ICHNIA 2008, Kracow, Poland, september 1-6, 2008

Stratinomy and taphonomy of trace fossils from Eocene to Miocene turbidites in Northern Apennines (central Italy)

Paolo Monaco ¹ & Alessio Checconi ¹

¹Dipartimento di Scienze della Terra,Università degli Studi di Perugia;  Piazza dell’Università 1, 06100 Perugia (Italy); e-mail: pmonaco@unipg.it, paleodot@unipg.it

                                                        

In turbidite and hemipelagite deposits of Northern Apennines (Italy) deep-sea ichnotaxa have been approached in order to describe the ichnocoenoses and to define their taphonomy and stratinomic value. Studied deposits range from Early Eocene to Late Miocene and are represented by shales and limestones with carbonate (Scisti Varicolori Beds and Scaglia Group) and siliciclastic turbidites (Macigno, Vicchio Marls, Cervarola and Marnoso Arenacea of Apennine foredeep). Stratinomic analysis allows to recognize five ichnia classes: A-type = hypichnia; B1-2-types = sandy or muddy endichnia; C-type = crossichnia of many beds and D-type = epichnia.

A-type category is characterized by pre-depositional traces preserved as hypichnia and comprises several networks of Paleodictyon (and variants as Squamodictyon) preserved as horizontal meshes or vertical shafts (e.g. P. hexagonum). This category also includes meandering-shaped graphoglyptids (e.g.  Desmograpton, Urohelminthoida, Paleomeandron, Protopaleodictyon, Megagrapton, Helicolithus, Cosmorhaphe and Helminthorhaphe) and spiral-shaped burrows (e.g. Rotundusichnium, Spirorhaphe and Spirophycus). Radiate Lorenzinia and Glockerichnus, plug-shaped Parahaentzschelinia and Bergaueria, and some bilobate Scolicia specimens may probably be considered pre-depositional traces.

B-type includes post-depositional trace fossils preserved as endichnia within sandy beds to exploit grains and nutrients transported directly by turbidity flows. They consist in branched tubes, straight or sinuous and more or less structured (e.g. Alcyonidiopsis, Sabularia (Granularia) simplex, Ophiomorpha annulata, O. rudis, Thalassinoides and  Palaeophycus), in bilobate-trilobate forms (Scolicia strozzii) and in simple short traces plunging in sand (Arthrophycus). Gordia, Helminthopsis, Strobilorhaphe, Subphyllochorda-Cardioichnus and Halopoa are doubtfully included in B-type category.

B2-type consists in muddy endichnia and comprises branched vertical burrows (e.g. Chondrites, Cladichnus and Trichichnus) that developed downward in calcilutites and in cm-thick muddy turbidites (e.g. Eocene Varicolori beds) and probably some pustulose Nereites forms.

C-type involves traces crossing more beds (crossichnia). Vertical burrows are commonly branched and structured (e.g. Ophiomorpha rudis and other subquadrate in transversal section). Crossichnia helicoidal or lobate structures, up 1 m wide (Zoophycos, Spirophyton groups), are also common in thick hemipelagic deposits of Eocene-Oligocene Scaglia Group and Early Miocene Bisciaro–Schlier Formations.

D-type category is represented by epichnia which develop at the top of thin-bedded (cm-thick) turbidites and correspond to preservational variants of Nereites and Neonereites (multiserialis, biserialis and uniserialis), to meandering forms (e.g. Scolicia prisca) or to similar trails which exhibit backfill meniscate structures (e.g. Taenidium and others), to simple sinuous traces such as Planolites and to radiate forms (? Glockerichnus).

These five peculiar ichnia are very sensitive to specific parameters of the substrate and morpho-structures of sea-floor (e.g. slope, fans, deep-sea plains) in hemipelagic to turbiditic conditions; heterogeneous and diversified ichnocoenoses reflect variations in environmental parameters as sedimentation rate, organic matter, oxygen content, consistency, temperature, and bottom traction-currents. This study also provided to identify twelve taphonomic features that are supposed to be the most important factors influencing mainly hypichnia preservation. They are induced by biogenic (bulldozing and burrowing) and physical agents (e.g. currents and creep). In studied thin-bedded and fine-grained turbidites both agents produced, for example, several three-dimensional oriented pre-turbiditic deformations of A-type burrows.

(oral session, sept. 2. 2008)