STRATINOMIC VALUE OF TRACE FOSSILS IN TURBIDITES AND HEMIPELAGITES FROM EARLY EOCENE TO LATE
MIOCENE IN THE NORTHERN APENNINES
Autori
MONACO PAOLO 1, CHECCONI ALESSIO 1, GABRIELLI MATTEO 2
presenter's e-mail: pmonaco@unipg.it
1 - Dipartimento di Scienze della Terra, piazza dell'Università 06100 Perugia.
2 - Fraz. Croce di Castiglione 9, Città di Castello, 06010 Perugia
Parole chiave: turbidites, trace fossils, Tertiary, Apennines
Abstract
In turbidite and hemipelagite deposits of the northern Apennines, deposited from Early Eocene to Late Miocene,
deep-sea trace fossils may be approached considering their stratinomic value which reflects their position in the
event bed: hypichnia (base of bed), endichnia (inside bed), crossichnia (more beds) and epichnia (top of bed).
Stratinomy allows to recognize five ichnia (A-type = hypichnia; B1- and B2-type = sandy or muddy endichnia; Ctype
= crossichnia of many beds and D-type = epichnia). Ichnia value in deep water turbidites is more useful and
easily observable than classic ichnofacies models described in literature (e.g. Nereites and Zoophycos ichnofacies).
Excellent stratinomic examples are represented by calcareous or siliciclastic turbidite beds and hemipelagites of the
Scisti Varicolori Beds, Scaglia Group (Rossa-Variegata-Cinerea), Bisciaro-Schlier marls and Macigno-Cervarola-
Marnoso Arenacea flysch deposits.
A-type category is characterized by pre-depositional traces preserved as hypichnia. They consist in Paleodictyon
networks (from large to small in size) preserved as horizontal mesh or vertical shafts (P. Hexagonum, P. strozzii, P.
majus, P. italicum, P. minimum, P. latum) and Squamodictyon. Other hypichnia are meandering-shaped (uniramous
and biramous) traces such as Desmograpton, Urohelminthoida, Paleomeandron, Protopaleodictyon, Megagrapton,
Helicolithus, Cosmorhaphe, and Helminthorhaphe. Spiral-shaped Rotundusichnium, Spirorhaphe and Spirophycus
have been found in siliciclastic turbidites of Cervarola. Probably also radiate (Lorenzinia and Glockerichnus) or plugshaped
(Parahaentzschelinia and Bergaueria) forms may be considered pre-depositional traces.
B1-type are typical endichnia developed within sandy beds and three-dimensionally distributed to exploit food
resources and external nutrients transported directly by turbidity flows or deep-sea currents (post-depositional
trace fossils). They consist in branched tubes, straight or sinuous and more or less structured (Alcyonidiopsis,
Ophiomorpha annulata, O. Rudis, Thalassinoides and Palaeophycus), bilobate-trilobate forms (Scolicia strozzii) and
maybe simple short traces plunging in sand (Arthrophycus). Uncertain position of some traces as Gordia,
Helminthopsis and Strobilorhaphe needs more analyses. In Cervarola arenites typical B1-type endichnia are
structured Subphyllochorda-Cardioichnus, Spongeliomorpha, Protovirgularia and Halopoa.
B2-type consists in muddy endichnia characterized by branched vertical burrows Chondrites and Trichichnus
downward developed in calcilutites and in cm-thick muddy turbidites (e.g. Varicolori beds).
C-type involves traces which cross more beds vertically and therefore may be considered crossichnia. They are
vertical burrows commonly branched and structured (50 cm long and subquadrate in transversal section and
Ophiomorpha). Crossichnia are also present in thick hemipelagic deposits of Eocene-Oligocene Scaglia Group and
Early Miocene Bisciaro–Schlier (Contessa-Gubbio area and S-E Umbria) as helicoidal or lobate structures up 1 m
wide (Zoophycos, Spirophyton groups).
D-type category is represented by epichnia such as those of the Varicolori Beds or Marnoso Arenacea, which
develop at the top of cm-thick turbidites and correspond to preservational variants of Nereites (multiserialis,
biserialis and uniserialis). Other epichnia belong to meandering forms (Scolicia prisca) or similar trails which exhibit
backfill meniscate structures (e.g. Taenidium), simple sinuous traces such as Planolites (e.g. P. Beverleyensis) or
radiate forms (?Glockerichnus) of fringe deposits in Verghereto High area.
These five peculiar ichnia are very sensitive to specific parameters of the substrate and topography of sea-floor (e.
g. slope, fans, deep-sea plains) and hemipelagic to turbiditic conditions, reflecting variations in sedimentation rate,
organic matter, oxygen content, consistency, temperature, and bottom traction-currents.