Journal of Asian Earth Sciences 69 (2013) 3–16

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Journal of Asian Earth Sciences

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The first record of Eocene tuff in a Paleogene rift basin near Nantou,Western Foothills, central Taiwan

1367-9120/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2013.02.022

⇑ Corresponding author at: Key Laboratory of Marginal Sea Geology, GuangzhouInstitute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China.

E-mail address: huangcy@mail.ncku.edu.tw (C.-Y. Huang).

Chi-Yue Huang a,b,⇑, Wen-Ron Chi a, Yi Yan b, Kenn-Ming Yang a, Ping-Mei Liew c, Min-Shyan Wu d,Jong-Chang Wu d, Chuanheng Zhang e

a Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Chinab Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwanc Department of Geosciences, National Taiwan University, Taipei, Taiwand Exploration and Development Research Institute, Chinese Petroleum Corporation, Miaoli, Taiwane School of Earth Sciences and Resources, China University of Geosciences, Beijing, China

a r t i c l e i n f o

Article history:Available online 21 March 2013

Keywords:TaiwanWestern FoothillsPaleogene rift basinSyn-rift sequenceBreak-up unconformity

a b s t r a c t

Microfossils and a U–Pb age dating on zircon grains in the tuff beds exposed in the axial part of the Tsuk-eng anticline along the Pinglin River in the Western Foothills near Nantou, central Taiwan, show anoccurrence of the Eocene volcanics unconformably beneath the uppermost part of the Latest OligoceneWuchihshan Formation. This is the first discovery of the Eocene tuff exposed in the Western Foothills.

The proposed Miocene ‘‘Tsukeng Formation’’ and ‘‘Takeng Formation’’ of Ho et al. (1956) named forsequences exposed in the Nantou area, Western Foothills, have to be abandoned and the standard Oligo-cene–Miocene lithostratigraphy used commonly in the Western Foothills of northern Taiwan is properlyapplicable in central Taiwan. The thick pink–brown–green colored volcanics unconformably beneath theuppermost Wuchihshan Formation is named for the first time as the Pinglin Tuff which contains LateMiddle Eocene calcareous nannofossils (Zone NP16) consistent with a U–Pb age dating (38.8 ± 1 Ma)on zircon grains in the tuff. The Pinglin Tuff is overlying the Middle Eocene Chungliao Formation whichcontains indigenous larger foraminifera Discocyclina dispansa ex. interc. sella-dispansa and calcareousnannofossils of Zones NP14–15. The Middle Eocene Pinglin Tuff and Chungliao Formation representthe Paleogene syn-rift sequence unconformably overlain by the Latest Oligocene–Miocene post-riftsequence. This is the first document with conclusive paleontological data and age dating showing anoccurrence of Paleogene marine rift basin exposed in the Western Foothills. This study also confirms sim-ilar Tertiary basin architecture between the Taiwan Strait–Pearl River Mouth Basin in the NE South ChinaSea and the Western Foothills onland central Taiwan.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Over the last seven decades, there has been a general consensusthat the Western Foothills in northern–central Taiwan is composedof Late Oligocene to Pleistocene shallow marine to fluvial stratacontinuously deposited on the stable passive Asian continentalmargin without significant stratigraphic break (Ichikawa, 1931;Chang, 1975; Chou, 1980; Huang, 1980a; Huang and Cheng,1983). In contrast, Eocene and older strata are exposed exclusivelyin the Hsüehshan Range and the Central Range east of the WesternFoothills (Fig. 1). Reconstruction of the fold-and-thrust strata inwestern Taiwan further suggests that there is a decollement withinthe Late Oligocene Wuchihshan Formation (Suppe, 1980). During

the plate convergence between the Asian continental margin andthe Philippine Sea Plate since the Latest Miocene, the Late Oligo-cene–Pleistocene strata above the decollement have been progres-sively folded and thrust westward to form the present-dayWestern Foothills (Huang et al., 2006). However, the strata beneaththe Wuchihshan Formation underlying the decollement are neverexposed and, therefore, their stratigraphic character has been hith-erto unknown.

In western Taiwan, tuff beds and larger foraminifera are com-monly used as key horizons and diagnostic fossils in field mappingof shallow marine sequences of the Western Foothills (Fig. 1; Yabeand Hanzawa, 1930). There are two tuff beds in the Miocene se-quences of the Western Foothills in northern Taiwan: the EarlyMiocene shallow-marine Kungkuan Tuff and the Late Miocenecoal-bearing Nanchuang Formation. Larger foraminifera of Lepido-cyclina, Miogypsina and Miogypsinoides commonly occur in theshallow-marine tuffaceous limestone of the Early Miocene

Fig. 1. Geological map near the Tuskeng Village, Nantou, central Taiwan (Ho et al., 1956). CR: Central Range; DF: deformation front; HR: Hsüehshan Range; K: Kuohsing; N:Nantou; PBH: Peikang Basement High; T: Taipei, WF: Western Foothills; TSK-1: Borehole near the Tsukeng Village; WG-1, PC-1: Boreholes above Peikang Basement High.

4 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

Kungkuan Tuff, but they are never reported in the Late Miocenecoal-bearing Nanchuang Formation. Consequently, when Ho et al.(1956) and Ho (1961) found the thick tuff sequence associatedwith larger foraminifera Discocyclina exposed in the axial part ofthe Tsukeng anticline (11 km east of the Chelungpu fault; Fig. 1)in the Western Foothills, central Taiwan, they regarded these tuffbeds as the Early Miocene strata equivalent to the Kungkuan Tuffin northern Taiwan (Fig. 2). Accordingly, the Eocene larger foram-inifera Discocyclina exposed in the axial part of the Tsukeng anti-cline were interpreted as reworked fossils in the young Miocenestrata. On the other hand, key coal beds in the Miocene sequencesof central Taiwan are so thin or even absent that Ho et al. (1956)could not apply the standard Miocene lithostratigraphy (marinefossil-bearing beds alternated with three coal-bearing units) estab-lished in northern Taiwan to map the sedimentary sequences incentral Taiwan. Therefore, they established a local lithostratigraph-ic scheme in which the Miocene ‘‘Tsukeng Formation’’ was pro-posed for the strata with thick tuff and the larger foraminiferaDiscocyclina exposed in the axial part of the Tsukeng anticline

(Fig. 1). The Miocene ‘‘Tsukeng Formation’’ was considered to beconformable between the overlying Miocene ‘‘Takeng Formation’’and the underlying Oligocene ‘‘Shuichangliu Formation’’ (Figs. 1and 2). However, according to the geological map in Ho et al.(1956) the Oligocene Shuichangliu Formation appears only in theHsüehshan Range (5 km east of the Tsukeng anticline), while theMiocene ‘‘Tsukeng Formation’’ is exposed exclusively in the Tsuk-eng area of the Western Foothills (Fig. 1). Therefore, the strati-graphic relationship between the ‘‘Tsukeng Formation’’ and theunderlying ‘‘Shuichangliu Formation’’ cannot be observed directlyeither in the Western Foothills or in the Hsüehshan Range.

