The gravel quarry at Niederweimar is situated south of Marburg in the central Lahn valley. It is one of the largest gravel quarries in Hesse. The middle reach of the Lahn cuts through a wide range of geological units such as the Rhenisch Massif and sandstones of Permian and lower Triassic age. Tributaries coming in from the east pass the basaltic Vogelsberg massif. This leads to a rather diverse gravel spectrum, dominated by greywacke, associated with radiolarites, sandstones, basalts and quartzites. The hard rock base of the gravel pit is formed by red to purple sandstones and claystones of upper Permian age (Zechstein). Sediments within the gravel pit have not only been deposited by the river Lahn, but also by the river Allna, a tributary flowing in from the west, sourced in the Rhenish Massif. More detailed information on the fluvial history of the Lahn valley near Marburg can be found in Heine (1970).

From a geomorphological point of view, the gravel pit is situated on the lower terrace of the Lahn. It is currently not inundated by floods, and its cover sediments are of Late Pleistocene and early Holocene age, as evidenced by the Laacher See tephra (LST; 12 900 ka, van den Boogard, 1995). Chronostratigraphically, the lower terrace would be assigned to the last glacial period. However, it appears that three gravel units are exposed in the pit, of which the lower ones seem to be much older than the last glacial. Elevation differences in the past and current floodplain of the Lahn are minimal (see Fig. 3); thus it is nearly impossible to distinguish different terrace levels from a geomorphological point of view. It therefore appears that at this location of the Lahn River, we are not dealing with a classical staircase of terraces, but with vertical stacking of terrace units, possibly due to (relative) tectonic subsidence in this part of the Lahn valley. So far, several radiocarbon ages, pollen and macrofossil assignments of the cover sediments as well as the gravel units have existed (e.g. Huckriede, 1982; Urz, 1995; Schirmer, 1999; Freund and Urz, 2000; Bos and Urz, 2003). But since large parts of the gravel units are older than 40 ka, numerical ages in particular of the older gravel units have been missing so far. New optically stimulated luminescence (OSL) and 14C ages for the gravels as well as the floodplain loams are presented on this field trip.

During more than 20 years of excavation by the State Archaeological Service of Hesse on ca. 70 ha of river floodplains and adjacent alluvial terraces, a large area of settlements has been detected, spanning from the Mesolithic (11.7 to 7.5 ka) and different periods of the Neolithic (7.5 to 4.2 ka), Bronze (4.2 to 2.8 ka) and Iron Age (2.8 to 2.0 ka) to the Middle Ages. Such an extensive colonization of a local river landscape is, as yet, unique. The possibility to settle on the drier terraces near the water, as well as the species-rich flora and fauna, made the river landscape of the central Lahn valley attractive to humans (Bos and Urz, 2003).

Already in 1994, two early Mesolithic sites were found during a gravel excavation. They were dated to around 10.5 ka cal BP (Bos and Urz, 2003). Pollen and macrofossil analyses, which were part of two research projects during the DFG (German Research Foundation) priority programme “Changes of the Geo-Biosphere during the last 15 000 years, continental sediments as evidence for changing environmental conditions”, suggest that forest-clearing due to deliberate burning by Mesolithic people occurred in the area (Bos and Urz, 2003). A reconstruction of the Mesolithic landscape in the central Lahn valley is shown in Fig. 4.

Since 2017, a DFG-funded research project has focused on plant remains from archaeological records as a source of information on the changing environmental conditions and agricultural systems within the prehistoric settlements near Niederweimar (Ralf Urz, Department of Geography, Philipps University of Marburg). Further details on the archaeology of Niederweimar can be found on the homepage of the archaeological survey of Hesse (https://lfd.hessen.de, last access: 11 July 2018).

The gravel unit can be divided into three subunits (Fig. 5). The oldest unit (Unit I) forms the base of the gravel pit. It is not present and/or visible in all parts of the pit and is of dark grey to dark reddish colour. Unit II consists of brown gravels with trough and horizontal bedding and with a strong overprint caused by precipitation of iron oxides. This unit can be further divided into two subunits, separated by a discontinuous layer of larger blocks. Unit III is formed by greyish gravels with marked horizontal and trough bedding and a block layer at its base. Unit II and III are separated by an erosional disconformity. Further information on the gravel units is given in Freund and Urz (2000) and Urz (1995). They assign the lower part of the gravels (our Unit II) to the early Weichselian, based on pollen and macrorest analyses, and the upper part of the gravels (our Unit III) to the last Pleniglacial. The latter is supported by one 14C age at Niederweimar of around 32 ka, and two further 14C ages between 30 and 40 ka at the gravel quarry Niederwalgern (Urz, 1995).

