RAPID SEDIMENT ACCRETION, STRATIFIED ARCHAEOLOGICAL DEPOSITS, AND FLOODPLAIN FORMATION WITHIN THE UPPER OHIO VALLEY

Geoarchaeological research in the Ohio River valley near Willow Island, West Virginia, revealed stratified archaeological deposits within a 9-10 m thick, rapidly accreted middle Holocene floodplain. Archaeology, sedimentology, and ¹⁴C age indicate that most sediment was deposited episodically during discrete flood events during two trending episodes of net sediment accumulation: 1) an early period of relatively rapid deposition between ca 4.5-3.5 kBP and 2) a subsequent period of gradual deposition between ca 3.0-1.0 kBP, with this shift in rates noted between ~3.5-4.0 mbs. Incipient terrace “surfaces” within this sequence were utilized by humans during intervals of non-deposition.

Limited pedogenesis occurred on these surfaces, reflecting brief near-surface residence-times. Ephemeral and discontinuous surface soil (ACb) horizons were generally identified by inclusive archaeological components (charred organics, artifacts and fire-cracked rock concentrations) that accumulated during episodic Late Archaic occupations. The archaeological remains, which included among the earliest pottery in North America, were often the principal markers of any depositional hiatus – the amount of such cultural material reflected both intensity and duration of occupation. Subsequent burial of these strata promoted preservation of the archaeological data within the rapidly accreted flood environment.

The rapid floodplain accretion and intensive flood sequence recorded at Willow Island may seem unique within the upper Ohio valley, but is consistent with, albeit slightly later than, middle Holocene vertical accretion sequences noted elsewhere. At Willow Island, this rapid accretion occurred within a significant side-valley channel (Cow Creek) that was deeply eroded into the Ohio River floodplain. Significant alteration of the drainage basin including the uplands drained by Cow Creek during the middle Holocene---likely related to climate change at the end of the Hypsithermal---probably resulted in greatly increased basin-wide erosion, increased sediment load and subsequent terrace-component construction near the mouth of the creek.