UNSTEADY LATE CENOZOIC EPEIROGENY OF THE CENTRAL APPALACHIANS ENCODED IN THE DELAWARE RIVER – ATLANTIC COASTAL PLAIN SOURCE TO SINK SYSTEM

We combine a model of base level change in the Delaware River and Brandywine Creek catchments, a new paleomagnetic and cyclostratigraphic-based age model for the mid-Atlantic inner Coastal Plain, and eustasy to reconstruct a 25-million year record of epeirogenic uplift and subsidence for the mid-Atlantic region.

Catchment base level change is predicted by a stream-power based linear inversion of fluvial topography that assumes uniform block uplift upstream of the Fall Zone and uniform, steady erosion of 30 m/Myr, but accounts for non-uniform rock erodibility. Several iso-elevation knickpoints in the Delaware and Brandywine catchments are consistent with upstream-propagating transients generated by unsteady base level fall at the Fall Zone. Model results show slow rates of Early Miocene basin uplift of ~10 m/Myr increasing to ~25 m/Myr in the Middle Miocene. An impulsive uplift to ~55 m/Myr at ~8 Ma decreases to ~30 m/Myr in the Early Pliocene, followed by higher frequency unsteadiness in base level change during the Pleistocene presumably linked to glacio-eustasy.

The corresponding subsidence and depositional record in the sink is recorded by the stratigraphy in the Bethany Beach core (Qj32-27 from ODP Leg 174AX), here re-evaluated in terms of a paleomagnetically-constrained cyclostratigraphic age model. Optimal sedimentation rates between published unconformities in the core are based on evolution of the correlation coefficient relating power spectra of a continuous gamma ray data log and expected astronomical forcings. Inferred polarity intervals based on characteristic remanent inclinations from demagnetization of core samples tie in with the geomagnetic polarity time scale from chron C5A (Late Miocene) through C1 (Middle Pleistocene) to delineate durations of extended hiatuses ranging from ~0.8 to ~1.9 Myr.

Excluding hiatuses, variable sedimentation rates fluctuate near the catchments’ median uplift rates since ~15 Ma. Increased core sedimentation rates are subsequent to initiation of basin uplift pulses beginning in the Late Miocene. These results are a rare example of linked uplift, erosion, sedimentation, and subsidence records over geologic time scales. Moreover, they are consistent with, and further refine dynamic mantle processes and rates proposed for the post-orogenic Appalachians.