Tectonic Relations of South Georgia Island to the Southernmost Andes
Rocks on South Georgia Island at the eastern end of the North Scotia Ridge are no older than late Mesozoic. The Cumberland Bay and Sandebugten graywacke and mudstone sequences there are comparable in general lithology and structural style to the Lower Cretaceous Yahgan Formation of the Beagle Channel area in southernmost South America. The Cumberland Bay rocks, which form most of South Georgia Island, were thrust northeastward over the Sandebugten sequence. The Cumberland Bay and Yahgan sequences contain Cretaceous fossils, whereas the Sandebugten rocks are unfossiliferous. The dominant dispersal of Cumberland Bay detritus was toward the northwest. The Sandebugten dispersal pattern was more complex but was dominated by a south-directed component. In Early Cretaceous time, however, the South Georgia microcontinent apparently was attached to South America along the present southern margin of the Burdwood Bank. The Cumberland Bay, Sandebugten, and farther westward along strike, the Yahgan, apparently were deposited in a marginal small ocean basin between a calc-alkalic volcanic arc built on a sliver of old South American continental crust and the main part of the South American continent from which the sliver moved away. According to this interpretation, deformation of the sediments occurred when the arc moved back toward the continent in middle Cretaceous time, and the basin was closed and uplifted with the arc.