Late Oligocene bunch grassland and early Miocene sod grassland paleosols from central Oregon, USA

Abstract

Fossil soils, burrows and mammals of the upper John Day Formation in central Oregon are evidence of bunch grasses and open, semiarid vegetation as old as late Oligocene (earliest Arikareean, 30 Ma). Root traces in these paleosols include both stout, tapering tubes, like roots of trees, as well as sinuous filamentous tubes, similar to roots of grasses. Paleosol structure is fine subangular blocky, with patchy distribution of grass-like roots, as in wooded grassland and sagebrush steppe with bunch grasses. Cursoriality in horses ( Mesohippus, Miohippus) and hypsodonty in rhinos ( Diceratherium) is also evidence for open grassy vegetation. Trace fossils of Pallichnus (dung beetle boli) and Edaphichnium (earthworm chimneys) are characteristic of wooded grassland paleosols, whereas Taenidium (cicada burrows) dominates desert shrubland paleosols, as has also been found in Quaternary paleosols and soils of eastern Washington. In both Oligocene and Quaternary paleosol sequences, arid shrubland and semiarid grassland paleosols alternate on Milankovitch frequencies (23, 41, 100 ka). The oldest known paleosols in Oregon with crumb structure and abundant fine fossil root traces characteristic of sod grasslands are dated by mammalian biostratigraphy as Hemingfordian (early Miocene, ca. 19 Ma). Wooded grassland habitats are indicated by scattered chalcedony-calcite rhizoconcretions from large woody plants, and by fossil chalicotheres ( Moropus), camels ( Gentilicamelus, “ Paratylopus”) and horses ( Parahippus). Silty texture and silcrete horizons are evidence of semiarid to arid paleoclimate, and are in striking contrast to highly calcareous, and clayey underlying paleosols of the John Day Formation. These silcrete paleosols may represent the Miocene onset of summer-dry (Mediterranean) seasonality, as opposed to a summer-wet (monsoonal) pattern of seasonality found in this region during the Oligocene. Oregon's early rangelands can be compared with those in the North American Great Plains. Granular-structured calcareous paleosols of the Brule Formation of South Dakota are evidence of dry, bunch grasslands as old as 33 Ma (early Orellan, early Oligocene), and crumb-structured paleosols of the Anderson Ranch Formation of Nebraska are evidence of sod grasslands as old as 19 Ma (late Arikareean, early Miocene). Although grasses were a conspicuous part of dry rangelands well back into the Oligocene, early and middle Miocene sod grasslands in North America were restricted to regions estimated to have had less than 400 mm mean annual precipitation.