The Pliocene and Pleistocene fluvial evolution in the northern Upper Rhine Graben based on results of the research borehole at Viernheim ( Hessen , Germany )

The research borehole drilled in 2006 by the Hessian Agency for the Environment and Geology (HLUG) north of Viernheim (Hessisches Ried) reached a total depth of 350 m, and penetrated high resolution fl uviatile and limnic-fl uviatile sediments (0 to 225 m) of Pleistocene age, and partially highly pedogenically overprinted limnic-fl uviatile sands, clays and silts of Pliocene age (225 to 350 m). The Pliocene sediments tend to be sourced locally. The sediments repeatedly show sourcing from the Odenwald which is characterised by a high percentage of green hornblende in the heavy mineral fraction. As part of the Heidelberg Basin research programme, one of the main purposes of this borehole was to analyse the Pleistocene “Normal Facies” of the northern Upper Rhine Graben, i.e. a sedimentary sequence subject to minimum disturbance, largely unaffected during the Pleistocene by material sourced from the graben margins or smaller tributaries. The Pleistocene sedimentary sequence consists of three units: a thin horizon with reworked Pliocene material is overlain by ten cycles each beginning erosively with gravely sandy sediments and ending with siltyargillaceous to in part peat-like sediments. Internal cycles can also be identifi ed, amongst other features. A characteristic aspect is the green-grey, strongly calcareous, micaceous and well sorted, fi ne to medium sands of the Rhine. These are dominated by the Rhine Group (garnet, epidote, green hornblende and alterite) in the heavy mineral fraction. These sediments are classifi ed as the “Rhenish Facies”. The upper Pleistocene sedimentary sequences at the top of the Viernheim research borehole are dominated by several fi ning-upward and in part coarsening-upward sequences. The deposits in this part of the well are dominated by gravel deposited by the Neckar. The heavy mineral distribution of the sand fraction reveals, however, that there was mixing with Rhenish sediments. Weichselian to Holocene aeolian sands form the topmost part of the well section. The stratigraphic classifi cation of the Pleistocene sedimentary sequences is still uncertain in parts. The Pliocene-Pleistocene boundary is placed at 225 m because of the characteristic change in facies. Due to lithostratigraphic correlations with sediments within the Lower Rhine Embayment, a larger unconformity at the depth of 225 m must be accepted. Research carried out in the area around the well indicates that the youngest fi ne-clastic section penetrated by the well between 39.76 and 58.55 m is of Cromerian age. [Die pliozäne und pleistozäne fluviatile Entwicklung im nördlichen Oberrheingraben unter besonderer Berücksichtigung der Forschungsbohrung Viernheim (Hessen, Deutschland)] Kurzfassung: Die 2006 durch das Hessische Landesamt für Umwelt und Geologie (HLUG) abgeteufte Forschungsbohrung nördlich von Viernheim (Hessisches Ried) hat mit einer Endteufe von 350 m hoch aufgelöst fl uviatile und limnisch-fl uviatile Sedimente (0 bis 225 m) des Pleistozäns und zum Teil stark pedogen überprägte limnisch-fl uviatile Sande, Tone und Schluffe des Pliozäns (225 bis 350 m) durchteuft. Die Liefergebiete der pliozänen Sedimente sind eher regional geprägt. Die Sedimente zeigen wiederholt Schüttungen aus dem Odenwald, die durch einen hohen Anteil grüner Hornblende in der Schwermineralfraktion gekennzeichnet sind. Als Teil des Forschungsprogramms „Heidelberger Becken“ zielte diese Bohrung insbesondere * Address of author: C. Hoselmann, Hessisches Landesamt für Umwelt und Geologie, Postfach 3209, D65022 Wiesbaden, Germany. E-Mail: christian.hoselmann@hlug.hessen.de The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 287 im Pleistozän auf die „Normalfazies“ des nördlichen Oberrheingrabens ab, das heißt: auf eine möglichst ungestörte Sedimentabfolge, die im Pleistozän Schüttungen von den Grabenrändern oder kleineren Zufl üssen weitestgehend ausschließt. Die pleistozäne Sedimentabfolge besteht aus drei Einheiten: über einem geringmächtigen Horizont mit aufgearbeitetem pliozänen Material folgen zehn Zyklen, die erosiv mit kiesig sandigen Sedimenten einsetzen und mit schluffi g-tonigen bis zum Teil torfi gen Ablagerungen abschließen. Mitunter sind interne Zyklen zu erkennen. Charakteristisch sind grünlich-graue stark carbonatische, glimmerführende und gut sortierte Feinbis Mittelsande des Rheins. In diesen dominiert die Rhein-Gruppe (Granat, Epidot, grüne Hornblende und Alterit) in der Schwermineralfraktion. Diese Sedimente werden als „Rheinische Fazies“ bezeichnet. In der hangenden letzten pleistozänen Sedimentabfolge der Forschungsbohrung Viernheim bestimmen mehrere fi ning-upward und zum Teil coarsening-upward Sequenzen das Sedimentationsgeschehen. Die Ablagerungen dieses Profi labschnitts sind Kies dominiert, der vom Neckar geschüttet worden ist. Die Schwermineralverteilung der Sandfraktion zeigt aber an, dass es zu einer Vermischung mit rheinischen Sedimenten gekommen ist. Weichselbis holozänzeitliche Flugsande schließen das Profi l ab. Die stratigraphische Einstufung der pleistozänen Sedimentabfolge ist in Teilen noch unsicher. Die PliozänPleistozängrenze wird auf Grund des charakteristischen Fazieswechsels auf 225 m gelegt. Eine überregionale Korrelation mit Sedimenten der Niederrheinischen Bucht spricht für eine Diskordanz mit größerer zeitlicher Lücke an der Plio-Pleistozängrenze. Untersuchungen im Umfeld der Bohrung sprechen für cromerzeitliches Alter des jüngsten feinklastischen Abschnitts der Bohrung zwischen 39,76 und 58,55 m.


