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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
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Early post-rift sequence stratigraphy of a Mid-Tertiary rift basin in Taiwan: Insightsinto a siliciclastic fill-up wedge
Neng-Ti Yu a,⁎, Louis S. Teng b, Wen-Shan Chen b, Li-Fan Yue b,c, Mien-Ming Chen d
a Department of Applied Science, National Hsin-Chu University of Education, 521, Nanda Road, 300, Hsin-Chu City, Taiwanb Department of Geosciences, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei, 10617, Taiwanc Southern Africa SBU, Chevron Africa/Latin America Expl & Prod Co., Rm 38-070, 1400 Smith Street, Houston, TX 77002, USAd Central Geological Survey, MOEA, ROC, 2, Ln.109, Huaxin St., Zhonghe Dist., New Taipei City, 235, Taiwan
a b s t r a c ta r t i c l e i n f o
Article history:Received 16 July 2012Received in revised form 25 December 2012Accepted 28 December 2012Available online 11 January 2013
Editor: B. Jones
Keywords:Early post-rift wedgeSequence stratigraphyTaiwanLower MioceneFilling-up processEustatic sea level control
In order to scrutinize the development, process and control of an early post-rift siliciclastic fill-up wedge,sequence stratigraphy was applied to the Oligo-Miocene coastal-shelf strata of a mid-Tertiary rift basin inTaiwan.Four sequences at the million-year/3rd-order scale, constrained by microfossil biohorizons and radioactivedates, were identified and correlated regionally. The lower sequence belongs to the fault-bounded Oligocenesyn-rift wedge. The middle two comprise the lower Miocene early post-rift wedge, onlapping onto theMesozoic basement highs. The upper sequence is the lower Miocene late post-rift drape on the continen-tal margin.The middle two sequences record a process of active topographic transformation through successive coastal-shelf progradations. At first, the remnant rift topographic low received drastic progradations from both theinner and outer highs. During the later progradations principally from the inner high, the topographic lowwas filled up for the successive drape.This wedge formation was characteristically short-lived and high in sedimentation rate, when accommoda-tion space and sediment supply both increased. The initial accommodation space of the remnant rift deepshelf was augmented by rapid thermal subsidence and long-term eustatic sea level rise. The sediment supply,in spite of a decrease in provenance exposure due to onlapping, was promoted by periodic exposures due to3rd-order eustatic sea level falls, and additionally by a warm and humid climate. These falls are age concor-dant with the sequence boundaries and thus evidently important for the sequence boundary formations andsediment dispersal.
Early post-rift deposits, onlapping onto basement shoulder highs,form a saucer-shaped rock body of parallel strata in rift basins dur-ing the transition from active faulting to thermal cooling (Whiteand McKenzie, 1988; Favre and Stampfli, 1992; Alves et al., 2003;Contreras et al., 2010). The stratal architecture may also vary, de-pending on the interplay between accommodation space and sed-iment supply, from a deepwater fine-grained infill to a shallowing- andcoarsening-upward fill-up wedge (Prosser, 1993; Nottvedt et al., 1995;Ravnas and Steel, 1998).
Although the early post-rift architecture is widely known, its devel-opment and control are less well understood, in particular regardingthe progradational fill-up wedge. Firstly, how the depositional system
and basin bathymetry transform into a smoothed depositional profileduring the wedge formation has been rarely demonstrated. The fine-grained infill has recently been shown composed of aggradational-stacking hemi-pelagic sequences with minor variations by a passiveinfilling process and a dominance of remnant rift topography in theEarly Cretaceous Norwegian North Sea (Zachariah et al., 2009). Second-ly, the role of sediment supply in the progradational wedge formation isless underscored. Usually, the progradation is attributed to the decreasein accommodation space that results from the subsidence decelerationfrom the syn-rift fault-controlled to the post-rift thermal-cooling set-ting (e.g., Simpson and Eriksson, 1989; Dinis and Trincao, 1995; Alveset al., 2003; Contreras et al., 2010). It is, however, quite common forthe accommodation space to remain and broaden, due to thermalsubsidence and also to residual faulting or base level rise (e.g., Dupreet al., 2007; Rosas et al., 2007; Contreras et al., 2010). The sediment sup-ply is, in contrast, confronted by the unfavorable condition of a decreasein provenance exposure and relief due to onlapping and prolonged
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syn-rift exhumation, respectively (Simpson and Eriksson, 1989; Ravnasand Steel, 1998; Persano et al., 2005; Burton and Wood, 2010). Theroles of tectonic uplift and climate have been rendered important in
sediment supply during the formations of breakup unconformity andthe fill-up carbonate wedge (e.g., Michalzik, 1991; Dinis and Trincao,1995; Rosas et al., 2007).
Fig. 1. Regional geology of Taiwan. (A) Geological map with locations of studied sections (outcrop in square and well log in circle). Inset shows the convergent boundary betweenthe Eurasian Plate and the Philippine Sea Plate. (B) Geological transect of NE Taiwan (vertically exaggerated).
In order to better constrain the development, infilling process andcontrol of the early post-rift fill-up wedge in a siliciclastic setting, wetake advantage of a well-exposed mid-Tertiary marine rift basin inTaiwan (Fig. 1A). The extensive outcrops and well logs of this coastalto shallow marine basinfill provide a wealth of facies and stratigraphicconstraints for applying sequence stratigraphy to specifying the earlypost-rift depositional system, stratigraphic architecture, and relativesea level change (i.e. sedimentary responses to accommodation space/sediment supply interplay). In this paper, we first present the faciesand sequence characteristics of the rift-drift transition deposits. Twostratigraphic transects are, afterward, synthesized so as to demonstratefacies distribution and sequence architecture in dip direction for deduc-ing the wedge formation, in terms of depositional and topographictransformation. Based on these results, as well as on other lines of evi-dence, we further evaluate the controls on the accommodation space/sediment supply interplay.
2. Tectonic setting
Taiwan is an orogenic island located on the convergent bound-ary between the Eurasian Plate and the Philippine Sea Plate (insetmap in Fig. 1A). It is the result of a collision between the SE Chinacontinental margin and the Luzon Arc (Suppe, 1981; Teng, 1990).The collision started in the late Miocene and has pushed upNNE–SSW trending mountain ranges and foothills on the island(Fig. 1A). The corresponding lithospheric flexure has pulled downthe foreland basin in the west, from the coastal plain to the TaiwanStrait.
Prior to the collision tectonics, the SE China continental marginwas subjected to episodic extension and rifting dating from the lateCretaceous (Zhou et al., 1995; Ren et al., 2002). A series of marinerift basins has been created and has accumulated thousands of metersthick siliciclastic deposits. One of these basins, termed the ‘HsuehshanTrough’, has been partly deformed by the collision and exhumed inthe orogenic mountain belts of Taiwan (Fig. 1B; Teng et al., 1991;Lin et al., 2003).
