Date of Award

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Joseph E. Hazel

Abstract

Late Pliocene (3.8-3.6 Ma) and early Pleistocene (1.5-0.9 Ma) marine ostracodes of the Rio Banano, Quebrada Chocolate and Moin Formations (Limon Group) provide a detailed paleoenvironmental record for the Limon Basin, Costa Rica. The marine ostracode assemblages recovered from the Limon region do not provide evidence that the tropics cooled dramatically during the late Pliocene when the Northern Hemisphere was entering the Modern Ice Age. Paleoenvironmental interpretations are based on cluster analyses and a modern tropical ostracode analog analysis. Diverse ostracode assemblages of the middle Pliocene Rio Banano Formation are dominated by genera typical of the inner to middle neritic zone (e.g., Basslerites, Pellucistoma, Puriana, and Radimella). The constancy of the ostracode assemblages suggests that environmental conditions did not change greatly during deposition. A shift in dominance of ostracode species, however, suggests that whereas no major environmental disturbances occurred, subtle change(s) in environmental conditions took place during deposition. The Assemblages of the Quebrada Chocolate formation document ostracodes coexisting with some of the oldest identified coral reefs in Costa Rica. These late Pliocene ostracode assemblages, occurring in a series of stacked coral reef buildups in this formation are characterized by unusually low simple diversity. The sediments of the Moin Formation contain ostracode assemblages representative of several environments of deposition. An assemblage of bathyal and neritic ostracodes is interpreted to indicate the presence of a coastal upwelling system active during the early Pleistocene (the Cangrejos Creek section). Previously, the mixed microfossil assemblage was interpreted to be the result of downslope transport. However, many of the same ostracodes that are common in the bathyal and outer neritic environments (e.g. Bradleya, Krithe, and Echinocythereis) are also found in the coral- and mollusk-rich Lomas del Mar section that caps the ridge near Limon. A modern ostracode analog analysis identified the closest match to these Moin samples to be Venezuelan continental shelf, where modern coastal upwelling brings cooler, nutrient-rich waters up onto the shelf. Ostracodes recovered from the Empalme Mollusk are interpreted to represent a shallow lagoonal environment and are dominated by Reussicythere reussi, Pellucistoma howei and Puriana aff. P. matthewsi.

ISBN

9780591905113

Pages

289

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