INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Low-gradient coastal environments of an exceptional Titanopodus sauropod tracksite at the lattermost cretaceous epicontinental sea way in northwest Patagonia (Southern Mendoza) Argentina: sequence stratigraphic, taphonomic and paleobiogeographic implicati
ASTINI, RICARDO A.; PRÁMPARO, MERCEDES; GONZÁLEZ RIGA, BERNARDO J.; PREVITERA, ELENA; CARIGNANO, ANA
Congreso; XIX Congreso Geológico Argentino; 2014
Asociación Geológica Argentina
The Loncoche Formation within the Malargüe Group yields an important sauropod track site in northern Neuquén Basin, northwestern Patagonia. Almost all the tracks (about 350 ichnites) were assigned to a new taxon named Titanopodus mendozensis González Riga and Calvo 2009. The trackmaker correspond to titanosaur sauropods of 10-13 m long. Understanding of the environmental context that favoured the preservation of the footprints and how the environment was changing through time seems crucial. The Loncoche Formation is integrated by a complex array of brackish, lacustrine and shallow-marine facies representing a mosaic of dynamic coastal environments developed within a very low-gradient marginal-marine setting, known as the Neuquén embayment. Global sea-level rise and Atlantic transgressions during the Late Cretaceous gave rise to an epeiric sea way that covered central-northern Patagonia, reaching the Neuquén foreland basin at the foothill of the emerging Andes. This region with a sustained subsidence allowed accommodation and preservation of a succession of rather unusual coastal lagoons, estuaries, tidal flats and deltaic to shallow-marine facies associations with mixed tide-wave influence. Within this setting, minor high-frequency sea-level fluctuations create retrograding-aggrading-prograding meter-scale stacking patterns composed of largely detrital facies assemblages, seldom capped by carbonates and culminating with subaerial exposure surfaces. A common low-rank cycle initiates with subaerial exposure and related incision (indicating negative accommodation) covered by a thin coarse-grained lag followed by subtidal bioturbated silty shales and cross-bedded lithic sandstones associated to bay-head deltas. Laterally linked verdine-rich tide-dominated subtidal sandwave complexes with intervals of fossil-rich bioturbated sandy to argillaceous greenish siltstones punctuated by coquina beds are developed when accommodation creates sufficient depth to allow detrital input. Cycles are frequently overlain by tabular cross-bedded grainy limestones containing a variety of components, largely oolites and bioclasts, but also reworked intraclasts, oncoids and bone fragments. These bed-sets represent backstepping of higher-energy granny shoals developed as restrictive spits rapidly moving across the environment during peak sea level. Alternatively, these limestone beds may be lacustrine. Sharp truncation and exposure at their tops indicate complete filling of the accommodation space during highstand stages, allowing microkarstic and calcrete development. Toward the top, cycles include salt casts and syneresis cracks indicating high-salinity, extremely-shallow ponds, and tidal flats that together with stromatolite-rich patches indicate relatively arid inter- to supratidal fringing environments within the embayment. Glossifungites icnofacies associated with omission surfaces are recorded during early stages of estuarine fill, reflecting short term interruptions between higher frequency sequences. Protracted subaerial exposure is outlined by rapid shifts toward reddish-purplish colors, pervasive mottling due to water-table fluctuation and development of palustrine root bioturbation patterns. Microfossils (ostracods and foraminifers) and palynological record as well as the opportunistic invertebrate faunas are consistent with a stressful brackish to restricted marine environment but no group, by its own, seems an unequivocal marker. However, minor changes and presence-absence criteria, particularly among the palynological record, seems to pinpoint the moments of complete continentalization between high-frequency marine flooding events. Altogether the transgressive estuary fills in the Loncoche Formation, shows limited lateral continuity and variations in thickness and internal architecture due to the point sourced supply-dominated accommodation and limited dispersal efficiency within the Neuquén embayment. Throughout the succession white-yellowish tuff layers, partly or largely reworked, constitute distinct event-layers derived from the active volcanic arc that have the chance of covering and enhancing preservation of dinosaur tracks. Variety of substrates, early cementation and volcanically influenced environments, altogether may have played an important taphonomic roll in the exceptional preservation of trackways during the dinosaur south-north migration through the narrow early-andean continental bridge across inlets of the Atlantic sea-way developed crosswise northern Patagonia.