Bioerosion is a common process in hard substrates. This study introduces an example from the rocky palaeoshore cropping out at a sea cliff on the Bozcaada Island. It includes bioerosion trace fossils preserved in limestone boulders of the shallow marine and lacustrine Alcitepe Formation of Late Miocene age. The ichnotaxa include borings produced by duraphagous drillers (Oichnus isp.), phonorids (cf. Conchotrema isp.), clionid sponges (Entobia cf. goniodes, Entobia geometrica, Entobia laquea, Entobia ovula, E. cf. solaris, Entobia isp.), endolithic bivalves (Gastrochaenolites torpedo, Gastrochaenolites lapidicus, Gastrochaenolites isp., Phrixichnus isp.), polychaete annelids (Maeandropolydora isp., Maeandropolydora sulcans, Maeandropolydora decipiens, Caulostrepsis taeniola, Caulostrepsis isp.), echinoids (cf. Circolites isp.) and spinculid worms (cf. Trypanites isp.). Barnacles are also common as encrusters. The borings can be ascribed to the Gastrochaenolites-Entobia assemblage, which is typical of Neogene rocky-shores. They belong to the Entobia ichnofacies indicating various conditions of light, energy, and depth. Therefore they can reveal environmental changes and play an important role in forming palaeo-rocky shores and wave-cut platforms during marine trangressive events.
La bio-érosion est un processus commun dans les substrats durs. L’étude présentée ici introduit un exemple de paléorivage rocheux affleurant au niveau d’une falaise marine sur l’île de Bozcaada. Il inclut des traces fossiles de bio-érosion conservées dans des blocs calcaires de la Formation Alçitepe lacustre et marine peu profonde, d’âge Miocène supérieur. Les ichnotaxa comportent des creusements provoqués par des organismes foreurs duriphages (Oichnus isp.), phonoriés (cf. Conchotrema isp.), éponges clionidées (Entobia cf. goniodes, Entobia geometrica, Entobia laquea, Entobia ovula, E. cf. solaris, Entobia isp.), bivalves endolithiques (Gastrochaenolites torpedo, Gastrochaenolites lapidicus, Gastrochaenolites isp., Phrixichnus isp.), annélides polychètes (Maeandropolydora isp., Maeandropolydora sulcans, Maeandropolydora decipiens, Caulostrepsis taeniola, Caulostrepsis isp.), échinoïdes (cf. Circolites isp.) et vers spinculidés (cf. Trypanites isp.). Les barnacles sont aussi des organismes encroûtants. Les creusements peuvent être attribués à l’assemblage Gastrochaenolites-Entobia, typique des rivages rocheux néogènes. Ils appartiennent à l’ichnofaciès Entobia indiquant des conditions variées de lumière, d’énergie et de profondeur. C’est pourquoi, ils peuvent révéler des changements environnementaux et jouer un rôle important dans la formation de paléorivages rocheux et de plates-formes d’érosion marine au cours des évènements marins transgressifs.
Bioerosion, Rocky-shore, Trace fossils, Late Miocene, Bozcaada, TurkeyBio-érosion, Rivage rocheux, Traces fossiles, Miocène supérieur, Bozcaada, TurquiepresentedPresented by Philippe TaquetIntroduction
Bioerosion, designed by Neumann (1966) as an abbreviation of ‘biological erosion’, identifies every form of biological erosion of hard substrates, including lithic and woody substrates (Bromley, 1992). Organisms can erode hard substrates by mechanical or chemical means, resulting in the formation of scrape traces, borings and other biogenic structures. Encrustation is the behaviour by which organisms attach to hard substrates. Encrusting and boring organisms represent a community that changes with the substrate that sustains them (Hohman, 1993).
Rocky-shores, as hard surfaces exposed in shallow marine environments associated with null or low sedimentation, offer excellent conditions for colonization by marine bioeroding organisms (Cachão et al., 2008). These environments represent a unique marine sedimentary environment where wave-cut marine erosion platforms are formed. Therefore, the identification of rocky-shores in the geological record is very important, because they represent major flooding surfaces and provide us with crucial information about palaeoshorelines and sea level changes. Bioerosional structures play a major role in their recognition (Ghibaudo et al., 1996, Jia-Yu and Johnson, 1996, Martinell and Domènech, 1995 and Santos et al., 2008).
