Middle Devonian reef‑derived calciturbidites of western Ossa‑Morena Zone (Portugal): conodont biostratigraphy of the Eifelian Pedreira da Engenharia Limestone

Abstract

The first biostratigraphical study on the complete Pedreira da Engenharia quarry section (Middle Devonian, SW Portugal) is presented. A total of 32 samples were collected from the three subsections (PE-A, quarry floor; PE-B and PE-C). This led to the identification of 15 species belonging to the genera Icriodus, Polygnathus and Tortodus. Four index species were identified: P. costatus; T. australis; T. kockelianus; and P. ensensis. The age for the Pedreira da Engenharia section was updated from the costatus zone (Boogaard, 1972) to the costatus-ensensis zones (lower to uppermost Eifelian). There is a thin part of the succession representing the kockelianus and, possibly, eiflius zones, together being represented by 1,5 m. A CAI value of 5-5.5 for the conodont elements indicates a maximum temperature of over 300 ºC. The Pedreira da Engenharia Limestone, together with the Odivelas Limestone, are evidence of the development of calciturbidite sedimentation associated with atoll-like structures (only preserved in Odivelas) along the SW boundary of Ossa-Morena Zone, being probably associated with the beginning of the subduction of the Rheic Ocean during Early to Middle Devonian times.