On the supposed Upper Paleozoic unconformity in North Sumatra

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 unconformity caused by transgression. In the latter case we would meet with an Upper Paleozoic folding (and transgressive Triassic) in the Area of Lake Toba, On the other supposition, we should have a conformable sequence from Paleozoic to Upper Triassic in Western Sumatra and Malaya”.