THE HYDROLOGIC AND TAPHONOMIC IMPLICATIONS OF OXIDE MINERALIZATION ALONG CALVERT CLIFFS, MARYLAND, USA

Active oxide mineralization along the Calvert Cliffs plays an important role in shaping the coastline along the western shore of the Chesapeake Bay in southern Maryland, U.S.A. The Calvert Cliffs can be broadly grouped into two chronostratigraphic units: a basal nearshore marine Miocene unit and an overlying backshore aeolian Pleistocene unit. The upper Pleistocene unit is characterized by coarse sands and gravel, with relatively high porosity and permeability. The underlying Miocene unit is characterized by alternating bioturbated clay-rich layers and shell beds. Through repetitive atmospheric precipitation and continuous groundwater flow, heavy metals, such as iron and manganese, are leached out of the overlying sands. The mineral-saturated fluids flow over the cliff surface and accumulate on terraces formed by the relatively impermeable Miocene clays. Oxidation occurs with prolonged exposure to air and, potentially, through biological mediation driven by microbial chemotrophs. The mineral-saturated fluids also penetrate deeper into the Miocene strata through fractures and burrows. The heterogenous nature of the shell beds is associated with very high porosity and permeability, which allows the mineral-saturated fluids to reach the lower clay-rich beds. Acidic water dissolves the carbonate fossils, resulting in compaction and cementation of the siliciclastic grains. This alters the fossils to a state of moldic preservation or destroys them entirely. Miocene carbonized driftwood is also re-mineralized, resulting primarily in wood impressions preserved in goethite. As the mineral-saturated fluids flow through the complex network of Miocene burrows, 3-dimensional molds and a rigid plumbing system for continued water flow are created. Many of these ironstone burrows preserve vertical and lateral flow structures that further illustrate the relationship between the hydrology of the cliffs and oxide mineralization, while also serving as geopetal structures. Oxide minerals present along the Calvert Cliffs include iron oxides (e.g., hematite, goethite, ferrihydrite) and manganese oxides (e.g., birnessite, romanechite, todorokite). Complete characterization of these minerals is still ongoing and a major question that remains is the role of microbial life in the formation of these minerals.