The coralline fl ora of a Miocene maërl : the Croatian “ Litavac ”

The fossil coralline fl ora of the Badenian bioclastic limestone outcropping in Northern Croatia is known by the name “Litavac”, shortened from “Lithothamnium Limestone”. The name was given to indicate that unidentifi ed coralline algae are the major component. In this fi rst contribution to the knowledge of the coralline fl ora of the Litavac, Lithothamnion valens seems to be the most common species, with an unattached, branched growth-form. Small rhodoliths composed of Phymatolithon calcareum and Mesophyllum roveretoi also occur. The Badenian benthic association is dominated by melobesioid corallines, thus it can be compared with the modern maërl facies of the Atlantic Ocean and Mediterranean Sea. Since L. valens still survives in the present-day Mediterranean, an analogy between the Badenian Litavac and the living L. valens facies of the Mediterranean is suggested.


INTRODUCTION
Since Roman times, a building stone named "Litavac" has been quarried at many Croatian localities, particularly on the SW slopes of the Medvednica Mts. (Fig. 1).The name derives from the shortening of the Croatian words litotamnijski vapnenac meaning "Lithothamnium Limestone".
A description of the stratigraphy and palaeogeography of Miocene deposits from the investigated area (SW Medvednica) and neighbouring Samoborsko gorje -Žumberak Mts. is provided by VRSALJKO et al. (2005).The sedimentology and palaeoenvironmental evolution of Upper Badenian and Sarmatian deposits of Mt.Medvednica is discussed by VR-SALJKO et al. (2006).
Despite the historical use of Litavac as a building stone (more than 2250 m 3 were used to build the present Zagreb Cathedral; VRSALJKO & al., 2007a,b), and its importance as a reservoir rock for water and oil and gas (LUČIĆ et al., 2001), the taxonomy of the coralline algae composing the limestone has never been investigated in detail.The aim of this paper is to provide a fi rst account of the fossil coralline fl ora of the northern Croatian Litavac, and its possible palaeoenvironmental interpretation.

MATERIAL AND METHODS
Sampling has been performed at the quarries of Gornje Vrapče and Bizek (NW of Zagreb, Medvednica Mt.; Fig. 1), in the bioclastic limestones facies of Upper Badenian age.Figured specimens are from the Bizek quarry where the Litavac crops out and is about 30 m thick (see VRSALJKO et al., 2007d).Observations of coralline anatomy and biometry have been made on thin sections.Thallus nomenclature follows BASSO et al., (1997).In particular, the terms cortex and medulla, in the description of protuberances, are used with a merely topographic sense, to distinguish the central cell fi laments running parallel to the main axis of the protuberance (= the medulla) from the derivatives diverging and bending toward the surface of a protuberance (= the cortex).The terms unattached branches and prâline are used according to BASSO (1998).Growthform nomenclature follows WOELKERLING et al., (1993).Percentage quantifi cation of total coralline algal thalli is based on visual estimate.

RESULTS
The Litavac from the studied localities is a calcareous rudstone, mainly composed of fragments of fruticose corallines (VRSALJKO et al., 2007d, fi g. 2).The Litavac corallines are free-living, unattached branches or small rhodoliths with various degrees of protuberance development.The Litavac underwent dolomitization and dissolution, preventing identifi cation of a large part of the algal remains.However, despite fossil diagenesis, numerous algal fragments still have their microscopical anatomy preserved, therefore allowing identifi cation.The lumpy growth-form of this species has been identifi ed in a single rhodolith, about 6 mm in diameter, among dominant branches of L. valens.The thallus shows a thin hypothallium, with cells L 6-11 x D 6-7 μm, a perithallium with cells L 8-11 x D 7-8 μm and rounded multiporate conceptacles with diam.125-150 x 75-90 μm in height.Although the epithallial cells are not preserved, the specimen is attributed to Phymatolithon calcareum on the basis of the correspondence of all the other known anatomical features (BASSO et al., 1997).
P. calcareum is distributed from the Oligocene to Recent (BASSO et al., 1997).
Rhodolith-forming plant with a lumpy growth-form shows stout protuberances reaching about 2 mm in length.Each thallus barely exceeds 450 μm in thickness, but several thalli are superimposed to give the total thickness.In thin sections, the protuberances appear to have originated from the superposition of fertile and sterile layers of the thallus, which thicken up in correspondence with the conceptacle chambers.Hypothallium coaxial is 100-200 μm thick, with cells L 22-25 x D 12-14 μm.Perithallial cells are L 9-16 x D 8-9 μm.Multiporate conceptacles abundant has D 275-425 x H 155-205 μm with roof thickness of 50-75 μm.Several conceptacle chambers show a partial infi ll of irregularly-shaped, large "cells".
The stratigraphic distribution of M. roveretoi ranges from the Upper Eocene to the Miocene of the Tertiary Piedmont Basin (FRAVEGA et al., 1987) and in the Badenian of the Leitha Limestone (Vienna Basin; CONTI, 1946b).

