Significance of indigenous Eocene larger foraminifera Discocyclina dispansa inWestern Foothills,...

Journal of Asian Earth Sciences 62 (2013) 425–437

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Journal of Asian Earth Sciences

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Significance of indigenous Eocene larger foraminifera Discocyclina dispansa inWestern Foothills, Central Taiwan: A Paleogene marine rift basin in Chinesecontinental margin

Chi-Yue Huang a,b,⇑, Yi Yen b, Ping-Mei Liew c, Dai-Jie He b, Wen-Ron Chi b, Min-Shyan Wu d,Meixun Zhao 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, Taiwan, Miaoli, Taiwane Key Laboratory of Marine Chemistry Theory and Technology of the Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China,Qingdao, China

a r t i c l e i n f o

Article history:Received 4 July 2008Received in revised form 28 September 2012Accepted 22 October 2012Available online 10 November 2012

Keywords:DiscocyclinaWestern FoothillsEocene rift basinBreak-up unconformityTaiwan

1367-9120/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.jseaes.2012.10.026

⇑ Corresponding author at: Key Laboratory of MargInstitute of Geochemistry, Chinese Academy of Scienc

E-mail address: [email protected] (C.-Y.

a b s t r a c t

The Western Foothills of Taiwan was known to be composed of Late Oligocene to Pleistocene shallowmarine strata continuously deposited on the stable passive Chinese continental margin without signifi-cant stratigraphic break. Here we present multiple micropaleontological evidences, including occurrenceof larger foraminifera Discocyclina dispansa ex. interc. sella-dispansa and calcareous nannoplanktons, toshow that there are Middle Eocene marine strata (first named as the Chungliao Formation) exposed inthe Tsukeng anticline of the Western Foothills, central Taiwan. Occurrences of intact tests with thin del-icate outer rims and well-preserved embryonic chambers suggest that the Discocyclina dispansa ex. interc.sella-dispansa (Lutetian to Bartonian in the Tethys region) are buried indigenously on shallow inner shelfduring an episodic transgression in the Early Middle Eocene. The conclusion is consistent with a biostra-tigraphy study of calcareous nannoplanktons (Zones NP14–15) in the shale/sandstone alternations over-lying the Discocyclina-bearing bed of the Chungliao Formation and calcareous nannofossils of Zone NP16integrated with an age dating of 38.8 ± 1 Ma (Late Middle Eocene) on zircon grains of the overlying Ping-lin Tuff. The Middle Eocene syn-rift sequences (Chungliao Formation and Pinglin Tuff) exposed along theTsukeng anticline are unconformably covered by the latest Oligocene–Miocene post-rift sequence, a sce-nario similar to what have been drilled in the East China Sea-Taiwan Strait-South China Sea. This riftbasin (named as the Nantou Basin) is sitting on the Peikang Basement High margin which further extendssouthwestward to the Central Uplift of the Pearl River Mouth Basin in the northern slope of the SouthChina Sea. The present work documents a hitherto unknown occurrence of the exposed early Tertiarymarine rift basin sequence in the Western Foothills of Taiwan. The study extends our knowledge ofthe Western Foothills geohistory from the Late Oligocene downward to the Early Middle Eocene. Theoccurrence of the Paleogene Nantou rift basin in the Western Foothills may also suggest that there couldhave similar Paleogene rift sequences exposed in other parts of the Taiwan mountain belt like the Hsüeh-shan Range and the Central Range east of the Western Foothills.

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1. Introduction

The Western Foothills in Taiwan is generally known to be com-posed of Late Oligocene to Pleistocene shallow marine strata con-tinuously deposited on the stable passive Asian continentalmargin without significant tectonic disturbance and stratigraphic

ll rights reserved.

inal Sea Geology, Guangzhoues, Guangzhou, China.

Huang).

break (Ichikawa, 1931; Chang, 1975; Huang, 1980a; Huang andCheng, 1983). In contrast, Eocene and older sediments are recog-nized restrictedly in the Hsüehshan Range and the Central Rangeeast of the Western Foothills (Fig. 1). One exception is in the cen-tral part of the Western Foothills near Nantou, where Eoceneforaminifera Discocyclina occur in the axial part of the Tsukenganticline, central Taiwan (Fig. 1). The Tsukeng anticline is located11 km east of the Shuangtung fault along which the Miocene strataare thrust westward over the Late Pleistocene foreland conglomer-ates during the active arc-continent collision in the last 2 Ma

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Fig. 1. Original geological map and stratigraphy in the Tsukeng anticline structure, near Nantou, of the Western Foothills, Central Taiwan (Ho et al., 1956). WF: WesternFoothills; HR: Hsüehshan Range; CR: Central Range; PBH: Peikang Basement High; KH: Kuanyin High; CF: Chuchih fault. T: Taipei; K: Kuohsing; M: Miaoli; N: Nantou. TB:Tungyintao Basin; NB: Nanjihtao Basin; PB: Penghu Basin.

