| Popis: |
Little is known about controls of clastic deposition in deep limestone caverns, particularly those with no known connection with the surface. Such caverns are occasionally discovered by tunneling, but seldom studied in detail. One of several caverns discovered during construction of St. Rok road tunnel in S. Velebit Mt. was largely filled with clastic sediment, and sampled shortly after its discovery in 1999, in approximately mid-part of the tunnel. The cavern sediment is of mixed carbonate-siliciclastic composition with alternating mm- to cm-thick layers of medium- to fine- and very fine-grained arenite. Individual arenite layers differ in grain-size, color, matrix and iron content. Some very fine-grained arenite layers are draped by thin red-brown clay laminae. The sediment organization indicates that the deposition was strongly controlled by seasonal (? annual) changes in water discharge. Medium-grained arenite was deposited from stream flow, apparently during periods of relatively high discharge, whereas fine-grained and very fine-grained arenites were deposited during slow percolation and stagnant water. This sediment pattern is very similar to clastic varves, and reflects climatic pattern in Velebit Mt. high parts, but ca. 300 m below the topographic surface. |
