Key features of mixed carbonate-siliciclastic shallow-marine systems ...

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Fig. 10 - Cartoon showing the Capo Colonna depositional system during co ralline algal framework growth. The system is characterized by siliciclastic sedimentation in proximal settings passing into mostly carbonate sedimentation seawards. Currents locally washedout the calcarenites associated to the coralline algal frameworks, and sili - ciclastic sandstones may be laterally juxtaposed to the latter. Transgressive strata are represented by the condensed deposits of the facies association A, whereas the bulk of the sedimentation occurred during highstand conditions. KEY FEATURES OF MIXED CARBONATE-SILICICLASTIC SHALLOW-MARINE SYSTEMS 377 Trough cross-bedded sandstone (Facies C2) This facies is present in the proximal sections, where its lower contact locally corresponds with the base of the terrace deposits (the Col 7 section, figs. 2 and 3), and its thickness reaches 3 m. The facies passes into Facies C1 in a seaward direction, locally with a sharp contact, and it is overlain by a fine-grained wedge derived from palaeocoastal cliff dismantling in subaerial conditions (figs. 2, 3 and 9A). Facies C2 consists of trough-cross stratified, medium- to very coarse-grained, mixed siliciclastic to bioclastic sandstone (figs. 3 and 9A). Cross sets commonly alternate with planar-laminated intervals (figs. 3 and 9A). Bioclastic layers are well cemented. Pectinid shells and burrows are occasionally present. Trough cross-sets are up to 30 cm thick and foreset laminae are inclined between 15° and 40°. Toe of cliff deposit (Facies C3) This facies is present only in the extreme proximal part of the transect, at its landward termination (the Col 8 section, figs. 2 and 3), and consists of cobble to boulder size (10 cm to 1 m) calcareous clasts. Cobbles and boulders are commonly bored by Lithophaga holes, and are associated with medium- to coarse-grained quartz sandstone, which becomes dominant seaward and shows flat to very low-angle lamination (fig. 3). Facies C3 passes seaward into Facies C2 (fig. 3). Interpretation of facies association C This facies association is representative of a mixed siliciclastic-bioclastic shoreface-shelf system, receiving the supply of both terrigenous and intrabasinal bioclastic detritus. The structures and distribution of Facies C1 suggest the accumulation in relatively lower energy condition in distal shoreface to inner shelf settings (READING & COLLINSON, 1996; CLIFTON, 2006). The facies replaces the coralline algal frameworks (Facies B1) in proximal settings, whereas its lower erosional contact above Facies B1 indicates an accumulation in more distal settings when algal growth ceased. The local lateral contact of Facies C1 with Facies B1 suggests the action of strong currents among the coralline algal framework patches, which were able to turn away the calcarenites (Facies B2) associated to Facies B1 and then mostly siliciclastic fine-grained sediment was deposited during calmer conditions. The trough cross-bedded Facies C2, located in more proximal settings, represents an upper shoreface deposit, where 3D dunes migrated in the surf zone due to the action of longshore and rip currents (MASSARI & PAREA, 1988; HART & PLINT, 1995; CLIFTON, 2006). The locally recognized sharp lower contact with Facies C2 is interpreted as the surf diastem (ZHANG et alii, 1997), bounding the upper shoreface zone and migrating seawards during coastal regression (CLIFTON, 2006). Facies C3 derives from the accumulation of coarse detritus coming from older marine terraces located on top of the partially eroded palaeo-coastal cliff that lies just behind the Capo Colonna terrace. This deposit, passing seaward into Facies C2, is inferred to have accumulated in a beachface context (e.g. POMAR & TROPEANO, 2001). DISCUSSION Present data highlight that facies associations reco - gnized in the CC2 cycle of the Capo Colonna terrace deposits form a mixed carbonate-siliciclastic system, showing shelf condensed deposits (facies association A) overlain by coralline algal frameworks and related calcarenites (facies association B), that pass landwards and upwards into shoreface-shelf clastic deposits (facies association C) (figs. 2 and 10). Current models of carbonate ramp sedimentation and architecture highlight the role of the interplay between accommodation creation and carbonate production/rate of reef growth in determining the slope and morphology of the ramp itself (WILSON, 1975; JAMES & CLARKE, 1997; WRIGHT & BURCHETTE, 1998; POMAR, 2001a,b; PEDLEY & CARANNANTE, 2006). Non-tropical carbonates are commonly characterized by a land-attached clastic wedge, an increase of bryozoans with depth, and by dominant coralline algal deposits in mid-ramp settings (CARAN- NANTE et alii, 1988; MARTÍN et alii, 1996, 2004; PEDLEY & GRASSO, 2002). NALIN & MASSARI (2009) considered the CC2 cycle as an example of non-tropical carbonate ramp deposit. They also noted that the asymmetric architecture of the CC2 cycle, consisting of thicker regressive deposits, shows