426 C.-Y. Huang et al. / Journal of Asian Earth Sciences 62 (2013) 425–437

(Huang et al., 2006; Fig. 1). The Tsukeng anticline axis runs in a N–Sdirection and plunges southward (Ho et al., 1956; Fig. 1). Discocyclinaoccur in a thin calcareous siltstone bed above thick bioturbatedsandstone dominant sequences of the lower unit of the so-called‘‘Tsukeng Formation’’ (Ho et al., 1956). This lower unit sandstone iscovered by thick tuff beds, the middle unit of the ‘‘Tsukeng Forma-tion’’ (Fig. 2), which was previously considered to be equivalent tothe Early Miocene Kungkuan Tuff in the Taipei area, northern Taiwan(Fig. 1; Ho et al., 1956; Ho, 1961). Accordingly, these Eocene disco-cyclinids in the Miocene ‘‘Tsukeng Formation’’ were considered tobe reworked from exposed Eocene strata into the Early Miocene se-quence during the ‘‘Puli Movement’’ (Chang, 1955; Ho et al., 1956).However, the correlation proposed by Ho et al. (1956) and Ho(1961) is problematic because the Kungkuan Tuff in northern Taiwancontains typically the early Miocene Burdigalian larger foraminiferaLepidocyclina, Miogypsina and Miogypsinoides (Yabe and Hanzawa,1930; Chang, 1960), but these were never reported from the ‘‘Tsuk-eng Formation’’.

In contrast to the reworked fossil model, based on fossil preser-vation Hashimoto and Kurihara (1974) and Hashimoto et al. (1979)argued that the Eocene Discocyclina (named as D. changi Hashimotoand Kurihara, 1974) exposed in the Tsukeng anticline were buriedin situ below a local unconformity within the ‘‘Tsukeng Formation’’.

Unfortunately, they did not provide any other independent micro-fossil evidence or age dating to support their observation and con-clusion. This indigenous model was later challenged by Chi (1979)who found Late Oligocene calcareous nannofossils (Zones NP24/25) in the ‘‘Tsukeng Formation’’.

We present herein results of a detailed micropaleontologicalstudy on the Discocyclina-bearing sequence that could help to re-solve the controversy over the occurrence of the Eocene largerforaminifera in the ‘‘Tsukeng Formation’’ of the Western Foothills,central Taiwan. Our considerations are: (1) If the Eocene Discocycli-na in the Tsukeng anticline axial part were reworked into the Mio-cene strata, their preservations would be very poor, and thereshould also occur some indigenous Miocene foraminifera of eitherbenthic or planktic species in the ‘‘Tsukeng Formation’’ and therewould be also highly probable to find Early Miocene diagnostic lar-ger foraminifera, such as Lepidocyclina, Miogypsina and Miogypsin-oides of the Kungkuan Tuff fauna in the ‘‘Tsukeng Formation’’;and (2) Conversely, if the Discocyclina were buried in situ, theirpreservation would be better, and one would expect to also findother Eocene microfossils in the strata.

If the Discocyclina are proved to be indigenous in the axial partof the Tsukeng anticline, this would expand our knowledge aboutthe geohistory of the Western Foothills.

Fig. 2. Correlation of stratigraphic units used in the Tsukeng anticline structure of the Western Foothills near Nantou, Central Taiwan. The low unit of thick sandstone withlarger foraminifera Discocyclina (in gray block) of the Miocene ‘‘Tsukeng Formation’’ of Ho et al. (1956) is revised as the Middle Eocene Chungliao Formation is this paper. Thegray blocks mark the same horizon but were assigned to different stratigraphic names and ages in the previous studies and the present work.

C.-Y. Huang et al. / Journal of Asian Earth Sciences 62 (2013) 425–437 427

2. Study site and method

The ‘‘Tsukeng Formation’’ includes three distinct lithologicalunits: shale dominant sequences of the upper unit, thick tuff bedsof the middle unit and thick bioturbated sandstones with Discocy-clina of the lower unit (Fig. 2; Ho et al., 1956; Chiu, 1972, 1975).The upper and middle units are best exposed along the Pinglin Riv-er in the north (Fig. 1) and the detailed stratigraphic study and cor-relation are reported in Huang et al. (2012). The lower unit of the‘‘Tsukeng Formation’’ is best exposed along the Tsukeng Riverand its tributary near Tsukeng Village, Nantou (Figs. 1 and 3). Thispaper will focus on the occurrence and preservation of the largerforaminifera Discocyclina and calcareous nannofossils in the lowerunit of the ‘‘Tsukeng Formation’’. Morphometric measurements onembryonic developments of free Discocyclina specimens are docu-mented in this work and an age dating on the overlying tuff unitprovides additional evidence (Huang et al., 2012) to evaluate ifthe Discocyclina were buried indigenously.

Field occurrences of Discocyclina at two localities in the axialpart of the Tsukeng anticline along the Gully A, a small tributaryof the Tsukeng River in north of the Tsukeng Village, are studied(Fig. 3). Free Discocyclina specimens on the weathered beddingplane in the lower unit of the ‘‘Tsukeng Formation’’ were collectedto make equatorial sections in order to examine their taxonomy,preservation, embryonic chamber arrangements and morphomet-ric measurements to compare with what had been reported fromthe Tethys region for understanding their geological age. TheGenus Discocyclina (Family Discocyclinidae of orthophragmines)is characterized by a discoidal test having an equatorial layer withalmost rectangular equatorial chamberlets and surrounding lateralchamberlets (Less et al., 2007). Most species of Discocyclina arelong-ranged and show progressively evolutionary changes inembryonic developments of megalospheric form. We follow themorphometric measurement methods used in the European disco-cyclinids (Fig. 4; Less, 1998; Less and Kovács, 1996; Özcan et al.,2007) to identify the Discocyclina (13 free specimens) from thelower unit of the ‘‘Tsukeng Formation’’. The measured morpholog-

ical parameters include: p, d, N, H and W, n, h and w (Fig. 4). Amongthem d value (cross-diameter of the deuteroconch) is the mostimportant for a taxonomic determination of artificial subspecies(Less, 1998). In addition, fossils in 50 thin sections of the Discocy-clina-bearing siltstone are examined to identify the larger forami-niferal assemblage if there is any early Miocene species such asLepidocyclina, Miogypsina and Miogypsinoides of the Early MioceneKungkuan Tuff fauna. Calcareous nannofossils in the shale/sand-stone alternation beds immediately overlying the Discocyclina-bearing siltstone (Figs. 5 and 6) are identified to provide additionaland independent evidence to further evaluate the results based onfield observation, thin section study and morphometric measure-ments on Discocyclina.

