Abstract:
The sediments of the Gal-Oya River basin were investigated in order to identify the
mineral compositions and depositional environments, as the river crosses the Highland
Vijayan litho-tectonic (HC-VC) boundary of Sri Lanka. A total of 14 surface sediment
samples were collected representing both HC and VC. Grain size analysis was performed
on all samples and consequently, the grain size parameters were calculated following the
standard procedures. SEM analysis was performed on selected quartz grains to interpret
the micro-textures to relate to the depositional environment. Heavy liquid mineral
separation and magnetic separation were used in mineral separations and petrographic
analysis was performed in identification. The results revealed an assemblage of heavy
minerals dominated by garnet, tourmaline, rutile, monazite, spinel, and zircon, indicative
of high-temperature-pressure metamorphic sources. The high concentrations of heavy
minerals in the samples collected beyond the HC-VC boundary suggest its contribution
to the enrichment of the heavy mineral contents in the basin. The identified garnet
variety- hessonite may be an indicator for the possible gem accumulations, which
requires further investigation. In addition, the presence of graphite nodules might be due
to the carbon influences in the metasedimentary processes of the source rocks or
hydrothermal influences during metamorphism. Most probably, the Senanayake reservoir
may act as a sediment trap for such mineral contents. The grain size distribution exhibits
the poorly sorted (1.3–1.7) and coarse skewed (-0.8 to -0.2) nature which suggests high
energy and mixed energy environments for sediment deposition in the region. The
mechanical surface micro-textures of the quartz grains such as medium to large
conchoidal fractures, cracks, graded arcs, and arcuate steps also reveal the high energy
transportation and depositional conditions in the basin. The gradient of the mountain
ranges in the southeastern region of the central highlands and the rectangular drainage
pattern may contribute to the mixed energy environments observed in the sedimentation
process. Further studies of the sediments in the Senanayake reservoir are recommended
to understand the trapping nature of valuable heavy minerals, which have already been
identified in the eastern coastal areas and deposited before the construction of the
reservoir.