The Hsuehshan Trough was a NE–SW striking half graben with themaster boundary fault system in the east, namely the Lishan Fault(Figs. 1, 2A). The footwall/outer high of the metamorphic Mesozoicbasement on the southeast is outcropped in Backbone Range. Thehanging-wall/inner Mesozoic high in the northwest remains buriedintact in the foreland basin, mostly in the Taiwan Strait. Based on aretrodeformed structural transect through northern Taiwan, the basinwas about 250 km wide before some 200% shortening in the process ofmountain building (Suppe, 1980; Teng et al., 1991).
The reconstructed Hsuehshan Trough is characterized by a three-fold, sandstone–mudstone–sandstone motif from the upper Eoceneto the lower Miocene (Fig. 2A; Teng et al., 1991; Teng and Lin,2004). During the late Eocene rift initiation (early syn-rift), con-glomeratic fluvial-coastal deposition dominated in the west of theLishan Fault, while the outer high on the east was actively rising.Through the Oligocene rift climax (syn-rift), a wedge of offshore-mud domination developed in the rapidly subsiding half graben.During the Miocene post-rift stage after the end of Lishan Fault activ-ity, the entire continental margin subsided, including the basementhighs. It was covered by coastal to offshore sandstones and mud-stones that extended northwesterly to the present-day SE Chinacoastline.
3. Rift-drift transition stratigraphy
The upper Oligocene to lower Miocene strata represent the rift-drift transition deposition of the Hsuehshan Trough (Fig. 2A). Theycomprise a saucer-like rock body that thickens up to 1000 m in thebasin center around Western Foothills and Hsuehshan Range, andthins toward the inner and outer basement highs (Chou, 1974; Lee,
1990; Huang and Lee, 1992). These strata are mostly stacked intoa mega sequence of upward coarsening and then fining in terms ofsandstone content (Fig. 2B). Above the basement highs, the megasequence is only fining upward.
The mega sequence is commonly divided into three rock unitsthat can be regionally correlated, including a lower and an uppermudstone-dominated unit, and a sandstone/mudstone interbeddedunit in the middle (Fig. 3). The lower mudstone-dominated unit isconfined to the east by the Lishan Fault, and grades northwesterly intoa conglomeratic sandstone/mudstone interbedded lithology (Fig. 2).The middle interbedded unit is traceable to the lower part of thefining-upward mega sequence above the basement highs (Fig. 2B).This lithostratigraphic correlation is based on the sandstone-rich natureof these strata, and on the commonoccurrences of basaltic volcanics andcoal seams. The upper mudstone-dominated unit is the most laterallycontinuous in this continental margin, and extends northwesterlyafter the disappearance of the middle unit (Fig. 2A). It is characterizedby coquina and glauconitic beds, marking a major regional marineflooding event (Fig. 3).
Owing to common shallow marine fossils, biostratigraphic divi-sion is also possible in the three rock units, such as mollusks, fora-minifers, and coccoliths (Fig. 2). Biozones of planktic and benthicforaminifers, and calcareous nannofossils have been established,and bear the most significant correlativity on regional scale onaccount of their extensive occurrences (Chi, 1981; Huang, 1982,1986; Huang and Cheng, 1983). Age delineation is further derivedfrom the planktic foraminifer and nannofossil biozones, by compar-ing the biozones with the global geological time scale. The timescale of Berggren et al. (1995) is followed for this study, becausethe NP25/NN1 biozone boundary is identified by the last occurrencedatum plane of Sphenolithus ciperoensis, Dictyococcites bisectus andZygrhablithus bijugatus rather than that of S. delphix in the laterscale of Gradstein et al. (2004).
Accordingly, a Late Oligocene age for the lower unit is based on theNP25 and P22 biozones. An Early Miocene age for the upper unit isbased on the NN2 and N5 biozones. The transition from Oligoceneto Miocene for the middle sandstone/mudstone interbedded unitis therefore indicated. This is also supported by the occurrences ofthe NN1 and N4 fossil assemblages in the middle unit. This overallbiochronostratigraphic framework is consistent with the age esti-mates derived from basaltic volcanics (23 to 19 Ma, zircon fissiontrack method; Chung et al., 1994), and from the sandstones abovethe outer basement high (26.4±1 Ma, detrital zircon U\Pb method;Feng, 2011).
4. Facies characteristics
A dominance of wave-related processes with tidal influences havebeen demonstrated in previous facies studies of the three rock units(Chou, 1974; Lee, 1990; Huang and Lee, 1992; Yue and Teng, 2000).By reexamining key outcrop sections and well logs (Fig. 1A), awave-dominated coastal to shelf system is here proposed for theOligo-Miocene deposition. Three facies associations of shoreface, bar-rier and lagoon are identified for the wave-dominated coastal to near-shore deposition. Two associations of offshore and offshore transitionare identified for the storm-wave-related shelf deposition. Details ofthese associations are listed in Fig. 4 and Table 1.
4.1. Offshore
4.1.1. DescriptionThis association is made up of massive-bedded mudstone with
thin beds of siltstone, fine sandstone and coquina (Figs. 4A, 5A). Thesiltstone and sandstone beds are planar and sharp-based with wavylamination. Bioturbation is moderate to intense by trace fossils ofthe Cruziana ichnofacies, such as Planolites, Chondrites, Scolicia and
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Zoophycos (Fig. 5B). Microfossils are abundant, including an outer shelfto bathyal benthic foraminifer assemblage of Lenticulina, Bathysiphon,Pullenia and Uvigerina (Chang, 1960; Huang and Cheng, 1983; Huang,1986).
4.1.2. InterpretationThe relatively deepwater microfossils and the Cruziana ichnofacies
indicate an open shelf environment. The mudstone is principally attrib-uted to suspension fallout in quiescent periods,while the sandstone and
Fig. 2. Stratigraphy of the mid-Tertiary Hsuehshan Trough rift basin. (A) Retrodeformation of the geological transect in Fig. 1B showing the full basinfill, modified from Suppe (1980)and Teng et al. (1991). The control sections (outcrop in square and well log in circle) are projected to the transect, according to their positions in the orogenic belts and forelandbasin. See locations of the control well logs and outcrops in Fig. 1A. (B) Oligo-Miocene rift-drift transition stratigraphy of the Hsuehshan Trough. Dashed lines mark the key strat-igraphic correlation. See location in Fig. 2A.
siltstone record deposition of low-energy storm-related processes. A rel-atively deep shelf environment close to storm wave base is interpretedfor the association.
