A THRUST FAULT ADJACENT TO
THE OCHOCO PASS HIGHWAY
Cretaceous and Tertiary strata have been intensely deformed within a relatively small area that is well exposed along highway 26, five miles north of Ochoco Summit.
This area is dominated by dip-slip, strike-sip, and thrust faults that have displaced Hudspeth mudstones and Gable Creek conglomerates as well as overlying rocks of the Clarno Formation. A road-cut outcrop in this area that has attracted attention of geologists for many years displays a section of turbidite sandstones that have been subjected to penicontemporain-eous folding,
overlain by a 12-foot bed of massive, crushed mudstone which, in turn, is overlain by a thick section of fractured Gable Creek conglomerate.
Rounded clasts of conglomerate, sandstone, and other Cretaceous lithologies, up to 2 feet in diameter are sandwiched between the crushed mudstone and underlying sandstone beds.
This unusual assembly of rocks was interpreted to be a thrust fault by Wilkinson (1959, p. 82), was later described by Oles (1973, p. 46) as the result of a large landslide, and was identified by Baldwin (1976, p. 94) as an angular unconformity between Cretaceous sedimentary beds and overlying rocks of the Clarno Formation. My observations support Wilkinson’s thrust-fault interpretation.
The massive mudstone unit lacks bedding because it has been intimately comminuted. It contains sparse pebbles whose surfaces are polished and display minute striations.
It is a thick fault gouge developed within incompetent mudstone that was overridden by well-indurated and very competent conglomerates. Some of the resistant lithologies were separated from their bedding sites and dragged along the interface until they became large rounded “ball bearings” at the base of the overriding plate.
Correct interpretation of these features requires a broader study that includes the distribution of faults and deformed rocks that are well exposed west along Highway 26. Reference to this appended geologic map will assist in organizing the following descriptions into a coordinated view of the many events that have occurred here.
The mudstone fault gouge that marks the base of the thrust plate dips 14 degrees to the southwest. The conglomerate unit that rests upon the fault gouge is increasingly deformed in outcrops to the west and consists of large blocks of different inclinations, separated by near-vertical faults.
These conglomerate blocks can be traced westward to a fault contact with bedded tuffs of the Clarno Formation. This fault, as exposed in a road cut of highway 26, strikes N. 15 W. and dips 85 degrees SW.
The tuffs are deformed and display westerly dips ranging from 60 to 90 degrees. Another parallel fault 1500 feet farther to the west, forms the western boundary of a graben that has dropped tuffs of the Clarno Formation at least 900 feet into the Cretaceous rocks. Striations on these fault surfaces are inclined down to the south 60 to 70 degrees.
West of the graben is a near-vertical section of Hudspeth mudstone which is displayed along Highway 26 for approximately 2700 feet before it is terminated by a strike-slip fault against Clarno basaltic lavas.
This fault is parallel to the highway and strikes N. 15 E. with a dip of 75 degrees to the west. Striations are especially obvious on this fault and trend 15 degrees down to the north. The basaltic lavas are seen for about 1800 additional feet southwest along both sides of Highway 26 which is now west of the strike-slip fault. Beyond the basaltic outcrops, a thick section of Clarno tuffs has been dropped in a graben similar in width to the graben previously described to the east. Geographic and geologic relationships between these many structural features are represented on this appended map:
The graben of Clarno tuffs is seen to have been cut by a left-lateral strike-slip fault and displaced 1.3 miles to the northeast. A section of Cretaceous rocks approximately 3000 feet broad has been carried with it as a shallow overthrust.
What geologic processes have been responsible for this extensive structural deformation in such a restricted area? Less than 1.25 miles south of this location is the largest Clarno intrusive body that I have found in the Mitchell region. It was emplaced in the form of a basaltic sill of gabbroid texture, more than 600 feet thick, which is exposed over a north-south distance of 2.5 miles at the interface between westward-dipping Cretaceous and Tertiary strata. The exposed east-west width is approximately 3000 feet, but much of the eastern part of the sill has been removed by erosion and an unknown western part extends into the subsurface. In road cuts of highway 26, 1.6 miles north of Ochoco Summit, a basalt sill is exposed that might be an apophysis from this main sill at depth.
It should be considered that a sill of such density, thickness, and extent could, in addition to lifting and invading its roof rocks, also exert lateral stresses sufficient to push aside some of the adjacent strata, especially where this could be accomplished by moving (thrusting) competent conglomerates over a bed of incompetent, easily deformed mudstone.