Pliocene and Quaternary stratigraphic architecture and drainage systems of the Big Lost Trough, northeastern Snake River Plain, Idaho

Abstract

The geometry, volcanic-sedimentary stratigraphic architecture, and distribution of clastic sedimentary facies reflect a complex tectonic setting and fluctuations in climatic conditions during the past 2.5 m.y. in the Big Lost Trough on the eastern Snake River Plain. Interaction of the migrating Yellowstone hotspot and developing Basin and Range structures controlled the spatial distribution of volcanic rift zones that define the margins of the Big Lost Trough, an arid, underfilled basin. The volcanic-sedimentary stratigraphy of the basin is characterized by basaltic volcanic units that offlap eruptive centers and downlap into the basin, and clastic sedimentary units that onlap adjacent volcanic rift zones. Climatically influenced interactions of a fluvial-playa-eolian depositional system of the Big Lost River and a lacustrine system of Lake Terreton are reflected in the composition and architecture of the sedimentary basin fill. Petrographic and U/Pb detrital-zircon geochronology analyses of subsurface sands compared with analyses of modern fluvial and eolian sands allow definitive determination of the provenance of the subsurface deposits. Petrographic and detrital-zircon data suggest that the Big Lost River has been the dominant source of sediment for at least the past 1 m.y. Big Lost River deposits found in the middle and northern parts of the basin suggest that the river system prograded northward during lowstands of Lake Terreton. Lowstands of Lake Terreton are also associated with development of an eolian system that reworked the fluvial deposits. The abundance of Big Lost River and eolian sands in the middle of the basin documents the effective damming of sediment by the volcanic rift zone that defines the northern basin margin. X-ray diffraction data suggest that subsurface playa or marginal lacustrine deposits along the northeastern basin margin contain abundant gypsum, indicating that ancient arid climate cycles were drier than the modern arid climate.