THE GEOLOGY IN THE SOUTHEAST CORNER OF THE CASCADE-SISKIYOU NATIONAL MONUMENT, OREGON AND CALIFORNIA

The Western Cascade Volcanic Series in the Agate Flat-Jenny Creek area consists of Oligocene basaltic andesite lavas, volcaniclastic sandstone, debris flows, and silicic tuff. Being slightly tilted to the northeast, these rocks are unconformably overlain by the 3.86 +/- 0.06 Ma High Cascade lava of Pinehurst Inn (Tpbpi). The upper sections of the Tpbpi lava are mostly diktytaxitic and vesicular, yielding blocky, rubbly outcrops with columnar and platy joints forming cliffs and colluvium aprons deposited over Western Cascade rocks. Steeply inclined northwest-trending faults affect both series. Basalt, andesite, and dacite intrusions formed along a zone paralleling the fault pattern. These intrusions range in age from late Oligocene to late Miocene, revealing a long-lived zone of weakness. Jenny Creek Falls is made up of intrusive bodies that trend northwest. Evidence shows (river gravel, pillow basalt, and palagonite) that Jenny Creek used to flow slightly east of its current location. The Tpbpi lava formed a pond in the ancestral creek, eventually filling the creek and flowing over into Agate Flat. Agate Flat contains dacitic and rhyolitic tuff, with heterolithic volcanic sandstone interlaced with thin lava flows. The recessive nature of these rocks explains the wide, topographically subdued terrain of Agate Flats.