Trigger mechanism recognition of the soft-sediment deformation in the upper Messinian deposits of the Gargano Promontory (Apulia, southern Italy)

Soft-sediment deformation structures have been found in some upper Messinian transitional deposits, probably equivalent to the widespread Lago-mare event (Cita, 1978, Cipollari et al., 1999, Iaccarino & Bossio, 1999, Clauzon et al., 2005), named as “calcari di Fiumicello” in the new geological map (CARG Project, Morsilli et al., in press). This lithostratigraphic unit, about 30-40? meter thick, crops out in the northern part of the Gargano Promontory near Cagnano Varano (Fig. 1). Four detailed stratigraphic sections have been measured and sampled along some rail cuts (Fig. 2). The succession consists of an alternation of well bedded limestone, marly limestone, marls and lime mud, with few conglomerate layers. Limestone texture are made by ooidal (mainly dissolved and recrystallized) or intraclastic grainstone to packstone, arranged in 10 to 120 cm thick beds frequently cross laminated and cross stratified (current and wave ripple, bidirectional and sigmoidal lamination at dune scale, low angle planar laminations). Grain size range from fine to very coarse sand. Marly limestone and marls, white to light brown in colour, shows a variable bed thickness from 2 to 50 cm with very thin, mm-scale, even laminations. Conglomerate layers, mainly 5 to 15 cm in thickness, are visible in various intervals of the stratigraphic sections. They consist mainly of othoconglomerate with clasts up to 2 - 6 cm, derived from the Serravallian-Tortonian interval of the Pietra Leccese formation. Paraconglomerate with a sandy matrix are visible only at the top of the section 1 where the bed thickness increase up to 30 cm. Elongated clasts reach the maximum size of about 10 cm. Exposure surfaces are testified by thick root traces visible in the marly intervals and also in some conglomerate bed of the Section 3. Macrofossils are not visible in the measured sections and micofossils in thin sections are mainly reworked and consists of fragments of benthic and planktonic foraminifera. There are also some fossils not reworked as miliolids and ostracods. The “calcari di Fiumicello” paraconformably overlay a thick red-matrix gonglomeratic unit interpreted as alluvial fan to fan delta deposits. The upper limit is not known. Tectonic deformation is very gentle with a general tilting toward NE and some broad folds with NNW-SSE axis. Only in the section 1 there are present some small normal faults with few decimetre downthrow. Some features like beds thickness change, small dykes and onlap in a small “graben like” structure seem to suggest the presence of synsedimentary deformation at least in this part of the succession. On the base of the lithologies, texture and sedimentary structures five main lithofacies has been recognized and interpreted as indicative of a barrier island-lagoon system like the modern Lake of Varano. The various sub-environments are: 1) very low energy lacustrine? with stressed salinity condition or Eh=0 at the water-sediment interface testified by the absence of epifauna and infauna (mm thick laminations); 2) shallow protected lagoon sometimes with terrestrial? plants (root traces); 3) small internal beaches made of pebbles or sand size grains (foreshore - low angle planar laminations); 4) small coarse grained deltas (paraconglomerate); 5) flood delta and/or small tidal bars (m-scale sigmoidal and bidirectional lamination). Soft-sediment deformation affects the entire late Messinian succession and occurs in different discrete intervals (more than 10 ?) separated by undeformed beds. The rail cuts allow us to follow the deformed beds along thousands of meters and the direct observation of lateral variations in the style and degree of deformation. By a morphological point of view, they are represented mainly by load-structures with minor water-escape features while only one deformed bed contains distorted lamination and irregular folds. The detailed analysis of the deformation features allows us to affirm that deformation occurred during and after sedimentation and that mechanisms of deformation were different: some are clearly related with liquefaction and/or fluidization (viscous-fluid behaviour in load- and water-escape structures) while some show plastic and/or viscous behaviour (distorted lamination and irregular folds). Excluding the action of overloading processes and the action of storm-waves, the reliable trigger mechanisms for the observed deformed beds are seismic shocks. Nevertheless, the main interest of this work is related with the lateral variation in the style and degree of deformation in the single deformed beds. In fact, one of the most accepted criteria to recognize seismic-induced deformations (seismites) is the large lateral extension of the deformed beds. In our example, locally, highly deformed beds pass laterally (without an appreciable preferred direction of variation) to totally undeformed beds or single deformed beds pass laterally to two thinner beds separated by an undeformed surface. Having excluded the presence of erosional surfaces, we think that lateral variations in lithology (texture, presence of matrix, porosity, water content, early-diagenetic features, etc.) could be able to control the lateral extension of the seismic shock effects on soft-sediments.