PROVENANCE OF TWO CRETACEOUS CONGLOMERATE UNITS OF THE EASTERN KLAMATH MOUNTAINS: COMPARISON OF THE BASAL HORNBROOK FORMATION AND THE PYTHIAN CAVE CONGLOMERATE, NORTHERN CALIFORNIA

The provenance of two Early to middle Cretaceous conglomeratic units that both rest unconformably on the Eastern Klamath terrane in northern California may prove critical to tectonic reconstructions by linking those units to source regions and to each other and thus providing constraints on their locations during their formation. The informally named Pythian Cave conglomerate contains dominantly porphyritic volcanic clasts, with less than 20% of the clasts derived from metamorphic, metavolcanic, or quartzitic sources, and a west to southwest paleocurrent direction, indicating a non-Klamath provenance to the east. In contrast, the Klamath River Conglomerate, basal member of the Hornbrook Formation, contains primarily quartzitic and metavolcanic clasts, with less than 20% porphyritic volcanic clasts, and a north to northeast paleocurrent direction, indicating derivation from the underlying metamorphic rocks of the Yreka subterrane of the Eastern Klamath terrane. Therefore, detrital zircon age signatures from the Klamath River Conglomerate should yield abundant pre-Mesozoic ages derived from the Antelope Mountain Quartzite and other metamorphic rocks in the Yreka subterrane. However, preliminary results indicate that both the Pythian Cave conglomerate and the Klamath River Conglomerate contain only Mesozoic zircon with peak ages of Early, Middle, and latest Jurassic. These peak ages are not inconsistent with provenance in the Klamath Mountains, but could reflect derivation from an eastern volcanic arc. In fact, the absence of pre-Mesozoic zircon grains, the significant distance between current exposures of Jurassic plutons and the conglomerate outcrops, and dramatic differences in petrology and paleocurrent directions between the two units all suggest the possibility of a non-Klamath provenance for at least the Pythian Cave conglomerate. Furthermore, detrital zircon age signatures from the Pythian Cave conglomerate and the Klamath River Conglomerate do not match those of the Great Valley Group near Redding, California, rocks of similar depositional age with probable Klamath provenance. Additional provenance analysis, including geochemical study of conglomerate clasts, should help address this uncertainty.