Sedimentology of the west Sicilian Jurassic.

This thesis examines the petrology, geochemistry, and palaeoecology of the west Sicilian Jurassic deposits and, where possible, compares them to similar facies elsewhere in the Alpine-Mediterranean region. The Liassic platform carbonates form the greatest part of the west Sicilian Jurassic in terms of thickness these are a series of white limestones and dolomites whose shallow-water origin is suggested by the presence of such structures as Stromatactis, birdseyes, and shrinkage cracks - and the component lithologies which include pelletal, oolitic and stromatolitic facies. The modern Bahamian pattern of sedimentation compares well with the reconstructed depositional environment of these rocks. Crinoidal limestones, which cap the Liassic platform carbonates, usually occur as discontinuous lenses, and these are interpreted as sand-waves deposited on oceanic seamounts - seamounts which were formed after the carbonate platform had disintegrated during the Lias. The Toarcian iron pisolites, which may also cap the white Liassic limestones, are considered to result from volcanic emanations that accompanied this disintegration. Fossil manganese nodules, which occur in condensed sequences of Middle Jurassic age, have been subject to detailed investigation, and these ancient concretions are comparable in their structure, mineralogy and geochemistry to Recent oceanic iron-manganese accumulations. The condensed sequences themselves - like the crinoidal calc-arenites and the iron pisolites - are interpreted as seamount deposits formed as the presence of stromatolites, boring algae and herbivorous gastropods suggests, in shallow photic zones. Seamount evolution in the Upper Jurassic followed two patterns: the seamounts either sank with the consequent formation of more basinal deposits such as red nodular limestones and radiolarites, or with possible uplift, were the site of more massive carbonate production, with the formation of oolitic and pelletal deposits. Finally, in the Tithonian and extending into the Neocomian, a more uniformly basinal coccolith ooze covered much of the area.