Liliane Janikian

The Ediacaran to Cambrian Rift System of Southeastern South America: Tectonic Implications

The tectonic evolution of southeastern South America from the Middle Ediacaran to the Early Cambrian is marked by a series of small fault‐bounded siliciclastic and volcaniclastic basins and voluminous coeval granites traditionally associated with the compressional or transpressional tectonics of the late stages of the Pan‐African‐Brasiliano orogeny. Most existing models consider these basins separately, with distinct tectonic evolutionary histories according to local geological settings. However, new and recently published age constraints, lithological similarities, and structural aspects point to the correlation of all Ediacaran to Cambrian basins in southeastern South America within a common basin system more than 1500 km long. The interpretation of a common origin for all Ediacaran to Cambrian basins of southeastern South America implies that all the different terranes of the Brasiliano orogenic collage in the region were already united in a single plate at approximately 600 Ma. An extensional origin for this basin system is interpreted from the recognition of basin‐forming normal faults (later reactivated as strike‐slip or inverse) feeding alluvial fans and from expressive basic to acidic volcanic successions in several basins. The occurrence of basic, intermediate, and acidic volcanic rocks and voluminous coeval granites indicates that mantle and crustal fusion were simultaneous with the extensional event. Raised temperatures may have caused the thermal weakening of the lithosphere, enabling both extensional deformation and recurring strike‐slip deformation that formed major shear zones in the region. This strike‐slip deformation has been mistaken for basin‐forming tectonics, but it occurred in the Early Cambrian, after the formation of the basins, and most probably was the result of the far‐field propagation of compressional stresses originating in younger collisional orogens at the plate margins.

Recurring extensional and strike-slip tectonics after the Neoproterozoic collisional events in the southern Mantiqueira province

In Eastern South America, a series of fault-bounded sedimentary basins that crop out from Southern Uruguay to Southeastern Brazil were formed after the main collisional deformation of the Brasiliano Orogeny and record the tectonic events that affected the region from the Middle Ediacaran onwards. We address the problem of discerning the basin-forming tectonics from the later deformational events through paleostress analysis of more than 600 fault-slip data, mainly from the Camaqua Basin (Southern Brazil), sorted by stratigraphic level and cross-cutting relationships of superposed striations, and integrated with available stratigraphic and geochronological data. Our results show that the Camaqua Basin was formed by at least two distinct extensional events, and that rapid paleostress changes took place in the region a few tens of million years after the major collision (c.a. 630 Ma), probably due to the interplay between local active extensional tectonics and the distal effects of the continued amalgamation of plates and terranes at the margins of the still-forming Gondwana Plate. Preliminary paleostress data from the Castro Basin and published data from the Itajai Basin suggest that these events had a regional nature.

Evolução Estratigráfica e paleogeográfica do grupo Santa Bárbara (Ediacarano) na sub-bacia Camaquã Central, RS

As exposicoes do Grupo Santa Barbara (Ediacarano) na Sub-bacia Camaqua Central sao representadas por uma sucessao de depositos arenosos e ritmicos marinhos/transicionais e depositos conglomeraticos de leques aluviais, que compoem ciclos progradacionais-retrogradacionais. Nesta sub-bacia, o Grupo Santa Barbara apresenta mais de 2.000 m de espessura, com excelentes exposicoes nas regioes das Minas do Camaqua e de Bom Jardim. Estudos estratigraficos de paleoambientes, paleocorrentes e de proveniencia realizados nestas regioes possibilitaram a seguinte subdivisao do Grupo Santa Barbara na Sub-bacia Camaqua Central, a partir da base: 1. Formacao Passo da Capela: unidade areno-siltitica subdividida em turbiditos distais de leque subaquoso (Outer-fan), e turbiditos proximais a distais de leque subaquoso (Inner-Fan a Outer-fan) e tempestitos de costa-afora (offshore); 2. Formacao Seival: arenitos e siltitos de depositos litorâneos e de planicie de mares; 3. Formacao Rincao do Mouras: unidade areno-conglomeratica formada por depositos de leques aluviais e de sistemas fluviais entrelacados e 4. Formacao Joao Dias: composta principalmente por arenitos interpretados como depositos litorâneos de antepraia e de face litorânea e tempestitos de face litorânea. Estes estudos permitiram ainda o reconhecimento de uma notavel correlacao entre as unidades encontradas nestas regioes legitimando, desta forma, a proposta de englobar os depositos da regiao das Minas do Camaqua, classicamente relacionadas a Formacao Arroio dos Nobres, no redefinido Grupo Santa Barbara.

A Formação Sete Lagoas em sua área-tipo: fácies, estratigrafia e

Detailed facies analysis and stratigrafic measurement, description and interpretation of 12 quarry and road-cut sections across a 80.5 km long transect lead to recognition of 11 sedimentary facies of the Sete Lagoas Formation, 2 facies of Carrancas Conglomerado and 3 facies at the base of the Santa Helena Formation. Such facies are comprised into 9 facies associations regarding 3 depositional sequences. The described stratigraphic sequences are characterized by transgressive and higstand systems tracts, with minor preservation of one lowstand system tract, being the first two correlated with the carbonatic Sete Lagoas Formation and the last to the overlying siliciclastic Serra de Santa Helena Formation. The stratigraphic distribution of facies associations reveals an evolution from a CaCO 3 oversaturated carbonate ramp to a storm dominated ramp that progressively becomes distally steepened, and finely a clastic-dominated margin in the last sequence. The geographic distribution of facies associations reveals a transition from shallow water environments in the West to deeper water environments towards the East of the studied area.