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Understanding local and regional reactions to the global Eocene-Oligocene climate transition is a continuing challenge. The White River Group in the North American midcontinent preserves dynamic fluvial, volcaniclastic and lacustrine facies that yield to aeolianites. To test whether this shift in sedimentation style was driven by climate change, 20 paleosols from 8 profiles were analyzed from the fluvial-aeolian Orella Member through the aeolian-dominated Whitney Member of the earliest Oligocene Brule Formation at Toadstool Geologic Park, NE. Paleosol morphology and geochemistry were used to assess the balance of aeolian vs. alluvial sedimentation at key stratigraphic intervals and lithologic transitions. Significant loess deposition began at least as early as the lower Orella Member but is masked in most settings by concomitant fluvial deposition. As fluvial influence on landscapes waned across the Orella-Whitney Member boundary, loess deposits predominated and became more recognizable. Paleosols follow different pedogenic pathways in direct response to depositional setting. Whereas all paleosols formed through top-down pedogenesis in alluvial settings, paleosols in aeolian-dominated settings formed though pedogenic upbuilding during aggradational phases and through top-down pedogenesis during depositional hiatuses. The disparity between each style of pedogenic development creates fundamentally different pedogenic associations that must first be understood before climatic interpretations can be drawn from macroscopic paleosol morphology alone. Microscopic analysis of loessic and alluvial paleosols indicates that pedogenic features do not greatly change across the Orella-Whitney Member boundary. Furthermore, results of climofunction calculations from five paleosol Bw and Btk horizons show mean annual temperature (ca. 9.0-10.5 °C) and precipitation (ca. 650-800 mm/y) do not significantly vary across the Orella-Whitney Member transition. Clay mineralogy and the presence of pedogenic carbonate and translocated clay corroborate paleoclimate estimates. However, geochemical paleosol profiles are uniform and do not reflect observed vertical associations of pedogenic features. Constant additions of aeolian sediment, which replenishes base losses through leaching, explain this phenomenon. Interpretations of paleovegetation from root trace morphology and paleosol taxonomy indicate that predominantly open canopy to savanna habitats were in place in the lower Orella Member and continued into the Whitney Member. Evidence for riparian partitioning exists in the lower Orella Member but disappears as fluvial deposits wane in the Whitney Member. Lacking evidence of climate change from paleosol analysis, changes in sedimentation style and vegetative biomes are most likely a reaction to increased aeolian deposition. / Geology

Detection of large-magnitude coastal events, especially in wave-dominated retrograding barrier settings, has traditionally relied upon lithological evidence, such as distinct overwash sand layers within the muddy backbarrier sequences. In tide-dominated environments, unconformities in marsh stratigraphy have been interpreted as rapid drowning or erosion events caused by large storms. In transitional mixed-energy backbarrier environments, however, clearly identifiable event horizons are rare, due to unfavorable conditions for peat formation or to a lack of overwash. To address these challenges, the present study utilized X-ray fluorescence (XRF), magnetic susceptibility (MS), and X-ray diffraction (XRD) techniques to identify anomalies within 4-to-7-m-thick sequences recovered from Sewell Point, Cape May, New Jersey. In these cores, at least five peaks were identified that exhibit up to three-to-four-fold increase in Fe (up to 6.2%) and Ti (up to 0.5%) concentrations and a substantial increase in MS values (> 200 μSI). The sand fraction at these intervals exceeds 40% and is represented by moderately well-sorted, negatively-skewed, fine-grained sand (2.7 φ). Fe, Ti, and MS are positively cross correlated due to the relatively high content of sand-sized heavy minerals such as magnetite, rutile, ilmenite and biotite, as well as phlogopite and muscovite mica. These minerals were also identified using a Rigaku Dmax/B X-ray diffraction device. Seven radiocarbon-dated in situ samples of intertidal gastropods and the few available rhizomes indicate that the Sewell Point sequence accumulated over the past 900-1,000 years, at an average sedimentation rate greater than 4.5 mm/yr, which is consistent with its proximity to a historically active tidal inlet. Lithological anomalies at Sewell Point are interpreted as the signatures of episodic large-magnitude sediment fluxes into the paleo-lagoon. The calibrated ages of organic remains (mollusks and rhizomes) constrain chronology and allow interpretation of these stratigraphic layers as event horizons with historical storms of 1594, 1743 and 1821, along with pre-historic storms in the 11th-13th centuries. This research indicates that geochemically diagnostic intervals offer an effective tool for detection of event horizons and their regional correlation in mixed-sediment backbarrier settings. / Geology

Assateague Island is a classic example of a retrograding barrier island, with its recent geological history punctuated by episodes of overwash and breaching. However, in addition to a number of historical inlets, parts of the island owe their origin to relict (pre-historic) channels. The present study was conducted north of the Virginia-Maryland border, focusing on a narrow segment of the island fronting the Green Run Bay. The site lies north of the historical Green Run Inlet that was active until 1880; however, there is no geological evidence of its migration along the island. More than 4 km of high-resolution (250 MHz) ground-penetrating radar (GPR) images, complemented with sediment cores and multi-dating techniques, were used to reconstruct the geological legacy of the Green Run Bay segment of the barrier and to test whether it was the site of an older channel. My findings indicate that a backbarrier paleo-channel still visible within the Green Run Bay corresponds to a large (>380 m wide, 3.0-3.5 m thick) channel cut-and-fill structure revealed in GPR images. The channel fill consists of tangential- to sigmoidal-oblique, southward-dipping reflections downlapping onto channel lag facies, which overlie subhorizontal bay-fill strata. Hummocky reflections in a shore-normal channel transect suggest partial preservation of inlet-related bedforms, believed to be associated with the channel closure. Mollusk shells from the bay fill yield radiocarbon ages of 4630-2400 cal BP (calibrated years before 1950). The paleo-channel facies overlying the bay deposits exhibit a fining-upward sequence, with a mean grain size range of 0.44-2.43 phi. The first set of optical dates indicates that the inlet fill is 660 +/- 70 cal BP (AD 1220-1360). The paleo-channel fill does not extend to the south and therefore is a separate relict feature that predates the historical Green Run Inlet. Based on geophysical and core data, the paleo-tidal prism of the relict channel is 17x10 6 m3. Following the closure of the inlet, a series of beach ridges have developed across the Green Run Bay segment and exhibit signatures of storm erosion in shore-normal GPR profiles. This punctuated barrier progradation took place during the historical period (post-1600), with optical dates of beach ridge and dune generations ranging from AD 1680 to 1920. In addition to geological evidence, dendrochronological records were examined for occurrences of abrupt thinning of tree rings as a proxy for intense environmental stress. Tree samples that exceed 50 years in age (n=7) display an abrupt ring thinning in 1962 that coincides with the Ash Wednesday extra-tropical storm of record. This study demonstrates that the historical stability of the Green Run Bay segment of Assateague Island is likely due to the influx and preservation of substantial sand volume related to a relict tidal inlet. / Geology