Introduction
The north-eastern part of the Upper Rhine Graben (URG) is known as the "Hessisches Ried" (Fig. 1).This area has become an important research area for Quaternary graben development in recent years.According to VAN GIJSSEL (2006), the URG is a non-glaciated type region for mid Central European large and medium sized upland basins.The historical landscape development in the late and post-glacial period in the northern URG was recently studied by BOS et al. (2008), DAMBECK (2005), DAMBECK & BOS (2002), DAMBECK & THIEMEYER (2002) as well as ERKENS et al. (2009).The geometry of the Quaternary sediment body was investigated using high resolution refl ection seismic by HAIMBERGER et al. (2005) andWIRSING et al. (2007).The interaction between tectonics, and fl uviatile and erosive processes, particularly along the western graben margin, was investigated by PETERS & VAN BALEN (2007a, b) as well as PETERS et al. (2005).The sedimentological structure of the Pleistocene sedimentary fi ll in the northern Upper Rhine Graben has, however, not been looked at systematically in recent times.This was one of the reasons why the Geological Survey of Hessen (Hessisches Landesamt für Umwelt und Geologie (HLUG)) drilled several research boreholes in the Hessisches Ried in recent years.In addition to scientifi c aspects concerning the sedimentology, sedimentary petrography, vegetation history, palaeontology and tectonic development during the Pleistocene and Upper Pliocene, the investigation was also interested in applied geological aspects, which require a thorough understanding of the structure of the graben fi ll for their clarifi cation.The focus of this aspect of the investigation was on hydrogeology, resource geology and geothermy.The boreholes were designed to reveal the geometry of the Pleistocene sedimentary body in more detail, and to specify each of the facies in the area of investigation.The thickness of Quaternary sedimentary fi ll increases from the western to the eastern graben margin.There is also an increase in thickness from north to south-east in the direction of the Heidelberg Basin (Fig. 2).At the eastern graben margin of the Hessisches Ried, the fl uviatile or limnic- Abb.1: Übersichtskarte mit den geologischen Strukturräumen und dem Untersuchungsgebiet "Hessisches Ried".
The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 289 fl uviatile deposits repeatedly contain interbeds of redeposited sediments of hard rocks lying on higher ground further east, i.e., from the Odenwald or the Sprendlingen Horst.This hard rock, consisting of Palaeozoic crystalline rocks and sedimentary rocks of Rotliegend age (Sprendlingen Horst) and Buntsandstein, was intensively weathered during the Mesozoic and Tertiary.The resulting unconsolidated rock was easily eroded and redeposited during the Pleistocene periglacial periods.Directly along the graben margin in particular, there are also very high proportions of redeposited sediments derived from these source areas.The 111 m deep C/04 B1 borehole drilled south of Gräfenhausen north-west of Darmstadt (R 3472203 -H 5531111 based on the Gauss-Krüger coordinates of German topographic maps), clearly shows an almost 20 m thick sequence of redeposited sediments aligned east-west in this part of the northern URG, forming part of the Quaternary sedimentary sequence with a total thickness of 83.58 m.These redeposited sediments can be traced for approx.5 km from the graben margin in the northern URG.The Pleistocene fl uviatile graben fi ll itself consists in the area of investigation of alternating fl uviatile sands and gravels mixed up in various proportions, as well as silts, clays and peat.This sequence is the "Normal Facies" of the northern URG.
In the north part of the Hessisches Ried, the Rhenish sediments interfi nger with the fl uviatile sediments deposited by the Main river.Several hundred boreholes were geologically analysed and recorded by HLUG between 2003 and2004 2008;PREUSSER et al. 2008;GEYH 2008) and to provide information on sedimentation rates where possible.The form of subsidence in this region means that large parts of the section can be expected to be preserved in superposition.The discussion in the following dispenses with detailed core descriptions.This material can be requested from the author by e-mail, along with photo documentation and analysis data.

