A charophyte flora from the Lower Pechelbronn Formation (?Upper Eocene/?Lower Oligocene) of Malsch south of Heidelberg (SW Germany)

The Lower Pechelbronn Formation (?Upper Eocene/?Lower Oligocene) of Malsch (clay-pit ‘Am Viehweg’) yielded a moderately abundant charophyte assemblage. One new species, Chara rhenana sp. nov., is erected, two more species, Harrisichara lineata and Nitellopsis (Tectochara) cf. wonnacotti, are recorded in the Rhine Graben area for the first time. The Lower Pechelbronn Formation may be correlated to the Stephanochara vectensis zone (Riveline, 1986) and the mammalian level MP 20 (reference locality: St. Capraise; Schmidt-Kittler, 1987). The associated ostracod fauna clearly indicates a limnic depositional environment.


INTRODUCTION
The Palaeogene block of Malsch is situated between the Tertiary sediments covered by mighty Quaternary deposits of the Rhine Graben to the west and the Mesozoic sediments of the Kraichgau to the east (Barth, 1970a), about 20 km south of Heidelberg (Fig. 1). This uplifted block, whose structure had been recognized for the first time by Moos (1934) -Roter Septarientonplatteis built up by Palaeogene sediments (Eocene Basal Clay, Lymnaea Marls, Pechelbronn Formation, Grey Marls Formation) with an overall thickness of 460 m maximum.
At the southern edge of this block, very close to the eastern peripheral fault of the Rhine Graben, several clay-pits are situated; in this area Lymnaea Marls and the Pechelbronn Formation are exposed at the surface. The first clay-pits had been laid out at the beginning of this century east of Federal Highway 3 (Heidelberg-Karlsruhe); these are currently refilled (clay-pits 'Guggen' and 'Dittmannstal'; Barth, 1970b). The latter was of particular interest for mammalian palaeontology since a well-dated fauna from the Lower Pechelbronn Formation had been described by Tobien (1949Tobien ( , 1968Tobien ( , 1987Tobien ( , 1988. West of Federal Highway 3 are located the clay-pits 'Reimschloch' and 'Am Viehweg'. The first indication of an occurrence of charophytes in the clay-pits of Rot-Malsch was given by Wilser (1922). She cited Chara petrolei from strata of the clay-pit 'Guggen', erroneously assumed by her to be Cyrena Marls; this charophyte species, however, may not be interpreted with certainty (Schwarz & Griessemer, 1992).
The only further information referring to the subject is Barth (pers. comm.) who succeeded in isolating gyrogonites from samples of the clay-pits 'Reimschloch' and 'Am Viehweg'; these specimens were later determined by Dr K. Madler (Hanover).

SAMPLING
The material for this paper was obtained from 15 samples taken by one of us (Th.G.) in 1988 from the clay-pit 'Am Viehweg', excavated by the 'Ziegelwerke Bott-Eder GmbH' at Rauenberg. Due to the clay-extracting methods using a bulldozer, no detailed collecting could be carried out. Detailed collecting was carried out by Barth in the 1960s, however, when clay was extracted by bucket chain excavating.

GEOLOGY
The clay-pit 'Am Viehweg', origin of the charophytes subsequently described, has been exploited since 1962. The sediments, exposed at present, nearly 125 m in thickness, are exclusively of ?Upper Eocene/?Lower Oligocene age. According to Barth (1970a: fig. 2) the strike angle varies between 5.5" and 85", and the dip angle between 22" and 40" to the NW. Thus, the age of the sediments decreases from SE (samples 1-3c) to NW (sample 13). The sampled sediments predominantly belong to the Lower Pechelbronn Formation, consisting of muds and calcareous muds with intercalated sandstones; only sample 13 may belong already to the overlying Middle Pechelbronn Formation, normally containing a rich ostracod and foraminifera1 fauna.
The collected samples show olive-green to grey colours (1-3c, 7-12), while samples 4-6 came from red to 'beige'-coloured muds. At the NE edge of the pit, close to the 'g' of 'Viehweg' on the map (Fig. lc), intense reddish-coloured muds with alabaster gypsum nodules can be observed. The stratigraphical position of these intensecoloured muds has not yet been clarified. There are indications that they may represent a special local type of facies somewhere within the sequence of the Lower Pechelbronn Formation. Alternatively, the aforementioned petrographic characters also allows one to regard them as the 'Rote Leitschicht'. This is a very important marker-level encountered by petroleum geologists in many borings in different oilfields in the middle part of the Rhine Graben. The 'Rote Leitschicht' is considered to be a separate stratigraphical unit underlying the Lower Pechelbronn Formation by some authors. Other geologists include it in the Lower Pechelbronn Formation, to represent the basal part of it, an opinion the authors of this paper would share.

MICROFAUNA
Besides the charophyte species, our knowledge about the palaeoecological demands of the accompanying microfauna is essential to the interpretation of the palaeoenvironment. Nearly all samples yielded a freshwater ostracod fauna, preserved in most cases with broken valves. The valves and fragments could be assigned to the following genera: Heterocypris?, Herpetocypris?, Ilyocypris, Paralimnocythere, Stenocypris and Virgatocypris. These genera are known to inhabit freshwater, salinity being less than 3%0. Stenocypris is regarded as a genus living in warm waters (Morkhoven, 1963: 55;Triebel, 1953: 12). Thus the whole fauna can be attached to the Candona-Cypridopsis assemblage (Keen, 1975), which lived in shallow lakes or lake edges. In addition, some internal moulds of snails have been found by picking the samples. The other fauna (Jurassic ostracodes, sclerites of sponges, columnalia of crinoids, spines of echinids, Jurassic? foraminifera) is reworked from Mesozoic sediments forming the border of the Rhine Graben between Heidelberg and Karlsruhe. However, no further investigations on this fauna were undertaken: for distribution of this fauna see Fig. 4.

