Island-arc carbonates: characterization and recognition in the ancient geologic record

Carbonates of island-arc origin that are preserved in Paleozoic-Mesozoic terranes of the North American Cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs. This study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities. Models are developed that show patterns in the long-term evolution of shallow marine organisms and the construction, evolution, and demise of carbonate platforms in island arcs. A suite of criteria is identified that may be used to differentiate island-arc carbonates from limestones that accumulated in other platform settings. graphic isolation, prolonged subsidence, steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism, impoverished normal marine faunas, complex provincial affinities, and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences, fringing and barrier reefs at the shelf margin, extensive lagoonal deposits and rapid lateral and vertical facies changes. Although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform, and particular facies types may not be represented (e.g., fringing and barrier reefs may be replaced by sand shoals at the platform, margin), many characteristics have potential value for identifying carbonates of island-arc origin in the ancient rock record. Apart from being associated with calc-alkaline volcanic and volcaniclastic assemblages, the most valuable suite of features for recognizing island-arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities, extraordinary thicknesses of platform (shallow marine) and periplatform carbonates, and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones. Recognizing that an ensemble of features is distinctive within island-arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins.