Late Eocene transgressive sedimentation in the western Swiss Alps: Records of autochthonous and quasi-autochthonous biofacies on a karstic rocky shore

Abstract

Carbonate karst unconformities represent primary sequence-stratigraphic boundaries but, where sealed by marine sedimentary successions, also signify ancient rocky shores. During the Late Eocene (Priabonian), a shallow sea flooded the deeply karstified and brecciated Cretaceous carbonate bedrocks of the western Swiss Alps. Transgression resulted in the formation of a rocky archipelago of basement highs with elevations of up to 20 m and dip angles varying from sub-horizontal to sub-vertical. Although rocky coasts are commonly subjected to vigorous waves, strong currents and storm surges, protected and restricted narrow embayments existed between these islands. The new habitats thus created were colonized by intertidal and shallow subtidal macrobiota, which can be grouped into four biofacies associations: 1) endolithic; 2) epilithic; 3) intertidal; and 4) shallow subtidal. The endolithic biofacies corresponds to an impoverished ichnoassemblage composed of Gastrochaenolites and Entobia borings. The epilithic biofacies is made up of well-preserved rocky-shore biota such as oysters and corals exhibiting an encrusting lifestyle. The intertidal biofacies is composed of wholly preserved shells of potamidid gastropods (Tympanotonos? sp.). The biota present in these biofacies are monospecific or show a very poor species diversity. This fact is in agreement with the pioneer character of the organisms and the sheltered and restricted environments they inhabited. The shallow subtidal biofacies has higher diversity and is characterized by dasycladaceans, oysters, cardiid bivalves, potamidid, ampullinid and Campanile giganteum gastropods, echinoids and solitary corals. The transgressive carbonates evolve upwards into sandy limestones and sandstones displaying sedimentary structures formed in high-energy conditions. This progressive lithofacies change coincides with the opening of the coastal system once the irregular palaeotopography was filled. The transgressive deposits reported here are of significance in that they contain remarkable examples of shallow subtidal and intertidal autochthonous and quasi-autochthonous biofacies associations generated in rocky coast and soft-sediment environments. To our knowledge, this is the first study documenting in detail a rocky shore in the Priabonian.

Trace fossils from the Lower Muschelkalk of Raciborowice Górne (North Sudetic Synclinorium, SW Poland) and their palaeoenvironmental interpretation

“…Gastrochaenolites is indicative of very shallow marine environments, intertidal-upper subtidal (Bromley and Asgaard 1993;Ekdale and Bromley 2001a;Wilson et al 2005;Carmona et al 2007;Checconi et al 2010;Bover-Arnal et al 2011;Caracuel et al 2011;Belaústegui et al 2012). Malpas et al (2005) regard this ichnogenus to be limited to waters of no more than a few metres deep.…”

Transgressive Eocene clastic-carbonate sediments from the Circum-Rhodope belt, northeastern Greece: implications for a rocky shore palaeoenvironment

“…Ancient rocky shores provide important information about Precambrian and Phanerozoic sea levels and palaeoshorelines as well as unusual fossil communities (Johnson and Baarli, 2012;Baarli et al, 2014) and trace fossil assemblages (de Gibert et al, 2012). This specific setting has often been developed on limestone substrates affected by various physical, chemical, and biological processes (Semeniuk and Johnson, 1985;Reading and Collinson, 1996;Sanders, 1997;Tropeano and Sabato, 2000;Sheppard, 2006;Bover-Arnal et al, 2011;Baarli et al, 2014). This specific setting has often been developed on limestone substrates affected by various physical, chemical, and biological processes (Semeniuk and Johnson, 1985;Reading and Collinson, 1996;Sanders, 1997;Tropeano and Sabato, 2000;Sheppard, 2006;Bover-Arnal et al, 2011;Baarli et al, 2014).…”

“…The redeposition of material derived from preexisting carbonate rocks is a common process in various sedimentary environments where extraclasts originate from the drainage of river systems in near-coastal regions, rock-falls at shores, supply by means of debris and turbidity flows, and by aeolian transport (Flügel, 2004). Such material may be trapped and reworked in shallow to deep marine and lacustrine settings where concomitant carbonate deposition takes place, resulting in mixed clastic-carbonate sediments (Tropeano and Sabato, 2000;Clifton, 2003;Titschak et al, 2005;Sheppard, 2006;Bover-Arnal et al, 2011). In particular, limestone conglomerates are typical products of wave-influenced beachface and shoreface environments where they record conditions of rapid erosion and burial (Flügel, 2004).…”

“…Many of them have been associated with gravelly beaches and nearshore regions as coarse psephitic deposits may be the only depositional record. This specific setting has often been developed on limestone substrates affected by various physical, chemical, and biological processes (Semeniuk and Johnson, 1985;Reading and Collinson, 1996;Sanders, 1997;Tropeano and Sabato, 2000;Sheppard, 2006;Bover-Arnal et al, 2011;Baarli et al, 2014).…”

“…The winnowed grain-supported fabrics with reworked and closely packed bioclasts, as opposed to the lack of micrite, lamination, and encrustations suggests that the depositional area was subjected to the action of continuous waves and/or currents above the fair-weather wave-base. Also, recent and other fossil examples show that the occurrence of associated red algae and larger benthic foraminifers is typical of more open marine settings (Johnson and Hayes, 1993;Johnson et al, 1996;Racey, 2001;Bassi, 2005;Payros et al, 2010;Bover-Arnal et al, 2011;Nebelsick et al, 2013).…”