Marine Palynology Across the Cretaceous-Tertiary Boundary in New Zealand

Abstract

A high-resolution palynological study of several eastern New Zealand Cretaceous-Tertiary boundary marine sections has resulted in a refined dinoflagellate cyst biozonation and the establishment of a paleoenvironmental model for this interval. The following six sections were examined in detail: Mead Stream, Branch Stream, mid-Waipara River, Fairfield Quarry, Wharanui Point and Grey River. The dinoflagellate cyst assemblages from Branch Stream, Wharanui Point and Grey River had not previously been examined. The dinoflagellate cyst assemblages allow the establishment of a new dinoflagellate cyst biozonation consisting of seven interval subzones: one in the latest Maastrichtian and six in the early Paleocene. The interval subzones span a ca. 4 Ma period and each has a duration of 0.2 Ma to 0.8 Ma. Mead Stream is selected as the stratotype section for the new biozonation. Seventeen of the 135 dinoflagellate cyst species observed are considered to be previously undescribed species. Several of the new species appear to be stratigraphically important and one of these, Carpatella sp. 1, is the primary index for the new latest Maastrichtian Carpatella sp. 1 Subzone. Furthermore, comparison of the New Zealand and Northern Hemisphere records shows significant similarities and differences in the dinoflagellate cyst ranges. Cerodinium striatum and A. margarita occur earlier in the New Zealand region than in the Northern Hemisphere. The last occurrence of P. pyrophorum is ca. 4 to 5 Ma earlier than in Northwest Europe whereas the time-ranges of the early Paleocene marker species S. inornata and C. cornuta are comparable to their northern Hemisphere ranges. No evidence is found for mass extinction of the cyst-forming dinoflagellates across the Cretaceous-Tertiary boundary. Instead, the boundary is marked by the first occurrence of T. evittii and S. inornata which are rapidly followed by the first occurrence of C. cornuta. Several characteristic latest Maastrichtian to earliest Paleocene species disappear ca. 0.5 Ma after the boundary event. Within the early Paleocene, from ca. 63.5 Ma to ca.62.2 Ma, ca. 26 species have their first occurrence. The palynofacies and dinoflagellate cyst assemblages were quantified in the Mead Stream, Branch Stream and mid-Waipara River sections, leading to the establishment of five palynodebris intervals: Palynodebris Intervals I and II in the latest Maastrichtian and Palynodebris Intervals III, IV and V in the early Paleocene. The onset of a regression is recorded in the latest Maastrichtian at the base of Palynofacies Interval II. A distinct change directly below the Cretaceous-Tertiary boundary, within Palynofacies Interval III, could reflect a period of global warming preceding the boundary event by ca. 0.3 to 0.1 Ma. The latest Maastrichtian regression is followed by a transgression in the earliest Paleocene, and the depositional environment became progressively deeper during the early Paleocene Palynofacies Intervals III to V. The dinoflagellate cyst composition indicates that a prolonged period of coastal upwelling occurred along the eastern margin of New Zealand from the latest Maastrichtian to early Paleocene. Six abrupt shifts in dinoflagellate cyst assemblages are observed in the early Paleocene. These shifts are interpreted to reflect a period after the Cretaceous-Tertiary boundary event, during which the sea surface water temperatures alternated between relatively warmer and colder conditions. In the New Zealand region this recovery period had a duration of ca. 1.8 Ma and it was probably a result of climatic instability resulting in long term eustatic sea level fluctuations. The recovery period was succeeded by well-oxygenated oligotrophic conditions by ca. 63.2Ma.