Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 19. 2001. (Budapest, 2001)

The porosity usually measures 15%, in thin section No. 5 it is as high as 20%. Due to compaction, adjacent grains meet in line contact, collapsed bioclasts (e.g. serpulid tubes) were observed regularly (Plate II: E, F). The flat contacts and collapsed bioclasts suggest postdepositional compaction. The majority of red algal clasts is highly fragmented but well-preserved branch fragments can also be observed. Occasionally red algal aggregates of ca. 10 mm occur, however, rhodolites were not found in the samples. Coralline algae form crusts around bioclasts, competition between algae and bryozoan colonies can be observed. Foraminifer genera observed in the thin sections are listed in Table 2. All identified foraminifera are benthic forms. All five thin sections showed the presence of miliolinas and agglutinated foraminifers of limited abundance. Only Asterigerina and Hlphidium are present in significant numbers in all samples. The latter genus occurs in a wide range of shelf communities including marginal marine environments (MURRAY 1976). Porcelaneous foraminifers or miliolinas {Quinqueloculina, Spiroloculina, Triloculinà) are abundant in shallow, subtropical seas and they are tolerant to reduced salt conditions (Agnes GÖRÖG, pers. comm.). Agglutinated foraminifers (TextttJarid) prefer cold and deeper waters (MURRAY 1976). In conclusion, the embedding rock of Ob-258 is composed of calcareous remains of shallow-water organisms, mosdy corallinacean fragments. The rock texture is grainstone and rudstone. The local bottom was soft uncemented sand. The lack of fine mud refers to deposition under strong current conditions. High porosity and coarse grain-size suggest rapid sedimentation. Algal grains are most probably redeposited fragments of branching colonies. Transportation of the corallinacean segments resulted in grain-sizes transitional between that of the maerl and the calcareous sand. Maerl usually develops at depths of 0—25 m but it is sometimes found at 40 m depth in the Mediterranean (BLANC 1968). Calcarenite forms under less energetic current conditions than maerl. It is possible that a seagrass meadow community was established on the calcareous sand, as concluded from the relative abundance of bryozoas. Sea-grass meadows usually develop on soft bottom in the infralittoral zone (ROS et al. 1985). Micritic envelopes around bioclasts are typical in shallow, euphotic environments. Diagenesis of the embedding limestone was also studied. Marine isopach rim cement is not present. The occurrence of thin micrite cement can not be excluded. If it is present, it is indistinguishable from the micritic envelope. The first observable cement generation is thin dogtooth calcite. It is common on all surfaces, including the fracture faces of the bioclasts. This fact evidences postcompactional origin. Intragranular drusy cement grows inside of the bioclasts, mosdy in the bryozoan chambers. Calcite cement filled the tiny pores of the coralline algae as well (best seen under the cathodoluminoscope). The last generation is the late, postcompaction syntaxial cement around echinoids. There is no vadose cement in the samples. Aragonitic fossils were not found in the limestone. The lack of aragonitic fossils indicates diagenetic dissolution, maybe due to meteoric water influence (see also DÜLLO 1983). Because no biomold cavities after aragonite shells are present, the dissolution must have been an early diagenetic process. Under the cathodoluminoscope all cement phases show bright orange luminescence (Plate II: E). The syntaxial, and in some cases, the intragranular cements are slightly zoned, while other cement generations show concordant bright luminescence. The calcite shows luminescence when bombarded with electron beam if Mn

2+ substitute the Ca

2+ in the crystal lattice (and no Fe

2+ is present in the system) (MARSHALL 1988). Bright luminescence of the cement refers to manganese substitution. Hence, reductive conditions during diagenesis are assumed. These observations suggest cementation under meteoric phreatic conditions. This is also supported by the stable isotope ratios. DÜLLO (1983) observed signs of early meteoric vadose aragonite dissolution and calcite cementation, from which the aragonite dissolution could easily be inserted into the diagenetical evolution of the studied samples. The diagenetic evolution of the sediment could be drawn as follows. Hypothetical early marine cementation took place at first: micritic or isopach rim cement was formed around the grains. Subsequendy, meteoric dissolution erased the marine cement and dissolved the aragonite grains. This was followed by compaction and weak cementation by dogtooth calcite under meteoric phreatic conditions. Finally, intragranular and syntaxial calcite cement appeared in the burial setting. Explanation to Plate II D Large red algal fragments in thin section No. 5. Crossed polarisers. Scale bar equals 10 mm. E Thin section No. 1 under cathodoluminoscope. Echinoid spine in scross section inserted between collapsed serpulide tube fragments. Bright luminescent thin dogtooth calcite covers the grains including fracture faces. Scale bar equals 5 mm. F Thin section No. 2 under cathodoluminoscope. Bryozoan colony on the right. Thin dogtooth calcite can be observed on the grains including fracture faces and the inside of the theca. Scale bar equals 5 mm.