FOLDED FORMAT: 1 color plate (approx. 54.5" x 77")
This 1:25,000 map covers 500 square kilometers of Archean rocks in the Barberton Greenstone Belt (BGB), South Africa—the oldest, best-preserved sedimentary and volcanic sequence on Earth. Five major tectonic cycles are represented in the BGB’s 3.55 –3.22 Ga history—komatiitic volcanism, forming lava plains, begins each cycle, and dacitic volcanism, likely related to an early style of subduction, ends each cycle. These cycles produce small yet stable protocontinental blocks that coalesce by magmatic and tectonic accretion. The BGB rocks provide direct information about the nature and evolution of early Earth and its biota. Carbonaceous cherts occur throughout the sequence, containing microfossils, microbial mats, and stromatolites. Evaporites represent shallow marine environments, whereas ferruginous chert and banded iron formation are indicators of deeper water settings. The BGB also contains a remarkable record of large asteroidal impacts, in eight distal impact layers, each more energetic than the K-T boundary event.