Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the history of thermohaline circulation

Partially buried giant drifts are located in the southern-most sector of the Argentine continental margin, generating a bathymetric jump at the base of the slope. They are characterised as giant, asymmetrical elongated, mounded contourite drifts, and are described in detail here for the first time. This description is mainly based on the bathymetric and multichannel seismic reflection profiles data. Their origin and evolution in the Argentine Basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the Atlantic and Antarctica are discussed. The buried giant drifts are divided into two zones by a large seamount. The major giant-drift (50 km wide, 300 km long, and having a sedimentary thickness of nearly 1 km) is located to the south of this seamount and oriented to the north, where its summit outcrops at present seafloor. Here, its asymmetric external shape is characterised by a steep, western side and a gently dipping, smooth eastern side, with internal reflections prograding eastward. Based on its position, morphology and internal characteristics, it has been deduced that this giant-drift was generated in the Argentine Basin by the southward branch of a confined large loop of the Antarctic Bottom Deep Water, from the Eocene-Oligocene boundary (coevally with the Drake Passage opening) until the middle Miocene. However, after a major palaeoceanographic change in the mid-to-late Miocene a new oceanographic scenario was established that fossilised these giant drifts. These changes produced a new depositional style in the margin, resulting in initiation of the present contourite depositional system and morphologic features over the slope. Extension of North Atlantic Deep Water circulation into the Southern Hemisphere and deepening of Antarctic Bottom Water circulation in the Argentine Basin are herein postulated as the factors that controlled said changes. These giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration, thus demonstrating the economic potential of contourite deposits in deep marine environments.