CLARNO LAVA FLOWS
Andesitic lavas are the most abundant volcanic rock in the Clarno Formation. They do not resemble the extensive lavas of plateau basalts but accumulated as many flows of limited extent. There are miles of these overlapping andesite flow rocks that seem to obliterate all other volcanic materials. This corresponds the abundant small andesite dikes and plugs that penetrate all pre-Eocene rocks in the Mitchell region. While young and well preserved, the andesite lavas were covered by scoria and cinders that were soon oxidized to the brick-red color still seen today.
When these andesite lavas lost heat and solidified, they contracted and developed parallel cracks that are referred to as “platy jointing.”
Under the wet, semi-tropical Eocene climate, these andesite lavas commonly became so deeply weathered that platy jointing disappeared and even the boundaries between flows became an indistinct mass of clay.
Additionally, the groundmass and enclosed phenocrysts of twinned plagioclase feldspar and monoclinic crystals of pyroxene were converted to smectite.
Crystals of hornblende that are so common in Clarno intrusive andesites have been converted in the lavas to a reddish oxiamphibole.
In well-preserved andesite lavas, clear phenocrysts of twinned plagioclase feldspar and monoclinic crystals of pyroxene are abundant.
Some Clarno andesites even contain crystals of quartz now surrounded by reaction rims of secondary pyroxene.
Other types of lava are much less abundant than andesite. Basalt lavas occur but were even more suseptible to alteration and now contain veins of secondary zeolite minerals. I have found only one remnant of a basaltic cinder cone in which small oxidized red volcanic bombs are still preserved.
While dacite lavas are rare, the dacitic plug of Sand Mountain appears to represent a large dome that issued marginal dacitic lavas now weathered to hydrated volcanic glass.
Rhyolite lavas, while few in number, do occur in the Clarno Formation. They were formed as domes on the surface and some represent small marginal flows in which it is still possible to recognize marginal columnar jointing, internal flow banding, spherulites, and at the base, obsidian now completely devitrified.