Trace fossil assemblages from Lower Toarcian-Lower Bajocian siliciclastic marine platform-Neuquén Basin, Argentina

BRESSAN, GRACIELA S.; PALMA, RICARDO M.

Hammamet (Tunez)

Simposio; 5th Internacional Symposium of IGCP 506 on: Marine and non-marine Jurassic: global correlation and major geological events; 2008

Faculty of Sciences of Bizerte and the Faculty of Sciences of Tunis

  The Bardas Blancas Formation is a siliciclastic platform developed in northern NeuquenBasin (34-41ºS, 70º30´W) during the Lower Toarcian to Lower Bajocian. The most abundant facies in these deposits are hummocky cross-stratified, laminated and massive fine and medium-grained sandstones, as well as laminated and massive mudstones and siltstones. Less abundant are massive or graded fine and medium-grained conglomerates. This facies association indicates an environment between the lower foreshore and the transition-offshore zone, where the storm processes have dominated. The sandy facies comprise a high diversity association of trace fossils, with elements from Cruziana and Skolithos ichnofacies. The former are characterized by the ichnogenus Chondrites (Fodinichnia), Thalassinoides (Fodinichnia), Palaeophycus (Domichnia), Planolites (Fodinichnia), Gyrochorte (Repichnia), Taenidium (Fodinichnia), Gordia (Pascichnia), Lockeia (Cubichnia) and Rhizocorallium (Fodinichnia). At ichnospecific level have been found Chondrites intricatus, Palaeophycus tubularis, Palaeophycus striatus, Planolites beverleyensis and Taenidium serpentinum. The Skolithos ichnofacies is characterized by the ichnogenus Skolithos, Ophiomorpha, Arenicolites and Diplocraterion, including the ichnospecies Skolithos verticalis. In mudstones and siltstones have been found isolated Chondrites and Thalassinoides. Ichnofabric indexes calculated for Skolithos and Thalassinoides are 1-2 (1: without bioturbation; 2: discrete, isolated trace fossils, with less of 10% of the original bedding disturbed). This can be attributed to an underestimation of the abundance of trace fossils or to a low biologic productivity. During the storms the erosion erased evidences of biogenic activity in bed surfaces (lower foreshore-upper shoreface). Besides, the amalgamation is high in some sections (lower shoreface), making impossible to appreciate the surface of the sandstone beds and the presence of horizontal traces. The distribution of the fauna in patches is another factor that conduces to the underestimation of traces. A low activity could be due to high energy levels in shallow depth waters and low oxygen levels in deep waters. The order of colonization can be determine through the presence of some elements from the Skolithos icnofacies next to element from the Cruziana ichnofacies, indicating evolution of the faunistic association from opportunistic communities to climax communities, suggesting a reduction in the energy of the environment and the stabilization of the environmental conditions. The tiering is showed through the presence of a mixed zone with superficial feeding structures like Taenidium and Gordia and a transition layer with deep Chondrites and Skolithos. The presence of Chondrites in the same level with superficial trace fossils like Palaeophycus, Planolites, and Gyrochorte emplaced before, and composite ichnotaxa, where Chondrites appears burrowing abandoned Thalassinoides and Palaeophycus, shows the sedimentary accretion characteristic of environments with high sedimentary rates. The ichnodiversity, the faunal succession and the tiering are typical of siliciclastic platforms storm dominated, in an environment where the distribution of the fauna is controlled by energy and oxygen levels.