Storm‐flood‐dominated delta succession in the Pleistocene Taiwan Strait

Storm‐flood‐dominated deltas are sedimentary systems in which a complex interplay of hydrodynamic processes occurs during storms (e.g. tropical cyclones) due to the coeval action of continental and oceanic processes. This paper reports on a superbly exposed, 135.5 m thick stratigraphic succession of the Pleistocene Cholan Formation exposed along the Da'an River, Taiwan. The sedimentary succession comprises alternating mudstone and sandstone, is mostly fine‐grained, and exhibits multiple event beds that record deposition during tropical cyclones and post‐depositional deformation features produced during earthquakes. Detailed facies analyses reveal that deposition towards the base of the succession occurred in the palaeo‐Taiwan Strait in storm‐flood‐dominated prodelta and delta‐front environments passing upwards into delta‐plain environments. Tropical cyclone beds are encountered throughout the subaqueous storm‐flood delta successions, and are identified by (i) trough cross‐stratified sandstone bedsets with erosive bases that contain both mud clasts and mudstone beds, (ii) sandstone with aggrading wave ripples and (iii) hummocky cross‐stratified sandstone with rare gutter casts filled with coal fragments and shell remains. Tropical cyclone deposits are either top‐down burrowed or capped by massive or laminated mudstone. Seismites are rare and are mainly recognised through soft‐sediment deformation of beds; they do not show evidence of slope failure. Compared to storm‐flood delta successions described elsewhere, the Cholan Formation shows significantly fewer oscillatory‐generated sedimentary structures and gutter casts. This difference is attributed to the Cholan Formation being deposited in and along the margin of a strait characterised by strong shore‐parallel currents and relatively small storm waves due to its position between Taiwan and mainland China. This study refines depositional process interpretations of the Cholan Formation, provides criteria for recognising storm‐flood delta deposits in tectonically active straits with multiple sediment sources fed by steep drainages and short river catchments, and provides additional criteria for recognising tropical cyclone deposits in shallow‐marine settings.


| INTRODUCTION
The Western Foreland Basin (WFB) of Taiwan (臺灣) formed in response to oblique arc-continent collision between the Luzon Arc on the Philippine Sea Plate and the Eurasian Plate (Lin & Watts, 2002).The uplift of Taiwan created the Taiwan Strait (i.e.WFB).Since its emergence from the Pacific Ocean at the Miocene-Pliocene transition, Taiwan became the dominant sediment source to the WFB (Dashtgard et al., 2021a;Hsieh et al., 2022), and hence, detailed analysis of the Pliocene and younger sedimentary fill of the WFB may uncover past environmental, climate and tectonic changes that affected the island.
This paper reports on and interprets a Lower Pleistocene stratigraphic section through the Cholan Formation from the WFB of Taiwan.The sedimentological, ichnological and biostratigraphic content of the studied interval are described.These data are used to define the range of palaeoenvironmental settings recorded in the succession and the complex range of processes that impacted sedimentation, including both tropical cyclones (TCs) and earthquakes.The distinctive character of Cholan Formation strata establishes an analogue for recognising storm-flood deltas in tectonically active straits.Strait-margin deltas are widely recognised and interpreted in both modern and ancient sedimentary systems (Ayranci & Dashtgard, 2016;Dalrymple, 2022;Longhitano & Steel, 2017;Rossi et al., 2017); however, storm-flood deltas remain to be identified in such settings, notably in ancient successions.
Event beds in stratigraphy reflect sporadic and/or energetic depositional conditions (e.g.storms, earthquakes, delta-front failures, tsunamis, floods, etc.) that impact sedimentation and differ from the prevailing background deposition.Multiple criteria have been proposed for identifying triggering mechanisms responsible for different types of event-bed formation.Storms (including TCs), for example, are mostly recognised as hummocky crossstratified (HCS) sandstone.Erosion occurs during the waxing phase of the storm, and HCS records the strong oscillatory currents and combined flows acting on the sea floor during the waning phase (Cheel & Leckie, 1992;Hayes, 1967;Jelby et al., 2020;Myrow & Southard, 1996;Quin, 2011;Snedden et al., 1988;Vaucher et al., 2017).While HCS is widely used as evidence of stormy conditions, its architecture generally oversimplifies the range of sedimentary dynamics and patterns that occur during the Cholan Formation being deposited in and along the margin of a strait characterised by strong shore-parallel currents and relatively small storm waves due to its position between Taiwan and mainland China.This study refines depositional process interpretations of the Cholan Formation, provides criteria for recognising storm-flood delta deposits in tectonically active straits with multiple sediment sources fed by steep drainages and short river catchments, and provides additional criteria for recognising tropical cyclone deposits in shallow-marine settings.

