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Fossil rocky-shores, submarine and polygenic discontinuity surfaces: characterization, eustatic and tectonic implications

Discontinuity surfaces (or discontinuities) resulting from breaks in sedimentation and representing hiatuses independent of their duration are common features in the sedimentary record (Clari et al. 1995 ; Hillgärtner 1998). Such surfaces are useful marker horizons for correlation of stratigraphic sections and are highly applicable in carbonate sequence stratigraphy (e.g., Wright 1994 ; Cachão et al. 2009 ; Buatois and Mángano 2011 ; Schwarz and Buatois 2012). Besides subaerial exposure surfaces, which are commonly distinguished from other discontinuity surfaces by a variety of palaeokarst and palaeosol features (Esteban and Klappa 1983 ; James and Choquette 1984 ; Wright 1994 ; Otoničar 2007 ; Brlek et al. 2014), several other types of discontinuity surfaces, such as fossil rocky-shores represented by unconformities and basal conglomerates, as well as composite and marine omission surfaces, can be recognized in the geological record and a clear distinction can be made from subaerial exposure surface based on the criteria given below. The transition between land and sea is the most abrupt and coextensive ecological boundary on Earth (Johnson and Baarli 2012). Overall, intertidal rocky shores may account for more than 33 per cent of the world’s present shore (Johnson 1988a), but they may have been more or less abundant during the past (Johnson 2006) representing a marine habitat of long temporal persistence (Johnson 1988a, b). The presence of marine trace fossils on a hard substrate, such as borings (Trypanites Ichnofacies with common elements of the Gastrochaenolites-Entobia assemblage ; Bromley 1994 ; de Gibert et al. 2012) at the unconformity surface or as direct encrustations on it, are the best evidence for the existance of a rocky shore (Johnson 1988a, 2006 ; de Gibert et al. 1998, 2012 ; Domènech et al. 2001 ; Santos et al. 2011 ; Johnson and Baarli 2012). Such trace fossils are also significant in reconstruction of the palaeoenvironments related to sedimentary discontinuities (Bromley 1975 ; Cachão et al. 2009 ; Santos et al. 2010). Identification of rocky shores (colonised by boring and encrusting organisms due to their reduced or null sedimentation rates) in the geological record is very important because they represent major transgressive surfaces and provide crucial information about palaeoshorelines and ancient sea-levels (de Gibert et al. 1998, 2012 ; Domènech et al. 2001 ; Santos et al. 2008 ; Johnson et al. 2011, 2012). They also give information needed for palaeoenvironmental and tectonic reconstructions (Cachão et al. 2009 ; Santos et al. 2010), including e.g., tectono-eustatic movements deduced from fossilized marine tidal notches (developed by bioerosion in the intertidal zones of rocky coasts) found raised above or submerged below the water line (e.g. Mediterranean region ; Spampinato et al. 2014 ; Surić et al. 2014 ; Boulton and Stewart 2015 ; Mourtzas et al. 2015). Some ancient rocky-shore deposits involve a basal conglomerate associated with an unconformity (Johnson 1988b ; de Gibert et al. 1998 ; Domènech et al. 2001 ; Santos et al. 2008, 2011 ; Johnson and Baarli 2012). The unconformity between the Mesozoic basement and the overlying Middle Miocene (Badenian) deposits (which belong palaeo-geographically to the south-western margins of the Central Paratethys, and geotectonically to the Pan¬nonian Basin System) in the NE Mt. Medvednica (N Croatia) is marked by basal Badenian conglomerates (Brlek et al. 2016). The Upper Cretaceous limestone lithoclasts occurring in basal conglomerates show abundant truncated Gastrochaenolites and Entobia borings (represented by an in situ rocky substrate community of bivalves and sponges, respectively), with Gastrochaenolites being the dominant ichnogenus (Brlek et al. 2016). Gastrochaenolites-Entobia ichnofossil assemblage related to the Entobia subichnofacies and in turn assignable to the Trypanites Ichnofacies, is very typical of Neogene rocky shores (Santos et al. 2011). This association characterizes littoral rockground environments indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) with a low or null rate of sedimentation. Erosion of a pre-existing Mesozoic basement rocky shore during a marine transgressive phase in these high-energy littoral conditions, which formed basal conglomerates analysed here, is also evidenced by truncation and the occurrence of Gastrochaenolites borings on all sides of limestone clasts (Brlek et al. 2016). The transgressive phase correlates to the main Badenian transgression in the Central Paratethys (Orbulina suturalis Zone, the 3rd order sequence TB 2.4) (Brlek et al. 2016). The significance of substrate-controlled trace fossil suites and calcretes for genetic interpretations of discontinuity surfaces has been emphasized by many researchers (e.g., Bromley 1975 ; Wright 1994 ; Hillgärtner 1998 ; Schwarz and Buatois 2012). Marine omission surfaces are clearly marked by substrate-controlled trace fossil suites (Bromley 1975 ; Knaust et al. 2012 ; Savrda 2012 ; Schwarz and Buatois 2012), with progressive hardening of the substrate demonstrated by cross-cutting vertical relationships between the pre-lithification Glossifungites Ichnofacies (firmgrounds) and the post-lithification Trypanites Ichnofacies (hardgrounds) (Bromley 1975 ; Savrda 2012). In addition, authigenic marine mineralization (e.g., phosphorites and glauconites) also reflect sedimentary condensation and is often associated with marine omission surfaces (e.g., Bromley 1975 ; Odin and Fullagar 1988 ; Föllmi 1996, 2016). Composite (polygenic) surfaces, which record both marine omission and subaerial exposure stages (Sattler et al. 2005 ; Rameil et al. 2012), are also commonly marked by substrate-controlled trace fossil suites and calcretes (e.g., Wilson et al. 1998), and indicate the complexity of hiatal surfaces in carbonate rock successions (Rameil et al. 2012). Several firmgrounds and composite surfaces were recorded in the Upper Cretaceous platform carbonate successions in central Dalmatia, Croatia (Adriatic-Dinaridic Carbonate Platform, ADCP) (Brlek et al. 2014). Thalassinoides (probably T. paradoxicus) box-work burrow systems of the substrate-controlled Glossifungites Ichnofacies characterize the documented firmgrounds and the composite (polygenic) surface (Brlek et al. 2014). Rhizogenic laminar calcretes developed subsequently inside burrows of the composite surface through diagenetic overprint of marine sediment that passively infilled the burrows (Brlek et al. 2014). The formation of the firmgrounds was probably caused by cessation of precipitation and/or deposition of calcium carbonate due to relative sea-level fall, with development of some firmgrounds probably correlative with the regionally recorded Upper Campanian Event that represents a global eustatic sea-level fall sensu Jarvis et al. 2002 (Brlek et al. 2013). However, the recorded trace fossils associated with the composite surface indicate that this surface developed through both submarine firmground and subaerial exposure stages probably caused by several episodes of regression and transgression (Brlek et al. 2014), and exemplifies the general complexity of hiatal surfaces in shallow-marine carbonate successions (Rameil et al. 2012).

rocky-shore deposits; discontinuity surfaces; firmground; polygenic surfaces; trace fossils

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