Implications of preliminary subsidence analyses for the Parnaíba cratonic basin

Cratonic sedimentary basins are underlain by thick lithosphere and subside over hundreds of millions of years. The driving mechanism for these basins remains enigmatic due to the paucity of crustal-scale constraints. To address this shortcoming, an integrative study of the Parnaiba cratonic basin of NE Brazil is underway. We summarize field observations, well-log information, seismic reflection profiles and subsidence analyses with the objective of shedding light on possible basin-forming mechanisms. Lithological descriptions and borehole logs reveal a stratigraphic succession that consists of interbedded shallow marine and continental deposits. On seismic reflection profiles, this succession is divided into packages of relatively undisturbed reflections bounded by basin-wide disconformities. These disconformities are manifest as bright, rugose reflections that probably represent buried palaeo-landscapes. Backstripped and water-loaded subsidence curves calculated from boreholes distributed throughout the basin show that tectonic subsidence decreases exponentially over c. 350 Ma. A simple model suggests that this subsidence pattern agrees with a thermal time constant of 70–100 Ma, placing a significant constraint on the likely basin-forming mechanism. This background trend is punctuated by minor increases in subsidence that follow basin-wide erosional disconformities. These departures could be indicative of transient epeirogenic uplift events caused by changing patterns of dynamic topography.