ORIGIN OF A FRESH-WATER-DIATOM-RICH PYROCLASTIC-DEBRIS-FLOW DEPOSIT IN A SHALLOW-MARINE TERTIARY FORE-ARC BASIN, NW OREGON

An unusual freshwater-diatom-bearing pyrodastic-debris-flow deposit is present within the shallow-marine upper Eocene to Oligocene Pittsburg Bluff Formation of northwestern Oregon. The subaerially generated pyr oclastic-debris flow rapidly debouched into the shallow-marine environ ment. The flow formed a wedge-shaped deposit, up to 3.5 m thick, that is mappable over several square kilometers. Thickness and maximum clas t size decrease offshore; near the distal margins the debris flow mixe d with seawater and became a high-density shelf turbidity current, Whe re emplaced above storm wave base, the deposit was locally reworked to form a thin lag conglomerate. Thalassinoides burrows are present in t he upper 30 cm of the deposit. The rhyodacite chemical and mineral com position, regional geologic setting, and thickness and clast-size vari ations indicate a nearby highly explosive calcalkaline volcanic source to the northeast (i.e., Western Cascade are). This deposit documents the earliest known Cascade are explosive event that directly affected sedimentation in the Tertiary forearc basin of western Oregon. A sanid ine- and biotite-bearing ash fall tuff 3 m above the debris-flow depos it was derived from a backarc eruptive source (e,g., Oligocene John Da y Formation). This tuff yielded an Ar-40/Ar-39 date of 29.83 Ma. The w ell-preserved diatom flora within the debris-flow deposit matrix exten ds the geologic range of some genera (e.g., Gomphonems) and widens the geographic distribution of others (e.g., Gomphopleura). Possible scen arios to explain the presence of exclusively freshwater diatoms in the debris-flow deposit in this shallow-marine section include: (1) a pri mary pyroclastic debris flow may have passed through a diatom-rich lak e between the site of eruption and its entrance into the sea; (2) fres hly erupted pyroclastics temporarily blocked a river drainage, creatin g an alpine lake that produced the debris flow upon dam failure; (3) p rimary pyroclastic debris may have been vented directly through a cald era or crater lake rich in diatomaceous sediment.