Correlation of Pleistocene sediments from boreholes in the Ludwigshafen area , western Heidelberg Basin

Cores from several boreholes in the Ludwigshafen area were analysed to investigate their sedimentology, palynology, palaeomagnetics, rock magnetics and heavy mineral composition. The preliminary results are presented from the new Ludwigshafen-Parkinsel borehole P35, which was drilled 500 m WSW of borehole P34, to a total depth of 300 m. Correlation between the two boreholes reveals similarities and dissimilarities in stratigraphy, structure and the thickness of the sediments. As a result of core documentation and the preliminary evaluation of the investigation results, a good correlation is established between the coarse and fi ne-grained sequences in both boreholes down to a depth of 122 m. However, the Plio-Pleistocene boundary in borehole P35 is much deeper than in P34. A fault throw of 42 m is assumed, attributable to young tectonics. The poor correlation between the thicknesses of the sediments in the lower sections of the two boreholes suggests that tectonism was particularly active in the Pliocene and Lower Pleistocene. The different occurrence of interglacial sequences in the two Ludwigshafen boreholes can be attributed to fl uvial dynamics and neotectonic events. Further palynological analysis is required to determine whether the alternation of at least fi ve interglacial periods determined in the Ludwigshafen-Parkinsel P34 borehole, can also be confi rmed in the P35 borehole. The information gained so far from the correlation of the already analysed Middle Pleistocene interglacials in the Ludwigshafen/Mannheim area, as well as the links with the primarily Lower Pleistocene sections in Schifferstadt, already suggest that this would allow a much better understanding of the changes in vegetation and climate during the Pleistocene. [Korrelation pleistozäner Sedimente aus Bohrungen im Raum Ludwigshafen, westliches Heidelberger


Introduction
To investigate hydrogeological aspects, numerous boreholes were cored in the Ludwigshafen/ Mannheim area in recent years.These cores were analysed using a wide range of methods (Fig. 1, Table 1).The results of the Ludwigshafen-Parkinsel P34 borehole in particular -which was drilled to a TD (total depth) of 300 m in 2002 with 95 % core recovery -provided a great deal of new information on the history of the river Rhine and the changes in vegetation and climate in the northern Upper Rhine Graben during the Pleistocene (WEIDENFELLER & KÄRCHER 2008).
Of particular value were the palynological analyses (KNIPPING 2008), heavy mineral analyses (HAGEDORN & BOENIGK 2008) and the palaeomagnetic and rock-magnetic analyses (ROLF et al. 2008).This made it possible for the fi rst time to reliably detect the Plio-Pleistocene boundary in the northern Upper Rhine Graben in a core, confi rmed by various independent methods.Another core of 300 m (P35) was drilled approximately 500 m WSW from borehole P34 in 2006.P35 was analysed using a very similar range of methods.The results are presented in this paper, including a preliminary correlation between the two boreholes.The results of the P34 and P35 cores are the key for the interpretation of the unconsolidated sedimentary sequences in the Ludwigshafen/ Mannheim area.The sequences in both cores are characterised by alternating coarse-grained and fi ne-grained horizons.They are stratigraphically subdivided using a hydrogeological terminology (BARTZ 1959, 1974, KÄRCHER 1987) into aquifers (Oberes Kieslager "OKL" (Upper Gravels) , Mittlere sandig-kiesige Folge (Middle Sandy-Gravely Series), Untere sandigsiltige Folge (Lower Sandy-Silty Series)) and Zwischenhorizonte (intermediate horizons) (Oberer Zwischenhorizont "OZH" (Upper Intermediate Horizon), Unterer Zwischenhorizont "UZH" (Lower Intermediate Horizon)).Reddish-brown sandy sediments were encountered in some sections and interpreted on the basis of heavy mineralogical analysis as fans deriving from the Pfälzerwald (HAGEDORN 2004, HAGEDORN & BOENIGK 2008).Whereas the stratigraphic position of the Lower Pleistocene sediments is uncertain, the upper gravels ("OKL") are conventionally attributed to a Weichselian age and referred to in the terminology of terrace division as a Lower terrace (KÄRCHER 1987).Accordingly, the interpretation of the upper aquitard (Oberer Zwischenhorizont "OZH") as an Eemian formation seems doubtful, which is confi rmed by the research of ENGESSER & MÜNZING (1991), RÄHLE (2005) and WEDEL (2008).