3. Results

3.1. Field survey

Among the three units of the ‘‘Tsukeng Formation’’, the lowerunit is restrictedly exposed in the axial part of Tsukeng anticlinealong a small tributary of the Tsukeng River, north of the TsukengVillage (Gully A in Fig. 3). However, along Gully A, exposures ofstrata between the middle unit (thick tuff beds) and the lower unit(thick bioturbated sandstones) are obscure due to a covering ofthick Holocene talus deposits. But in several other outcrops, forexample in an agriculture field south of the Tsukeng Village(Fig. 5A–E), the middle tuff unit is unconformably overlain by theshale dominant sequence of the upper unit, a scenario like whatwas observed along the Pinglin River section in the north (Huanget al., 2012). Distributions of the middle tuff unit in the TsukengRiver section along the Tsukeng anticline is irregular due to the riv-er erosion and an occurrence of a transpressive fault which runs inN–S direction parallel to the anticline axis (Fig. 3), therefore thetrue thickness of the middle unit is not clear. However, it is at least30 m in thickness as measured in the agriculture field south of theTsukeng village (Fig. 5). Glauconitic grains are commonly foundwithin the upper unit (g in Fig. 5D).

Fig. 3. Revised geological map in the Tsukeng anticline axis area, Tsukeng village near Nantou, Central Taiwan, showing also two localities (Locality 1 and 2 with �mark) withDiscocyclina in the Middle Eocene Chungliao Formation. WF: Western Foothills; HR: Hsüehshan Range; CR: Central Range; PBH: Peikang Basement High; N: Nantou. A2–5,B1–3, and C5–7 and D1–3 mark the sample localities of Chi (1979). The broken line marks a transpressive fault parallel to the Tsukeng anticline axis.

Fig. 4. Measurement system for megalospheric orthophragmines (Özcan et al.,2007). Measured parameters: p and d: outer diameter of the protoconch anddeuteroconch perpendicular to their common axis; n: number of adauxiliarychamberlets; H and W: height and width of the adauxiliary chamberlets; n: numberof annuli within 0.5 mm distance measured from the deuteroconch along the axis ofthe embryon; H and w: height and width of the equatorial chamberlets around theperipheral part of the equatorial layer.


3.2. Field occurrence of Discocyclina

Discocyclina are found in the lower unit of the ‘‘Tsukeng Forma-tion’’ along the Tsukeng anticline axis of Gulley A at two localities,about 700 m and 200 m north of a water supply station (=formerprimary school; Fig. 3), Tsukeng Village (Chiu, 1972).

At Locality 1, Discocyclina occur in a calcareous siltstone layer(�30 cm thick; Fig. 6C; sample TK-1 in Figs. 6D and 7) at the topof a thick, intensively bioturbated sandstone bed (�5 m in thick-ness; Fig. 6A) underlying the shale/sandstone alternation beds(Fig. 7). After weathering, free Discocyclina individuals can bepicked up by hand from the bedding surface of the calcareous silt-stone (Fig. 6B and C). However, Early Miocene larger foraminiferaLepidocyclina, Miogypsina, or Miogypsinoides of the Kungkuan Tufffauna are not found at this outcrop. Although Nummulites were re-ported by Ho et al. (1956) from this locality, we did not find anyspecimen of Nummulites neither in the field nor in thin sections.Discocyclina tests are very thin (0.12 cm), slightly curved in sideview, rounded in plane view, 1.2–1.5 cm in diameter, no deforma-tion at all. They are well preserved even after weathering (Figs. 6and 8). In the overlying shale/sandstone alternation beds, thereare abundant calcareous nannofossils and well-preserved smallerforaminifers without any abrasions or damaged features.

Fig. 5. Exposures observed in an agriculture field south of the Tsukeng Village (A–E) and an outcrop at upper reach of Gully A (F). (A) A general view of the disturbed shale ofthe Latest Oligocene Wuchihshan Formation unconformably overlying the M Eocene Pinglin Tuff; (B) Enlarged view of the irregular contact between the WuchihshanFormation (Uppermost) and the underlying Pinglin Tuff; (C) Tuffaceous sandstone bed in the disturbed Pinglin Tuff; (D) Glauconitic sandstone blocks (g) in the shearedWuchihshan Formation(Uppermost); (E) The Wuchihshan Formation (Uppermost) overlies the Pinglin Tuff; (F) Irregular contact between the Wuchihshan Formation(Uppermost) and the Pinglin Tuff along the upper reach of Gully A north of the Tsukeng Village (circle marks a hammer as scale) .


