All DSDP pelagic sediment cores penetrating the Cretaceous-Tertiary boundary 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. Furthermore, calcareous nannofossil biostratigraphy indicates that the magnitude of the Cretaceous-Tertiary unconformity is nearly identical 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 measurements based on inferred sedimentation rates suggest a hiatus of 10<sup>5</sup>—10<sup>6</sup> 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 105-10<sup>6</sup> 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 magnitude 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.