Floodplain loams overlie the Pleistocene gravels and show a very detailed stratigraphy, with alternating sand and silt layers in the lower part, one or several light grey layers of varying thickness in the middle and upper part, and a further dark palaeosol horizon in the uppermost part. From field observations, it is tempting to assign the greyish layers to the (relocated) Laacher See tephra, which would place the lower part of the floodplain loams in the late glacial and the upper part of the section mainly in the Holocene. This stratigraphy is supported by detailed pollen and macrofossil studies as well as radiocarbon dating carried out by, e.g. Urz (1995), Bos and Urz (2003) and Schirmer (1999). However, this stratigraphic interpretation contradicts the findings on the gravel unit investigated in the current study. Here, one OSL age places the uppermost part of the gravels in the Younger Dryas. Heavy mineral analyses also confirm that their deposition took place after the Laacher See event. In order to gain further insight into the chronostratigraphy of the site, further 14C and OSL dating and palynological, granulometric and heavy mineral analyses on the floodplain loams were carried out.

Particle size distribution was determined by classical pipette and sieve procedures without decarbonation according to Köhn (ISO 11277). In order to provide a first chronology of the section, OSL dating was applied to the underlying gravels of the lower terrace and to the upper part of the overlying fine sediments. For this purpose, the coarse grain quartz fraction was analysed. Due to incomplete bleaching, the De values of the floodplain fines are based on a minimum age model. In contrast, material from a sand lens within the underlying gravel was well bleached. Thus, the Central Age Model was applied for deriving the mean De. Results are summarized in Table 3 and Fig. 8.

The investigated section comprises three units: the gravel unit (I) at the base of this site yields an OSL age of 8.5 ± 0.6 ka. In comparison to a 14C age obtained by Urz (1995), which places this unit in the Younger Dryas, our OSL age appears to be too young. Further investigations on this issue will be undertaken in the near future.

The intermediate layer (II) is composed of floodplain loams with a dominant sand fraction. The unit terminates with a dark soil complex, in which the clay content increases to around 40 % in the uppermost sample. This soil can possibly be correlated with the so-called black floodplain soil which is widespread in the area. The formation of this floodplain soil in middle Hesse is assigned to the early Holocene (Mäckel, 1969; Houben, 2002; Urz, 2003) or to the early to mid-Holocene (Rittweger et al., 2000). So far, precise numerical ages of this horizon have been sparse, and the site at Niederwalgern offers the potential for improving the chronology by undertaking further OSL analyses.

The sediments of the uppermost unit (III) are dominated by silt. Charcoal pieces and fragmented ceramics are also abundant, indicating strong anthropogenic impact. At the current stage of research, it is however not clear whether this sediment layer is a colluvium from the small slopes further to the west or an alluvial sediment. Five OSL ages assign the unit to the Early Medieval Period in the lower part and the High Medieval Period in the upper part. As in many other parts of Germany, the High Medieval Period was characterized by deforestation and intensive farming, not only in the lowlands, but also on the hillslopes of low mountain ranges. This led to intense soil erosion and deposition of material at toe slopes and floodplains as colluvial and alluvial deposits.

The section is situated on a slope within a former brickyard on the east side of the Wetter River (50∘26′ N, 08∘46′ E; 198 m a.s.l.), in the northern part of the Wetterau basin within the Hessian Depression (Fig. 9). The basin's topography is characterized by a gently rolling landscape, flanked by the northern Taunus mountains to the west and the basaltic Vogelsberg massif to the east. During the Tertiary, tectonic subsidence created a mosaic of small-scale depressions, accompanied by the deposition of marine, fluvial, limnic and aeolian sediments (Bibus, 1974, 1976). Therefore, the lithology of the study area is dominated by unconsolidated Miocene sediments consisting of sands, gravels and clays. Additionally, Miocene basalts and intensively saprolized rock form the subsurface of the northern part of the Wetterau, characterizing the lithology of the study area (Kümmerle, 1981; Sabel, 1982).

Under periglacial conditions during the Pleistocene, the river Wetter formed terraces above the present-day river bed. These terraces were later covered by calcareous aeolian sands and reworked loess-derived clayey silts. On northeast-facing slopes and geomorphologically sheltered positions, loess was deposited and has been preserved to thicknesses of up to 10 m (Schönhals, 1996). Farming in the area already started in the early Neolithic, ca. 7500 years ago, favoured by a moderate climate and fertile soils. Because of this long-term cultivation, the present-day soilscape of the area is characterized by truncated soil profiles and anthropogenic colluvium, e.g. truncated Luvisols, Cambisols and Regosols (Houben, 2012; Lang and Nolte, 1999; Schrader, 1978).

