Quaternary Subrosion and Transformation of Stassfurt Potash Seam (Zechstein 2) at Top of Gorleben Salt Dome, Federal Republic of Germany

Otto Bornemann, Reinhard Fischbeck

The Gorleben salt dome is located in the eastern part of Lower Saxony in northern Germany. The dome consists of salt layers of cycles 2, 3, and 4 (z2-z4) of the Zechstein.

Above-ground studies have been conducted since 1979 to determine whether the Gorleben salt dome is suited for the permanent disposal of radioactive waste.

During the Elster glacial stage, the Gorleben salt dome was covered by up to 1,000 m-thick ice. The meltwater eroded up to 300-m deep runoff channels into the rock beneath the glacier.

One of these Quaternary channels runs across the salt dome; locally, the channel cuts the cap rock down to the rock salt. The Stassfurt potash seam cuts across at the bottom of this channel under Quaternary sediments. The extent of leaching and the stages of transformation of the potash seam were explored by six boreholes sited perpendicular to the strike. The potassium minerals of the potash seam had been completely dissolved down to 90-130 m beneath the salt surface. The cavities formed by leaching were filled with sand and clay material from the overlying sediments. Only at about 170 m below the top of the salt dome was the potash seam, encountered in the form of carnallite, unaffected by water from the overlying sediments.

Between 90 and 170 m below the top of the salt dome, the potash seam was encountered in different stages of transformation. The sequence from bottom to top was as follows: unchanged carnallitic rock, followed by a zone of kainitic rock, ending with a potash-free halitic rock containing clastic material from the overlying sediments.

Diagenesis of the clastic Elsterian sediments interbedded in the rock salt blocked further migration of solutions into the potash seam. No evidence has been found for further subrosion of the potash seam since the Elsterian (i.e., the last 300,000 years).

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.