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

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KEY FEATURES OF MIXED CARBONATE-SILICICLASTIC SHALLOW-MARINE SYSTEMS 373 Fig. 4 - The two sections measured along the southern side of the Capo Colonna promontory (see fig. 1B for location). These shelf deposits sharply overlie the Cutro Clay, which represents the substrate of the marine terrace. that original, indicating that the terrace has not been tilted. Such a WRS is associated to a Glossifungites ichnofacies where it truncates the upper part of the CC1 cycle (ZECCHIN et alii, 2009) (fig. 3). Regressive clastic shoreface deposits pass distally into mostly coralline algal frameworks and associated calcarenites (NALIN & MASSARI, 2009; ZECCHIN et alii, 2009) (figs. 2-4). The general stratigraphic architecture of both cycles highlights relatively thin transgressive and thick regressive intervals (fig. 2). The latter can be top-truncated due to wave ravinement during a subsequent erosional transgression. FACIES ANALYSIS OF THE CC2 CYCLE The CC2 cycle is composed of three facies associations composing a shoreface-shelf depositional system. According to the sequence stratigraphic interpretation by ZECCHIN et alii (2009), the sharp surface that marks the base of the cycle, interpreted as a wave-scoured ravinement surface and paved by the deposits of facies association A, erodes both the substrate of the terrace (the Cutro Clay) and the upper shoreface deposits forming part of the CC1 cycle (figs. 2 and 3). The boundary between the facies associations A (below) and B-C (above) is interpreted as the boundary between transgressive deposits accumulated during siliciclastic starvation (facies association A) and normal plus forced regressive deposits, i.e. a maximum flooding surface (figs. 2 and 3). The boundary between normal and forced regressive deposits is thought to coincide with the erosional contact between Facies B1 (below) and shallower clastic deposits of Facies C1 (above), that is found in distal locations (figs. 2 and 3). BASAL CONDENSED DEPOSITS (FACIES ASSOCIATION A) - TST Basal conglomerate (Facies A1) This facies is locally present at the base of the terrace deposits (Col 10 section, fig. 4), and consists of a granule-
374 M. ZECCHIN & M. CAFFAU Fig. 5 - Well cemented shell bed (Facies A2) and bryozoan-bearing calcirudite (Facies A3), which form the rigid substrate of the coralline algal frameworks of Facies B1. The outcrop is located close to the Col 3 section (see fig. 1B for location). to cobble-size (up to 10 cm) conglomerate up to 10 cm thick. Clasts are well rounded and commonly elongated, and are composed of quartz and lithic fragments. The matrix consists of fine-grained quartz sandstone. The base of the facies is highly erosional and locally irregular at the outcrop scale, and corresponds to the sharp contact between the substrate (the Cutro Clay) and the terrace deposits (fig. 10). Shell bed (conquina) (Facies A2) Facies A2 (up to 40 cm thick) displays a lenticular shape in dip sections, thinning toward both onshore and offshore (fig. 3). It consists of a cemented and burrowed calcirudite made of pectinid and oyster fragments, with minor bryozoans (figs. 3-5). A mixed siliciclastic-carbonate fraction, consisting of medium- to coarse-grained sandstone, is present. The deposit is structureless to flat-laminated, and its base is flat and erosional. Facies A2 lies in the lower part of the CC2 cycle, above Facies A1 or directly at the contact with the underlying CC1 cycle, and is abruptly overlain by Facies A3 or Facies B1 (figs. 3-5). Bryozoan-bearing calcirudite (Facies A3) As observed in Facies A2, also Facies A3 displays a lenticular shape that thins both offshore and onshore (the Col 3 section, fig. 3). Facies A3 (up to 50 cm thick) consists of a highly packed bryozoan-bearing calcirudite with a minor amount of pectinid shell fragments and quartz granules (figs. 3 and 5). The facies is locally cha - racterized by an unidirectional cross-stratification partially masked because of the coarse grain size (fig. 3). Sets are 20 cm thick, and foreset are 10° to 15° inclined. The base is sharp and erosional, whereas the top consists of a non-erosional contact with Facies B1 (figs. 3 and 5). A term intermediate between Facies A2 and A3 is present in the Col 5 section (fig. 3). Here, a planar-stratified and locally burrowed calcirudite composed of mixed oyster valves and bryozoans is visible at the base of the terrace deposits. Fragments of calcareous algae are mixed with bryozoans in the upper part of the facies (fig. 3). This deposit is overlain by Facies B1 and B2 (fig. 3). Interpretation of facies association A The conglomerate of Facies A1, found at the base of the terrace deposits, is interpreted as a transgressive lag deposit accumulated after wave erosion and reworking of clasts from the substrate (DEMAREST & KRAFT, 1987; NUMMEDAL & SWIFT, 1987; KIDWELL, 1991). Deposits of Facies A2 and A3 record a phase of generalized silicicla - stic sediment bypass and starvation promoting the concentration of skeletal material in distal shoreface to shelf settings (e.g. KIDWELL, 1991; NAISH & KAMP, 1997). Moreover, Facies A3 resulted from the colonization of a stable substrate by epifaunal benthic communities (KONDO et alii, 1998), then transported by shelf currents to form cross bedding. Facies A2 and A3, therefore, formed an extensive carbonate pavement in relatively distal settings, which was subjected to early cementation providing a rigid substrate for the growth of the coralline algal frameworks of Facies B1. CORALLINE ALGAL FRAMEWORK AND ASSOCIATED DE - POSITS (FACIES ASSOCIATION B) - HST Coralline algal framework (Facies B1) This facies is well represented in the middle to outer part of the measured transect, where it is at least 3.5 m thick (the Col 4 section), and thins both offshore and onshore (fig. 3). Moreover, the facies shows a patchy di - stribution in landward locations, where abrupt lateral contacts with clastic deposits are recognizable (Col 5 and Col 10 sections, figs. 2-4). Facies B1 consists of a laterally extensive carbonate body that commonly form well cemented frameworks (figs. 3-7). The primary framework builders are coralline algae that form a compact structure, whereas bryozoans, corals (Cladocora caespitosa), serpulid-worm tubes, mollusc shells (pectinids, oysters and gastropods) and fragments of echinoids represent accessory components (figs. 3, 4, 6A, 6B and 8). Although the encrusting succession is unclear, four species were identified in the algal assemblages in variable proportion: Titanoderma pustulatum (Lamouroux), Mesophyllum alternans (Foslie) CABIOCH & MENDOZA, and minor Lithophyllum incrustans Philippi and Spongites sp. (fig. 8). Locally, the framework is less massive and contains also a siliciclastic fraction, consisting of quartz sandstone to granule clasts (Col 2 and Col 10 sections, figs. 3 and 4). Irregular cavities, filled by sediments of Facies B2 (see below), are very common (figs. 3, 4 and 7). Near its landward termination, Facies B1 consists of large patches, some tens of meters wide, laterally confined by sediments of Facies B2 (see below) (figs. 2, 3 and 6D). The coralline algal framework typically lie upon the pavement formed by Facies A2 and A3, which offered a rigid substrate for the growth of the bioconstructions (figs. 2-5). Calcarenites and calcirudites (Facies B2) Facies B2 consists of well cemented bioclastic deposits filling irregular cavities into the coralline algal frameworks of Facies B1, or accumulated adjacent to them (figs. 2, 3, 4, 6A, 6C, 6D and 7). The bioclastic detritus is mostly composed of mollusc shells, bryozoans and
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