Permian non-marine bivalves of the Falkland Islands and their palaeoenvironmental significance

Abstract

Simões, M.G., Quaglio, F., Warren, L., Anelli, L.E., Stone, P., Riccomini, C., Grohmann, C.H. & Chamani, M.A.C. December 2012. Permian non-marine bivalves of the Falkland Islands and their palaeoenvironmental significance. Alcheringa 36, 543–554. ISSN 0311-5518.

We describe the occurrence of non-marine bivalves in exposures of the Middle Permian (Capitanian) Brenton Loch Formation on the southern shore of Choiseul Sound, East Falklands. The bivalves are associated with ichnofossils and were collected from a bed in the upper part of the formation, within a 25 cm thick interval of dark siltstones and mudstones with planar lamination, overlain by massive sandstones. The shells are articulated, with the valves either splayed open or closed. At the top of the succession, mudstone beds nearly 1.5 m above the bivalve-bearing layers yielded well-preserved Glossopteris sp. cf. G. communis leaf fossils. The closed articulated condition of some shells indicates preservation under high sedimentation rates with low residence time of bioclasts at the sediment/water interface. However, the presence of specimens with splayed shells is usually correlated to the slow decay of the shell ligament in oxygen-deficient bottom waters. The presence of complete carbonized leaves of Glossopteris associated with the bivalve-bearing levels also suggests a possibly dysoxic-anoxic bottom environment. Overall, our data suggest that the bivalves were preserved by abrupt burial, possibly by distal sediment flows into a Brenton Loch lake, and may represent autochthonous to parautochthonous fossil accumulations. The shells resemble those of anthracosiids and are herein assigned to Palaeanodonta sp. aff. P. dubia, a species also found in the Permian succession of the Karoo Basin, South Africa. Our results confirm that (a) the true distributions in space and time of all Permian non-marine (freshwater) bivalves are not yet well known, and (b) there is no evidence for marine conditions in the upper part of the Brenton Loch Formation.

Key words:

• Bivalvia
• Anthracosiidae
• Permian
• non-marine
• Brenton Loch Formation
• Falkland Islands
• Gondwana

Acknowledgements

The authors are indebted to Mr Len Clifton of Walker Creek Farm for allowing us to collect and search for fossils near Rory's Creek, on the southern shore of Choiseul Sound, East Falkland, in 2011. Thanks are due to Mrs Phyllis Rendell, Director of the Mineral Resources Department, Falkland Islands Government, for information and support during our respective stays in the Falkland Islands and for arranging the necessary export permits for the fossil specimens. PS contributes by permission of the Executive Director, British Geological Survey (NERC) and thanks Mr Brian Aldridge for assistance at Walker Creek Farm in 2001 and 2004. T.L. Dutra and R. Ianuzzi helped with the identification of the plant remains. Funds were provided by the Brazilian agencies FAPESP and CNPq. We also appreciate the valuable comments of both reviewers (N.H. Trewin and M.A. Pagani) and the time, careful revision, corrections and critical comments of the Editor (S. McLoughlin), which substantially improved the paper.