Geology Reference
In-Depth Information
Stratigraphy and structure in the vicinity of Picture
Gorge and Sheep Rock in Grant County
brites". These.hot, gas-charged clouds of ash poured
down the flanks of volcanoes then across the landscape green and buff intervals reflect a succession from lakes
for miles, destroying everything on the way, before and rivers giving way to more open grasslands as the
settling to the ground and annealing into a solid episode progressed.
crystal-studded, glassy mass. Within the western part of Following the John Day deposition, the soft
the basin, vents pumped out silica-rich rhyolites, ash rocks of the formation were lightly dissected by erosion
flow tuffs, and lavas, to build up domes, whereas higher before the vast basalt flows of the Columbia River
temperature alkali-olivine basalt and andesite lavas lavas covered much of the Blue Mountains region in
erupted from volcanoes farther east. the middle Miocene between 17 and 12 million years
Rapid deposition of volcanic ash and mud in ago. Very fluid basaltic lavas began to erupt and pour
every depression on the landscape was optimum to from cracks and fissures in northeastern Oregon,
preserve remains of the semi-tropical vegetation and southeastern Washington, and western Idaho. As much
mammals which lived in the John Day basin. Because as 400 cubic miles of lava were extruded in single flows,
this period of fossil accumulation dates back only to and eruptions lasted for weeks at a time. Even though
the Oligocene and Miocene, from 18 to 36 million some later flows can be traced all the way to the Pacific
years ago, all of the plants and most of the mammals Ocean, most of the Columbia River basalts in central
look somewhat familiar, although none of the species Oregon are from dikes near Monument and Kimberly,
exactly correspond to those living today. where they covered a distance of less than 20 miles.
As climatic and volcanic conditions altered, the Throughout the Columbia basin over 40
nature and color of volcanic sediments of the John Day separate flows of the Miocene basalt are recorded,
changed as well. Highly oxidized, deep red ash layers of
However, it is primarily the Picture Gorge lavas that
the lower John Day Big Basin member are only poorly
overlie the John Day Formation. Lavas of this group
fossiliferous. These are succeeded by the pea green
were very liquid, and the eruptions rapidly filled in
Turtle Cove layer dominated by clay and rich in fossils.
valleys and low spots within the Blue Mountains prov-
The Turtle Cove is, in turn, followed by the fossilifer-
ince. Prior to the eruption of the Columbia River
ous, cream or buff-colored layers of the Kimberly and
basalts, the Miocene topography had a relief of as
Haystack rock members. Throughout the deposition of
much as 1,000 feet from the valley floors to the crests
these layers in streams, lakes, and floodplains, volca-
of the hills. As volcanism continued, flows lapped over
noes dispersed incandescent clouds of glowing ash that
each other until only the highest exposures of earlier
melted and flowed together. John Day fossils in the
rocks projected above the youngest layers of basalt.
