Magnetostratigraphy and rock magnetic cyclostratigraphy for a part of the Miocene passive margin deposits at Calvert Cliffs, Maryland, USA

A high-resolution cyclostratigraphic age model was constructed from paleomagnetic and rock magnetic (anhysteretic remanent magnetization, ARM, and magnetic susceptibility, χ) data for 926 cm of section from the Miocene Calvert and Choptank formations exposed at the Calvert Cliffs, Maryland, USA.  The paleomagnetic data are noisy due to a weak depositional remanence plagued by viscous overprinting.  Thermal demagnetization shows that most of the section is reversed polarity, with three normal polarity intervals, one near the base of Shattuck bio-lithozone 11 and two at the base of zone 13.  These data, combined with biostratigraphic constraints, indicate that the normal sections fall in sub-chrons C5C, C5Bn.1n and C5AD.  Time missing in an unconformity at the base of zone 12 spans ~200 kyr, cutting out sub-chron C5Bn.2n and an unconformity below the base of zone 14 spans ~400 kyr based on the missing normal polarity sub-chron C5AC.  Overall, the 11.5 m of section studied spans ~2.5 Myr, with ~0.6 Myr missing in the unconformities, resulting in a mean sediment accumulation rate (SAR) of ~0.6 cm/kyr.  Time series analysis of the ARM and χ time series identifies periodicities that are likely driven by orbitally-forced global climate change. Statistically significant spectral peaks with stratigraphic wavelengths of 230-270 cm, 22-14 cm and 8-10 cm are identified as long eccentricity (405 kyr), obliquity (~40 kyr) and precession (~20 kyr), respectively. Using Acycle software, eCOCO analysis of the χ time series suggests unsteady SARs of ~0.4-1.0 cm/kyr below Shattuck zone 12, an increase to ~0.6 cm/kyr for zone 12 and the base of zone 13, and a decrease to ~0.3-0.4 cm/kyr for the upper part of zone 13, and zones 14 and 15.  These results show that the continental margin subsided slowly, punctuated by long-eccentricity driven eustasy that shifted the locus of silici-clastic deposition. The coarsening- and shallowing-up of the Calvert Formation, particularly above the younger unconformity, indicates that either regional subsidence or terrestrial sediment flux, or possibly both, were unsteady, indicating complex interactions of recent dynamic uplift and climate change across the U.S. Atlantic margin.