378 M. ZECCHIN & M. CAFFAU closer affinities with siliciclastic systems (see ZECCHIN, 2007) than non-tropical carbonate systems, as the latter commonly feature limited inner shelf deposition (RYAN et alii, 2008). NALIN & MASSARI (2009) explained this discrepancy as the consequence of minor hydrodynamic levels during accumulation of the Capo Colonna deposits with respect to other contexts. In the Capo Colonna deposits, an important silicicla - stic component, becoming dominant landwards, characterizes the system, and this must be considered to justify the observed stratal architecture of the CC2 cycle. Due to the action of waves and shelf currents, clean siliciclastic fine-grained sandstones of Facies C1 could be transported offshore and accumulated adjacent to coralline algal frameworks (fig. 9B). Moreover, also coarser siliciclastic particles were transported offshore during storms, composing a minor fraction of mostly carbonate deposits. Thus, the dominance of mixed clastic sedimentation in proximal settings justifies the strong resemblance of the architecture of the considered cycle with that of silicicla - stic systems, which are typically characterized by the R cycle architecture (i.e. dominated by the regressive component of the cycle; ZECCHIN, 2007) in sequences controlled by late Quaternary glacio-eustasy (ZECCHIN et alii, 2009; ZECCHIN et alii, 2010). It should also be noted that the deposits of the Capo Colonna terrace overlie a fine-grained terrigenous unit, and that the overall dip of the shoreface and shelf during deposition was inherited from the dip of the basal WRS (fig. 2), not controlled by carbonate productivity. The geometry of the WRS is in turn controlled by wave energy during transgression, rate of relative sea-level rise, sediment supply and inherited physiography (CATTANEO & STEEL, 2003; ZECCHIN, 2007). Taking into account the present evidence, therefore, in contrast to NALIN & MASSARI (2009) it is highlighted that the CC2 cycle of the Capo Colonna terrace cannot be considered as a carbonate ramp deposit, and it should be referred to a temperate mixed carbonate-siliciclastic shallow-marine depositional system. The main features of this mixed system include (figs. 2 and 10): dominance of coralline algal deposition in distal settings; mixed siliciclastic-bioclastic sedimentation in proximal settings; pe - culiar lateral juxtapositions between siliciclastic deposits and coralline algal frameworks due to hydrodynamic processes; comparability of the stratal architecture of the clastic-dominated part of the system to that of siliciclastic shelf/ramp systems. Moreover, it is important to note that the gradient of the studied shoreface-shelf system is not controlled by carbonate productivity but is inherited by the processes acting before and during transgression. CONCLUSIONS Facies and architectural analyses of the CC2 cycle, forming most part of the Capo Colonna terrace deposits, show a mix of carbonate and siliciclastic sediments accumulated in a shoreface-shelf setting. Key diagnostic features of these mixed deposits are (i) the prevalence of clastic deposits landwards and coralline algal frameworks seawards, (ii) a stratal architecture comparable to that of siliciclastic systems, that is the dominance of regressive deposits in late Quaternary sequences, (iii) the dominant role of the inherited physiography and processes typifying the transgression rather than carbonate productivity in shaping the shelf profile, and (iv) the local lateral juxtaposition of siliciclastic sandstones and coralline algal frameworks in shelf settings due to hydrodynamic processes. ACKNOWLEDGMENTS This study was carried out within the CARG Project (official geological cartography of Italy, scale 1:50,000) for the geological mapping of the Ionian Calabria. 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