Examination of Latest-Pleistocene to Holocene Stream Terrace Deposits Using Ground-Penetrating Radar Stratigraphy and Sedimentology
The upper terrace contains eolian sands, pebble-gravel bedload deposits, and sandy overbank deposits. Most of this material was likely deposited around the Latest Pleistocene transition from glacial to meteoric water sources. However, archaeological remains and a radiocarbon date of 870 years BP from the uppermost parts of this terrace show late-Holocene deposition 50 ft above the modern river, suggesting extreme-magnitude floods.
GPR profiles and cores from the middle terrace show three units bounded by
laterally-extensive unconformities. The upper two units are interpreted as overbank deposits due to their fine-grained, laterally-continuous sediments infilling topographic lows. GPR images of the basal unit show point-bar architecture; cores reveal poorly-sorted, angular pebble-gravels.
Radiocarbon dates indicate the middle terrace was deposited starting around the beginning of the Holocene. If the coarse-grained basal sediments are glacially-derived, they were in the valley for as much as 8000 years before the Holocene, without being sorted and rounded by fluvial re-working. This suggests the early terrace was stable for much of the latest Pleistocene, and once it began eroding, its material was re-deposited rapidly.
Alternatively, these sediments may suggest a postglacial influx of immature sediment was delivered to the stream locally, then rapidly deposited and sheltered from further reworking.
Sediments of the lowest terrace are similar to those of the middle terrace. However, samples from 10 ft. below the surface gave radiocarbon dates of about 210 and 270 years BP. This relatively young age is consistent with the scarcity of Native American remains in this terrace in the upper part of the valley. Thus, a large volume of sediment was likely deposited since European colonization, suggesting extremely high sedimentation rates.