4.2. Offshore transition
4.2.1. DescriptionThis association is composed of thick-bedded sandstone/mudstone
interbeds, very thick- to massive-bedded sandstone, and thin-beddedcoquina (Fig. 4B). Upward coarsening sequence from the sandstone/
mudstone interbeds to the massive-bedded sandstone is frequentlyencountered. Hummocky cross stratification (HCS) and planar andwavy lamination are common (Fig. 5C-D). Bioturbation is usuallymoderate to intense by diverse ichnofabrics of the Cruziana ichnofacies,including Thalassinoides, Skolithos, Planolites, Macaronichnus, Teichichnusand Gyrochorte. Larger benthic foraminifers are present, such asMiogypsinoides, Miogypsina and Lepidocyclina (Chi, 1981). Microfossilcontent is low, including a littoral to neritic benthic foraminifer assem-blage of Textularia, Hanzawaia and Ammonia (Chang, 1960; Huang andCheng, 1983; Huang, 1986).
Fig. 3. Stratigraphic division and correlation of the Oligo-Miocene strata of the Hsuehshan Trough. See locations of Outcrop 15 and Well 13 in Fig. 1A.
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4.2.2. InterpretationThe Cruziana ichnofacies and foraminifer assemblages indicate a
shallowneritic environment. The sandstone/mudstone interbeds recordsand deposition in major storm periods and subsequent mud settling infair weather periods. Themassive-bedded sandstone is attributed to de-position of similar processeswith abundant sand supply. A shallow shelfenvironment subject to intense and frequent storm-wave processes isinterpreted for the association.
4.3. Shoreface
4.3.1. DescriptionThis association is dominated by massive-bedded, well-sorted,
coarse- to fine-grained sandstones with very thick- to massive-bedded muddy sandstone (Fig. 4C). Upward coarsening sequence isobserved from the muddy sandstone to the well-sorted sandstone.Planar stratification, and planar, trough and swaley cross stratifications
Fig. 4. Facies associations and sequence stratigraphic delineation of the Oligo-Miocene strata of the Hsuehshan Trough (SQ: Sequence; UO: Upper Oligocene; 1, 2: Lower Miocene).See geographic location of Well 11 in Fig. 1A and stratigraphic location in Fig. 7A.
are well developed in the well-sorted sandstone (Fig. 5E). The muddysandstone is dominated bywavy laminationwith planar cross stratifica-tion and HCS (Fig. 5F). Bioturbation is slight by trace fossils of theSkolithos ichnofacies, such as Ophiomorpha, Skolithos and Planolites.
4.3.2. InterpretationThe well-sorted and -stratified sandstone reflects traction flows
induced by asymmetric oscillatory fair weather waves. The laminatedmuddy sandstone records low-energy fair-weather-wave depositionand subordinate storm-wave deposition. The Skolithos ichnofaciesindicates a littoral to sublittoral environment. A shoreface environ-ment between mean low tide level and fair-weather-wave base isinterpreted for the association.
4.4. Barrier
4.4.1. DescriptionThis association consists of massive-bedded, well- to medium-
sorted, coarse- to fine-grained sandstones (Figs. 4D, 6A). Upward finingsequence with channel-like basal erosion surface and conglomerate iscommon. Planar stratification, flaser lamination, SCS, and high-anglecross stratification are omnipresent. The foresets of high-angle crossstratification usually contain double mud drapes and form sigmoidalbundles (Fig. 6B). Bioturbation is sporadic, dominated by Ophiomorpha.
4.4.2. InterpretationThe well-sorted and -stratified sandstone, similar to the shoreface
sandstone, reflects fair-weather-wave process. The medium-sortedcross-stratified sandstone, associated with mud-draped foresets andsigmoidal bundles, record deposition of time-velocity asymmetrysubtidal currents. The fining-upward sequence represents abandonedtidal channel fill with a flow velocity decrease from subtidal channelthalweg to adjacent point bar. A mixed wave-and-tidal, barrier-related environment of inlet channel, tidal delta and washover fan isinterpreted for the association (Kumar and Sanders, 1974; Galloway,1986).
4.5. Lagoon
4.5.1. DescriptionThis association is composed of carbonaceous heterolithics of
massive-bedded mudstone and muddy sandstone (Fig. 6C). Wavyand flaser laminations with bi-directional foresets are pervasive(Fig. 6D). Fining-upward sequence is observed, and may grade intomottled mudstone and coal seam with desiccation cracks and rootletstructures (Fig. 6C). The mottled mudstone changes upward in tex-ture from gray to leaching white, and from well laminated to massiveand mottling. Bioturbation is of various extents, by trace fossils ofDiplocraterion, Planolites and Skolithos.
Table 1Facies characteristics of the Oligo-Miocene siliciclastic strata.
Facies association Lithology Electric log shape Sedimentary feature and fossil content
Offshore 1) Dominance of massive-bedded mudstone (3–20 m)2) Thin-bedded interbeds of coquina, siltstone and
fine sandstone (s/m ratio: 0.25–1; interbedthickness: 3–15 cm)
3) Glauconitic pellet enrichment in the siltstoneand fine sandstone
1) Smooth to serrate, slim cylinder2) Resistive streak
1) Wavy lamination in the mudstones2) Wavy lamination, planar and sharp base in the
siltstone and sandstone interbeds3) Moderate to intense bioturbation of Cruziana
ichnofacies4) Common to abundant benthic foraminifers:
Lenticulina, Bathysiphon, Pullenia and UvigerinaOffshore transition 1) Dominance of thick-bedded sandstone/mudstone
3) Wavy lamination dominance in the muddysandstone
4) Common bioturbation of Skolithos ichnofaciesBarrier 1) Dominance of massive-bedded sandstone (3–10 m)
2) Basal conglomerate of rip-upmud clasts and sandstonepebbles (20–100 cm)
3) Common plant fragments and carbonaceous lamina4) Sand size: fine to very coarse
1) Chunky and serrate bell 1) Well-developed and diverse physical structures:planar stratification, trough and planar crossstratification, SCS, wavy and flaser laminations
2) Common sequence from basal erosion surface,stratifications to laminations
3) Common occurrences of herringbone foresets, andreactivation surfaces and double mud drapes onforesets
4) Bioturbation of Skolithos ichnofaciesBackbarrier 1) Dominance of very thick-bedded sandstone and
mudstone (mudstone: 3–10 m, sandstone: 1–5 m)2) Abundant carbonaceousmaterials andplant fragments3) Fining-upward sequence from the sandstone to the
mudstone4) Common occurrences of coal seams (5–60 cm) and
rip-up conglomerates (20–50 cm)5) Sand size: fine to coarse
1) Pervasive wavy, flaser and ripple cross laminationswith herringbone foresets
2) Trough and planar cross stratifications also in thesandstones with mud-draped foresets
3) Common texture transition from laminated tomassive/mottled, and from grayish/fresh toyellowish white or rusty red (30–100 cm)
4) Bioturbation of Skolithos, Planolites and Diplocraterion
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4.5.2. InterpretationThe carbonaceous laminated heterolithics and the bioturbation
are attributed to intertidal deposition of low-energy waves and tidalcurrents, and to suspension fallout in shallow tidal slack water. Themottled mudstone records incipient pedogenesis, possibly due tolowered water table and/or subaerial exposure (Retallack, 1988; Millerand Eriksson, 1999). An intertidal to supratidal, backbarrier environmentof lagoon and wetland is interpreted for the association.
