Paleoenvironmental reconstruction of microbial mud mound derived boulders from gravity-flow polymictic megabreccias (Visean, SW Spain)

摘要

The Upper Visean outcrops from the Guadiato Valley (Cordoba, SW Spain) provide a well-preserved record of the mud mound factory, which was developed in a mainly siliciclastic synorogenic foreland basin during the oblique sinistral collision of two terranes (Ossa Morena and Central Iberian blocks). The first onset of mud mound development has been recorded as microbial mud mound-derived boulders in polymictic megabreccias as result of strong tectonic activity. The Upper Visean record from the Mississippian central band at Guadiato Valley starts with lower heterolithic units (up to 180 m thick) and shows two major tectonically-controlled cycles: a fining upwards interval (FU) followed by a coarsening upwards interval (CU). These cycles are linked to two active margins with gravelly fan delta development and different source areas. Mud mound-derived boulders occur in the CU interval and are formed by peloidal primary and secondary (reworked) automicrites and allomicrites, showing a diverse faunal and floral assemblage, although never as the main skeletal framebuilders. However, the observed coeval richness in sponges (lyssacinose hexactinellids and non-lithistid demosponges) and the diverse calcareous algae assemblage in mud mound derived boulders are not common in other Visean buildups. The growth cavities display changes in the geopetal relationships between fillings and the secondary cavities containing sand to gravel fillings reflecting a complex pre-boulder and mud mound derived boulder history. Detailed mapping, sampling, stratigraphic and microfacial analyses have allowed the reconstruction of the mud mounds sedimentary environment prior to the collapse, transport and emplacement as boulders with polymictic gravels.