Occurrence of shallow gas in the easternmost Lago Fagnano (Tierra del Fuego)

Abstract

High-resolution seismic profiles acquired in the eastern sector of Lago Fagnano, the southernmost ice-free lake in the world, have shown the presence of very shallow gas-bearing layers in the upper sedimentary sequences. The gas-related features observed on seismic profiles include a typical, very strong reflection with reversed polarity, multiple reflections and acoustic blanking that hide subsurface sedimentary and structural features. The top of the acoustically high-amplitude layer is located between 0.3-1.7 m below the lake floor. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration, which could be related to the lithology of the overlying sediments. To confirm the presence of gas, some gravity cores were recovered in places where the blanking effect was most relevant and in the supposed gas-free zone. Sediment core analyses have highlighted the occurrence of significant organic-rich components within the uppermost, largely unconsolidated sedimentary layers, in correspondence of the seismically-detected gassy zone, whereas only a few organic layers were found in the gas-free zone. We assume that the main origin of gas is linked to the presence of a shallow, thin peat-rich layer of Middle-Late Holocene age. In fact, the mapped gassy zone occurs in correspondence of the outlet of the Rio Turbio, the principal tributary of Lago Fagnano, which discharges waters coming from a relatively small sag pond located immediately to the east of the eastern shore of the lake. To date, this is the first evidence of shallow gas in Tierra del Fuego lakes.