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Landslide Susceptibility of Portland, Jamaica: Assessment and Zonation - page 5 / 16

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GEOLOGIC UNIT Colluvium

SYMBOL -

LITHOLOGICAL DESCRIPTION

GEOTECHNICAL GROUPING COLLUVIUM

Loosely or poorly consolidated rock, debris and soil material.

Alluvium

Qa

Unconsolidated or partly consolidated river sediments

ALLUVIUM

Elevated reef Low Layton Lava

Ql MP1 MP

Elevated reef Basalt pillow lavas and breccia

COASTAL GROUP TERTIARY VOLCANICS COASTAL GROUP

Buff Bay and Manchioneal Formations

Fossiliferous reefal limestone, sandstone and marls

Montpelier Formation

Mm

Chertiferous planktonic chalk and micrite

WHITE LIMESTONE

Gibraltar-Bonny Gate Formation

Egb

Evenly bedded chertiferous chalky micrites

WHITE LIMESTONE

Fonthill Formation

Ef

Well bedded impure micritic limestone

YELLOW LIMESTONE

Newcastle Volcanics Halberstadt Volcanics Richmond Formation Chepstowe Limestone

En Ev Er El

Lavas, volcanic breccias and tuff Basaltic lava including pillow lavas Well-bedded sandstone and siltstone

TERTIARY VOLCANICS TERTIARY VOLCANICS SHALE YELLOW LIMESTONE

impure limestone and interbedded shale

Wagwater Formation

Ew

Red and purple conglomerates and sandstones

CONGLOMERATE

Bowden Pen Conglomerate

Kbp

Polymictic conglomerate and tuff

CRETACEOUS SEDIMENTARY

Bonny View Formation (or Spring Bank Andesite)

Ksb

Plagioclase feldspar porphyry

CRETACEOUS VOLCANICS

Cross Pass Shale

Kcp

Brown weathered shale, siltstone, sandstone and conglomerate

CRETACEOUS SEDIMENTARY

Spanish River Formation

Ksr

Purple andesitic tuffs, lava and polymictic conglomerate with limestone

CRETACEOUS VOLCANICS

Rio Grande Formation

Kr

Weathered green volcanic rocks and grey recrystallized limestone

CRETACEOUS VOLCANICS

Ginger House Formation St. Helen’s Gap Formation

Kgh

Tuffaceous conglomerate

CRETACEOUS VOLCANICS

Ksg

Dark green trachytes to olivine basalts; deeply weathered in some areas

CRETACEOUS VOLCANICS

Bellevue Formation

Kbe

Mudstone, siltstone, porphyry, tuffs and limestone

CRETACEOUS SEDIMENTARY

Back Rio Grande Limestone

Kbr

Rubbly limestone and conglomerate with volcanic sediments

CRETACEOUS SEDIMENTARY

METAMORPHIC

Westphalia Schist Granodiorite

INTRUSIVE

Minor porphyritic intrusions

Maastrichtian

PLANTAIN GARDEN GROUP

Lower

WAGWATER

Eocene

GROUP

Miocene to Mid-Eocene

WHITE LIMESTONE

YELLOW LIMESTONE

Catalina Formation

CRETACEOUS VOLCANICS

METAMORPHICS IGNEOUS INTRUSION IGNEOUS INTRUSION

S. BHALAI Landslide susceptibility of Portland, Jamaica

Recent to Miocene

COASTAL GROUP

CORN HUSK GROUP

Kc

Kw KG g

Tuffaceous andesite and conglomerate Felsic and basic schists

Hornblende granodiorite

Table 1. Lithostratigraphic record of Portland. Groups of similar geotechnical behaviour used in the landslide susceptibility analysis are also listed. Data extracted from Mines and Geology Division (1997a-d).

Lower Maastrichtian to Upper Campanian

ALLIGATOR CHURCH GROUP

AGE

Recent

GEOLOGIC GROUP

applied simple map combination in their study of part of the Rio Minho valley in the parish of Clarendon. Multivariate statistical techniques have been employed by Unit for Disaster Studies (1999) for modelling the Kingston district.

This assessment of Portland represents medium- scale (1:50,000) analysis of landslide susceptibility

utilizing a bivariate statistical approach, specifically simple map combination. It is based on the fundamental principle that “the past and present are keys to the future” (Carrara et al., 1995). This simply means that the conditions that generated landslides in the past and even those occurring today are likely to generate slope failures in the

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