The content of this section is the result of my retirement from teaching students how to apply field geology to the study of many limited areas. Upon retirement, I was able to consider investigation of more widespread geologic interests. Chief among these was the distribution and deformation of the Picture Gorge Formation. Your review of the section “Picture Gorge Formation” might be of assistance here.
Several post-PGB events occurred east of Mitchell that reveal how the Mitchell area developed during the Neogene. In the John Day Valley, volcanic silt, sand, and ash-flow tuffs accumulated in deposits known as the Mascall Formation.
Isotopic determinations of Mascall Formation age do not distinguish it from the 15 Ma of Picture Gorge basalt but the Mascall rests conformably upon the PGB. These formations and all of the older rocks of the region were uplifted approximately 2000 feet on the south side of the nearly 80-mile-long east-west John Day Fault. We know the approximate time of this faulting because extensive aprons of alluvium (known as the Rattlesnake Formation) accumulated at the base of the escarpment on top of the Mascall and that an ash-flow tuff of 7.05 Ma (the Rattlesnake Ignimbrite) is interbeded within these alluvial gravels.
The Rattlesnake Ignimbrite spread from a southeast origin, entered the John Day Valley probably by the South Fork near Dayville and then moved downstream where it was partly deflected northward as far as Monument. However, at the point of deflection, it met the drainage system of Deep Creek and continued to flow westward in a direction toward Mitchell.
At that time Deep Creek and its tributaries drained the north flank of the Ochoco Mountains and occupied a very broad and low-level valley extending west beyond the Mitchell area.
I became aware of this when I traced the ignimbrite nearly 20 miles upstream in the Deep Creek system to an elevation of 4320 feet, 1020 feet above its starting point on the John Day River. Moreover, I have found the ignimbrite at 3200 feet elevation as part of a landslide block west of Baldy Mountain in the Richmond Quadrangle and exposures of the Rattlesnake Ignimbrite also occur on the west summit of Sutton Mountain at 3700 feet elevation, about 32 miles from the John Day River starting point. Ignimbrites are known to have moved upstream for limited distances; but this instance appears to be an extreme limit. Could the inclined distribution of the ignimbrite be a result of uplift? How can the extent and frequency of deformation in this area be determined?
It is important to recognize that the late Oligocene John Day surface consisted chiefly of deeply altered tuffaceous soils containing a widespread ash-flow tuff (the Picture Gorge Ignimbrite) that was deposited upon (and largely buried) irregular Clarno Formation topography. It was this relatively level surface that was covered 7 million years later by the mid-Miocene Picture Gorge Basalt.
Later sheets of lava numbered 24 to 30 and varied in thickness from 25 to 180 feet. These individual lava sheets can seldom be traced for more than 2 or 3 miles before being covered by later sheets. Each sheet provides a record of original horizontality during eruption; if they now deviate from horizontality, they can reveal the occurrence of faulting or folding that would otherwise be obscure. However, the lava-pile does not provide marker horizons that can be traced over wide areas.
I have found many instances in which individual lava sheets can be traced for a mile or more before having been overlain or “pinched-out” by other flows. Where a single lava flow can be carefully mapped around a hill or on both sides of a valley, the dip and strike of the entire stack of lavas can be determined. I have placed symbols that I call “dip-arrows” on appropriate maps to indicate where this has been applied with success.
I have developed other methods to determine the original horizontality of the PGB lavas. Three unusually thick units at the base of the formation are distinctive and can be easily traced where the John Day River and its tributaries have exposed them. Because they spread over a low land of at least 20 miles in width, the third flow (thickest, most distinctive and widespread) represents an original horizontal profile of the entire stack of lavas.
I have surveyed the distribution of this third maximum-thickness lava up to 10 miles north and south from the John Day River Canyon (see illustration below) and find that the elevations above sea level (relative to modern contours) of the base of PGB have been significantly disturbed after deposition.
One of these disturbances has long been recognized north of Mitchell as the Sutton Mountain Syncline and traces of axial location have been published on several geologic maps in slightly different positions and with no indication of dip magnitudes. It can now be seen that the axis of this syncline is at least 50 miles long, trends in a N65E direction, and that the axial floor is 2000 to 1800 feet below the rim areas with marginal slope inclinations between 3 and 5 degrees.
Subsidence of this syncline might have influenced the position of an ancestral John Day River as erosion excavated the overburden of basalt. One location of this river or a tributary is indicated by a thick and extensive deposit of river gravel (TJDG) overlain by Picture Gorge Basalt. It is depicted on my map “Sutton Mountain SE” as representing an example of the Haystack Valley Member of the John Day formation.
Inclination of the Picture Gorge Basalts on the north side of the syncline decreases to modest dips that are usually less than 2 degrees and this continues 12 miles northward to the axis of the Blue Mountains Uplift. It has been suggested (Robinson, P. T., Walker, G. W., and E. H. McKee, 1990) that following deposition of the first John Day ignimbrite (Member A) this uplift acted as a barrier to later ash-flow tuffs such as the Picture Gorge Ignimbrite and early sheet lavas of the Picture Gorge Formation.