Subdivision of the sedimentological record
A strongly simplifi ed description of the borehole section is shown in Figure 3  The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 293 ends with fi ne-clastic silts and clays.Other sequences can be identifi ed within these cycles.The description repeatedly mentions sediments in "Rhenish Facies" (Table 1).This is a well to very well sorted grey to greenish-grey fi ne to medium grained sand.The carbonate content The cycles I-X reveal fl uviatile and limnic-fl uviatile sediments which were mainly deposited by the Rhine.Mixing with Neckar sediments occurs in the gravely parts.Units A-C are, however, much more strongly dominated by gravel, and the infl uence of Rhenish sedimentation reduces upwards.Gravel analysis was carried out at various depths to determine the provenance of the gravel (Table 2).There are basically no signifi cant changes in the gravel content.The values are similar to those for the area of the Neckar alluvial fans described by e.g.LÖSCHER (1988).The fractions analysed are dominated by Keuper limestones and sandstones transported by the Neckar, as well as red sandstones from Buntsandstein areas.The interesting fi nding is that the limestones in the lowest sample in particular are partially dissolved.In addition to the Neckar pebbles, there is also mixing with a local component from the Odenwald.This is revealed by the relatively high proportion of red sandstones derived from the Buntsandstein, as well as crystalline pebbles.These are less well rounded, which indicates shorter transport distances.

CHRISTIAN HOSELMANN
The limestones are well to very well rounded because of their longer transport distances and lower hardness.The portions of up to 26.6 % quartz, and lydite occurring as an accessory component, indicate mixing with Rhine pebbles.These were transported long distances from either the Black Forest or the Alps.

Heavy Mineral Analysis
Various systematic investigations of the heavy mineral composition of Pliocene and Pleistocene sediments in the Upper Rhine Graben have been undertaken previously (e.g.VAN ANDEL 1950;MAUS in BARTZ 1982;BOENIGK 1987;HAGEDORN 2004;HAGEDORN & BOENIGK 2008).Heavy mineral analysis is an important instrument for characterising fl uviatile Pliocene and Pleistocene fi ne sands from the Rhine system as part of work conducted in the middle Rhine area, the Lower Rhine Embayment and the Netherlands.The composition of the heavy mineral distribution provides information on the source of the sediments as well as the weathering of the sediments.171 samples from the Viernheim research borehole were analysed for their heavy mineral content.The main analytical method was that of BOENIGK (1983).Sodium polywolframate (Na 6 (H 2 W 12 O 40 )*H 2 O) with a density of 2.85 g/ cm 3 was used as the heavy liquid to separate the heavy from the light fraction in a centrifuge.The samples were boiled with concentrated hydrochloric acid prior to centrifuging to remove iron and manganese hydroxide crusts which would complicate the identifi cation.The disadvantage of this method is the dissolution of carbonate, apatite and parts of monazite and olivine (BOENIGK 1983).This was deemed acceptable because of the benefi t of being able to make comparisons with our own and other previous analyses.
Generally, the sediments in the borehole from a depth of 350 to 225 m are dominated by the stable heavy mineral group (Fig. 4).These are mainly zircon, but also tourmaline and rutile.CHRISTIAN HOSELMANN

