PRE-CRETACEOUS BASEMENT ROCKS
The basement rocks that occur in the Cretaceous inlyer of central Oregon are a mˇlange of low-grade metamorphic lithologies similar to those of the Baker Terrane in eastern Oregon whose age is assumed to be late Mesozoic and derived from an oceanic provenance of unknown tectonic origin.
In the Mitchell area they have been found in three localities: Meyers Canyon, Toney Butte, and Scott Butte. These ancient rocks have been brought to view by uplift and erosion of the Mitchell Anticline and occur along the axis of that fold. In addition, these basement rocks are associated with many large Tertiary andesite and rhyolite intrusions which probably contributed to their uplift and appearance at the surface.
The following geologic maps should confirm the melange character of these outcrops in which individual lithologies have been so deformed and intermixed by metamorphic processes that individual rock groups are represented only by isolated bands of color within a generallized "PK".
Phyllite (a fine-grained, micaceous, schistose rock) is dominant and is represented by fresh, altered, and contorted varieties.
Other common representatives are thick assemblages of bedded cherts which upon close inspection are seen to consist of chert layers with interbedded phyllite.
Metabasaltic rocks in which pillows are associated with mafic tuffs and breccias clearly represent deposition in an oceanic environment.
Several large pods of marble crop out and they are often associated with bedded, recrystallized limestones.
Glaucophane-bearing phyllites (often described as blueschists) occur in limited outcrops at both Meyers Canyon and Toney Butte localities.
Serpentinites with traces of pyroxenite are common near Toney Butte and Scott Butte but are absent at Meyers Canyon. Much of the serpentinite near Toney Butte has been altered to a light-gray mixture of Ca-Mg hydrates while the serpentinite near Scott Butte contains pyrite and has been altered to a dark brown color.
Exposures of depositional contact between basement rocks and Hudspeth Formation sediments often reveal evidence of localized, energetic, and sometimes perplexing interactions. For example: at Meyers Canyon there are thin Hudspeth sand layers entirely composed of serpentine grains but no serpentinite has been found there.
Adjacent outcrops of Hudspeth mudstone completely enclose bodies of coarse breccia composed of all metamorphic lithologies found in the Meyers Canyon area, except for fragments of a very distinctive non-metamophosed plagioclase, biotite, quartz, and garnet-bearing rhyolite which closely resembles a 51-Ma dike found in Hudspeth mudstones near Scott Butte.