Stratigraphic Correlation and Isopach Maps of the Western Taiwan Basin

Subsurface geologic reports, paleontological data and stratigraphic cor­ relation studies of the 88 onshore and offshore exploration wells of the west­ ern Tai,van Basin completed by earlier researchers "''ere incorporated. Six regional stratigraphic profiles and the isopach maps of each f onnation above the Wuchihshan Formation were constructed in t. his study. During the Oli­ gocene, the depositional center was located to the east. of the Hsinchu­ Taoyuan-Taipei belt, and the thickness of the Oligocene strata increased from the west to the east. During the Miocene, the depositional center was situated to the east of the Miaoli-Hsinchu belt and the sediments were mainly from the northwest of Taiwan although part of the sediments was also de­ rived from the Peikang basement high in the early l\tliocene. The configura­ tion of the sedimentary basin and the source of sediment during the deposi­ tion of the Kueichulin and Chinshui Formiations "·ere not too different from those in the Miocene. Ho\vever, during the late Pliocene, the sedi­ ments were from the east and with quite thick elastic sequence deposited in \\'estern Tai,van. The depositional center of the Cholan Formation is lo­ cated to the east of the Taichung-Miaoli-Hsinchu belt. In the Toukoshan stage of the Pleistocene the sediments \Vere also form the east and very thick elastic strata were deposited in western Taiwan.


INTRODUCTION
The purpose of this study is to describe the stratigraphy· ot· the late Cenozoic elastic se quence in western Taiwan and elucidate the soL1rces ()f the sediment, the depositional cente.r at· the basin as well as the ccJastline during the highest sea level of· this reg . ion during that time.
Tertiary · sedimentary basins in western Tai 'Ai7an have been the n1ain targe .
ts for petroleum exploration, with hundreds of deep exploration v\,.el1s being drilled onshore and offshore in those areas. Abundant subsu1·t' ace geological data are no\v a\1ailable, and some t1f the explora tion wells have reache. d the Pre-Miocene "economic basement". Most of the "basement" rocks encountered are comprised of · sedimentar)' rocks, ranging in age. f' rom the Paleogene. to Meso-• ZOIC.