Extensive seismic surveys over the Coastal Plain above the Peik-ang Basement High (PBH in Fig. 1) in central Taiwan and its adja-cent offshore were conducted by the CPC Corporation, Taiwan,during 1960–1980s for hydrocarbon explorations. Geophysical sur-veys and drillings confirmed the occurrence of Early Tertiary riftbasins in the Taiwan Strait and also beneath the Coastal Plain inthe Peikang Basement High. In these Paleogene rift basins, thesyn-rift sequences with rifting volcanics are unconformably

Fig. 2. Correlation of stratigraphic units used by Ho et al. (1956) and Chiu (1975)(left panel) and the present study (right panel) in the Western Foothills near Nantou, centralTaiwan. Correlation between the three different lithostratigraphic units of the so-called ‘‘Tsukeng Formation’’ (Ho et al., 1956) and their equivalent new revised stratigraphyare shown in different gray shading.

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 5

between the overlying Miocene post-rift strata and the underly-ing Mesozoic pre-rift basement (Huang and Chi, 1979; Ting,1979; Sun, 1982; Huang, 1982; Hsiao et al., 1991; Yang et al.,1998). Meanwhile, the geology of the Tsukeng anticline in theNantou area was also studied by petroleum geologists to explorethe stratigraphic significance of the ‘‘Tsukeng Formation’’ for itscharacteristic volcanics-bearing sequence like what had beendrilled (the Shuangchi Red Beds) in the Early Tertiary rift basinin the Peikang Basement High (Chiu, 1972, 1975; Chang andWang, 1986). Integrating field mapping and borehole data (WellTSK-1 in Fig. 1), they agreed with Ho’s consideration (i.e., re-worked fossil model) for the strata exposed in the Tsukeng anti-cline and also believed that the stratigraphic sequences in theNantou area were conformable successions without significantstratigraphic break, although there may have a minor diastem

Fig. 3. Sampling locations with a revised stratigraphy in the axial part of the Tsukenstratigraphic distributions of consecutive lineages of benthic foraminifera Gaudryina specomparison. The dash lines immediately east of break-up unconformity represents a tra

above the tuff bed (Fig. 2; Chiu, 1972, 1975). However, their con-clusions about the stratigraphy of the ‘‘Tsukeng Formation’’ arecontroversial. For example, Chiu (1972) first correlated the ‘‘Tsuk-eng Formation’’ in central Taiwan to the Early Miocene Shihti For-mation in northern Taiwan, instead of the Kungkuan Tuff as waspreviously suggested by Ho et al (1956). The Shihti Formation is acoastal to lacustrine coal-bearing unit above the Early Mioceneshallow-marine Taliao Formation/Kungkuan Tuff (Fig. 2). Later,the ‘‘Tsukeng Formation’’ was further interpreted to be equivalentto the upper part of the Oligocene–Miocene (OM) ‘‘ShuichangliuFormation’’ conformably underlying the Shihti Formation (Fig. 2;Chiu, 1975). Such a controversy about the stratigraphy of the‘‘Tsukeng Formation’’ has resulted from the lack of detailedmicrofossil study and precise age dating on the tuff unit of the‘‘Tsukeng Formation’’.

g anticline along the Pinglin River section, Nantou, central Taiwan, showing alsocies. The stratigraphy proposed by Ho et al. (1956) is also shown on top of map fornspressive fault cutting into the lowest part of the Wuchihshan Formation.

Fig. 4. Field outcrops at various horizons along the Pinglin River section. (A) brown–pinkish shallow-marine sandstones of the Earliest Miocene Mushan Formation (shale/sandstone ratio < 1); (B) Conformable boundary between the Mushan Formation and the upper part of Wuchihshan Formation (shale/sandstone ratio > 5); (C–F) Break-upunconformity between the Late Oligocene Wuchihshan Formation (uppermost part) and the underlying Late Middle Eocene Pinglin Tuff along road cuts (C and D) and PinglinRiver (E and F).

6 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

In contrast to the re-worked fossil and conformable stratigra-phy model, Hashimoto and Kurihara (1974), Hashimoto et al.(1979) and Huang et al. (1991) documented that there existed atleast an unconformity within the ‘‘Tsukeng Formation’’. However,except field observation, no independent line of conclusive evi-dence was documented.

To understand the geological significance of the rock succes-sions exposed in the Tsukeng anticline, we have conducted a de-tailed biostratigraphic study of foraminifera incorporated withcalcareous nannofossils and age dating on the ‘‘Tsukeng Forma-tion’’ since 2003. We establish the stratigraphic scheme for se-quences exposed in the Nantou area of the Western Foothills,central Taiwan, using traditional stratigraphic names based on de-tailed lithostratigraphic survey, biostratigraphic study on bothforaminifera and calcareous nannoplanktons and a U–Pb datingon the tuff unit in the ‘‘Tsukeng Formation’’. We then explore thegeological significance of the results from an island-wide consider-ation of regional basin development. This paper focuses on theforaminiferal and calcareous nannofossil stratigraphy of the upperand middle units of the ‘‘Tsukeng Formation’’ exposed along the

Pinglin River section, while in a separated paper (Huang et al.,2013), geological significance of the Eocene larger foraminiferaDiscocyclina in the lower unit of the ‘‘Tsukeng Formation’’ isdocumented.

2. Methodology

Foraminifera in a total of 43 samples (each sample 200 g of sed-iments) of the ‘‘Takeng Formation’’ and ‘‘Tsukeng Formation’’ ex-posed along the Pinglin River are identified (Fig. 3). For a moredetailed study on the stratigraphy of the ‘‘Tsukeng Formation’’, cal-careous nannoplanktons in tuff beds of the ‘‘Tsukeng Formation’’are determined to compare with results of the foraminiferal studyin siliciclastic successions and an independent U–Pb age dating onzircon grains separated from the top of thick tuff beds (Sample A inFig. 3).

