Geological map of part of the Arroio América Sheet - SH-22-Y-A-IV-1 (MI 2995/1) – Explanatory note

Abstract

This explanatory note refers to the geological map of part of the Arroio América Sheet (SH.22-Y-AIV-1 / MI 2995/1) and is based on the results obtained during the Arroio América Project, carried out in 2003 by professors and undergraduate students of the Geology Course, Federal University of Rio Grande do Sul. Seve ral lithostratigraphic units were recognized, some of which are newly-defined and formally named. The main large-scale structure recognized and named is the Arroio América Shear Zone (ZCAA), composed of several NE-trending, subvertical mylonitic zones of sinistral transcurrent kinematics given by shallow-plunging stretching lineation. Brittle reactivations of old trends are significant, as well as NW-trending brittle structures which affect basement and sedimentary rocks from Neoproterozoic to Permian age. The oldest basement sequences are orthogneisses of the Cambaí Complex, and supracrustal rocks of the Arroio Marmeleiro Metamorphic Complex (CMAM). The Arroio Branquilho Diorites form a newly-defined unit of foliated biotite-hornblende diorites and quartz-diorites and comprise two co-genetic subunits - the medium-grained equigranular facies, and the fine-grained facies. Structural relations indicate that their emplacement predates the ZCAA. The Arroio dos Carros Tonalites (TAC) and Rincão dos Encerrados Granitoids (GRE) are also newly-defined units, correlated to the Lagoa da Meia Lua Suite. The TAC comprises medium- to coarse-grained, foliated hornblende tonalites, and their magmatic flow foliation grades into a mylonitic one due to its syntectonic character relative to the ZCAA. The GRE are equigranular, fine to medium-grained biotite tonalites to granodiorites which are late-tectonic relative to the same shear zone. The Passo da Chácara Granite, also newly-defined, is the youngest granitic unit, and comprises weakly-foliated to massive biotite leucosyenogranites. The Maricá Formation is composed of lithic arkosean sandstones with subordinate interleaving of shale and conglomerates. Local evidences of anchimetamorphic conditions and slaty cleavage in pelitic compositions are found. Two facies units are described, the first resulting from a braided river system and the second formed in shallow marine conditions. The Tapera Monzonite, as well as Hilário, Cerro da Pedra and Acampamento Velho formations are sub-units of the Bom Jardim Group. In the Tapera Monzonite, four different facies are recognized, based on compositional and textural characteristics: monzonitic plutonic, leucodioritic plutonic, dioritic plutonic and hypoabyssal monzonitic facies. In the enclosing CMAM rocks, contact metamorphism of hornblende-hornfels conditions is described. The Hilário Formation comprises andesitic flows, as well as andesitic and lamprophyric dikes. The Cerro da Pedra Formation is presently defined to describe conglomerate packs formed in alluvial-fan systems. A proximal facies is recognized as oligomitic para- and orthoconglomerates, while a distal facies is represented by polimitic orthoconglomerates. The Acampamento Velho Formation is represented by a rhyolitic lava-dome and related dikes. The Permian Rio do Sul Formation, belonging to the Itararé Group, is restricted to the northern extreme of the mapped area, where fine-grained sandstone layers are interleaved with siltstones.