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Giornale di Geologia


In addition to the predominant marly lithologies, the Serravallian-Tortonian sequence at Monte dei Corvi (MDC), south of Ancona, Italy, contains at least 85 thin, dark calcareous shales. Such shales, averaging 14 cm in thickness, comprise 9% of the total Serravallian sequence. Sixteen of them were sampled for a preliminary organic geochemical evaluation. All the MDC shales appear to have been deposited during periodic anoxic events, as demonstrated by the presence of significant quantities of organic matter and authigenic pyrite. The degree of anoxicity (and thus the amount of organic matter preserved) appears to have differed from one event to the next. The quantity of organic matter preserved is highly variable, with Rock Eval S2 ranging from 0.4 to 27.6 mg/g. However, the type is remarkably consistent from sample to sample, as evidenced both by analytical pyrolysis-gas chromatography/mass spectrometry (GC/MS) of the solid organic matter and by the GC/MS analysis of the extractable material. This implies a regeneration of similar microfloral/microbial assemblages and depositional conditions during each anoxic event. The dominant organic matter types are marine, including several types of fossil algae and amorphous material (largely the product of bacterial reworking of organic matter). There is evidence of minor terrestrial input. The presence of isoprenoid hydrocarbons (prist-1-ene) in the pyrolyzates and 17β(H),21β(H) hopanes in the extractable organic matter attest to the low level of thermal alteration of the MDC shales. The MDC organic matter appears to be partially oxidized, probably from weathering at the outcrop, which partly obscures its original nature. To avoid this problem in any future organic geochemical study of the MDC sequence, the authors recommend that fresh, unweathered samples be obtained, preferably by coring.

Published Citation

Kruge, M. A., Stankiewicz, B. A. and Montanari, A., 1994, Serravallian shales in the Monte dei Corvi pelagic sequence (Ancona, Italy): An organic geochemical perspective. Giornale di Geologia 56:173-182.