The facies
On the basis of the lithology and fossil components (relative abundance of prâlines, more or less fragmented coralline unattached branches, small and large benthic/planktic foraminifera and molluscan remains) at least three different facies can be distinguished within the Bizek limestone.
Branches dominate in facies A, small rhodoliths dominate in facies B, corallines are fragmented and probably were not autochthonous in facies C. Facies A and B were also recognized at Gornje Vrapče (Fig. 1), though their geometry and palaeoenvironmental / stratigraphic relationships need further investigation.
Facies A (Pl. 1, Fig. 7) has been sampled at about 50 cm above the Triassic base.It represents a micro-breccia with angular and sub-rounded clasts of dolomite mixed with bioclasts.Coralline branches and their fragments are dominant in the limestone.Other components include centimetre to millimetre-sized fragments of bryozoan colonies and small benthic foraminifera (Lobatula lobatula, Textularidae, Elphidiidae, Miliolidae).External moulds of large Arcidae and Glycymeris sp.randomly occur.Dolomitization of bioclasts is extensive, and was followed by dissolution, both of bioclasts and lithoclasts.The resulting mouldic porosity involves about 20-30% of the rock.
The well preserved algal fragments (about 20-40% of the total algal fragments) are irregularly cylindrical, 1-5 mm in diameter and up to about 1 cm in length.Corallines with lumpy to fruticose growth-forms compose sparse, small prâlines (sensu BASSO, 1998), 1-3 cm in diameter.Most algal remains belong to L. valens, with the contribution of P. calcareum and M. roveretoi.
Two other facies B and C (Pl. 1,Figs. 8,9) appear as lateral variations at the same stratigraphic level, at about 15 m height within the Upper Badenian strata.In the algal biocalcarenite of facies B (Pl. 1, Fig. 8), which represents the most typical Litavac, the fragments of coralline branches become less abundant and smaller, while small rhodoliths become common.The coralline fl ora is composed of Lithothamnion spp.and Mesophyllum spp.Large benthic foraminifera (Amphistegina and Planostegina) are the co-dominant components.
In facies C (Pl. 1, Fig. 9), an evident orientation of the grains is observed.It represents a mud supported micro-conglomerate with small sub-rounded dolomitic clasts in a matrix of fi ne grained crushed bioclasts.The dominant components are the large benthic foraminifera, mostly as fragments, together with small benthic and common planktic foraminifera.Other components are small gastropods and fragments of bivalves together with annelid tubes.Millimetre-sized fragments of uncommon corallines also occur.

DISCUSSION AND CONCLUSIONS
The accumulation of prevalently loose-lying non-geniculate, branched corallines corresponds to the modern concept of maërl.Maërl is a Breton word defi ning a benthic association dominated by free-living, branching algal thalli.Along the north-western coast of France, at many localities in Ireland, and in Galicia, maërl covers large areas of the infralittoral seafl oor at < 20 m of water depth in a macrotidal environment, thus exposed to oscillating tidal currents (CABIOCH, 1969;BIOMAERL team, 2003).In the Mediterranean, maërl occurs infrequently at about 40 m of water depth, in current-swept, coastal bio-detritic, benthic environments of the upper circalittoral zone (BASSO & BRUSONI, 2004).
The available literature reports analogues of fossil algal facies in the Miocene of Europe.The "branching algae facies" reported by STUDENCKI (1988) 1988).This facies has been interpreted as the fi rst step of an ecological succession leading to a rhodolith pavement along a shallowing trend (STUDENCKI, 1988).
A cool-water, algal rudstone/fl oatstone microfacies composed of whole and fragmented red algae has been described for the Hungarian Leithakalk (RANDAZZO et al., 1999).Corallines appear in numerous growth-forms, can be referred to maerl or prâlines and are associated with benthic foraminifera (Amphistegina and Heterostegina) and worm tubes.They are reported to belong to several coralline genera, including Lithothamnion, Sporolithon (quoted as Archaeolithothamnion), Meso phyllum and Lithophyllum.Unluckily, the absence of palaeontological descriptions and insuffi cient illustration prevent confi rmation and more detailed identifi cation of the reported taxa (RANDAZZO et al., 1999).
These observations support the conclusion that in Paratethys, a Badenian maërl was predominantly composed of Lithothamnion (free-living branches) and Mesophyllum, with locally subordinate Sporolithon, Spongites and Phymatolithon (in rhodoliths).
We can attempt a comparison of this Badenian fossil association with the present-day maërl.The Mediterranean maërl has a diversifi ed algal fl ora, with the occurrence of several species of Lithothamnion, Phymatolithon, Lithophyllum and Mastophoroideae.Presently, L. valens is a Mediterranean endemic, infrequently occurring at considerable depth (50-70 m), mainly in the Western Mediterranean (BASSO, 1996;BABBINI & BRESSAN, 1997).In the tropics, an analogous maërl (based on the algal growthform) is represented by the accumulation of unattached branches of Neogoniolithon in seagrass meadows (BASSO et al., in press).The Badenian maërl has no obvious analogies with the tropical Neogoniolithon facies.The Atlantic and the Mediterranean maërl and their Badenian fossil counterparts are dominated by melobesioid corallines.However, one of the most characteristic and abundant species in the Badenian maërl, Lithothamnion valens, does not occur in the present-day maërl association living in the Atlantic.Since L. valens still occurs in relatively deep Mediterranean waters, we conclude that the temperate-water, modern Mediterranean maërl is fl oristically the most similar analogue of the Badenian Litavac.However, direct palaeodepth interpretations based on the Mediterranean occurrence of L. valens would be speculative, until further palaeoecological investigations based on the whole benthic association and sedimentary environment of the Litavac have hopefully clarifi ed the matter.