NEOGENE HISTORY OF THE GRANDE RONDE VALLEY, NE OREGON

Fieldwork, seismic profiling and examination of cuttings from water wells suggest that the Grande Ronde Valley is a mature half-graben that began forming ~9-m.y. ago. Sediments started accumulating in the basin after Columbia River basalts and the 14.5- to 9-Ma sequence of olivine basalt, andesite and dacite lavas of the Powder River Volcanic Field were erupted during Barrash and others’ (1983, Am. J. Sci., v. 283, p. 897-935) tectonic stage 1. The bedrock underlying the valley sediments is divided into a series of slice blocks formed by step-and-scissor faulting. Quaternary movement along these fault blocks appears to be responsible for the rectangular course of the Grande Ronde River across the valley floor and may be responsible for Sand Ridge, a subdued topographic feature in the northern part of the valley.

The presence of two major coarsening-upward cycles in the valley sedimentary sequence suggests that the subsidence rate has not been constant over the past 9 m.y. Each cycle began with deposition of diatom-rich marsh and lacustrine sediments, followed by the deposition of fluvial and alluvial fan/fan-delta sediments. This pattern suggests initially rapid subsidence followed by slower subsidence and basin-filling. The timing of these cycles, from ~9- to 4-Ma and ~4- to 0-Ma, coincides with Barrash and others’ (1983) tectonic stages 2 and 3.

The dominant components of the gravels deposited in the Grande Ronde Valley between 9-4 Ma changed from metachert fragments to diorite and granite fragments and finally to volcanic rock fragments. This pattern suggests that the source of the Grande Ronde Valley sediments shifted from fragments derived from erosion of older Late Cretaceous to Paleocene(?) sediments and older volcanic arc and intrusive rocks in the nearby Elkhorn and Wallowa Mountains to fragments derived from local volcanic sequences during tectonic stage 2. Evidence based on fish fossils and diatoms suggests that the Grande Ronde Valley was linked to Lake Idaho to the south and the Pasco Basin to the north shortly after the start of tectonic stage 3 (Van Tassell and others, 2001, Or. Geol., v. 63, p. 77-96). The link to Lake Idaho was cut off by ~2-Ma as deposition in the valley gave way to alternating periods of deposition and erosion during the Pleistocene.