Bioerosional evidence is also excellent for the study of the evaluation of marine communities in these types of environments (Johnson, 1992) since their remains are often preserved in situ, connected to the hard surface itself (Brett, 1988, Johnson, 1992 and Taylor and Wilson, 2003). The case of ancient marine hard substrate communities has been described from the Lower Cambrian (e.g. Kobluk, 1981a, Kobluk, 1981b, Kobluk and James, 1979 and Kobluk et al., 1978) to the Neogene (e.g. Bromley and D’Alessandro, 1987, Bromley and Asgaard, 1993a, Bromley and Asgaard, 1993b, Domènech et al., 2001, Gibert et al., 1998, Martinell and Domènech, 1995, Radwański, 1970 and Watkins, 1990).
Although bioerosional structures are very important in revealing the effects of biological factors on coastal morphology, there are very few publications on bioerosion in Turkey. The first paper related to bioerosion in Turkey, ‘Neogene borings from rocky-shore’, is Uchman et al. (2002), who explained the relative sea level changes recorded in borings from a Miocene rocky-shore of the Mut Basin, southern Turkey. The present study uses bioerosional structures to contribute to the interpretation of palaeoenvironmental conditions in the rocky-shore of the Late Miocene depositional sequence of the Alcitepe Formation. It firstly focuses on the identification and the description of bioerosion structures with their tracemakers and then analyses the borings in terms of their substrate nature.
Geological setting
The studied rocks of the Alcitepe Formation are located along the southwestern coast of Bozcaada Island (Fig. 1A–C), on the southern edge of the Thracian Basin (e.g. Göncüoglu, 2010). Previous palaeontological and sedimentological studies on the Neogene deposits of the island are quite limited (e.g. Çağatay et al., 2006, Druit, 1961, Kesgin and Varol, 2003, Önem, 1974, Sakınç et al., 1999, Siyako et al., 1989 and Temel and Çiftçi, 2002). These Neogene sediments are composed of various coeval lithofacies with lateral and vertical transitions (Dermitzakıs and Papanikolaou, 1981, Sakınç and Yaltırak, 1997, Sakınç et al., 1999, Yaltırak, 1996, Yaltırak and Alpar, 2002, Yaltırak et al., 1998 and Yaltırak et al., 2000).
Regarding previous studies, the lowermost unit in the generalized stratigraphical section (Fig. 2) is represented by the Eocene-Oligocene Ayvacik Volcanics, mainly comprising andesitic lavas and pyroclastics. Pillow lavas at some localities characterize quiescent submarine volcanic activity. The Ayvacik Volcanics are disconformably overlain by the Gazhanedere Formation, which is characterized by mudstone and conglomeratic sandstone (Middle Miocene–Lower Pliocene), representing a fluvial and lagoonal environment. On top of the Gazhanedere Formation, with lateral and vertical transitions, rests the Kirazli Formation. This consists of massive and semi-consolidated sandstone representing beach and back-beach lithofacies of the Late Miocene (Sakınç et al., 1999). Both formations are overlain by the Alcitepe Formation (Late Miocene) with lateral and vertical transitions. This formation consists of bioclastic and oolitic limestone with basal clastic rocks, and contains molluscan and ostracod faunas in the western outcrops. These faunas indicate deposition in shallow, brackish to fresh water environments. On the other hand, previous studies (Çağatay et al., 2006) on faunal and palaeomagnetic analyses of the Alcitepe Formation in other parts of the Aegean Sea show that the formation represents chron C3r (6.04–5.24 Ma). The ostracod analysis also indicates that during the deposition of the Alcitepe Formation salinity increased from brackish in the lower part to more saline conditions in the upper part (Çağatay et al., 2006).
In the study area, a non-depositional and denudational period continued until the Pleistocene. During the transgression in the Middle-Late Pleistocene (Yaltırak et al., 2002), the valleys and broad coastal plains were affected by warm sea conditions. The terrace deposits formed during this transgression include conglomerates and sandstones, which are rich in Ostrea edulis shells (Sakınç et al., 1999, Yaltırak, 1996 and Yaltırak et al., 2002). In general, they overlie the Miocene rocks with an angular unconformity in this area.
The borings described in this article occur within the Alcitepe Formation.
Description of the locality
The Alcitepe Formation is quite extensive in Bozcaada Island and shows up as elevations and hills due to its physical features (e.g. erosion-resistence). The formation shows different characteristics from place to place. For instance, along the northern coast of the Gulf of Saros, the formation consists of Mactra-bearing bioclastic and cross-bedded limestones, interbedded with Cardium-bearing sandstones and Ostrea banks. These sediments indicate fluctations in salinity from brackish water to normal marine conditions. In the Gelibolu and Biga peninsulas the formation consists of mudstone and marl in the lower part, and bioclastic and oolitic limestones with marl, mudstone, and sandstone intercalations in the upper part (Çağatay et al., 2006).
