GSA Annual Meeting, November 5-8, 2001

Paper No. 0
Presentation Time: 1:30 PM-5:30 PM


LOEWEN, Mark A., Utah Museum of Natural History and Department of Geology and Geophysics, Univ of Utah, 1390 E. Presidents Circle, Salt Lake City, UT 84112-0050, SAMONDS, Karen E., Department of Anatomical Sciences, Stony Brook Univ, Health Sciences Center, Stony Brook, NY 11794-8081 and RAMAROLAHY, Myria F., Département de Paléontologie et d'Anthropologie Biologique, Universite d'Antananarivo, Faculte des Sciences, Antananarivo, BP 906, Madagascar,

Lake Tsimanampetsotsa in southwestern Madagascar represents a dynamic modern analog for the spectacular lagerstatten-producing alakaline playa lakes of the fossil record. Carbonate playa lakes are relatively rare today; this lake provides a unique opportunity to study lacustrine sedimentation in a modern setting.

The lake sits amongst the last vestiges of the pristine southern spiny forest of Madagascar and is frequented by a highly endemic vertebrate fauna. The lake directly precipitates carbonate in a closed evaporite basin defined by cliffs of Eocene marine limestone to the east and a wide strip of aluvium capping low outcrops of limestone to the west. The extremely low gradient within the basin produces dramatic changes in arial extent in response to seasonal changes in precipitation. These fluxuations create broad hypersaline mudflats, which form a broad apron around the perimeter of the lake.

Prior to the orally recorded history of the region, a paleolake of much greater size and depth occupied the basin. Expansive strand lines ring the extreme edges of the basin and deltaic sediments produced by a paleodrainage system are present as well. Strandlines along the cliffs to the west are marked by a 10 meter wide band of oncolites, representing the maximum extent of the paleolake. Many of these present and recent depositional characteristics mimic those Cenozoic lacustrine systems such as the Green River Formation of North America.

The composition of the modern community and the sedimentation patterns of this lake are analyzed for comparison with past lacustrine ecosystems, both in Madagascar and in fossil deposits elsewhere. Lake Tsimanampetsotsa provides an unparalled glimpse of modern sedimentation in a rapidly changing ecosystem.