Sedimentological study of the karstic landforms and associated cart-ruts at Misrah Ghar il-Kbir, T'Alla w Ommu, and Dwejra sites

Abstract

A pilot study was initiated in order to assess the vulnerability of the Maltese cart-ruts to modification by natural meteoric dissolution and recementation. In order to carry this out it was necessary to sample 3 key sites in the islands and tie them into their respective geological settings. It was also hoped that the study would shed light on the relative ages of the cart-ruts against ancient quarrying activities. The Misrail. Giiar il-Kbir (aka Calpham Junction) site is located in the SE area of the Rabat Plateau about 1.5km SE of Dingli village. It lies on the upper part of the Mtarfa Beds of the Upper Coralline Limestone Formation. This member consists of a pale cream to pale grey coloured, pure carbonate succession principally composed of fine bioclastic debris bonded in and lime mudstone (micrite) matrix. The strata occurs in 0.5 to 2m thick beds which dip eastwards at up to 8 degrees. In contrast, the T'Alla w Ommu site is located about 0.6km N of the village of T'Alla w Ommu, eastern Malta. The site lies on the Xlendi Member of the Lower Coralline Limestone Formation. The member consists of a pale grey, medium- to coarse-grained, massive bedded, bioclastic carbonate succession. The rutted surface lies on subhorizontal bedding plane surfaces inclined gently towards the WNW The substrate is affected by faults and it is apparent from the presence of water sinks that there is considerable cavernization within the substrate which is associated with these fractures. The site lies immediately south of the Victoria Lines Fault. The Dwejra site lies 1.1km W of the town of San Lawrenz, Gozo. The cart-ruts lie on west facing slopes associated with the Lower Globigerina Limestone Member of the Globigerina Limestone Formation. This strata is a pale brown, fine-grained carbonate composed of planktonic microfossils set in a lime mudstone matrix. These strata outcrop in bedding plane controlled steps on the hillside (see map in Pedley and Bennett, 1985). Background information on general karst terminology used in this article is to be found in Sweeting (1972) and Jennings (1971). The petrological terminology used in this account can be found in Tucker and Wright (1994) and the standard rock preparation techniques in Tucker (1989). All grid references in this account relate to co-ordinates on the 1:25000 scale geological maps of the islands.