Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician-Silurian boundary and early Silurian deposits from Argentina

ESTEFANIA ASURMENDI; SANCHEZ MARIA LIDIA; SUSANA, HEREDIA

GEOLOGICAL JOURNAL (CHICHESTER)

JOHN WILEY & SONS LTD

Lugar: LOndres; Año: 2018

0072-1050

The La Chilca Formation is exposed at the Poblete Norte and Ancha sections, Talacasto, Central Precordillera. Detailed facies analysis on field outcrops suggest a tide-dominated tidal plain model on the Silurian siliciclastic ramp, integrated by supra- to subtidal plain deposits. The lower part of the La Chilca Formation (upper supratidal plain) is herein attributed to reworked glaciomarine deposits that occurred during interglacial Hirnantian age, under conditions of sea rise and the action of tidal amplification. The inter-supratidal depositional environments are represented by tidal channel, scours and fill, tidal dunes and hummocky cross-stratified sandstones. The estimated tidal conditions for the tidal plain was meso- to macrotidal, under action of combined flows, attributed to the interaction of tidal currents and orbital low-velocity wave. The identification of MISS (Microbially Induced Sedimentary Structures) in these deposits, such as petee polygonal structures, pitted texture, imploded volcanoes sand, Astropolithon sp., Manchuriophycus sp., domal buildups, biofilms, elephant skin, and trace fossils such as, Chondrites sp., Zoophycos sp., Undichnia sp., Planolites sp., Monomorphichnus sp. and gastropod traces may have played a protection and stabilization role on the clastic sediment surfaces against erosion of the La Chilca Formation environment. The prolific growth of microbial mat on the intertidal to supratidal zone occurs explicitly at the turning points of transgression?regression cycles in shallow environments under stable base-level conditions. The medium-upper section of the La Chilca Formation shows non-cyclic rhythmites that are replaced by the cyclic rhythmites, suggesting an open tidal plain subject to an orbital/climatic rather than a tectonic control of cyclicity (Milankovitch cycles). The aggradation in this section indicates a shallowing towards the top of the formation. The La Chilca Formation records two transgressive events of the third order and a shallowing event. The transgressive events are found in the base and top of the formation and are marked by the presence of oolites, while the shallowing event is determined by the presence of MISS.