Results
The Upper Gravels (OKL) consist of arenaceous medium gravel to coarse gravel of up to 20 m in both cores.The OKL can be divided into two sections in both boreholes.The upper section consists of coarse gravel, extends in each case to a depth of 13 m, and is characterised by fi ning upwards from 10 m to 1 m depth.This is underlain by interbedded coarse sands and fi ne to medium gravels down to a depth of 19.5 m.A coarse-grained facies is only present in the Upper Gravels.The underlying sediments are distinctly fi ner grained (sandygravelly in the middle aquifer, and sandy-silty Correlation of Pleistocene sediments from boreholes in the Ludwigshafen area  Nearly 1200 samples were taken from the P34 cores for pollen analysis.120 samples were initially selected for a preliminary analysis and prepared at the laboratory operated by LGB Rheinland-Pfalz.This preliminary analysis Correlation of Pleistocene sediments from boreholes in the Ludwigshafen area involved a quick estimation of the pollen content, or counting only a minor number of pollen grains.1600 pollen samples were taken from the P35 borehole, and 60 samples were looked at.38 samples from the Maudach A 36 borehole also underwent preliminary analysis to roughly determine the pollen content, or subjected to counting a small number of pollen grains (Fig. 4, Fig. 5).The pollen sequences are mostly incomplete due to the variable tectonic subsidence of the URG and the fl uvial dynamics which didn't allow a regular sedimentation.
The preliminary investigation of borehole P34 revealed at least parts of 5 interglacials and se- In the fi rst analysis of borehole P35, which involved the zone down to a depth of 93 m, there were at least three interglacial sequences alongside several glacial pollen spectra (Fig. 4).An analogous interglacial sequence was found at a similar depth to that in borehole P34.In the lower part of this sequence (30.4 -30.7 m) pollen from Abies, Carpinus and Quercus were identifi ed, along with massulae from Azolla.
In the upper part (26.2 -29.0 m), Fagus pollen was also found.The decline in pollen from thermophilous taxa (from 26.5 m) and the marked increase in Pinus pollen appears to show the end of the interglacial.A pollen

