Quantitative biostratigraphy of Middle Jurassic-Early Cretaceous foraminiferal events from the Arctic

摘要

Arctic foraminiferal events are of great interest for biostratigraphical understanding of the Boreal Realm,but Greenland, Svalbard, Barents Sea and Siberia have limited published studies in the Middle Juras-sic-Early Cretaceous. Selected foraminiferal events data have been compiled and entered into a database.This database has been subjected to quantitative analysis with the help of the RASC program (RAnkingand SCaling). In total, 740 foraminiferal events from 13 outcrops and wells were used to build a large-scale zonation for the Arctic with seven tentative zones, using the probabilistic zonation feature of RASC.Published foraminiferal zones, lithostratigraphic units, and palaeoenvironmental interpretations for eacharea are compared and discussed, and their correlations to the calculated ranked and scaled optimumsequences of events contrasted. The chosen marker species in the literature and their correlative relia-bility over the Boreal Realm is investigated, and foraminiferal event arrangement in the ranked optimumsequence is compared to the published age ranges for the taxa. These investigations have highlightedforaminiferal events, which are reliable and traceable through more than one basin, such as the first andlast occurrences of Recurvoides obskiensis and Schleiferella emeljanzevi, the first occurrence of Recurvoidesscherkalyensis, and the last occurrence of Trochammina septentrionalis. Statistical tests performed by theRASC program have revealed other events, which could be unreliable, for example the first occurrencesof Cribrostomoides subretusus and Ammoglobigerina canningensis, and the last occurrence of Eomarssonellaparaconica. The reliability of published zones on a local scale depends on various factors, and they canprove to be too detailed if the goal is to correlate foraminiferal events to other areas. The ranking andscaling of the RASC program is an extremely valuable tool in quantitative stratigraphy, and with its helpthe first step has been taken