engleski

Provenance of Beach Sands from the Island of Rab: Evidence from Heavy Minerals

Introduction: Studying heavy mineral assemblages of beach sands and possible source rocks can often gain useful insight concerning supply patterns by which beaches are nourished with detritus. Beaches represent a delicate environment, one that is easily subject to destructive processes if imbalance is created between the supply of detritus and erosion. Such insights are thus of particular importance considering the rising pressure the Adriatic natural coast is experiencing with increased development of tourism and industry in recent years. Previous research on beach sands from the Island of Rab (CRNJAKOVIĆ et al., 1998) has shown that considerable variability exists in their heavy mineral compositions. The proposed sources of detritus were Eocene and Pleistocene siliciclastic rocks, which are present over a considerable area of the Island. In the current study new data is being obtained from both ancient rocks and recent beach sands with the intent of more precisely defining the source-beach relationship. Materials and Methods: Beach sands have been sampled at 8 beach locations (Figure 1). Several additional samples have been taken from Eocene and Pleistocene arenites to test similarities in heavy mineral associations. Obtaining heavy mineral concentrates was performed by commonly used procedures (MANGE & MAURER, 1992). Additionally, particle composition of arenites has been studied using thin sections made from artificially cemented sands. Results: Transparent heavy mineral associations from beach sands show specific differences. Most conspicious are the differences in the proportion of garnet and the ultrastable minerals zirkon, tourmaline and rutile. Garnet proportion varies between 12% and 72%, while ultrastable minerals vary from 11% to 67%. A higher proportion of garnet relates to a lower proportion of ultrastable minerals, and vice versa. Among the other most common heavy minerals are epidote/zoisite, Cr-spinel, staurolite and kyanite. Discussion and conclusion: The consistent variability in the garnet versus ultrastable mineral proportions in heavy mineral associations of the studied beach sands seems to be a consequence of the derivation of these sands. A comparison of Eocene and Pleistocene arenites shows similar difference. While the Eocene heavy mineral associations resemble that of the garnet-rich and ultrastable mineral-poor beach sands, the Pleistocene heavy mineral associations are similar to those beach sands rich in ultrastable minerals and poor in garnet. These relationships suggest that some beaches are dominantly supplied from source areas consisting of Eocene sandstones, some dominantly from source areas consisting of Pleistocene sands, while several beaches show HM associations suggesting a mixed source. Such relationships between beach sands and Eocene/Pleistocene arenites have previously been suggested based on the study of three beaches of the Island of Rab (CRNJAKOVIĆ et al., 1998), and the beaches in northern Dalmatia (PAVIČIĆ et. al., 2000). This study confirms previously proposed interpretations and shows that the relevant consistencies in heavy mineral variability in beach sands represent a rule for the northern half of the Croatian Adriatic coast. As the spatial relationship of beaches and their interpreted type of source areas show close vicinity, it follows that the supply of detritus to the beaches is greatly controlled by immediate hinterland erosion. Other possible supply paths such as longshore drift or a derivation from a distant source are much less significant. In conclusion, the source areas and the natural nourishment pathways of beach sands for the studied coasts can be successfully identified using a heavy mineral approach. The result of this is useful knowledge gained pertaining to the problem of sustaining Croatian beaches (PAVIČIĆ et. al., 2000). The need for evaluating how relevant natural processes will change with accelerated development of the coastal zone is growing. Otherwise, ignorance will inevitably bring damage to this natural and economic resource of the country.

provenance; beach sands; heavy minerals; Rab; Croatia

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