RECONSTRUCTING LANDSCAPES ACROSS THE EARLY TO LATE CRETACEOUS TRANSITION USING PALEOSOL AND SPHAEROSIDERITE PROXIES

Potomac Formation sediments provide a mid-Cretaceous record of the coastal plain along the eastern margin of North America. We use 3 coreholes from New Jersey and Delaware to evaluate landscape evolution from the Early Cretaceous into the Late Cretaceous. The formation boundary between the Potomac and overlying units are placed using lithology and geophysical well log data (gamma and resistivity); the Potomac Formation is subdivided into three distinct lithologic units (I-III) recognized as candidate sequences with a succession of lower medium to fine sands overlain by finer grained clays and silts. Correlation of the units among sites is aided by pollen biostratigraphy, which provides units with a coarse age resolution. We described 103 paleosol profiles from the Potomac units I-III and grouped into five pedotypes ranging in pedogenic maturity: 1) weakly developed, poorly drained, immature soils; 2) moderately developed soils forming under wet/dry conditions; and 3) well-developed, well-drained mature soils. A morphology index and two geochemical proxies (Nb and Ba/Sr) provide further information on paleoprecipitation, lessivage, and drainage conditions. We develop a conceptual model linking the Nb paleoprecipitation proxy and Ba/Sr drainage proxy to determine landscape changes as a result of paleoclimate versus base level. These paleosol and proxy data are complimented by stable oxygen isotope data from sphaerosiderites to aid in the understanding hydrologic changes within Units I-III, and as a check of the conceptual model. Paleosol, proxy and stable isotope data show the change from climate to base level as the major influence on landscape conditions from Unit I (?Berrisian-Aptian), to Unit II (middle to late Albian), to Unit III (early Cenomanian). Given the location of these sites on a low gradient passive margin this influence and change is therefore governed largely by sea level. Paleosol morphology was used in conjunction with assigned fluvial aggradation cycles (FACs) to place candidate sequence boundaries within all three units. The use of FACs to place a sequence stratigraphic framework on these sites offers an additional correlatable surface between corehole sites within the units.