To the south, after thick megaflows had been buried, thin PGB sheet-lavas came to rest upon topographic elevations consisting of older formations. In the southeast part of the Toney Butte Quadrangle, there is little evidence of this because PGB lavas have been removed by erosion. However, PGB lavas are better preserved farther east in the Richmond and Johnson Heights Quadrangles, where irregular topographic contacts are revealed. Still farther east, very thick sections of PGB have been so well preserved that the rock inclinations can be best shown by dip-arrows obtained from surface measurements.
At the southwest corner of the Masiker Mountain Quadrangle the synclinal pattern is modified by an elongate structure that is known as the Donnelly Basin. It is an uplifted dome that has been eroded to expose a core of John Day basaltic lavas, overlain by Picture Gorge ignimbrite, John Day bedded tuffs, and the Picture Gorge basaltic lavas. Was the Donnelly Dome formed prior to or after subsidence of the Sutton Mountain Syncline? The dome is much more inclined on the northwest side than to the southeast where the strata are nearly horizontal. This suggests that Donnelly Dome was uplifted before the syncline was formed.
The valley of the John Day River between Mount Misery and Miller Flat Quadrangles is known for the well-preserved Oligocene fossil animals and plants that have been extracted from the John Day tuffs and their display at the John Day Fossil Beds National Monument. The colorful John Day features known as Cathedral Rocks, Blue Basin, and Sheep Rock are popular photo sites.
The impressive PGB cliffs on the east and west rims of this valley are less well known and seldom brought to the attention of visitors.
These cliffs descend to the valley floor downstream where Brown and Thayer ( 1966) designated this valley to be underlain by a doubly-plunging syncline. However, the structural inclination of the PGB cliffs and adjacent highlands clearly show that this is an area that has been uplifted to form a broad dome extending south to Picture Gorge and the Ochoco Mountains. Headward erosion by the John Day River was able to remove the soft tuffaceous beds, undermine cliffs of PGB, and bring down large landslide blocks of the basalt.
At the southwest margin of the Miller Flat quadrangle I have discovered several significant dip-slip faults that have displaced PGB. These faults are close to the valley rim and will eventually be undermined by the erosional processes that have been so effective in forming this canyon.
Faulting was more common to the south and is represented by the obvious fault cutting Sheep Mountain, the Middle Mountain Fault, and several other NW-SE trending faults that occur near the well-known John Day Fault. This fault, south of the map, was described by very experienced U.S. Survey geologists, C. E. Brown and T. P. Thayer in 1966, who showed that a full column of PGB south of the fault was lifted 1500 to 2000 feet where it now caps the Aldrich Mountains.
This southern half of the canyon displays the same inclination of marginal PGB cliffs to the valley floor as was described for the northern half of the canyon, except the decent is now toward the south. Here near Picture Gorge, the southerly dips are obvious even from the highway and can also be seen in highlands to the west. In the quadrangle immediately west of the Picture Gorge W quadrangle (Tubb Spring, not shown), dips arrows are dominantly 5 to 8 degrees toward the southeast. The over-all structure is that of a broad uplift pronounced to the west toward Mitchell.
With regard to the condition of Rattlesnake Ignimbrite in the Deep Creek system, it represents a slope of only 1. 8 degrees resting on PGB dips as high as 12 degrees in the same direction. Consequently, most uplift of this region must have been underway prior to 7 million years ago.
Consequently, we need to itemize the last 15 million years of geologic events in the Mitchell region as consisting of:
1) PCB eruptions spread sheets of basalt over a broad region of relatively low relief until all preexisting rocks and landforms were completely buried. Even the Mitchell area, the Ochocos, and parts of the Blue Mountains Uplift were included. We do not know how far these basalts spread to the west.
2) This was followed by 8 million years of erosion that did not remove all of the PGB but occurred during uplift of the Donnelly Dome and subsidence of the Sutton Mountain Syncline. The course of an ancestral John Day River was influenced by this subsidence to assume an eastern path. A new drainage in the Mitchell area also flowed east along a proto Deep Creek system to the John Day.
3) The John Day Fault became active, lifting the PGB lavas to form part of the Aldrich Mountains. Alluvial fans covered the Mascall beds and the John Day Valley. At this time the Rattlesnake Ignimbrite entered the Deep Creek drainage and spread west beyond Mitchell to Sutton Mountain.
4) A broad uplift occurred over the entire Mitchell area. This uplift extended east beyond the John Day River causing the river to incise the PGB to form high cliffs on each side. Downstream, a meandering John Day River cut deeply into the basalts from the locations of current Kimberly to Service Creek.
5) South of Mitchell, the uplift was responsible for the south-dip of PGB lavas that cap the Ochoco Mountains.
6) Many less obvious geological features are probably related to the Mitchell Uplift: (A) Drainage from the Mitchell area is now to the west. (B) Many lines of evidence that the Mitchell area has undergone an accelerated rate of erosion. (C) The Mitchell area contains the largest exposure of Cretaceous rocks in central Oregon. (D) The extensive cross-hatched pattern of fractures displayed on U.S.G.S. geologic maps that are so obvious on aerial photographs of the Ochoco Mountains:
It has been suggested that these fractures were caused by an uplift of PGB (resting upon deformable John Day tuffs) producing their modern south inclination. If PGB moved slightly southward over the tuffs, a system of stress developed and was responsible for the fracture pattern (Lawrence, R. D., 19790).