Discussion
It should be stressed in advance that the following correlations are only tentative in character because they are based on preliminary analysis, and complete analysis of the samples still remains to be done.The pollen sequences recorded to date are often fragmentary and there are uncertainties when correlating these with other pollen sequences.Publications covering the boreholes from the Mannheim area (KNIPPING 2004(KNIPPING , 2008) ) already include some of the results of the preliminary analysis of the P34 borehole, which provided an invaluable addition to and confi rmation of the biostratigraphic classifi cation of the penetrated sequences.
For the interglacial or temperate interstadial pollen spectrum at a depth of 13.6 m in the OKL in borehole P34, a Holocene or Eemian age can be rejected, but a correlation with either one of the Early Weichselian interstadials or an interglacial older than the Eemian is possible (KNIPPING 2008).No analogous pollen spectrum has so far been found in P35.P34 contains pollen spectra (26.5 -29.05 m) which are correlatable with a high degree of probability with the Middle Pleistocene Mannheim Interglacial (KNIPPING 2008).Borehole P35 (Fig. 4) also revealed pollen spectra at a similar depth which, because of the presence of Carpinus, Abies and Azolla in the lower part (30.4 -30.7 m), and the additional occurrence of Fagus in the upper part of the interglacial (26.2 -29.0 m), correlates with a high degree of probability with the interglacial in P34 (Lu-I-B + C) and therefore with the Mannheim Interglacial (Fig. 4, Fig. 5).According to the pollen spectra identifi ed to date, the upper part of the interglacial is represented in P35 by a section with a thickness of 2.8 m, and is thus much thicker than in P34 where the thickness is only 0.3 m.The lower part of the interglacial has a similar thickness in both boreholes with a thickness of approx.0.3 m.
There are some similarities with the Mannheim Interglacial, but Celtis which is present in the Mannheim Interglacial as well as the Kärlich Interglacial is not present at Meikirch.The interglacial found in the OZH in P34 at 36.9 -38.4 m (Lu-I-D) is primarily characterised according to the current degree of analysis by the absence of Carpinus, Abies and Fagus.
There are some similarities to the pre-Rhume thermomere described by MÜLLER (1992) and the interglacial found by BLUDAU ( 2001) at approx.47 -52 m in the Mannheim Ergo BK 3 borehole.Interglacial pollen spectra without Carpinus, Abies and Fagus were only found in two horizons at 42 m in borehole P35.It is not certain at this stage whether these correlate with Lu-I-D.Should this correlation be confi rmed, then there is a very marked difference in the thickness of the sequence in P34 (1.5 m) and the 0.1 m thick sequence in P35.
Another cored borehole in the Ludwigshafen area (Maudach A36) was also included in this preliminary analysis (Fig. 1, Table 1).An interglacial with high proportions of Carpinus is reported in the OZH at a depth of 9 -11.2 m.Based on the current status of the preliminary analysis, this cannot be correlated with the  (WELTEN 1982(WELTEN , 1988)), but accounts for nearly 40 % in Maudach.The Upper Pleistocene position of the interglacial at Maudach at a depth of 9 -11.2 m cannot, however, be excluded on the basis of the current status of the pollen analysis.
Interdisciplinary analysis in the northern part of the Upper Rhine Graben, which in addition to looking at the lithostratigraphy, also looks at "Upper Pleistocene" mammal remains, gastropods, wood remains and pollen, provided no reliable stratigraphic classifi cation of the material (KOENIGSWALD 1988).Dating of parts of the Upper Gravels (OKL) and the "Oberer Ton" (Upper Clay = OZH) to the last interglacial (Eemian) has, however, been favoured by some authors.However, the latest analysis by KNIPPING (2002KNIPPING ( , 2004KNIPPING ( , 2008) ) 2000) have so far been identifi ed in the boreholes in Mannheim, Schifferstadt or Ludwigshafen.The Schifferstadt Interglacial described in the Schifferstadt borehole (KNIPPING 2002), is also probably equivalent to the Ferdynandowian Interglacial (JANCZYK-KOPIKOWA 1975), and has also not been identifi ed so far in the two boreholes in Ludwigshafen.
Increased subsidence of sub-blocks during the Plio-Pleistocene in the northern Upper Rhine Graben around the Mannheim/Heidelberg area is not only indicated lithostratigraphically, but also by pollen analysis.According to the initial investigation, the P34 and P35 boreholes penetrated sediments down to a depth of 102 m in which at least 3 superposed interglacials have been partially identifi ed (KNIPPING 2004(KNIPPING , 2008)).
Tsuga occurs from approximately 103 m in P34 and 92 m in P35, which currently still indicates a Lower Pleistocene age.Notable in this context is the single identifi cation of a Tsuga pollen grain in the lower part of the Mannheim Interglacial (UVB-3) (Fig. 5) (KNIPPING 2008).
Indications that Tsuga could also be present in the Cromerian Complex are supported by the analysis conducted by BLUDAU (2001)   been looked at in the Schifferstadt borehole.
Various interglacial and glacial vegetation sequences were described here in the older sections (KNIPPING 2002) Because the upper 122 m of the P34 and P35 boreholes can be very well correlated in terms of thickness, as well as sedimentary facies and gamma logs, it is assumed that tectonism was particularly active prior to the start of sedimentation of the Lower Intermediate Horizon (UZH), and that it then weakened considerably.Because the OZH is assigned to the Cromerian Complex on the basis of current fi ndings (KNIPPING 2008, RÄHLE 2005, WEDEL 2008), and because the UZH is also assigned to the transition zone between the Lower and Middle Pleistocene, the phase of tectonism giving rise to the approximately 42 m throw is considered to have taken place during the latest Pliocene and Lower Pleistocene.It seems clear that this period was characterised by more rapid subsidence of the Heidelberg Basin, and thus the Ludwigshafen-Mannheim area as well, and that this rapid subsidence was compensated for by high rates of sedimentation.