298
Subordinately also occur heavy minerals of the Rhine Group (garnet, green hornblende and epidote) (cf.also Fig. 5).However, some samples have very high percentages of dark green hornblende (up to 86 %).
The composition of the heavy mineral spectrum changes signifi cantly above sample 222,9-222,8 m: zircon, the TiO 2 -group and tourmaline become insignifi cant.These are replaced by garnet, epidote and green hornblende which determine the composition of the heavy minerals in the sand fraction, although the distribution overall remains relatively homogenous.This is emphasised even more when the heavy minerals are divided into separate groups (Fig. 5).The Rhine Group dominates with proportions between 60 and 80 %.Instable minerals have sometimes undergone strong alteration or are strongly dissolved.Garnet shows some evidence of corrosion, and green hornblende has been strongly altered in parts.Accessory minerals include the volcanic heavy minerals pyroxene and brown hornblende.
Boiling with concentrated HCl leads to the dissolution of some heavy minerals such as carbonate and apatite.A test run with 16 samples studied at the effect of the concentrated HCl treatment on the heavy mineral distribution.
Figure 6 shows the distribution of selected samples with and without HCl treatment.Generally, the heavy mineral distribution remains the same with or without treatment.The differences lie within the range of statistical error.However, there is a signifi cant rise in opaque heavy minerals (no HCl boiling), that is explained by the iron and manganese hydroxide encrustations on some grains which can therefore not be identifi ed in transmitted light under a polarised microscope.Apatite now occurs in proportions up to 8 %, however, only in samples between 163.55 and 5.5 m.In general, counting during heavy mineral analysis becomes much more diffi cult, and some grains can only be identifi ed after a lot of work.The difference for the sediments in the Upper Rhine Graben is, however, not as signifi cant because of the presence of carbonate-rich sands which have not been subject to any very intensive weathering processes after sedimentation (cf.Chapter 3.2).Heavy mineral analysis without boiling with HCl is very diffi cult in the noncalcareous Pleistocene terrace deposits with no overburden to protect them from weathering, which are characterised by strong iron and manganese hydroxide deposits, and are frequent in the Middle Rhine and Lower Rhine areas.This can give rise to erroneous results.Decisions to change the methodology should therefore be looked at very critically because changing the methodology would considerably restrict direct comparison with most of the old analysis data collected in the Rhine area.

Analysis of the Carbonate Content
The carbonate content was analysed in 136 sand samples from the Viernheim research borehole.The samples were initially ground up in a mortar before being ground down to ca. 100 μ in a ball mill.The carbonate content was determined using the method described by Scheibler (DIN ISO 10693), which involved two to three measurements.The results were used to derive an average number.Total carbonate content was determined in each case.
The carbonate in sediments from the Upper Rhine Graben primarily consists of calcite (CaCO 3 ), and to a lesser extent also dolomite (CaMg(CO 3 ) 2 ).The analysed sediments are almost all calcareous above the boundary at 225 m.Concentrations fl uctuate signifi cantly between 0.96 and 37.17 % (Fig. 7).VAN ANDEL 1950 andVINKEN 1959), used for provenance analysis.
Abb. 5: Verteilung der Schwerminerale nach Schwermineralgruppen (nach VAN ANDEL 1950und VINKEN 1959) CHRISTIAN HOSELMANN Rhenish Facies, as in the Viernheim research borehole.A similar picture is shown by another research borehole (C/00-BK01), lying between Biblis and Groß-Rohrheim (Fig. 1), approx.18 km north-west of the Viernheim research borehole (R 3460250 -H 5508325).The Quaternary base lies almost 100 m below ground level.The carbonate concentrations in the 42 samples fl uctuate between 1.39 and 22.71 %.They are therefore lower in general than the boreholes further to the south.However, the C/00-BK01 research borehole does not have the silty and clayey beds with fi ne sand intercalations which usually contain the calcareous sediments.Future work should investigate whether the distribution pattern of the carbonate concentrations can be correlated between boreholes.

