geological field trips 2010 -2(2) i n d e x geological field trips 2010 -2(2) i n f o r m a t i o n s 4 Riassunto L'escursione offre una panoramica delle associazioni di facies e dell'evoluzione stratigrafico-sedimentologica di sistemi deposizionali carbonatici di età giurassico-cretacica, affioranti nella Sardegna orientale (Golfo di Orosei) e nella Nurra (Sardegna nord-occidentale). La successione della Nurra si è deposta in un contesto di mare epicontinentale a partire dal Giurassico sino a buona parte del Cretacico, i depositi della Sardegna orientale (Baiociano-Berriasiano) evidenziano un ambiente più aperto e strutturato per la presenza di bacini intrapiattaforma, verosimilmente prospiciente la Tetide alpina. La presenza di successioni parzialmente coeve nei due settori permette una prima comparazione fra i diversi sistemi deposizionali in esame, integrata da un confronto con eventi riconosciuti a scala regionale e globale. Infatti, sebbene le successioni in esame mostrino notevoli affinità con le serie coeve del dominio pirenaico-provenzale (Nurra), sono presenti sensibili differenze fra le successioni in esame sia in termini di facies sia per numero ed ed estensione temporale di lacune stratigrafiche (Golfo di Orosei). Queste differenze sono principalmente ascrivibili ad un diverso contesto paleogeografico e deposizionale tra i diversi settori.

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Arragoni et al. (2016) suggest in their paper published on tectonics that the carbonate succession of Eastern Sardinia represents a Cenozoic fold‐and‐thrust belt, related to the Alpine orogenesis. According to these authors, this supposed fold‐and‐thrust belt represents the southward continuation of the Alpine Corsica collisional chain and the missing link between the Alpine Chain and the Calabria‐Peloritani domain. Field evidence and the published literature document instead that all the surfaces that Arragoni et al. interpret as thrust are actually stratigraphic contacts. The balanced geological section of Arragoni represents thus a geometric exercise missing the basic data needed to nurse the proposed model, and it does not reflect the geology of Eastern Sardinia. The data provided by Arragoni et al. (2016) do not support the presence of an Alpine thrust‐and‐fold belt in Eastern Sardinia, and this paper may suggest to the geological community a misleading interpretation of the geodynamic evolution of the Alpine and Mediterranean area.

Quantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represen...

•We propose a quantitative analysis of a carbonate platform using 3d numerical models.• Parameter definition is critical to produce realistic 3d numerical models.• Modelling parameters were defined by architectural and sedimentological studies. •The obtained model has been tested by comparison with outcrop and laboratory data.• 3d numerical models may help to parameterise petrophysical properties of geobodies.Key words: carbonate platform, forward modelling, Triassic, facies analysis, diagenesis 2 AbstractQuantitative analysis of depositional systems using numerical models can provide insights into the interaction of sedimentological and diagenetic processes, difficult to interpret from observation of the geological products alone. Numerical models can highlight the interactions between controlling parameters, and generate best estimate simulations of case studies from the geological record. Different data sources (e.g., outcrop data, uniformitarian constraints, paleogeography) need to be considered in the process of defining the parameters required to produce models that honour individual case studies. We present a best-estimate forward model of a high-relief Triassic carbonate platform (Southern Alps, Italy) generated using CARB3D+, a 3D process-based forward modelling package able to simulate sedimentary facies, geometries and early diagenesis of isolated carbonate platforms. The parameters used for the presented forward model have been deduced by the study of architectural, sedimentological and diagenetic features of the platform, and by data from modern and ancient analogues. Sensitivity analysis is used to tune values of the parameters within ranges defined from field, laboratory and uniformitarian constraints. Critical evaluation of the model allowed 1) verification of the validity of the used parameters; 2) evaluation of the interaction between controlling parameters; 3) development of a numerical model that can be used for quantitative elaborations. Furthermore, the process of generating the numerical model required a detailed interpretation of field and laboratory data, crucial to define the required input parameters. Modelling the studied carbonate platform also enabled evaluation of the effects of the interaction between productivity, environmental energy and creation of accommodation space, promoting the understanding of their role on the architecture of carbonate systems. Forward modelling also allows the integration of data from conventional data sets with data from presentday observations and seismic geometries, promoting a transition from a qualitative to a semiquantitative/quantitative sedimentological approach. Forward models can further be used to parameterise petrophysical properties of complex geobodies, with potential application for the management of georesources.