According to Bibus (1974), the investigated loess section can be subdivided into 17 units, including several palaeosols showing different intensity of pedogenesis, reaching a thickness of up to 10 m. However, the chronostratigraphic interpretation by Bibus (1976) was based solely on palaeopedological criteria, whereas there has been no numerical age control so far. Therefore, the existing loess profile has been extended, described and sampled in several field campaigns since summer 2013. Magnetic susceptibility measurements were conducted in the field with a SatisGeo Kappameter KM-7 at a 10 cm depth interval, recording five measurements per depth interval. Samples for sedimentological analyses were collected at high resolution (5 cm), yielding 180 bulk samples, based on the continuous column sampling method described in Antoine et al. (2009). Sedimentological analyses included determination of particle size distribution by classical pipette and sieve procedures without decarbonation according to Köhn and gas-volumetric determination of carbonate using the Scheibler method. Additionally, spectrophotometric analysis for determination of colour and lightness was conducted using a Konica Minolta CM-5 spectrophotometer at the laboratory for Physical Geography of RWTH Aachen. Based on the colour values, the Redness Index (RI) was calculated as a proxy for soil rubification and changes in hematite content (Barron and Torrent, 1986). For luminescence dating, 16 samples (Fig. 10; red circles) were taken at night-time by direct sampling into opaque plastic bags, after removing the light-exposed outer sediment layer of the profile wall. Samples for dosimetry measurements were collected within a 30 cm radius of each luminescence sample. Sample preparation and post-IR IRSL measurements, following a modified post-IR IRSL225 protocol originally proposed by Buylaert et al. (2009), were carried out at the Luminescence Laboratory of Giessen University. Further information can be found in Steup and Fuchs (2017). A total of 15 undisturbed samples were collected from the profile for micromorphological analyses (Fig. 10; red boxes).

For the interpretation of relative variations in the geochemical composition along the loess section, XRF analyses were performed on a ITRAX XRF core scanner at Bremen University. The results are presented as element log ratios (Fig. 12) to characterize weathering intensity and dust provenance.

The division of the loess section into 14 pedostratigraphic units (Fig. 10) is based on field observations including identification of major discontinuities and variations in colour, grain size distribution, magnetic susceptibility and carbonate content, as well as quantitative analyses of grain size distribution, carbonate content, spectrophotometric colour measurements and age estimates obtained from luminescence dating. Five units are distinguished based on sedimentological and pedological characteristics from the basal stratum (I) of the sequence to the surface of the recent soil (V): I.

basal soil complex

Unit 2 marks a transitional stage between subsequence I and II, showing several indices for translocation, e.g. coarsening substrate, only partial decalcification and diffuse boundaries (Fig. 11c, d). It is superimposed by 1 m of homogeneous yellow-grey, calcareous (11–12 % CaCO3) and silty loess (Unit 3) with incorporated CaCO3 concretions (Ø 5–6 cm). Unit 4 differs clearly from the underlying and overlying typical calcareous loess layers (Units 3 and 5) in the occurrence of reworked yellowish brown to grey silt loams and sandy layers, both containing Fe / Mn concretions and erosive and translocated structures. The uppermost laminated calcareous loess (Unit 5) of subsequence II is infiltrated with large calcareous nodules up to 15 cm in diameter and marks the boundary towards the overlying light brown reddish silt loam (Unit 6), with a tabular structure and a lower carbonate content compared to the loess sediments. Luminescence ages of the under- and overlying sediments confirm a gap of ∼ 100 ka between subsequence II and IV, implying deposition of SS II during MIS 6 (GI 146: 167.5 ± 21.9 ka). The overlying subsequence IV represents MIS 2 and is characterized by the alternation of yellow sandy loess sediments with intercalated coarser brownish sand layers (Units 8, 10, 12) and greyish yellow horizons with higher silt and lower sand contents, reflecting incipient pedogenesis (Units 7, 9, 11, 13). Transitions between sandy loess and bleached tongue horizons are represented by disturbed boundaries accompanied by redepositional features, such as rounded Fe / Mn nodules and the highest coarse sand contents of the entire sequence. In the uppermost loess (Unit 12) a greyish-black layer of volcanic material 1–2 mm thin is observed, showing deformation features through solifluction processes. Based on the post-IR IRSL ages (GI 154 and GI 155), the volcanic ash layer can be attributed to the Eltville tephra, which serves as an important marker horizon and thus enables us to correlate the Münzenberg loess section with other sequences from central Germany containing this ash layer. The superimposed Unit 14 of subsequence V corresponds to the modern surface soil.