DEVELOPMENT OF GEOLOGIC EVENTS
(Describing progressive changes in central Oregon landforms, climates, and living conditions over geologic time)
Pre-Cretaceous development of north central Oregon is known chiefly by study of the many “accreted” Triassic and Jurassic terranes that are exposed to the east. Chief among these is the Baker Terrane which is represented near Mitchell by a mélange of oceanic volcanic rocks, including pillowed metabasalts, serpentinite, bedded-cherts, metacarbonates, phyllites, and blueschists, that had been metamorphosed and brought to the continent by plate-tectonic processes. The landscape that was underlain by these rocks must have been gradually destroyed by erosion and advance of Pacific shorelines over most of central Oregon, probably during late Jurassic to middle Cretaceous time. This process is represented in the Mitchell area by the sandy basal member of the Hudspeth Formation which was deposited upon the basement rocks. The basal member contains fossil plant fragments, shells of small pelecypods, pebbly sandstones, and beds of granule conglomerate. It is even possible to find thin sandstone beds that are composed almost entirely of serpentine grains. All of this implies near-shore marine conditions with deeper sea extending to the west.
INTRODUCTION OF MARINE CRETACEOUS ROCKS
During a period of many millions of years, the crust of central Oregon became relatively stable as shorelines migrated eastward and the ocean floor subsided. Erosion of the eastern landscape produced sediment that was discharged by rivers into the sea and deposited in broad sheets of mud and sand that are now represented by the black and thin-bedded mudstones and sandstones of the Hornbrook Formation of northern California and the Hudspeth Formation of central Oregon. In both places, fossils of ammonites (which floated in the sea and fell into the mud upon their death) and belemnites (which were pencil-shaped hard parts that supported squids) are abundant in these rocks. Fossils of ichthyosaurs (marine reptiles) and pterosaurs (flying reptiles) have also been found in the Mitchell area. Commonly, these sediments display a variety of features (called sole marks) on their bedding surfaces which provide clues to the direction of transport; such as a flute cast which forms when a small depression is excavated by flowing water and is then filled in by a different bed, or a tool mark which is formed as a resistant object is dragged over a bed. I have examined many sole marks at the base of Hudspeth mudstones and found the depositional currents to have been chiefly moving from NE to SW. Frequently, the mud and sand were carried with such energy over the seafloor as to be deposited as turbidites, which can be recognized in beds that were formed by settling of grains in water that had been turbulent. Occasionally, ocean currents moved with sufficient energy to transport pebbles with the sand and silt, introducing thin beds of conglomerate. As a result of all these processes, several thousand stratigraphic feet of the Cretaceous Hudspeth Formation accumulated in the region near Mitchell. They probably do not represent a large near-shore delta system as the modern Mississippi River has produced, but a deeper-level environment as on a submarine shelf.
The Hudspeth Formation which is dominated by mudstones and sandstones (with some conglomerates), is overlain by the Gable Creek Formation which is dominated by conglomerates (with some mudstones and sandstones). The individual conglomerate beds can be more than 200 feet thick and can be traced for more than 10 miles before they are obscured by younger formations or removed by erosion. Many of them were deposited in broad submarine channels trending from northeast to southwest. Igneous rock types ranging from coarse granites and granodiorites to fine-grained rhyolites and densely welded tuffs are far more abundant as clasts than metamorphic or sedimentary rocks. There is even a volcaniclastic bed containing coarse pumice fragments in northeast outcrops that has been traced to southwest outcrops with smaller pumice fragments. These characteristics suggest that the conglomerate provenance was underlain chiefly by igneous rocks and located to the northeast. It should be noted that I have been unable to find counterparts in eastern Oregon and adjacent Idaho to the most distinctive volcanic and intrusive rocks that are common in the Gable Creek conglomerates. There have been many published hypotheses commonly known as the “Baja-BC Model” of northward tectonic migration of a large part of the western North American continent as far as 1200 miles during upper Cretaceous time. If this occurred, the Gable Creek clasts could have been derived from as far south as the Sierra Nevada province or even Northern Mexico.
