The geologic sequence of Jeju Island is generally divided into Basement,
Paleo-Jeju Volcanic rocks, Seogwipo Formation, Pyoseolli Basalt Group,
Tamna Formation, Basalt dyke complex, Hallasan Basalt Group,
Baengnokdam Trachyte Group, Volcanic debris-avalanche deposit, Sinyangni
Formation and Paleobeach-embayment sand deposit in ascending order. The
Tamna Formation is fluvial deposits consisting conglomerate, sandstone and
mudstone and widely distributed in Jeiu Island. The origin of constituent
minerals was investigated by analyzing occurrence and mineral composition
of mudstone layer that is evenly distributed in the Tamna Formation. The
geological survey, sample collection, X-ray diffraction analysis, petrographic
microscopic observation and SEM analysis were conducted.
Large quartz crystals having 1~3cm in diameter were observed in the
Tamna formation. These quartz had very similar characteristics with those
observed in quartzite and quartz vein. XRD analysis indicates that the
mudstone in the Tamna formation was composed of quartz, plagioclase,
K-feldspar, micas, magnetite and hematite and clay minerals such as illite,
smectite, kaolinite, chlorite, 10Å-halloysite. The constituent minerals can
be largely classified into the six groups of mineral assemblage. Clay
minerals can also be grouped as the eight mineral assemblages. The
characteristic occurrence of these mineral assemblages was
comprehensively analyzed.
In the central area, large quartz was distributed and their content was
generally high. The contents of kaolinite, vermiculite and chlorite were
slightly higher in the west area showing high quartz content. The contents
of kaolinite and vermiculite were higher in the west than in east areas.
The results suggest that the source materials of the Tamna Formation
are related to the Precambrian Okcheon Metamorphic Belt. In other words,
the formation of the Tamna Formation can''t be explained by the volcanism
in Jeju Island. The Tamna Formation is considered to be formed in totally
different geological and geographical environments from now. Main source
of sediments contributing to the Tamna Formation is seemed to be rocks
at the Okcheon Metamorphic Belt. The Tamna Formation is considered to
be formed by these sediments with the addition of materials derived from
pre-existing basement rocks and volcanic rocks. Various minerals were
formed by depending on the content of source materials and the degree of
post-processes such as weathering and alteration.