Late Eocene detrital laterites in central Oregon: Mass balance geochemistry, depositional setting, and landscape evolution

Abstract

Detrital laterites interbedded with clayey, Ultisol-like paleosols in the late Eocene strata of central Oregon record periods of soil erosion, colluvial concentration of iron-cemented soil nodules, and deposition of these weathered products in hillslope settings. Two sets of lateritic paleosols are extensively exposed in the Painted Hills area of Oregon and span the transition from Eocene Clarno Formation andesitic volcanism to the initiation of late Eocene–Miocene pyroclastic volcanism of the John Day Formation. These late Eocene lateritic paleosols developed along the margins of several different lava flows where they formed local accumulations of iron-rich strata, which are now exposed in deep-red and ocher-colored badlands along the exhumed flow margins. Stratigraphically, the lateritic paleosols are above upper Clarno Formation rhyodacite flows, below the thick tuffaceous Oligocene–early Miocene part of the John Day Formation, and sandwich the welded tuff of member A that defines the base of the John Day Formation. In each of several cases from different lava flows studied, a similar sequence of detrital laterites and clayey paleosols rest on weathered lava flow breccia. The basal paleosol of these sequences consists of a thick (5–10 m), very strongly weathered saprolite zone developed in lava flow breccia and an overlying clayey B horizon. This paleosol is overlain by 8–12 m of alternating clayey, kaolinite-rich paleosols (Ultisol-like paleosols) and weakly developed paleosols with iron-rich, claystone breccia fragments (detrital laterites). The iron-cemented claystone fragments are up to 35% Fe 2 O 3 , very base poor, and weather-resistant, and they contain abundant cross-cutting clay skins and clay-filled pedotubules indicative of polycyclic weathering. The lower of the two sets of detrital laterites is associated with a thick rhyodacite flow in the upper Clarno Formation and has an up-section increase in the degree of weathering and concentration of resistate constituents, as determined by mass-balance geochemical analysis. The time span represented by this well-developed weathering trend is estimated to be between 2 and 4 m.y. based on estimates of the time of formation of interbedded paleosols. This long-lasting weathering trend is probably the result of a lack of soil rejuvenation resulting from the late Eocene hiatus between Clarno and John Day volcanism. A developmental model for the formation of the detrital laterites and Ultisol-like paleosols involves alternating episodes of soil formation and soil erosion in which iron-rich soil nodules are concentrated as a colluvial lag deposit on the toe slope of hills. Subsequent colluvial pulses of iron-cemented gravel were increasingly weathered and rich in resistate constituents because of longer residence time in up-slope soils. During periods of landscape stability, slow vertical accretion of soils by small additions of volcanic ash and dust produced the strongly developed, but nonlateritic, Ultisol-like paleosols. The episodes of soil erosion probably correspond to periods of climatic change during the late Eocene climatic deterioration. The John Day Formation detrital laterites and clayey paleosols are very similar to the Clarno formation laterites except for the presence throughout the section of 1%–3% pyrogenic feldspar crystals. No up-section increase in weathering is observed in the John Day detrital laterites, perhaps because of rejuvenation of soils by volcanic ash. The similar textures and chemistries of the two groups of detrital laterites, despite the onset of John Day pyroclastic volcanism, indicate that climate remained subtropical and humid up to the Oligocene-Eocene boundary.