Three-dimensional burrow systems and taphofacies in shallowing-upward parasequences, Lower Jurassic carbonate platform (Calcari Grigi, Southern Alps, Italy)

Analysis of parasequences of carbonate platform origin from the Calcari Grigi Formation (Rotzo Member, Lower Jurassic) in the Venetian Pre-Alps of Northern Italy reveals intense burrowing affecting warm lagoonal and protected shelf environments of the Trento carbonate platform. The trace fossil pattern suggests a strong biogenic activity by many unknown organisms (episodic surficial bioturbation) and by decapod crustaceans (deep burrowing) during the Sinemurian - Pliensbachian. Burrow networks of Thalassinoides (T suevicus, type I to IV) and Ophiomorpha form three-dimensional systems (3DBS), up to 150 cm in thickness, that reveal well developed taphonomic characteristics. Occasionally, also rare body fossil remains (exuvia) of Phlyctisoma (Erymidae) are preserved, although it is very difficult to attribute to this decapod crustacean a real tracemaker activity. In a total of 18 shallowing-upward taphosequences, each 2.3-2.6 m in thickness, the lower, subtidal part (BOP: base of parasequence), is invariably characterized by 3DBS. Three parts of a 3DBS are taphonomically relevant: a) the enlargement at bifurcation points (the so-called turning chambers) with related tunnel sections in horizontal networks; b) the distribution of vertical tunnels and their openings, locally associated with biogenic mounds; and c) the infilling with coarse-grained skeletal debris of abandoned burrows (tubular tempestites). Case (a), although well known in the literature, is nevertheless peculiar because of the exceptional diameter (up to 22 cm) and shape of bifurcations of Thalassinoides suevicus. Therefore their taphonomic analysis helps to refine shallowing-upward taphosequences. Case (b) and (c), instead, are exceptional because traces of the activity of decapod crustaceans at the sea-floor are very rare in the geologic record. The 3DBS involve taphofacies that are peculiar of the upper part of the Calcari Grigi across western Venetian Prealps. The analysis of 3DBS in all Lower Jurassic parasequences will aid sequence stratigraphy and the paleoenvironmental reconstruction of the carbonate platform setting.