Subaqueous Gravity Processes on the Front of Cretaceous Deltas, Recôncavo Basin, Brazil
Delta-front sedimentation dominated deposition of the Cretaceous Pitanga Member of the Candeias Formation and the Caruacu Beds of the Marfim Formation, Reconcavo Basin, Brazil. The Pitanga Member is subdivided into three sedimentary facies according to differences in lithology and sedimentary structures, whereas the Caruacu Beds are subdivided into ten such facies. The three sedimentary facies of the Pitanga Member are (1) Disrupted Sandstone-Mudstone Facies, containing slump fragments, pull-aparts, and small-scale faults and folds deposited by slumping and sliding as a cohesive sediment; (2) Massive Sandstone Facies, poorly sorted sandstones containing dispersed clasts of coarse sand, granules, and mudstone blocks deposited by mass flow and slurrying during cohesion loss; and (3) Dish-structured Sandstone Facies; poorly sorted sandstones with dish structures and steplike load structures deposited by grain flow. The ten facies of the Caruacu Beds were deposited in distributary channels (Quartzose Sandstone Facies; Micro–cross-laminated Facies), interdistributary bays (Parallel-laminated Mudstone Facies), delta fronts and delta-front troughs by subaqueous slumping as cohesive sediment (Disturbed Sandstone-Mudstone Facies, Pebbly Mudstone Facies), mass flowand slurrying due to cohesion loss (Massive Sandstone Facies), and grain flow (Dish-structured Sandstone Facies, Conglomerate Facies). These facies can be grouped into a delta-plain association, a high delta-front association (slump dominated) and a low delta-front trough association (massflow, slurry, and grain-flow dominated). Their areal distribution indicates both southward progradation of the deltas and repeated slumping. Triggering mechanisms of subaqueous slumping, mass flow and grain flow in this example include growth faulting, slope instability in response to local diapiric upwarp, and loading of water-saturated muddy sediments with overlying sand and gravel transported over short distances by high-gradient streams. Deposition occurred predominantly in delta-front troughs.