ALBERT

Description: The processes that govern formation and development of cratonic basins are poorly understood, both individually and as a class. The cratonic Silurian–Jurassic Parnaíba Basin in NE Brazil is less well-studied than North American examples such as Williston, Illinois and Michigan but offers an opportunity to study stratal architecture both in outcrop and the subsurface. Published stratigraphic compilations emphasize basin-wide unconformities separating layer-cake, basin-wide packages; analysis of geological maps indicates this interpretation is over-simplified, with at least 12 local unconformities, concentrated along the SW basin margin. Comparison of basin-margin and intrabasinal well-correlation panels shows that unconformities are more common in marginal areas, so the current exposure margins may have some validity as near-original margins. Palaeocurrents suggest a consistent SW–NE transport direction for the Serra Grande and lower Canindé groups. Supporting heavy mineral analysis from Silurian–Carboniferous strata on the SW and NE basin margins show that assemblages are dominated by ultra-stable zircon, tourmaline and rutile with minor amounts of staurolite, especially in the SW and older units. Tourmaline varietal results are remarkably consistent across the basin. U–Pb age spectra from detrital zircons are dominated by Neoproterozoic grains, with subsidiary Meso-Paleoproterozoic provenance. We infer that the source terrane was a medium-grade regionally metamorphosed mica schist, probably the Araguaia Fold Belt. These results are indicative of a large sediment routing system feeding material across an evolving crustal sag, analogous to observations from North American cratonic basins; this suggests that cratonic basins may not have strongly shaped or controlled the routing system. Supplementary material: High-resolution geological map of the Parnaíba Basin as a PDF. This shows all of the basement units and small details of the stratigraphic relationships between different basin-fill formations. This map is derived from the ArcGIS files of CPRM (2004). Supplementary material available at https://doi.org/10.6084/m9.figshare.c.4183472