Watsonella crosbyi and Definition of the Base of Terreneuvian, Stage 2 (Lower Cambrian) in South Australia

Abstract

Comprehensive regional (basinal) biostratigraphy which help construct global schemes are dependent upon the reliability, applicability and repeatability of the stratigraphic and geographic ranges of key taxa which compose the scheme. Inconsistencies resulting from taphonomic or taxonomic issues or inaccuracies related to the diachronism of First Appearance Data, undermine regional and global correlation, ultimately hampering establishment of concise temporal subdivisions. The Cambrian Period remains one portion of the International Geological Timescale that has only been partially formally subdivided and defined. The first appearance of the stem micromollusc, Watsonella crosbyi has been suggested as a potential means to define the base of pre-trilobitic Terreneuvian Series, Stage 2, though this has caused considerable controversy. This taxon has a widespread distribution, and can be globally correlated across lower Cambrian successions of both carbonates and siliciclastics in South China, Siberia, Mongolia, France and Newfoundland. The taxon often co-occurs with the dextrally coiled shells of the stem group mollusc, Aldanella attelborensis, which also shares a similar geographical distribution and has also received attention as a potential candidate to define the base of Stage 2. The occurrence and biostratigraphic range of Watsonella crosbyi in Australia has previously been very poorly documented; some recent papers have (erroneously) suggested the stratigraphic range of W. crosbyi extends into the Parabadiella huoi trilobite Zone. This would confound global correlation of the lower Cambrian and ultimately question the viability of this taxon as an index fossil. However, previous work on this (and associated) taxa have been hindered by a paucity of material, lack of detailed systematic description and unresolved stratigraphic context in South Australian successions. To resolve this, new stratigraphic sections were measured through the lower part of the Normanville Group on Fleurieu Peninsula, eastern Stansbury Basin, South Australia, revealing an assemblage of micromolluscs including a sinistral form of Aldanella (A.cf. golubevi), Bemella sp., Paraisanella sp., and (though rare) Watsonella crosbyi. The Wastonella assemblage of South Australian correlates closely with the temporal ranges of occurrences in China, France and Siberia (though not Newfoundland), supporting Watsonella crosbyi as a good candidate for defining the base of the Cambrian Stage 2.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015