Ichnology of mixed carbonate-siliciclastic tidal flats, Lower Cretaceous, Neuquen Basin, Argentina.

PAZOS, P; FERNANDEZ, D; LAZO, D; TUNIK, M; MARSICANO, C; AGUIRRE-URRETA, M.B

Krakovia

Conferencia; II ICHNIA. 2008; 2008

Tidal flats are complex environments where the interplay of contrasting sedimentary processes takes place. They usually occur in low gradient coasts with dominant tidal action, wave influence and variable fluvial discharges. The interplay of tidal currents with fluvial discharges reinforce or reduce flood or ebb controls but also will imprint the palaeocurrent pattern, salinity fluctuations, turbidity, among other physical parameters. The analysis of several outcrops which correspond to the top of the Agrio Formation (Lower Cretaceous) allowed identifying a particular internal facies arrangement. It marks a change in the palaeontological content and sedimentological features in relation to the downward stratigraphic section of the unit. For instance, typical marine fauna that characterises a normal marine platform disappears and is replaced mainly by gastropods, bryozoan and reworked bioclasts, and a strong tidal influence dominates. Facies distribution exhibits rapid lateral shifting between nearby localities. In general subtidal, intertidal and less common supratidal deposits are distinguishable and arranged in a very repetitive cyclicity. Subtidal facies are carbonatic and flood dominated,  mainly composed of oolitic grainstones, with reworked bioclasts (mainly gastropods) incrusted by bryozoans; oolitic bars, chanellised deposits with herringbone and cross stratification with reactivation surfaces or siliciclastic containing hiperpicnical deposits are present in the deeper areas. Ichnologically this facies is depauperated in the carbonatic areas with J-shaped tubes, as the result of stressful conditions and loose substrates, or they present a record similar to intertidal cases in the siliciclastic examples. Intertidal facies are more variable but characterised for a strong heterolithic alternation of rippled beds and channels sometimes with IHS structures. Channels exhibit lateral accretion structures, and sometimes bipolar palaeocurrent patterns while intertidal facies are, in general, fining upward that confirm the typical prograding tidal stacking pattern. Ichnologically, this interval is very rich, particularly the non-channelized deposits, and contains a complex cross-cutting record evidencing the shallowing upward trend. Chondrites intricatus, Rhizocorallium jenense, Ophiomorpha igen., Teichichnus rectus, Arenicolites, Helminthoidichnites, Gordia marina, Treptichnus pollardi, and arthropods ploughing marks are very frequent. The ichnoassemblage resembles a mix of typical marine with nonmarine ichnogenera. Mud cracks are also present, and flat shaped ripples and syneresis cracks indicate exposition and salinity fluctuations.  Soupy substrates and firm ones coexist in this interval. Hillichnus is a special ichnogenus related to a sandy and ebb dominated interval accompanied by Lockeia igen., Gyrochorte comosa and Ophiomorpha. Carbonates in all cases are related to the transgressive part of each cycle, regardless the presence of theropod tracks which, like laminites, indicate the earliest stages of transgression. Substrate consistency is related to early dolomitization process. Laminites may appear immediately overlying siliciclastic intertidal flats.  In our scheme they are part of the earliest stages of a transgressive cycle where clastic influx was restricted.