Timing of lateritization on the Guiana Shield: synthesis of paleomagnetic results from French Guiana and Suriname
Abstract A paleomagnetic study was carried out on representative duricrust paleosurfaces of the Guiana Shield in French Guiana (Cayenne region and Kaw Mountain) and Suriname (Moengo and Bakhuis Mountains). The age of weathering of ferruginous and bauxitic duricrusts was estimated from comparison of paleomagnetic poles determined from these duricrust horizons with the South American apparent polar wander path (APWP). In most of the studied profiles, hematite is the dominant magnetic carrier, as indicated by unblocking temperatures that exceed 580°C for about 20–60% of the natural remanent magnetization (NRM). Magnetite and goethite may also be present and contribute to the overall NRM, but contribute little or nothing to the characteristic remanent magnetization, which is estimated from principal component analysis of the medium to high unblocking temperature component. Paleomagnetic directions from the duricrust profiles generally cluster well (dispersion parameters >30) and give paleomagnetic poles that fall between the Cenozoic and the present-day portion of the APWP. Recent paleomagnetic directions are usually detected in the uppermost part of the duricrust profiles which may have sustained some recent reworking. In contrast, the middle part of the duricrust, of massive facies, usually shows ancient directions. The bauxitic surfaces of Bakhuis Mountains at 400 m elevation, Kaw Mountain (upper surface, 300 m elevation) and probably the Cayenne Unit 1 (75–200 m elevation) can be attributed to the ‘Sul Americano’ lateritization cycle of Paleocene–Eocene age, with respectively average relative ages of 60, 50 and 40 Ma. The bauxitic paleosurface of Kaw Mountain (lower surface, 220 m), the coastal Moengo deposit (40 m) and the ferruginous surface of Cayenne Unit 2 (20–50 m) may be related to the ‘Late Velhas’ lateritization cycle of Miocene age with relative average ages of 13 and 10 Ma. The Fe-duricrust of Cayenne Unit 3, close to sea level and occupying most of the coastal plain, is geomorphologically the youngest duricrust paleosurface, but also seems related to a Late Miocene lateritization event (5–13 Ma). The Oligocene appears to represent a major break in the development of weathering profiles, with erosion as the predominant process which stripped 50–80 m from the landscape between the two main lateritization phases of Paleocene–Eocene and Miocene.