GEOLOGICAL SETTING

Trace-bearing levels belong to the lower portion of the Rotzo Formation (Sinemurian-Pliensbachian; Calcari Grigi Group, see Avanzini et al. (2007) and references therein; Figure 2) represented in this geographic sector by a shallowing upward sequence composed by cycles, each characterized by the transition from a low energy subtidal unit to high energy subtidal deposits up to inter- and supra-tidal deposits (see Monaco and Giannetti 2002; Giannetti and Monaco 2004).

In the studied section, the whole sedimentary succession is developed through the vertical stacking of cyclic, meter-scale, shallowing-thickening-upward packages showing a characteristic asymmetric profile due to the different weathering profiles of their component lithofacies.

At its base, marking the beginning of the marine transgression, each cycle starts with a decimetre-thick transgressive basal lag comprised of reworked intraclasts and bioclasts and/or intraformational breccias with flat pebbles ripped from the stromatolitic levels at the top of the underlying cycle.

Above this lag, the main part of the subtidal portion of the cycle is alternatively made of oolitic-bioclastic calcarenites that, in some cases, occur as graded storm layers or of grey, peloidal packstone/wackestone. The subtidal parts of the cycles may include gastropod and bivalve lags both in primary position and as reworked, disarticulated valves.

The inter-supratidal, upper portion of each cycle is represented by thin reddish marly levels or sometimes by stromatolitic dolostone that includes a wide range of depositional structures and diagenetic modification produced during subaerial exposures of the sediments, from sheet and polygonal cracks, tepee and birdseye structures to microstalactictic cements, and other products of vadose diagenesis.

Microfossils analysis allowed the recognition of various taxa such as Paleodasycladus mediterraneus, P. fragilis, Siphovalvulina spp., Pseudopfenderina butterlini, Ammobaculites sp., Spiraloconus sp. This microassemblage strongly indicates a middle-late Sinemurian age that seems coherent with the stratigraphical location of the studied levels (Petti et al. 2009b).