Clays that form the matrix of melange deposits such as the Joes River Formation in Barbados are characterised by a texture comprising of a high density of anastomosing, curved, polished shear surfaces of low shear strength. This texture gives the unweathered clay a scaly appearance and is believed to be related to the geological processes by which the melange deposit has been emplaced in the crust, generally in accretionary prisms. A series of experiments were conducted on undisturbed samples of scaly clay to deduce their physical and mineralogical properties. This paper will present the results from this experimental programme, and also results from previously published data on triaxial and direct shear tests conducted on stiff, fissured clay.
The Joes River Formation in the Scotland District of Barbados is representative of a melange deposit. It consists of a number of large olistoliths embedded in a dark green to brown, silty clay matrix, which upon further investigation appears inhomogeneous, containing clay rich and silt rich areas. It also contains wisps of uncemented, fine grained sand and streaks of a solid hydrocarbon known locally as "manjack". To add to this inhomogeneity, there are nodules of highly plastic, light green clay, which are of variable diameter. However the most distinctive feature of the melange clay is its pervasive fracture pattern. The fracture surfaces are curved, highly polished and of a low shear resistance. As a result of the high density of fractures, the unweathered matrix readily breaks down into sub-lenticular blocks, which have the appearance of scales. This gives rise to the descriptive name for the rock, Scaly Clay. Scaly clays have been identified both on and offshore in many neo-tectonic areas. The characteristic texture of the material is believed to be related to geological processes by which it was emplaced in the crust (Barber 1986). Similar clays have been found on the islands of Timor (Bobonaro Scaly Clay), Cyprus (Moni Melange), and in Italy (Ariglle Scagliosi). All of these areas have a history of tectonism associated with subduction, and scaly clays are always found in melange formations (Barber 1986, Yassir 1989). However, scaly clays have also been recovered in DSDP cores (Casey Moore et al. 1986, Cowan 1984, Schoonmaker 1986), and in these instances are associated with fault structures. Seismic surveys and sonar images of the sea floor to the east of Barbados indicate the presence of mud diapirs associated with underconsolidated sediments at depth, and it is these sediments that arc thought to form the diapiric melange, of which the Joe's River Formation is a pan (Brown and Westbrook 1987, Stride et al. 1983, Biju-Duval et al. 1982). The stratigraphy of Barbados was divided into five formations by Senn in 1940. The geology of the Scotland group is shown in Figure 1. For the purposes of laboratory testing and classification of the Joe's River Formation undisturbed block samples were collected form three sampling localities within the Scotland District. The locations are labelled A, B and C in Fig. 1.
