The thesis area consists of 44 square miles in southeast Harney
County, Oregon. The Pueblo Mountains are located in the northwest
part of the Basin and Range structural province, and are characterized
by normal faults of large displacement that produced the tilted fault
block mountain range.
The oldest rocks exposed are mafic lava flows, and marine
sedimentary rocks, which were subjected to regional metamorphism
probably during Jurassic time. A thin sequence of late Oligocene to
early Miocene silicic volcanic rocks, arkosic sedimentary rocks and
andesite unconformably overlies the metamorphic sequence. These
rocks are disconformably overlain by a thick sequence of Miocene
basalt flows. Included in the basalt sequence is a unit of silicic volcanic
rocks. Overlying the basalt sequence with slight angular unconformity
is a sequence of tuffaceous sedimentary rocks, tsillari,
and welded tuff. A fan.glomerate unit crops out in the northeast
corner of the area Quaternary deposits include lacustrine, alluvial
fan, and dune deposits.
Major normal faulting, which began in late Miocene time, has
continued into the present.