Plant Fossils in the Clarno Formation, Oregon

The Clarno formation was originally described by Merriam (1901) from outcrops at Mitchell and Clarno's Ferry, Oregon. Since that time the formation has been found to be very widely distributed throughout central Oregon (Wilkinson, 1959). It is composed of an unknown thickness of volcanic rocks and interfingering terrestrial sediments which contain a newly discovered vertebrate fauna and an abundance of fossil plant remains. Wilkinson (1959) and Taylor (1960) describe several distinctive rock types that make up the Clarno formation. Of special importance are the beds of tuff ranging from coarse gritty tuffaceous sandstones to fine-grained tuffaceous shales, some of which are water-laid and contain fossil leaves. Another important unit in which plant remains are found, is composed of volcanic conglomerates and breccias of mud-flow origin. Associated wi th the conglomerates and breccias are andesite and basal t flows. Welded tuffs occur at several horizons. Plugs, dikes, and sills of andesite, rhyolite, and dacite are characteristic of the Clarno formation and are topographically expressed on the landscape as prominent buttes and ridges. The range in age of the Clarno formation has not been determined with certainty. In the few places where its base is observed, it lies unconformably on Cretaceous deposits, and it is overlain unconformably by the upper Oligocene-lower Miocene John Day formation. Up until recently, writers have generally favored a middle to upper Eocene age for the Clarno formation, the assignment being based almost entirely on fossils found in the vicinity of the type locality at Clarno's Ferry. Thus, R. A. Scott (1954) in his excellent monograph on the fossil nuts and fruits occurring 1! miles east of Clarno's Ferry, suggests an age somewhat older than upper Eocene. Until recently, animal fossils were thought to be of such rare occurrence in the formation that Stirton (1944) used a single tooth, . presumably belonging to the genus Hyrachyus, to establish a middle Eocene age for the CI~rno formation. R.ecently an extensive deposit of animal fossils near the type locality was discovered by the late A. W. Hancock and is being studied by J. A. Shotwell of the University of Oregon. Prel iminary work suggests an upper Eocene or, possibly, lower Oligocene age for these fossils, but the stratigraphic relationship of the vertebrate bed to the plant locality is problematical (Taylor, 1960). Plant fossils have been found at many other places in the Clarno formation in addition to those occurring at the type locality (see accompanying map). Comparison clearly shows the non-contemporaneity of many of the f10rules and suggests that the age of the Clarno formation is of an appreciably greater range than that proposed by many previous authors. Since some of these local ities have not been described in detail in the literature previously, it would seem to be of value to summarize our present state of knowledge of the Clarno flora. Descriptions of the local ities are presented below in the order of decreasing age of the