Tuff sample were ground to 0.1–0.2 mm in size and zircongrains were separated from the ground tuff by magnetic,electromagnetic and heavy mineral dressing processes, and then

Fig. 5. Field outcrops of (A) the Wuchihshan Formation with bioturbated brown–pink-colored sandstone; (B) Ichofossils Ophiomorpha in the Wuchihshan Formation; (C) Atranspressive fault cut into the lower part of the Wuchihshan Formation; (D) Ichnofossils Ophiomorpha in the Mushan Formation; (E and F) Thick, pink–green–gray coloredPinglin Tuff below the break-up unconformity.

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 7

hand-picked at random under a binocular microscope. Zircongrains were mounted in epoxy, polished, and imaged with athodoluminescence (CL) to reveal textural details such as zoning andmetamorphic overgrowths, and determine the analyses area forin situ zircon U–Pb dating. U–Pb ages were measured using aComPex102 ArF 193 nm excimer laser ablation system, coupledto an Agilent 7500a quadrupole ICP-MS based within the StateKey Laboratory of Continental Dynamics facilities at the North-west University, China. Repeated measurements of external Har-vard standard zircon 91500 and NISTSRM610 silicate glass wereused to correct for instrumental mass bias and depth-dependentinter-element fractionation of Pb, Th and U. 29Si was selected asinternal standard element. The compromise laser ablation spotsize is 30 lm. Real time data were processed using GLITTER laserablation data reduction software version 4 (Macquarie Univer-sity). Ages and probability density functions were calculatedusing Isoplot (Ludwig, 2003).

Combined these independent evidences from field survey,microfossil study and age dating, the results are then correlatedwith the well-studied standard Tertiary sequences onland north-ern Taiwan and offshore Taiwan Strait to recognize a proper lithos-tratigraphy in the Nantou area, central Taiwan, for an island-widestratigraphic correlation.

3. Results

3.1. Field survey

The field survey along the Pinglin River confirms that the ‘‘Tak-eng Formation’’ of Ho et al. (1956) is conformably underlain by theupper unit of the ‘‘Tsukeng Formation’’ in the east limb of the Tsuk-eng anticline (Fig. 3). The Miocene ‘‘Takeng Formation’’ includesthe shale-dominant Tanliaoti Member in the upper part and thesandstone-dominant Shihszeku Member in the lower part(Fig. 2). In the eastern limb of the Tsukeng anticline, these stratastrike in N20�–45�W and 25�–50� dip to the east (Fig. 3). The Tan-liaoti Member shows a coarsening upward sequence starting fromthick shales in the lower part to thin alternations of sandstone/shale in the upper part, which is further conformably overlain bythick sandstones of the Shihmen Member of the ‘‘Shuilikeng For-mation’’ (Ho et al., 1956). The base of the Tanliaoti Member is char-acterized by an occurrence of a 3-m thick glauconitic siltstone bedwith mollusks Amussiopecten yabei (Nomura). In addition, there areabout 15 m thick units of pinkish-brown sandstone (Fig. 4A) thatcontains Ophiomorpha and Thalassinoides burrows, suggesting thatthese rocks are coastal to shallow marine in origin (Fig. 5D). Theshale/sand ratio of the Shihszeku Member is about 1.

Fig. 6. (A and B) Field sketches of the unconformity between the Latest OligoceneWuchihshan Formation and the underlying Late Middle Eocene Pinglin Tuff byHuang et al. (2001) and (C) by Hashimoto and Kurihara (1974). The width of (b) and(c) is about 3 m.

8 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

The ‘‘Tsukeng Formation’’ is composed of three lithologicalunits. Only its upper and middle units are exposed along the Ping-lin River (Fig. 3). The upper unit is of a shale dominant sequence

Table 1Distributions of planktonic foraminifera for samples collected along the Pinglin River sect

with several brown–pink-colored sandstone beds and the middleunit consists of thick tuff beds (Fig. 2). The shale/sand ratio ofthe upper unit is more than 5. Each sandstone bed is 1–1.5 m inthickness (Fig. 4B). They are either a greenish because of glauconitegrains, or are pinkish-brown because of oxidation by weathering(Fig. 5A). They are also burrowed by Ophiomorpha ichnofossils(Fig. 5B). Parallel to the Tsukeng anticline axis, there is a transpres-sive fault with thick fault gauge (�3 m wide) cutting into the verylower part of the upper unit (Fig. 5C). The shale dominant unit alsoappears in the west limb of the Tsukeng anticline north of the Hos-hin Primary School (Fig. 3).

The middle unit of the ‘‘Tsukeng Formation’’ is characterized bya distinct occurrence of thick, variegated, and massive tuff beds(green, pink, red, gray, brown and white colors) without much ter-restrial siliciclastic sediments (Fig. 4E–F and 5E–F). A distinctunconformity is marked by an erosion surface between thebrown–pink-colored sandstone bed of the upper unit and the thicktuff of the middle unit (Fig. 4C–F). Right above the unconformity,there are several greenish or reddish-colored tuff blocks erodedfrom the thick tuff unit below the unconformity (Fig. 6; Huanget al., 2001). Fossils Ostrea are observed in the brown–pink-col-ored, cross-bedded sandstone immediately overlying the uncon-formity (Fig. 6b) which has also been described by Hashimotoet al. (1979; Fig. 6c). Unfortunately, this unconformity exposurealong the Pinglin River has been covered by talus deposits since2007. The unconformable contact can only be observed now alongthe trail south of the Pinglin River (Fig. 4) and also in several out-crops south of the Tsukeng Village (Huang et al., 2013).

