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The volcano-sedimentary Picada das Graças Alloformation is the top of the Ediacaran Bom Jardim Allogroup in the Rio Grande do Sul State, southernmost Brazil. It was firstly distinguished at the Caçapava do Sul region. It is suggested to be found also at the Passo da Areia locality at Lavras do Sul and Arroio Carajás map-sheets. The sequence overlays the volcanic Hilário Alloformation (∼593-586 Ma) and underlies the Acampamento Velho Alloformation (∼550 Ma) of the Cerro do Bugio Allogroup.
The Bom Jardim Allogroup is part of the Camaquã Supergroup, a thick pile of molassic volcanic and volcanosedimentary rocks which occur in the Rio Grande do Sul Shield. The Camaquã Supergroup was deposited into a fault-bounded molassic basins of Camaquã and Santa Bárbara, during the final stage of the Brasiliano Orogenic Cycle.
Ten distinct lithofacies (A-J) were distinguished in the Picada das Graças Alloformation at Lavras do Sul:
Facies A: Oligomictic conglomerates, representing a proximal part of an alluvial fan, deposited by debris-flows;
Facies B: Polymictic conglomerates, pebbly sandstones and coarse-grained sandstones, featuring an intermediate part of a alluvial-fan;
Facies C: Represents a distal part of an alluvial-fan, consisting of sandstones and lenses of shale. Facies B and C have features that indicate deposition in a braided river system;
Facies D: Pebbly mudstones and peperites. These diamictites were probably deposited by hot lahars in an alluvial fan-lacustrine environment;
Facies E: Rhythmically intercalated fine-grained sandstones, siltites and argillites, indicating a deltaic-lacustrine environment;
Facies F: Laminated silt- and mudstones, evidencing the deepest part of a lacustrine basin.
Facies G: Grey diamictite, with angulous to subrounded volcanic clasts, deposited on a rhyolitic bed, evidencing deposition by debris-flow and/or glacial process;
Facies H: Interlayered sandstone, shale, and conglomerate, showing alternation of depositional energy in a fluvial environment ;
Facies I: Grey, rhythmic shales, evidencing flooding and stagnant, subaquous conditions in a lacustrine setting. Shales have some linear feetures suggesting ichnofossils or tool marks;
Facies J: Interlayered shales and basalts, revealing coeval volcanism.
The provenance of the sediments was mainly from the underlying volcanic Hilário Formation. Facies D-F are unfossiliferous, others were not yet investigated. A conspicuous feature of facies E and F is the presence of lonestones immersed in rhythmic shales, including a bullet boulder, that suggest dropstones or volcanic ejecta. Its four diamictite levels may have been deposited by debris-flow and/or within a glacial influence.
The sequence represents an alluvial fan-delta system, strongly controlled by faulting and volcanism at source area, with a probable glacial influence. Based on its age and characteristics, it is tentatively correlated with the Gaskiers or Moelv glaciation in the form of probable local, ice-clad volcanoes.
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