K E Y W O R D S
ichnology, sedimentology, stratigraphy, tropical cyclones, Western Foreland Basin intense storms including TCs (Dashtgard et al., 2020;Grundvåg et al., 2020;Jelby et al., 2020;Mitchell et al., 2005;Snedden et al., 1988).Intense storms in marine and marginal-marine environments are expressed differently in the preserved sedimentary record depending on the context in which they formed.In deltaic environments, when storms occur, a simultaneous response of enhanced marine processes and river discharge due to elevated hinterland precipitation leads to complex sedimentation patterns that reflect a mix of oscillatory and unidirectional flows, and hyperpycnal and hypopycnal discharge (Allison & Neill, 2003;Bhattacharya et al., 2020;Bhattacharya & MacEachern, 2009;Collins et al., 2017;Lin & Bhattacharya, 2021).Sedimentation patterns in deltas are also impacted by allogenic processes such as sediment supply, eustasy and tectonics (Jervey, 1988).
Earthquake-induced event beds remain challenging to decipher in the rock record.Seismically generated beds are commonly interpreted from soft-sediment deformation structures resulting from the liquefaction of unconsolidated sediments that may partially or entirely obliterate primary sedimentary structures (Davenport & Ringrose, 1987;Gibert et al., 2011;Gladkov et al., 2016;Moretti & Ronchi, 2011).Their recognition is difficult because several processes, including rapid sedimentation, and groundwater and slope movements, can lead to the same deformational patterns (Chiarella et al., 2016;Owen et al., 2011).Because the WFB is subject to frequent TCs and earthquakes it is an ideal location to investigate the sedimentary signatures of these events in shallow deltaic settings.

| GEOLOGICAL SETTING
Taiwan (Figure 1) is a small, tectonically active mountainous island located in the north-west Pacific Ocean that formed in response to oblique collision between the Philippines Sea Plate and the Eurasian Plate (Lin et al., 2003).The onset of plate convergence led to Taiwan's emergence ca 6.5 Ma, creating the WFB and consequently, the Taiwan Strait (Chou & Yu, 2002;Covey, 1984;Lin & Watts, 2002;Yu & Chou, 2001).Taiwan is divided into five main geological regions, including (from west to east): Coastal Plain, Western Foothills, Hsuehshan Range, Central Range and Coastal Range (Lin & Chen, 2016).The studied section of the Cholan Formation crops out in the Western Foothills (Figure 1A,B,C).The Western Foothills consist of fold-and-thrust belts that expose pre-collisional and post-collisional rocks deposited in passive margin, rift, post-rift and foreland basin stages of the Eurasian Plate margin/WFB (Lin et al., 2003(Lin et al., , 2021;;Lin & Watts, 2002;Mouthereau et al., 2002).

