Diagenetic Evolution of the Cangulo Formation Pre-Salt Carbonates (Onshore Namibe Basin, Angola)

Abstract

The Cangulo Fm., exposed onshore within the Namibe Basin of Angola, belongs to the Pre-Salt succession, characterized by fluvial to marginal marine sediments deposited during Early Aptian times along the entire West African margin. This unit is composed of two types of carbonate separated by an erosional unconformity. Prevailing carbonate facies below the unconformity are peritidal-like dolostones deposited during a transgressive period. The petrological and geochemical data points to slightly evaporated marine waters responsible for the early dolomitization of these carbonates. In addition, oxidation of organic matter occurred during this time as suggested by the highly depleted values of ¿13C. The erosive surface recorded in the Cangulo Valley is related to a major regressive event. The development of karstic systems affecting the peritidal-like carbonates together with the calcitization of the dolomite cements and Fe-rich mineralization, indicate that meteoric fluids circulated during this period. The drop-off of the phreatic level caused the spring flow onset from basement highs and precipitation of tufa deposits. The ¿18O and ¿13C values together with the Mg/Ca molar ratios of these deposits are coherent with the precipitation from low temperature meteoric waters. The progressively depletion of ¿13C and the decreasing trend of Mg/Ca molar ratio indicate an input of soil-derived CO2 in the system and the transition from phreatic to a vadose diagenetic environment. Tidally-influenced sandstones onlap the tufa deposits. The geochemistry of the cements together with the values from the partially dolomitized tufa carbonates, suggest that mixed marine and meteoric waters were circulating at this time. The end of the transgression is marked by deposition of the Aptian evaporites (Bambata Fm.) representing the maximum transgressive stage in the Cangulo Valley. The tectono-sedimentary and diagenetic reconstruction of the Pre-Salt succession in the Namibe Basin demonstrates a periodic marine influence in the sedimentary record, linked to at least two transgressive events. These are separated by a major forced regression that generated an erosional gap, the consequent drop-off of the phreatic level and the tufa system development. These new data appear to indicate that the latest Pre-Salt succession is not a simple transgressive succession, but rather there is a more complex transgressive-regressive story that needs to be examined in more detail.