Intermediate- and high-resolution seismic reflection profiles (1129 km) along with piston cores were used to investigate the morphotectonics and sedimentology of the South Chile margin between 52°30' and 54°30' Lat.S. The examined latitudinal transect shows an accretionary prism with a terraced forearc basin, up to 40 km large, mostly coinciding with the Fuegian Terrace. The trench is completely filled by a thick, well reflective turbiditic sequence with local resedimentation bodies fed by the rare canyons characterizing the lower slope. A discontinuous outer high at the top of the accretionary prism forms the trenchward limit of the forearc basin. North of 53°S the basin rapidly narrows losing its bathymetric expression, even though its structural and stratigraphic occurrences are still recognizable on the nothernmost seismic profiles. The forearc basin records evidence of both sediment deposition and erosion. Three mound-shaped sedimentary bodies, which are interpreted as turbiditic lobes, have been detected on a seismic section near the inner side of the basin. The lobes are fed by small canyons cut across the steep (6°-8°) upper continental slope on which gravity driven deposits, such as slides and slumps, are detected. The surface of the basin also shows gullies, low-relief incisions and widespread erosional surfaces. On the continental shelf off the Magellan Strait a thick prograding sequence testifies the depositional effects of the Magellanic ice tongue during the Plio-Pleistocene glacial advances. The four cores recovered from the forearc basin have been X-rayed and grain-size analysed. Radiocarbon datings on several levels have also been performed along with carbonate and TOC content analyses. In three cores a type-sequence has been recognised, consisting of three depositional units linked to the late Quaternary climatic variations. The lower unit is made up of turbiditic sequences deposited before 20 000 yr BP, when a widespread glacial cap affected the southernmost South America. The intermediate unit dates between 20 000 yr BP and 16 950 yr BP, during or immediately after the LGM: It is characterised by an increasing hemipelagic sedimentation with episodic turbiditic events and ice-rafted debris supply. The upper unit is younger than 16 950 yr BP, it is related to a post-glacial to present-day hemipelagic sedimentation with increasing carbonate content that reflects the Holocene climatic amelioration. The last core, recovered within an incision at the surface of the forearc basin, records three carbonate-rich turbiditic events, most likely of Late Glacial-Holocene age. Since no evidence of the Magellanes transform plate boundary has been found on our seismic profiles, we suppose that: a) The margin does not record any evidence of this transform fault; b) Its effects escape the present network of profiles.
Morphostructural and Sedimentological Aspect of the South Chile Margin between 52° 30' and 54° 30' lat. S
DEMURO, SANDRO;
2001-01-01
Abstract
Intermediate- and high-resolution seismic reflection profiles (1129 km) along with piston cores were used to investigate the morphotectonics and sedimentology of the South Chile margin between 52°30' and 54°30' Lat.S. The examined latitudinal transect shows an accretionary prism with a terraced forearc basin, up to 40 km large, mostly coinciding with the Fuegian Terrace. The trench is completely filled by a thick, well reflective turbiditic sequence with local resedimentation bodies fed by the rare canyons characterizing the lower slope. A discontinuous outer high at the top of the accretionary prism forms the trenchward limit of the forearc basin. North of 53°S the basin rapidly narrows losing its bathymetric expression, even though its structural and stratigraphic occurrences are still recognizable on the nothernmost seismic profiles. The forearc basin records evidence of both sediment deposition and erosion. Three mound-shaped sedimentary bodies, which are interpreted as turbiditic lobes, have been detected on a seismic section near the inner side of the basin. The lobes are fed by small canyons cut across the steep (6°-8°) upper continental slope on which gravity driven deposits, such as slides and slumps, are detected. The surface of the basin also shows gullies, low-relief incisions and widespread erosional surfaces. On the continental shelf off the Magellan Strait a thick prograding sequence testifies the depositional effects of the Magellanic ice tongue during the Plio-Pleistocene glacial advances. The four cores recovered from the forearc basin have been X-rayed and grain-size analysed. Radiocarbon datings on several levels have also been performed along with carbonate and TOC content analyses. In three cores a type-sequence has been recognised, consisting of three depositional units linked to the late Quaternary climatic variations. The lower unit is made up of turbiditic sequences deposited before 20 000 yr BP, when a widespread glacial cap affected the southernmost South America. The intermediate unit dates between 20 000 yr BP and 16 950 yr BP, during or immediately after the LGM: It is characterised by an increasing hemipelagic sedimentation with episodic turbiditic events and ice-rafted debris supply. The upper unit is younger than 16 950 yr BP, it is related to a post-glacial to present-day hemipelagic sedimentation with increasing carbonate content that reflects the Holocene climatic amelioration. The last core, recovered within an incision at the surface of the forearc basin, records three carbonate-rich turbiditic events, most likely of Late Glacial-Holocene age. Since no evidence of the Magellanes transform plate boundary has been found on our seismic profiles, we suppose that: a) The margin does not record any evidence of this transform fault; b) Its effects escape the present network of profiles.
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