3.2. Planktonic foraminifera

Along the Pinglin River section, the thick shale 9 m above thebase of the Tanliaoti Member of the ‘‘Takeng Formation’’ is richin planktonic foraminifera of the Earliest Miocene species, includ-ing Globigerinoides quadrilobatus (d’Orbigny), Globigerinoides saccu-lifer (Brady), Globigerinoides altiaperturus Bolli, Globoquadrinadehiscens (Chapman, Parr & Collins), Globoquadrina venezuelana(Hedberg) and Globorotalia siakensis LeRoy of Zone N5 (Table 1).No planktonic foraminifera are found in the Shihszeku Member.In contrast, Oligocene species such as Globigerina ciperoensis Bolliand Globigerina angulisuturalis Bolli are found in the shale-domi-nant upper unit of the ‘‘Tsukeng Formation’’ (Table 1). However,no planktonic foraminfera are found in the thick tuff unit of the‘‘Tsukeng Formation’’.

ion.

Table 2Distributions of benthic foraminifera in the section exposed along the Pinglin River section.

G.k.Z.: Gaudryina kokuseiensis Zone.

Fig. 7. (Left) Plots of stratigraphic distributions of three key benthic foraminifers of Genus Gaudryina on Ho’s (1961) base figure for a regional biostratigraphic correlation inthe Western Foothills between northern and central Taiwan, and (right) on the newly revised stratigraphy in the Tsukeng anticline, near Nantou, central Taiwan.

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 9

Table 3Calcareous nannofossils in the Middle Eocene Pinglin Tuff, Pinglin River section.

Table 4Results of U–Pb age determinations on zircon grains in the Pinglin Tuff.

Zircon grains Isotope ratio Age (Ma)

Pb207 1r Pb207 1r Pb206 1r Pb208 1r Pb207 1r Pb207 1r Pb206 1r Pb208 1rPb206 U235 U238 Th232 Pb206 U235 U238 Th232

09PL-01-01 0.04333 0.00704 0.03543 0.00568 0.00593 0.00017 0.00293 0.00012 0.1 212.06 35.3 5.57 38.1 1.07 59.2 2.4109PL-01-02 0.04722 0.00564 0.03931 0.00458 0.00604 0.00017 0.00175 0.00006 59.9 262.64 39.2 4.47 38.8 1.1 35.3 1.3109PL-01-03 0.05027 0.0079 0.07347 0.01138 0.0106 0.00031 0.00375 0.00022 207.3 328.18 72 10.76 68 1.98 75.7 4.3909PL-01-04 0.78229 0.03425 0.78493 0.02603 0.00728 0.00022 0.00503 0.00024 4889.8 61.04 588.3 14.81 46.7 1.43 101.4 4.7309PL-01-05 0.04784 0.00377 0.11672 0.00889 0.01769 0.00041 0.00548 0.00019 90.4 177.89 112.1 8.08 113.1 2.6 110.4 3.8709PL-01-06 0.05366 0.01096 0.04709 0.00948 0.00636 0.00023 0.00232 0.00021 356.5 405.38 46.7 9.2 40.9 1.47 46.9 4.2909PL-01-07 0.04733 0.00639 0.12431 0.0165 0.01904 0.00051 0.00661 0.00035 65.4 293.76 119 14.9 121.6 3.25 133.2 6.9809PL-01-08 0.05517 0.00344 0.16853 0.00997 0.02215 0.0005 0.0069 0.00018 419.1 133.6 158.1 8.66 141.2 3.15 138.9 3.6309PL-01-09 0.05353 0.0075 0.0459 0.00632 0.00622 0.00018 0.00209 0.00012 351 289.22 45.6 6.13 40 1.15 42.2 2.4309PL-01-10 0.04163 0.00921 0.03251 0.00713 0.00566 0.00018 0.00176 0.00014 0.1 233.46 32.5 7.01 36.4 1.13 35.4 2.8909PL-01-11 0.05767 0.00355 0.177 0.0103 0.02226 0.00051 0.01028 0.00038 516.9 129.95 165.5 8.89 141.9 3.21 206.8 7.5209PL-01-12 0.04931 0.00199 0.10373 0.00378 0.01526 0.00031 0.00464 0.0001 162.5 91.51 100.2 3.48 97.6 1.96 93.6 1.9609PL-01-13 0.0517 0.00967 0.04352 0.008 0.0061 0.00022 0.00196 0.00018 272.1 380.21 43.3 7.79 39.2 1.43 39.7 3.5909PL-01-14 0.04884 0.00225 0.111 0.00472 0.01648 0.00034 0.00533 0.00015 140.2 104.74 106.9 4.32 105.4 2.18 107.4 3.0709PL-01-15 0.0724 0.00976 0.06382 0.00834 0.00639 0.00022 0.00264 0.00016 997.1 252.1 62.8 7.96 41.1 1.43 53.3 3.1609PL-01-16 0.05654 0.00498 0.15061 0.01283 0.01932 0.00049 0.00627 0.00027 472.7 184.73 142.5 11.32 123.3 3.11 126.3 5.4609PL-01-17 0.07016 0.00774 0.18953 0.02018 0.01959 0.00061 0.00704 0.00036 932.9 211.2 176.2 17.22 125.1 3.86 141.9 7.2809PL-01-18 0.0508 0.00906 0.04389 0.0077 0.00626 0.00022 0.00202 0.00016 231.6 366.91 43.6 7.49 40.3 1.39 40.8 3.1409PL-01-19 0.04359 0.00482 0.04089 0.00443 0.0068 0.00017 0.00221 0.00009 0.1 119.13 40.7 4.32 43.7 1.08 44.6 1.8809PL-01-20 0.04126 0.00699 0.04677 0.00783 0.00822 0.00023 0.00221 0.00017 0.1 111.23 46.4 7.59 52.8 1.48 44.7 3.4409PL-01-21 0.05041 0.00228 0.12193 0.00507 0.01754 0.00036 0.00561 0.00014 214 101.35 116.8 4.59 112.1 2.31 113.1 2.9109PL-01-22 0.05627 0.00352 0.17826 0.01055 0.02297 0.00053 0.00881 0.00032 462.2 133.07 166.6 9.09 146.4 3.36 177.3 6.3809PL-01-23 0.10183 0.00923 0.18025 0.01559 0.01284 0.00038 0.00527 0.00018 1657.8 159.15 168.3 13.42 82.2 2.42 106.3 3.6709PL-01-24 0.04949 0.00415 0.09006 0.00732 0.0132 0.00031 0.00423 0.00012 171.2 184.85 87.6 6.82 84.5 1.98 85.2 2.4309PL-01-25 0.04873 0.00196 0.1315 0.00479 0.01957 0.0004 0.00609 0.00015 134.7 91.85 125.4 4.3 124.9 2.52 122.6 3.0209PL-01-26 0.05971 0.01154 0.0538 0.01022 0.00653 0.00024 0.00254 0.00019 593.3 371.11 53.2 9.85 42 1.56 51.2 3.909PL-01-27 0.04675 0.00215 0.15809 0.00673 0.02452 0.00051 0.00726 0.00022 36.1 106.73 149 5.9 156.2 3.2 146.2 4.4409PL-01-28 0.05291 0.01064 0.04473 0.00889 0.00613 0.00021 0.00215 0.0002 324.9 401.91 44.4 8.64 39.4 1.33 43.4 4.0709PL-01-29 0.04466 0.00461 0.11598 0.0117 0.01883 0.00047 0.00585 0.00021 0.1 160.69 111.4 10.64 120.3 2.98 117.9 4.2309PL-01-30 0.05063 0.00313 0.12764 0.0075 0.01828 0.00041 0.0188 0.00043 223.9 136.96 122 6.76 116.8 2.59 376.4 8.5309PL-01-31 0.04331 0.00964 0.03698 0.00813 0.00619 0.00023 0.00189 0.00016 0.1 326.81 36.9 7.96 39.8 1.45 38.2 3.14