At locality 2, calcareous sandstones with Discocyclina are ob-served in a small exposure about 200 m north of the water supplystation (Fig. 3). Discocyclina are also found in a siltstone bed over-lying an intensively bioturbated sandstone layer. Individual Disco-cyclina could also be collected by hand from the weatheredcalcareous siltstone bed. At locality 2, several sandy limy blockswith abundant Discocyclina and mollusks scatter along this smallexposure. Preservations of Discocyclina tests at locality 2 are simi-lar to those at locality 1. Thin quartz veins (<1 cm thick) penetratedthe limy blocks.

3.3. Thin section observation

Under stereomicroscope (�60), neither abrasion nor deforma-tion feature is observed on the individual test of Discocyclina(Fig. 8). Discocyclina fossils occur generally parallel or sub-parallelto each other (Fig. 6C). Embryonic chambers and the delicate thinmargins of individual Discocyclina are well preserved (Fig. 8). These

observations strongly suggest that Discocyclina fossils are unlikelyof a displaced thanatocoenosis assemblage nor reworked fauna,but are buried indigenously in a relatively stable and low energyenvironment during an episodic transgression event (Fig. 7).

Microfossils found in 50 thin sections of the Discocyclina-bear-ing calcareous siltstone (sample TK-1, Fig. 7) are listed in Table 1.In addition to algae, mollusks, echinoderm spines and largerforaminifera Discocyclina, well-preserved benthic foraminifers ofGenra Heterolepa, Quinqueloculina, Ammonia, Textularia, and Amph-istegina are observed. They are also found in the overlying shale/sandstone alternation beds. Among the benthic foraminiferal Gen-era found in the thin sections, Ammonia and Amphistegina com-monly live in shallow depth of the East China Sea and theTaiwan Strait (<100 m). In offshore southern Taiwan modernAmphistegina and Heterolepa live in warm water environmentswith annual sea-surface temperature �25 �C (Fan, 1991), the envi-ronment similar to what probable algae-symbiosis larger foramin-ifers like Discocyclina lived in Paleogene tropical-warn temperate

Fig. 6. Field occurrence of Discocyclina at Locality 1: (1a-3): Well-preserved tests of individual Discocyclina; (A) bioturbated Discocyclina-bearing calcareous sandstone bed(5 m in thickness) at Locality 1 about 700 m north of the water supply station; (B) individual tests of Discocyclina from the weathered surface of the bioturbated sandstonebed; (C) Discocyclina occur in a siltstone bed overlying the bioturbated sandstone bed; (D) A total of 11 samples were collected for foraminiferal study. Lithological column ofthis exposure (Samples TK1–11) is shown in Fig. 7.


regions. Therefore their presence is consistent with the indigenousoccurrence of Discocyclina. However, in these thin sections, Mio-cene larger foraminifers of Miogypsina, Miogypsinoides and Lepido-cyclina of the Kungkuan Tuff fauna are never found.

3.4. Morphometric measurements of Discocyclina

The tests of the study discocyclinids materials are thin (lessthan 0.1 cm), saddle shape, up to 1.5 cm in diameter. The cross-diameter (d) of the deuteroconch of free Discocyclina specimenswere measured following the method described in Less (1998)and Özcan et al. (2007). The results (Table 2; deuteroconch dimen-sion, d = 300–460 lm, average 379 lm, based on 11 specimens)show that the specimens are a monospecific species of an interme-diate stage between D. dispansa sella (d = 290–400 lm) and D. dis-pansa dispansa (d = 400–520 lm). Therefore, it is properlydenominated as Discocyclina dispansa (Sowerby, 1840) ex. interc.sella (d’Archiac, 1850) – dispansa (Sowerby, 1840). In Tethyan scale,the biometric result is referable to Middle Eocene (Lutetian to Bar-tonian; Serra-Kiel et al., 1998; Less and Kovács, 1996; Less et al.,2007; Özcan et al., 2007).

3.5. Smaller foraminifers and calcareous nannofossils

Smaller foraminifers are rather rich in the Discocyclina-bearingcalcareous siltstone and the overlying shale/sandstone alternationbeds at Locality 1 (Fig. 7). In this interval, the benthic foraminifersare shelf fauna predominated by Hanzawaia asterizana (Fichtel &Moll), Heterolepa praecincta (karrer), Heterolepa dutemplei (d’Orbi-gny), Gaudryina hayasakai Chang, Baggina saitoi Asano & Murata,Nonionella kankouensis Chang, Planularia okinoshimaensis Murata,Cribrostomoides yeni Chang and species of Genera Quinqueloculina,Cyclammina and Bathysiphon. They are commonly found in the Oli-gocene Shihtsaoan sequences in the Hsüeushan Range, northernTaiwan (Chang, 1962; Huang, 1984). However, new elements ofbenthic foraminifers like Heterolepa cf. solida (Terquem) and Karr-eriella? sp., that occur restrictively in the early Eocene and Paleo-cene strata in the East China Sea (Wang et al., 1989) – TaiwanStrait (Huang, unpublished) are also found, representing a faunaltransition from the typical Paleocene–Early Eocene fauna to thetypical Oligocene Shihtsaoan fauna.

The shale/sandstone alternation beds overlying the Discocycli-na-bearing siltstone also contain some calcareous nannofossils

Fig. 7. Lithological column, depositional environment, sample horizons of Discocy-clina-bearing sandstone and the overlying strata at the fossil locality 1 along GullyA, 700 m north of Tsukeng village.


