The 1974 geologic map of Taiwan, presented on a scale of 1:250,000, is divided into four sheets. It
summarizes the results of geologic mapping in Taiwan since the 1:300,000 map was published in 1953. The
rocks of Taiwan are divided into 37 separate units with additional symbols for limestone layers, coal beds,
and tuff lenses. As compared with the older maps, a more detailed subdivision of the Neogene sediments into
a larger number of rock units was made to illustrate the different geologic environments in the three
geologic and tectonic provinces of Taiwan. Rocks of the same age in these three provinces are of different
lithologic characters and structural features, and therefore are of different significance in the geologic
development of Taiwan. They are represented by different units and color patterns in the legend so that a
better understanding of the geology of Taiwan can be attained. This should be of value to those who use the
geologic map for mineral exploration, earthquake or engineering studies, and other useful purposes. The
subdivision of the rock units in the legend is based mainly on stratigraphic-lithologic sequences and not
merely on the age of the rocks as most of the older maps did. These units are, therefore, chiefly
cartographic units.
The legend is divided into two major divisions: sedimentary and metamorphic rocks, and igneous rocks.
There are 29 units in the former division and eight units in the latter division. Except the mafic to
ultramafic igneous rocks in eastern Taiwan and a small exposure of quartz porphyry in the Taiwan Strait, the
main igneous rocks are volcanic rocks. Both central eruptions and fissure eruptions are known. Small and
scattered tuff and lava lenses are placed in the host sedimentary formations depending on field association.
Most stratigraphic names in the legend are derived from those already defined in published literature
and maps, and are well-known stratigraphic terms. Three of the stratigraphic names, however, are new and
have not been previously described. It is necessary to validate these new formational names that make their
first appearance in the legend of this map. Definition of the new formations or groups is given later in
this explanatory text together with brief descriptions of the older names.
Although a good deal is now known of the broad aspects of the geology of Taiwan and precise data are
available for many areas, there remains considerable uncertainty in some areas, especially in the more
inaccessible parts of the Central Range where both the stratigraphy and the structure are little known. In a
large part of the Central Range, representation of geologic data on the map is much generalized because of
lack of detailed mapping or because the geology is imprecisely known. Inevitably the geologic data of some
areas are dealt with in more detail than those of others. To guide the reader, a reliability diagram is
added to the map. A mapping scale diagram is printed at the lower-right corner (No. 3 sheet) of this map,
and illustrates the different scales of the original geologic maps used for the compilation of the present
map. These scales range from 1:10,000 to 1:250,000. The reader may thus have some idea of the reliability of
the data in different parts of the map.
The information and data on the map was derived from both published literature and unpublished
articles. Some generalization was necessary in a number of areas in order to reconcile different
interpretations of the geology by different authors. It was the responsibility of the chief compiler and the
members of the editorial committee to screen the data from different sources and, where there are
differences, to decide which should be used for the map compilation. In a compilation such as this, adoption
of a particular point of view cannot be avoided. It is inevitable that some geologists will not fully agree
with the views presented in this publication. However, this map represents the fruits of the work of a large
number of field geologists in Taiwan as well as the general viewpoint of the editorial committee.
In the first edition (1974) of the 1:500,000 geologic map of Taiwan, a total of 26 stratigraphic
units are presented in the legend. In this revised edition (1986) of the map, 30 stratigraphic units are
given in the legend. The increase in number of units is necessary in view of the increasing and advanced
stratigraphic knowledge of Taiwan. Some newly proposed and revised formational names that make their first
appearance in the legend of this revised map need to be validated and well-established. They are defined and
discussed more fully in this revised text.
|
