MAASTRICHTIAN 87SR86SR STRATIGRAPHY AND A WORLDLY (NON-IMPACT) EXPLANATION FOR SOME SLUMPS IN LATE CRETACEOUS SECTIONS FROM THE WESTERN NORTH ATLANTIC

Both the Chicxulub impact event at the Cretaceous/Tertiary (K/T) boundary and its causative relationship to the K/T mass extinction are widely accepted, but details of the effects of the impact remain murky. One predicted consequence of the impact is large scale seismicity. Lack of evidence for such seismicity was a problematic aspect of the K/T record for many years, but recent studies in the Gulf Coast and western North Atlantic have documented mass wasting that was apparently coincident with the K/T boundary and, thus, consistent with impact induced shaking. This interpretation could have important implications for Late Cretaceous stratigraphy in the North Atlantic. On the other hand, gravity flow deposits are common features throughout the geologic column, and it is difficult to demonstrate rigorously either their exact timing or their cause.

We have used ⁸⁷Sr/⁸⁶Sr analyses to test stratigraphic completeness of the Maastrichtian on Blake Nose (western North Atlantic) including study of the detailed record across two slumps (one each at ODP Sites 1050 and 1052). These slumps have been attributed to K/T shaking, but our data suggest they occurred during the Maastrichtian. There is no evidence for repetition of pelagic strata; pelagic chalk below the slumps is older than chalk above the slumps. The difference in ⁸⁷Sr/⁸⁶Sr ratios across the slumps indicate ~1 and 1.5 x 10⁶ years of pelagic accumulation are missing at Sites 1050 and 1052, respectively. At a third locality (Site 1049), the interval in which these slumps occur is represented by a hiatus. Slumped material is relatively coarse grained and at least at Site 1052 contains foraminifera older than those in the subjacent chalk. Also at Site 1052 the slump’s upper portion is extensively burrowed. Pelagic deposition punctuated by gravity flows provides a simple explanation for these observations whereas an impact based explanation requires a complicated depositional history. Although K/T related slumping seems well supported in some cases, our results demonstrate that not all Maastrichtian slump features can or should be attributed to the K/T boundary event.