The northern part of the Southeast Three Sisters quadrangle straddles
the crest of the central High Cascades of Oregon. The area is
covered by Pleistocene and Holocene volcanic and volcaniclastic rocks
that were extruded from a number of composite cones, shield volcanoes,
and cinder cones. The principal eruptive centers include Sphinx Butte,
The Wife, The Husband, and South Sister volcanoes. Sphinx Butte, The
Wife, and The Husband are typical High Cascade shield and composite volcanoes
whose compositions are limited to basalt and basaltic andesite.
South Sister is a complex composite volcano composed of a diverse assemblage
of rocks. In contrast with earlier studies, the present investigation
finds that South Sister is not a simple accumulation of andesite
and dacite lavas; nor does the eruptive sequence display obvious evolutionary
trends or late stage divergence to basalt and rhyolite. Rather,
the field relations indicate that magmas of diverse composition have
been extruded from South Sister vents throughout the lifespan of this
volcano. The compositional variation at South Sister is atypical of the
Oregon High Cascade platform. This variation, however, represents part
of a continued pattern of late Pliocene and Pleistocene magmatic diversity
in a local region that includes Middle Sister, South Sister, and
Broken Top volcanoes. Regional and local geologic constraints combined
with chemical and petrographic criteria indicate that a local subcrustal
process probably produced the magmas extruded from South Sister, whereas
a regional subcrustal process probably produced the magmas extruded from
Sphinx Butte, The Wife, and The Husband.
All of the volcanoes in the field area are probably less than
720,000 years old. Sphinx Butte, The Wife, and The Husband are older
than South Sister and have been subjected to at least two glaciations.
Late Pleistocene glaciers covered all but the upper ridges and summit
of South Sister; however, evidence for multiple glaciation is obscure
and it is possible that the bulk of South Sister is younger than the
second-to-last Pleistocene glaciation. Glaciated andesite lavas at the
summit of South Sister are capped by a veneer of basaltic andesite
lavas. The basaltic andesite lavas were extruded prior to 6840 yrs.
B.P., but are probably of late Pleistocene rather than Holocene age.
At some time between 12,000 and 2300 yrs. B.P., basaltic andesite lavas
and cinders were extruded from the Le Conte vent at the southwest base
of South Sister. The Le Conte lavas may bear only a spatial relation
to South Sister. Between 2300 and 1900 yrs. B.P., a series of rhyodacite
domes and block flows were extruded from flank vents on South Sister.
Future eruptive activity is likely at this volcano.