engleski

Role of taphonomic features in paleoecological interpretation of Eocene carbonates from the Paleogene Adriatic carbonate platform (PgAdCP)

The Paleogene (Early Ypresian, SBZ 11 to Bartonian, SBZ 17) carbonates originated on the Adriatic carbonate platform are dominated by various larger foraminiferal facies with subordinate coralline red algae facies. Foraminiferal tests (studied from standard thin-sections) show evidences of mechanical, biological or chemical breakdown with intensity that varies between foraminiferal groups and depositional settings along the shelf gradient (from proximal restricted/lagoonal to outer ramp settings). Effects observed include breakage, cutting of test edges (poles), removal of surface layering, chamber-space infillings, borings, encrustation, corrosion, pitting and their occurrences vary from sporadic to very common across the groups. Breakages ranging from damage to the terminal chamber/whorl to disintegration into sandand fine-size fragments are recorded within larger benthic foraminiferal tests in alveolinid-, nummulitid- and orthophragminid-dominated facies. Partially broken tests are most common in alveolinids. The co-occurrence of wide size range of clasts and unequal preservation of alveolinid tests attest to high- to moderate-energy settings, with possible post-mortem reworking (bombardment with sand-sized grains including more persistent nummulitid tests) within a short distance transport. Bioerosion traces in alveolinids are present in forms that were post-mortem transported over a greater distance and settled down in lower energy environments (mid to outer ramp). The mid-outer ramp environments characterize a predominance of rotaliid foraminifera over miliolids. Nummulites tests show a number of mechanical damages (cutting of tests edges, exfoliation of the youngest whorls, broken tests) as result of post-mortem seaward transportation and/or predation. Coralline red algae are significant biotic constituent in Nummulitic packstones to wackestones, and multilayer algal encrustation stands out as the most common feature. The post-depositional alternations consist of filling the pore spaces by micrite mud or by sparite, of redistribution of the tests by transport and/or by bioturbation. The upper-most part of the Eocene shallow-water carbonate succession is represented by Orthophragminae-bearing limestones (ranging in age from the Lutetian to Bartonian). They are mainly composed of orthophragminid and nummulitid tests spread in mud matrix, the former with abundant traces of bioerosion. A statistical analysis of affected specimens reveals that 10% of them are with boring marks. These non–randomly distributed traces point out to the predators or parasites responsible for leaving them, because drilling attacks are size selective and a position of traces within the test suggesting that they were made to gain access to the inside of the host. The occurrence of traces was controlled by environmental parameters such as low sedimentation rate, mesotrophication, lower photic zones and low energy, parameters that characterize outer ramp settings. As subordinate features, glauconitic infillings are recorded in some chambers spaces, as well as mechanical damages of tests (accompanied with the extensive micritic envelope), altogether suggesting the possible post-mortem transport of from shallower part of ramp towards the upper slope.

Paleogene; taphonomy; larger benthic foraminifera; shallow-water deposits

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