10 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

180

140

100

60

20

0.000

0.004

0.008

0.012

0.016

0.020

0.024

0.028

0.00 0.04 0.08 0.12 0.16 0.20 0.24207Pb/235U

206 Pb

/238 U

data-point error ellipses are 68.3.% conf

09PL-01:13potst(206Pb/238U)= 38.8±1.0Ma

MSWD = 1.8

34

36

38

40

42

44

Pb20

6/U

238

data-point error symbols are 1σ

Mean = 38.8±1.0 [2.6%] 95% conf.Wtd by data-pt errs only, 0 of 13 rej.MSWD = 1.8, probability = 0.042

Fig. 8. (Upper) Concordia plot of U–Pb age on zircon grains of sample A from thePinglin Section. (Lower) U–Pb age distributions of 13 zircon grains of the youngestgroup obtained by LA-ICPM method of the Pinglin Tuff. Analytical uncertainty isrepresented by 2r error.

Table 4 (continued)

Zircon grains Isotope ratio Age (Ma)

Pb207 1r Pb207 1r Pb206 1r Pb208 1r Pb207 1r Pb207 1r Pb206 1r Pb208 1rPb206 U235 U238 Th232 Pb206 U235 U238 Th232

09PL-01-32 0.05666 0.00548 0.04717 0.00443 0.00604 0.00016 0.00202 0.00007 477.4 201.44 46.8 4.29 38.8 1.01 40.8 1.4709PL-01-33 0.05338 0.00613 0.04361 0.00489 0.00592 0.00016 0.002 0.00009 345 240.77 43.3 4.76 38.1 1.01 40.4 1.8209PL-01-34 0.05025 0.00302 0.10242 0.00583 0.01478 0.00033 0.00469 0.0001 206.7 133.55 99 5.37 94.6 2.07 94.6 2.0909PL-01-35 0.05442 0.00434 0.04491 0.00344 0.00598 0.00015 0.00191 0.00008 388.4 169.71 44.6 3.35 38.5 0.94 38.5 1.5509PL-01-36 0.05575 0.00514 0.15992 0.01426 0.0208 0.00055 0.0062 0.0002 442.2 193.32 150.6 12.49 132.7 3.46 124.9 3.9209PL-01-37 0.0477 0.00258 0.10556 0.00535 0.01605 0.00033 0.00465 0.0001 83.4 124.38 101.9 4.91 102.6 2.1 93.7 2.1109PL-01-38 0.05498 0.00687 0.04294 0.00524 0.00566 0.00016 0.00188 0.00009 411.2 257.7 42.7 5.1 36.4 1.04 37.9 1.7709PL-01-39 0.05286 0.00227 0.11149 0.00434 0.01529 0.00031 0.00489 0.00011 322.9 94.43 107.3 3.96 97.8 1.95 98.6 2.1309PL-01-40 0.25681 0.01041 0.26845 0.0092 0.00758 0.00018 0.0078 0.00018 3227.2 62.57 241.5 7.37 48.7 1.15 157 3.5209PL-01-41 0.05659 0.01125 0.05007 0.0097 0.00642 0.00029 0.00224 0.00019 474.8 388.78 49.6 9.38 41.2 1.89 45.2 3.84

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 11

3.3. Benthic foraminifera

Benthic foraminifers are commonly found in the siliciclastics ofthe study section (Table 2), but they are absent in the thick volca-nic tuff unit of the ‘‘Tsukeng Formation’’. The Tanliaoti Member ofthe ‘‘Takeng Formation’’ contains the typical Early Miocene indicesof benthic foraminifera Gaudryina kokuseiensis Ishizaki, Gaudryinapseudohayasakai Chang, Lenticulina taluensis Chang, Baggina denta-toapertura Chang, Heterolepa praecincta (Karrer), Nonionella kanko-uensis Chang, Uvigerina nitidula Schwager, Nodosaria rapana (Linne)and Amphicoryna scalaris (Batsch). Foraminifera of the ShihszekuMember of the ‘‘Takeng Formation’’ are similar to what are foundin the upper shale-dominant unit of the ‘‘Tsukeng Formation’’ (Ta-ble 2 and Fig. 7). Both units are dominated by benthics of Ammoniabeccarii (Linné), Gaudryina hayasakai Chang, Gaudryina collinisielongata Chang, Nonion kankouensis Chang, Pullenia compressiusculaReuss and Cribrostomoides yeni Chang of the Oligocene Shihtsaoanfauna (Chang, 1962; Huang, 1984, 1986; Fig. 10).