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TA O, Vol. 7, N(J . 3, Septembe1 .. 1996 Previous studies involving the stratigraphic analyses of the Western Taiwan Basin have 111�1i111 �/ been t'ocused \\1ithin onshore or offshore. Tw·o examples include, Geological and ge<lChL�111ical studies t�o r the exploration of hydrocarbon in the Peikang area, Taiwan ti (Hu et ct/._ 1981) and '' Stratigraphic correl ation of the Tertiary formations, the Chia-Nan Plain in the southern Tai\V·an area " (Hsu, 1979), though there are others. The only study encompassing every basin in \\1estern Tai\van is 1'Tertiary · sedimentary basins in Taiwan'' (Ho, 1971), the result of' which suggests that the Neoge . ne fo rmations in central-so·uthern Taiwan thin out from the east to the west in a V\1edge-shape, while the formations also pinch out toward and onlap the lithologic units in the west. Bosum et al. (1970) suggested that the Peikang basement high located in the . central part of western Taiwan extends for only a short distance to the east and then te1�minates or is truncated. The whole western Coastal Plain has been affected by the Kuanyin basement high in the north and the Peikang basement high in the central south, and · the formations between the highs show slight undulation. In his study on the e\1olution of the tertiary basins, Sun ( 1982) proposed nomenclature for various structural units. He suggested that the Paleogene basins had been initiated by a series of horst and graben structures and included the Tung)'intao Basin, South Penchiayu Basin, Nanjehtao Basin, Taihsi Basin, Taichung Basin, Tungyintao Ridge, Kuanyin base.ment high, Penghu basement high and the Peikang Ridge. He also postulated a possible extension of the Peikang ridge to the north. The structural mechanisms for the formation of the Neogene basins were very different from those of the Paleogene basins, according to Sun (1982), who also noted that the Penghu platform di\1ided the Neogene sedi · mental)r basins into south�rn and northern basins, which are named the Tainan Basin and the Taihsi-Taichung Basin, respectively.
Three distinct cycles of� sedimentation are recognized in the Oligocene and Mioc.ene. elas tic se.quences of western Taiwan. The first cycle consists of the Wuchihshan and Mushan Formations, the. Kungkuan Tuff and the Taliao Formation; the second of the Shihti, Tsouho, and Nankang Formations; and the third of the Nanchuang and Kueichulin Formations. The Wuchihshan, Mushan, Shihti and Nanchuang Formations, all of continental-paralic-neritic deposits, are mainly composed of well winnowed protoquartzite, subgraywacke and shale, inte1�calated with se\'eral thin coal seams. These formations are mainly of stable shelf sedi ments. The Taliao, Tsouho, Nankang and Kueichulin Formations, all of a neritic-shallow bathyal en'v·ironment, consist chiefl)' of less \vinnowed subgraywacke and lithic graywacke units interbedded \\1ith siltstone and shale and )1ielde. d marine fossils (Chou, 1976(Chou, , 1980. Four major structural mo\,.ements have bee. n identified in the Tertiary Basins (Sun, 1982), and they are: the Crustal mov·ement at the end of the Cretaceous which uplifted the area, and w·as then subjected to folding and erosion. The subsequent advance of the sea and volcanic activities in the Paleogene resulted in the deposition of the Paleogene sediments. A second uplift caused the Paleogene deposits in the high areas to be eroded, and the subsidence in other areas, such as the Penghu area, initiated the deposition of the Eocene sediments. Growth fau lts, \\Ihich are in the trend of the. Taiwan Strait, developed and extended from the southeast toward the northwest during this period . The third crustal acti\rit)' developed in the early Oligocene, and the 1·eju,renated uplift resulted in the e. rosion of most of the upper Eocene and part of the middle Eocene sediments .. The period from the middle of the Miocene. to the Pliocene in \\1est ern Tai\\1an experienced the fourth structural movement whic.h \\'as characterized by· a vertical 335 uplift clnd n()rmal faulting. In contrary, intensive t' olding and metamorphism took place . in the eastern part of Taiwan island. Owing to this signit' icant dift' e14ence in geological history, there is a striking change ()f' stratigraphy acr<)SS the bound,11·)1 ()f the \\1estern and the eastern Taiwan Basins.

Stratigraphic Correlations
The stratigraphic correlation Profile A (Figure 2 . ) was constructed based on the subsurface ge< .

Isopach Maps and Discussion
The locations of the wells used in this study are shown in Figure 8. The correlation of the stratigraphic divisions in Taiwan (after L-in, 1982;Huang et al., · 1979;Chi, 1980) is shown in Table 1.

Pre-Oligocene
Most of the exploration Vt1ells ""·ere not deep enough to encounter Paleocene formations and, in fact, the paleontological evidence indicates that the Paleocene f o. rmations were only present locally in the northern offshore of Taiwan and the Wangkung-Taihsi-YTH region. The Eocene formations are also present in the abo\re mentioned areas as well as in the Penghu Basin. Data is not sufficient to enable the constrution of the regional Oligocene and Eocene isopac. h maps.

Wuchihshan Formation
The Oligocene formations are generally absent in the offshore, the west of YKL-1, YTP-1 and YML.-1 and north of Block E areas; they are also absent in the YIT-1 and Y A-1 regions. The Pe.ikang basement high was gradually formed during this stage, and it extended to the Paochung-Tienchung area in the north and to Chiali in the south. This basement high sepa rated the Western Taiwan Basin into two parts: the Northern and Southern Basins. The depocenter of the Wuchihshan Formation (Figure 9) in the Northern Basin was in the Hsinchu Taoyuen-Eastern Taipei belt, and the thickness of the sediment increases from the northwest to the southeast and fr om the south (north of the Peikang baseme. nt high) to the north. The maximum thickness of the \\tl'uchihshan Formation in the basin reaches to more than I OOOm in the depositional center, and it was derived mainly fr om the northwest of Taiwan but also partly fr om the Peikang baseme.nt high.

