Paleoenvironments of sedimentary interbeds in the Pliocene and Quaternary Big Lost Trough, eastern Snake River Plain, Idaho

Thick successions of Pliocene and Quaternary sediment recovered from core 2-2A in the central part of the Big Lost Trough on the Idaho National Engineering and Environmental Laboratory (INEEL) record deposition in two climatically controlled depositional systems. During latest Pliocene and early Pleistocene time (2.5-1.5 Ma), wetter climates plus a higher regional water table produced lakes and lake deltas. The eastern Snake River Plain groundwater level was adjusted to the level of Lake Idaho to the west, which drained by middle Pleistocene time. During relatively dryer late Pleistocene time, sediment was deposited in cyclically aggrading playa-lunette systems with a fluvial and loess sediment source. The central Big Lost Trough was a persistent low area that trapped and preserved sediment. Thus, unlike cores and wells in the northern INEEL around Test Area North, core 2-2A contains a substantial sedimentary record including four upper Pliocene to upper Pleistocene stratigraphic intervals. A thick interbed at depth 292-350 m (958-1148 ft) is dated by a combination of magnetostratigraphy and radiometric ages as ca. 1.7-1.9 Ma, and records infilling of a lake that was probably ice free. In contrast, an overlying interval of earliest Pleistocene age (estimated as ca. 1.5- 1.6 Ma) contains both laminated silts and clays (lake-bottom, prodelta facies) and massive silt with abundant pebble- to cobble-sized clasts. This diamictite facies is interpreted as the result of sedimentation during cooler times when profundal suspension-fallout deposits were mixed with clasts of ice-rafted origin (shore and river ice). Furthermore, three distinct sequences of seasonal-ice and ice-free conditions are recognized in this interval and could correspond with the 40 ka glacial-interglacial cycles recognized from marine records. An upper sedimentary interbed is separated from the diamictite interval by a thick succession of basalts. The Matuyama-Brunhes reversal (0.78 Ma) probably occurs near the base of this interbed, making the strata middle and late Pleistocene in age. These strata contain playa deposits, lunette dunes, fluvial and eolian sands and silts, paludal-lacustrine deposits, and weakly developed paleosols. Taken together, the change from lacustrine-dominated deposition during the late Pliocene and early Pleistocene to eolian-, playa-, and soil-dominated deposits in the late Pleistocene indicates drying, plus lowering of regional hydrologic base level. Carbon and oxygen isotope analysis of a well-developed, stage III calcareous paleosol, as well as isotope data from weakly developed paleosols and silty lake beds indicates that the dominant source of calcareous loess, ubiquitous in these deposits, was Paleozoic and Mesozoic carbonate units.