Gênese e evolução da Lagoa Dourada, Ponta Grossa, PR

Abstract

The Lagoa Dourada belongs to the furnas system of the Vila Velha State Park, state of Paraná, southem Brazil. It is a roughly circular depression in the Furnas Sandstone (Devonian of the Paraná Basin), with a diameter of about 200 m and a water column up to 5.4 m deep. It contains at lea9t 12.2 m of sediments of the Holocene and end of the Pleistocene. In the last 12,000 years the Lagoa Dourada has been filled by sands brought by sources of underground waters as well as by mud brought by the floods of the Guabiroba River, a tributary of the upper Tibagi River. The furnas are sinkholes formed in the Furnas Sandstone. Lagoa Dourada is considered to be a silted-up furna, a consequence of invasion by floodwaters of the Guabiroba River. Data from a core sample of the sediments show that the grain size is predominantly silt and clay in the southem and southeastem two thirds, changing to sand in the northem and northwestem third. The occurrence of gypsum and pyrite in the sediments, both authigenic, suggests periods of increase in the water salinity, related to greater evaporation in drier climates. "Cycles" of increase in total carbon rates in the sediments coincide with the increase in the isotopic ratios 13C/ 12C (δ13C), indicating variations in the vegetation, with greater participation of grasses and consequently an increase in δ13C,suggesting trends to drier climatic phases and/or variations in the salinity of the pond water. The sedimentation rates should reach 0.53 mm/year between 11.9 and 10.6 m deep, and 1.22 mm/year above 10.6 m deep. These rates are similar to those of other Holocene flood plain ponds in southeastem Brazil. Lagoa Dourada is older than 11,170 years, as judged from dates of plant remains in the base of the core sample, which did not reach the bedrock. On the other hand, some of the furnas must be not old features, as suggested by furna nº 4 of the Vila Velha State Park, which is placed in the stream bed of a brook, dry downstream from the furna. It can be speculated that the subterranean erosion that originates the furnas has more efficiency during phases of pronounced loweringof the water table, which happen during the drier climates associated to Pleistocene glacial periods.