STRATIGRAPHY OFPARANÁ VOLCANICPROVINCE FLOWS IN WESTERN RIO GRANDE DO SUL, BRAZIL, BASED ON BOREHOLES, GAMMA SPECTROMETRIC LOGGING AND FIELD GEOLOGY

Abstract

The Serra Geral Formation has a wide distribution in the western region of Rio Grande do Sul (200 x 200 km) and consists of six tholeiitic lava flows which constitute the Alegrete facies. This number of lava flows was evidenced through geological mapping integrated with lithological and scintillometric description of 19 boreholes drilled for groundwater by the Geological Survey of Brazil (CPRM) in the years 1983-1986. The general thickness of the Alegrete facies is <100 m, locally reaching 300 m in the borehole UR-13. The Botucatu desert dunes were partially buried by the Mata Olho (basalt) flow, followed by the Catalán (andesite), Cordillera, Muralha, UR-13 and Coxilha (basaltic andesites) flows. The scintillometric measurements are typical for each one of lava flows, varying between 45-120 cps and act as a stratigraphic guide, specially when integrated with field work and lithochemistry. This study enables the systematic geological mapping of each lava flow on a regional scale, with great significance for the comprehension of the geology of the Serra Geral Formation.