Trace fossils from a middle Eocene tide-dominated estuary, Man Aike Formation of Patagonia,

NADINE PEARSON, M. GABRIELA MÁNGANO, MARTÍN RODRÍGUEZ RAISING, SILVIO CASADÍO Y LUIS A. BUATOIS

Cracow, Polonia

Congreso; Second International Congress on Ichnology; 2008

The middle Eocene Main Aike Formation of Patagonia was formed in a tide-dominated estuary incised into marine deposits of the underlying Upper Cretaceous Calafate Fomation. The interplay of tectonism and sea-level fall created significant accommodation space that was filled mostly during the subsequent transgression. Pebble conglomerate and very coarse- to coarse-grained sandstone occur along the valley axis representing deposition in low-sinuosity rivers, characteristic of the lowstand systems tract. Above these fluvial deposits and lateral to valley axis, a thick unit of pebbly sandstone and medium- to very coarse-grained sandstone with abundant marine fossil fragments is present. This interval records deposition in sandbars and channels mostly formed at the estuary mouth, representing the transgressive systems tract. Trace fossils are relatively common in the estuary-mouth complex, but intensity of bioturbation and ichnodiversity levels remain low. The ichnofauna is dominated by robust crustacean burrows (Thalassinoides). The complex trace fossil Patagonichnus, as well as simple facies-crossing forms, such as Palaeophycus and Planolites, is also abundant. Asterosoma and Rosselia are subordinate and apparently restricted to certain horizons. Ophiomorpha is relatively rare. The presence of Patagonichnus in the Man Aike Formation is consistent with previous occurrences, because this ichnogenus has been documented in tidal-channel deposits formed in the outer regions of estuarine systems in other regions of Patagonia and Antarctica. Overall, the Man Aike ichnofauna displays most of the characteristics of brackish-water trace-fossil assemblages, namely low ichnodiversity, forms typically found in marine environments, dominance of infaunal traces rather than epifaunal trails, mixture of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, abundance of some ichnotaxa, and presence of monospecific suites. However, size reduction has not been observed, and some of the structures (e.g. Patagonichnus) are extremely complex, revealing climax strategies rather than opportunistic behavior by trophic generalists. This may reflect less stressed environmental conditions typical of the estuary mouth. An increase in the degree of bioturbation is apparent towards the top of the formation, suggesting a transition to open-marine environments during the late phase of the middle Eocene transgression.