EVOLUTION AND TAXONOMIC STUDY OF THE CRETACEOUS PLANKTIC FORAMINIFERAL GENUS HELVETOGLOBOTRUNCANA REISS, 1957

摘要

The evolution and taxonomy of the planktic foraminifer genus Helvetoglobotruncana Reiss, 1957, are investigated to improve stability in the concepts of the biomarker species Hv. praehelvetica and Hv. helvetica, which have been used to correlate uppermost Cenomanian-middle Turonian (Upper Cretaceous) marine sediments worldwide. The study focuses on exquisitely preserved specimens obtained from Turonian marine claystones drilled on the coastal margin of Tanzania, and these are compared with specimens reported from a number of Cenomanian/Turonian boundary sequences worldwide. Absence of discernable shell recrystallization in the Tanzanian specimens provides an unprecedented opportunity for detailed observations of the external wall texture and wall microstructures, allowing for a more concise characterization of similarities and differences between the two species. Abundance counts of Turonian planktic foraminifer assemblages from one Tanzania borehole reveals that Hv. helvetica occurs in trace abundance early in its range and reaches maximum abundance of 35-45% of the total assemblage shortly before the simultaneous and abrupt extinction of both helvetoglobotruncanid species. The ratio between Hv. helvetica and Hv. praehelvetica increases gradually up-section, with a 1:1 ratio reached nearly halfway above the lowest occurrence level of Hv. helvetica and 9:1 dominance by Hv. helvetica near the top of its range. The helvetoglobotruncanid and mixed whiteinellid species yield δ~(13)C and δ~(18)O values that are nearly identical and slightly more enriched in δ~(18)O than co-occurring Globoheterohelix paraglobulosa, and they are slightly more enriched in δ~(13)C and more depleted in δ~(18)O than co-occurring species of Dicarinella and Praeglobotruncana. These isotopic results indicate that helvetoglobotruncanids lived at relatively shallow depths in the mixed layer. Using an age model developed for the Rock Canyon Anticline Cenomanian/Turonian Global Stratotype Section and Point (GSSP) sequence near Pueblo, Colorado, the age of the first occurrence of Hv. helvetica is calculated as 93.52 Ma. The extinction of Hv. helvetica has not been properly calibrated, but it must be younger than 92.77 Ma, which is the age of its occurrence at the top of the Rock Canyon Anticline GSSP section.