The third order sequence stratigraphy of the Itararé Subgroup (late Paleozoic) across Paraná Basin

THE THIRD ORDER SEQUENCE STRATIGRAPHY OF THE ITARARÉ SUBGROUP (LATE PALEOZOIC) ACROSS PARANÁ BASIN^*

JOSÉ R. CANUTO^**, PAULO R. SANTOS AND ANTONIO C. ROCHA-CAMPOS

Instituto de Geociências, USP, São Paulo, SP.

The Itararé Subgroup (late Paleozoic) on the eastern margin of the Paraná Basin is subdivided into seven 50-80m thick, type I, third order sequences that correspond to fining-coarsening upward cycles. Sequences are bounded at base by an erosional unconformity that at the eastern margin of the basin is marked either by glacially striated and/or glaciotectonised bedrock, or intraformational, glacially striated surfaces.

Our depositional model considers that each erosional surface, usually overlain by subglacial tillite, records an advance of a grounded glacier. Higher up the glaciogenic section includes glacioterrestrial, glaciomarine and marine beds (tillites, flow diamictites, proglacial rhythmites with dropstones, outwash sandstone, etc.), deposited during retreat of the ice margin. The sequences culminate with marine shale documenting short-lived post-glacial transgression, and shallow-marine progradacional fluviatile-deltaic sandstone, associated to glacio-isostatic rebound that followed retreat of the glacial margin.

This type of sequence was earlier identified in outcrop and is now recognized in the subsurface of the Paraná Basin, along an east-west cross-section trough wells 1-PT-1PR, 1-RO-1-PR, 2-NA-1-PR and AMst-1-MT in Brazil, and Asunción 2 in Paraguay. Correlation of sequences in subsurface and outcrop was established lithostratigraphically from top to bottom, with reference to a datum represented by a widespread marine marker bed intercalated in the Rio Bonito Formation. Absence of basal S1 sequence in well 1-RO-1-PR may be related to its location on a basement high. Only the uppermost part of S1 is recorded in well AMst-1-MT due to the intercalation of a thick diabase sill (Cretaceous) between Itararé beds and the underlying Ponta Grossa Formation.

With exception of the two cases, the subsurface section shows a full sequential correspondence. Most sequences, however, thicken from SE-NW, almost doubling values found on the eastern margin of the Paraná Basin. This suggests higher subsidence rate towards west, that compensated the positive tectonic behavior of the adjacent Asunción arch and created adequate space to accommodate sediments supplied from the eastern flank of the Asunción arch. — (December 14, 2001).

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002