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Drainage Development in the West Totes Gebirge ( Austria)

Preliminary observations

by Mike Luckwill

The Totes Gebirge are a complex of many kinds of limestone and dolomite situated east of Bad Ischl and north of the Dachstein massif, in Upper Austria. Bounded to the west by the Traun river, which takes most of the drainage from the Dachstein, the western third of the mountains is dominated by the Schönberg: a ridge attaining a height of more than 2,000 metres.  To the south of the Schönberg a gently sloping plateau is the site of the many entrances of the Raucherkar System, and to the north, the Schönberg drops steeply into the tributary valleys of the Traun (Fig.1).

 

Figure1.  North – South Section from the Schönberg

The area under particular consideration is that delineated by the northerly drainage of the Schönberg, and is mainly on the Dachstein Limestone (Fig. 2).

 

Figure 2.  Sketch map of the environs of the Schönberg.
Solid lines:  Contours at 1900, 1700, 1500 and 1100 metres.
Broken lines: geological boundaries: dL – Dachstein limestone: L – Lias:  L+ - Lias plus others: d- dolomite.
Fuzzy large dots indicate Peaks.   Dots indicate cave entrances.

 

Figure 3.  Joints in the north face of the Schönberg

The drainage of the limestone is joint controlled, except where superficial water from the soil is cutting channels in the rock which represent the initial stages of clint and gryke formation.  There are two types of joints in the limestone which for the purposes of this article will be called A – joints and B – joints.  They are both illustrated in Fig. 3.

The A – joints consist of three mutually perpendicular families of joints with separations of the order of a few feet.  Fig. 4 is an attempt to show alignments of these joints, which will be called A1, A2 and A3 joints.  In the locality of the Schönberg the A1 joints strike 030o – 210o and dip about 80o – 90o in a westerly direction.  The A2 joints dip 10o – 15o along 030o, that is along the strike of A1 (true dip is about NE).  The third set A3 are nearly vertical and strike 120o – 300o; they are poorly developed and are an aid to erosion rather than a controlling factor.  The B – joints are fault features although little movement has occurred along them in this area.  Their strikes tend to run about 020o and their angle of dip varies considerably from joint to joint and also down each joint.

 

Figure 4.  Spatial distribution of A joint families

Surface water is supplied from two sources: run-off from rain and snow, and melt water from permanent or semi-permanent snow patches.  Run-off water is quickly channelled into a drainage system which, under the influence of the A – joints, runs along the intersection of the A1 and A2 joints: it thus bears along 030o and at the same time sinks about 15o. The snow patches on the other hand, promote the development of pits.  The A1 aligned sides of these form smooth, vertical walls, frequently 30 metres deep; whereas, the other two sides, formed by lesser developed A3 joints, tend to be step like.  The result is a rectangular pit with cross section as shown in Fig. 5.  Formation of these pits and other dolines on the Schönberg plateau concentrates the run-off from the area into a number of focal points where it then develops a cave down the intersection of the A1 and A2 joints. As can be seen from Fig. 6, the cross-section of these caves is closely controlled by the jointing and some of these simple, A-caves appear to have developed lengths of as much as 1,500 metres.

 

Figure 5.  Vertical section of rectangular pits.

Unfortunately, the beautifully simple picture of surface pits at about 2,000 metres feeding water to long, simple A – cave has been complicated by glacial erosion.  The major effects of the multiple glaciations that occurred during the Pleistocene period were two in number.  Firstly the changing temperatures and the changing topography frequently altered the supply and nature of surface waters; and secondly the periodic lowering of valley floors and hence the base-levels altered the erosive power of these waters.  Unravelling the timetable of these events required the analysis of a considerable amount of data and is not helped by the fact that each glaciation frequently removed the evidence of previous glaciations!

The last glaciation, the Wurm IIc (Wurm III of some workers) was responsible for the erosion of the Fuertal and the Hinterglas, the two valleys immediately north of the Schönberg and running approximately NW – SE.  This resulted in the tri-section of the A – Caves (see Fig. 6) and left the entrance to the Ahnenschacht, the largest system in the area, stuck on top of a narrow ridge!

 

Figure 6.  Section through Schönberg and Ahnenschacht (not to scale).

Previously to this, the A – joint drainage had intersected a B – joint and erosion down the dip of this joint resulted in the formation of the Ahnenschacht.  For a depth of some 300 metres this superb cave follows the same joint, which is always visible in the cave.  Occasional shifts to the north along the B – joint indicate the influence of the A – joints on inlet waters.  Little deposition of calcium carbonate has occurred in the cave (except in one rift, See Thomas) and at the present time what little formations one can find are rotting.  A sequence of calcite deposition and consequential rotting, located at a depth of about 30 metres appears to correlate with Wurm glaciations.  Some indication of the conditions extent during these times may also be derived from the alterations of phreatic and vadose features as one proceeds down the cave.  Three distinct processes have occurred.  Phreatic conditions have produced tubes and half-tubes above the joint, leading eventually to anastomoses.  Vadose conditions involving little water have modified this development, frequently causing collapse; and vadose conditions involving large quantise of water (supplied for example by melting snow) have formed canyons and vertical pitches and have also caused the transport of collapsed material and other fill.

The existence of a steady base level for a considerable length of time allowed the development and enlargement of an A – cave below the Fuertal which bears about due north and dips about 15o.  The extension of this system would bring one to the intermittent-spring line in the Aibl-grube.  Luckily a minor joint, developed by percolating waters to form a sloping rift, has connected this A – cave with the B – cave at a height of about 1,500 mettes above sea level, thus facilitating its exploration.

 

Figure 7.  A- caves on north face of Schönberg.  Distance apart of A – joints may be as much as 6 metres but often is only 1 metre.

At the present time drainage is being modified by the annual weather cycle which, in Spring, introduces into a system the melt water from as much as 20 metres of snow.  Snow patches lasting throughout the summer in protected hollows and pits create vertical inlet features and ensure a constant supply of water to the lower parts of the cave, regardless of weather conditions. The resulting waters are at present creating a system, presumably A – joint controlled at a depth of 100 metres below the older system.  As yet nothing is known about this system, except that its extension northwards brings one top the Ursprung Brucke: the permanent spring in the Aibl-grube.

Further exploration and accurate surveying of the Ahnenschascht should lead to the correlation of many surface features with their subterranean counterparts and for this reason extremely fascinating.

REFERENCE: Thomas, A.R., ‘Ahnenschaschat 1968’. BB Vol.22 No.9 pages 103-114.