28 May, 2015

Short anatomy of a bentonite deposit

Bentonite deposits in Bavaria, Southern Germany, are located in a 10 by 40 km wide belt between Mainburg, Moosburg and Landshut, roughly northeast of Munich. Among non-geologists the region is more famous for its hop fields, and bentonite mines are scenically located between hop and gently rolling green hills. Underneath the hop, bentonites are hosted in fluviatile-lacustrine gravels of the Nördlicher Vollschotter (Northern Main Gravel) unit.

In the image you see one of the many small, lenticular bentonite deposits. This particular deposit was  part of my last Bachelor student's thesis in 2014 and the samples were a good test for a portable X-ray fluorescence unit a few months later. The pit nicely shows how a thin lenticular bentonite is hosted in micaceous footwall sands and hanging-wall fluviatile sand and gravels of the Nördliche Vollschotter. The region is blanketed by Quaternary loess of high agricultural value.

Bentonite in unconsolidated Miocene sediments. Bucket excavator and Bachelor student (left from center) for scale.

The sand and gravels are themselves a wonderful sedimentogical units with textbook-like depositional structures. Igneous, metamorphic and sedimentary clasts are derived from Mesozoic, Cenozoic and crystaline rocks of the Bavarian Forest, Swabian Alb, and from the Alps.

Cross-cutting sedimentary structures in fluviatile, sand-rich gravels. A sharp erosional contact separates loess and gravel.

In this deposit gray-greenish bentonite is sandwiched between thin floodplain marls and hanging-wall gravels. Upper contacts in bentonites are almost always erosional in nature. What is very unusual in Bavarian bentonite deposits is that they contain abundant authigenic dolomite. Two up-coming publications are dealing with those, and I can already reveal that dolomite and calcite formation are syn- to very early postgenetic to bentonitization. Using micromorphological, carbon and oxygen stable isotopes we can also show that bentonite-associated carbonates formed in pedogenic, palustrine and groundwater settings.

Bentonite, rather unspectacular, sandwiched between marl (part of the sand unit below) and gravel.

Last but not least a dolomite nodule from underneath the bentonite. Most of the sediments are rich in swelling clay minerals (smectites) and show typical shrink-swell features, such as slickesides. Carbonate precipitation is often related to these structures. The outer shapes of nodules often are perfect imprints of slickensides surfaces.

A medium-sized dolomite nodule from the marl bed. Macroscopically identical to those in bentonite.

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