The best exposure of the Alcitepe Formation in the studied area is located along Habbeli Bay in the southwestern part of the island (Fig. 1C), between N 39° 49’ 107; E 025° 59′ 026 and N 39° 49’ 2877; E 025° 59′ 369, along the cliff of Ayazma beach in Habbeli Bay, and southeast of the few inland localities which are represented by carbonate and siliciclastic units (Fig. 3 and Fig. 4).
At Habbeli Bay, the Alcitepe Formation is approximately 30 metres thick and rests on the Kirazli Formation. The formation starts with Mactra-bearing, beige-grey coloured, medium-bedded, pebbly to coarse-grained sandy limestone, and continues with conglomerates alternating with sandstones, succeeded by marls (18 cm thickness) in the study area. The marls of the formation contain a brackish water fauna with Mactra shells (Fig. 5).
Towards the top of the section, Ostrea- and Pecten- bearing calcareous marine sandstones occur. Directly on top of this level is a 70-cm thick coquin composed of bivalve and gastropods shells, which can be traced as a marker. The bivalves are almost completely dissolved with only their moulds remaining. The unit continues as whitish grey coloured, thin-medium layered limestone, sandy limestone and mudstone towards the top.
Oolitic limestone can be observed in some localities, as can large- scaled tabular cross stratification (Fig. 6) and lightly cemented spherical sand clumps. The presence of large-scaled cross stratification and widespread ooids indicates high-energy levels with agitated shallow marine or lacustrine deposition progressing towards the open sea.
Towards the top of the section, Ostrea- and Pecten-bearing calcareous marine sandstone occurs. The uppermost part of the formation is made up of an alternation of Mactra-bearing limestones and sandstones containing Ostrea, followed by another alternation of Mactra- and Ostrea-bearing limestones. The presence of Ostrea the upper part of the Alcitepe Formation indicates normal marine depositional conditions.
The section from the middle to top of the succession is rich in fossils, represented by bivalves (Fig. 7: 1–9; 12), some echinoids (Fig. 7: 10, 11) and gastropods (Fig. 7: 13, 14). Bivalves produce Gastrochaenolites torpedo (Fig. 8), and Gastrochaenolites lapidicus is produced by some bivalves of the genera Lithophaga. They all are observed on a heavily karstified and brecciated Miocene carbonate hard-substrate bioeroded surface, on limestone boulders and cobbles.
In the study area, the most conspicuous trace fossil is Gastrochaenolites, which was produced by bivalve bioerosion preserved in epirelief and hyporelief (G.torpedo and G.lapidicus), and on the boulder surfaces (Fig. 9a, b). Gastrochaenoliteslapidicus casts (Fig. 10) and clavate borings Phryxichnus-like Gastrochaenolites have also been recognized. These borings are perpendicular to the surface, are truncated, and some include the trace-producing organisms (Fig. 11).
Entobia is the second dominant trace fossil that is mostly observed from penetrating outer and inner parts of shells and boulders. In some cases, it is mostly seen together with Gastrochaenolites. The polychaete annelid borings Caulostrepsis and Maeandropolydora are observed occasionally. Trypanites was identified in the substrate from the walls of G.torpedo. Also, some skeletons of encrusting worms were frequently observed.
Relationship between substrate and borings
The bioerosional structures in the study area occur together with fossil assemblages including bivalves, gastropods, echinoids and also remains of encrusting organisms. Their traces are widely distributed within carbonate boulders, and shells of ostreids, and were formed by the activities of macrobioeroders. The borings are preserved in concave hyporelief and epirelief morphology, and display intense biogenic activity with abundant skeletal remains of endo and epilithic organisms. Most of the borings are common on the upper parts or the lateral surfaces of the boulders. They are perpendicular to the surface and are proximally truncated. The outer parts of the borings have been destroyed by the marine erosion. Their ethological categories, tracemaker interpretations and the relationship between the substrates and borings in the study area are summarized in Table 1.