(Table 3; Fig. 9). However, the nannoflora represent a mixedassemblage of various ages, including the Late Cretaceous species:Micula decussata, Ceratolithoides aculeus, Cretarhabdus conicus, Pre-discosphaera cretacea, Watznaueria barnesae, Braarudosphaera bige-lowii; the Paleocene–Early Eocene species (NP5–NP12): Toweiuspertusus, T. eminens, Discoaster multiradiatus; Early Eocene species(NP10–15): Tribrachiatus bramlettei, T. orthostylus, Discoaster lodo-ensis, D. cf. distinctus; some species restricted to Eocene: Discoasterbarbadiensis (NP10–20), Sphenolithus radians (NP11–19), Discoastersaipanensis (NP15–20), and some wide-range species with theirlowest occurrence in Eocene (Sphenolithus moriformis, Ericsonia for-mosa, Micrantholithus spp., Cyclicargolithus spp, Reticulofenestraspp., Zygrhablithus bijugatus). The collective assessment of theoccurrence of these nannoflora suggests a Middle Eocene age(Zones NP14–15).

4. Discussions

4.1. Significance of indigenous Discocyclina and the revisedstratigraphy of ‘‘Tsukeng Formation’’

The establishment of the indigenousness of Discocyclina dispan-sa ex. interc. sella-dispansa fossils in the Tsukeng anticline is impor-tant because it would indicate that the lower unit of the ‘‘TsukengFormation’’ is Middle Eocene age instead of the Early Miocene ageas it was suggested previously (Ho et al., 1956; Ho, 1961). Thiswould prove a definite occurrence of marine Eocene strata in theWestern Foothills. The excellent preservation of the Discocyclina,occurrences of Eocene smaller and larger benthic foraminifersand calcareous nannoplanktons in the Tsukeng anticline providedirect and strong evidence that these fossils were buried in situin a stable and low energy environment during an episodic trans-

gression in Middle Eocene (Fig. 7). Lacks of Miocene indices likelarger foraminifera Miogypsina, Miogypsinoides and Lepidocyclinaand smaller foraminiferal species of Gaudryina pseudohayasakaiChang and G. kokuseiensis Ishizaki in the axial part of the Tsukenganticline, also suggest that the ‘‘Tsukeng Formation’’ is not Mioceneage (Huang, 2009; Huang et al., 2012). Thus, the stratigraphic nameof the so-called Miocene ‘‘Tsukeng Formation’’ should be aban-doned (Fig. 2; Huang et al., 2012). By study of both planktic andbenthic foraminifers, especially the evolutionary lineage of Gaudry-ina hayasakai–G. pseudohayasakai–G. kokuseiensis integrated withspotty occurrences of planktic foraminifers in the strata exposedalong the Pinglin River section, Huang et al. (2012) have docu-mented that the upper unit of the ‘‘Tsukeng Formation’’ is actuallycoeval with the latest Oligocene strata of either the uppermostWuchihshan Formation in the Western Foothills or the uppermostShuichangliu Formation in the Hsüehshan Range, northern Taiwan(Huang, 2009), while the middle unit of thick tuff beds (named asthe Pinglin Tuff; Huang et al., 2012) unconformably underlain be-low the Wuchihshan Formation (uppermost part) is Late MiddleEocene age. The Pinglin Tuff bears calcareous nannoplanktons ofZone NP16 and zircon grains in the upper part of the Pinglin Tuffwas determined to be 38.8 ± 1.0 Ma (Fig. 10). These microfossilsand age dating of the Pinglin Tuff (Late Middle Eocene) is a litteryounger than the underlying the Discocyclina-bearing lower unit(Zones NP14–15). Similar Middle Eocene tuff/basaltic volcanic lay-ers have been drilled within the middle Eocene shallow marineshale sequences of the Penghu Basin and Taihsi Basin offshore Tai-wan Strait (Hsiao et al., 1991a). The lower unit of the ‘‘Tsukeng For-mation’’ is certainly not Miocene. Instead, biometricmeasurements of Discocyclina dispansa ex. interc. sella-dispansa(late Lutetian to Bartonian age) and occurrence of middle Eocenenannoplanktons (Zone NP14–15) indicate that the Discocyclina-bearing sequences exposed in the Tsukeng anticline axis, WesternFoothills, would represent a Paleogene lithological unit hithertounknown. It is herein named as the Chungliao Formation (Fig. 2).

Discocyclina distribute globally in Eocene sequences of the Eur-ope, Middle East, North America, Turkey (Less and Kovács, 1996;Özcan et al., 2007), Tibet (Wan, 1990), the Western Pacific includ-ing in the Indonesian and Philippine archipelagos (Hashimotoet al., 1979), the Ryukyu Islands (Ujiie and Miyagi, 1973) and theEast China Sea subsurface sequence (Wang et al., 1989). Amongthe various species of Discocyclina, D. dispansa has a wide geo-graphic distribution from the Europe to the Fiji Island in the Pacific(Özcan et al., 2007). In Taiwan Discocyclina were documented fromthe Eocene sequences unconformably underlain below the Mio-cene strata in the southern Central Range (Yabe and Hanzawa,1930; Chang, 1972; Huang, 1980b; Chu et al., 1995), a scenariosimilar to what we present here in the Western Foothills, centralTaiwan (Huang et al., 2012).

