Paleotemperature of external Ligurian units based on mineralogical and organic maturity data: preliminary results

Crystallographic and compositional data from clay minerals are currently used in orogenic belts and accretionary wedges as temperature and pressure indicators in sedimentary rocks, with special regards to the interval from diagenesis to low metamorphism. These methods are suitable to give information on the different burial conditions of rock units involved in subduction processes world wide. The External Ligurian units are a part of the Cretaceous-Eocene, Alpine accretionary wedge of the Northern Apennines. These units presently crops out as a part of the so-called Ligurian nappe and can be subdivided in several structural units (Cerrina Feroni et al., 2002). This study reports the preliminary thermal maturity data from clay minerals and organic matter obtained along a transect in the Bologna area. In references to the Cerrina Feroni et al. (2002), the Lower Ophiolite unit (Oi) and the Samoggia-Sillaro (SmS) unit have been sampled. Samples have been collected in the strongly deformed to completely stratally disrupted stratigraphic units. Samples come from the Palombini Shales of the Oi (POi), their stratigraphic equivalent into the SmS, subdivided in a lower member (Neocomanian-Barremian) and an upper member (Aptian-Albian) (LC1 and LC2, respectively), and the Upper Cretaceous Varicolori Shales unit (Cenomanian-Turonian) of SmS (Av). Paleotemperatures and diagenetic degree were evaluated by mineralogical assemblage of clay fraction (< 2 micron), illite-smectite ratio in mixed layer (I/S) and measurement of illite Kubler index (KI) carried out by X-ray diffraction. The thermal maturity of organic matter (Tmax) was determined by Rock-Eval Pyrolysis analysis. The occurrence of a regularly ordered mixed layer I/S and KI values of about 0.65 (°2) indicate for the POi, which is the structurally and geometrically highest unit, and for the Av and LC1 units temperature of about 130 degree, whereas the LC2 unit is characterized by a randomly ordered mixed layer I/S consisting with paleotemperatures lower than 60°C. Mineralogical data are also supported from organic maturity data. The difference between the lower Ofiolitic unit and the Samoggia-Sillaro unit are somewhat of expectable, in reason of the different structural position, the inferred different paleogeographic position and the marked differences at the outcrop. The Ophiolitic unit show a minor degree of stratal disruption in respect with the SmS. Moreover, the ubiquitous scaly fabric of the latter is not present in the Ophiolitic unit, the outcrop of which are characterized by a penetrative piano-parallel cleavage by compaction. The marked difference among the different stratigraphic levels in the Samoggia-Sillaro are much more difficult to explain and, thus, intriguing. The only explanation so far is that the apparently unique structural unit represents two different structural units, or at least, two thrust sheets. The samples with different thermal results in fact pertain to two different belts of tectonosomes outcrop striking grossly NW-SE, which are separated by a thrust surface (Pini, 1993, 1999). As a partial, and preliminary confirmation of this assumption we can also report that different temperatures of maximum burial inside SmS correspond to changes in style of deformation, different characters of clayey matrix and to stratigraphic characters of the two structural units or sub-units.