SEDIMENTOLOGY AND ICHNOLOGY OF THE TRACE BEARING CYCLE

Here follows a detailed ichnological description of two shallowing upward cycles including both Asteriacites and dinosaur traces (Figure 3).

The base of this sedimentary portion is represented by a peloidal mud followed by sandy layers with parallel to slightly irregular upper and lower contacts and internal laminae varying from parallel to undulatory. This heterolitic stratification (sensu Damicco and Hardie 1994), or flaser-lenticular bedding (sensu Reineck 1960) was considered by Reineck and Wunderlich (1968) as quite common in shallow water carbonate platform deposits. When tidal flats are covered during ebb or flood tides, currents or waves produce irregularly rippled sands and silts. During slack water or calm seas, mud suspended in the water column settles out, burying the ripples to various degrees. The mud compacts and is thus not easily eroded during the next current cycle. Alternatively, as appears in the Coste dell'Anglone dinosaur ichnosite, such interlaminated structures can be produced by the preferential trapping of mud by microbial mats during storm flooding and then deposition of sand as the current wane (Damicco and Hardie 1994). The top of the peritidal cycle shows a 30 cm thick inter- to supra-tidal sequence made of a interlaying of thinly laminated dark peloidal mudstone and reddish millimetre-thick sandy-dolostone layers. The presence of thin stromatolite layers with subaerial exposure evidence, makes reasonable that in our case the tidal flat, after the accumulation of the peloidal mud, became exposed to the subaerial environment with pedogenetic rubefaction, mud cracking, and dolomitisation. Among these finely laminated layers and immediately below the supratidal interval, a monospecific Asteriacites lumbricalis assemblage was recognised (Figure 4).

The lower part of the subsequent cycle (deposited in the lower-middle subtidal environment of a carbonate lagoon) is intensely burrowed. Centimetre-sized elliptical spots and vertical tunnels are infilled by peloidal grainstones and mixed skeletal debris. These tubular concentrations may be related to open burrow networks of crustaceans burrowed in a firm substrate (Monaco and Garassino 2001). In other overlying parasequences, burrowing of this basal portion of the cycle is evidenced by turnarounds at nodes and branched tunnels indicating deposit feeding activity by decapod crustaceans (Thalassinoides and Ophiomorpha). Commonly, cylindrical branches Y to T shaped show diameters up to 5 cm. Mazes in general are regularly tiered, and burrowing can be recognised as a primary factor in the nodularity of this basal part of the cycles.

The middle and upper part of the cycle is much less burrowed. Small trace fossils (0.5 to 1 cm in diameter) include Chondrites, Planolites and Skolithos. These traces are associated to ripple marks and reworked bivalve and brachiopod skeletal fragments. Among taphonomic characters abrasion, reorientation, disarticulation, winnowing, and shelter porosity suggest the dominance of recurrent physical agents on the biological ones (Monaco 1999). These taphonomical features are related to a middle subtidal environment with a shallowing trend from the bottom that, at the top, reaches the normal wave base level and the inter-tidal/supra-tidal environment.

Vertebrate tracks (an apparent monospecific theropod ichnoassociation) are slightly impressed on the top of the cycle consisting of alternating stromatolitic laminae and dark gray bioclastic grainstone covering a set of meandering shallow tidal channels infilled by coarse grained and imbricated oncoidal floatstones/rudtones. They are associated only to a few Planolites and locally to irregularly shaped depressions probably related to both biological and physical erosional processes. Similar irregularly shaped depressions have been described by Kvale et al. (2001) on modern intertidal tidal flats. They were interpreted as the destruction of the stabilized algal mats owing to bioturbation processes (e.g., infaunal organism burrows opening on the sediment surface) during the low tide. The result is the erosion or removal of a section of the microbial mat through a combination of current and wave action during a subsequent high tide, forming a shallow and irregular depression on the tidal flat surface.