4.2. Comparison of calcareous nannofossil study on the ‘‘TsukengFormation’’

Calcareous nannofossil assemblages in the Tsukeng anticline se-quences were first reported by Chi (1979) who claimed that the‘‘Tsukeng Formation’’ was late Oligocene age. Accordingly, therewas no significant unconformity within the ‘‘Tsukeng Formation’’.However, Chi (1979) indicated that his samples were collectedfrom four horizons: Horizon (I) shale dominant unit above the tuffunit (samples B1–B3; Late Oligocene; Fig. 3); Horizon (II) shale be-tween tuff unit and Discocyclina-bearing sandstone unit (samplesA2–A5, Late Oligocene; Fig. 3); Horizon (III), Discocyclina-bearingsandstone (samples D1–D3, mixed ages of Late Oligocene, Eocene,Paleocene and Cretaceous; Fig. 3); and Horizon (IV), reddish tuffunit near the Tsukeng anticline axis (C1, C2 and C3 from the Pinglinriver section in the north and C5, C6 and C7 from Tsukeng river

Fig. 8. Embryonic chambers and equatorial chamberlets in the equatorial layer of Discocyclina dispansa ex. interc. sella-dispansa in the Chungliao Formation (upper fourfigures) and thin sections of Discocyclina-bearing siltstone (lower two figures).


section in the south; mixed ages of Oligocene, Eocene, Paleoceneand Cretaceous; Fig. 3). Although his samples were all assignedto the same age of late Oligocene (Zones NP24/NP25), there aretwo distinct calcareous nannoplanktic assemblages (Table 4): Fos-sil assemblage A restrict of Late Oligocene age, and Fossil assem-blage B of mixed Late Oligocene (very rare-absence), Eocene(predominant), Paleocene (rare) and Cretaceous (rare) nanno-planktons. If the three units of the ‘‘Tsukeng Formation’’ weredeposited in the same age of Late Oligocene (Zones NP24/25) asit was claimed by Chi (1979), why there were two different fossilassemblages occurred in the same ‘‘Tsukeng Formation’’ (Hoet al., 1956) in a short age range (Zones NP24/25)?

Because Chi (1979) did not properly separate the ‘‘Tsukeng For-mation’’ into different stratigraphic units, his results becameambiguous. In view of our new study, Chi’s records and interpreta-

tions should be re-examined according to the new established stra-tigraphy as summarized in Table 4. Excluding those uncertaintytaxa and samples without definitely identifiable fossils from Chi’s(1979) fossil chart, samples with fossil assemblage A were thosecollected from the uppermost Wuchihshan Formation (Fig. 3 andTable 4) including samples of B2 of Horizon I and A1, A2, A3, A5of Horizon II, whereas samples with fossil assemblage B wereeither collected from the Pinglin Tuff (Fig. 3 and Table 4; includingsamples of C1–C3 and C5, C6 of the Horizon IV) or from the Chun-gliao Formation (Fig. 3 and Table 4; samples of D1–D3 of HorizonIII and sample C7 of Horizon IV). The uppermost Wuchihshan For-mation is characterized by occurrence of Sphenolithus ciperoensisBramlette & Wilcoson and Triquetrorhabdulus carinatus Martini(Zone NP24–25; Table 4) and lack of any old fossil (Fossil assem-blage A, Table 4). In contrast, the Pinglin Tuff and the Chungliao

Table 1Distributions of fossils observed in thin sections of Discocyclina-bearing calcareous siltstone.

Discocyclina Heterolepa Quinqueloculina Ammonia Textularia Amphestigina Mollusks Ech. spines

1 Rich Rare2 Rich Rare Rare3 Rich4 Rich5 Rich Rare6 Rich Rare Rare Rare7 Rich8 Rich Rare9 Rich Rare

10 Rich Rare11 Rich Rare12 Rich Rare13 Rich Rare Rare14 Rich Rare15 Rich16 Rich17 Rich18 Rich19 Rich20 Rich21 Rich Rare22 Rich23 Rich Rare Rare24 Rich25 Rich Rare Rare Rare26 Rich Rare27 Rich Rare Rare28 Rich Rare Rare29 Rich30 Rich Rare31 Rich Rare32 Rich Rare33 Rich Rare34 Rich35 Rich36 Rich Rare37 Rich Rare38 Rich Rare39 Rich Rare40 Rich Rare41 Rich Rare42 Rich43 Rich Rare Rare44 Rich Rare Rare Rare45 Rich46 Rich Rare47 Rich48 Rich49 Rich50 Rich

Table 2Results of biometric measurements on embryonic chambers of Discocyclina dispansa ex. interc. sella-dispansa in the Chungliao Formation of the Tsukeng anticline axial part, nearNantou.

Sample no P1 D1 N 0.5 H W h w Taxon

DIS-1 340 14 45–55 30–50 50–55 30–40 Discocyclina ex. interc. sella-dispansaDIS-2 160 330 11 55–70 30–50 55–60 20–45 Discocyclina ex. interc. sella-dispansaDIS-3 140 315 11 40–70 50–85 Discocyclina ex. interc. sella-dispansaDIS-4 190 385 Discocyclina ex. interc. sella-dispansaDIS-6 200 375 11 50–70 25–35 Discocyclina ex. interc. sella-dispansaDIS-8 260 460 12 Discocyclina ex. interc. sella-dispansaDIS-9 190 435 11 Discocyclina ex. interc. sella-dispansaDIS-10 200 430 11 45–75 Discocyclina ex. interc. sella-dispansaDIS-11 180 460 13 Discocyclina ex. interc. sella-dispansaDIS-12 195 375 11 55–70 Discocyclina ex. interc. sella-dispansaDIS-13 430 Discocyclina ex. interc. sella-dispansa


Formation contain very rich Eocene species in association withsome old reworked Paleocene and Cretaceous fossils of the assem-

blage B (Table 4). Because (1) our new study did not find any def-inite Oligocene indices of nannoplanktons in the Pinglin Tuff

Table 3Distributions of calcareous nannoplanktons in the shale/sandstones above the Discocyclina-bearing unit of the Chungliao Formation. The list marked by gray scale represents thelate Mesozoic species.

