FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Ichnology ox mixed-carbonate siliciclastic tidal flats, Lower Cretaceous, Neuquén Basin, Argentina
Congreso; Ichnia, Second International Congress on Ichnology; 2008
Tidal flats are complex environments where the interplay of contrasting sedimentary processes takes places. They usually occur in low gradient coast 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 paleocurrent pattern, salinity fluctuations, turbidity, between other physical parameters. The analysis of several localities oat the top of the Agrio Formation (Lower Cretaceous) permitted to identify an internal facies arrangemetn. It marks a change in paleontological content and sedimentologial features with respect to the downward stratigraphic section of the unit. For instance, typical marine fauna that characterizise a normal marine platform disappear and it is replaced mainly by gastropods, briozoan and reworwed bioclasts and strong tidal influence dominates. Facies distribution exhibits rapid lateral shifting between close localities. In general subtidal, intertidal and less common supratidal deposits are distinghishable and arranged in a very repetitive cyclicity. Subtidal facies are coarbonatic and flood dominated,  mainly composed of oolitic grainstones, containing reworked bioclasts (mainly gastropods) incrusted by bryozoans, oolitic bars, chanellised deposits with herringbone and cross stratification with reactivation surfaces or  siliciclastic and containing hiprpicnical deposits in deeper areas. Ichnologically this facies is depauperated in carbonatic areas with J-shape tubes, as result of stressful condition and loose substrates or present a record similar to intertidal examples 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 exhibits lateral accretion structures, and sometimes bipolar palaeocurrents pattern 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, arthropods ploughing marks are very frequent. The ichnocasamblege resemble a mixing of typical marine with nonmarine ichnogenera. Mud cracks are also present and flat shaped ripples and syneresis cracks indicates exposition and salinity fluctuations.  Soupy substrates and firm ones coexist in this interval. Hillichnus is a special ichnogenera 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 theropod tracks, like laminites indicates 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 a part of earliest stages of a transgressive cycle where clastic influx was restricted.