The conglomerates are characterized by clast support in which very little of smaller mud, sand, or pebbles is found between the larger cobbles. Other features of these conglomerates such as boulders up to 2 feet in diameter and 5-foot angular fragments that have been ripped up from underlying sediment are difficult to imagine being transported over the seafloor for great distances by the same marine currents that had been responsible for the Hudspeth sands and muds. It may well be that landward conditions such as mountain building brought very coarse sands and gravels to the edge of the submarine shelf where they accumulated while finer materials continued to deeper levels. Each time the coarse fraction became unstable, it could have moved en-masse toward deposition further down the submarine slope. Moreover, these conglomerates contain many features such as coarse-grained sole marks and graded bedding that characterize very turbid conditions of transport.
THE VOLCANIC EOCENE CLARNO FORMATION
Again, many millions of years transpired during deposition of Cretaceous submarine muds, sands, and gravels before a gradual uplift established a broad coastal plain over much of Paleocene central Oregon. In the Mitchell area, these Cretaceous rocks were disturbed by a major north-south dip-slip fault (the Narrows Fault). If this fault and others produced irregular topography, erosive processes must have reduced it to a surface of modest relief with gently inclined resistant cuestas of Gable Creek conglomerate. Details of this surface are revealed by the first volcanic deposits of the Eocene Clarno Formation. Clarno volcanism brought an impressive variety of lavas, volcanic mudflows, ash deposits, and welded tuffs to cover all of the earlier landscape. Andesitic lavas spread far and wide mostly from limited vents and cones because evidence of only a few large volcanoes has been found. In contrast, small andesitic dikes and plugs stand in relief over nearly all of the Cretaceous rocks. In the vicinity of Mitchell, two large intrusive plugs have been exposed: White Butte dacite which was intruded as a solidified body, forcing Cretaceous conglomerate aside into vertical layers, and Black Butte andesite which was intruded as a more liquid mass that lifted Cretaceous strata above itself.
Many Clarno intrusive bodies were more silica- and volatile-rich than andesite; they fed violent eruptions of rhyolitic volcanic ash. In response to the tropical wet climate of that time, surface ash and lava fragments were incorporated into mudflows that swept up everything in their path including trees of the jungle and blocks of Cretaceous conglomerate (some of which were over 30 feet in diameter). In many of these mudflow bodies, trunks of petrified trees still stand in growth position. East of Mitchell, total thickness of superimposed mudflows is near 1000 feet. Much of the Clarno ash was washed into marshlands or standing lakes where thin beds of lacustrine tuff now reveal near-perfect fossil leaves and fish. Extensive records of Clarno plant and animal life has been discovered at localities northwest of Mitchell near the Clarno type location on the John Day River and it is likely that the same life forms lived in the Mitchell area. They include palm, pine, black walnut, sycamore, magnolia, avocado, figs, cinnamon, and large equisetum stems, with petrified woods, which are closely related to plants in modern southeast asia. Animal fossils include oreodons, rhinoceros, crocodilians, titanotheres, and tapirs among many other recovered fossils that have not yet been identified. The most violent eruptions gave vent to pyroclastic flows which were chiefly confined to valleys and lowlands where they destroyed nearly all combustible materials. Some of these welded tuffs still contain nodules of carbonized wood now completely enclosed in volcanic glass.
All of these structures and rocks were involved in folding of the NE–SW Mitchell Anticline which is evident to anyone traveling along highway 26 near Mitchell. This anticline is 22 miles long with east and west limbs dipping 20 to 30 degrees. Folding of this anticline was followed by displacement of the E-W trending strike-slip Cherry Fault that displaced the Narrows Fault, the Mitchell Anticline, and all but the final deposits of the Clarno Formation.
THE JOHN DAY FORMATION
Clarno volcanism gradually stopped at about the time that a Western Cascade volcanic belt became active. Consequently, the Eocene surface rocks in central Oregon were subjected to long-time erosion and extensive weathering before they were covered by volcanic ash that drifted eastward from the Cascades during the Oligocene. These ash beds now make up the John Day Formation. The John Day ash beds total about 2500 feet in thickness and accumulated over a period of about 15 million years, which might imply that only 1.4 inches of ash was deposited each one-thousand years. However, the internal structure of the formation demonstrates that ash clouds appeared intermittently, each contributing a few inches of ash that were subjected to long periods of weathering and soil production. This implies that Western Cascade eruptions were infrequent but extremely violent.