Taxonomic description
The Late Miocene rocky-shore of the studied area is intensively affected by bioerosion. Bioerosional structures are widely distributed on the tops of the limestone boulders and on shells. The most common bioerosional traces belong to macrobioeroders, e.g., phoronids (Conchotrema), polychaete worms (Caulostrepsis, Maeandropolydora), clionid sponges (Entobia), and boring bivalves (Gastrochaenolites).
Nine ichnogenera have been recognized. Taxonomic description is prepared according to their ethology and morphology distinguished by Bromley (1992), Ekdale et al. (1984), Martinell (1989), and recently, Taylor and Wilson (2002). All material figured in this article is housed in the Natural History Museum, General Directorate of Mineral Research and Exploration, MTA Ankara, Turkey (B1. 09).
ConchotremaTeichert, 1945
cf.Conchotremaisp. (Plate 1, Fig. A)
Description: A system of straight to slightly curved branched grooves. The grooves are 1.5–2, 0.1–0.25 mm in wide. The surfaces of the boulders were cut by their dense scrapings and they cross each other.
Remarks: Generally, these are subsurface borings, which have been exposed at the surface by erosion. Voigt (1975) suggested that Conchotrema is produced by endolithic phoronids.
CaulostrepsisClarke, 1908
Caulostrepsisisp. (Plate 1, Fig. B)
Description: Caulostrepsis is a pouch-shaped boring resembling a very tight U-gallery with limbs. The limbs merge towards the aperture. The aperture is 1 mm in diameter.
Remarks: Bromley and D’Alessandro (1983) revised the systematics of this ichnotaxon and recognized several ichnospecies. It is produced by polychaetes (Radwański, 1969).
Caulostrepsis taeniolaClarke, 1908
(Plate 1, Fig. C, D)
Description: This is a smooth, elongate and narrow U-shaped gallery with distinct limbs and an interconnecting vane. The trace fossils range from 5 mm long, 0.5 mm wide (Fig. C), to 7 mm long, 0.8 mm in wide (Fig. D).
Remarks: This trace is seen on the exterior surfaces of Ostrea shells. Caulostrepsis is mainly considered to be produced by the polychaetes of the genus Polydora (Radwański, 1969).
Caulostrepsis occurs in shallow water environments, e.g., Caulostrepsis cretaceaVoigt, 1971 was found between 7 and 15 m water depth (Wisshak et al., 2005).
CircolitesMikuláš, 1992
cf.Circolitesisp. (Plate 1, Fig. E)
Description: Circolites is a subcircular, bowl-shaped boring 3 to 4.5 cm in diameter.
Remarks: This boring is interpreted as a dwelling structure of regular echinoids (Warme, 1975), and is always restricted to very shallow waters.
MaeandropolydoraVoigt, 1965
Maeandropolydoraisp. (Plate 1, Fig. F)
Description: Maeandropolydora occurs as long, shallow and sinuous borings on boulders and shells that extend like ‘U’-shaped pouches in the study area. It is 0.2 mm in diameter.
Remarks: This boring is produced by polychaetes of various families (Bromley and D’Alessandro, 1983).
Maeandropolydora sulcansVoigt, 1965
(Plate 1, Fig. G)
Description: M.sulcans is a sinuous, cylindrical boring found in Ostrea shells in the study area small in diameter (0.5 mm) and with a narrow vane.
Remarks: It is usually attributed to annelid worms (Bromley and D’Alessandro, 1983). M.sulcans is commonly preserved in semirelief on rock or shell surfaces as a result of the erosion of very shallow borings.
Maeandropolydora decipiensVoigt, 1965
(Plate 1, Fig. G)
Description: A single specimen is preserved on a small fragment of an oyster shell and comprises a sinuous, looped and continuous, smooth, unbranched, internally structureless gallery that parallels but clearly penetrates the shell surface. The looped limbs do not touch or intersect and the intervening oyster shell material is unaffected. Diameter of the gallery, relatively constant at both levels, is approximately 0.5 mm.
Remarks: Though incomplete, the specimen can clearly be assigned to the ichnotaxon Maeandropolydora, discussed in detail by Bromley and D’Alessandro (1983). Maeandropolydora is interpreted as the domichnion of suspension-feeding polychaete annelids (Bromley, 1994).
EntobiaBronn, 1837
Entobiaisp. (Plate 1, Fig. H)
Description: This boring shows camerate morphology and consists of networks of chambers in planar arrangement. The shapes of chambers are from subrounded to subrectangular. The chambers are 3 mm wide.
Remarks: Entobian borings are produced by endolithic sponges. Such borings can be made by several species of Cliona (Bromley and D’Alessandro, 1989).