l\1ushan Formation
The Mushan Formation (Figure l 0) is absent in the offshore areas, west of YTP-1, YML l, YCI-5 and YIT-1, and in the onshore PC-I and TC-1 wells. Its thickness is generally unde. r I OOm in the Peikang basement high but ma)' reach to nearly 800m in the depocenter of the Northern Basin to the east of Miaoli. The sediment was derived from the northwest and south west of Taiwan.

Taliao Formation
The Taliao Formation (Figure 11) is virtually absent from the offshore areas, west of the YC 1-1 .4.5 and YIT-1 wells. The thickness of this formation is about 1 OOm in the Peikang b<.1�cn1�11t high but ma)1 reach to about 600m in the depocenter of the Northern Basin in the east ot' the IV1iaoli-Hsinchu belt. The sediments were mainly from the north"''est of Taiwan, and the Peikang basement high became a prominent barrier seperating the Northern and Southern Basins.

Shihti Formation
The thickness of' the Shihti Formation ( Figure 12) is about 200 m in the YSS and YTH ai·eas, while that in the Peikang basement high is under I OOm. This phenomenon indicates that the Peikang basement high (extending from Paochung-Tienchung to Chiali) \\1as still a barrier bet\:veen the Northern and Southern Basins. About 700m thick, the Shihti Formation (684 m at CS-69) was deposited in the depocente. r located in the east of· th. e Miaoli-Hsinchu belt, with its SC)t1 rce.s in the west and north \\1est of Tai \\'an. The average thickness of the Peiliao Formation ( Figure 13) in the KY-1, YSS and HL areas is 200 to 300 m but increases toward the east and may reach up to 800-900 m in thickness in the depocenter in the east of the Miaoli-Hsinchu-Taoyuan belt. The sedime . nt was deri'\'ed from the west and northwest of Taiwan, and the effect barrier of the Peikang basement high has diminished gradually . The isopach map of the Talu Formation ( Figure 14) reveals that the thickness of the for mation maximizes in the YSS and TCS areas \Vhere . the thickness ranges t� rom 400 to 500m and decreases toward the east, the south and the north. Based on the distribution of the Talu Formation, it may be interpreted that the formation represents a large fan delta.
. 352 TAO, Vol. 7, No. 3, September 1996 3.2.8 Ku�n)1inshan Sandstone and Nanchuang Formation As it is diffic ult to di fferentiate these two formations (Figure 15), they are discussed here as one unit. The thickness of these two formations in the Taichung-Pachanghsi area is below 250 m, but this gradually increases in the direction the Northern and Southern Basins. The thickness in the YSS to TCS areas is bet�1een 500 m and 700 m and inc.reases toward the depocenter located to the east of the Hsinchu area �,he. re the thickness may reach up to nearly 1500 m. The Kuanyinshan Sandstone and Nanchuang Formation were derived from the west and northwest of Taiwan.  The isopach map of the Kueichulin Fo1·mation ( Figure 16) shows that the sediment thick ness in the KY-1, YSS to TCS areas is 200-400 m. It then increases toward the northeast and ma)1 reach more than 800 m in the depocenter in the east of the Hsinchu-Taoyuan area. The sediments were derived from the \vest and northwest of Taiwan. Mountain building movements were. acti\i1e during the Cholan stage, on uplifting the Cen tral Range and subjec.ting it to erosion. As a result, a tremendous amount of se.diment (thicker than 1500 m) derived from the uplifted Central Range was deposited in \v estern Taiwan (Fig  ure J 8). The thickness of the sediment gradually decreases to the \\1est, and in the present coastal area its thickness is about 750 m. Further to the west in the area west of the YSS and THS-6 wells, its thickness decreases abruptly' to under 25 0 m. As stated, the sediment was obviously from the east, and the depoc.enter was located in the Miaoli-Taichung belt. Mountain building on the island of Tai\van continued in the Toukoshan stage and has resulted in the deposition of huge amounts of sediment in western Taiwan (over 