Fe–Mn-encrusted “Kamenitza” and associated features in the Jurassic of Monte Kumeta (Sicily): subaerial and/or submarine dissolution?

An unusually jagged dissolution surface, capped by a thick Fe–Mn crust is well exposed in small quarry-cuts of the Jurassic of Monte Kumeta. It was formed on a crinoidal limestone substrate of Pliensbachian age, and is covered by Upper Bajocian Ammonitico Rosso-type sediments, all cross-cut by several generations of neptunian dykes. This peculiar surface is more or less coeval with hardgrounds, Fe–Mn-capped dissolution surfaces and associated neptunian dykes described from other localities of the Western Tethys and currently subject to fierce debates as to their purely submarine (or perhaps partly subaerial) origin. The major goal of this paper is to add new arguments to this debate by revealing the finest details of field relationships at a site particularly adapted to the study of this phenomenon. Field observations are supported by petrography and, to a lesser extent, by geochemistry. Results are as follows: (i) vertical dissolution grooves, pointing to dissolution by gravitationally controlled waters, were detected on the sides of several micro-topographic highs; (ii) extensive intergranular dissolution (predating the formation of the Fe–Mn crust) was proved in the substrate both on the micro- and meso-scale; (iii) intense (micro)bio-erosion and local phosphate enrichment were detected immediately underneath the crust; (iv) a Toarcian fauna was identified from the hollows of the irregular surface; (v) synsedimentary faults and fractures clearly predating the major Fe–Mn-encrusted surface were observed, and (vi) a meso-scale synsedimentary growth structure, post dating the Fe–Mn crust, which controlled the Liassic depositional environment of Monte Kumeta is documented. Our conclusion is that the studied surface records at least three separate events of dissolution and precipitation/sedimentation each having either erased or overprinted the effects of the previous one and therefore not permitting the exact reconstruction of all the details of the complex story. To form the irregular surface, in addition to a transient phase of subaerial exposure, a complex history of bio-erosion and submarine dissolution by fluids of widely different chemical composition is proposed. To permit the mixing of sea-water with fault-controlled waters of higher temperature and with groundwaters introduced by deep circulation, a scenario of down-faulted blocks and an adjoining, distant subaerially exposed region is invoked. Such a region provided the hydraulic drive for the postulated circulation. The ultimate cause for the unusual phenomena under scrutiny was the combined effect of tectonics (the local manifestation of Early Liassic rifting in the Western Tethys) and the well-known Pliensbachian–Toarcian sea level-rise.