Counting rational plane curves

Abstract

The Passa Dois Group is divided into formations and members, representing variations of lithofacies separated by unconformities or correlative conformities.The contact between the Passa Dois and Tubarão Groups is represented by a paraconformity, and the contact between the Passa Dois and São Bento Groups corresponds to a regional unconformity or correlative conformity.Intracratonic basins differ physiographically from passive margin basins, for which the concepts of Sequence Stratigraphy were originally developed; so some adjustment has to be done for the application of the theory.The 3rd order chronostratigraphic framework for the Passa Dois Group begins by offshore marine sedimentation of the Taquaral Member upon the coastal marine regressive sediments of the upper part of the Palermo Formation, through a conformable contact representing a transgressive surface. After this, a highstand phase prevails, bounded at its top by the maximum flooding surface, that is coincident with the lower conformable contact of the Assistência Member, which represents a new lowstand phase. Sea level rises again, through transgression, depositing the offshore sediments of the Serra Alta Formation. Deposition of the sediments of the Teresina Formation follows, suggesting a new lowstand phase with wave and tidal structures, than passes to the Serrinha Member of deeper waters with hummocky structures indicating a new highstand phase. A new lowstand phase occurs with well-sorted sandstones showing cross-stratification with medium to high angles, with clay drapes, suggesting the recurrence of tidal conditions, corresponding to the base of the Morro Pelado Member, that passes, by transition, to continental deposits, bounded at their top by a regional unconformity or a correlative conformity considered between the Passa Dois and the São Bento Groups, corresponding to a 2nd order sequence.The thicknesses of each pair of units, deposited in shallower waters at its base and by deeper waters at its top, do not exceed 500 m. Considering a rate of deposition of the order of 1 m per each 10,000 years, they correspond to 3rd order sequences. The bounding surfaces that separate the sequences are of conformable type 2. Depositional systems characteristic of steeper slopes, or of floor basin fans or slope fans, that are common in sequences of type 1 are not observed. The duration of deposition of the related pairs varied from 1 to 10 M.a., well within the known duration of stages, which strengthens the hypothesis that they represent 3rd order sequences. - ( December 10, 1999 ) .