Entobiacf.goniodesBromley and Asgaard, 1993a
(Plate 2, Figs. A, B)
Description: This boring is observed as a system of small and camerate to nodular chambers. The chambers are 1–1.5 mm in diameter.
Remarks: E.goniodes is produced today by Cliona viridis and rarely Cliona schmidti in the photic zone of the Mediterranean Sea. C.viridis is found at the 20 m water depth (Bromley and Asgaard, 1993a).
Entobia laqueaBromley and D’Alessandro, 1984
(Plate 2, Figs. C, D)
Description: This bioerosional structure is composed of a system of tunnels and chambers in well-developed growth stages A and C sensu Bromley and D’Alessandro (1984). It is seen here as growth stage C and is represented by irregular, oval, elongate to subangular chambers, 1.5–2 mm in diameter.
Remarks: This ichnospecies covers the surface of boulders. Entobia laquea is produced by sponges of the genus Cliona in the photic zone of the Mediterranean Sea at the present day (Bromley and Asgaard, 1993a).
Entobia ovulaBromley and D’Alessandro, 1984
(Plate 2, Fig. E)
Description: This trace fossil is preserved in the A to C growth stages exposed on the surface of the boulders. The structures of stage A occur as a narrow tunnel system, with tunnels less than 1 mm in diameter. The structures of stage B are composed of curved rows of elongate chambers that are 2 mm in wide. Stage C is developed as oval, closely spaced chambers, which are 3 mm wide.
Remarks: The ichnogenus Entobia is produced mostly by sponges of the genus Cliona. Its taxonomy was discussed by Bromley and D’Alessandro (1984). Entobia ovula is produced in the Mediterranean today by C.schmidti, Cliona vermifera or Cliona vastifica (Bromley and Asgaard, 1993a).
Entobiacf.solarisMikuláš, 1992
(Plate 2, Fig. F)
Description: This boring is preserved as an irregularly hemispherical depression. It also has straight radiating tunnels. The tunnels are about 1 mm in width.
Remarks: Entobia solaris has been described from the Late Cretaceous of the Czech Republic (Mikuláš, 1992). These borings are produced by sponges.
Entobia geometricaBromley and D’Alessandro, 1984
(Plate 2, Figs. G, H)
Description: This is a camerate ichnospecies. Apertures are of two distinct size groups. The larger apertures are circular and 4–5 mm in diameter, the smaller are between 1 mm and 2 mm in diameter.
Remarks: E.geometrica differs from E.cretacea in several respects (Bromley and D’Alessandro, 1984). The apertures of E.geometrica are larger and the chambers to a greater extent leave thinner dividing walls than E.cretacea in which the chambers are normally connected by single intercameral canals.
GastrochaenolitesLeymerie, 1842
Gastrochaenolitesisp. (Plate 3, Fig. A)
Description: Gastrochaenolites is a flask-shaped boring with a narrow aperture and larger, round, elongate, straight, ovoid chambers. It is preserved as large, elongate, straight openings, steeply inclined to the host substrate. Lengths are up to 49 mm and a single observed opening is 5 mm in wide.
Remarks: This ichnogenus is produced by boring bivalves (Kelly and Bromley, 1984) such as Pholadidae, Gastrochaenidae, and Mytilidae (Fisher, 1990a, Fisher, 1990b and Warme, 1975).
Gastrochaeonolites lapidicusKelly and Bromley, 1984
(Plate 3, Fig. B)
Description: G.lapidicus displays a rounded bottom. It is a smooth ovate chamber with an apertural neck and is circular throughout cross-section. The neck is also circular in cross-section or elliptical. The boring nearly 6 mm in diameter.
Remarks: This type of boring is produced by some bivalves of the genus Lithophaga (Kelly and Bromley, 1984).
Gastrochaenolites torpedoKelly and Bromley, 1984
(Plate 3, Figs. C, D)
Description: This is a smooth, strongly elongate chamber, 49 mm long, up to 17 mm in depth and 8–16 mm in diameter (Fig. C).
Remarks: G.torpedo may show a calcite lining (Jones and Pemberton, 1988) which is not observed in the studied material. This type of borings is produced by some bivalves of the genus Lithophaga and Gastrochaena (Kelly and Bromley, 1984), and in the Mediterranean region by Lithophaga lithophaga (Linnaeus). G.torpedo has been reported from Miocene rocky-shores of many regions in Europe and neighbouring areas (Radwański, 1969).
