A microstratigraphic analysis of a shallow water carbonate succession of Aptian age, cropping out at Monte Tobenna (southern Apennines, Italy), has allowed the recognition of eight lithofacies grouped into four lithofacies associations that imply inner carbonate platform environments. A sedimentological study on a 32-m-thick section, carried out on a centimetre scale, reveals a clear cyclic organization of the lithofacies; 7 to 135-cm-thick beds, frequently alternating with cm-thick green clayey levels, de®ne the 53 recognized cycles. Each cycle represents an Elementary Depositional Unit (EDU)whose vertical lithofacies evolution generally records either a transgressive-regressive trend (T/REDUs)or a regressive trend (R-EDUs). Generally, features related to emersion are represented by small dissolution cavities with geopetal ®lling. Early diagenetic processes in a vadose zone commonly characterize the subtidal deposits forming the EDU tops. Finally EDUs formed by sediment representing a same sub-environment (aggradation) and showing emersion-related features on top(A-EDUs) have also been recognized in the succession. The vertical organization of recognized lithofacies and related early diagenetic overprint clearly point to oscillations of depositional environments repeatedly interrupted by emersion, suggesting high-frequency relative sea-level oscillations. The measured succession includes the well known Orbitolina level which represents an important biostratigraphic marker horizon of southern Apennine carbonate platform deposits of Aptian age. In the ®rst 20 m of the studied section the EDU stacking leads to 10 bundles of a higher order (average thickness 205 cm) composed of 2-5 elementary cycles. In turn the recognized bundles join into 2 superbundles formed by 4 and 3 bundles with thicknesses of 753 cm and 549 cm respectively. The EDUs display a hierarchical stacking probably related to composite eustatic sea-level fluctuations in the Milankovitch frequency band. By contrast, a few metres above the Orbitolina level it was not possible to recognize any hierarchical organization of EDUs, and both fresh/brackish water deposits and green clayey levels reach their greatest abundance, testifying to a possible long-lasting relative low stand of sea-level and consequent cycle condensation. Nevertheless, some EDUs, composed of restricted lagoonal deposits, record higher frequency sea-level oscillations temporarily allowing for subtidal conditions. Unlike other Cretaceous intervals in the southern Apennines, data obtained from spectral analysis of the hierarchically organized portion of this sequence do not always agree with data from cyclostratigraphic analysis, probably because the sequence discussed does not represent a continuous record of the sedimentary processes, and also because phenomena such as amalgamation and condensation cannot be easily detected by mathematical treatment of sedimentological data. Therefore, spectral analysis of such shallow water carbonate successions should be used carefully and in conjunction with other techniques.
Microfacies analysis of shallow water carbonates and evidence of hierarchically organized cycles: Aptian of Monte Tobenna, southern Apennines, Italy
Raspini Arturo
1998
Abstract
A microstratigraphic analysis of a shallow water carbonate succession of Aptian age, cropping out at Monte Tobenna (southern Apennines, Italy), has allowed the recognition of eight lithofacies grouped into four lithofacies associations that imply inner carbonate platform environments. A sedimentological study on a 32-m-thick section, carried out on a centimetre scale, reveals a clear cyclic organization of the lithofacies; 7 to 135-cm-thick beds, frequently alternating with cm-thick green clayey levels, de®ne the 53 recognized cycles. Each cycle represents an Elementary Depositional Unit (EDU)whose vertical lithofacies evolution generally records either a transgressive-regressive trend (T/REDUs)or a regressive trend (R-EDUs). Generally, features related to emersion are represented by small dissolution cavities with geopetal ®lling. Early diagenetic processes in a vadose zone commonly characterize the subtidal deposits forming the EDU tops. Finally EDUs formed by sediment representing a same sub-environment (aggradation) and showing emersion-related features on top(A-EDUs) have also been recognized in the succession. The vertical organization of recognized lithofacies and related early diagenetic overprint clearly point to oscillations of depositional environments repeatedly interrupted by emersion, suggesting high-frequency relative sea-level oscillations. The measured succession includes the well known Orbitolina level which represents an important biostratigraphic marker horizon of southern Apennine carbonate platform deposits of Aptian age. In the ®rst 20 m of the studied section the EDU stacking leads to 10 bundles of a higher order (average thickness 205 cm) composed of 2-5 elementary cycles. In turn the recognized bundles join into 2 superbundles formed by 4 and 3 bundles with thicknesses of 753 cm and 549 cm respectively. The EDUs display a hierarchical stacking probably related to composite eustatic sea-level fluctuations in the Milankovitch frequency band. By contrast, a few metres above the Orbitolina level it was not possible to recognize any hierarchical organization of EDUs, and both fresh/brackish water deposits and green clayey levels reach their greatest abundance, testifying to a possible long-lasting relative low stand of sea-level and consequent cycle condensation. Nevertheless, some EDUs, composed of restricted lagoonal deposits, record higher frequency sea-level oscillations temporarily allowing for subtidal conditions. Unlike other Cretaceous intervals in the southern Apennines, data obtained from spectral analysis of the hierarchically organized portion of this sequence do not always agree with data from cyclostratigraphic analysis, probably because the sequence discussed does not represent a continuous record of the sedimentary processes, and also because phenomena such as amalgamation and condensation cannot be easily detected by mathematical treatment of sedimentological data. Therefore, spectral analysis of such shallow water carbonate successions should be used carefully and in conjunction with other techniques.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
