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Kuphus beds at the 34th IAS Meeting of Sedimentology

Kuphus beds in an Oligocene carbonate platform (Sierra de la Argüeña, Southeastern Spain): palaeoecological context and relationship with shallowing-upward cycles” by Alice Giannetti and Santiago Falces-Delgado

Although the Oligocene carbonate platforms saw the development of Kuphus-dominated assemblages, few studies addressed the role of these bivalves in ancient shallow-water ecosystems. Kuphus is a genus of peculiar teredinid burrower bivalves that begun to be common in the Oligocene, reaching their maximum abundance in the Oligocene-Aquitanian time interval. Nowadays, they are represented only by the species K. polythalamia, present in the Western Pacific. Recent studies revealed that the physiology of K. polythalamiais based on symbiosis with chemoautotroph sulphur bacteria, this producing new interest about the role of Kuphus in ancient carbonate platforms. Kuphus siphons are embedded in concentric calcitic sheets forming long subcylindrical tubes, which accumulation produces thick beds with good preservation potential. In the Sierra de la Argüeña (Alicante province, Southeastern Spain), different types of Kuphus beds were recovered in nearshore carbonate platform deposits, preserved either as resedimented material or in situ assemblages. Five facies were distinguished: i. marly limestones rich in fragments of Kuphus tubes, corals, irregular echinoids, bivalves and gastropods; ii. miliolid-rich wackestones/packstones with small turriculate gastropods and the branched coral Stylophora, representing protected environments close to the coast; iii. small coral bioconstructions formed by laminar and domed colonies of Porites, Montipora, Actinastrea and different favids; iv. packstones rich in red algae and echinoid fragments with lepidocyclinids and miliolids, filling the space between coral bioconstructions; v. fine-grained greenish silty sands, locally associated to Microcodium. These facies are repeated throughout the section to form regressive cycles from the lagoon to coral patch reefs developed in the inner platform and then to nearshore areas, in proximity of the emerged coast. Beds with resedimented Kuphus tubes are found in the lowermost or in the uppermost part of the regressive sequences; in both situations, preferred orientation is absent. In the first case, they are made up of tube fragments mixed with other bioclasts and have been interpreted as perireefal accumulations. In the second case, the beds are made up almost exclusively of Kuphus tubes, show quite a high density of tube fragments and wide areal development. These have been interpreted as product of physical reworking, e.g. by storms. Several patches of tubes in life position vertically cross this bioclastic rudstones, suggesting that some bivalves survived the event and resumed the construction of the tube after the storm ceased. At the very top of the section, anterior ends are preserved in life position inside the fine-grained silty sands of facies v. This last level is of particular interest, since it represents the original undisturbed environment of the pioneer colonies. So far, accumulation of organic matter and plant remains similar to those present in the habitat of modern-day Kuphus were not observed in this bed, this suggesting the possibility that Oligocene Kuphus could have retained normal filter feeding mechanisms and were not obligate chemotrophic organisms. This would recommend prudence with the use of modern-day Kuphus polythalamia as an ecological analogue of fossil Kuphus and that further study is needed before considering the latter a proxy for poorly oxygenated environments.

Cite as: Giannetti, A., and Falces-Delgado, S. (2019): Interaction of boring organisms with corals. Entobia ichnofacies development in a transgressive nearshore scenario. 34th IAS International Meeting of Sedimentology, Rome 10-13 September 2019. “Sedimentology to face societal challenges on risk, resources and record of the past.  Sesion 1.A-989.