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Remote sensing for resources development and environmental management
Damen, M. C. J.

Figure 1. Location map
1: emerged area, 2: oceanic crust, 3: active
subduction, 4: inactive subduction
To detect structural elements, morphological
features were used such as wide thalwegs, strong
deviations in drainage patterns, rectilinear scarps,
and watershed deviations (Fig. 2a). In Fig. 2c
structural elements are shown as faults only if they
separate different geological units, if offsets on
crests and boundaries can be recognized, or if they
correspond to known tectonized zones of faults.
Mapped lineaments are rendered on the images by
rectilinear thalwegs and fractures. Tonal anomalies
are less reliable and were not used. Discrete
anomalies of great extension or strong influence on
the drainage pattern might indicate buried faults
associated with lineaments. Sometimes, directions
of motion can be deduced from associated folds,
scarps, or deviated crests. However, these
observations must be confirmed by the stress pattern
obtained from microtectonic studies in the field.
Due to the complexity of the region the evaluation
of folding and imbrication requires more careful
observations. Hard rocks only once tectonized
forming herringbones and crests are good indicators
of local dip. Small crests are generally due to a
harder layer interbedded in a soft sequence. Its
geometric aspect might indicate folding and a kind
of motion when associated with a fault (Rizal
Fault). Regions with several tectonic phases are
more difficult to interpret. It seems that the
first folding can be recognized by bedding, small
ridges, and fine elements, whereas the second phase
is often evidenced by the orientation of main
Palawan island is interpreted as a tectonic ridge
related to the collision between a volcanic arc and
the Chinese continental margin which was displaced
southward during the early Oligocene to early middle
Miocene opening of the South China Sea.
Until Eocene times, the sedimentation area of the
present-day island of Palawan started on the Chinese
Continental Margin. Southward in the adjacent
oceanic basin, a first overthrusting of the
ophiolites was due to Pacific plate movements. From
late Eocene to middle Oligocene, the rifting of the
South China Sea is marked (in Reed Bank and North
Palawan continental area) by tilted blocks bounded
by normal faults trending N60E (seismic data).
From Middle Oligocene to Middle Miocene, the
rifting of the South Sea caused part of the Chinese
Continental margin to drift southward.
Concommitently, calcareous deposition occured on the
tilted blocks previously described. Nowadays, these
limestones are eroded in the north, overthrust in
Middle Palawan and underthrust in the south.
During the Middle Miocene, a north-south collision
with a volcanic arc (tuffaceous marls and Aba-Aba
andesites) occured. This major tectonic event
developed a prism which imbricates Cretaceous and
Cenozoic sediments deposited along the previous
margin. Ultra-basic elements, coming from the
Cagayan Arc are also integrated. Microtectonic
measurements give a N120E strain direction. Studies
of the detritai series in the wells offshore from
North Palawan indicate deposition evolution from
north to south, accompanied by erosion decreasing
from NE to SW. This explains the major
organizational differences between both sides of the
Ulugan Fault.
In the late Miocene, the second tectonic event
( N67E) was induced by movements occurring around
Mindoro in the Philippines Archipelago. It reworked
the structures previously elaborated. Left lateral
shear along N-S faults has been recognized on
offshore seismic profiles and proved by field work.
This third tectonic event seems to be of Upper
Pliocene-Pleistocene age.
3.1 Lithofacies
For the entire island, the Quaternary deposits are
easily distinguished by their flat smooth texture.
The gray level on the picture is linked to the
moisture of the alluvium.
In the southern part of Palawan, the oldest
sediments are dated from the Paleogene. Thick
turbiditic series of late Paleocene to early Miocene
age are strongly tectonized (folded) and imbricated
with slices of ophiolites, pillow lavas, cherts and
volcanic breccias of oceanic crust origin. They are
unconformably overlain by late Miocene-early
Pliocene series.
In the Quezon area, several units, dated late
Miocene-early Pliocene (C. Muller, 1984), are
differentiated. On top of the series, a brillant
surface bounded by scarps corresponds to thick
limestones with marls interbedded (Nc). The
topographically underneath surface is rough
(numerous small hills), with a very light grey tone:
it is related to the reefal limestones often as
buildup (Nm). A small adjacent plateau is assumed
to be the westward extension of the one previously
described (Nc), but boundaries are not very
understandable and should require acurate field
data. High limestone ridges in Bulanjao Range are
attributed to the same formation. They were
uplifted and slightly tilted before Pleistocene.
The Paleogene-early Miocene formations are
considered as a whole since they consist in a very
thick, quite homogeneous turbiditic sequence. On
the images, they are hardly distinguished from
alluvial deposits, but differences in photofacies
seem to be linked to the proportions of siltstones
and sandstones. In the southernmost part of the
island, a detrital unit (Ml) is characterized on the