TY - JOUR
T1 - Significance of Upper Triassic nannofossils from the Southern Hemisphere (ODP Leg 122, Wombat Plateau, N.W. Australia)
AU - Bralower, Timothy J.
AU - Bown, Paul R.
AU - Siesser, William G.
N1 - Funding Information:
The authors acknowledge helpful discussions with W. Balch, W. Brenner, J. Compton, B. Haq, H. Jenkyns, P. Swart and U. von Rad. We are grateful to J. Bergen, K. Perch-Nielsen and H. Thierstein for reviews of an earlier manuscript. We thank J. Compton (Univ. South Florida) and P. Swart (Univ. Miami) for providing XRD and stable isotopic data respectively. We are grateful to B. Haq, U. von Rad, S. O'Connell and R. Merrill for permission to publish outside the Initial Reports. We thank N. Romer, P. Blackwelder, N. Chasens and J. Leal for assistance with SEM analysis and J. Arfstrom, C. Botelho and J. Davy for laboratory and darkroom support. Research funded by USSAC (PO # 20278 ) and NERC.
PY - 1991/3
Y1 - 1991/3
N2 - A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofossils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagenetic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated byPrinsiosphaera triassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, withCrucirhabdus primulus as the ancestor. This species gave rise toC. minutus andArchaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences ofC. primulus andEoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphometric changes in this time-interval which possess biostratigraphic utility: (1)P. triassica increases in diameter from an average of 6 μm to over 9 μm; (2)E. zlambachensis evolves from a stubby to an elongated shape; and (3)C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).
AB - A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofossils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagenetic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated byPrinsiosphaera triassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, withCrucirhabdus primulus as the ancestor. This species gave rise toC. minutus andArchaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences ofC. primulus andEoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphometric changes in this time-interval which possess biostratigraphic utility: (1)P. triassica increases in diameter from an average of 6 μm to over 9 μm; (2)E. zlambachensis evolves from a stubby to an elongated shape; and (3)C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).
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U2 - 10.1016/0377-8398(91)90025-2
DO - 10.1016/0377-8398(91)90025-2
M3 - Article
AN - SCOPUS:0025958539
SN - 0377-8398
VL - 17
SP - 119
EP - 154
JO - Marine Micropaleontology
JF - Marine Micropaleontology
IS - 1-2
ER -