The Cretaceous-Tertiary Boundary Event in the Ocean

All DSDP pelagic sediment cores penetrating the Cretaceous Tertiary bOllndary exhibit a major unconformity between the Cretaceous and Tertiary The hiatuses in these cores are greater than in most marine shelf sections although all are paleontologically unconformable Further more calcareous nannofossil biostratigraphy indicates that the magnitude of the Cretaceous Tertiary uncon fOlmity is nearly idcnti al in many shelf sections The lack of a paleontologically continuous section across the boundary prohibits direct measurement of the span of this missing interval However indirect measllrements based on inferred sedimentation rates suggest a hiatus of 10 106 years for shelf sections and more for pelagic ones It appears likely that high carbonate production in the Maastrichtian coupled with near base levelling of the continents produced a nutrient and carbonate poor ocean having a shallow CCD along with an oxygen rich cloud free atmosphere characterized by sub arid conditions and strongly differentiated climatic belts The mass extinctions ending Cretaceous time would have occurred during the inferred 10 106 year interval when severe nutrient and carbonate depletion would have caused the CCD to approach the photic zone thereby leaving only a thin layer of non calareous deposits to record this spectacular event in the earth s history The observed mag nitude of the unconformity as indicated by calcareous fossils would seem to be a function of paleobathymetry with pelagic sections having been below compensation depth longer than shelf sections and consequently exhibiting greater hiatuses