From outcrop studies up to SEM examinations: integrated ichnological/sedimentological research of Lower Cretaceous, marginal-marine units from northern Patagonia

FERNÁNDEZ, DIANA E.; PAZOS, PABLO J.

Rio de Janeiro

Congreso; 12º TWAS-ROLAC Young Scientist Regional Conference y Avances y Perspectivas de la Ciencia en Brasil, América Latina y el Caribe; 2012

Academia Brasilera de Ciencias

During the past few years, the first integrated ichnological/sedimentological research of Lower Cretaceous, marginal-marine units from the Neuquén basin (Argentina) has been carried out.             The Agrio Formation (Late Hauterivian) is a classical unit of this basin. It contains some of the most complete ammonoid and nannoplankton biostratigraphic zonations which was recently confirmed as accurate with precise zircon geochronology. The upper member of the Agrio Formation has been traditionally interpreted as a storm-dominated depositional setting. However, the last detailed analyses of the uppermost sections of the unit, in several localites of the basin, documented tidal influence in marginal-marine depositional settings.             Along with the extensive work on sedimentological characteristics, through which these marginal-marine signatures have been found, trace fossils have been included in the study. Ichnosystematic interpretations allow a complete paleoecological reconstruction. Several ichnogenera have been identified (e.g. Chondrites; Nereites; Bolonia; Cochlichnus Thalassinoides; Trypanites; Hillichnus). The ichnofauna here reported enhances the paleodiversity since it opens a window to some inhabitants that have not been documented by body-fossil remains. Some of the ichnotaxa, such as Kouphichnium (xiphosurid trackways) and Therangospodus (a type of theropod dinosaur tracks) are also important as paleoenvironmental indicators: they provide useful information to interpret exposure in areas considered basinal facies in paleogeographic maps. Ethological inferences have also been possible through the use of trace fossils.             The scale of the analyses includes from outcrop studies up to SEM examinations. Microbially induced sedimentary structures (MISS), and therefore, the presence of microbial mats, are known from tidal flats. The appearance of MISS on some trace fossil-bearing levels pointed towards the involvement of microbial mats. Samples of the surface of these levels were taken and photographed under a scanning electron microscope (SEM). Between the sand particles of the uppermost millimetres of the surfaces, filament-like microstructures were observed. They resemble ensheathed forms of cyanobacteria, a common morphotype involved on mat and biofilm formation commonly described in tidal flats. These filamentous mats are considered a necessary condition for the exceptional preservation of the traces, through the binding and biostabilisation of the layer, preventing erosion. This constitutes another example of the importance of integration of sedimentological and ichnological data.             Studies of the Lower Cretaceous Mulichinco Formation (Neuquén basin) have recently started, and their preliminary results show that in some localities marginal-marine environments where also widespread. Particularly, some facies have also been interpreted as tidally-dominated depositional settings.             The ichnological characteristics found show that these deposits bear more complex ethological and paleodiversity information than shown by previous works. The paleoenvironmental inferences for these units are also important for reviewed paleogeographic reconstructions, which can be applied in the oil industry as paleobatimetric controls.             This is the contribution C-41 of the Instituto de Estudios Andinos Don Pablo Groeber.