THE NARROWS FAULT
The Narrows Fault is a dip-slip fault that can be traced 19 miles NNE along the west limb of the Mitchell Anticline.
In 1984 I first recognized and studied the fault adjacent to highway 26 in the walls of the canyon known to early settlers as “the Bridge Creek Narrows”. The Narrows Fault has not been found to have displaced any Clarno rocks. This is verified by the common occurrence of Clarno volcanic rocks resting on the fault and by the many Clarno intrusive bodies that coincide with the trace of the Narrows Fault and probably used the fault zone as a channel of least resistance toward the surface. Several other dip-slip faults exist parallel to the Narrows Fault and some of them have also been invaded by long, narrow dikes.
The Narrows Fault traces a nearly straight path across very irregular terrain which suggests that the fault plane is vertical. This is best demonstrated where the fault crosses Meyers Canyon and Bridge Creek Canyon (A) and Bridge Creek Canyon (B); relief at both localities is in excess of 600 feet.
North of Meyers Canyon, horizontal slickensides are exposed on the surface of the Narrows Fault. This probably represents only the last movement of the fault because the entire length of the west side of the fault was displaced down relative to the east side. This is made evident by beds of conglomerate on the east side of the fault that have been dragged down to a near-vertical orientation.
The magnitude of displacement on the Narrows Fault is not well known. However, on the east limb of the Mitchell Anticline a distinctive bed of marine volcaniclastic rock composed of ash and pumice clasts serves as a marker horizon between Gable Creek conglomerates located 200-300 stratigraphic feet above the main Hudspeth mudstones. Although this marker bed has been eroded from the crest of the anticline, it’s pre-erosion conformation can be reconstructed from the dip of underlying rocks. When this is done from east to west across the anticline, the former level of the marker bed is found to be 900 to 1100 feet above existing outcrops of the marker bed on the west side of the Narrows Fault. Consequently, the dip-slip component of Narrows Fault displacement is estimated to be approximately 1000 feet, down on the west side.
As the Narrows Fault is traced south from Bridge Creek, it is increasingly obscured by lavas and intrusive rocks of the Clarno Formation. It is completely covered where it meets the right-lateral Mitchell Fault and has been displaced to a location 3.26 miles west near the position of a large hornblende-bearing dacite intrusive plug near Cougar Gulch, adjacent to the Mitchell Fault.
Immediately south, large blocks of Gable Creek conglomerate display near-vertical dips which suggest that the southern continuation of the Narrows Fault is nearby to the west beneath a cover of Clarno volcanic rocks. It is not known if the buried Narrows Fault extends further south in the subsurface.
However, one mile to the east is a fault that appears to be a splay of the Narrows Fault because it has the same orientation, the same sense of displacement, and the same relationship to surrounding rocks.
Where Highway 26 crosses this fault, the deformed rocks are exposed in road cuts. Gable Creek conglomerates on the west side of this fault have been broken into large blocks of diverse orientations with many polished slickensided surfaces.
These conglomerates have been dragged up along the fault plane and have, in the process, compressed less-competent adjacent sandstones and mudstones.
Many of these sandstones and mudstones are now vertical and some display isoclinal folds.
The obvious differences in deformation of rocks near the Narrows Fault at Bridge Creek and the splay fault at Highway 26 are probably a result of different depth of erosion of the same fault at the two localities.