Subdivision of the Viernheim research borehole
The Viernheim research borehole can be divided into four sections (cf.Chapter 2).The zone from 350 to 225 m contains clay-rich, densely layered limnic-fl uviatile deposits laid down on a fl oodplain.Some of these deposits have been subjected to strong post-sedimentary pedogenic overprinting giving rise to soils similar to plinthosols.These soils were formed by stagnant water or under the infl uence of groundwater, and have gley and pseudo-gley characteristics.Rust and bleach spots are typical of plinthosols.They are formed under humid-tropical climatic conditions.The fi ne clastic sediments contain calcareous clusters at some places and repeated intercalations of sandy deposits.Sections 8 to 9 (Fig. 3) comprise a fi ning-upward sequence topped by an organic sediment (lignite).The sands have partly high proportions of green hornblende indicating a local sedimentary source from the Odenwald situated to the east (Fig. 8).BOENIGK 2008).Sections 1 to 10 (Fig. 3) of the well section are classifi ed as Pliocene.
The next section (Section 11) is a transition zone which follows discordantly from 225 to 221.21 m and is interpreted as a reworked horizon.This section contains deposits derived from the underlying sediments which have been signifi cantly reworked.In terms of heavy minerals, it contains a rather localised stable spectrum as well as a typical Pleistocene spectrum dominated by the Rhine Group.This section is preliminarily classifi ed as Pliocene to Pleistocene.
The zone from 221.21 to 39.76 m in the Viernheim research borehole is made up of ten basically repeated suites (I-X) which begin autocyclically and discordantly with gravely sands and end with clayey sediments or peat.The thickness of each cycle fl uctuates between 6.17 m (IV) and 48.19 m (IX).Additional partial sequences were identifi ed in each cycle.The sediments (I-X) are almost exclusively medium to strongly calcareous.A typical feature is the repeated occurrence of very well sorted sands of the "Rhenish Facies" (cf.Chapter 2.2).These were formed by fl uviatile fl ood sediments from the Rhine.The whole section in the well contains sediments dominated by the Rhenish Facies, with a gravel fraction which includes pebbles from the Neckar, and subordinately local components with crystalline pebbles from the Odenwald and red sandstones from the Buntsandstein.However, high percentages of quartz and the presence of lydite also show that the gravel fraction contains material sourced from areas to the south of the Neckar (Tab. 1 and 2).The fl uviatile part of the section is topped from 39.76 to 3.1 m by highly gravely sands and The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 303 CHRISTIAN HOSELMANN 304 gravels mainly laid down by the Neckar.The dominance of the Rhine Group in the heavy mineral fractions in the sands, however, indicates mixing with Rhenish sediments around Viernheim.This is also indicated by the quartz constituents and lydite in the gravel fraction (Tab.2).This section is also marked by dominant fi ning-upward and subordinately coarsening-upward sequences.The top of the section from 3.1 m to the ground surface is formed by Weichselian to Holocene aeolian sands.

R h e in
The unequivocal identifi cation of the Rhine signal in the heavy mineral fraction of the sands can be deduced from the proportions of heavy minerals in the sediments.Unlike the Pleistocene Neckar sediments which only contain around 0.04 % heavy minerals, the Rhine sediments contain approx.0.5 % heavy minerals.This means that the fl uviatile Rhenish sediments have around 10 times more heavy minerals than the Neckar sediments (Fig. 9).HAGEDORN ( 2004) reports similar findings.
The change in provenance in sample 222.8 m show a strongly alpidic dominated heavy mineral spectrum, indicating the connection of the Rhine to the Alpine drainage system.The general homogenous structure of the heavy mineral distribution (Fig. 4 and 5) in the Pleistocene shows that lateral sources of sediment in the Viernheim area are almost completely absent.This locality is therefore different to that of the Ludwigshafen-Parkinsel borehole, for instance, which is marked by the periodic deposition of local sediments from the western graben margin (HAGEDORN & BOENIGK 2008).This confi rms the assumption by HAGEDORN & BOENIGK (2008), that the Rhine tended to flow in the eastern part of the northern URG during the Pleistocene.Subordinate proportions of volcanic heavy minerals (pyroxene and brown hornblende) are probably not associated with the initiation of Middle Pleistocene East Eifel volcanism prior to approx.700 ka (BOGAARD & SCHMINCKE 1990).In principle, the Neckar could also be the source for the pyroxenes as well as the brown hornblendes, although a strong decrease in the pyroxenes is observed towards its confl uence with the Rhine as a result of dissolution and dilution by tributaries ( VAN ANDEL 1950).
The importance of heavy mineral analysis for provenance analysis in the northern Upper Rhine Graben is highlighted by a comparison of the unstable heavy minerals (garnet, green and brown hornblende, and pyroxene) with the carbonate content (Fig. 7).The post-sedimentary weathering of sediments with a pedogenic overprint initially gives rise to a decrease in and relocation of the carbonate content.If the section has also undergone pedogenic overprinting, which was not obvious in the section, this would also have led to a decrease in the concentration of unstable heavy minerals.However, this is not the case in the Viernheim research borehole because zones with lower carbonate contents for instance have raised levels of unstable heavy minerals.No correlation is shown between low carbonate contents and low concentrations of unstable heavy minerals.This in turn means that the Pleistocene sediments have almost preserved their original heavy mineral association and are therefore ideal for provenance analysis.Some of the samples show signs of dissolution and traces of corrosion.