3.4. Calcareous nannoplankton

Latest Oligocene calcareous nannoplanktons of Zone NP25 flora,including Zygrhailithus bijugatus (Deflandre), Sphenolithus ciperoen-sis Bramlette and Wilcoxon, Sphenolithus moriformis (Brönnimannand Stradner), Discoaster deflandrei Bramlette and Riedel were re-ported from the shale-dominated upper unit of the ‘‘Tsukeng For-mation’’ by Chi (1979). This planktonic floral age is consistentwith the planktonic foraminiferal age (Late Oligocene). Althoughmost thick tuffs of the middle unit of the ‘‘Tsukeng Formation’’do not contain any foraminifera, some calcareous nannoplanktonsare fortunately found in 2 of 19 samples (samples J and Q in Fig. 3and Table 3) collected from the middle tuff unit. In addition tothose indeterminable and questionable species like Sphenolithussp. and Coccolithus sp., nannoplanktonic flora in these two samplesare composed of three groups (Table 3): Group (A) of long rangespecies (from Eocene upward), including Cyclicargolithus floridanus(Roth and Hay), Dictyococcites bisectus (Hay, Mohler and Wade),Coccolithus pelagicus (Wallich), Cyclococcolithina formosus (Kampt-ner), Zygrhablithus bijugatus (Deflandre), S. moriformis (Bronni-mann and Stradner), Reticulofenestra umbilica (Levin); Group (B)of Middle Eocene species: Chiasmolithus grandis (Bramlette andRiedel), Triquetrorhabdulus inversus Martini, Sphenolithus radians(Deflandre), Shenolithus furcatolithoides Locker, Shenolithus spinigerBurkry, Discoaster cf. sublodoensis Bramlette and Sullivan, Neococ-colithus dubius (Deflandre); and Group (C) of re-worked Paleocenespecies of Heliolithus riedelii Bramlette and Sullivan, Fasciculithustympaniformis Hay and Mohler and the Cretaceous species ofMicula decussata Vekshina and Watznaueria barnesae Black. Theassemblage (Table 3) suggests an age of Late Middle Eocene (ZoneNP16; Fig. 11) for the middle tuff unit.

3.5. Zircon U–Pb dating

Zircon U–Pb geochronology of a tuff sample of the middle unitis applied to further constrain the tuff age. Forty-one zircon grainswere analyzed in which ten grains show a relatively high degree ofdiscordance due to Pb loss and thus are eliminated. Thirty-onegrains get reliable age with high degree of concordance. The resultsare shown in Table 4 and presented on concordia plots in Fig. 8.

Fig. 9. Correlation of break-up unconformity between the Penghu Basin in offshore Taiwan Strait and boreholes (WG-1 and PC-1) onland the Coastal Plain above the PeikangBasement High as well as the exposures in the Tsukeng Anticline structure in the Western Foothills near Nantou, Central Taiwan. See Fig. 1 for well and outcrop locations.

12 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

Two groups of U–Pb ages are obtained from these 31 analyses ofzircon grains. The first group is older than 100 Ma with a mean of120 ± 20 Ma which may represent inheritance from the source re-gion of the magmas, while the second group of thirteen zircongrains yields a younger, weighted mean age of 38.8 ± 1.0 Ma, sug-gesting the best estimation of crystallization age of the volcanictuff (Fig. 8).

4. Discussion

4.1. Stratigraphy and correlation

The Tertiary sequences of the Western Foothills in northern–central Taiwan contain characteristic foraminifera occurred insome specific stratigraphic intervals. Among them the evolutionarylineage of benthic foraminifera G. hayasakai Chang – G. pseudoha-yasakai Chang – G. kokuseiensis Ishizaki plays the key role for sucha stratigraphic correlation (Chang, 1959, 1960, 1962; Huang, 1986,1989, 1990; Huang and Cheng, 1982, 1984). G. hayasakai is thediagnostic species of the Oligocene–Earliest Miocene ShihtsaoanStage (equivalent to planktonic foraminiferal Zones P20-N4). G.pseudohayasakai occurs restrictedly in the Early Miocene (or ZoneN5) in northern Taiwan, but it may extend to the Middle Miocene

in central Taiwan (Huang and Cheng, 1982), while G. kokuseiensisappears from the Early to Middle Miocene (or Zones N5–N12;Fig. 7). Along the Pinglin River section, the base of the TanliaotiMember is characterized by the first appearance of G. pseudoha-yasakai (sample 27 in Fig. 3 and Table 2), while G. kokuseiensishas the lowest occurrence from the horizon about 15 m abovethe base of the Tanliaoti Member (sample 28 in Fig. 3 and Table 2).In addition, G. hayasakai appears downward from the upper unit ofthe ‘‘Tsukeng Formation’’ (Table 2).

Using Ho’s (1961) correlation column as a base map, Fig. 7shows stratigraphic distributions of these key benthic taxa in theWestern Foothills of the Taipei area in northern Taiwan and theTsukeng anticline of the Nantou area in central Taiwan (Fig. 7 leftpanel) with reference to the stratigraphic correlation of Ho (1961).The right panel of Fig. 7 marks a new stratigraphic correlation con-trolled by planktonic biostratigraphy of foraminifera and calcare-ous nannofossils based on the present work and what has beenpublished in the Kuohsing area (Huang and Ting, 1979; Chi,1983; Huang, 1986). The results indicate that the two ‘‘Members’’of the so-called ‘‘Takeng Formation’’ (Tanliaoti Member in theupper and the Shihszeku Member in the lower) are actually coevalto the Early Miocene Taliao Formation/Kungkuan Tuff and the Ear-liest Miocene Mushan Formation in northern Taiwan, respectively(Figs. 2 and 7). The Taliao Formation/Kungkuan Tuff is also charac-

Fig. 10. Foraminifers from the strata in axial part of the Tsukeng anticline. 1, 11. Flouilus boueanus (d’Orbigny), sample 26, Wuchihshan Formation; 2, 6. Globoquadrinadehiscens (Chapman, Parr & Collins), sample-28, Taliao Formation; 3. Globigerinoides altiaperturus Bolli, sample-28, Taliao Formation; 4–5, 14–15 Globigerinoides quadrilobatus(d’Orbigny), sample-28, Taliao Formation; 7. Globigerina angulisutralis Bolli, sample 32, Wuchihshan Formation; 8. Marginulina cf. hantkeni Bandy, sample 16, WuchihshanFormation; 9. Amphicoryna scalaris (Batsch), sample 27, Taliao Formation; 10. Baggina saitoi Asano and Murata, sample 35, Wuchihshan Formation; 12. Heterolepa preacenta(karrer), sample 31, Taliao Formation; 13. Globoquadrina venezuelana (Hedberg), sample 28, Taliao Formation; 14. Gaudryina hayasakai Chang, sample 16, WuchihshanFormation; 16. Gaudryina hayasakai Chang, Sample 25, Wuchihshan Formation; 17, 22. Hanzawaia asterizana, (Fichtel and Moll), sample 31, Taliao Formation; 18, 23, 25.Gaudryina pseudohayasakai Chang, sample 28, Taliao Formation; 19, 24. Gaudryina kokuseiensis Ishizaki, sample 31, Taliao Formation; 20. Martinottiella communis (d’Orbigny),sample 28, Taliao Formation; 23. Textularia sp., sample 14, Wuchihshan Formation.

