THE TRANSITION FROM RESURGE TO SECULAR SEDIMENTATION AT THE MARINE LOCKNE AND CHESAPEAKE BAY IMPACT STRUCTURES EXPRESSED IN CARBON AND OXYGEN ISOTOPE VALUES

When the depth of the water masses surging back after a marine-target impact exceeds the rim height of the seafloor crater the water can enter and eventually fill the crater. Depending on the relation between the water depth and the crater diameter this process can be more or less forceful. The resurging water brings back ejecta and rip-up material representing much of the affected parts of the target succession. With decreasing transport energy of the waning resurge, secular sedimentation will again dominate the deposition in the area. The transition may appear abrupt (i.e., Chesapeake Bay impact structure (CBIS)) or gradual (i.e., Lockne crater) in visual inspection. However, a distinct grain-size boundary does not necessarily represent the compositional boundary between the resurge and secular sediments. It merely reflects the shift in transport energy. In the case of a gradual transition the boundary can be difficult to distinguish visually.

Here we have used stable isotopes of oxygen (d18O) and carbon (d13C) in carbonate to trace the resurge/secular sedimentation boundary at the CBIS and the Lockne crater with the priori assumption that the bulk composition of the mixed resurge deposits will have different values than the sediments deposited out of the water column after the onset of secular sedimentation. Bulk samples were taken across the previously estimated boundaries between resurge deposits and secular sediments. The d18O and d13C values are within the range of typical seawater at the time of the impacts. At both craters there is a sharp difference in d18O and d13C near the resurge/secular boundary. This shift occurs at a stratigraphic level slightly above the visual grain size shift and coincides with a decrease in reworked calcareous nannofossil specimens. Probably, there was a period of time before all of the finest resurge material had settled out of suspension and secular sedimentation began to dominate.