Abstract

Four contrasting carbonate sequences developed in a tectonically active, extensional setting in the Lower Cretaceous (Aptian-Albian) Castro Urdiales platform of north Spain. The four carbonate phases (P1-P4; early Aptian, late Aptian, earliest Albian, and early-late Albian, respectively) are identified across the platform-to-basin transition on the basis of sedimentary geometries and internal facies patterns. They are vertically separated by four incipient drowning events (D1-D4; middle Aptian, Aptian-Albian boundary, early Albian, and late Albian). The first carbonate stage (P1) corresponds to a low-relief carbonate platform characterized by widespread shallow-marine depositional environments with a nearly flat organization. The second platform stage (P2) developed as a rimmed carbonate platform. The third carbonate stage (P3) evolved from a low-relief aggradational platform (substage A) to a progradational offlapping platform (substage B), and back to a backstepped, aggradational rimmed platform (substage C), responding to changes in differential subsidence and carbonate production. At the beginning of the Albian the Castro Urdiales carbonate platform was broken up by extensional tectonic movements with formation of fault blocks bounded by NE-SW and NW-SE trending faults. On the crests of uplifted blocks meteoric processes were active, whereas grabens and half grabens acted as channel conduits for terrigenous-rich deposits. Subsidence rates increased during the late early Albian-late Albian. The crests of rotated blocks formed paleo-highs that nucleated small residual carbonate platforms (carbonate stage P4) that were surrounded by deeper-water basins. This study suggests that the evolution of carbonate platform sequences in the Lower Cretaceous of Castro Urdiales was primarily controlled by changes in accommodation space, which were most likely driven by intraplate stresses related to extension in the north Spanish continental margin during the opening of the North Atlantic. Drowning events in the Castro Urdiales platform coincide with major crises of Cretaceous carbonate production worldwide, suggesting that regional tectonism and global oceanic changes interacted to cause phases of platform demise.

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