Depositional sequences and stratigraphy of the Colón carbonate platform: Bocas del Toro archipelago, Panama

摘要

The Colón platform, located in the Bocas del Toro region of Panama, was studied as a near-modern analog for buried reefal platforms. The platform has an areal extent of ~ 90 km~2 and formed in a convergent tectonic setting (backarc) under a tropical climatic regime. The attached platform formed as part of a discontinuously rimmed shelf along the Caribbean side of the Isthmus of Panama and consisted of mixed carbonates and siliciclastics. Based on field relations and strontiumisotope stratigraphy, three depositional sequences were identified in the near-surface deposits of the uplifted platform. A Miocene to late Pliocene lithic sandstone unit (sequence 1, Old Bank Formation) forms the foundation of the platform. Coralrich limestone and coral-rich siliciclastic units were deposited during both the late Pliocene (sequence 2, Isla Colón Formation) and early Pleistocene (sequence 3, Urracá Formation). In sequences 2 and 3 the sediment composition and fauna show distinct differences between the windward and leeward sides of the platform, with the main lithologic variable being the amount of siliciclastic sediment that was admixed with the biogenic carbonate sediments. The siliciclastic deposits (mud rich) in sequences 2 and 3 exhibit lower permeability and relatively minor diagenetic modification, and they lack major karst features. Conversely, the more carbonate-rich deposits are well cemented and strongly affected by meteoric dissolution and karstification. This facies-selective dissolution occurred when middle Pleistocene regional uplift exposed the platform to a tropical climate with high rainfall. Earthquakes and associated uplift of the platform over the past ~ 1 Myr intensely fractured the limestone. Fracture porosity has contributed to rapid infiltration of meteoric water and pervasive dissolution that resulted in numerous vertical sinks as well as an extensive network of horizontal conduits. A large, incised channel feature connects several of the pre-existing caves and conduits. The Colón platform limestone is a product of an equatorial humid carbonate system and is of size similar to platforms of Oligo-Miocene age near Malaysia (Sarawak), Indonesia, and farther north in the South China Sea.