Earth and Environmental Science Transactions of the Royal Society of Edinburgh

Research Article

The development of an atypical Hirnantia-brachiopod Fauna and the onset of glaciation in the late Ordovician of Gondwana

Owen E. Sutcliffea1 p1, David A. T. Harpera3, Abdallah Aït Salema4, Robert J. Whittingtona2 and Jonathan Craiga5

a1 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK; email: osutcliffe@badley-ashton.co.uk.

a2 Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK

a3 Geologisk Museum, Oester Volgade 5–7, 1350 Koebenhaven K, Denmark; email: dharper@savik.geomus.ku.dk.

a4 34 Avenue Al Fadila, 10050 Rabat, Morocco.

a5 ENI LASMO, 101 Bishopsgate, London EC2M 3XH, UK; email: jonathan.craig@lasmo.com.

ABSTRACT

The development of an atypical Hirnantia Fauna in the late Ordovician of Gondwana was coeval with a slow eustatic fall induced by the abstraction of water into a growing ice sheet. This event is dated as early Hirnantian in age and occurred in tandem with the start of a major mass extinction. A tectonic episode in the Caradoc-Ashgill of North Africa differentiated the continental shelf into highs and lows and may have formed the land required for the accumulation of a permanent snow cover. Depositional lows were filled by regressive shallow-marine deposits in the early Hirnantian. During the mid-Hirnantian, advance and retreat of an ice sheet on the continental shelf resulted in the deposition of glaciomarine sediments above these regressive deposits. The demise of an atypical Hirnantia Fauna is attributed to deglaciation and the associated flooding of the continental shelf by a stratified anoxic water column. This glacioeustatic sea-level rise occurred in the late Hirnantian.

(Received January 21 2000)

(Accepted June 02 2000)

KEY WORDS

  • Ashgill;
  • biostratigraphy;
  • Hirnantian;
  • ice-sheet growth;
  • Libya;
  • Lower and Upper Second Bani formations;
  • Melez Chograne Formation;
  • Morocco;
  • sea-level fall

Correspondence

p1 Badley Ashton & Associates, Winceby House, Winceby, Horncastle, Lincolnshire LN9 6PB