Stratigraphy
The  Abb. 9: Schwermineral-und Kiesschüttungen während des Pleistozäns im nördlichen Oberrheingraben.CHRISTIAN HOSELMANN (2007), andPREUSSER (2008) and coincide with the palaeomagnetic Gauss/Matuyama boundary.The deposition of Rhine Group alpine heavy minerals in the URG already is of Late Pliocene age (HAGEDORN & BOENIGK 2008;PREUSSER 2008).Investigations in the LRE confi rm this: a fi rst shift from a stable heavy mineral spectra to unstable spectra of alpine type was detected in the Kieseloolite Formation (KEMNA 2005(KEMNA , 2008 a, b) a, b).The Öbel beds (Latest Pliocene) also contain unstable heavy mineral spectra (BOENIGK & FRECHEN 2006;DONDERS et al.2007;KEMNA & WESTERHOFF 2007).In some parts of the LRE the Öbel beds cover the Reuver Clay s.s. and thus defi ne the top of the Pliocene sedimentation.
In the Viernheim research borehole, no typical alpine heavy mineral spectra can be found below a depth of 225 m.Consequently, these sediments are correlated with the Pliocene.A larger unconformity at this depth must be accepted.There is no doubt that a marked change in sedimentation took place at that time.At the present stage of the investigation a further stratigraphic subdivision of these Pliocene sediments is not possible.This characteristic change at 225 m in the Viernheim research borehole is comparable to the boundary at 176 m in borehole P 34 Ludwigs-hafen-Parkinsel (WEIDENFELLER & KÄRCHER 2008;WEIDENFELLER & KNIPPING 2008).There are also indications in the P 34 borehole of a palaeomagnetic change from the Gauss to Matuyama-Chron (ROLF et al. 2008).ROLF et al. also proposed that a characteristic change of the magneto-minerals from goethite (Pliocene) to greigite (Pleistocene) can be interpreted as a climatic signal.Pollen analysis from organic sediments just beneath and above the boundary at 225 m are planned to provide important information on the age of this section in the Viernheim research borehole.Another key zone in the borehole is the Xb section (58.55 to 39.76 m).According to the hydrostratigraphic classifi cation, this uppermost signifi cant fi ne-clastic section of the borehole corresponds to a high resolution Obere Zwischenhorizont (OZH) in the sense of the hydrogeological mapping in the Rhine/Neckar zone (HGK 1999) or the sequence-stratigraphic equivalent to the Ladenburg Horizon (Symbolschlüssel Geologie Baden-Württemberg 2007, VILLINGER 2005).This section has been classifi ed as part of the Cromerian Complex since ENGESSER & MÜNZING (1991) on the basis of its mollusc fauna.Palynological investigations by KNIPPING (2004KNIPPING ( , 2008) ) on various sections in the Mannheim-Ludwigshafen area led to the conclusion that the OZH contains several interglacial fl ora which are assigned to the Cromerian Complex.A correlation with the Eemian interglacial is ruled out.This means that Section Xb in the Viernheim borehole can also be assumed to be of Cromerian age.Important biostratigraphic indications of the Lower Pleistocene sections in particular are provided by the investigations conducted by WEDEL (2008).No other chronostratigraphic interpretations of the Viernheim research borehole are possible at the current stage of the research.

