Geology

Geology

As stated above, there are differences between the geographical and the geological division of the Black Forest.

The geological division follows hercynian fault and suture zones, which are not of the same direction as the tertiary fault zones the recent rivers follow.

Instead of speaking of a northern, middle and southern part in a geological sense one can rather call it complexes or gneiss masses (the latter only applicable to the middle and southern parts).

There are two main suture zones of hercynian age (~ 330-290 million years b.p.):

1. The Baden-Baden-Zone, which is part of the suture between two main microplates/crustal blocks in hercynian times, the saxothuringian and the moldanubian zone. This suture reaches from France in the west (outcropping in the Vosges Mts.) to Bohemia in the east.

2. The Zone of Badenweiler-Lenzkirch (BLZ) which is named after two towns in the Southern Black Forest. This zone is of SW-NE striking direction and is somewhat a miracle. Neither from the Vosges nor from Bohemia similiar features have been reported. This zone consists of early variscian granitoids (deformed to gneisses in variscian orogeny), metablastic gneisses (like augen gneisses) and sedimentary/volcanoclastic rocks typical of deep water such as greywackes).

The geological division follows these zones. Whereas the northern part has no special name, the central and southern part can be described as Central Black Forest Gneiss Mass/Complex and  Southern Black Forest Gneiss Complex/Mass.

In the next part  the geological events in the Black Forests are described in a chronological order.

1. Precambrian basement complex
This basement complex can be found in the Central and Southern Black Forest Gneiss Complex in vast areas, whereas it is rare in the northernmost part. This is due to tectonic uplift in the neogene (see chapter about neogene tectonics below).
The basement complex is described by METZ (1980) especially for the southern part to be formed in 5 subsequent steps:
1. Formation of sediments (protoliths of paragneisses) in early precambrian times
2. Formation of shists, possibly higher metamorphic event afterwards
3. Intrusion of mafic magmatic bodies, today preserved as serpentinites, gabbros, norites and amphibolites
4. Intrusion of granitoid magmatic bodies (later orthogneisses) to a vast extent, mingling with paragneisses, first anatexis
5.kinetic overprint of the whole gneiss complex by assyntian orogeny

Today, the prehercynian gneiss complexes are built up of paragneisses and orthogneisses as well as mixed gneisses, formed by contamination of granitoids due to crustal melting processes. Those mixed gneisses, mostly anatectic, are widespread. They often contain plagioclase, K-spar, biotite and quartz. Paragneisses are often anatectic as well, containing either cordierite (especially in the southernmost part of the Southern Black Forest Gneiss Complex and the area east of Freiburg, where cordierite blasts reach up to 6 cm in size), or garnet or sillimanite. Granulitic types are widespread, some of them do contain garnet and kyanite.
Orthogneisses are present, too. They mainly are granitic/granitoid in composition, consisting of plagioclase, biotite, quartz and K-spar. Some types contain garnet or allanite, rarely orthopyroxene bearing types have been found (e.g. near Todtmoos).
Smaller, lens-shaped bodies of eclogites, calc-silicate rocks, amphibolites (former marls or basic rocks) are present, too. In the southernmost part, some lenses of mafic to ultramafic rocks (especially norites and pyroxenites) are present, most of them serpentinized in hercynian times.

2. Prehercynian (upper devonian-lower carboniferous) sediments and volcanoclastics
The occurrence of these rocks is limited to three areas:
1. The Baden-Baden trough, forming the westernmost part of the Baden-Baden Zone
2. The Zone of Badenweiler-Lenzkirch (BLZ)
3. small outcrops E of Offenburg (central part)

Mostly consisting of sediments of upper devonian to lower carboniferous age. These sediments are greywackes and shales (mostly consisting of clay) as well as fine-grained volcanoclastic sediments. Conglomerates (with granitoids and shales as clasts) can be found, too. In the Sulzburg area they contain gold.

They are preserved nearly as they have been formed. There is by no means a strong  hercynian overprint. Only elongation of clay flakes in greywackes can be found. Fossils are rare, only a few goniatites have been reported from Schönau.

This sequence of rocks seems to have been formed in a deep marine environment by submarine volcanic activities. The Zone of Badenweiler-Lenzkirch is described by some authors as a subduction zone of upper devonian/lower carboniferous age. The closure took place in early hercynian times, leading to a granitoid magmatism, which is preserved as an orthogneiss band, the so-called Rand Granite.

In the Offenburg area carboniferous sedimentary rocks can be found near Berghaupten and Hohengeroldseck, consisting of sandstones and coal seams. Near Berghaupten coal has been mined up to mid 20th century. Here fossils such as petrified wood have been found.

3. Hercynian metamorphic event
The second main event in the development of the basement complexes took place in hercynian times (carboniferous to lower permian, ~330-270 million years b.p.).

In these times a second anatexis took place. The main impact on geology has been the intrusion of vast bodies of granitoid rocks.

