An Ocean Opens

Editorial study. This essay describes no lot offered for sale. Carbonado’s origin is unresolved and debated. ASA Digital Art’s specimens are not identified as carbonado; no origin, space or otherwise, is claimed for any ASA Digital Art specimen or Luxurious Specimens lot.

Haggerty links the separation of the São Francisco and Congo blocks to the breakup of Gondwana, which began about 180 million years ago; the two blocks finally parted when the South Atlantic opened in the Early Cretaceous^[1,2]. The South Atlantic opened in stages: seafloor spreading began in the south around 135 to 130 million years ago and propagated northward^[2,3].

A 2009 reconstruction traced the opening and the division of an Aptian salt basin between the two margins^[3]. Studies of the southern South Atlantic examine its late rifting phase and continental breakup^[2].

As the rift propagated, it cut through the cratonic bridge, leaving one half in Brazil and the other in Africa^[4]. That is why two deposits now separated by an ocean can be treated as one geological province.

References

Gems & Gemology (GIA), Summer 2017, “Carbonado Diamond: A Review of Properties and Origin.” https://www.gia.edu/gems-gemology/summer-2017-carbonado-diamond

“The late rifting phase and continental break-up of the southern South Atlantic” (2016). Geological Society, London, Special Publications 420. https://doi.org/10.1144/sp420.2

“A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin” (2009). Geophysical Journal International. https://doi.org/10.1111/j.1365-246x.2009.04137.x

Alkmim, F. F. et al. (2006). Kinematic evolution of the Araçuaí-West Congo orogen in Brazil and Africa: Nutcracker tectonics during the Neoproterozoic assembly of Gondwana. Precambrian Research. https://www.sciencedirect.com/science/article/abs/pii/S0301926806001483