Regional Trends
Regional trends can only be investigated at on the basis of characteristic horizons or boundaries which extend over larger areas.The horizons selected for this purpose are the Quaternary base as well as the top and base of Section Xb.This of course presupposes that the same unit or boundary could be selected for the correlation in the various wells and correlation could only use those boreholes with reliable stratigraphic logs, and gamma logs where possible (Tab.3 and Fig. 10).Despite a large number of boreholes in the Hessisch part of the area of investigation, only eight boreholes in the Viernheim-Bensheim area are currently suitable for such a correlation.The top of Section Xb dips to the south towards the centre of subsidence of the Heidelberg Basin.The thickness of the unit increases accordingly.The minor amount of data available on the Quaternary base indicates a similar picture with a major increase in thickness from north to SSE.Care must be taken for the difference in the quality of the samples.The absence of Section Xb in the Einhausen research borehole highlights a fundamental problem when look-The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 307 ing at the Pleistocene sedimentary fi ll in the northern Upper Rhine Graben: some horizons are very diffi cult to correlate, particularly towards the north.Despite high subsidence rates in this part of the northern URG as well, horizons are absent and can no longer be lithologically correlated even over short distances.Although correlation seems to function reasonably well in the centre of the Heidelberg Basin, there are also some fi ne-clastic horizons here which are only of restricted use for lithostratigraphic correlations.For instance, a fi ne-clastic horizon occurs in the Deponie Hirschländer borehole GWM1/03 at a depth of 21.58 to 25.65 m (thickness 4.07 m).This horizon is not seen in any of the surrounding boreholes.This is interpreted as indicating the presence of a very local channel deposit cut into the older sediments.Although easier to correlate regionally, the fl ood deposits of Section Xb become thinner as the distance from its subsidence centre increases, and even disappear in some cases.The conclusion drawn from this is that the use of sequence-stratigraphic models -as used for instance by LANG (2007) for the Hanau-Seligenstadt Basin (Fig. 1), a parallel structure to the URG -are more promising.Determining the A/S rates (A= accommodation space available for the sediment; S: sediment supply), and the interpretation of A/S domains can be used successfully in sedimentary basins with low subsidence rates.This approach needs to be tested fi rst in sedimentary basins with higher subsidence rates, such as the northern URG.

Comments on supra-regional correlation and outlook
The Rhine as the link between the Alpine and North European glaciated areas also linked up different areas of provenance, accumulation areas and tectonic structural areas during the Quaternary (PREUSSER 2008;WESTERHOFF 2008).The typical sediments of the Rhenish Facies which are wide-spread in the northern Upper Rhine Graben also have equivalents downstream.At the northern edge of the URG in Wiesbaden (Fig. 1), deposits called the Mosbach Sands occur which have supraregional signifi cance because of their rich and typical fossil content (BOENIGK 1977/78;KAHLKE 2007;KELLER 1999) & SEMMEL 1977).
No other correlatable sediments are known in the other terraces of the Middle Rhine or in the Lower Rhine Embayment.The question therefore remains whether the sediments either did not accumulate in other times or whether they did not survive because they are easy to erode.In the area of the lower Middle Rhine, fl uviatile terrace sediments occur in a series of terraces so that the uplift of the Rhenish Massif exposed the sediments to much stronger postsedimentary erosion than the deposits in the Heidelberg Basin.
A more detailed explanation is required in the future of the type of mechanism which controlled the depositional cycles.A key role is played here by the uplift of the German Mittelgebirge and the subsidence of the URG, and in particular in this case the Heidelberg Basin.
The cause for the start of a new cycle could also have been controlled by climatic conditions because periglacial environmental conditions were required to make these sediments available, particularly the coarse clastic sediments, which then accumulated in proximal locations.
The interaction of sea level rises and falls on accumulation behaviour in the URG needs to CHRISTIAN HOSELMANN  The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 311 be clarifi ed for this purpose.Rapid rises in sea level can also cause water to back up in the hinterland and possibly trap accumulation in front of the rising Rhenish Massif and lead to increased coarse clastic sedimentation in the southern Upper Rhine Graben and in the northern URG north of the Karlsruhe High to dominant fi ne-clastic sedimentation.Answering these questions requires detailed analysis of cores in the Heidelberg Basin because understanding the depositional processes and palaeogeographic development of this region and the fl uviatile Rhine system is only possible in the high resolution deposits which are only found here in the thickest Quaternary sedimentary succession in the URG.

Fig. 1 :
Fig.1: Location map with the geological structural zones and the area of investigation "Hessisches Ried".

