Caracterização de moluscos em sistemas deposicionais do tipo laguna-barreira pleistocênicos no sul da planície costeira do Rio Grande do Sul: Uma ferramenta para interpretações estratigráficas e paleoambientais

Abstract

In the southern sector of the Coastal Plain of Rio Grande do Sul (PCRS), between Lagoa dos Patos estuary and the Chuí Stream mouth, three of the four lagoon-barrier depositional systems are well preserved which form the PCRS: the Pleistocene II and III and the Holocene IV. These systems consist of marine, eolic, lagunar and fluvial deposits, and also paleosols. The marine deposits of the Pleistocene systems are often covered by thick eolic facies. For this reason, such deposits are accessed only through limited outcrops and/or recovery of core samples and samples of drilling. Recently, several drills have been made in the southern portion of PCRS, in Santa Vitória do Palmar and Chuí towns, as part of installation of the Complexo Eólico Campos Neutrais. These drills have recovered fossil shells of marine molluscs in subsurface layers, at depths ranging from 5 to 15 meters. In geomorphological terms, the drilling area corresponds to the Lagoon-Barrier System III. Stratigraphically, it is suggested that these drills penetrate marine facies of Barriers II and III. This work aims to chacracterize the fossil remains in order to contribute to a better understanding of the coastal evolution of this sector of the PCRS. For this, purposes were analized samples collected in drillings and SPT (Standard Penetration Test) data intended for foundations of eolic towers. These analyzes included fossils taxonomic identification, taphonomic and paleoecological characterization, as well as analysis of granulometric and compaction data of SPT drill. The taxonomic composition suggests an upper shoreface environment and demonstrate a higher sea level during the Pleistocene, under warmer climatic conditions than today in Rio Grande do Sul coast. The taphonomic analyzes allow the following conclusions: 1) Most of the specimens remained exposed above the sediment-water interface for a short period of time, and 2) Shortly after the death and disarticulation, the organisms were buried at a sufficient depth (below the Taphonomic Active Zone, or TAZ), where have not undergone of dissolution, abrasion and fragmentation processes or action of bioerosive agents. A taxonomic, taphonomic and paleoecological characterization of fossils allied to SPT drilling data proved to be a very useful tool for stratigraphic interpretations and may contribute to a better understanding of the evolution of Pleistocene depositional systems in the southern end of the PCRS.