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Clay Mineralogy of Shale-Limestone Rhythmites in the Scaglia Rossa (Turonian-Eocene), Italian Apennines
Mark J. Johnsson, Robert C. Reynolds
The Scaglia rossa (Turonian-Eocene), part of the miogeoclinal series exposed in the Italian Apennines, consists of pelagic limestones and marly limestones rhythmically punctuated by thin shale partings. The variations in lithology in part reflect variations in carbonate productivity and/or preservation during deposition. Samples of the shale partings and the intercalated limestones were collected from the Botaccione Gorge, near Gubbio. Semiquantitative methods of analysis, described herein, were used to characterize the clay mineralogy of the two lithologies. The mineralogical compositions of the shales differ qualitatively and quantitatively from those of the limestones, suggesting that the clay fractions of the two lithologists were derived either from different sources or from diff rent proportions of the same multiple sources. Because variations in provenance occur concurrently with variations in carbonate productivity/preservation, a climatic mechanism is most likely responsible for the shale-limestone alternations. The rhythmic nature of these alternations is interpreted as evidence for cyclic climatic variability in the late Mesozoic Mediterranean Tethys.
Stratigraphic variation in clay mineralogy, independent of lithology, is limited to a brief but significant influx of kaolinite at the Cretaceous/Tertiary boundary. This influx is not related to a single, geologically instantaneous event, but may correspond to continental margin progradation and retreat, shifting depositional patterns, climatic change, and/or exploitation of new source terrains.
The clay mineralogy of the shale parting lying at the Cretaceous/Tertiary boundary, the so-called "boundary-layer clay," does not differ qualitatively or quantitatively from that of shales above and below the boundary. It is accordingly deemed unlikely that the layer is composed entirely of transported, impact-derived material exotic to the Gubbio section, although the possibility that a small proportion of the layer is made up of such material cannot be excluded.
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