Environmental Significance of the Supercontinent Columbia

The supercontinent Columbia reached maximum packing at about 1.9 Ga. Prior to that time, the dominant magmatism produced highly deformed TTG (tonalite-trondhjemite-granodiorite) suites. Although some GG (granite-granodiorite; calcalkaline) batholiths formed in old cratons before 1.9 Ga, these suites became common after 1.9 Ga and are now typical of modern subduction zones.

Two different configurations of Columbia have been proposed, based largely on the position of the North China (Sino-Korean) craton. One model places the southern margin (present orientation) of North China against the eastern margin of North America, particularly the Penokean belt. A second model places the North Hebei belt along the northern margin of North China against the western margin of India and correlates the Central Indian Tectonic Zone with the trans-North China orogen.

Three changes in the surface environment occurred at the same time as the maximum packing of Columbia at 1.9 Ga: 1) atmospheric oxygen concentrations had increased sufficiently to cause a switch from the deposition of reduced iron formations to oxidized red beds; 2) the composition of ocean water changed from carbonate rich to sulfate rich; and 3) the oldest eukaryote biomarkers (steranes) occur in sediments.

Development of oxygen- and sulfate-rich seawater could only have occurred in shallow water. The area of Columbia that was below sealevel is unknown, but presumably much of it was covered by shallow water. This shallow ocean would have been an ideal place for the evolution of eukaryotes.