SYSTEMATIC DESCRIPTIONS
The material examined and figured is kept in the micropalaeontological collections (collection GRIES-SEMER) of the Geological Survey of Rheinland-Pfalz at Mainz, Germany. From Chara rhenara sp. nov. some isotypes have been given to the British Museum (Natural History), London. Description. Oval to elliptic gyrogonites, as a rule constantly narrowed towards the base; gyrogonites thus occasionally with a somewhat conical appearance. Apical pole in general slightly depressed or even conspicuously flattened, not domed equally, as in the case of many Chara species. In a great number of specimens (70%) the apical centre has broken off, which is evidence for rather decreased thickness of the spiral cells in the apical periphery. Spiral cells usually slightly concave to almost flat, rarely markedly concave or even convex; in the apical region invariably concave. Intercellular ridges barely prominent, periapical constrictions not, or at most poorly, discernible; apical ends of spiral cells, however, distinctly widened. Basal pore mainly within a shallow basal crateriform depression. Grambastichara, differ from the new species firstly by the noticeably periapically modified spiral cells, secondly by an f equally domed apex and thirdly by an elevated number of convolutions, Remarks. Riveline (1986: 68) points out the strong morphological similarities that exist between the genera Chara Vaillant, 1719 and Psifochara Grambast, 1959 from the Upper Eocene onwards and thus the difficulties which can arise in the generic assignment of some species; as an example she refers to Chara hamsteadensis. Likewise in Chara rhenana sp. nov., pecularities of both genera are combined: the oval, sometimes slightly conical shape of the gyrogonites points to the genus Chara, whereas the periapically, barely modified, spiral cells as well as the somewhat flattened apical region, are to be found within the genus Psilochara. A future revision of both genera would be highly desirable, especially since the diagnosis of Psilochara does not sufficiently separate that genus from Chara .
Genus Gyrogonu Lamarck, 1804 ex Lamarck, 1822, emend. Grambast, 1956 Gyrogonu sp. (PI. 2, figs 6-7) Samples 4 and 10 yielded some isolated and fragmentary, flat to barely concave spiral cells of great width (up to 170pm) and thickness. Such cells are characteristic of the genus Gyrogona. Specific assignments on the basis of spiral cells alone are, of course, always delicate, but it may be presumed with some certainty that they belong to Gyrogona wrighti.

BIOSTRATIGRAPHY
The charophyte association from the Lower Pechelbronn Formation of Malsch comprises exclusively those species that also exist at other ?Upper Eocene/?Lower Oligocene localities in the Rhine Graben system (Schwarz, in prep.).
Decisive for the biostratigraphic position of the locality Malsch/'Am Viehweg' is the synoptic representation of the hitherto known maximum vertical extension of all involved taxa.
Chara rhenana sp. nov. appears in the Rhine Graben from the Lymnaea Mark up to the Upper Pechelbronn Formation (Schwarz & Griessemer, 1992) with increased density records from the Lower Pechelbronn Formation (Table 1).
Harrisichara lineata is confined, in the Rhine valley, to the Lymnaea Mark and the Lower Pechelbronn Formation (pers. obs.). To date, this species has been described only from western Europe (Riveline, 1986); there, its stratigraphic levels extend from the 'MarinCsien moyen' (Chara friteli zone) to the 'Ludien moyen rCcent 2i supCrieur'
Sphaerochara pygmaea, a less frequently observed species, occurs in the Rhine Graben from the Lymnaea Mark up to the Upper Pechelbronn Formation (Schwarz & G riessemer , 1 992).
Tolypella pumila is a quite rarely encountered species in the Rhine Graben, which occurs from the Lymnaea Mark to the Upper Pechelbronn Formation (Schwarz & Griessemer, 1992). In the Paris Basin it has been recorded from several localities in the Marnes bleues d'Argenteui1 and the Calcaire de Brie (Cavelier et al., 1972;Riveline, 1973).
Tolypella caudata has been quoted from the Lymnaea Mark of the Rhine Graben by Breuer & Feist (1986). Originally, this species was described from the 'Tongrian' of Hoogbutsel (Belgium), but it also exists in the Paris Basin (Cavelier et al., 1972;Riveline, 1973  Limestone du Gypse for the biostratigraphic interpretation of the sedimentary sequence studied here. The possibility of a more circumscribed correlation is offered solely by the two 'exotics' of the assemblage, Harrisichara lineata and Nitellopsis (Tectochara) cf. wonnacotti, occurring together in sample 4 (Fig. 4): Stephunochara uectensis zone (Riveline, 1986).
In the Hampshire Basin the Sfephanochur.u uectensis zone covers the whole series of the Bembridge Beds (Bembridge Limestone and Bembridge Marls); in the Paris Basin it is represented by the Premiitre Masse du Gypse and the Marnes supragypseuses (Riveline, 1986).
It may be deduced easily that within the charophyte zonation the Lower Pechelbronn Formation parallels the lower parts of both the Bembridge Mark and the Marnes supragypseuses (i.e. the Marnes bleues d'Argenteuil). This concept, although in contrast to the opinion of Breuer & Feist (1986; Table 1) in some aspects, would take into consideration equally well all data available for the mammal zonation; these data indicate concordantly an MP 20-age (reference locality: St. Capraise) for the following lithological units (Table 1): Lower Pechelbronn Formation: Tobien (1987Tobien ( , 1988. Bembridge Marls: Bosma (1974), Gad et al. (1990), Schmidt-Kittler (1987).