A series of Miocene to Pliocene bedded pyroclastic units in the Western Cascades of Oregon, previously interpreted as being of fluvial and lacustrine origin, are described and reinterpreted as subaerial pyroclastic surge and palagonite tuff cone deposits. The pyroclastic surge units are characterized by (1) an abundance of transparent unaltered, blocky sideromelane fragments with low vesicularity; (2) a constant relative proportion of minerals; (3) limited plagioclase composition; (4) trace quantities of palagonite and secondary minerals; (5) a lack of accidental rock fragments not enveloped in sideromelane; and (6) absence of subaqueous sediments and fossils. The palagonite tuffs are characterized by: (1) massive lower portions that grade upward to finely laminated tops in each sequence of tuff beds; (2) the presence of vesicles; (3) the presence of angular, unaltered, vesicular basalt clasts; (4) an apparent quaquaversal structure displayed by the palagonite tuff bed attitudes; and (5) interstratification with the pyroclastic surge beds. Because these beds are primary volcanic features, the bedding angles are not useful in interpreting post-depositional tectonic flexuring or tilting, and cannot be used as evidence for a down-drop to the east structure in this part of the Cascades as has been proposed by earlier workers.
ASJC Scopus subject areas
- Geochemistry and Petrology