The Cretaceous/Tertiary boundary event in New Zealand: Profiling mass extinction

Abstract

Of over 20 known Cretaceous/Tertiary (K/T) boundary sections in the New Zealand region, 6 in the northern South Island were selected for detailed biostratigraphic and paleoenvironmental study because of their apparent stratigraphic completeness and the range of depositional environments represented. These sections represent the only known southern high‐latitude (55–60°S) transect of the K/T boundary transition from continental slope to terrestrial mire. The K/T boundary is marked by deposition of an impact fallout layer in four of these sections, and a disconformity representing a hiatus of < 100 000 yr in the remaining two sections (Mead and Branch Streams). In the terrestrial Moody Creek Mine and shallow marine mid‐Waipara sections, rapid and widespread destruction of forests is shown by abrupt replacement of mixed forest pollen and spore assemblages by assemblages dominated by few species of ground ferns. Foraminiferal and radiolarian biostratigraphy indicates that within 100 000 yr these ferns were replaced by a cool‐temperate conifer‐dominated assemblage that persisted until c. 63.5 Ma. In the marine sections, an abrupt decrease in carbonate content reflects a major decline in carbonate production associated with mass extinction of calcareous plankton, although several smaller species of planktic foraminifera persist for c. 100 000 yr into the Paleocene. In four sections in Marlborough (Flaxbourne, Woodside, Mead, and Branch), which represent a transect across a coastal upwelling zone, paleoproductivity proxies (excess SiO2, excess Ba, Ba/Al, δ13 C) indicate that the decline in calcareous plankton was partly offset by sustained siliceous plankton productivity. Extinctions of thermophilic foraminifera, survival of cosmopolitan foraminifera, and expansion of diatoms and spumellarian radiolarians indicate that pronounced cooling occurred across the K/T boundary. A subsequent decline in planktic foraminifera and in carbonate content, associated with a progressive increase in siliceous plankton productivity, indicates that further cooling resulted in dominantly biosiliceous sediment being deposited in the core of the Marlborough upwelling zone from c. 64.9 to 63.5 Ma. The onset of this biosiliceous event at c. 64.9 Ma and a later major increase in biogenic silica in the deep‐water sections (Flaxbourne, Woodside) at c. 64.6 Ma are correlated with sea‐level falls in the shelf section (Branch) and are also linked to eustatic events recorded in Europe and North and Central America. These early Paleocene regressive events within a cool climatic regime are the probable cause of incomplete K/T boundary records in most of New Zealand. It remains uncertain if they are directly related to the K/T bolide impact, the short‐term effects of which may have disrupted the global climate system for millions of years, or are related to other factors such as tectonism or fluctuating CO2 emissions from Deccan volcanism.