ENVIRONMENTAL SETTING
The north-west Pacific Ocean is a region where many of the most powerful TCs form (Milliman et al., 2017).For the last 35 years, an average of 3.5 TCs have passed over Taiwan or within 50 km of the coastline per year (Dashtgard et al., 2020).Super-TCs (i.e.category 4-5 on the Saffir-Simpson Hurricane Intensity Scale; Kelman, 2013) impact Taiwan approximately every 4 years, and TCs account for approximately 48% of annual rainfall (Chen et al., 2010).Simultaneously, the continued rapid uplift and active tectonism of Taiwan results in frequent earthquakes and mass-wasting events that liberate an abundant supply of sediment from the orogen into the adjacent rivers (Dadson et al., 2003;Lin et al., 2003;Milliman et al., 2007;Milliman & Kao, 2005).During TCs, rainfall over Taiwan is responsible for eroding soil and floodplain sediment, and bedrock and sediment that was released through landslides.The massive volume of sediment derived from these various sources is transported and deposited in the adjacent seas (Chen et al., 2018;Jin et al., 2021;Milliman et al., 2007Milliman et al., , 2017;;Milliman & Kao, 2005).At the same time, strong winds associated with TC rotation leads to the development of storm waves around Taiwan with wave heights in excess of 4 m in the Taiwan Strait (compared to less than 1 m under fair-weather conditions; Dashtgard et al., 2020).Because the sedimentary routing system of Taiwan consists of steep and short rivers (mainly less than 100 km in length), drainage systems respond rapidly to changes in hinterland precipitation (Castelltort & Van Den Driessche, 2003;Vaucher et al., 2021).This results in the transport of close to 400 Mt of sediment annually to the adjacent Pacific Ocean and Taiwan Strait (Milliman et al., 2017;Milliman & Meade, 1983).
The Taiwan Strait is affected by semi-diurnal tidal circulation that flows dominantly southwards and alongshore (Jan et al., 2002;Korotenko et al., 2020;Wang et al., 2003).In addition to tidal currents acting in the Taiwan Strait, other alongshore currents, such as the South China Sea Warm Current, the Kuroshio Branch, the Taiwan Warm Current and the Zhejiang-Fujian Coastal Current also operate (Huh et al., 2011;Jan et al., 2002).Together, these currents drive sediment reworking in the Taiwan Strait and sediment dispersal towards the South China Sea and East China Sea (Milliman et al., 2007;Nagel et al., 2018;Figure 1A).

| MATERIALS AND METHODS
The 135.5 m thick studied stratigraphic section comprises alternating mudstone and sandstone, and crops out along The Taiwan Strait with the main oceanic currents acting therein and the geological map of Taiwan with the main geological units.1: Coastal Plain; 2: Western Foothills; 3: Hsuehshan Range; 4: Central Range; 5: Coastal Range; 6: Tatun volcano group (modified after Lin & Chen, 2016).SWC, South China Sea Warm Current; KB, Kuroshio Branch; KC: Kuroshio Current; TWC, Taiwan Warm Current; ZFCC, Zhejiang-Fujian Coastal Current.Orange arrows denote north-directed currents, and the blue arrow shows south-directed current.(B) Detailed geological map of the studied area (modified after Lin & Chen, 2016).The orientation of (D) is shown by the white camera.The orange line shows the position of the studied interval of the Cholan Formation.(C) The chronostratigraphic framework for the northern part of the Western Foothills (Cohen & Gibbard, 2019;Horng & Shea, 2007;Pan et al., 2015;Vaucher et al., 2021).(D) Drone view of the Cholan Formation outcrop along the Da'an River (大安溪) in the Zhuolan Grand Canyon (卓蘭大峽谷).Canyon width is ca 20 m.

A B C D
the Da'an River (大安溪) in the Zhuolan Grand Canyon (卓蘭大峽谷), south of the city of Zhuolan Township (卓蘭鎮), Taiwan (Figure 1D).The studied interval belongs to the Pleistocene Cholan Formation (Figure 1), and the section was described and sampled in late 2019.
The interval was logged at a decimetre-scale.Stratal thicknesses were measured using a digital Jacob's staff, and descriptions were made of bed geometries, bounding contacts, grain size, sedimentary structures, ichnology and body fossils (Figure 2).Drone photographs were acquired using a DJI Mavic Pro 2. Ichnological descriptions include both ichnogenera and the degree of burrowing, using the bioturbation index (BI; Taylor & Goldring, 1993).Three samples were taken from mudstone intervals to assess the nannofossil content for age estimations of the studied stratigraphic interval (Figure 2; Table 1).Smear slides were prepared from bulk sediments and were examined for calcareous nannofossil identification under an optical microscope with 1,000× magnification.Nannofossil analyses were performed in the Department of Earth Sciences, National Cheng-Kung University, Taiwan.