MICHAEL WEIDENFELLER & MARIA KNIPPING 4 Conclusions
Fluctuating fl uvial dynamics over very short distances leave behind strongly differentiated sequences in the geological record which can only be approximately interpreted with the information gained from several cores.The differences in the nature of the sediments between the P34 and P35 boreholes highlights that the interpretation of the climatic and fl uvial history of an area on the basis of the analysis of a few boreholes becomes very diffi cult because the sediments are not only affected by fl uvial dynamics, but also by small-scale tectonics.
The complex fault pattern in the northern Upper Rhine Graben is partially responsible for the preservation and character of the sediments.Unlike major cycles which can still be recognised and correlated across several boreholes, closer analysis reveals the presence of frequent gaps which make interpretation more diffi cult.Sedimentation is often controlled by young tectonics where small-scale fault block movements can result in the deposition of very different sequences of sediments, as recorded in the cores.The clear characterisation and stratigraphic identifi cation based on several independent methods, and on pollen analysis in particular, provides the information required to make reliable interpretations of the sedimentation history infl uenced by palaeoclimatic fl uctuations and tectonic movements.The differences identifi ed by comparing the two boreholes P34 and P35, which are only 500 m apart, emphasise that the large-scale correlation of boreholes separated from one another by several tens of kilometres, will only succeed when the sediments in the cores are analysed using analogous methods and can be stratigraphically classifi ed.

Fig. 1 :
Fig. 1: Position of the investigated boreholes in the Vorderpfalz, on the Frankenthal Terrace and in the Ludwigshafen area.
in the Mannheim Ergo BK3 borehole.The author describes several isolated occurrences of Tsuga in an interglacial and discusses a possible Cromerian age.In addition, in the Schifferstadt MICHAEL WEIDENFELLER & MARIA KNIPPING borehole lying further to the west (KNIPPING 2002), which is also described as containing a Middle Pleistocene interglacial (Schifferstadt Interglacial), several occurrences of Tsuga have been confi rmed at depths below 36 m.This section has therefore been preliminarily correlated with the Lower Pleistocene down to the Plio-Pleistocene boundary at approximately 91 m.The Middle-Lower Pleistocene boundary and Quaternary base deepen in these boreholes from west to east in the direction of the centre of the Heidelberg Basin.The Quaternary base in the Viernheim research borehole is at a depth of 224 m (HOSELMANN 2008).Lower Pleistocene and Tertiary sediments from the northern Upper Rhine Graben have mainly

Fig. 6 :
Fig. 6: Assumed position of a fault between the Ludwigshafen P34 and P35 boreholes, depicting the position of the Quaternary faults, interpreted on the basis of industrial seismic and river seismic profi les (after WIRS-ING et al. 2007).

Table 1 :
Investigations of cores from boreholes Ludwigshafen-Parkinsel P34, P35 and Ludwigshafen-Maudach A36, TK 25 6516 Mannheim-Südwest MICHAEL WEIDENFELLER & MARIA KNIPPINGin the lower aquifer).The OKL is underlain by the Upper Intermediate Horizon (OZH) which is widely distributed in the northern Upper Rhine Graben, in the same way as the OKL.
m and 60 m depth, and between 51 m and 66 m depth in borehole P35.This is followed by a medium to coarse sandy section which reaches a depth of approximately 87 m in both boreholes, underlain in P34 by fi ne-grained sediments with organic horizons down to a depth of about 92 m; and down to a depth of 98 m in P35.A distinct intermediate horizon, termed the Unterer Zwischenhorizont (Lower Intermediate Horizon) (UZH), occurs at a depth of 101 m to 122 m in both boreholes, and also contains several organic horizons.The sequence below 122 m is dominated by interbedded fi ne to medium-grained sand with intercalations of clayey-silty horizons.Borehole P35 has slightly thicker fi ne-grained sections between 151 m to 163 m, 167 m to 175 m, 183 m to 187 m and from 201,5 m to 204 m.According to the current status of the analysis, the fi negrained sections between 122 m to 183 m in borehole P35, are much thinner or completely absent in borehole P34.Coarser horizons with fi ne to medium gravel fi rst occur between 162 m to 167 m in P34 and between 175 m to 183 m in P35.There is a good correlation between the sediments in both boreholes from 0 m to 122 m depth -as also highlighted by a comparison of the Gamma Ray logs run in both boreholes (Fig.2).This changes between 122 m to 183 m where the boreholes have different lithofacies profi les and Gamma Ray logs.The distinct change of the facies at 177 m depth in borehole P34, which is also interpreted as the Plio- Correlation of Pleistocene sediments from boreholes in the Ludwigshafen area younger part of the Mannheim Interglacial because no Fagus has been found.It is possible that this interglacial correlates with the older part of the Mannheim Interglacial.