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Chronostratigraphy Across a Conjugate Margin Source-to-Sink: UruguaySPAN and NamibiaSPAN—Why Stop at Basement?

Abstract

The continental margins of the southern South Atlantic, particularly the Uruguayan margin, remain underexplored, while the conjugate Namibian rifted margin has begun to reveal significant potential as an exploration province. Potential targets range from near-shore shelf carbonates to deep-water siliciclastics, although hydrocarbons in commercial quantities have yet to be discovered by recent wells. However, all elements of functioning petroleum systems have been encountered by wells drilled on both margins. These conjugate margins were at one point a single basin and so share similar geological histories; the challenge remains to constrain their subsidence histories and how these have influenced sedimentation styles across the margins through time. Utilizing ION's long-offset, long-record 2D seismic data, we take a bottom-up approach to evaluating geochronologic timing of crustal formation, including structure and tectonics that controlled basin (and eventual margin) development. We relate basin fill, sediment and enigmatic seaward-dipping reflector (SDR) chronostratigraphy on each margin to the crustal interpretation. This approach provides unique insight into evolution of Earth's crust at the juncture of continent and ocean, as well as the nature and timing of control on overlying sedimentary basin formation. We reconstruct the regional chronostratigraphy within the sedimentary section as well as within what is conventionally viewed as basement. This includes SDR packages interpreted to link the processes driving magmatic rifting/stretching of the continental crust which lead to the formation of oceanic crust, to sedimentation (extrusion) patterns observed along the rifted margins. Initially we generate a HorizonCube (closely-vertically-spaced horizon interpretation) on offshore 2D seismic profiles from conjugate margins Uruguay and Namibia. The many horizons generated within the HorizonCube are flattened to create a chronostratigraphic “Wheeler”-style representation of sedimentation (including syn-rift volcaniclastic SDRs) along that survey. This full, geochronological representation of the seismic data permits comparative analysis of evolution of both stratigraphic as well as ‘crustal’ architecture. We find that crustal processes active during continental break-up control overlying basin evolution. Constraining the timing of basement evolution then permits predicting formation of key petroleum systems elements across the southern South Atlantic.