The Pre-Silurian Riachao Basin: A New Perspective On The Basement Of The Parnaiba Basin, Ne Brazil

We present a new interpretation of the basement immediately below the pre-Silurian unconformity in the mid-western part of the Parnaiba cratonic basin. Regional seismic reflection data, constrained by well and gravity data, have revealed the remnants of a pre-Silurian basin named the Riachao basin. It has a north-south trend and is about 120 km wide with a minimum area of c. 35 000 km(2) and a maximum thickness of 4 km. Seismic interpretation shows the Riachao basin is defined by erosional unconformities at its top (the pre-Silurian unconformity) and base (the top of the pre-Riachao basement). The basin fill is formed of three stratigraphic sequences - Riachao I, II and III - separated by seismic discontinuities and laterally bounded by monoclinal folds and basement uplifts related to thrust faults. Exploration wells show that the Riachao sediments are Ediacaran to Lower Cambrian, clastic and a minimum of 1 km thick, unconformably overlying a heterogeneous metamorphic basement. The mechanism of formation of the basin is unclear. However, our seismic stratigraphic interpretation suggests that Riachao II and III are remnants of marine, clastic, foreland basin sequences of opposed polarity, related to late Neoproterozoic tectonics. The banded, high-amplitude, Riachao I sequence is interpreted as a carbonate platform preceding the main subsidence and clastic input of the Riachao basin.