PhrixichnusBromley and Asgaard, 1993a
Phrixichnusisp. (Plate 3, Fig. E)
Description: This is a smooth, strongly elongated clavate chamber. The chamber is 24 mm long and 13 mm in diameter. Its clavate shape is clearly seen.
Remarks: A clavate, Gastrochaenolites- like boring but exhibiting a very particular ornamentation on the walls consisting of arcuate or concentric grooves in two gently concave or flat areas that meet along one edge of the boring.
Phrixichnus was previously recorded in the Pleistocene of Rhodes (Bromley and Asgaard, 1993a). It was also recognized on the Middle Miocene rocky-shores of Catalonia, Spain (Domènech et al., 2001).
OichnusBromley, 1981
Oichnusisp. (Plate 3, Figs. F, G, H)
Description: Smooth, vertical, circular to subcircular holes with axes oriented perpendicular to host substrates (gastropods and bivalves), completely or incompletely penetrative. Diameters range from 1.5–2 mm.
Remarks: Oichnus are generally interpreted as praedichnia of the gastropod families Naticidae and Muricidae respectively (Bromley, 1981 and Pickerill and Donovan., 1998).
TrypanitesMägdefrau, 1932
cf.Trypanitesisp. (Plate 3, Fig. D)
Description: Trypanites is a simple, vertically to obliquely oriented boring that can curve slightly and have rounded terminations. The boring extends into the substrate from the walls of G.torpedo and is 1 mm diameter and up to 7 mm in length.
Remarks: Trypanites is generally considered to have been produced by sipunculid worms (Bromley, 1992 and Pemberton et al., 1980). This trace occurs in marine carbonate deposits, including firm- to hardgrounds, pebbles and skeletal substrates (Bromley and D’Alessandro, 1987).
Discussion
The Miocene succession in the northeastern Aegean region indicates the first Mediterranean marine transgression affecting these areas. Frequent marine flooding occurred during the Late Miocene in the northeastern Aegean region. This period in the study area is represented by brackish to fresh water carbonates with some marine sandstone-siltstone interbeds of the Alcitepe Formation, with endolithic bivalves, endolithic clionid sponges, echinoids, polychaetes, annelids and ostracods.
The foremost trace fossil in the studied area is Gastrochaenolites, which is a typical hardground colonizer. Two ichnospecies of Gastrochaenolites have been identified on the basis of the shape of the distal part of the boring. G.lapidicus, which displays a rounded base, and G.torpedo, which is larger with a sharper base. Some of the borings contain the trace-producing organism in living position, and in some the proximal part of Gastrochaenolites is truncated by submarine erosion. Phrixichnus has also been observed for the first time. Phrixichnus is a clavate boring resembling Gastrochaenolites.
Entobia is the second most dominant ichnogenus in the studied material. Five ichnospecies are identified in the study area. It is frequently observed together with Gastrochaenolites. The inferred polychaete annelid borings, sipunculid worms, mollusc drill holes, and duraphagous scars are also present with Gastrochaenolites and Entobia.
The rocky-shore in the study area is covered with limestone boulders, which correspond to the bioeroded surfaces of wave-cut platforms. These result from cliff retreat, which is on relatively large sub-horizontal rocky surfaces. As a stratigraphic point of view, the above mentioned bioeroded rockground surfaces reflect hard substrate marine flooding surfaces (transgressive surfaces). The hardground surface shows well-developed macrobioerosional structures. According to previous workers (Santos et al., 2010) the hardground marine flooding surfaces (transgressive surfaces) are characterized by the presence of Gastrochaenolites borings, especially vertical G.torpedo.
Therefore, the rocky-shores at Bozcaada Island, including bioerosional structures with an abundance of the borings Gastrochaenolites, especially G.torpedo, can be interpreted as horizontal wave-cut platforms with low or no sedimentation. Moreover, the occurrences of this ichnogenus (Gastrochaenolites) characterize shallow water environments (Bromley, 1992, Bromley and Asgaard, 1993a and Bromley and Asgaard, 1993b), according to Bromley (1992), indicating only a few metres of water depth. Water depth is restricted to 1–2 m in the Neogene of the Mediterranean areas affected by G.torpedo (Kleemann, 1973). However, unpublished work has recently revealed that Gastrochaenolites may have occurred in several tens of meters water depth in the past. G.R. Bromley and U. Asgaard (personel communication) have observed Gastrochaenolites in ancient Greek marble statues from a Roman shipwreck at an approximate depth of 45 m. Also Danian Gastrochaenolites in Denmark seems to have inhabited deeper–water (Kleemann, 1973).