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 13

terized by occurrence of planktonic foraminifera of Zone N5 con-taining prosperous species of Genus Globigerinoides and benthicforaminiferal fauna of G. pseudohayasakai Zone and G. kokuseiensisZone, while the benthic foraminifera of G. hayasakai Zone were alsofound in the Mushan Formation (Table 2; Huang and Cheng, 1983,1984). This suggests that the two members of the ‘‘Takeng Forma-tion’’ can be upgraded to ‘‘Formation’’ level and the proposed ‘‘Tak-eng Formation’’ can be abandoned. This conclusion is consistentwith what had been documented already from the independentstratigraphic studies by using calcareous nannoplanktons andforaminifera in the Kuohsing area, central Taiwan (20 km northeastof the Pinglin River section; K in Fig. 1; Huang and Ting, 1979; Chi,1983; Huang, 1986).

In addition, the ‘‘Tsukeng Formation’’ has to be revised (Fig. 2).Since the ‘‘Shihszeku Member’’ (Ho et al., 1956) in central Taiwan isequivalent to the Earliest Miocene Mushan Formation in northernTaiwan, the shale dominant unit in the upper part of the so-called‘‘Tsukeng Formation’’, which conformably underlies the EarliestMiocene Mushan Formation, cannot be equivalent to the EarlyMiocene Kungkuan Tuff in northern Taiwan (Figs. 2 and 7) as

was proposed by Ho et al. (1956). Instead, the foraminiferal stratig-raphy indicates that the shallow marine shale dominant unit of theupper part of the ‘‘Tsukeng Formation’’ (Zone NP25 of calcareousnannoplanktons or benthic foraminifer G. hayasakai Zone) is actu-ally of the uppermost part of the Late Oligocene Wuchihshan For-mation in northern Taiwan (Figs. 2 and 7) or the uppermost part ofthe Late Oligocene Shuichangliu Formation in the Kuohsing area ofthe Hs}uehshan Range, central Taiwan. However, the ‘‘ShuichangliuFormation’’ was considered by Ho et al. (1956) to be underlying theproposed ‘‘Tsukeng Formation’’ (Fig. 2). Accordingly, there is astratigraphic conflict in the lithostratigraphic scheme establishedby Ho et al. (1956) and Ho (1961). Consequently the stratigraphyof the ‘‘Tsukeng Formation’’ has to be revised (Fig. 2). Here we fol-low the traditional stratigraphy names used in the Western Foot-hills to revise the local ‘‘Takeng Formation’’ and ‘‘TsukengFormation’’ stratigraphy of Ho et al. (1956) along the Pinglin Riversection in descending order: the Taliao Formation, Mushan Forma-tion and the uppermost Wuchihshan Formation unconformableoverlying the Late Middle Eocene Pinglin Tuff. The Pinglin Tuff(discussed below) is first named herein which lies unconformably

Fig. 11. Calcareous nannoplanktons in Sample Q, Pinglin Tuff. Eocene fossils: 1–4. Cyclococcolithus formosus Kamptner; 5–7. Reticulofenestra umbilica (Levin); 8–9.Dictyococcites bisectus (Hay, Mohler & Wade); 10. Cyclicargolithus floridanus (Roth & Hay); 11. Zygrhablithus bijugatus (Deflandre); 12. Discoaster sp.; 13–14. Sphenolithusradians (Deflandre); 15. Sphenolithus moriformis (Brönnimann & Stradner); 16. Coccolithus pelagicus (Wallich); Cretaceous secondary fossils: 17–18. Watznaueria barnesaeBlack; 19–20. Micula decussata Vekshina; Paleocene secondary fossils: 21. Heliolithus riedelii Bramlette & Sullivan x1000.

14 C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16

beneath the uppermost Wuchihshan Formation (Late Oligocene).The Pinglin Tuff also overlies the Middle Eocene Chungliao Forma-tion (Huang et al., 2013) exposed in the southern part of the Tsuk-eng anticline (Figs. 2, 6 and 7).

4.2. Age of the Pinglin Tuff

Before the present study, calcareous nannofossils of the ‘‘Tsuk-eng Formation’’ were studied by Chi (1979). He followed the stra-tigraphy of Chiu (1975; Fig. 2), instead of Ho et al. (1956), to putthe three different lithological units beneath the Mushan Forma-tion (=Shihszeku Member of Ho et al., 1956, or the Shihti Formationof Chiu, 1975) as the ‘‘Shuichangliu Formation’’. Based on his find-ing of calcareous nannoplanktons Spenolithus ciperoensis in theshale-dominant upper unit of their ‘‘Shuichangliu Formation’’, heconsidered that the ‘‘Shuichangliu Formation’’ of Chiu (1975) tobe Late Oligocene age. However, in addition to the normal nannofl-ora of Zone NP25 found in the shale-dominant upper unit, Chi(1979) also noted that the thick tuff middle unit contained pre-dominant Eocene species in association with some reworkedPaleocene and Upper Cretaceous nannofossil assemblages.

For a more precise age determination for the Pinglin Tuff, we re-collected 19 samples (A–M in Fig. 3) of the Pinglin Tuff along thePinglin River section for both detailed calcareous nannoplanktonstudy and age dating. This new calcareous nannofossil study con-firms the predominant Middle Eocene species in association withreworked Paleocene and Cretaceous species in the Pinglin Tuff(samples J and Q; Table 3). The nannofossil assemblage from thePinglin Tuff (Zone NP16) is almost similar to the underlying unit(the Chungliao Formation, Zones NP14–15) which also containsthe indigenous Middle Eocene larger foraminifera Discocyclina

dispansa ex. interc. sella-dispansa (Huang et al., 2013). Both thePinglin Tuff and the Chungliao Formation contain also the similarreworked Paleocene and Cretaceous nannoflora. Such a character-istic mixture of the in situ Middle Eocene calcareous nannofossilassemblage with the reworked Paleocene and Cretaceous specieshad been also reported previously from the Middle Eocene Assili-na-bearing slates (Zones NP14–15) in the Hsüehshan Range(Huang, 1980b) and also in the Penghu Basin, Taiwan Strait. The re-worked Paleocene and Cretaceous nannoplanktons in the PinglinTuff could be derived from the Early Tertiary rift basin and the Cre-taceous pre-rift basement in the horst on the Peikang BasementHigh.