5. Sequence characteristics
Based on the facies associations, the rift-drift transition stratawere predominantly offshore and offshore transition deposits inter-calated with shoreface, barrier, and lagoon deposits. They documentfrequent coastal progradations and retrogradations on a continentalshelf as a result of shoreline shifts driven by relative sea level change.The sequential and lateral variations in facies among these strata are
Fig. 5. Outcrop facies reflecting wave-dominated processes in the Oligo-Miocene strata of the Hsuehshan Trough (the offshore facies in Photos A, B; the offshore transition facies inPhotos C, D; the shoreface facies in Photos E, F). Photo A is from Outcrop 18 and Photos B–F from Outcrop 4. See geographic locations in Fig. 1 and stratigraphic locations in Fig. 7B.(A) Massive-bedded mudstone. (B) Glauconitic sandstone with ichnofabric network of Glossifungites. The base of the sandstone (dashed line) is interpreted as maximum floodingsurface (interpretation in text). (C) Thick-bedded sandstone/mudstone interbeds with well-developed hummocky cross stratification (HCS). The mudstone interbeds are wavy-laminated.(D) Amalgamated HCS, massive-bedded sandstone. Dashed lines outline the hummocky surfaces of the offshore transition facies. (E) Massive-bedded, well-sorted and -stratified sandstonewith cross bedding. (F) Muddy sandstone in well-developed wavy lamination.
demonstrated here by a basinward outcrop section and a landwardwell log (Fig. 7).
Transgressive–regressive (T–R) depositional sequences (cf. Emeryand Myers, 1996) are applied to these strata, in order to delineatethe major trends of these shoreline shifts of various magnitudes andcyclicities. Each T–R sequence is bounded by sequence boundary(SB) of maximum regressive surface, and divided by maximumflooding surface (MFS) into retrogradational/transgressive (TST) andprogradational/regressive systems tracts (RST).
acterizes the SB of maximum regressive surface. Overlying the lagoon,barrier or shoreface deposits of RST, the surface is also superimposedby subaqueous erosion caused by the successive transgression.
The subaerial reworking is recorded by the mottled mudstoneof pedogenesis atop the lagoon sandstone/mudstone heterolithics(Fig. 6C). The unconformable surface atop the Mesozoic basementhighs is also the subaerial-exposed surface of SB. In the outer high,
the surface is associated with an orange weathered zone and an over-lying conglomerate with pebbles of the Mesozoic schist and paleosol(Suppe et al., 1976). In the inner high, the SB is a surface of angularunconformity (Chiu, 1973).
The subaqueous reworking is identified by the Glossifungitesichnofacies of Ophiomorpha, Skolithos, and Thalassinoides that pene-trate down through the erosional surface of SB into the RST barrierand shoreface sandstones (Fig. 8A–D). Filled with pebbly coarsesands, shell fragments, and glauconitic pellets, these trace fossils arerelated to a firm-ground substrate after the removal of surficial un-consolidated sediment (Savrda et al., 2001; Rodriguez-Tovar et al.,2007).
The superimposed transgressive erosion is marked by the upwardfacies deepening across the SB, and by the basal erosion contact of over-lying strata (Figs. 6C, 8A and C). The transgressions are associated withdifferent amounts of upward facies deepening, including from thelagoon heterolithics to the barrier or offshore transition sandstone,and from the shoreface or barrier sandstone to the offshore transitionsandstone. The basal erosion contact of the overlying offshore transitionsandstone is referred to as awave ravinement surface ofwave reworkingprocesses in shoreface/nearshore zone (Nummedal and Swift, 1987;
Fig. 6. Outcrop facies in Outcrop 4 reflecting tidal influences in the Oligo-Miocene strata of the Hsuehshan Trough (the barrier facies in Photos A, B; the lagoon facies in Photos C, D).See stratigraphic locations of photos in Fig. 7B. (A) Cross-stratified and tidal-bundled sandstone (barrier) above HCS sandstone/mudstone interbeds (offshore transition). Dash lineshows the sharp and erosive contact between the two facies, a surface of forced regression (interpretation in text). (B) Close-up of Photo A. Dashed lines mark the sigmoidal tidalbundles. (C) Fining-upward sequence from flaser-laminated sandstone to paleosol of mottled mudstone (lagoon) below HCS sandstone/mudstone interbeds (offshore transition).Dash line shows the sharp and erosive contact between the two facies, a surface of sequence boundary (interpretation in text). (D) Flaser-laminated sandstone with bi-directionaland mud-draped foresets of the lagoon facies association (coin diameter: 25 mm). Arrows point to foreset directions.
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Fig. 7. Sequence delineation, schematic correlation and lateral facies change of the Oligo-Miocene strata of the Hsuehshan Trough. The two selected sections (outcrop in square andwell log in circle) form a WSW–ENE profile, which is apparently strike-directional on the regional scale (Fig. 1). According to the geological provinces, Well 11 is in the forelandbasin close to the inner high, while Outcrop 4 is in the orogenic belts close to the outer high. The facies thus deepen laterally from Well 11 to Outcrop 4.
Allen and Posamentier, 1993). The basal erosion contact of the overlyingbarrier sandstone is referred to as a tidal ravinement surface ofwave andtidal reworking processes.
5.1.2. Maximum flooding surfaceThe MFS is identified by an erosion surface with upward facies
deepening above the TST, i.e. a transgressive wave or tidal ravinementsurface. It is also associated with the stacking pattern change fromretrogradation to progradation. The stacking patterns are describedin detail below (see Section 5.2).
The MFS of tidal ravinement is the erosion base of massive-beddedbarrier sandstone that overlies the lagoon sandstone/mudstoneheterolithics (e.g., SQ1 in Fig. 4). The base marks a shoreline retreatinto the backbarrier coastal plain, whereas the massive-bedded sand-stone records the successive voluminous subtidal deposition. The as-sociated stacking pattern change is from the barrier-dominated TST tothe lagoon-dominated RST, marking the turn of major shoreline shiftfrom landward to basinward.
The MFS of wave ravinement is the erosion base of glauconiticsandstone between the underlying offshore transition sandstone/mudstone interbeds and the overlying massive-bedded offshoremudstone (MFS of SQ UO in Fig. 7B). The surface is penetrated bythe Glossifungites ichnofacies filled with shelly lag deposits and glau-conitic pellets (Fig. 5B). The authigenic sediment fill is related to aperiod of sediment starvation in the offshore shelf (Savrda, 1995;Hesselbo and Huggett, 2001). The starvation is attributed to the in-crease in coastal accommodation during a major shoreline retreatthat traps most of the terrestrial sediment. The associated stackingpattern change is from the deepening-upward coastal-to-offshore TSTto the shallowing-upward offshore-to-coastal RST. The change marksthe major turn of relative sea level from transgression to regression.