Age Eocene (NP14–15)

Sample 1A 11 9 8 7 6 5 4 3 2

Cocolithus pelagicus (Wallich) r r r r r r r r rCoccolithus sp. r r

Sphenolithus moriformis (Bronnimann & Stradner) r r r

Sphenolithus radians (Deflandre) rCyclococolithina formosa (Kamptner) r r r r

Cyclicargolithus floridanus (Roth & Hay) r

Cyclicargolithus sp. r r rDiscoaster barbadiensis Tan r r r r r r r r rDiscoaster saipanensis Bramlette & Riedel r r r rDiscoaster cf. distinctus Martini r r rDiscoaster lodoensis Bramlette &Riedel r rDiscoaster cf. lodoensis Bramlette & Riedel rDiscoaster multiradiatus Bramlette & Riedel r r rTribrachiatus orthostylus (Stradner) r

Tribrachiatus bramlettei (Bronnimann & Stradner) r

Reticulofenestra umbilica (Levin) r r rReticulofenestra spp. (medium) r rToweius pertusus (Sullivan) rToweius eminens (Bramlette & Sullivan) rToweius craticulus Hay & Mohler rZygrhablithus bijugatus (Deflandre) r rMicrantholithus spp. r rMicrantholithus flos Deflandre rLithostromation operosum (Deflandre) rBraarudosphaera bigelowii (Gran & Braarus) r r rNeococolithues dubius (Deflandre) rWatznaueria barnesae (Black) rPrediscosphaera cretacea (Arkhangelsky) rCretarhabdus conicus Bramlette & Martinil rCeratolithoides aculeus (Stradner) rMicula decussata Vekshina r


(Huang et al., 2012) or the Chungliao Formation (Table 2), and (2)the fossil assemblage B in the Pinglin Tuff and Chungliao Formationare similar to each other, we concluded:

(1) The youngest fossil age of Middle Eocene (Zones N14–15)could represent the true deposition age of the Chungliao For-mation, whereas the Pinglin Tuff contains Dictyococcitesbisectus (Hay, Mohler and Wade), which occur from ZoneNP16 to Zone NP25, is assigned as Zone NP16 (Huanget al., 2012). The old nannoplanktons of the Paleocene andCretaceous ages in the Chungliao Formation would be thereworked ones;

(2) The late Oligocene age of the uppermost Wuchihshan For-mation should not be confused with the middle Eoceneage of the Pinglin Tuff and the Chungliao Formation;

(3) The Early Middle Eocene age of the Chungliao Formationwith calcareous nannofossils of Zone NP14–15 is consistentwith an independent study of age determination of zircongrains (38.8 ± 1 Ma by using ICPMS U-Pb dating method;Fig. 10) and discovery of Zone NP16 calcareous nannofossilsin the overlying Pinglin Tuff (Huang et al., 2012).

4.3. Significance of the reworked Paleocene and Cretaceousnannoplanktons in the Pinglin Tuff and Chungliao Formation

In the Hsüehshan Range, near Nantou of central Taiwan, Huang(1980c) also found the poorly-preserved Mesozoic, Paleocene andearly Eocene calcareous nannofossils mixed with the youngestMiddle Eocene calcareous nannofossils of Zones NP14–15 in thesandstone–siltstone sequence, where the Eocene larger foraminif-era Assilina were found (Yen, 1973). Therefore, it could be a com-

mon phenomenon that during the Middle Eocene there wereMesozoic – Early Tertiary strata subaerially exposed as sourcesfor erosions. However, there are no definite Paleocene and Creta-ceous strata onland Taiwan. The possible sources to provide theseold Mesozoic, Paleocene and Early Eocene fossils to the Middle Eo-cene Pinglin Tuff and the Chungliao Formation could be:

(1) The Cretaceous-Paleogene hosts west of Tsukeng on thePeikang Basement High beneath the Coastal Plain onland centralTaiwan or hosts closed to the Taihsi Basin – Penghu Basin in theTaiwan Strait (Fig. 1; Hsiao et al., 1991a; Yang et al., 1998), wherethe Paleogene rift basins filled with Eocene–Paleocene shallow-marine strata and pyroclastics developed unconformably overlyingthe pre-rift shallow-marine Cretaceous basement (Matsumotoet al., 1965; Sun, 1982; Huang, 1978; Huang and Chi, 1979); and/or (2) The Paleocene–Early Eocene syn-rift sequences in the Ou-Jiang Depression, southern East China Sea (Wang et al., 1989), orthe Tungyintao Basin, northern Taiwan Strait (Hsiao et al.,1991b). When the Paleogene rift basin was uplifted due to thermaleffect, part of the early syn-rift sequence of the Paleocene age andthe Cretaceous pre-rift basement could be exposed for such a sub-aerial erosion (Huang, 1980b) and then re-deposited in the MiddleEocene syn-rift strata.