Fig. 2 :
Fig.2: Thicknesses of the Quaternary sediments in the northern URG; redrawn fromBARTZ (1974),HAIM- BERGER et al. (2005)  as well as a great deal of new information from this work.The thickness of the Quaternary sediments increases from north to south as well as from west to east.The Heidelberg Basin begins in the Hessisches Ried south of Gernsheim.Around Heidelberg, a more strongly subsided subbasin revealing approx.400 m of Quaternary sediments is documented.

HeidelbergFig. 8 :
Fig. 8: Heavy mineral deposits during the Pliocene in the northern Upper Rhine Graben.Dominated by locally reworked sediments with stable heavy minerals and subordinate unstable heavy minerals derived from the Black Forest and the Vosges.Repeated local deposition from the graben margins which dominantly supply green hornblende in the eastern part of the northern URG.Abb.8: Schwermineralschüttungen während des Pliozäns im nördlichen Oberrheingraben.Dominant sind lokal aufgearbeitete Sedimente mit stabilen Schwermineralen und untergeordnet instabilen Schwermineralen, die aus dem Schwarzwald und Vogesen stammen.Wiederholt kommt es zu lokalen Schüttungen von den Grabenrändern, die im östlichen Teil des nördlichen URG dominant grüne Hornblende liefern.
Fig. 9: Heavy mineral and gravel deposition during the Pleistocene in the northern Upper Rhine Graben.

Fig. 10 :
Fig.10: Distribution of various boreholes drilled recently in the southern Hessisches Ried used to evaluate "regional trends".The well locations are plotted on the geological base map of Hessen 1:300,000.The graben margin at the boundary to the crystalline Odenwald can be seen in the eastern part of the map; in the southeast with sedimentary rock remains of the lower Buntsandstein.B Xb = Base section Xb in m below ground level; B qp = Base Pleistocene in m below ground level Abb.10: Verteilung verschiedener in den letzten Jahren abgeteufter Bohrungen im südlichen Hessischen Ried, die für die Auswertung "regionaler Trends" verwendet wurden.Die Bohransatzpunkte wurden in die Geologische Übersichtskarte 1:300.000von Hessen eingetragen.Im östlichen Teil des Kartenausschnitts ist an der Grenze zum kristallinen Odenwald der Grabenrand zu erkennen; im Südosten mit Sedimentgesteinsresten des unteren Buntsandsteins.B Xb = Basis Sequenz Xb in m unter GOK; B qp = Basis Pleistozän in m unter GOK The thickness of the Quaternary sediments increases from north to south as well as from west to east.The Heidelberg Basin begins in the Hessisches Ried south of Gernsheim.Around Heidelberg, a more strongly subsided subbasin revealing approx.400 m of Quaternary sediments is documented.
A characteristic feature is the distinctive presence of mica.The light mica fl akes can have diameters up to several millimetres.This part of the well section has the following geological setup (cf.Figure3):

Table 2 :
Summary of the gravel analysis at various depths in the Viernheim research borehole.The fi gures are percentages and add up to 100 % in total.200 to 500 pieces of gravel were counted in the fraction higher than 5 mm.Tab.2: Zusammengefasste Schotteranalysen aus verschiedenen Teufenbereichen der Forschungsbohrung Viernheim.Die Werte geben Prozent an und summieren sich insgesamt auf 100 %.Gezählt wurden zwischen 200 und 500 Kiese der Fraktion größer 5 mm.
The Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 297 , die zur Liefergebietsanalyse verwendet wurden.Pliocene and Pleistocene fl uvial evolution in the northern Upper Rhine Graben 301 Fig. 7: Carbonate distribution from 136 sand samples from the Viernheim research borehole.The carbonate content is shown in relation to the group of unstable heavy minerals which weather easily in post-sedimentary environments.Abb.7: Carbonatverteilung von 136 Sandproben der Forschungsbohrung Viernheim.Der Carbonatgehalt wurde in Bezug zur Gruppe der instabilen Schwerminerale gesetzt, die postsedimentär leicht verwittern.
staurolite Fig. 6: Heavy mineral distribution in 16 selected samples from the Viernheim research borehole with and without HCl treatment.Apatite was counted separately in the samples prepared without boiling in HCl.Abb.6:Schwermineralverteilung von 16 ausgewählten Proben der Forschungsbohrung Viernheim mit und ohne Behandlung durch HCl.Bei der Probenaufbereitung ohne Kochen mit HCL wurden Apatite separat mitgezählt.The