ROCK MAGNETIC CYCLOSTRATIGRAPHY AND PALEOMAGNETIC STUDY OF THE PLUM POINT MEMBER OF THE CALVERT FORMATION, CALVERT CLIFFS, MD

Unconsolidated, marine, Miocene siliciclastics exposed at the Calvert Cliffs, western shore of Chesapeake Bay, MD preserve an excellent record of U.S Atlantic margin subsidence, eustasy, sediment supply, and paleoenvironments. The accepted lithostratigraphic (Shattuck, 1902, 1904), biostratigraphic (DeVerteuil and Norris (1996)) and sequence stratigraphic (Kidwell, 1997) models are used as beginning assumptions for rock magnetic cyclostratigraphy and paleomagnetic investigations. Our goal in this exploratory study is to identify astronomically-forced climate cycles and build a high resolution chronostratigraphic model. Based on the previous studies, an average sediment accumulation rate of 0.4 cm/kyr was assumed, fixing the sampling interval of unoriented rock magnetic samples at 4 cm (~10 kyr) and oriented paleomagnetic samples at 40 cm (~100 kyr). Two exposures between Parker’s Creek and Western Shores were sampled, discontinuously, through Shattuck Beds 13-15. Time series analysis (MTM) of magnetic mineral concentrations measured by susceptibility and anhysteretic remanent magnetization (ARM) reveal cycles with stratigraphic thicknesses of 100 cm, 31 cm, 14 cm, and 11 cm for Bed 13, and cycles of 100 cm and 14 for Beds 14 and 15. These thicknesses correspond to long eccentricity (100 cm), short eccentricity (31 cm) and obliquity (11-14 cm), assuming normal nested bundling of Milankovitch periodicities and sediment accumulation rates of ~ 0.25 cm/kyr rather than the original rate of 0.4 cm/kyr. Preliminary paleomagnetic results indicate that all of the samples are reversed polarity except the topmost part of Bed 15. Additional alternating field demagnetization work will confirm this observation. Isothermal remanent magnetization (IRM) acquisition experiments suggest that the magnetic mineral carrying the ARM and susceptibility signal has a mean coercivity of 70-100 mT, indicating that it is likely a secondary iron sulfide acquired soon after deposition. Our work shows that astronomically-forced cycles may be preserved in this slow, inner Coastal Plain setting and can be used to refine existing stratigraphic models, with broader implications for the steadiness of uplift and subsidence of this well-studied passive margin.