5.2. Systems tracts
5.2.1. Transgressive systems tractThe retrogradational TST includes three characteristic facies succes-
sions that record the landward, intermediate, and basinward depositionwith respect to the transgressive shoreline.
The landward succession is dominated by the massive-beddedbarrier sandstones intercalated with the lagoon sandstone/mudstoneheterolithics (TST of SQ1 in Fig. 4). The barrier sandstones, concentratedin the lower TST, are attributed to frequent and significant, subtidalincursion and deposition in the backbarrier coastal plain during a seriesof shoreline retreats. The intercalations of lagoon heterolithics repre-sent the relict deposits of high-frequency relative sea level regression.Thickening upward in the TST, these intercalations further indicate anincrease in the preservation of high-frequency regressive deposits,and a decrease in the transgressive tidal erosion. The increase and de-crease are related to rapid shoreline retreat and/or accommodation cre-ation (Nummedal and Swift, 1987; Cattaneo and Steel, 2003; Catuneanuet al., 2009). This landward succession is thus interpreted as the accu-mulation in a transgressive coastal plain where the accommodationspace rapidly increases and outpaces the sediment accumulation.
The intermediate succession is characterized by an overall upwardchange of facies deepening and sandstone decreasing (TST of SQ 2in Fig. 4). The lower part is similar in facies to the landward/barrier-lagoon TST succession, while the upper is dominated by theoffshore transition sandstone/mudstone interbeds intercalated withthe shoreface sandstones. The upward deepening from coastal toshelf deposition indicates the passage of landward migrating shore-line away from the observed location. The intermediate successionis invariably observed above the SB atop the inner and outer base-ment highs (Locations 8, 9, 12 and 19 in Fig. 9), and above the land-ward RST (SQ 1 in Fig. 4).
Commonly intercalated with coquina and glauconitic beds, thebasinward succession is also characterized by an overall upward change
of facies deepening and sandstone decreasing (TST of SQ1~3 in Fig. 7B).The succession is dominated by the offshore sandstone/mudstone inter-beds with the shoreface sandstones in the lower part, and the offshoremudstones in the upper. The succession thus records an increase inthe relatively deepwater deposition and accommodation space. Due tothe lack of barrier-lagoon deposits, the succession is interpreted as theaccumulation landward of the transgressive shoreline.
5.2.2. Regressive systems tractThe progradational RST, likewise, includes three characteristic facies
successions that record the landward, intermediate, and basinwarddeposition with respect to the regressive shoreline.
The landward succession is dominated by the lagoon sandstone/mudstone heterolithics with the barrier sandstones in the lower part(RST of SQ1 in Fig. 4). The dominance of the lagoon deposits recordsthe backbarrier outbuilding and an overall basinward shoreline shift.The barrier sandstones in the lower part are attributed to the subtidalincursion and deposition after the major shoreline retreat at the MFS.
The intermediate succession is characterized by an overall upwardchange of facies and sandstone increasing (RST of SQ1–2 in Fig. 7B).The succession is dominated by the offshore transition sandstone/mudstone interbeds with the offshore mudstones in the lower part.The upper part contains common intercalations of the shoreface andbarrier sandstones, and the lagoon sandstone/mudstone heterolithics.The shoreface and barrier sandstones are usually sharp- and erosive-based, and overlie the offshore mudstone and the offshore transitionsandstone/mudstone interbeds, respectively (Figs. 5A, 10). The inter-mediate facies to form gradual upward shallowing are missing, whichindicates high-frequency forced regression of relative sea level (Huntand Tucker, 1992; Plint and Nummedal, 2000). Soft-sediment deforma-tion is usually observed below the erosion surface of forced regression(Fig. 10B), recording sudden overloading of the barrier/shoreface sanddeposition on the fluid-laden muddy offshore substrate (Walker andPlint, 1992).
The basinward succession is similar to the intermediate RST, onlyin the lack of the barrier-lagoon intercalations in its upper part (RSTof SQ UO in Fig. 7B). Also common are the features of high-frequencyforced regression and soft-sediment deformation. The succession isinterpreted as the accumulation mainly basinward of the regressiveshoreline.
6. Sequence architecture
Three SBs (SB 1–3) are identified in the rift-drift transition strata,which are thus divided into four T–R sequences, namely SQ UO of theupperOligocene, and SQ1, 2 and 3 of the lowerMiocene (Fig. 7). Regionalcorrelation and approximate ages of the SBs are delineated, based on thebiostratigraphy of foraminifers and nannofossils (Fig. 2B). Conspicuousonlapping of the sequences onto the basement highs appears in thetwo synthesized dip-directional transects (Fig. 9), in which the projectedcontrol sections and well logs are repositioned, in order to untangle thefold and thrust structures, according to the reported retrodeformationtransect (Fig. 2A; Suppe, 1980; Teng et al., 1991).
The tectonostratigraphic and sedimentary significance of the SBsare evaluated by integrating the geometry and facies distribution ofthe sequences shown in the transects. Also evaluated are the accom-modation space/sediment accumulation interplay and the topographictransformation during the rift-drift transition.
6.1. Sequence boundary significance
The SB 1 is located above the biohorizons a and b, and below thebiohorizon c (Fig. 9), recording a relative sea level fall around theOligocene/Miocene boundary of 23.8 Ma (Berggren et al., 1995). It isamalgamated with the unconformity surface atop the Mesozoic base-ment when the underlying SQ UO disappears.
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The SB 1 is referred to as ‘breakup unconformity’ of the HsuehshanTrough on account of the overlying SQ 1 that starts to onlap onto theinner and outer highs. The SB, therefore, marks the onset of the earlypost-rift deposition.
The SB1 is, furthermore, associated with an upward change insequence geometry and facies distribution, and thus also marks amajor change of depositional system in the basin. The underlyingSQ UO is characterized by prominent lateral deepening in facies,and forms a southeasterly-thickening wedge bounded by the LishanFault. The conglomeratic barrier-lagoon strata are concentrated inthe northwest area close to the inner high, and soon grade into theoffshore-mud-dominated strata in the southeast. Limited coastalexposure and progradation in the end of SQ UO deposition is thusalso indicated. The SQ 1 and 2, onlapping onto the basement highs,are characterized by an overall shallower facies composition and asmaller extent of lateral variation. The sequences are dominated bythe offshore transition sandstone/mudstone interbeds with the lesssignificant offshore mudstones in the southeast. Much extensivecoastal exposure and progradation after the SQ 1 and after the SQ 2deposition are also indicated by the widely distributed barrier-lagoon strata in the RST of the two sequences. In addition, the rareRST barrier-lagoon progradations from the outer high are observedin the basinward part of the SQ 1 (Sections 7-10-8 and 17-18-19 inFig. 9), further characterizing the depositional system change acrossthe SB1.
