Geologic map and provenance of the Clarno formation in the Whitney 7.5' Quadrangle, NE Oregon

Brian Nicholas Spall

The Clarno Formation records mid to late Paleogene volcanism in central and eastern Oregon, covering an area of ~5500 km2 with a maximum thickness of ~2 km. This lithostratigraphic unit is characterized by interstratified basaltic-andesitic lavas and related volcanogenic sedimentary deposits. Extensive exposures are located along and to the south of the axis of the Blue Mountain anticline with smaller separated bodies to the east of the majority of the deposit. Most previous work on the Clarno Formation has been focused on its western deposits and primary volcanics, with detailed studies of the volcanogenic sedimentary deposits and eastern outcrop bodies lacking. Though the number and distribution of volcanic centers and the volcano-tectonic history of these deposits are still debated, detailed examination of the sedimentology and stratigraphy of these volcanogenic sedimentary deposits offers potential for reconstructing the volcano-tectonic history of the region. This research remapped the Whitney 7.5' quadrangle in northeastern Oregon, at a larger (1:24,000) scale than previous maps and examined the sedimentology and stratigraphy of the volcanogenic sedimentary deposits of the Clarno Formation to reconstruct its provenance. Stratigraphic assessment reveals the dominance of crudely stratified to massive, poorly- to moderately-sorted, angular to subrounded, matrix-supported, lithologically heterogeneous boulder to pebble conglomerates indicating that deposition was predominantly from mixed hyperconcentrated and noncohesive sediment-gravity flows in the Whitney quadrangle. The presence of large boulders, tree casts, juvenile clasts, scarcity of tephra and intercalated beds of silty sandstone and lava flows suggests that deposition occurred on the medial part of broad alluvial apron(s). Major and trace element data and the lithologies of the Whitney Clarno deposits correspond to Urbanczyk (1994) and Lilligren's (1992) upper sequence of eastern Clarno Formation. Chemostratigraphic data do not reveal any evidence of systematic unroofing. Geochemical analysis suggests a mixing of multiple volcanic sources that supplied material to the Whitney quadrangle. Paleocurrent measurements within the quadrangle, though limited in number and spatial and temporal distribution, support at least two volcanic upland sources beyond the boundaries of the Whitney quadrangle, one to the NW and one to the SE.

Geologic map and provenance of the Clarno formation in the Whitney 7.5' Quadrangle, NE Oregon

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