Pelagic Oncoids and Ooids in the Middle-Upper Jurassic of Eastern Sardinia

Abstract

In eastern Sardinia a Middle to Upper Jurassic marine sequence more than 1000 m thick rests unconformably on the Palaeozoic basement and consists in the coastal area of three main lithostratigraphic units: the Dorgali Formation, consisting for the most part of dolostones (Bathonian), the ammonite-bearing Mt. Tului Limestone (Lower Callovian to Uppermost Kimmeridgian) and the Mt. Bardia Limestone (Portlandian to Berriasian). The second unit its gradually replaced westwards by oolitic and oncolitic dolostones, and is therefore incorporated in the first one (Fig. 1 A, C). A transgressive trend can be recognized in the lower part of the Jurassic sequence, from Bathonian to Upper Kimmeridgian. During this time span asymmetric carbonate shelf cycles mostly consisting of upward shoaling oolite sequences were formed. They indicate the existence of a wide shallow shelf with a system of tidal oolitic bars periodically prograding seawards and separating an internal shelf lagoon largely open to marine influences from an outer shelf area. The latter during the Early Callovian-Late Kimmeridgian was the deposi-tional setting of the Mt. Tului unit which in its lower part is represented by an oncolitic facies, mainly occurring in the more condensed and pelagic sequences.