On the other hand, all endolithic organisms, especially Entobia borings (endolithic sponges), require low rates of sedimentation. In this context, the studied ichnoassemblage suggests a rocky-shore with a low rate of sedimentation. In addition, low sedimentation or non-deposition are revealed by bioeroded hardgrounds (Cachão et al., 2008). The bioeroded surfaces, recognized in the field on the wave-cut platform, being hardground transgressive marine flooding surfaces, are characterized by Gastrochaenolites borings. The assemblages found on wave-cut platforms are composites resulting from the overprinting of successive ichnocenoses that could replace each other during a transgression. As the transgression progressed and the substrate was exposed to deeper conditions, an initial bivalve-dominated community was progressively replaced by a sponge-dominated community.
Conclusions
1. The study of the Late Miocene rocky-shore in the Habbeli Bay area (western part of Bozcaada Island) in the Alcitepe Formation revealed an assemblage consisting of the following 19 ichnotaxa: Oichnus isp., cf. Conchotrema, Circolites isp., Entobia isp., E.goniodes, E.geometrica, Entobia laquea, Entobia ovula, Entobia cf. solaris., Gastrochaenolites isp., G.torpedo, G.lapidicus, Phrixichnus isp., Maeandropolydora isp., Maeandropolydora sulcans, Maeandropolydora decipiens, Caulostrepsis isp., Caulostrepsis teniola, and cf. Trypanites.
2. The ichnoassemblage associated with the limestone boulders collected from the rocky-shore is dominated by the trace of endolithic bivalves, endolithic sponges, echinoids, polychaetes, and annelids. These are marine organisms, which belong to a marine hard substrate community showing the existence of an ancient wave-cut platform.
3. Gastrochaenolites and Entobia are dominant borings in the studied locality.
4. Although bivalve borings of the ichnogenus Gastrochaenolites and the sponge boring Entobia are dominant in this region, in the shallower part, Gastrochaenolites is more dominant (Bromley and Asgaard, 1993a and Martinell and Domènech, 1995). As the transgression progressed and substrate was exposed to deeper conditions, an initial bivalve-dominated community was progressively replaced by a sponge-dominated community. During this period of time, bioerosion combined with physical erosion lowered the substrate, thus destroying a great part and truncating the Gastrochaenolites and Entobia.
5. Phrixichnus has been for the first time reported in Turkey.
6. The bioerosional assemblage found in boulders consists of small Gastrochaenolites and Entobia. They indicate that endolithic communities on unstable substrates were affected by physical factors.
7. The borings can be classified as a Gastrochaenolites-Entobia assemblage, which is typical of Neogene rocky-shores and belongs to the Entobia ichnofacies. This characterizes littoral rockground environments indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) in the stratigraphical record.
Acknowledgements
This work was supported by the Research Project 2009-08-16-01-2 of the Natural History Museum General Directorate of Mineral Research and Exploration Ankara, Turkey. The author is grateful to Prof. Dr. Richard Bromley (Copenhagen University, Denmark) for critical remarks and improvement of the manuscript, and to Prof. Dr. Ana Santos (Universidad de Huelva, Spain) who made valuable suggestions. Also acknowledged is Dr. Ceren Küçükuysal, Ayşe Demirci, and Tuncay Temiz for their technical support during the preparation of this paper, and Prof. M.C. Goncuoglu for linguistic improvements.
Finally, I wish to thank Dr. Michel Laurin (Editor) and reviewers for the constructive discussions.
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A, B. Geological map of the northeastern Aegean region (modified from Sakınç and Yaltırak, 2005). C. Close up view of the study area (modified from Temel and Çiftçi, 2002).
A, B. Carte géologique de la région égéenne nord-orientale (modifié selon Sakinç et Yaltirak, 2005). C. Encart montrant la zone d’étude (modifié selon Temel et Çiftçi, 2002).
Generalized measured section of the study area (modified from Sakınç and Yaltırak, 2005).
Section générale de la zone étudiée (modifié selon Sakinç et Yaltirak, 2005).
Wave-cut platform produced by wave erosion in the Alcitepe Formation (Habbeli Bay).
Plate-forme d’érosion marine produite par l’érosion des vagues dans la Formation Alçitepe (Baie d’Habbeli).