The Middle Eocene age of the Pinglin Tuff is further supportedby a U–Pb age of zircon grains (Fig. 8; 38.8 ± 1 Ma; Late Middle Eo-cene) determined by LA-ICP-MS from the uppermost part of thePinglin Tuff (sample A in Fig. 3).

4.3. First documented Paleogene syn-rift basin in the Western Foothills

We argue that the Middle Eocene Pinglin Tuff and the underly-ing Chungliao Formation (Huang et al., 2013) exposed in the Tsuk-eng area is part of the Paleogene syn-rift sequence. Theaccumulation of the Pinglin Tuff could be originated from the vol-canism when the Asian continent crust was stretched during thedevelopment of the Paleogene rift basins. Similar Eocene or Paleo-cene volcanics unconformably beneath the Late Oligocene–Mio-cene post-rift sequence were also documented in the PenghuBasin in the Taiwan Strait (Hsiao et al., 1991; Lin et al., 2003),the Peikang Basement High in the Coastal Plain onshore Taiwan(Yuan et al., 1985), and the Pearl River Mouth Basin offshore south-ern China (Li and Rao, 1994; Zou et al., 1995). The Paleogene

C.-Y. Huang et al. / Journal of Asian Earth Sciences 69 (2013) 3–16 15

eruptions are all tectonically related to the opening of the SouthChina Sea (Huang et al., 2012). The break-up unconformity be-tween the syn-rift and post-rift sequences marks the ‘‘Puli Move-ment’’ in the Taiwan area (Chang, 1955, 1972; Huang et al.,2012). In the Borehole PC-1 on the Peikang Basement High, thethick tuff of the Shuangchi Red Beds (for at least 300 m thick;Lee, 1962; Chiu, 1972; Yuan et al., 1985) at 1953 m deep has beendated as 44.1 ± 2.2 Ma (Middle Eocene) by using K–Ar dating meth-od (Yuan, 1981). Middle Eocene syn-rift sequences of the Shuang-chi Red Beds are unconformably overlain by the earliest Miocenepost-rift Piling Formation (=Taliao Formation in northern Taiwan;Yuan et al., 1985). In the offshore Penghu Basin, Taiwan Strait, Eo-cene tuff and basalt for more than 400 m thick have been drilledbetween thick marine Middle Eocene shales (NP15–16; Hsiaoet al., 1991; Huang, 1982). This Middle Eocene marine shale isunconformably covered by the Earliest Miocene post-rift strata(Sun, 1982; Hsiao et al., 1991). Thus, the stratigraphic successionsat the Tsukeng anticline, Peikang Basement High onshore Taiwanas well as the Penghu Basin in the Taiwan Strait are comparable,representing the Paleogene syn-rift sequence unconformably over-lain by the Late Oligocene–Early Miocene post-rift strata (Fig. 9).

5. Conclusions

Field survey and paleontological study indicate that the stratig-raphy in the axial part of Tsukeng anticline near Nantou, centralTaiwan, has to be revised. The two members of the ‘‘Takeng Forma-tion’’ (Tanliaoti and Shihszeku Members) of Ho et al. (1956) can beupgraded to formation level and revised as the Early Miocene Ta-liao Formation (Gaudryina kokuseiensis /Gaudryina pseudohayasakaiZone and G. kokuseiensis Zone) and the Earliest Miocene MushanFormation (uppermost G. hayasakai Zone), respectively, as theyare commonly used in the Western Foothills of northern Taiwan.In addition, the ‘‘Tsukeng Formation’’ has also to be revised andabandoned. The ‘‘Tsukeng Formation’’ includes three lithologicalunits separated by an unconformity between the upper unit (theuppermost Wuchihshan Formation of Late Oligocene Zone NP25or G. hayasakai Zone) and the middle unit (Late Middle EocenePinglin Tuff of Zone NP16) exposed along the Pinglin River section.

The Pinglin Tuff, firstly named in this paper, is currently dated as38.8 ± 1 Ma. It contains Late Middle Eocene nannoplanktons (ZoneNP16) with the reworked Paleocene and Cretaceous fossils, similarto what have been found in the underlying Discocyclina-bearingChungliao Formation (Zones NP14–15; Middle Eocene; Huanget al., 2012, 2013). The reworked Paleocene and Cretaceous nanno-planktons in the Pinglin Tuff could be derived from the pre-rift base-ment in hosts or Early Tertiary syn-rift basins onshore the PeikangBasement High and its offshore counterpart. Similar Eocene tuffbeds have been drilled in the Eocene syn-rift basins of the PenghuBasin in offshore Taiwan Strait and in the PC-1 Well on the PeikangBasement High onshore central Taiwan. The sequence in the Nantouarea, Western Foothills, is correlated with the rift history what havefound in the Taiwan Strait–Pearl River Mouth basin in the NE SouthChina Sea. The unconformity between the Latest Oligocene Wuchih-shan Formation and the Middle Eocene Pinglin Tuff corresponds tothe regional break-up unconformity between the post- and syn-riftsequences related to the opening of the South China Sea in the EarlyTertiary (Huang et al., 2012). This regional break-up unconformitycorresponds to the Puli Movement in the Taiwan region (Chang,1955, 1972; Huang et al., 2012).

This is the first document to show that in the Western Foothillsunconformably below the Late Oligocene Wuchihshan Formation,there are the Eocene strata and also shows the first discovery ofa Paleogene rift basin exposed in the Western Foothills. This studythus extends our understanding of the Western Foothills

geohistory from the Late Oligocene downward at least to the Mid-dle Eocene.

Acknowledgements

The authors appreciate H.T. Chiu, J.T. Chou and H.H. Ting of theCPC Corporation, Taiwan, for constructive discussions and com-ments during the study. This study was supported by grants fromthe National Science Council, Taiwan (NSC97-ET-7-006-007-ET)and National Science Foundation of China (No. 91128211) toC.Y. Huang, and Chinese Academy of Sciences (No. Y133071302) toY. Yan.

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