179 9. The Deffend hydrogeological model Bajocian and Bathonian formations of the Grands Causses in France (Charcosset et al., 2000) and in the Betic Cordillera in Spain (Molina et al., 1999). These discontinuities are defined by some authors as inception horizons (Filipponi et al., 2010) and play a key role in controlling karst development (Bosák, 2008). On the Poitou Threshold, deposits ranging from the Aalenian to the Bathonian are primarily controlled by eustatic variations (Branger, 1989; Gonnin et al., 1992). Each depositional sequence is bounded by discontinuities characterized by the absence of one or several ammonite zones (Gabilly, 1962; Gabilly and Cariou, 1974; Gabilly et al., 1985; Branger, 1989). The model that best explains sedimentation on the Poitou Threshold platform is the Catch-up/Keep-up model (Emery, 1996). The Emery Catch-up / Keep-up model describes the response of carbonate platforms and reefs to relative sea-level rise. In the keep-up mode, the vertical accretion rate of the platform equals or exceeds the rate of sea-level rise, allowing the carbonate surface to remain close to sea level continuously. In the catch-up mode, carbonate growth is initially too slow to match rapid sea-level rise, leading to partial drowning. A third outcome, often referred to as the give-up mode, occurs when carbonate accumulation cannot compensate for accommodation creation, resulting in permanent drowning of the platform. Emery’s Catch-up / Keep-up model can be directly related to the systems tracts of sequence stratigraphy in carbonate platforms. Keep-up platforms, where carbonate production matches relative sea-level rise, are expressed as aggradational successions typical of the transgressive systems tract (TST) or the highstand systems tract (HST). In contrast, keep-up platforms, which drown permanently, correspond to low systems tract (LST) or major sequence boundaries, often overlain by condensed pelagic deposits or siliciclastics. In each depositional sequence, a lowstand systems tract formed along the edges of the platform, either on the outer platform or in basins bordering the threshold. On the platform itself, limited accommodation space prevented the development of lowstand systems tracts. The low accommodation was expressed by early lithification, the presence of benthic faunas, bioturbation, and abrasion by intertidal currents. During this low sea level phase, the limestones were dolomitized by continental freshwater or rainwater according to the Coastal Mixing Zone Dolomite theory (Humphrey and Quinn, 1989) or the reflux model (Patterson and Kinsman, 1982). During transgression, when the sea level rose, the platform was drowned, and pelagic facies were deposited. Along the platform margin, these are represented by marly–calcareous condensed beds, forming thin strata enriched in ammonites and belemnites, which typify drowning intervals within the carbonate succession. Then, a highstand systems tract developed across the platform, characterized by increased carbonate production, consistent with the hypertrophic late sequence model of Gabilly and Cariou (1974). Concurrently, slope deposits thinned as carbonate accumulation declined in deeper, light-limited environments. The resulting highstand wedge subsequently served as a substrate for benthic colonization during the following sea-level fall.
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