Regional Evaluation of Mesozoic Chronostratigraphy, Paleoenvironment and Source Rock Systems in the Southern Ocean (South Atlantic)

Abstract

A re-evaluation of the chronostratigraphy, paleoenvironment and organic geochemistry of eight Southern Ocean DSDP-ODP sites (south of the Agulhas-Falkland Fracture Zone) was carried out with an aim to provide new insights into the age of the syn-rift section along with variation in the development and termination of dysoxic-anoxic conditions and the subsequent distribution of organic matter (OM)-rich intervals. This study integrated existing information from previous publications and new data from reanalysis of microfossils and organic geochemistry of selected intervals from Sites 327, 330, 511, 328 (Falkland Plateau), Site 361 (Cape Basin, SW African margin), Site 249 (Mozambique Ridge), and Sites 692 and 693 (Weddell Sea, East Antarctica).

A regional framework generated through the graphic correlation-based chronostratigraphic analyses warrants an age revision of the oldest recovered sediments into the Middle Jurassic (Callovian or older) in the Falkland Plateau region, which impacts the current understanding and assessment of Jurassic petroleum systems in the region by improved correlation of sections through enhanced time resolution. Anoxic-dysoxic bottom water conditions persisted during the Middle Jurassic–Aptian interval within a restricted, deepening basin in the Falkland Plateau area bearing bottom-water stagnation. Oxygen depletion ceased around the Aptian/Albian boundary in the area south of the Agulhas-Falkland Fracture Zone. The termination of anoxia-dysoxia is observed to be gradual (Late Aptian–Albian) which was documented via microfossil assemblages, decreasing OM content and changing principal OM type, from an anoxic-marine to a residual OM dominated composition with no petroleum generation potential.

Source rock evaluation in the region south of the Agulhas-Falkland Fracture Zone bears evidence of a significant, syn-rift dominated, oil-prone Middle Jurassic–Valanginian source rock system with good to excellent TOC content and excellent generative potential. The true regional extent of this source rock system is not fully evaluated, although it appears to extend into adjacent Antarctic basin areas and, possibly, beyond the Mozambique Ridge (Site 249) into the southwest Indian Ocean.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018