I. Stratigraphic constraints on the number of discrete neoproterozoic glaciations and the relationship between glaciation and ediacaran evolution. II. The Kwichup Spring thrust in the northwestern Spring Mountains, Nevada : implications for large-magnitude extension and the structure of the Cordilleran thrust belt

Abstract

Part I: Stratigraphic constraints on the number of discrete neoproterozoic glaciations and the relationship between glaciation and ediacaran evolution. Stratigraphic and structural observations in the Death Valley region provide new insights into two topical problems. First, stratigraphic observations provide a better understanding of the number of discrete Neoproterozoic glaciations and the relationship between glaciation and the diversification of the first animals. Detailed stratigraphic investigations reveal incised valleys within the Neoproterozoic Johnnie Formation. The size (>150 m) and regional extent of the valleys, and the carbon isotope signature of underlying carbonates show that the valleys are probably glacioeustatic in origin. The incised valleys help to complete the Neoproterozoic glacial record in the western United States. The incised valleys and a pair of glacial diamictites in the underlying Kingston Peak Formation represent two to three discrete Neoproterozoic glaciations. This record of two to three glaciations matches the global Neoproterozoic glacial record. The incised valleys provide the youngest evidence for large-scale Neoproterozoic glaciation in the western United States. Correlation of the Johnnie valleys with incised valleys in the uppermost Caddy Canyon Formation of Idaho and Utah shows that this glaciation occurred before 580 Ma. These findings suggest that diverse Ediacaran faunas post-date the youngest major Neoproterozoic glaciation by tens of millions of years. Second, structural and stratigraphic observations provide new constraints on the magnitude of extension in the Death Valley extended domain. These observations reveal a thrust fault in the northwestern Spring Mountains, NV. Correlation of this thrust fault with thrusts in other ranges constrains the translation of those ranges relative to the Spring Mountains. While these correlations are not unique, the most plausible correlations require large-magnitude extension in the Death Valley area and north of Las Vegas Valley. Correlation of contractile structures in the northwestern Spring Mountains and Specter Range with Permo-Triassic structures in the Cottonwood Mountains may provide a link between the Permo-Triassic thrust belt in the Cottonwood Mountains and the Central Nevada thrust belt. The Central Nevada thrust belt may be at least in part Permo-Triassic in age, and may represent a foreland fold and thrust belt which developed inboard of the Golconda allochthon during its emplacement. Part II: The Kwichup Spring thrust in the northwestern Spring Mountains, Nevada : implications for large-magitude extension and the structure of the Cordilleran thrust belt. Approximately 170 km(2) of new mapping in the northwestern Spring Mountains reveals contractile structures related to a regional thrust fault called the Kwichup Spring thrust. The Kwichup Spring thrust involves at least 1.4 km of stragraphic throw. Along much of its length, the Kwichup Spring thrust has been reactivated or excised by a normal fault. Stratigraphic and structural evidence suggest that the Kwichup Spring thrust correlates with the Montgomery thrust in the Montgomery Mountains. Correlation of these two thrusts requres a reappraisal of the geometry of the Cordilleran thrust belt in Nevada and southeastern California. The Kwichup Spring - Montgomery thrust probably correlates with either the Clery thrust in the Funeral Mountains or the Panamint thrust in the Panamint Mountains. Both possible correlations require large (>115 km) west-northwest translation of the Panamint and Cottonwood Mountains with respect to the Spring Mountains during Neogene extension. Before this large-scale translation, the Panamint and Cottonwood Mountains were positioned along the southern projection of the Central Nevada thrust belt of Taylor and others (1993). Since contractile structures in the Cottonwood Mountains are Permo-Triassic, the Central Nevada thrust belt may be at least in part Permo-Triassic.