The major change from the SQ UO to the SQ 1–2 is attributed to adecrease in shelf gradient during the rift-drift transition. A continen-tal shelf with a considerable gradient is recorded by the wedge shapeand prominent facies deepening of SQ UO, as the result of the rapidand differential subsidence, i.e. the Lishan Fault activity. The conti-nental shelf became shallower and gentler, recorded by the less sig-nificant facies deepening in the SQ 1 and 2, as the result of muchuniform subsidence of thermal contraction. The end of the LishanFault activity and the onset of much uniform thermal subsidence inthe basin are also pinpointed by the RST coastal progradations fromthe outer high in the SQ 1.
The SB 2 is constrained by the biohorizons c and d, thereforeapproximately within the age range from 23 to 22 Ma (Figs. 2, 9). Itis amalgamated with the unconformity surface atop the Mesozoicbasement when the underlying SQ 1 disappears. The overlying SQ 2extends both further landward and basinward from the SQ 1, markingthe successive onlapping after the SQ 1 deposition. The SB 2 thus re-cords a relative sea level fall within a period of progressive subsidenceand submergence of the basement highs, i.e. within the early post-riftstage.
The SB 3 is located closely below the biohorizons d and e/f/g,therefore approximately within the age range from 21.5 to 20.5 Ma(Figs. 2, 9). It onlaps onto the unconformity surface atop the innerbasement further landward from the SB 2. In the studied sections,the overlying SQ 3 TST is invariably present, and forms a sheet-like
Fig. 8. Outcrop characteristics of sequence boundary in the Oligo-Miocene strata of the Hsuehshan Trough. See stratigraphic locations in Fig. 7B. (A) Sequence boundary of maximumregression surface (dashed line) at the erosive base of the offshore transition bioturbated sandstone. It overlies the well-sorted coarse-grained shoreface sandstone. (B) Close-up ofPhoto A. Note the Glossifungites network filled with glauconitic pellets (coin diameter: 30 mm). (C) Sequence boundary of maximum regression surface (dashed line) at the erosivebase of the offshore transition bioturbated sandstone. It overlies the well-laminated and -stratified barrier sandstone. (D) Close-up of Photo C. Arrows point to Glossifungites trace fossilsfilled with lag deposits of coarse-grained sands and shell fragments, and penetrating down through the sequence boundary.
Fig. 9. Synthetic dip-directional stratigraphic transects showing Oligo-Miocene sequence stratigraphic framework of the Hsuehshan Trough. The north (A) and south transect (B) areoriented respectively in NW–SE and WNW–ESE directions (Fig. 1), owing to local variations of depositional dip (Chou, 1974). Both transects are repositioned according to theretrodeformed transect of Suppe (1980). The control sections (outcrop in square and well in circle) are projected to the transects, according to their positions in the forelandbasin and orogenic belts.
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rock body dominated by the offshore mudstone. This envisages an ex-tensive gentle-sloped outer shelf and a total submergence of thebasement highs due to thermal subsidence. The SB 3 is thus termedthe ‘drift onset unconformity’ in this study, marking the onset of amature passive continental margin setting.
6.2. Early post-rift sedimentation and development
Specified by the breakup and drift onset unconformities, the earlypost-rift deposits of the Hsuehshan Trough are composed of the SQ 1and 2 (Fig. 9). Overall, the sequences form a wedge thinning out land-ward, with the lower part bounded by the outer high. The wedge issandwiched by the underlying Oligocene syn-rift wedge and base-ment highs, and by the overlying late post-rift sheet-like drape(Fig. 2A; Teng et al., 1991; Teng and Lin, 2004).
The early post-rift wedge is also superimposed by a progradationalstacking pattern from SQ UO, 1 to 2, and demonstrates an overall dom-inance of sediment accumulation over accommodation space. Thesuperimposed stacking is highlighted by the RST barrier-lagoon strata,which increase in their basinward distributions from the SQ UO, 1 to 2(Fig. 9). The dominance of sediment accumulation is supported by thevariations in sedimentation rate among these sequences. The SQ 1 and2 deposition is estimated at a rate of about 250 m/myr, according toSection 7 near the outer high (750 m thick; Fig. 1A), while the SQ UOand 3 is slower at 150 and 50–90 m/myr, respectively (Chang and Chi,1983; Teng et al., 1991; Chou et al., 1994).
The SQ 1, geometrically, is a lens-like rock body with prominentvariations in thickness (Fig. 9). Aside from onlapping onto the innerand outer highs, the sequence characteristically thins in the basinalareas around Section 5, 7, and 17. Also devoid of the barrier-lagoonstrata, the basinal areas thus represent the underfilled part of theremnant rift topographic low after the SQ 1 deposition. Based onthe coexistence of the underfilled areas and the common coastalprogradations from the inner and outer highs in the SQ 1, a temporarybalance between sediment accumulation and accommodation spaceis inferred during the SQ 1 deposition.
The SQ 2, likewise, forms a wedge-like rock body with prominentvariations in thickness (Fig. 9). Aside from onlapping onto the innerhigh, the sequence characteristically thickens toward the basinalareas around Sections 5, 7, and 17, and also thins toward and coversthe outer high. The underfilled remnant rift topographic low in theSQ 1 is, therefore, shown being eventually filled up by the SQ 2
deposition. The filling up evidently indicates a dominance of sedi-ment accumulation over accommodation space during the SQ 2 depo-sition. In addition, the RST barrier-lagoon strata in the SQ 2 onlyextend from the inner high, likely due to the well submergence andburial of the outer high during deposition.
6.3. Infilling history
On the basis of the Oligo-Miocene sequence stratigraphy, aninfilling history model of the Hsuehshan Trough is proposed, withfocuses on the evolution of tectonic subsidence, depositional systemand sediment supply (Fig. 11). The model includes four substagesfrom the SQ UO, 1, 2 to 3, that are at the million-year/3rd-orderscale (cf. Haq et al., 1987), since the SQ 1 and 2 were depositedapproximately from 23.8 to 21 Ma and in an average time span of~1.4 myr. Through the substages, the depositional system and topog-raphy of this half graben are shown actively transformed by theprogradational fill-up wedge formation.
In the late syn-rift time of the Late Oligocene (SQ UO), theHsuehshan Trough was subject to the active normal faulting of theLishan Fault (Fig. 11A). A relatively deep and steep shelf was presentbetween the inner and outer shoulder highs, due to the fault-controlleddifferential subsidence. Southeasterly deepening and thickening infacies was prominent, as the rapid increase in accommodation spaceprimarily of tectonic origin outpaced the sediment supply mainlyfrom the northwest.
The remnant rift topography after this substage may reach some200 m in depth, according to the late Oligocene fossil assemblagesand basin geometry. The paleontological data indicate an outer shelfto bathyal environment, including the Cruziana ichnofacies of Planolites,Chondrites, Scolicia and Zoophycos, and the foraminifer assemblage ofLenticulina, Bathysiphon, Pullenia andUvigerina. A pteropods assemblageof Limacina lini and Clie mai is also documented in Section 6 (Fig. 1A),suggesting a depth range from 200 to 1000 m (Chen and Huang,1990). In addition, given a common shelf gradient of 1/1000–1/500(Reading, 1996), the water depth of this ~250 km wide half grabencould range between 250 and 500 m.
