ERUPTION-CONTROLLED EPICLASTIC SEDIMENTATION IN A DEVONIAN TRENCH-SLOPE BASIN - EVIDENCE FROM SANDSTONE PETROFACIES, KLAMATH MOUNTAINS, CALIFORNIA

The Devonian Gazelle Formation comprises sparsely fossiliferous shale, siltstone, siliceous mudstone, volcaniclastic sandstone, and conglome rate whose cumulative thickness is at least 1.25 km. Two end-member sa nd compositions combined locally to form a third ''mixed'' petrofacies . The volcaniclastic petrofacies was derived from an undissected magma tic arc, whereas the chert-mudrock petrofacies reflects derivation fro m melange of the underlying accretionary prism. The stratigraphically restricted mixed petrofacies is compositionally discrete and records t horough mixing of these two compositional end members. Data from sands tone petrography are consistent with independent geologic evidence ind icating that the Gazelle Formation represents the fill of a Devonian t rench-slope basin that formed above an east-dipping subduction zone. O ur interpretation of the Balaklala Rhyolite as the source of the volca niclastic petrofacies requires proximity of the Yreka and Eastern Klam ath terranes during the Early Devonian, and indicates that the eastern Klamath province has behaved as a single crustal block (sensu lato) e ver since. Field study and petrographic analysis indicate that the del ivery of coarse terrigenous detritus to the basin was controlled princ ipally by episodic eruptions of rhyolitic tephra in the arc source and also by one discernible episode of change in relative sea level. This study indicates that caution must be used when invoking changes in re lative sea level to explain the stratigraphic distribution of epiclast ic turbidites in island-arc settings. Identification of compositionall y and stratigraphically discrete mixed petrofacies in other ancient or ogens may permit similar inferences regarding controls on stratigraphi c development that have operated on poorly fossiliferous, immature con vergent margins.