EDIACARAN-EARLY CAMBRIAN CORDILLERAN VOLCANISM AND INCISION: EVIDENCE FOR RIFTING AND EUSTATIC LOWSTAND?

Regionally extensive basalt-bounded erosional unconformities exist in terminal Ediacaran and basal Cambrian strata of western North America. These stratigraphic associations provide an opportunity to constrain the timing and nature of rift-related volcanism and regional eustatic changes along the Cordilleran margin, and may offer an opportunity to link tectonics to evolutionary changes in marine communities.

Perhaps the best-mapped example of these major erosional unconformities occurs near the base of Unit 1 of the Early Cambrian Puerto Blanco Formation, in Sonora, Mexico. This unconformity can be traced laterally for ≥10s of km, and is characterized by m- to dm- of incision into marine shales and dolostones of the base of the unit, or incision into the top of marine dolostones of Unit 4 of the underlying Ediacaran-Early Cambrian La Cienega Formation. The unconformity is capped by laterally extensive 1-9 m thick volcanic- and carbonate-megabreccias and conglomerates dominated by cm- to m-sized subrounded to angular clasts of dolostone, basalt, and quartzite in a gravelly to pebbly matrix. These features are consistent with a major regional lowstand associated with cratonic erosion and bypassing. Unit 1 of the overlying Puerto Blanco Fm. and Units 1 and 3 of the underlying La Cienega Fm. have approximately ten horizons of cm- to dm- thick vesicular to massive mafic basalt which are interbedded between intertidal marine shales and dolostones, purple polygonally cracked shales, laminated sandstones, and volcaniclastic conglomerates. Basalts are laterally extensive across ≥10s of km and suggest widespread but episodic subaerial volcanism, including volcanism landward of the continent margin. Potentially correlative unconformities and mafic basalts have been logged in Ediacaran-Early Cambrian sequences from California (Reed-Deep Springs Fms.), Nevada (Stirling-Wood Canyon Fms.), and Utah (Prospect Mountain Qtz.). Together, these sequences offer the opportunity to test hypotheses about the within-plate vs. arc-related origin of volcanism in this region and its relationship eustatic and environmental changes of this interval.