What is the Origin of Carbonate Rich Chevron Dunes in Southern Madagascar?
Monday, 14 December 2015
Poster Hall (Moscone South)
Southern Madagascar contains extensive sets of chevron dunes along the coast. The largest dunes extend up to 185 meters above sea level and are more than 40 km in length. While most researchers have assumed that the sand in the dunes was transported inland by the wind, we instead have proposed that the deposits are from a megatsunami event. The present data appear to support our hypothesis. We used a coulometer to determine the bulk carbonate content of 22 samples from the Ampalaza and Fenambosy dunes and their vicinity. We found that the median CaCO3 content of the dunes was over 40% with a typical range from 22 to 58% CaCO3. At off dune sites, where the substrate was exposed, and at a few sites closer to the ocean, the CaCO3 content ranged from 0% to 9%. Local beach sands contained ~ 52-54% CaCO3. At a site on the Fenambosy chevron (elevation~180 meters, location ~22 km along strike of the dune and 8.0 km in a direct line to the ocean) at the edge of a steep escarpment, the sand contained abundant marine microfossils and ~52% CaCO3. Just below the escarpment (4.7 km from the ocean) the sand contained ~58% CaCO3. Microprobe analyses of 10 carbonate fossils from the distal end of the Ampalaza chevron (elevation 68 meters, location ~42 km along strike of the dune) showed MgO contents of 1 to 6% and CaO contents of 49 to 54%. SiO2 contents are typically less than 0.1%. The bulk sand at this latter site contained ~49% CaCO3. In all cases, the carbonate fossils appear as single shells with no signs of being eroded from pre-existing, lithified rock. The low bulk MgO and SiO2 contents of the carbonate fossils are consistent with unlithified marine sediment as their source. Individual mineral grains in the dune sand are relatively angular. The sands contain abundant heavy minerals and are poorly sorted. The maximum grain size at some sites exceeds 2 mm. These observations are most consistent with a tsunami origin for the chevron dunes. AMS 14C age dates on well-preserved marine microfossils are in progress and will be reported at the meeting.