Icnofósseis das Formações Tamengo e Guaicurus, Grupo Corumbá: implicações paleoambientais e paleoecológicasGICAS

Abstract

Ichnofossils are sedimentary structures of biological origin directly associated with the behavior of the organism that produced them. Using specific tools between sedimentology and biology, it is possible to recognize the producer, its behavior, environmental factors that influenced the activities, or the impact that these activities caused through bioturbation. The objective of this work is to identify trace fossils from the Tamengo and Guaicurus formations, Corumbá Group, analyze the stratigraphic distribution of trace fossil associations, their link with recognized sedimentary facies and determine their influences and limitations. The Tamengo Formation presents intercalation of carbonate and siliciclastics, facies attributed to the upper Ediacaran, and holds records of metazoans, macroalgae, microfossils and ichnofossils. The Guaicurus Formation corresponds to the top of the Corumbá Group, it has homogeneous pelitic sedimentation with the record of macroalgae, microbial mats and ichnofossils. From analyzes with hand samples, petrographic slides, computerized microtomography scans and energy dispersive spectroscopy, it was possible to obtain data related to morphology, bioturbation index and chemical composition of trace fossils and microbial mats. Three ichnotaxons linked to locomotion and feeding activities were recognized, Planolites isp. nov. and Palaeophycus tubularis that characterize fodinichnia and domichnia ethologies and Multina minima with pascichnia ethologies. The bedding plane bioturbation is low, with a maximum value of 10%, and the bioturbation index in the laminations is level 2, the most common for this period. Sub-horizontal to slightly vertical trace fossils were found, indicating the beginning of colonization in infaunal ecospaces. EDS analyzes on trace fossils indicate a high value of Fe and C in the filling and pelitic film compared to the host rock. Microbial mats were found only in the Guaicurus Formation where the bioturbation is lower. The association of mats with ichnofossils is an indication of the preference of excavations in these places, exerting a control over the preservation of metazoans. The combination of the higher degree of bioturbation and hydrodynamic flow in the Tamengo Formation prevents the development of mats and the ability to live in community with the organisms, different from the Guaicurus Formation, where the depositional environment was predominantly calmer.