General view of the Alcitepe Formation (Habbeli Bay).
Vue générale de la formation Alçitepe (Baie d’Habbeli).
Mactra shells in the grey coloured marls at Alcitepe Formation (Bozcaada Island, Turkey).
Coquilles de Mactra dans les marnes grises de la Formation Alçitepe (île de Bozcaada, Turquie).
Shallow marine-shore face cross lamination, Alcitepe Formation.
Stratification entrecroisée sur une face de rivage marin peu profond, Formation Alçitepe.
Well-preserved marine macrofossils at Alcitepe Formation (Scale: 1 cm).
Macrofossiles marins bien conservés de la Formation Alçitepe (échelle = 1 cm).
Plan view of Gastrochaenolites torpedo.
Vue de Gastrochaenolites torpedo.
View of Gastrochaenolites torpedo (a), Gastrochaenolites lapidicus (b).
Vue de Gastrochaenolites torpedo (a) et de Gastrochaenolites lapidieus (b).
Casts of Gastrochaenolites lapidicus.
Moulages de Gastrochaenolites lapidieus.
Gastrochaenolites with its producer inside.
Gastrochaenolites avec l’organisme à l’intérieur du trou.
Images of inferred polychaete, phonorid, echinoid, bivalve, annelid and clionid sponge borings from the Alcitepe limestone boulders and shells (All scales: 1 cm). A.Conchotrema isp. B.Caulostrepsis isp., and internal surface of Gastrochaenolites showing a sculpture of concentric lines or rugae. C.Caulostrepsis taeniola.D.Caulostrepsis taeniola. E. cf. Circolites isp. F.Maeandropolydora isp. G.Maeandropolydora sulcans (M1), Maeandropolydora decipiends (M2), Entobia laquea (E1), Gastrochaenolites isp (G). H.Entobia isp.
Images de creusements supposés de polychètes, de phonoridés, d’échinoïdes, de bivalves, d’annélides, et d’éponge clionidée dans des blocs calcaires et coquilles de la Formation Alçitepe (toutes échelles = 1 cm). A.Conchotrema isp. B.Caulostrepsis isp., and internal surface of Gastrochaenolites showing a sculpture of concentric lines or rugae. C.Caulostrepsis taeniola.D.Caulostrepsis taeniola. E. cf. Circolites isp. F.Maeandropolydora isp. G.Maeandropolydora sulcans (M1), Maeandropolydora decipiens (M2), Entobia laquea (E1), Gastrochaenolites isp (G). H.Entobia isp.
Field photographs of inferred clionid sponge borings from the Alcitepe limestone boulders and shells (All scales: 1 cm). A.Entobia cf. goniodes. B.Entobia goniodes. C.Entobia laquea. D.Entobia laquea. E.Entobia ovula. F.Entobia cf. solaris. G.Entobia geometrica. H.Entobia geometrica.
Photos sur le terrain de creusements supposés d’éponge clionidée, dans des blocs et coquilles calcaires de la Formation Alçitepe (toutes échelles = 1 cm). A.Entobia cf. goniodes. B.Entobia goniodes. C.Entobia laquea. D.Entobia laquea. E.Entobia ovula. F.Entobia cf. solaris. G.Entobia geometrica. H.Entobia geometrica.
Images of inferred bivalve borings and drill holes-duraphagous scars from the Alçitepe limestone boulders and shells (All scales: 1 cm). A.Gastrochaenolites isp. B.Gastrochaenolites lapidicus. C.Gastrochaenolites torpedo. D.Gastrochaenolites torpedo with Trypanites isp. E.Phrixichnus isp. F.Oichnus. isp. G.Oichnus isp. H.Oichnus isp.
Images de trous et creusements supposés de bivalves-traces de duriphages dans des blocs et coquilles calcaires de la Formation d’Alçitepe (toutes échelles = 1 cm). A.Gastrochaenolites isp. B.Gastrochaenolites lapidicus. C.Gastrochaenolites torpedo. D.Gastrochaenolites torpedo with Trypanites isp. E.Phrixichnus isp. F.Oichnus. isp. G.Oichnus isp. H.Oichnus isp.
Relationship between substrate and borings, identified at the Habbeli Bay/Ayazma beach (LB: Limestone Boulders; S: Shell).
Relation entre substract et creusements, déterminée sur l’exemple de la baie d’Habbeli/plage d’Ayazma (LB : blocs calcaires ; S : coquille).