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Sedimentology of Ancient Coastal Wetlands: Insights From A Cretaceous Multifaceted Depositional System
Coastal wetlands are prominent modern environments widely studied in geomorphology and ecology, but the term “coastal wetland” is still barely used for the sedimentological classification of ancient deposits. The depositional system studied here (Leza Formation, Cameros Basin, Early Cretaceous, N Spain) includes diverse carbonate and clastic facies deposited at the sea–land transition, and is an illustrative example of the wide array of sedimentary environments that may occur in coastal wetlands systems. The studied system was composed mainly of carbonate water bodies whose salinity ranged from fresh to brackish and near-marine, and which had variable input of clastic material due to their lateral connection with alluvial fans. In addition, the system also included carbonate water bodies with stronger marine influence, tide-influenced oolitic areas, and relatively restricted evaporative settings. The deposits of all these environments occur alternating with each other throughout a unit 30–280 m thick, and they range from continental to marine conditions in a relatively small area (approximately 10 km × 30 km). Thus, this sedimentological study of the Leza Fm provides an ideal opportunity to investigate challenging ancient deposits with both continental and marine features. Comparison with other modern and ancient coastal settings allows the conclusion that “coastal wetland” may be the most accurate sedimentological classification for the Leza Fm, since it was not part of a major coastal system (e.g., delta or estuary). A series of general sedimentological characteristics of coastal wetland deposits are gathered from the Cretaceous case study and from the modern and ancient examples examined. These characteristics include: predominance of shallow-water facies; common subaerial exposure and edaphic features; great variety of interrelated continental, transitional, and marine environments with contrasting hydrodynamic and hydrochemical conditions; and low-diversity biotic communities, including both continental and marine fossils, as well as fossils of ambiguous affinities.
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