GEOCHRONOLOGY AND STRATIGRAPHY OF HOLOCENE AND RECENT SEDIMENTS OF THE WHITE CLAY CREEK VALLEY, SOUTHEASTERN PA

To better define the historical context for contemporary sediment production and storage, we document the stratigraphy and chronology of valley bottom sediments. Vertical sedimentary sequences were described from core samples and exposed river banks; ages were determined using dendrochronology, analyses of fallout radionuclides, radiocarbon dating, and interpretation of buried soil horizons. Samples were obtained from reaches of varying slope to define storage and non-storage reaches. Lithostratigraphic relationships are generally consistent with two published stratigraphic models for mid-Atlantic stream valleys (Jacobson and Coleman, 1986, hereafter JC; Walter and Merritts, 2008), but neither model fits all of our sites. Deposits can be placed into one of 3 age categories (following JC): basal Holocene (and older) deposits formed before European colonization (pre-Settlement, hereafter pS), deposits formed after European settlement but before the early 20^th Century (post-Settlement, hereafter pS), and deposits formed during the 20^th and 21th Century (Very Recent, hereafter VR). Our interpretations are supported by radiocarbon dating of basal organic-rich deposits (537 – 1699 yrs. B.P.), buried pS-age A horizons, and VR sedimentation rates determined from dendrochronology and fallout radionuclides. At two of our sites (slopes < 0.004), VR floodplain accretion rates average ~ 1 cm/yr, and the thickness of VR overbank deposits is greater than the thickness of pS “legacy” deposits. As slope increases to ~0.005, rates of vertical accretion decrease to 0.2 cm/yr. When the slope exceeds 0.006, VR floodplain accumulation rates become negligible. Our results indicate that 1) overbank deposition is an active process along many reaches of the White Clay Creek, 2) pS “legacy” sediments can be the least abundant age category at some locations, and 3) a threshold slope of 0.006 separates floodplain storage and transport reaches.