摘要

I. Glacigenic deposits in the Death Valley region occur within the Neoproterozoic Kingston Peak Formation. In the Panamint Range, immediately west of Death Valley, these strata are ≥1000 m thick and are continuously exposed for nearly 100 km along the strike of the range. Although these strata are variably metamorphosed and locally exhibit pronounced ductile strain, original sedimentary textures are well preserved throughout the range. Diamictic strata occur in two distinct intervals, a lower one comprising the Limekiln Spring and Surprise Members, and an upper one known as the Wildrose Sub-member of the South Park Member. Each of these intervals are succeeded by well defined cap carbonates, which, from oldest to youngest, are the Sourdough Member of the Kingston Peak and the Sentinel Peak Member of the overlying Noonday Formation. Between the two glacial successions, the Sourdough and sub-Wildrose South Park units comprise a ~300 m thick interglacial succession that includes platform carbonate deposition. Sparse lonestones and striated clasts, along with the impressive lateral continuity of diamictic units, support a glacial origin. Chemostratigraphic profiles of δ13C through the Sourdough (-3‰ to +2‰, increasing upward) and Sentinel Peak (-3‰ +/- 1‰) suggest correlation with the Sturtian and Marinoan caps, respectively. Potentially economic U deposits (secondary brannerite) occur in graphitic schists of the Limekiln Spring Member and sub-economic U and Th (hosted by detrital monazite) occur within quartz-pebble conglomerates in the South Park Member. The strata contain no fossils, radiometric age control, or primary magnetizations.ududII. The Neoproterozoic-Cambrian succession in the Death Valley region of SW Laurentia is among the best exposed and easily accessible in the world, and comprises one of the most complete sections in Laurentia. The largest single exposure of these strata occurs in the Panamint Range on the west flank of Death Valley, but this area has received little attention in comparison to numerous exposures to the east of Death Valley, primarily because of structural complexity and metamorphism. The eastern strata, although unmetamorphosed, occur in isolated fault-bounded exposures and are relatively thin and incomplete compared to the Panamint stratigraphy. These factors, combined with a lack of fossil or radiometric age control, has hindered confident regional correlation, as well as placement in the context of hallmark Neoproterozoic events observed in the South Australian, Namibian and other successions around the globe. New geological mapping, measured sections and high-resolution C-isotope data reported here from the Noonday Formation in the Panamint Range delineate its regional stratigraphic architecture and establish its age through correlation with section with radiometric age control. Carbon isotopic trends in the Panamint Range match to within 1-2‰ reproducibility previous results obtained for correlative strata in the eastern sections, indicating that metamorphism did not significantly alter C isotopic ratios.ududThe combined lithostratigraphic and chemostratigraphic data form the basis for a revised, more complete stratigraphic framework for the Noonday Formation. A composite section shows that, where most complete, the Noonday consists of three members, from the base upward, the Sentinel Peak, Radcliff, and Mahogany Flats members. New mapping and chemostratigraphic data permit robust regional correlation of a thin dolostone marker horizon at the base of the Noonday in the Panamint Range as little as 2 m thick (Sentinel Peak Member) with a tube-bearing microbial dolostone in the eastern Death Valley region more than 200 m thick. The data also reveal that the Radcliff Member is bounded by disconformable surfaces and their correlative conformities. These surfaces are recognizable throughout the region and are used to construct a regionally unified stratigraphic nomenclature.ududA key finding of this study is the construction of a chemostratigraphic profile spanning most of Noonday time. This was greatly aided by the discovery of carbonatebearing strata in the lower part of the Radcliff Member in the Tucki Mountain area of the Panamints, and relating their stratigraphic position to upper Radcliff and younger Noonday strata in the Wildrose Canyon area. The chemostratigraphic profile is a remarkable match for the Maiberg cap carbonate sequence in Namibia, including the decline to a minimum at -5‰, a recovery to near 0‰, and then subsequent decline to -2‰. Globally, profiles through many post-Marinoan sequences are either too condensed or lack sufficient carbonate to record these features, including the sections in the eastern Death Valley region. (Halverson et al. 2005). As such, the Panamint profiles represent the first relatively complete record of the post-Marinoan C-isotopic recovery outside of southern Africa. Correlation of these curves (1) firmly places the Noonday at the base of the Ediacaran Period, (2) indicates deposition of ~200 m of Sentinel Peak and Radcliff strata occurred between 635 and 632 Ma, (3) supports the hypothesis that the Wildrose Diamictite of the Kingston Peak Formation, which lies in sharp contact below the Sentinel Peak Member, represents at least part of the Marinoan glacial interval; (4) helps identify correlative cap carbonate sequences in key Laurentian sections, which include the Ravensthroat Formation in the MacKenzie Mountains, dolostones capping the upper diamictite of the Pocatello Formation in eastern Idaho, and the middle part of the Mina el Mezquite Formation in Sonora. The Noonday C-isotopic profile confirms that the details of relatively rapid, complex variations in ocean chemistry observed in basal Ediacaran strata in Namibia are globally reproducible.ududIII. The Kingston Peak Formation in the Panamint Range represents the stratigraphically most complete section of Cryogenian strata along the SW margin of Laurentia. Two glacigenic diamictites and their associated cap carbonates, the older Surprise Member and Sourdough Member and the younger Wildrose Member and Noonday Formation (Sentinel Peak Member), provide timing constraints to bracket the inter-glacial succession to between ca. 713 Ma and 635 Ma, the ages of inferred correlative glacial cap carbonate rocks dated elsewhere. This timing constraint is further strengthened by the presence of a sharp decline in C isotopes in the Thorndike Member, which occurs immediately beneath the Wildrose Member; this decline is readily correlated with the global Trezona anomaly.ududWithin the inter-glacial succession, new mapping in the northern Panamints has documented the presence of a previously unrecognised suite of coarse sedimentary rocks herein defined as the Argenta Member of the Kingston Peak Formation. The Argenta consists largely of poorly-sorted breccias and conglomerates containing an assemblage of gravel-sized clasts dominated by granitic gneiss, schist, feldspar augens, vein quartz and quartzite fragments, and locally carbonate rocks. These compositions indicate derivation from a basement provenance and record deposition in alluvial-fan to coarse-braided fluvial settings; their textural and compositional immaturity implies relatively short distances of transport. Mapping shows that the Argenta defines wedge-shaped packages as much as 200 m thick and that the base of the Argenta is a significant angular unconformity. Combined, these features are evidence that deposition occurred during a phase of extensional tectonism interpreted as recording the initial dismemberment of the Rodinia supercontinent. Best estimates place the timing of this tectonism at ca. 650 – 700 Ma.ud