4.4. Significance of the Eocene strata in the Western Foothills

Based on multiple microfossil evidence, the present study doc-uments the occurrence of Eocene marine strata exposed in theWestern Foothills, Central Taiwan. The strata exposed in the axialpart of the Tsukeng anticline represent an Early Tertiary rift marinebasin (named as the Nantou basin) whose sequence is comparableto what have been reported from the East China Sea-Taiwan Strait

Fig. 9. Calcareous nannoplanktons in the Chungliao Formation: 1. Sphenolithus spiniger Bukry, sample TSK-6; 2. Reticulofenestra umbilica (Levin), sample TSK-1; 3.Cyclococolithina formosus (Kamptner), sample TK-1; 4. Neococolithues debius (Deflandre), sample TSK-6; 5. Neococolithes debius (Deflandre), sample TK-1; 6. Cocolithus pelagicus(Wallich), sample TK-2; 7. Sphenolithus spiniger Bukry, sample TSK-6; 8. Zygrhablithus bijugatus (Deflandre), sample TSK-6; 9. Braarudosphaera begelowii (Gran & Braarus),sample TSK-6; 10. Sphenolithus radians Deflandre, sample TK-6; 11. Neococolithes debius (Deflandre), sample TSK-6; 12. Micrantholithus flos Deflandre, sample TSK-6.

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 P

b/23

8 U

data-point error ellipses are 68.3% conf.

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

MSWD = 1.8

Fig. 10. U–Pb age dating of zircon grains of the Pinglin Tuff (after Huang et al.,2012).


and below the Coastal Plain above the Peikang Basement Highonland central Taiwan (Huang et al., 2012). The uppermostWuchihshan Formation and the overlying Miocene strata markthe post-rift sequence that unconformably overlies the Eocenesyn-rift sequence of the Pinglin Tuff and Chungliao Formation(Fig. 2; Huang, 2009; Huang et al., 2012). The unconformity be-tween the uppermost Wuchihshan Formation and the Pinglin Tuffmarks a regional break-up unconformity between the overlyingpost-rift and the underlying syn-rift sequences. This break-upunconformity was developed during the early opening phase ofthe South China Sea oceanic lithosphere (Huang et al., 2012).Comparison of the passive margin stratigraphy in the WesternFoothills-Hsüehshan Range and the subsurface stratigraphy inthe East China Sea-Taiwan Strait indicates that the Nantou extensionbasin should not be the only Early Tertiary rift basin onland Taiwan.More detailed stratigraphy study of the Western Foothills, Hsüeh-shan Range and even the Central Range could discover more Paleo-gene rift sequences onland Taiwan (Huang et al., 2012). This wouldgreatly expand our knowledge about the geohistory of Taiwan.

Table 4New interpretation of Chi’s (1979) calcareous nannofossil records according to the new revised stratigraphy in the Tsukeng anticline of the Western Foothills, central Taiwan.

Chi’s (1979) results and interpretation New interpretationThis paper

Group, Samples and Stratigraphy Main calcareous nannofossils Assigned age Formation in Fig. 2 Assigned age

Horizon I Samples B1 to B3 Oligocene Late Oligocene Wuchihshan Formation(Uppermost)

Late Oligocene

Horizon II Samples A2 to A5 Oligocene Late Oligocene Wuchihshan Formation(Uppermost)

Late Oligocene (NP25)

Horizon III, Samples D1 to D3 Mixed fossils of Oligocene, Eocene (major)Paleocene and Mesozoic ages

Late Oligocene Chungliao Formation Middle Eocene (NP14–15)

Horizon IVC1, C2, C3 (Pinglin River) C5–C6(Tsukeng River)

Mixed fossils of Late Oligocene (?),Eocene, Paleocene, Mesozoic

Late Oligocene Pinglin Tuff Middle Eocene (NP16)

C7 (Tsukeng River) Chungliao Formation Middle Eocene (NP14–15)


5. Conclusions

Field occurrence and faunal preservation status indicate thatthe Discocyclina dispansa ex. interc. sella-dispansa in the newly-named Chungliao Formation of Early Middle Eocene age (calcare-ous nannoplanktic Zones NP14–15) in the Western Foothills nearNantou, central Taiwan, were buried indigenously instead of beingreworked in the Miocene strata. The Discocyclina appear in a bio-turbated calcareous siltstone, which is on top of the bioturbatedsandstones but is overlain by shale/sandstone alternations duringan incipient transgression episode. They are well-preserved with-out any abrasion or deformation. The Chungliao Formation is over-lain by the Pinglin Tuff, representing the Paleogene syn-riftsequence on the Peikang Basement High. The study extends theknowledge of the geohistory of the Western Foothills from the lateOligocene to the Eocene Period. The result of this study suggeststhat a Paleogene extension basin (Nantou Basin) exists beneaththe Late Oligocene–Miocene post-rift sequence in the WesternFoothills. The occurrence of the Paleogene Nantou rift basin inthe Western Foothills may also suggest that there could have sim-ilar Paleogene rift sequences in other parts of the Taiwan mountainbelt like the Hsüehshan Range and the Central Range east of theWestern Foothills.

Acknowledgements

The authors appreciate Dr. Mark Jiang for identification of cal-careous nannofossils. We also thank two reviewers Dr. Xiao-QiaoWan of the China University of Geosciences and Dr. Ercan Özcanof the Istanbul Technical University for their critical review andconstructive comments and suggestions the morphometric studyof megalospheric Discocyclina. This study was supported by grantsfrom the Chinese Petroleum Company, Taiwan (91-F0107-5-7) andNSFC Grants (#91128211) to Huang and (#40730844) to Zhao.

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