The SQ 1 time after the breakup unconformity at the Oligocene/Miocene boundary is the initial fill-up period (Fig. 11B). The conti-nental shelf became gentler and shallower with a decrease in lateralfacies deepening, when thermal subsidence started to dominate thecreation of accommodation space. Such a gentle shelf profile also
Fig. 10. Outcrop characteristics of forced regression surface in the Oligo-Miocene strata of the Hsuehshan Trough. (A) Forced regression surface (dashed line) at the erosive base ofthe shoreface swaley-cross-stratified sandstone in Outcrop 15 (Fig. 1A). It overlies the offshore mudstone. (B) Forced regressive (dashed line) surface at the erosive base of thecross-stratified barrier sandstone in Outcrop 6 (Fig. 1A). It overlies the offshore transition sandstone/mudstone interbeds. Arrows point to the common soft-sediment deformationof ball and pillow structures in the interbeds.
Fig. 11. Schematic Oligo-Miocene infilling history of the Hsuehshan Trough. See further descriptions in text (Section 6.3). (A) SQ UO: late syn-rift underfilled deep shelf in the lateOligocene time. The depositional setting is a relatively steep shelf gradient, due to the active normal faulting of the Lishan Fault. (B) SQ 1: initial, early post-rift balanced shallowshelf during the Oligocene-Miocene transition. After the end of normal faulting, a relative balance forms between accommodation creation and sediment supply, and allows thebasin center to be underfilled and the gentle and shallow shelf profiles to establish from both the inner and outer high. (C) SQ 2: final, early post-rift overfilled shallow shelf inthe early Miocene. The sediment accumulation outpaces the accommodation space, despite the burial of the outer high leading to further increase in accommodation space anddecrease in sediment supply. (D) SQ 3: late post-rift underfilled shallow shelf in the early Miocene. The accommodation space outpaces the sediment supply, primarily due tothe total submergence and burial of inner and outer basement highs.
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established across the inactive Lishan Fault, and deepened from theouter high to the basin depression. The accommodation space inthis substage remained in balance with the sediment accumulationsuch that the basinal areas kept underfilled even with the commonprogradations from the surrounding basement highs.
The SQ 2 time is the final fill-up period of the Hsuehshan Troughwhen the sediment accumulation outpaced the accommodationspace (Fig. 11C). The gentle, southeasterly-deepening shelf becamemuch more extensive, and covered the outer high which had subsid-ed below the offshore shelf after the SQ 1 time. The remnant rifttopographic low was filled up progressively, despite the decreasein sediment supply primarily due to the submergence and burial ofthe outer high. After the basin depression was largely filled up, theexcess sediment was likely transported and deposited into the NESouth China Sea.
During the period of SQ 3 transgression, this continental margin,including the inner high, became widely submerged and buried(Fig. 11D). The majority of terrestrial sediment supply was thus cutoff, leaving the shelf underfilled. The shelf profile remained gentlein gradient, while the water depth increased as thermal subsidencecontinued. The offshore-mud deposition therefore prevailed in thecontinental margin and sealed the rift basinfill.
7. Discussion
7.1. Sedimentation of the early post-rift fill-up wedge
The newly established Oligo-Miocene sequence stratigraphy ofTaiwan characterizes the short-lived and rapid-infilling nature of theearly post-rift progradational fill-up wedge in the siliciclastic setting,with the sedimentation rate at about 250 m/myr over a period of ~2.8myr (23.8 to 21 Ma; Fig. 12A–C). It well exemplifies one end member
of the early post-rift deposit, compared to the other end of the ‘slow’
deepwater fine-grained infill (cf. Nottvedt et al., 1995) demonstratedby the Early Cretaceous Norwegian North Viking Graben (50 m/myrover a period of tens of myr; Gabrielsen et al., 2001; Zachariah et al.,2009). In the geological record, comparable wedge development isapproximately ranged in rate from 100 to 250 m/myr, and in durationfrom a couple to a dozen of myr (e.g., Alves et al., 2003; Dupre et al.,2007; Doust and Noble, 2008; Papini and Benvenuti, 2008; Kuhlmannet al., 2010).
The fill-up process of such a short-livedwedgemay be attributed, ina simplified scenario, to the decrease in accommodation creation fromrapid fault-controlled subsidence to asymptotic thermal-cooling subsi-dence (e.g., Simpson and Eriksson, 1989; Dinis and Trincao, 1995;Alves et al., 2003; Burton and Wood, 2010; Contreras et al., 2010). Theprerequisite is to complete the process before the decline of sedimentsupply in the late post-rift stage when the source area is largely sub-merged and buried. The early post-rift accommodation space is, how-ever, found rapidly increasing in the Hsuehshan Trough and in manyother rift basins (discussed below). It is suggested that an increase insediment supply is also required to prevail in the accommodation/accumulation interplay.
The initial accommodation space from the remnant rift topo-graphic low is also vital in the early post-rift sedimentation. Reaching500–900 m in depth, it played a dominant role in the underfilledEarly Cretaceous Norwegian North Viking Graben (Zachariah et al.,2009). It was less significant in many continental to coastal nearshorerift basins, and thus favorable for the fill-up process to complete overa short time period (e.g., Rosas et al., 2007; Papini and Benvenuti,2008; Kuhlmann et al., 2010). The initial accommodation space ofthe Hsuehshan Trough, estimated up to 200 m in depth, was relativelyvoluminous and apparently disadvantageous for the fill-up wedge for-mation. The requirement of an increase in sediment supply is, therefore,believed not to be overstressed in this particular basin.
Fig. 12. Oligo-Miocene sequence stratigraphy, and tectonic stages, climate and sedimentation rates of the Hsuehshan Trough, and eustatic sea level records. The sequence stratigraphicscheme (A) is based on Fig. 9A. The biochronostratigraphy follows Berggren et al. (1995). Sedimentation rate (C) is based on Section 7 for SQ 1 and 2 (750 m), and Chang and Chi (1983)for SQ 3, and Chou et al. (1994) for SQ UO. The palynofloral assemblages are based on Shaw (1992), Li et al. (2003) and Wu et al. (2003). The Exxon Chart (D) is adapted from Haq et al.(1987) and the modified chart of Miller et al. (1996). The δ18O record is adapted from Miller et al. (1996).
A rapid increase in the early post-rift accommodation space of theHsuehshan Trough is documented in the results of backstrippinganalysis (Chou et al., 1994; Lin et al., 2003). It is principally of tectonicand eustatic in origins, according to the comparison results betweenthe backstripping results and the conceptual thermal model (Lin etal., 2003), and to the global sea level records, respectively. It receiveslittle contributions from residual fault-controlled subsidence, due tothe lack of post-breakup activities of the Lishan Fault and associatedfault systems in the early Miocene (Teng et al., 1991; Chen et al.,1994; Lin et al., 2003).
