Samstag, 29. August 2009

Quaternary stratigraphy

The quaternary has a long-established tradition of sediment sequences divided on the basis of climatic changes influencing the deposited sediments, particularly sequences in central Europe and North America where divided with this approach.

The German pioneers PENCK & BRÜCKNER divided on the basis of glacial diamicton and nonglacial deposits the terrestrial stratigraphical sequence in the Alps into four main glacial (glaciation), and corresponding interglacial periods. This scheme was based on the identification of glaciofluvial sediments - attributed to glacials- that could be traced upstream until corresponding moraines. The fossil soils between these deposits were interpreted as interglacial weathering.
Although later workers added earlier events to this first sequence, the basic structure remained dominant for more than half a century - and seemed to be recognised all over the world.

In North- and Central Germany the relative division of the Quaternary is based - following the tradition- mainly on glaciofluvial gravels, fossil soils, Loess, moraines and limnic sediments.
The extensive mining for coal and gravel provided exceptionally vast outcrops to study these sediments.

Map with the discussed localities and the main recognized ice sheet stands.
See also: Gamsenberg / Westeregeln / Hundisburg / Bilzingsleben / Ehringsdorf /Weimar and Pennickental

The gravel pit of "Wallendorf" in Thuringia exposes typical, 12 to 5m thick gravel deposits - denominated "Wallendorfer Schotter". These gravels, with rocks outcropping in the Thuringian basin and Thuringian forest, were transported to this locality by the small rivers Ilm and Unstrut, that joined the greater Saale river.

Gravel pit with the "Wallendorfer gravel", attributed to the Holstein interglacial (ca. 350-300kyr).

The Wallendorfer gravels overlay with an unconformity a moraine (Elster) and varved sediments, and are overlay by remains of a younger moraine (Saale), so the unit was attributed to the "Hauptterassenkomplex", the sequence of fluvial terraces deposited during the classical Holstein - interglacial.
The decalcification of the lower part of the partly calcareous gravels, and also the studied mollusc assemblage, seems to confirm a temperate climate with ongoing weathering and soil development.
But the presence of lithic artefacts and bones of ice age steppe animals, li
ke mammoth, auerochs and reindeer, also with ice wedge casts, demonstrates that the upper part of the gravels were already deposited during cold conditions and the beginning of a glacial.

Ice wedge cast in the upper sequence of the "Wallendorfer gravel". This features are typical for periglacial conditions.

Varved sediments and the moraine of the Elster glacial - the next "deeper" sediments in the stratigraphic column- can be observed in the gravel pit "Rehbach".

Gravel pit "Rehbach".

Here, in a proglacial lake, fine clayey sediments were deposited until the advancing ice sheet run down the previously deposited sediments. On the basis of the moraine reworked, probably transported in frozen state, clasts can be observed.

Varved lake clay -deposited in a proglacial lake.

The contact between the varved lake sediments (note the brown layers) and the massive, loamy moraine.

Sandy clast with some pebbles in the transition area between lake sediments and moraine, probably reworked and fluvial transported (in frozen state) material then incorporated at the basis of the moraine.

What seems like an easy recognisable alternation of glacial and interglacial sediments, can however present local conditions that can be very tricky. In the gravel pit Karsdorf a succession of polymict gravels is suddenly overlay by sand with gravel lenses. This seems inconsistent with any sudden climatic change, or stratigraphic pattern in the region.

Outcrop in the gravel pit "Karsdorf", sand is overlay by large scale lenses of pebbles and coarse grained rubble.

Shear imbrication typical for debris flows overlay sandy sediments of fluvial orogin.

Only the study of the large outcrop, the topography and the underlying geology solved this apparent mystery.
The upper sand-gravel-rubble unit represents an alluvial fan, that coming from the circumjacent limestone hills joined the former pathway of the Unstrut river, and probably forced the river to change flow direction.

The discussed division is fundamentally lithological. However sediments are not unambiguous indicators of climate (for example the "Wallendorf gravels extend from a "warm" climate to a "cold"), so that other evidence, such as fossil assemblages, characteristic sedimentary structures (including periglacial structures) or textures, and soil development must be also considered. Local and regional variability of climate complicates additionally the attribution of sediments to climatic phases. The correlation between stratigraphy of different localities is mostly only possible were large outcrops are present, or very distinct sediments were found.

The climatic subdivision of the Quaternary is not without problems, in contrast with the rest of the geological column, which is divided using time (chronostratigraphy). The relative correlation between terrestrial glacial and interglacial periods is difficult and restricted mostly geographically - especially the correlation with the well dated and accepted marine records, that at least shows 40 climatic oscillations during the Pleistocene, is still at the first steps.


GIBBARD, P.L. (2006): Climatostratigraphy. In (ed): ELIAS, S.A.: Encyclopedia of quaternary science. Elsevier : 2819-2847
BERNHARDT, W.; THUM, J.; SCHNEEMILCH, M. & RUDOLPH, A. (1997): Flußschotter als Schaufenster in die Zeit der ältesten Besiedelung Mitteldeutschlands. Archäologie in sachsen-Anhalt Nr.6. Archäologische Gesellschaft in Sachsen-Anhalt, Halle
WEBER, T. (1996): Das Paläolithikum und das Mesolithikum in Mitteldeutschland. Archäologie in sachsen-Anhalt Nr.6. Archäologische Gesellschaft in Sachsen-Anhalt, Halle

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