A VOLCANIC MAAR ORIGIN FOR THE AVIAN PALEONTOLOGICAL LOCALITY AT FOSSIL LAKE, OREGON

The paleontological locality at Fossil Lake, Oregon, has been known to science since the late nineteenth century. Fossil Lake is an important avian locality and contains at least 70 species of fossil birds, as well as large and small mammals, fossil fish, and other aquatic organisms. Few radiometric dates have been reported from Fossil Lake, and previous workers have been confounded by inconsistencies among various biostratigraphic interpretations of the age of the deposit.

Most previous workers have proposed deposition in a Pleistocene pluvial lake-dominated environment. However, the geometry, lithostratigraphy, and faunal and taphonomic evidence at Fossil Lake, as well as its location within the extensive Fort Rock basin maar field, all support the hypothesis that the locality occupies a volcanic eruption crater or crater complex. A phreatomagmatic maar-style eruption or series of eruptions during created a shallow crater partially surrounded by a low cone of locally derived basaltic and glassy fragments. The eruptive facies is unfossiliferous and shows chaotic low- to high-angle crossbedding formed during the eruption. The debris cone is most prominent north and northeast of the crater, implying subaerial eruption and distribution by prevailing winds. The crater floor consists of blocks of Pliocene diatomite containing fossil fish and birds of that age. Subsequently, airfall ash layers and other sediments have been deposited on the diatomite above a major discontinuity. The maar hosted environments that were attractive to many species of birds, especially water birds, and many of the avian fossils accumulated in aquatic or semi-aquatic environments within the maar, during low stages of the pluvial lakes that occupied the Fort Rock Basin. Influx of water-borne sediment has been limited by the flat valley floor, tilted and built-up crater rim, and low sediment inputs under both lacustrine conditions at high levels of the pluvial lakes in Fort Rock Valley and past and present-day arid conditions. The Fossil Lake maar crater has remained as a depositional basin since its formation, with low sediment input, resulting in a time-averaged fossil assemblage at the locality.