The first one has been the intrusion of trondhjeimitic magma into the northern margin of the BLZ. This is considered as a calc-alcaline magmatic event at a subduction zone (dipping to the northwest) by some authors. This granite had been metamorphosed to a orthogneiss in the main metamorphic event in hercynian times.

Granite bodies formed during the main event are not metamorphosed afterwards. They can be considered as the main heat source for the second anatectic event. Some authors describe intrusions into the lower crust or even its base (magmatic underplating) as main heat source for the hercynian orogeny.

Granitoids formed in that stage are I-type (igneous type) granitoids, such as diorites, syenites and biotite granites. They do mostly consist of feldspar (plagioclase and K-spar to various amounts), biotite and quartz, sometimes (in diorites) amphibole is present. Some of these rocks do contain big orthoclase xls up to 10 cm.

Later on, to the end of the main event younger granitoids were formed, such as the Triberg Granite in the central part or the Bärhalde Granite in the southern part. Those are of mixed origin (magma mixing) or S-type granitoids, consisting of crustal melts. The composition of those bodies is more complicated, often being composed of different types. These rocks are often coarse-grained, rich in quartz and orthoclase, poor in mafic minerals of which only biotite is of some importance. In the  Triberg Granite some minor dioritic parts are present to the margins, surrounding light-coloured leucogranites with less than 10 % of biotite. Muscovite is present in some of these rocks, too. Stoping (i.e. wallrock falling into magma chambers) is present sometimes. Some zones (for example Niederwasser, Menzenschwand) which have been near the top of the magma chamber contain pegmatitic zones or so-called greisen zones (i.e. zones with a pneumatolytic overprint in late hercynian times), containing cassiterite, white mica and topaz. Miarolitic cavities are present in these areas as well as pegmatites. Those do contain schorl, quartz, scheelite, sometimes powellite or wolframite. Hematite is a common latestage mineral in these rocks, giving those granites a reddish colour.

Tectonically the hercynian orogeny consisted of the formation of a crustal plate from several microplates situated at the southern margin of former supercontinent Laurentia. The Black Forest consists of two microplate fragments, called saxothuringian and moldanubian.

4. Late/post-hercynian volcanic activity
The granitoid magmatism was subsequentially followed by a permian volcanic phase. In this phase layers or covers of volcanoclastic rocks and thin floodings were formed, being rhyolitic in composition. Outcrops can be found in the northern part near Baden-Baden and in the Lierbach Valley as well as Ottenhöfen. In the central part they can be found near Ohlsbach, Gengenbach and Schweighausen (Mt. Geisberg). To the south massive rhyolites are abundant in the Münstertal Valley. There late variscian fault breccia can be found, too. This rock is highly silicified, forming steep outcrops today.

Some of the rhyolites contain thundereggs filled with agate. Famous localities are near Baden-Baden, in the Lierbach Valley and Mt. Geisberg.

5. Late-stage/post hercynian erosion
The high levels of the orogenic body have been exposed to the surface in late hercynian times. In upper permian times top-levels of granitoid bodies have been exposed, leading to the formation of arcoses, showing fresh K-spar clasts. Such arcoses can be found outcropping between Maulburg and Kandern in the southwestern corner (15 km north of Basel). Sandstones and dolomites, some with chert horizons, can be found, outcropping in the Schramberg area.

6. Mesozoic sedimentary cover
Only triassic and jurassic sediments can be found, whereas in cretaceous times only erosion took place.
In Germany, the triassic is subdivided into three parts, called Buntsandstein, Muschelkalk and Keuper. The lowest triassic rocks are the red sandstones and conglomerates of the Buntsandstein. The bedrock is often weathered and bare of a upper permian covering, sometimes a layer of carnelian chert marks the lowermost layer of mesozoic rocks. The permian-triassic boundary is also marked by a layer of carnelian chert.
The Buntsandstein can be divided into three parts, of which the middle and upper are of special importance in the Black Forest. The lower part outcrops in some valleys in the northern Black Forest, e.g. the Enz Valley near Pforzheim. The middle part consists of red sandstones and conclomerates (pebbles up to cm in size). It is used in construction of buildings and walls, an example is the market place of Freudenstadt. The middle Buntsandstein covers vast areas in the northern, especially northeastern part and the southeastern part. The upper Buntsandstein consists of hard silicified sandstones (formerly used as material for millstones), a dolomitic horizon with carnelian nodules (carnelian dolomite) and red clay and outcrops in the northeastern and southeastern part of the Black Forest. 
 Tectonically sheared blocks of Buntsandstein can be found in the Upper Rhine Valley between Freiburg and Lahr.   

