Lithofacies and Transport of Clastic Sediments in Karstic Aquifers

Abstract

Karst aquifers demand continuous transport of clastic sediments if the conduit system is to remain open. Sediments are injected into the aquifer by sinking surface streams and through sinkholes, vertical shafts, open fractures, and other pathways from the land surface of sufficient aperture to permit gravity- and inwash-driven transport. Much transport of clastic sediments tends to be episodic with sediment loads held in storage until moved by infrequent flood events. Although the overall mix of clastic material depends on material available in the source area, distinctly different facies are recognizable depending on the flow dynamics within the conduit system. The facies are most clearly recognized when the source areas provide a wide variety of particle sizes from clays to boulders. In order of decreasing stream power, one can distinguish (i) diamicton facies: masses of unsorted, unstratified clays through boulders carried as a slurry during high energy flood events, (ii) thalweg facies: coarse gravel to cobble size material, well winnowed, forming armoring on underground streams that moves only during flood flow, (iii) channel facies: usually well sorted and often well stratified silt though gravel carried as bedload at intermediate stream powers, (iv) slackwater facies: mostly clay and silt carried as suspended load and deposited from floodwaters backfilled into the conduit system, (v) backswamp facies: mostly clay derived from the insoluble residue of the limestone, deposited under phreatic conditions with little lateral transport.