INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Paleontologic and sedimentologic evidence of forced regression at an active rift margin - the Triassic Los Rastros Formation, Ischigualasto-Villa Unión Basin, Argentina
MANCUSO, ADRIANA CECILIA; PALMA, RICARDO MANUEL; MARSICANO, CLAUDIA; KRAPOVICKAS, VERÓNICA
Congreso; 18 International Sedimentological Congress; 2010
The Triassic Ischigualasto-Villa Unión Basin is a rift basin located in the western margin of Gondwana; itsorigin is associated with the breakup of Pangea during the earliest Triassic. A lacustrine-deltaic succession existsin the Los Rastros Formation of this basin and consists of several coarsening-upward cycles of black shales, siltstones,and fine- to coarse-grained sandstones. Cycles thickness range from 10 to 180 m and are interpreted asinfluenced by tectonic and climatic variation. Only in the northwestern part of the basin, near the fault-boundedmargin, the base of the Los Rastros succession is dominated by deltaic facies (delta front and plain facies) linkedto synchronous marginal palustrine facies, both interbedded with offshore facies. Evidence of subaerial exposureis present in these three facies (Mancuso and Marsicano, 2008; Marsicano et al., 2010). Delta mouth-bars (deltafront facies) show the cross-cutting relationships among the ripple marks, footprints, and invertebrate traces ofthe Scoyenia ichnofacies that suggests a palimpset bedding surface, and some horizons with desiccation mudcracks.After this depositional gap, lake transgression is indicated by delta front and delta plain facies on the topof the delta front track-bearing beds. Laterally, these track-bearing deposits interfinger with several palustrinecarbonate bodies, developed at the end of each cycle. The peloidal and intraclastic textures in these bodies definea granular wackestone that has been described in palustrine carbonates (Alonso Zarza, 2003). Mottled textures inthese wackestones suggest the remobilization of iron as a consequence of changes in the Eh of fluctuating watertables (Freytet, 1973). Evidence for a relative groundwater table fall is revealed by a decrease in water contentwithin the upper wackestone at the time of track making and the presence of desiccation cracks in the overlyingwackestone. Some offshore black shales containing mudcrack are interbedded with delta front and palustrine facies,giving evidence of subaerial exposure. These exposed shales are also covered by offshore black shaleswithout subaerial exposure features. Upsection, the development of thick offshore facies over all these deposits(delta, palustrine and offshore) indicates the onset of a transgression over the lake margin facies. Thus, the LosRastros succession located on the western fault-bounded margin preserves subaerial exposure features in theshoreline facies (delta front and palustrine) as well as in the offshore facies as the lake level fluctuated. Therefore,these subaerial exposure features are related to a relatively rapid withdrawal of the water basinward probablydue to a forced regression perhaps related to rapid pulses of tectonic subsidence during early rifting of basinevolution.