A COMPARATIVE STUDY OF LEGACY SEDIMENTS IN THE FERGUSON CREEK WATERSHED: 1937 TO 2016
This study reinvestigates a transect (Happ 1945) across the floodplain of Ferguson Creek near its confluence with the South Tyger River. The original study focused on topographic profiles, eight boreholes and cut bank observations. This study relocated transect datums and the 1930s boreholes. In addition to the analytical techniques used in the 1930s, such as hand auguring, probing, and soil analysis, this study used techniques not available then, such as vibracoring, magnetic susceptibility analysis, Ground Penetrating Radar (GPR), and radiocarbon dating (RC).
Comparisons of 1937 and 1940 topographic profiles with one from 2016 indicate minimal deposition and erosion has occurred with exceptions near levees and toe slopes respectively since 1940. The most marked changes are widening of the stream 2.5 m to the east and deepening of channel by 0.6 m. Stability is probably due in large part to drastic reduction in agricultural activity in watershed since the 1940s.
General stratigraphic interpretations of 1930s were confirmed. Considerable deposition took place prior to the 1930’s, with at least 2.5 m of LS laid down between 1760s and mid-1930s. Radiocarbon ages from detrital carbon 1.7 and 2.4 m below surface have a >68% probability between 1806 and 1919 AD. Much of LS exhibits an upwardly fining sequence, consistent with gradual aggradation. Beneath LS is 1.5 m of older sediment. Two buried organic-rich layers (A horizon soils?) were reported in 1930s, one at the contact of legacy and older sediments (not confirmed), the other in the older deeper sediments (confirmed). This lower organic-rich stratum, 3.5 m below surface with a >68% RC probability of occurring between 692 and 543 AD, suggests slack water deposition. GPR data indicate abandoned channels. Detrital Carbon from sandy deposits 3.7 m below surface has a >95% probability of occurring between 5788 and 5527 BC suggesting the presence of at least one unconformity in the stratigraphic sequence.