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• 1. [Article] The structure and stratigraphy of a portion of the Trout Creek Mountains, Harney County, Oregon

  The thesis area is located in the southeastern corner of Harney County, Oregon. This portion of Oregon is situated in the northern part of the Basin and Range province, and is characterized by both mafic ...

  Citation × Citation Citation Abstract Copy Title: The structure and stratigraphy of a portion of the Trout Creek Mountains, Harney County, Oregon Author: Carlton, Richard W. The thesis area is located in the southeastern corner of Harney County, Oregon. This portion of Oregon is situated in the northern part of the Basin and Range province, and is characterized by both mafic and silicic extrusives, and localized nonmarine sediments and tuffs ranging in age from middle (?) Miocene to Recent. The rocks of Miocene and some Pliocene age have been cut by normal faults forming typical Basin and Range structure. The base of the stratigraphic sequence within the thesis area is formed by 775 feet of thin, highly porphyritic basalt flows. On the basis of petrographic and field similarities, these basalts have been correlated with the Steens Basalt of Fuller (1931). Overlying the porphyritic basalts are 980 feet of fine-grained, andesitic basalt flows, and flow breccias. These basalts superficially resemble the Steens Mountain volcanic series of Fuller but they occupy a higher stratigraphic position and may be correlative with the Owyhee Basalt of Renick (1931). Covering the andesitic basalt, but generally poorly exposed are approximately 155 feet of vitric and lithic-vitric air fall tuffs. These tuffs and the overlying ash flow tuffs may be genetically related and probably represent a period of silicic extrusion within the thesis area. Four ash flow tuffs are recognized and described. They appear to have been extruded over a short period of time and may represent a compound cooling unit. The basal ash flow (unit one) is 330 feet thick and characterized by large flattened pumice fragments in the middle welded zone. Covering the basal ash flow is a 230 foot thick ash flow (unit two) composed of a nonwelded base, a dense vitrophyre, and a thick partly welded upper zone. A third ash flow (unit three) whose maximum thickness is 225 feet overlies unit two and is characterized by extremely flattened pumice fragments and platiness. A fourth ash flow (unit four) found only on Red Mountain overlies unit three and has a maximum thickness of 10 feet. The ash flow tuffs are probably not related to the Canyon Rhyolite of Merriam (1910) and are middle (?) to late Miocene in age. Locally overlying the ash flow tuffs is a thick sequence of varicolored, thickly bedded vitric tuffs. These tuffs are correlated with the Trout Creek Formation on the basis of similar stratigraphic position. Stratigraphically above the Trout Creek Formation is a series of coarse angular conglomerates and fine lacustrine sediments which were deposited in stream channels, and as alluvial fans and slope wash during periods of high stream discharge. These sediments are correlated with the Turn-Turn Conglomerates found on the west side of Pueblo Valley. The Turn-Turn Conglomerates are covered on the west by Pliocene and Pleistocene terrace deposits composed primarily of unconsolidated volcanic gravels and pebbles. Structurally the western half of the thesis area is part of the Pueblo Valley graben while the eastern half forms the Trout Creek Mountains. Most of the faults cut the ash flow tuffs and strike within 30° east or west of due north. Donath postulates that this fault pattern may represent original conjugate strike slip shears caused by a north-south maximum principal stress. At a later time block faulting occurred along the previously formed shear zones forming the present topography. Title: The structure and stratigraphy of a portion of the Trout Creek Mountains, Harney County, Oregon Author: Carlton, Richard W. The thesis area is located in the southeastern corner of Harney County, Oregon. This portion of Oregon is situated in the northern part of the Basin and Range province, and is characterized by both mafic and silicic extrusives, and localized nonmarine sediments and tuffs ranging in age from middle (?) Miocene to Recent. The rocks of Miocene and some Pliocene age have been cut by normal faults forming typical Basin and Range structure. The base of the stratigraphic sequence within the thesis area is formed by 775 feet of thin, highly porphyritic basalt flows. On the basis of petrographic and field similarities, these basalts have been correlated with the Steens Basalt of Fuller (1931). Overlying the porphyritic basalts are 980 feet of fine-grained, andesitic basalt flows, and flow breccias. These basalts superficially resemble the Steens Mountain volcanic series of Fuller but they occupy a higher stratigraphic position and may be correlative with the Owyhee Basalt of Renick (1931). Covering the andesitic basalt, but generally poorly exposed are approximately 155 feet of vitric and lithic-vitric air fall tuffs. These tuffs and the overlying ash flow tuffs may be genetically related and probably represent a period of silicic extrusion within the thesis area. Four ash flow tuffs are recognized and described. They appear to have been extruded over a short period of time and may represent a compound cooling unit. The basal ash flow (unit one) is 330 feet thick and characterized by large flattened pumice fragments in the middle welded zone. Covering the basal ash flow is a 230 foot thick ash flow (unit two) composed of a nonwelded base, a dense vitrophyre, and a thick partly welded upper zone. A third ash flow (unit three) whose maximum thickness is 225 feet overlies unit two and is characterized by extremely flattened pumice fragments and platiness. A fourth ash flow (unit four) found only on Red Mountain overlies unit three and has a maximum thickness of 10 feet. The ash flow tuffs are probably not related to the Canyon Rhyolite of Merriam (1910) and are middle (?) to late Miocene in age. Locally overlying the ash flow tuffs is a thick sequence of varicolored, thickly bedded vitric tuffs. These tuffs are correlated with the Trout Creek Formation on the basis of similar stratigraphic position. Stratigraphically above the Trout Creek Formation is a series of coarse angular conglomerates and fine lacustrine sediments which were deposited in stream channels, and as alluvial fans and slope wash during periods of high stream discharge. These sediments are correlated with the Turn-Turn Conglomerates found on the west side of Pueblo Valley. The Turn-Turn Conglomerates are covered on the west by Pliocene and Pleistocene terrace deposits composed primarily of unconsolidated volcanic gravels and pebbles. Structurally the western half of the thesis area is part of the Pueblo Valley graben while the eastern half forms the Trout Creek Mountains. Most of the faults cut the ash flow tuffs and strike within 30° east or west of due north. Donath postulates that this fault pattern may represent original conjugate strike slip shears caused by a north-south maximum principal stress. At a later time block faulting occurred along the previously formed shear zones forming the present topography. Close