Retallack, Gregory , Kirby, Michael .
Middle Miocene vegetation of Panama.
FOSSIL soils, leaves and woods have been newly exposed in western foundation works of the Puente Centenario bridge over the Panama Canal near Paraiso, from the upper Cucaracha Formation, which has yielded a Barstovian (middle Miocene) mammal fauna. Fossil leaves are similar to the flora described by Berry in 1918, dominated by thick, entire-margined dicot leaves of medium size (10-15 cm long). Some of these may have been mangroves, because associated paleosols have abundant pyrite nodules and carbonate-permineralized pencil roots. Unmineralized woods include palms and a variety of ring-porous dicots, with smooth to dimpled bark characteristic of tropical regions. A dozen different kinds of paleosols were recognized and represent a variety of vegetation types: mangrove, freshwater and marine-influenced swamp, early successional riparian woodland, colonizing forest, dry tropical forest and woodland. A surprise for such a low latitude coastal site is that many of the paleosols have pedogenic calcareous nodules, and some have pedogenic barite nodules. Depth to carbonate and thickness of paleosol with carbonate, both corrected for burial compaction, indicate mean annual precipitation from 610-906 mm and mean annual range of precipitation from 27-65 mm, in at least 12 Milankovitch scale climatic oscillations. Chemical analyses of Bt horizons of the paleosols compared with climofunctions for North American soils confirm warm dry climate, with mean annual precipitation of 296-1142 mm and mean annual temperature of 15-16o C. Dry climate paleosols probably reflect a rain-shadow behind volcanic mountains, rather than cold marine currents incompatible with underlying fossil coral reefs. Fossil mammals of the Cucaracha Formation include North American Barstovian genera Texomys, Archaeohippus, Anchitherium, Menoceras, Merycochoerus, and Paratoceras. This fauna and its enclosing paleosols are evidence that the Caribbean Sea was enclosed by a mountainous connection to North America during the middle Miocene, with only a narrow marine strait to the Pacific Ocean.
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1 - University of Oregon, Department of Geological Sciences, Eugene, Oregon, 97403-1272, USA
2 - Geobiological Research Laboratories, 265 Cross Street, Middletown, Connecticut, 06457, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Tuesday, August 1st, 2006
Time: 11:00 AM