Geological investigation of the Berakvam quarry and reserve area for hard rock aggregates, Jelsa, Rogaland

Abstract

Geological mapping and interpretation has shown that the rocks in Berakvam operating quarry can be subdivided in four NW-trending zones, which continue at depth down to the northeast. The southwestern and northeastern zones consist almost exclusively of prophyritic granodiorite while the two central zones consist of prophyritic granodiorite with bodies of gneissic granite and fine-grained grey gneisses with subordinate sheets of pegmatite, respectively.\rSince Norsk Stein A\/S wish to extract rocks down to a level of -50 m in the present quarry, four boreholes supplemented geological mapping of the quarry. The interpretation concludes that the rock volume between the quarry's present level and the -50 m level in the southwestern 3\/5 of the quarry predominantly consists of porphyritic granodiorite of known good mechanical quality. In the remaining northeastern 2\/5 of the quarry, the rock volume down to the -50 m level consists of porphyritic granodiorite intercalated with c. 20% fine-grained grey gneisses. The later will possibly raise the average mechanical properties, as argued from the petrological analysis. The fine-grained grey gneisses contain scattered 5 m wide sheet-like bodies of pegmatite a 100 m wide zone in the southwest, which locally could reduce the homogeneity and quality.\rThe geological investigation of the Berakvam reserve area shows that the rockshere must have similar or better mechanical properties than in the present quarry. The mapping showed that the northern and eastern parts of the reserve area predominantly consist of uniform porphyritic granodiorite with plenty of lenses of fine-grained grey gneisses, and some of small-porphyritic diorite. Particularly interesting is a large irregular body of fine-grained grey gneisses that dominates the southern half of this part of the reserve area. As for the quarry, the petrological analysis of rocks from the area indicate that the fine-grained grey gneisses may show better mechanical properties than those obtained for the porphyritic granodiorites.\rAt the moment, virtually no data exist to document the assumed better mechanical property of the fine-grained grey gneisses. Because they are so important in volume in the western part of the reserve area, close to the present quarry, mechanical testing of these gneisses is recommended.