Paleoenvironmental changes across the Cretaceous/Tertiary boundary in the northern Clarence valley, southeastern Marlborough, New Zealand

Abstract

Strata outcropping in Mead and Branch Streams, northern Clarence valley, provide important records of pelagic‐hemipelagic sedimentation through the Cretaceous‐Paleocene transition in a southern high‐latitude, upwelling system flanking a carbonate platform. The two stream sections, <10 km apart along‐strike, comprise similar stratigraphic successions with differences mainly due to Branch Stream being situated closer to land (outer shelf to upper bathyal) than the mid‐bathyal Mead Stream section. Age control is based on foraminiferal and radiolarian biostratigraphy. A Cretaceous/Tertiary (K/T) boundary clay is not preserved in either section. The siliceous microfossil record indicates that basal Paleocene sediments at Branch Stream, although slightly enriched in Ni and Cr, were deposited after a significant relative sea‐level fall. A basal Paleocene claystone at Mead Stream is not enriched in K/T impact‐derived elements and was probably deposited after sea‐level fall. Earliest Paleocene sediment may be preserved as burrow fill in an uppermost Cretaceous bioturbated zone, which is enriched in Ni and Cr and contains a foraminiferal assemblage indicative of Paleocene Zone P0. Zone PO‐Pα foraminiferal assemblages within the basal Paleocene sediments in both sections indicate that sea level fell within 100 000 yr of the K/T boundary event. The K/T boundary at both sites coincides with an abrupt change in lithofacies from calcareous to siliceous ooze. Biosiliceous sediment dominates the sedimentary record over the first 1.5 m.y. of the Paleocene, which corresponds to 45 m of strata at Branch Stream and 20 m in the more condensed Mead Stream section. A trend from diatom‐poor to diatom‐rich and finally radiolarian‐rich microfossil assemblages over the lower 5 m of Paleocene strata at Branch Stream is consistent with progressive deepening at the landward margin of an upwelling zone. A second regressive pulse at c. 64.5 Ma, followed by prolonged transgression from 5–50 m above the K/T boundary, is inferred from an initial increase in the frequency of mudstone beds, followed by a similar trend from diatom‐poor to radiolarian‐rich microfossil assemblages. Within the upper part of this interval, an increase in carbonate marks a return to lithofacies, and probably also paleodepth, equivalent to the underlying Cretaceous. In the deeper Mead Stream section, variation in diatom and radiolarian assemblages is mainly due to variable preservation in highly recrystallised lithologies. High overall abundance and little change in paleoproductivity indicators (Ba, δ13C) indicate that high biological productivity continued across the K/T boundary and through the biosiliceous episode. Siliceous plankton thrived in the Marlborough upwelling zone during the early Paleocene. Fluctuations in abundance and lithofacies can be related to significant changes in sea level, which may be the result of local tectonic or global climate changes. The delayed recovery of calcareous plankton after mass extinction at the K/T boundary, in both outer neritic and bathyal settings, indicates a relatively cool oceanic regime for the first 1.5 m.y. of the Paleocene.