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The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre-scale and patchy, dome-like gypsum accumulations are well-exposed within strongly incised present-day river valleys along the eastern margin of the Namibe and Benguela basins (south-west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution-related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin-spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the current at-surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross-cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics. [ABSTRACT FROM AUTHOR]

The evaporites of the Aptian Loémé Formation occur along the stable structural domain of onshore Congo as horizontally well-stratified depositional halite interbedded with Mg-poor potash minerals including carnallite, bischofite, tachyhydrite and sylvite. Eleven correlative depositional recharge-to-evaporation cycles are composed of suites of organic-prone shale followed by primary halite and carnallite beds, sometimes punctuated by a bischofite-tachyhydrite end-member. A sedimentological and petrographical analysis performed on cores and thin sections allowed an interpretation of nine facies associations, named as FA1 to FA9, relying upon the occurrence of bands of fluid inclusions, grain-size, dissolution features, detrital sediments, minute inclusions and sedimentary structures. A depositional model reconciles the spatial distribution of primary textures, hydrology, brine saturation and palaeo-depth during a standard regressive evaporation cycle. Shallow burial eo-diagenesis led to compaction-driven dissolution and recrystallization into fine-grained halite and carnallite laminites but poorly affected the overall preservation of primary deposits. Measurements and profile analysis of bromine trace element concentrations reveal a palaeo-salinity increase of the parent brine for cycle II to base IX caused by a relative brine level fall and a gradual confinement of the salt basin, likely associated with an excess of saline elements inflow from hydrothermal vents. Similar depositional textures and bromine concentration variations are observed in the lower cycles of the Aptian evaporites from the counter-part Sergipe Basin in the northern part of the Brazilian margin, testifying of a progressive brine salinity rise due to a palaeogeographic confinement along the northern segment of the South Atlantic Ocean. In Congo, the sylvite displays horizontal "pseudo-stratifications" that unconformably overlay the crests of anticlines and propagate along flanks as a sharp-based caprock. The flushing and transformation of depositional carnallite – halite into secondary sylvite – halite is interpreted as having been caused by down-stepping incremental dissolutions sourced by younger pervasive connate to phreatic waters. In Sergipe, the sylvite is interpreted as secondary but further results from syndepositional dissolution of carnallite and tachyhydrite due to a regional North Atlantic Ocean ingression. [ABSTRACT FROM AUTHOR]

The Aptian Pre-Salt sedimentary succession cropping out in Cangulo palaeovalley onshore Namibe Basin (Angola) was studied by a combination of field and analytical techniques to constrain the sedimentary and diagenetic evolution of the uppermost sag sequence of the South Atlantic passive margin. Field observations allows definition of four transgressive-regressive cycles characterised by fluvial to tidal-influenced mixed clastic-carbonate and carbonate-dominated deposits, that locally show evidence of evaporite dissolution; highlighting that evaporite deposition started earlier than deposition of the regional South Atlantic Loeme-Bambata evaporite formations. Two separate pre-salt carbonate units have been differentiated within the Cangulo Fm; i) a lower transitional to marginal marine, and ii) a younger upper non-marine freshwater travertine system, that is documented for the first time in the west African margin. Transgressive-regressive cycles control the early diagenesis of the tidal carbonates that include dolomitization due to mixing fluids during transgressions, and karstification due to evaporite dissolution by meteoric water circulation during regressive events. Clastic supply appears to have been completely shut down during carbonate deposition, suggesting major climatic change associated with carbonate deposition. During the lowstand between the two carbonate units, fluid flow through Cangulo palaeovalley was re-established resulting in extensive karstification and formation of a large-scale erosional unconformity that is interpreted to be time equivalent to an intra Chela-Cuvo Fm. event. The top of the studied succession corresponds to the transgressive deposits of the Bambata evaporites that are not preserved in the Cangulo palaeovalley due to its erosion but are regionally developed. The results of this study can be directly linked to along strike age equivalent Pre-Salt successions cropping out in the Namibe, Benguela and Kwanza basins, and directly offshore Angola and Brazil using well and seismic data. These new data shed important new light and constraints on the depositional and diagenetic evolution of the complex Pre-Salt reservoir systems of the South Atlantic, and the depositional and bathymetric setting at the time of onset of the main south Atlantic evaporite deposition. [Display omitted] • This manuscript describes the sedimentology and diagenesis of pre-salt (upper sag) deposits onshore Namibe Basin (Angola). • It includes a detailed study of the pre-salt carbonates including non-marine freshwater travertine systems. • This work presents new paleogeographic reconstructions of the Pre‐Salt succession of the onshore Namibe Basin. • It provides new constraints on the overall setting from the sag to main evaporite deposition period in the South Atlantic. [ABSTRACT FROM AUTHOR]