Carbonate-Siliciclastic Mixing from a Small Platform: Pliocene-Pleistocene, Bocas Del Toro Archipelago, Panama

Laura C. Gillespie¹, James S. Klaus¹, Donald F. McNeill², and William A. Morgan^3
1Geological Sciences, University of Miami, Coral Gables, FL
²Marine Geology & Geophysics, RSMAS Univ. Miami, Miami, FL
³ConocoPhillips, Houston, TX

Isla Colón in the Bocas del Toro archipelago is a small uplifted platform that records shallow-water deposition during the early Pliocene (siliciclastic) and late Pliocene/early Pleistocene (mixed carbonate-siliciclastics). To quantify the platform depositional stages, we dated the key lithologic units using strontium-isotopes and magnetostratigraphy. The key lithologies include three formations (Old Bank, La Gruta, unnamed Pleistocene). The siliciclastic Old Bank Fm ranges from ~5 to 2.5 Ma, the carbonate/siliciclastic La Gruta Fm ranges between 2.4 and 1.6 Ma, and the carbonate/siliciclastic unnamed Pleistocene unit is between 1.2 and 0.8 Ma. A hiatus from 1.5 to 1.2 Ma may mark the beginning of island uplift. Carbonate deposition is first recorded in the La Gruta Fm at ~2.4 Ma. The La Gruta Fm is composed of a coralline limestone and a siliciclastic facies containing corals and molluscs. The spatial distribution of the two facies is controlled by: precursor topography, likely due to faulting; and windward exposure of the platform. The siliciclastic facies of the La Gruta is largely restricted to the leeward side of the platform. The second phase of mixed carbonate/siliciclastic deposition is coincident with the onset of Pleistocene high amplitude sea levels that started at ~1.0 to 0.8 Ma. This depositional package consists of a basal, transgressive siliciclastic unit and a coral- and skeletal sand-rich seaward prograding unit deposited during the highstand. This coastal reef unit marked the final marine deposition, as uplift began at ~0.8 Ma. This study demonstrates the spatial and temporal heterogeneity of carbonate lithofacies on a small platform influenced by tectonics and changes in sea level, as well as the importance of good chronostratigraphic resolution to understand stratigraphic relations that may be difficult to ascertain from limited field exposures.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas