A preliminary investigation of the lithological characteristics of the Troutdale Formation in portions of the Camas, Sandy, Washougal, and Bridal Veil quadrangles

Abstract

The study area encompasses a region of about 80 square kilometers, centered about Corbett, Oregon. Sieve analyses suggest that the Troutdale Formation is composed of a conglomerate, a coarse-grained sandstone, and a siltstone lithofacies. Measured sections data show that 56 to 60 percent of the Troutdale Formation's exposures are composed of the conglomerate lithofacies. The coarsegrained sandstone lithofacies makes up 34 to 38 percent, while the siltstone lithofacies makes up only 4 to 8 percent. Pebble count data show that the representation of basalt varies from 69 to 100 percent, with an average of 85 percent, while quartzite varies from 0 to 25 percent, with an average of 9 percent. Geochemical data show that at least one formal member and two informal members of the Columbia River Basalt Group (Frenchman Springs, Grande Ronde High Mg, and Grande Ronde Low Mg) compose part of the detritus. The geochemical data also show that rocks of the Boring Lavas or similar lavas are intercalated with the Troutdale Formation, high in the section. Grain mount petrographic data show that volcanic rock fragments are the dominant component in the coarse sand fraction of the Troutdale Formation. Many of the volcanic rock fragments contain brown glass with an index of refraction of 1.583 +/- 0.001, which indicates a mafic composition {silica content of 50 +/- 1 percent). The representation of volcanic rock fragments varies from 29 to 98 percent, with an average of 75 percent. Sedimentary rock fragments, such as chert, vary from 0 to 17 percent, with an average of 5 percent. Biotite ranges from 0 to 12 percent, with an average of 2 percent. Two-thirds of the samples which contain sedimentary rock fragments also contain biotite. Orthopyroxene varies from 0 to 18 percent, with an average of 5 percent. 2 The samples with the three greatest amounts of orthopyroxene do not contain sedimentary rock fragments or biotite.