THE MITCHELL ANTICLINE
Records of Late Cretaceous and Early Tertiary structural deformation in central Oregon are best observed in marine sedimentary rocks of the 133 square-mile Cretaceous inlier near the town of Mitchell and in surrounding volcanic rocks of the Clarno Formation. The most obvious structural feature is the Mitchell Anticline that extends NNE for 29 miles with limbs that dip 10-20 degrees away from the fold axis.
This anticline was first described by J. C. Merriam (1901), was presented on a geologic map by W. D. Wilkinson (1950, p. 81), and was named by W. D. Wilkinson and K. F. Oles (1968). Erosion on the crest of the Mitchell Anticline has provided excellent exposures of metamorphosed basement rocks overlain by Cretaceous marine mudstones (the Hudspeth Formation), Cretaceous marine conglomerates (the Gable Creek Formation), and Tertiary volcanic rocks of the Clarno Formation, all disturbed by folding of this anticline.
Folding of the Mitchell Anticline has been interpreted differently as geologic investigations have been published. It had been assumed that cretaceous marine rocks were subjected only to regional epirogenic uplift and were not deformed until after Eocene deposition of the Clarno Formation and folding of the Mitchell Anticline (for example: Walker and Robinson, 1990). However, small and slightly different eastward dips of Cretaceous and Tertiary rocks are evident along highway 26 near Mitchell.
But road cuts along the same highway on the west limb of the anticline display equal high-angle dips of the two formations:
This caused Oles and Enlows (1971) to propose that an angular unconformity exists within a “Clarno Group” and they represented separate Upper and Lower Clarno Formations on their geologic map of the Mitchell Quadrangle. Walker and Robinson (1990, page 13) disagreed with this interpretation. Also, after completing several hundred chemical analyses and petrographic examinations of appropriate Clarno lithologies, I have been unable to identify criteria that serve to trace a widespread boundary between upper Clarno and lower Clarno stratigraphic units as it was defined by Oles and Enlows.
Although it is difficult to recognize, a modest angular unconformity does exist regionally between the Cretaceous and Tertiary rocks, even though both have been inclined together during anticlinal folding. Dip and strike of thin-bedded sandstones and mudstones can be measured accurately throughout the Cretaceous marine section. In contrast, dip and strike are difficult to obtain within the Clarno Formation because the attitude of Clarno lava flows, volcanic mudflows, and clastic deposits was determined by pre-existing land forms. However, lacustrine deposits of fine-grained volcanic ash provide accurate dip and strike of the Clarno rocks. When appropriate Clarno dip and strike is compared with nearby dip and strike of Cretaceous beds, especially on the limbs of the anticline, an angular unconformity of 10+/- 5 degrees is revealed. Because dips of Cretaceous rocks are greater than Clarno dips, and both have been disturbed by anticlinal folding, it is evident that minor folding was active during late Cretaceous or Paleocene time and was continued during Eocene Clarno time. It should be recognized that within the Cretaceous inlier a complete hiatus exists between the youngest Cretaceous rocks and the oldest Clarno rocks. Consequently, the age of modest pre-Clarno folding described here could range from post Cenomainian through the Paleocene, an interval of approximately 50 Ma.
However, there are more pronounced angular unconformities that were produced by deformation of Clarno rocks on the west flank of the Mitchell Anticline. They are associated with relatively small doubly-plunging anticlines and synclines that involve interbedded Clarno lava flows, ash deposits, and welded tuffs (age 49 Ma, dips up to 70 degrees) that were unconformably overlain by later andesitic lavas (age 42 Ma, dips near 12 degrees).
These volcanic units are all inclined west as an extension of the over-all Mitchell Anticline and provide clear evidence of a prolonged interruption of anticlinal folding, followed by unconformable deposition of late Clarno lavas. Consequently, the intra-Clarno unconformity as proposed by Oles and Enlows (1971), does exist but not as depicted on their 1971 geologic map. Additional evidence that an intra-Clarno unconformity was produced by different episodes of folding is revealed by study of the Cherry Fault. That fault (described elsewhere) displaced the Mitchell Anticline and all of the folded Cretaceous sediments and Clarno volcanics except for the uppermost volcanic rocks that buried the fault and were not affected by it.
Development of the Mitchell Anticline was long-lasting and associated with different types of deposition, deformation, volcanism, and erosion. The complex interaction of these processes and their influence upon the development of the Mitchell area is illustrated by the following eight E-W cross sections of the anticline as located A-B-C on this map:
A – B – C correspond to west-east positions across the Mitchell Anticline.
#1 GENRALIZED CROSS SECTION OF THE EARTH’S CRUST DURING LATE CRETACEOUS TIME. Depicting marine deposits of mudstones, sandstones, and conglomerates all overlying metamorphosed basement rocks in the vicinity of Mitchell, Oregon.
#2 DURING PALEOCENE TIME, cretaceous sediments were uplifted to form a broad coastal plain and the crust was fractured by the dip-slip normal Narrows Fault. The marine rocks were dragged along the fault, vertically down on the east side, vertically up on the west side. The dashed lines represent possible locations of other faults now covered by post-cretaceous rocks.
#3 DURING EOCENE TIME, uplift continued, the Pacific shoreline moved westward, and the Mitchell region was subjected to early folding and erosion of the Mitchell Anticline. Folding of this anticline probably tilted the Narrows Fault closer to a vertical orientation.
#4 ERUPTIONS of many Clarno Formation lavas, volcanic mudflows, and clouds of ash were deposited unconformably upon the cretaceous sediments. Development of the Mitchell Anticline was accentuated with addition of smaller-scale folding to the west.
#5 After folding of the Mitchell Anticline had ceased, continued eruption of Clarno volcanic products unconformably covered all of the earlier rocks with extensive andesitic lavas.
#6 Erosion of exposed Clarno and Cretaceous rocks occurred prior to deposition of air-fall ash of the Oligocene John Day Formation which included four main members and eruption of the Picture Gorge Ignimbrite.
#7 Eruption of the Miocene Picture Gorge basaltic lavas covered and buried all previous rock formations. The uppermost thin red line depicts advance of the Rattlesnake Ignimbrite at 7 Ma.
#8 Profound uplift and erosion during the last 5 million years exposed rock units at the level of modern day topography.