Abstract The lower 1100 feet of the 2300-foot John Day Formation near Mitchell, Oregon, consists largely of massive tuffaceous claystones and vitric tuffs. These deposits, of early Miocene to late Oligocene age, are an air-laid accumulation of silicic (dominantly dacitic) ash on a land surface that was initially hilly, but later became almost level. Much of the vitric ash weathered to montmorillonite before burial; nearly all the unweathered ash was subsequently altered to mont-morillonite and clinoptilolite. The principal pyrogenic minerals are at least locally altered in the lower 350 feet of the formation, and clinoptilolite, orthoclase, vermiculite, kaolinite, fluorapatite, cristobalite, and quartz are among the authigenic minerals formed within these beds. Authigenic orthoclase has been noted within the John Day Formation over an area of about 600 square miles, and, with one exception, it has not been observed in overlying and underlying volcanic rocks. The orthoclase is a relatively pure K feldspar; it occurs as a pseudomorph after plagioclase and forms between a trace and 8 per cent of the claystones. A potassium-argon age of 22 m.y. was obtained from authigenic orthoclase in a bed 65 feet above the base of the formation, and seemingly reliable ages of 31 and 24–25 m.y., respectively, were obtained from pyrogenic materials from beds 165 and 1110 feet above the base. Authigenic orthoclase was probably formed at burial depths between 400 and 2200 feet, and at a temperature of 20–40°C. K and Si needed to form orthoclase were supplied by the alteration of glass. Laboratory studies suggest that a K+/H+ ratio of 109 may be necessary to form orthoclase within its stability field at temperatures of 20°–40°C, but the geologic data suggest that the K + /H + ratio in the subsurface water was probably no more than 2.5 × 10 6.5 at the time of formation of the authigenic orthoclase of the John Day Formation.

ABSTRACT The John Day Formation of central and eastern Oregon, contains a widespread assemblage of both ash-flow and airfall tuffs, yet only a few corresponding caldera sources have been identified in the region. Investigators have long speculated on the sources of tuffs in the John Day Formation and have suggested that these pyroclastic rocks were vented from now buried eruptive centers in or marginal to a nascent Cascade Range. Recent detailed geologic mapping in the John Day and Clarno Formations, however, indicates the presence of at least three large-scale rhyolite caldera complexes centered along the northeast-trending axis of the Blue Mountains. This field guide describes a three-day geologic transect, from the scenic high desert of central Oregon eastward across the axis of the Blue Mountains, that will examine the physical volcanology and geologic setting of the 41.50-39.35 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, the 29.56 Ma Crooked River caldera at Prineville, and the 29.8 to 28.1 Ma Tower Mountain caldera near Ukiah.

Two new genera and species of plagiomenids (Mammalia, ?Dermoptera, Plagiomenidae) are described from the North American Uintan (middle Eocene) and Chadronian (early Oligocene). A third genus and species, Ekgmowechashala philotau, from the early and middle Arikareean (late Oligocene) of the northern United States, is removed from the primate family Omomyidae and placed in the Plagiomenidae. All three newly recognized plagiomenids are placed in the Ekgmowechashalinae, sister subfamily to the subfamily Plagiomeninae (new rank). Ekgmowechashaline plagiomenids are somewhat primate-like, as is the plagiomenine genus Worlandia, but the Plagiomenidae are usually considered to be allied to the living colugos of southeast Asia, order Dermoptera. Analysis of that relationship is placed outside the scope of this paper. Tarka stylifera, the earliest known ekgmowechashaline, occurs in the type section of the Tepee Trail Formation, early Uintan (Shoshonian: late medial Eocene) of northwestern Wyoming. This locality falls in paleomagnetic Chron C20R, interpreted to be close to 47.5 Ma in age. A second, more primitive but later-occurring ekgmowechashaline genus and species, Tarkadectes montanensis, is from a nominally early Oligocene level (Chadronian) in the Kishenehn Formation of northern Montana. Ekgmowechashala is known from lower dentitions from the early Arikareean Sharps Formation of South Dakota and probably from an upper dentition reported from middle Arikareean rocks in the John Day Formation of Oregon. Ekgmowechashala is placed with the other two genera because of lower cheek-tooth morphology, but it lacks the enlarged incisor of Tarka. Ekgmowechashalines are hypothesized here to be primarily frugivores, folivores, and nectar- and exudate-feeders. Until now, known undoubted plagiomenids were restricted to the Paleocene and early Eocene (Wasatchian). The newly recognized post-Wasatchian occurrences are all in the northern part of the United States and are in keeping with previously known plagiomenid geographic distribution, which ranged from northern Wyoming to the Canadian arctic and possibly beyond.