Evaluating the segmented post-rift stratigraphic architecture of the Guyanas continental margin

Abstract

Segmentation of the Guyanas continental margin of South America is inherited from the dual-phase Mesozoic rifting history controlling the first-order post-rift sedimentary architecture. The margin is divided into two segments by a transform marginal plateau (TMP), the Demerara Rise, into the Central and Equatorial Atlantic domains. This paper investigates the heterogeneities in the post-rift sedimentary systems at a mega-regional scale (>1000 km). Re-sampling seven key exploration wells and scientific boreholes provides new data (189 analysed samples) that have been used to build a high-resolution stratigraphic framework using multiple biostratigraphic techniques integrated with organic geochemistry to refine the timing of 10 key stratigraphic surfaces and three megasequences. The results have been used to calibrate the interpretation of a margin-scale two-dimensional seismic reflection dataset, and to build megasequence isochore maps, structural restorations and gross depositional environment maps at key time intervals of the margin evolution. Our findings revise the dating of the basal succession drilled by the A2-1 well, indicating that the oldest post-rift sequence penetrated along the margin is late Tithonian age (previously Callovian). Early Central Atlantic carbonate platform sediments passively infilled subcircular-shaped basement topography controlled by the underlying basement structure of thinned continental crust. Barremian–Aptian rifting in the Equatorial Atlantic, caused folding and thrusting of the Demerara Rise, resulting in major uplift, gravitational margin collapse, transpressional structures and peneplanation of up to 1 km of sediment capped by the regional angular Base Albian Unconformity. Equatorial Atlantic rifting led to margin segmentation and the formation of the TMP, where two major unconformities developed during the intra Late Albian and base Cenomanian. These two unconformities are time synchronous with oceanic crust accretion offshore French Guiana and in the Demerara–Guinea transform, respectively. A marine connection between the Central and Equatorial Atlantic is demonstrated by middle Late Albian times, coinciding with deposition of the organic-rich source rock of the Canje Formation (average total organic carbon 4.21%). The succession is variably truncated by the Middle Campanian Unconformity. Refining the stratigraphic framework within the context of the structural evolution and segmentation of the Guyanas margin impacts the understanding of key petroleum system elements. Supplementary material: Photographs of sandstone petrography thin sections (Fig. S1); calcareous nannofossil plates (Fig. S2); palynology reports for A2-1 and FG2-1 (Fig. S3); taxonomy description of new species; sample table and organic geochemistry results (Table S1); and nannofossil distribution charts (Table S2) are available at https://doi.org/10.6084/m9.figshare.c.5280490