Shallow water carbonate platforms (Late Aptian–Early Albian, Southern Apennines) in the context of supraregional to global changes: re-appraisal of palaeoecological events as reflectors of carbonate factory response

摘要

This paper discusses the palaeoenvironmental significance of the"Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich depositsencased in the Aptian/Albian shallow water carbonate platform strata ofMonte Tobenna and Monte Faito (Southern Italy). These facies show a peculiarfield appearance due to their color and/or fossil content. In the shallowwater carbonate strata, the Late Aptian "Orbitolina Level" was formedduring a period of decreasing accommodation space. Microbial carbonatesoccur in different levels in the composite section. They reach their maximumthickness around the sequence boundaries just above the "Orbitolina Level"and close to the Aptian–Albian transition, and were not deposited duringmaximum flooding. S. dinarica-rich deposits occur in the lower part of the MonteTobenna-Monte Faito composite section, in both restricted and more openlagoonal sediments. S. dinarica has its maximum abundance below the "OrbitolinaLevel" and disappears 11 m above this layer.On the basis of δ13C and δ18O values recorded atTobenna-Faito, the succession has been correlated to globalsea-levelchanges and to the main volcanic and climatic events during the Aptian.Deterioration of the inner lagoon environmental conditions was related tohigh trophic levels triggered by volcano-tectonic activity. Microbialcarbonates were deposited especially in periods of third-order sea levellowering. In such a scenario, periods of increased precipitation during theGargasian induced the mobilization of clay during flooding of the exposedplatform due to high-frequency sea-level changes, with consequentterrigenous input to the lagoon. This and the high nutrient levels made theconditions unsuitable for the principle carbonate producers, and anopportunistic biota rich in orbitolinids (Mesorbitolina texana and M. parva) populated the platform. Inthe more open marine domain, the increased nutrient input enhanced theproduction of organic matter and locally led to the formation of blackshales (e.g. the Niveau Fallot in the Vocontian Basin).It is argued that the concomitant low Mg/Ca molar ratio and highconcentration of calcium in seawater could have favoured the development ofthe low-Mg calcite skeleton of the S. dinarica green algae.During third-order sea-level rise, no or minor microbial carbonates formed inthe shallowlagoonal settings and S. dinarica disappeared. Carbonate neritic ecosystemswere not influenced by the environmental changes inferred to have beeninduced by the mid-Cretaceous volcanism.The "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits inthe studied Aptian/Albian shallow water carbonate strata are interpreted tobe the response to environmental and oceanographic changes in shallow-waterand deeper-marine ecosystems.