Environmental stress and diagenetic modifications in inoceramids and belemnites from the Upper Cretaceous James Ross Basin, Antarctica.

ELORZA, J.; OLIVERO, E. B.; J. GOMEZ ALDAY

FACIES

SPRINGER

Lugar: Berlin; Año: 2001 vol. 44 p. 227 - 242

0172-9179

New petrographic and isotopic data from inoccramidbivalve shells and belemnite rostra from the lower Campanianand belemnite rostra from the mid-upper Maastrichtianof the Marambio Group, James Ross Basin, Antarcticaare presented. Most of the inoccramid data were processedfrom shell fragments of the largc li~rm Antarcticeramusrabotensis (Crame and Luther) at the stratigraphic levelmarking the extinction of the inoccramids in the JamesRoss Basin (uppermost early Campanian-basal late Campanian).Standard transmitted light microscopy andcathodoluminescence (CL) studies in thin sections of A.rabotensis show clear evidence of environmental stress,which is reflected as marked growth breaks in the shellbanding of this large inoceramid bivalve. At RedondaPoint, CL and the mean oxygen isotopic v~duc (81So=-3.11%,(PDB); n=ll; t~176 indicate a varied degree ofdiagenetic modification, but without any evidence ofneomorphism along the prismatic microstructures. EarlyCampanian belemnite rostra are much less diagcncticallymodified (at the Brandy Bay section; and the Santa Martasection; 8180=-0.50%o(PDB); n=5; t~176 ancl8180=-0.94%o(PDB); n=21 ; t ~ 15.80(, ") and are non luminescentexcept for localized, organic-rich bands. Themean oxygen isotopic value h~r mid-late lVlaastrichtianbelemnitc rostra (at the Seymour Island scction; blsO=-0.11%,(PDB); n=5; t~176 indicates a substantial drop in thesea-water paleotemperature, suggesting a causal relationshipbetween the early extinction of the inoceramid bivalvesin high latitudes of the Southern Hcmisphcrc andthe falling sea-water tcmperaturc.