The subsequent part is called Muschelkalk. The Muschelkalk itself consists of three parts, the lower and upper being calcitic, micritic limestones rich in fossils like clams (hence the name), brachiopods and ceratites, whereas the middle part is made up by salinar sediments such as dolomitic marls and limestones, anhydrite (weathered to gypsum often) and salt. The sediments are of shallow marine origin.
Muschelkalk mostly can be found  in the southernmost area. The Dinkelberg area (isolated hillside near Basel) consists of Muschelkalk. In Switzerland rock salt had been produced from the Muschelkalk. Other outcrops can be found to the east (Neckar Valley near Tübingen, reaches to the west to Freudenstadt) and north of the Black Forest in the Kraichgau hills. Tectonically sheared Muschelkalk is found in the southern part of the Upper Rhine Valley near Sulzburg and Staufen.

The last sequence of the triassic, the keuper can only be found north of the Black Forest in the Pfinzgau area (near Pforzheim), southwards it is eroded.

In the area of interest, jurassic rocks can only be found in the southwestern part as tectonically emplaced rocks, forming the hillside of the Markgräfler Land. Similiar rocks are found at the southwestern border of the Black Forest near Schaffhausen, too. This area is called Klettgau.
The rocks are mid to upper jurassic in origin, occurring as oolitic, shallow marine limestones. Iron rich middle jurassic oolites had been mined at Ringsheim near Lahr and in northern Switzerland. Massive white limestones from Istein, Markgräfler Land bear white to grey nodules of agate-like chert, which have been used in prehistoric times and in the middle ages.

Cretaceous rocks are not preserved in the area.

7. Subsequent tectonics, formation of the Upper Rhine Valley
In upper jurassic times the alpine orogeny started. Rapid uplift of the alpine orogen due to northwestern movement of the adriatic plate (Italy) led to pressure in the northern margin.
This led to a beginning uplift and erosion of the Black Forest/Vosges complex, acting as a rigid shield. It has been further uplifted during paleogene times, leading to high tension. The center of uplift had been the southern part (the Southern Black Forest is more uplifted than the northern part) near Freiburg. The Kaiserstuhl marks the area of highest tension and crustal thinning. In the paleogene the uplift led to the formation of a SW-NE directed fault zone, subsequentially leading to the formation of a inter-crustal rift zone. The vertical escarpment has been up to 800-1000 m. In most parts the margins are not made up by one normal fault zone only, but a series of parallel faults, forming horst and graben structures.
Parts of the sedimentary covers slipped down, forming hillsides west of the main fault.
The Upper Rhine Graben is part of a trans-european zone, reaching from southern France (Avignon) to the Oslo Graben in Norway. Continental rifting began in neogene times, leading to crustal thinning with maximum in the Kaiserstuhl area (~28 km thickness). The high, steep shoulders led to subsequent, fast erosion, filling the Upper Rhine Graben with lots of debris.
The formation of faults perpendicular to the main fault, with a strike direction of NW-SE led to broader valleys, such as the Münstertal Valley or the lower Kinzig Valley. Those are graben zones.
 In oligocene times the Upper Rhine Valley had been under sea level and had been flooded by a shallow sea. That led to the formation of clay, marls and even salt deposits. Up to th 1960'ies potassium salt (sylvite, carnallite) had been mined in Buggingen near Freiburg and in France.
Weathering of the rocks along the flanks took place in early paleogene under subtropical to tropical, humid conditions.
This led to the formation of iron-rich residual clays from jurassic, oolithic limestones. This clay contains limonite nodules called bean-ore. In some areas red to yellow jasper nodules can be found, formed from residual SiO₂ from the limestones. These eocene jaspers should not be mixed up with the jurassic, diagenetic chert nodules.
In the miocene the extension led to the formation of ultrabasic to basic melts in the center of spreading. The eruption led to the formation of the Kaiserstuhl Mts..

8. Weathering in Neogene and Quartary
Weathering in the neogene took place under humid, warm conditions in an area with high relief. High relief led to high energy in streaming water which led to rapid formation of narrow, steep valley, some are formed as canyons, especially those going to the river Rhine between Lake Bodensee and Basel.
In those times the River Rhine fled to the Mediterranean Sea through Burgundy and the Rhone Valley. The northern part of the Alps, the Jura hasn't been folded. The rivers north of the Kaisersthl fled to the Northern Sea through the actual bed of the River Rhine.

Later on the breakthrough of the River Rhine through the Kaiserstuhl height and the folding of the Jura led to the closure of the Rhone-Rhine and the formation of the actual stream bed.

The Quartary began with vast and widespread fluviatile sedimentation, leading to the formation of debris fans at the points the rivers led out of the mountains. The Rhine Valley had been flooded by sand and gravel. Erosion took place overall.

In Pleistocene times glaciation took place, leading to formation of glaciers in the Black Forest, especially in the Feldberg area. The actual shapes of the mountains are an effect of glaciation. A lot of the valleys especially in the Feldberg area had been formed by glaciers, as well as kar lakes and some bigger lakes, like famous lakeTitisee . In some areas, the sediments of the Upper Rhine Graben are covered by periglacial aerial sediments (löss).

(c) of the text: Sebastian Möller

Some information taken from: METZ,R. (1980): Geologische Landeskunde des Hotzenwaldes. Schauenburg Verlag, Lahr (in German and out of print)