Tectonic, eustatic and environmental controls on mid-Cretaceous carbonate platform deposition, south-central Pyrenees, Spain



Cenomanian–Turonian strata of the south-central Pyrenees in northern Spain contain three prograding carbonate sequences that record interactions among tectonics, sea level, environment and sediment fabric in controlling sequence development. Sequence UK-1 (Lower to Upper Cenomanian) contains distinct lagoonal, back-margin, margin, slope and basin facies, and was deposited on a broad, flat shelf adjacent to a deep basin. The lack of reef-constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. Sequence UK-2 (Upper Cenomanian) contains similar shallow-water facies belts, but syndepositional tectonic modification of the margin resulted in a steep slope and deposition of carbonate megabreccias. Sequence UK-3 (Lower to Middle Turonian) records a shift from benthic to pelagic deposition, as the shallow platform was drowned in response to a eustatic sea-level rise, coupled with increased organic productivity. Sequences UK-1 to UK-3 are subdivided into lowstand, transgressive and highstand systems tracts based on stratal geometries and facies distribution patterns. The same lithologies (e.g. megabreccias) commonly occur in more than one systems tract, indicating that: (1) the depositional system responded to more than just sea-level fluctuations; and (2) similar processes occurred during different times throughout sequence development. These sequences illustrate the complexity of carbonate platform dynamics that influence sequence architecture. Rift tectonics and flexural subsidence played a major role in controlling the location of the platform margin, maintaining a steep slope gradient through syndepositional faulting, enhancing slope instability and erosion, and influencing depositional processes, stratal relationships and lithofacies distribution on the slope. Sea-level variations (eustatic and relative) strongly influenced the timing of sequence and parasequence boundary formation, controlled changes in accommodation and promoted platform drowning (in conjunction with other factors). Physico-chemical and climatic conditions were responsible for reducing carbonate production rates and inducing platform drowning. Finally, a mud-rich sediment fabric affected platform morphology, growth geometries (aggradation vs. progradation) and facies distribution patterns.