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Mineralogy and geochemistry of Mesozoic black shales and interbedded carbonates, southeastern Sicily: evaluation of diagenetic processes

Published online by Cambridge University Press:  01 May 2009

E. Azzaro ,
A. Bellanca  and
R. Neri
E. Azzaro
Affiliation:
Istituto di Mineralogia, Petrografia e Geochimica, Università di Palermo, Via Archirafi 36, 90123-Palermo, Italy
A. Bellanca
Affiliation:
Istituto di Mineralogia, Petrografia e Geochimica, Università di Palermo, Via Archirafi 36, 90123-Palermo, Italy
R. Neri
Affiliation:
Istituto di Mineralogia, Petrografia e Geochimica, Università di Palermo, Via Archirafi 36, 90123-Palermo, Italy
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Abstract

Upper Triassic/Lower Jurassic organic-rich shales and interbedded carbonates (Rhaetian → Sinemurian) are widespread in the subsurface of southeastern Sicily where important oil fields have been found hosted in Triassic reservoirs. Core samples from wells drilled offshore and onshore were studied from petrographie and geochemical viewpoints.

In the Hettangian/Sinemurian shale-carbonate sequences, which accumulated in a rapidly subsiding basin, the micritic aragonitic mud is still largely preserved. Mixed-layer I/S has remained randomly interstratified to a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finely crystalline, pore-filling dolomite and/or calcite. The carbonate component exhibits a high Sr content and fair amounts of Fe and Mn. Carbon and oxygen isotopic values suggest a subsurface interstitial formation for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters were retained in the sediment pores for a long time after deposition, thus enhancing the preservation of significant amounts of the original organic matter.

In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I/S and a carbonate mineralogy dominated by low-Mg calcite in the uppermost beds and by near-stoichiometric dolomite in the lowermost ones. Petrographie, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence pervasive dolomitization in a sabkha-type environment.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 2 , March 1993 , pp. 191 - 202
Copyright
Copyright © Cambridge University Press 1993

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