ABSTRACT Pinnacle reef tracts are geomorphic features of carbonate systems that originated in the early Silurian and display an episodic distribution into the Cenozoic. Detailed study of Silurian pinnacle reefs of the United States midcontinent demonstrates repeated motifs, but most enigmatic is the coincidence of carbonate carbon isotope (δ 13 C carb ) excursions and reef pulses. Silurian δ 13 C carb excursions were associated with environmental changes and extinctions, and reefs appear to mark a resurgence of conditions favorable to biomineralizers following those extinction events. Previous workers in the region identified six discrete reef origination events in the United States midcontinent during the Silurian. Our reevaluation of outcrops and cores, conodont collections, and the generation of considerable new chemostratigraphic data across the region are clarifying the age relations of these events and their relationships to perturbations of the global carbon cycle.

ABSTRACT This field guide describes stops in the Oregon Klamath Mountains that visit near-complete ophiolite sections, pre- and post-accretion arc plutons, greenschist- to amphibolite-grade metamorphosed wallrocks, arc volcanic rocks, and interbedded chert, argillite, and olistostromal deposits. Structural features at these stops include local- and regional-scale folds and faults, as well as penetrative metamorphic fabrics such as slaty cleavage, gneissic layering, and mineral lineations. The geologic history here reveals a period of Late Triassic and Jurassic ophiolite and oceanic-arc formation followed by Middle Jurassic terrane accretion, tectonic mélange formation, and continued oceanic arc magmatism. Rifting from ca. 165 to 160 Ma produced the Rogue-Chetco arc, Josephine ophiolite, and remnant arc comprised of older Klamath Mountains terranes. Deformation and magmatism during the Late Jurassic Neva-dan orogeny accreted this active arc–inter-arc basin–remnant arc triad to western North America, producing the lithotectonic belts observed today. The Oregon Klam-ath Mountains therefore provide an exceptional opportunity to examine the deep to shallow levels of multi-phase oceanic lithosphere and deformational features related to the accretion of these terranes to the continental margin.