Journal of Conference Abstracts

Variations in the Sedimentological Record within the Lagoon of Mayotte During the Holocene (Comoro Archipelago, SW Indian Ocean)

Jens Zinke (jzinke@geomar.de)¹, John J. G. Reijmer (jreijmer@geomar.de)¹,

Wolf-Christian Dullo (cdullo@geomar.de)¹ &

Bernard A. Thomassin (thomassi@com.univ-mrs.fr)²

¹ Wischhofstr.1-3, Geb. 4, 24143 Kiel, Germany

² Centre d'Oceanologie de Marseille, (CNRS/UMR n.6450 Dimar), Station marine d'Endoume, rue de la Batterie-des-Lions, 13007 Marseille, France

The volcanic island of Mayotte is surrounded by a nearly continuous barrier reef system which rims a large lagoon of 1500 km² of up to 80 m depth. Eleven gravity cores from various settings in different bays were studied to determine the Holocene evolution of the sedimentary environment. The response of the sedimentary environment within the lagoon of Mayotte to the postglacial sea-level rise shows large variations in the petrophysical (Magnetic Susceptibilty), mineralogical (Carbonate, C org., Aragonite/Calcite) and biological (bioclastic composition, facies) parameters. The change in these parameters is controlled by the pre-Holocene topography, the sedimentary system (inner shoreline, outer reef domain), the structure of the bay (open or restricted), the stratigraphy of the hinterland (weathering potential, terrigenous input) and the exchange with open ocean water. The accommodation space created by the postglacial sea-level rise seems to be less important than the previously mentioned parameters. Seismics show that the Holocene sediment thickness has no correlation with accommodation space created by the Holocene sea-level rise. The sediment thicknesses show the largest values nearshore resulting from higher terrigenous input (8-14 m). Further offshore a rather chaotic thickness distribution (2-6 m) can be observed. This also holds for the sedimentation rates within the different bays. They vary from 0.14 mma^-1 in the pure carbonate facies to 1.96 mma^-1 in the mixed terrigenous-carbonate facies. Extraordinary high sedimentation rates occur in mangrove muds with 3.76 mma^-1.When using the Recent and Holocene sedimentation rates and the average durations of the last interglacials (approximately 20 ka) it becomes clear that sedimentation will not fill up the available accommodation space created by the postglacial sea-level rise. A lowering in sea-level will lead to different cycle preservation in seperate settings within the same lagoon ultimately. It will be impossible to evaluate absolute sea-level changes from the sediment thickness distribution of an area of limited lateral extent within the lagoon of Mayotte. Therefore it is concluded, that an unique interpretation of sedimentary cycles within the lagoon of Mayotte is not possible. It can be misleading to use sediment thickness alone as a proxy for accommodation space. Detailed studies of the sedimentary patterns and their lateral extensions in the Mayotte lagoon show clearly, that local autocyclic processes are of great importance. Processes of shorter time-scales than Milankowitch cycles (Decade-to-Millenial-scale), like climatically induced variations in humidity and aridity, seem to play a major role for the sedimentary processes in this latitude. So far, the Holocene sedimentation is the product of a synthesis of autocyclic and allocyclic forcing. These results are in good agreement with other studies of Holocene and older sedimentary deposits (Strasser, 1994; Boss & Rassmussen, 1995; Esker et al., 1998; Satterley, 1996; Larcombe & Carter, 1998). Our results also confirm the predicted sedimentation models dealing with the importance of autocyclic perturbations of the sedimentary record and concepts of chaotic sedimentation processes (Soreghan & Dickinson, 1994; Boss & Rassmussen, 1995; Peper & Cloetingh, 1995).

Boss SK & Rasmussen KA, Geology, 23/3, 221-224, (1995).

Esker D, Eberli GP & McNeill DF, AAPG Bulletin, 82/11, 2075-2109, (1998).

Larcombe P & Carter RM, Sedimentary Geology, 117, 97-121, (1998).

Peper T & Cloetingh S, Geology, 23/10, 937-940, (1995).

Satterley AK, Earth-Science Reviews, 40, 181-207, (1996).

Strasser A, Spec. Publs. Int. Ass. Sediment, 19, 285-310, (1994).