Factors controlling evolution of karst conduits in sandy limestone and calcareous sandstone (Turnov area, Czech Republic)
Abstract
Karst potential is rarely studied in calcareous sandstones and sandy limestones. The minimum content of carbonate necessary for the development of conduits is unknown, and the mechanisms of the origins of conduits in these rocks are poorly understood.
This article focuses upon these questions in an important regional aquifer. Samples of limestones and sandstones were taken from outcrops in vertical profiles, and were subjected to a rapid dissolution test with 10% HCI.
The degree of disintegration of the samples after dissolution of the carbonate was observed in order to estimate which portions of the rock enabled formation of karst conduits. Carbonate content was measured by calcimetry.
Distribution of calcite cement and other sources of cohesion were studied by scanning electron microscopy. Tracer tests in the conduits were evaluated.
Calcite is the dominant carbonate mineral and main cementing agent in the Jizera Fm. Aquifer (mean content 51%).
However, calcite content was found to be a weak predictor of karst potential. Most of the samples disintegrated completely if the calcite content exceeded 55%.
On the other hand, some rocks with a calcite content up to 70% did not disintegrate due to microcrystalline/amorphous silica, which bound the quartz grains together and/or formed a foam-like supporting structure. After in situ calcite dissolution, the rock in karst-prone zones is turned to residuum, which consequently is washed out by fast flow (0.01-0.2 m/s, based on tracer tests).
Microscopic study of cementing agents of the siliciclastic-carbonate sediments and dissolution tests are essential for understanding the mechanisms of conduit origin in ghost-rock karst.