Abstract

Potential reservoir facies represented by lacustrine shoreline grainstones and rudstones are typically relatively thin compared to those from marine basins because of limited fetch and reduced wave action producing a shallow wave base. This is especially the case in low-gradient endorheic lakes in which rapid lake-level oscillations preclude the development of a stable shoreline. However, the closed lake deposits of the Barra Velha Formation locally have thick (decameter-scale) continuous packages of grainstones and rudstones comprising fragments of crystal shrubs, spherulites, intraclasts, and, in some cases, peloids and volcanic fragments. Grainstones and rudstones of this type occur on the escarpment and dip slopes of tilted fault blocks along which a marked thinning of the Barra Velha Formation is evident. They mainly consist of sharp-based, decimeter-to-meter-scale, fining-upward packages with well-sorted and well-rounded grains, planar/low-angle lamination and, less commonly, cross-lamination and planar cross-bedding. Those that occur on dip slopes are generally finer than those associated with escarpment slopes, the latter also being texturally less mature. At the formation scale, grainstone-dominated successions show radial depositional dip azimuth patterns orientated normal to paleoslope. The grainstones are interpreted as wave-dominated fan-delta shoreline deposits. Although much effort has focused on the origin of the in situ components of the Barra Velha Formation, such as crystal shrub facies, detrital deposits of the type documented here constitute significant potential targets.