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

The new shell-armoured ichnogenus Ereipichnus geladensis and associated trace fossils from the Albian of Serra Gelada (Prebetic of Alicante, Spain)

Paolo Monaco ¹, Alice Giannetti ² & Jesus E. Caracuel ³

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

² Steinmann-Institut für Geologie, Mineralogie und Paläontologie   -  Nußallee 8  •  D-53115 Bonn; e-mail: giannetti@geo.uni-bonn.de

³  Depto. Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo.99, 03080 Alicante, Spain;. e-mail: jesus.caracuel@ua.es

Carbonate-rich and thickening upward parasequences from the Sácaras Formation of the Serra Gelada succession (Albian, Lower Cretaceous, Prebetic of Alicante, southeast Spain) are analyzed because of their extraordinary abundance of shell-armoured trace fossils. These shallowing upward sequences are characterized by marls, nodular limestones and calcarenites and they were deposited in an inner to middle shelf. This succession shows a cyclic ichnodensity and ichnodiversity that seems to reflect sea level variations. Ichnocoenoses are mostly characterised by tubular trace fossils that exhibit typical features in their external coating. The first group is represented by a shell-covered, structured burrows, up to 5 cm in diameter and up to 40 cm in length, built horizontally just below the seafloor/water interface. Their pattern varies from rectilinear (type 1) to gently curved (type 2) and usually envelopes an unstructured pipe of grey silty sediment. The coating is characterized by imbricated, flat particles (mainly orbitolinid foraminifers and other planar and sorted bioclasts) forming thin concentric layers. In cross section the bioclast arrangement produces typical plumed and imbricate structures. These trace fossils have been included in a new ichnogenus and ichnospecies, named Ereipichnus geladensis (Monaco et al., 2005). Particle arrangement of the external coating is similar to that of tubes produced by terebelloid worms; however Ereipichnus occurs as a typical horizontal trace fossil, while structured worm tubes are mostly vertical. Ichnodensity of these forms is very high and can reach 80-100 specimens/m² and up to 80% of specimens are isooriented. Strong and persistent unidirectional currents on the sea floor were the main source of suspension nutrients that were used by organisms that colonized and formed shell-armoured trace fossils; the peculiar system of shallow-water currents of the Serra Gelada during the Albian was strengthened by a presumed narrow passage.

The second type is a grain-coated trace fossil, tubular in shape, with a simple internal structure. The coating is often reddish with respect to the surrounding dark grey sediment and shows a slightly coarser-grained texture, which envelopes the internal muddy pipe. This burrow-type was produced by irregular or heart-shaped sea-urchins (spatangoids) as evidenced by the frequent finding of echinoid thecas and it is attributed to traces similar to Subphyllochorda and Cardioichnus.

Serra Gelada ichnocoenoses are characterized by Ereipichnus, Subphyllochorda and Cardioichnus, and they are locally enriched by other types of branched trace fossils primarily represented by Thalassinoides suevicus and Ophiomorpha nodosa. O. nodosa is only present with abundant vertical specimens in calcarenitic sand waves of the upper part of the parasequences. T. suevicus is commonly associated with Ereipichnus but it developed in deeper tiers while Ereipichnus is confined to most superficial tiers of the same ichnocoenosis.

Monaco P., Giannetti A., Caracuel J.E. and Yébenes A., 2005. Lower Cretaceous (Albian) shell-armoured and associated trace fossils from the Sácaras Formation, Serra Gelada area, southeast Spain. Lethaia, v. 38, p.333-344.

(Poster session)