GEOLOGY OF THE FRONTEIRA OESTE RIFT, SOUTHERNMOST BRAZIL – A FIELD GUIDE

Abstract

The Fronteira Oeste Rift located in the state of Rio Grande do Sul, southern Brazil, covers an area of 15,000 km2 and constitutes both a rhombohedral structural province and a paleohot spring field. These major features can be observed during a two-day field trip. A 4x4 vehicle is recommended. The weather is cold (freezing), humid climate during winter and hot (35 °C), varying from humid to dry during summer-. Excellent access to the outcrops is possible through paved and unpaved roads. Outcrops described in this field guide are stratigraphically located above the Botucatu Formation (paleoerg) and below the Serra Geral Group (volcanics) of the Paraná Basin. Sandstones of the Botucatu Formation were quickly filled by rainwater and after covered by a basalt (Mata Olho Flow) and a quartz andesite (Catalán Flow), thus originating the Guarani Paleoaquifer. The third Cordillera Flow (basaltic andesite) occurs in places, eventually covered by the Muralha Flow (basaltic andesite). Two additional basaltic andesites (UR13 and Coxilha Flows) were described in a drill hole. Aquifer heating by raised geothermal gradient led to pre-compaction cementation of sandstones by iron-rich minerals. Eolian quartz-rich sediments were deformed and hydrothermally altered in the composite paleodunes, showing thousands of circular, elliptical and curvilinear structures. Compaction caused upward motion of hot water + liquefied sand leading to the formation of many sand injectites and extrudites. Simultaneous doming of overlying rocks caused both large- and small-scale rhombi in sandstones and volcanic rocks. Post-compaction silicification occurred together with the formation of a myriad of 0.1-1.0 m-large hydrothermal vents at the surface of the paleoerg and also the first three lava flows. Features resulting from these processes can be visited, as described in this Geological Field Guide. A shorter geotrail is also described.