Anomalously rapid thermal subsidence in this continental marginprior to 21 Ma is shown in the backstripping results. The rapid subsi-dence has been related to the basaltic magmatism and mantle convec-tion induced by the Eo-Oligocene rifting in the SE China continentalmargin (Fig. 2B; Chung et al., 1994; Lin et al., 2003). The concomitanceof igneous activity and anomalous rapid subsidence is widely reportedin the coeval rift basins in the northern South China Sea margin (Xieet al., 2006; Shi et al., 2008).
The Early Miocene was a period of long-term eustatic sea leveltransgression by a magnitude of several tens of meters, as revealedby the eustatic proxies of global stratigraphic and isotopic records(Fig. 12E–F; Haq et al., 1987; Miller et al., 1996, 2005). This transgres-sion is theoretically able to enhance the accommodation space ofa marine basin like the Hsuehshan Trough, especially during thethermal-cooling stage when the entire continental margin is subsid-ing. It is supported by the omnipresent marine microfossils in theHsuehshan Trough, which evidence a well connection to open oceanduring basin development (Fig. 2).
7.3. Controls on sediment supply and sequence boundary formation
The sediment supply and accumulation in the early post-rift stageis subject to numerous factors, such as provenance area, relief, lithol-ogy and climate in source provenance, and sediment transportationand dispersal (Prosser, 1993; Nottvedt et al., 1995; Gawthorpe andLeeder, 2000). In the case of the Hsuehshan Trough, the relative sealevel fall is one of the most effective candidates for rejuvenating theprovenance exposure and relief, and thus promoting the sedimentsupply. Lines of evidence indicate that the fall is possibly of eustaticin origin. Also indicated is the Early Miocene climate as a subordinaterole in sediment yield. The provenance area and relief, without exceptionin a long term,were decreased by the prolonged syn-rift exhumation, theearly post-rift onlapping and submergence, and the slow Cenozoic upliftin the continental region in SE China (Fuh, 2000; Chen et al., 2002; Renet al., 2002; Lin et al., 2003; Shu et al., 2007).
The Early Miocene climate in the continental margin became humidand then warm after the Oligocene cool and dry state, according topalynofloral studies (Fig. 12D; Shaw, 1992; Lei and Huang, 1997; Wuet al., 2003). It is concordant with the global trend (Zachos et al.,2001), which is associated with a sedimentation increase in the globalmajor sediment sinks, including east Asia (Clift, 2010). The increase ispossibly reflected by the high early post-rift sedimentation rate in theHsuehshan Trough (Fig. 12C). This favorable climatic condition is, how-ever, probably offset by the decrease in provenance exposure due toonlapping. The most significant offset took place after the SB 3 of driftonset. The climate was warm and humid, while the sedimentationrate declined and the basement highs were largely submerged.
The relative sea level fluctuations during the wedge formationare common and usually basin-wide, according to the 3rd-order T–Rsequences and the high-frequency coastal progradations and retro-gradations (Figs. 7 and 9). As the relative sea level lowered, the base-ment highs and adjacent depositional areas were re-exposed, andthe overall relief of the distant provenance in SE China rebounded(Fig. 12A). An increase in sediment yield is thus possible in such a
rejuvenated condition, also taking the favorable Early Miocene climateinto account. In addition, the relative sea level lowering provides ameans for sediment transportation and dispersal into the basin center,by resulting in the basinward shoreline shifts in this wave-dominatedcoastal-shelf system.
The relative sea level fluctuations in the post-rift marine HsuehshanTrough are theoretically sensitive to the global sea level fluctuations(Prosser, 1993; Nottvedt et al., 1995). It is validated by comparingthe T–R sequences with the Exxon cycle chart and the global δ18Orecord (Fig. 12E–F; Haq et al., 1987; Miller et al., 1991, 1996). Threeshort-term/3rd-order events of eustatic sea level falling, by the magni-tudes of several tens of meters, are age concordant with the SB 1–3. Itthus supports the casual relationship between the eustatic sea levelfall and the increase in sediment supply and accumulation, and theeustatic origin of SB formation in the Hsuehshan Trough. The variationsin tectonic subsidence and sediment supply are also important, espe-cially for the SB 1 of breakup unconformity. The SB 1 is associatedwith the major change in tectonic subsidence, and also punctuated bythe onset of the sediment-productive climate in the Early Miocene.
8. Conclusions
The Oligo-Miocene sequence stratigraphy in Taiwan elaboratesthe depositional and topographic transformation of the HsuehshanTrough when the early post-rift progradational wedge rapidly devel-oped and filled up this marine half graben. This active infilling processis recorded by the 3rd-order depositional sequences of SQ 1 and 2.At first, when the normal faulting ended, the continental shelf in thebasin became gentler in gradient. The coastal-shelf progradationsstarted to approach from both the inner and outer highs. As the outerhigh gradually subsided below the offshore shelf, the progradationfrom the inner high became dominant, and eventually smoothed theremnant rift topographic low.
The sequence stratigraphy also demonstrates the prominent relativesea level fluctuations in this continental margin with the anomalouslyrapid thermal subsidence. The fluctuations are an important means forretaining the provenance exposure and relief, and for promoting the sed-iment supply to outpace the accommodation space. The three sequencesboundaries of SB 1–3 highlight these fluctuations and the coastal-shelfprogradations. They are related to the 3rd-order eustatic sea level fallsin the Early Miocene. The SB 1 is also related to the tectonic breakupevent and thewarmandhumid climate change in the continentalmargin.
The depositional sequences of thefill-upwedge are characterized bythe superimposed progradational stacking pattern and the prominentthickness variations. They are distinguished from the aggradational-stacking sequences of the slow fine-grained deepwater infill withminor variations in thickness (Zachariah et al., 2009). Accordinglyshown clearly are the differential sedimentary responses to the domi-nance of either sediment accumulation or accommodation space duringthe rift-drift transition.
Since such rapid and short-lived wedge formations are widely ob-served in marine rift basins in the geological record, similar infillingprocess governed by relative sea level change is therefore expected.The process may also apply to continental-coastal rift basins that usu-ally became subject to marine influence in the post-rift stage (Prosser,1993; Nottvedt et al., 1995; Ravnas and Steel, 1998). Nonetheless, itrequires more studies to examine the applicability on account of thewell-known diverse nature of rift basinfill.
Acknowledgments
The authors would like to thank A.T. Lin for the subsurface data andP.C. Tai for valuable comments on the early version of the manuscript.We also acknowledge the field photo supplied by Y.C. Hsieh and thefieldwork aid of H.C. Chen. The study is financially supported by NSCGrants: NSC101-ET-E002-015-ET and NSC102-ET-E002-001-ET.
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