S. BHALAI – Landslide susceptibility of Portland, Jamaica
Figure 6. Digital Terrain Model of Portland with landslide inventory.
Sedimentary and igneous rock types dominate though metamorphic lithotypes are present (Table 1; Mines and Geology Division, 1997a-d). These are of Cretaceous to Neogene age (Figure 7). The Cretaceous rocks occur in the Blue Mountains inlier and include quartzo-feldspathic and basic schists, tuffs, lava flows and granodiorite intrusions. Cretaceous lithologies comprise the upper sections of the Blue Mountains extending from Silver Hill Peak in the west to the left bank of the Rio Grande in the east. These extend northwards as far as Port Antonio.
Paleocene to Miocene sedimentary rocks encircle the Blue Mountain inlier. Paleocene-Eocene rocks
from conglomerates, to impure to pure
limestones. The John Crow Mountains comprise shale capped by deep-water micrites. There are also minor Miocene volcanic rocks (tuffs and lava flows) in the north central section of the parish. Elevated rocks of the Coastal Group fringe the coastline. Fluvial deposits extend from the coastal areas inland along some of the river valleys. Large masses of colluvium consisting of rock and debris drape the landscape in many areas such as
Shrewsbury, Tranquility, Milbank and Cornwall Barracks.
The dominant structural feature of Portland is the northern extension of the Blue Mountain Inlier superimposed with an intricate fault pattern reflective of polyphase deformation due to overprinting of tectonic events. The mountainous terrain consists of densely fractured rocks that have been exposed to long periods of deep weathering, and are highly susceptible to landslides.
3. METHODOLOGY Many approaches have been employed in Jamaica for landslide susceptibility assessments (Miller et al., 2007; Northmore et al., 2000; Unit for Disaster Studies, 1999). These generally involve combining direct and indirect methods. The methods are similar, with field surveys conducted initally and statistical approaches used in the analytical stage. The differences arise in the specific statistical techniques chosen. Miller et al. (2007) and Northmore et al. (2000) use bivariate statistical analyses in their studies. Miller et al. (2007) used Bayesian conditional probablility in zoning the parish of St. Thomas and Northmore et al. (2000)