| In Umbgrove’s “Geological History of the East Indies”, published in 1938 in the Bulletin of the American Association of Petroleum Geologists, we find on page 7 the following remarks about the possible extistence of an Upper Paleozoic unconformity in North Sumatra: “According to Zwierzicky (1929) the Triassic near Prapat on Lake Toba (Sumatra) rests unconformably on phyllites, which he considers to be Upper Paleozoic. As this is the only angular unconformity of the sort that has been established with certainty in the entire western part of the Archipelago, we shall — until this important question has been definitely settled by field research — have to consider two possibilities: either a local, more recently developed structural discordance, or a real... |