During the long intervals between ash falls, plant and animal life evolved in response to a gradual change in climate from that of modern wet-tropical, southeast Asia to that of current forest lands of our eastern states. Climate change during John Day weathering is also reflected in the dominant red (oxidized) color of the lower ash beds, followed by midsection greenish tints, and buff-colored upper sections. Construction of a high-standing Cascade Range might also have produced a “rain shadow” effect upon central Oregon. A remarkable assemblage of fossil plants and animals (too extensive to list here) have been recovered from the upper parts of the John Day Formation and include many trees: maple, birch, alder, oak, pine, and metasequoia (dawn redwood) that suggest well-developed open woodlands that were occupied by many animals: horses, oreodons, camels, rhinoceroses, tapirs, bears, pigs, dogs, and cats. Fossils of smaller animals have also been recognized: salamanders, frogs, birds, and even bats.
Many pyroclastic flows (ignimbrites) are included in the John Day Formation and two of them occur in the Mitchell area. Eruption of these welded tuffs was separated by 15 million years but each must have temporarily destroyed all organic materials in their path. The lowermost, designated “member A” is of rhyolitic composition and defines the base of the formation; while at a higher level, the “Picture Gorge Ignimbrite” is a widespread silicic ash-flow sheet that extends broady from the Ochoco Mountains on the south to the Blue Mountains Uplift on the north. A source vent for neither of these units has yet been located. It has been suggested that the northeast-trending, broad anticlinal structure of the Blue Mountains Uplift was produced at this time, preventing all of the many western John Day ash-flows except member A from reaching the Mitchell area.
ERUPTION OF THE PICTURE GORGE BASALTS
The post John Day landscape in the Mitchell area must have been relatively level after long continued erosion of ignimbrite deposits and burial by ash beds. Plants and animals probably lived and evolved in a stable environment for long periods. However, a series of brief events starting about 15 million years ago produced new volcanic hazards that must have completely destroyed all life forms over much of the John Day surface, requiring reintroduction of biota each time. Fissures opened in the crust near the current town of Monument and very hot, fluid basaltic magma issued so rapidly that vast areas of several hundred square miles were repeatedly buried under thin sheets of lava (now referred to as Picture Gorge Basalt, PGB). It does not appear evident that production of extensive ash accompanied these eruptions. The upper surface of many lava sheets was oxidized to a deep red color but there is little evidence of volcanic ash, soils, or long-lasting quiet between eruptions. We can see these lava flows today in walls of the John Day River canyon and nearby mountain slopes. They average about 40 feet in thickness and up to 23 separate layers can be seen in vertical succession. Clearly, most relief of the John Day surface was rapidly changed to a level platform of extensive basalt. Even today as we stand on PGB-covered Mount Pisgah, south of Mitchell, we look down on the 5620 summit of the Clarno White Mountain intrusive, one of the highest points of land above the ancient John Day landscape. Clearly, extensive erosion has occurred since all of the landforms (and probably White Mountain) were buried under these lava flows.
Volcanic ash and ash flow deposits of the Mascall Formation accumulated in the Picture Gorge Valley and were deformed while the PGB was subjected to folding and uplift. The east-west trending John Day Fault, located east of Mitchell was uplifted on the south side at least 2000 feet, lifting the PGB to the summit of the Aldrich Mountains.
CONTINUED EROSION AND UPLIFT
Erosion and crustal deformation have been the chief agents of geologic change in the Mitchell area during the last 15 million years. While these changes have been slow, the results have been profound. While PGB lavas still cap the Ochoco Mountains south of Mitchell and high ranchlands to the east, the area north of Mitchell was subjected to a gentle downwarp that produced the long recognized (and rather gentile) Sutton Mountain Syncline. Fluvial erosion has laid bare the underlying Tertiary and pre-Tertiary formations between these areas, and probably carried their overburden all the way to the Pacific Ocean. This erosion was so pervasive that the Mitchell area came to occupy a broad lowland that drained eastward to the ancestral John Day River, rather than westward as it does now. During this time the Mitchell area was invaded by the Rattlesnake Ignimbrite, while extensive erosion continued.
During the last 7 million years, a broad and slow uplift has occurred which invigorated erosive action to produce the incised and meandering John Day river canyon north of Mitchell and raise the Ochoco Mountains. On the local Mitchell scene, streams have dissected resistant intrusive rock bodies instead of eroding soft tuffaceous beds around them. Even the Rattlesnake Ignimbrite has been raised to the summit of Sutton Mountain.