3000 m thick in the east of the Taichung area). Sediment thickness, however, decreases abruptly to the west, and in the YSS and YTH-3 and 4 areas, the formation is less than 500 mi n thickness ( Figure  19). The Toukoshan Formation in the Taiwan Strait was deposited in a stable environment and had little impact ot" the thrust fau lting initiated by the mountrun building process. Again, the sediment was from the east, and the thickest place shifted to the Taichung-Nantou areas . The subsurface geologic reports, paleontological data, and stratigraphic correlation stud ies of 42 onshore and offshore exploration wells completed by earlier researchers were incor porated in this study. Six regional stratigraphic p1·ofiles were thereb)1 constructed. Profiles A and B trend north-south, whereas Profiles C, D, E and F are all east-west trending. The comple tion of these six regional stratigraphic profiles provides a better understanding of the distribu tion and thickness variation of every form ation in Taiwan.
In addition to the geological data of the 42 wells studied in the construction of the six stratigraphic profiles, those of the other 18 onshore and 28 offshore wells were also studied in the construction of the isopach maps of each formation above the Wuchihshan Formation. This means the geological data of 88 wells were utilized in this study. With the aid of the isopach maps, it is possible to postulate the depositional center of the basin during each stage.
During the Oligocene, the depositional center was located to the. east of the Hsinchu Taoyuan-Taipei belt, and the thickness of the Oligocene formation increases from the west to the east. The sediment th . ickness inc.reases from the south, i.e. the area to the north of the Peikang basement high, to the north and may reach to more than 1. 000 mi n the depositinal center of the basin. The sediments were derived mainly fr om the northwest of Taiwan and partly fr om the. Peikang basement high.
During the Miocene, the Peikang basement high acted as a sediment barrier and divided the Taiwan region into the Northern and Southern Basins. A tremendous amounts of sediment were deposited in each of these basins. For example, the total thickness of the Miocene forma tions may exceed 4000 m, w· hile those in the Peikang basement high are less than 1000 mi n thickness. The depositional center for the Miocene was situated to the east of the Miaoli Hsinchu be.It, and the source region of the Mioce.ne sediments was situated northwest of the Island of Taiwan in tl1e present Taiwan Strait. Those sediments were transported southward and southeast�1ard down a paleoslope of gentle inclination in directions approximately norrr1al to the isopachs of the Miocene elastic sequence (Chou, 1980). The configuration of the sedimentary basin and the source of the sediment of the Kueichulin and Chinshui Formations were not too different from those in the. Miocene time in the North ern Basin. HO\\lever, due to mountain building on Taiwan du14ing the. late Pliocene, the sedi ment originated in the east and a huge thickness of sedimentary rock was recorded in western Taiwan. The depositional center of the Chalan Formation was located to the east of the Taichung Miaoli-Hsinchu belt.
The mountain building movement continued in the Toukoshan stage of the Pleistocene, sediments more than 3000m thick were deposited in western Taiwan. The sediment was mainly from the east, forc ing the depositional center shifted to the Taichung-Nantou belt.

Chinshui Formation
The thickness of the Chinshui Formation ( Figure 17) in the KY-1, YSS, TCS to PKS-2 and TC-I areas is between 100 to 200 m and increases toward the northeast where the maxi mum thickness may be in excess of 300 m. The depocenter is situated in the east of Hsinchu Taoyuan. The sediments were from the west, northwest and southwest of Taiwan. The Chinshui Fonnation is generally absent in the offs hore YML, YTH and YCI-3 areas and onshore PCC-1 areas.