Graduate Thesis Or Dissertation
 

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  • In the area surrounding Clarno, Oregon, pre-Cretaceous metamorphics, Cretaceous marine sediments, and a varied assemblage of Tertiary volcanic rocks are known. The oldest exposures are of an homogeneously fine-grained, mildly metamorphosed, limy shale to which the term "Muddy Ranch phyllite" is applied. Non-metamorphosed, Cretaceous marine shales and graywackes, approximately 1500 feet thick, exist beneath the surface at Clarno. The beginning of Tertiary vulcanism is recorded in the Clarno formation of probable lower Eocene to lower Oligocene age. This is succeeded unconformably by upper Oligocene to lower Miocene tuffs and welded tuffs of the John Day formation. Middle Miocene lavas of the Columbia River basalt formation were the last major products of igneous activity in the area. Unconsolidated Pleistocene and Recent deposits occur along drainage channels. The Clarno Basin was a depository of volcanic sediments from nearby centers of igneous activity throughout Clarno time. Study of these rocks reveals the following succession: The lower two-thirds of the Clarno formation consists of a colorful series of volcanic conglomerates and breccias indicating widespread conditions of mudflow. While dominant in volume, these sediments record only brief interruptions of a prevailing fine-grained lacustrine and floodplain deposition. Andesitic and basaltic lavas of widely varying types were frequently spread over the Basin near the mid-point of mudflow development. This first stage of Clarno vulcanism was brought to a close by the eruption of distinctive white and yellow leucoandesites. A time of quiescence is recorded in a widespread sheet of very fine tuff which was deeply weathered as it accumulated. A flow of basaltic lava poured over this surface, creating a lake in which a jungle-like flora was preserved. When the "fossil soil" had become nearly 200 feet thick at Clarno, a blanket of welded tuff reached the Basin from a probable point of origin near' the Horse Heaven amines. This was followed, after prolonged weathering, by the eruption of pyroxene basalt which eventually rivalled the inter inter-mudflow lavas in thickness and distribution. As Clarno vulcaniscr, diminished, the terrane was reduced to a low relief and thick, tuffaceous soil again accumulated. A recent and remarkable discovery of lower Oligocene mammal remains was made in this horizon. The extravasation of an unusually gas-rich andesite entombed this surface beneath a resistant cap of ignimbrite. With the appearance of another welded tuff sheet, probably from the northeast, Clarno activity ceased. A regional trend of northeast-southwest folds was first developed during the late Cretaceous or Paleocene and was accentuated during lower or middle Oligocene time. This structural system strongly influenced the distribution of rocks formed within it. At the beginning of the Tertiary, Cretaceous sediments were deeply eroded from the anticlines, exposing older rocks. Clarno volcanics were deposited unconformably on this surface. Post-Clarno uplift accelerated erosion of Clarno rocks from the same anticlinal regions. Because structure also determined topographic character, John Day tuffs were concentrated in the synclines. A gentle warping of the older folds during Pliocene time involved the Columbia River basalts and is in part responsible for their superior preservation in synclinal areas. Present-day relief features in the Clarno Basin have been in process of formation since the upper Miocene. Two distinct periods of rejuvenation are recorded, one during the upper Pliocene, another during the later Pleistocene. The Pliocene warping was not a necessary factor in rejuvenation but may have been contemporaneous, increasing its effect in places, decreasing it in others.
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