| Biostratigraphy
The three samples taken for biostratigraphic analyses yield rare or absent nannofossils (Table 1).The two lower samples (DCG19-A at 2 m and DCG19-B at 49 m; Figure 2) lack late Miocene to Pleistocene index fossils but contain a few of Pseudoemiliania lacunosa, Reticulofenestra minuta and Gephyrocapsa sp.(small type; Table 1).Therefore, the ages of these samples are assigned to the NN-16 ~ NN-18 Zone of the Early Pleistocene (Pan et al., 2015).The upper sample (DCG19-C at 118 m; Figure 2) contains no complete nannofossils, and only two fragments of P. lacunosa (Table 1); consequently, DCG19-C probably belongs to the NN-16 ~ NN-18 Zone, but it is possible that this sample is from the NN19a Zone (Pan et al., 2015) because it is stratigraphically higher in the section.Based on the nannofossils identified, the studied stratigraphic interval corresponds to the NN-16 ~ NN-18 Zone of the Cholan Formation and the uppermost part possibly reaches the NN19a Zone.

| Sedimentary environments
The Cholan Formation (Figures 2 through 5) consists of mainly clayey siltstone interbedded with cross-stratified and cross-laminated sandstone.Ten sedimentary facies are defined (Table 2) and are grouped into two facies assemblages: subaqueous and subaerial deltaic environments.

Interpretation
The sedimentary structures displayed in F1-F5 (Figure 3B,C,G) suggest that the dominant sediment transport mechanism was unidirectional traction flow that drove the migration of bedforms such as current ripples, dunes and upper-stage plane beds (Vaucher & Dashtgard, 2022).Local symmetrical and combined-flow cross-lamination is also observed in F4 and reflects the superimposition of oscillatory motion via waves (Figure 3E,F;Dashtgard et al., 2021b;Vaucher et al., 2017).Inverse and normal grading also frequently occurs in F1-F4 and records the action of waxing and waning density flows, probably corresponding to turbidity and hyperpycnal flows during elevated river discharge (Bhattacharya et al., 2020;Bhattacharya & MacEachern, 2009;Saleh et al., 2022).Mud drapes on foresets (F4, F5) reflect either periods of reduced energy conditions allowing mud to settle from suspension or coeval deposition of mud aggregates with sand during floods (Dashtgard et al., 2020;Gugliotta et al., 2015;Lamb et al., 2020).Sharp-based, massive to laminated mudstone layers are interpreted as the product of fluid muds likely sourced from nearby rivers (Ichaso & Dalrymple, 2009;La Croix & Gingras, 2021).The overall low to moderate degree of bioturbation and diversity of trace fossils recognised in F1-F5 (Figure 4) suggests that these assemblages belong to the recently defined Phycosiphon and Rosselia Ichnofacies, which are considered to develop in deltaic systems (sensu The 135.5 m thick section of the Cholan Formation records shallow-marine environments (delta front, prodelta) that pass vertically upwards into terrestrial (delta plain) environments.The main sedimentary structures and bedding patterns are shown, as well as the position of three samples (DGC19-A, -B, -C) taken for biostratigraphic analyses.See Table 2 for sedimentary facies description.Note the occurrence of TC beds (F8, F9) and earthquake-induced liquefaction beds (F10) in the lower part of the section.Env., Environment; d., delta.MacEachern & Bann, 2020).The low to moderate BI suggests stressful environmental conditions such as high sediment supply, turbid waters and salinity fluctuations.Together, these observations indicate that deposition took place in shallow-marine deltaic environments and record deposition in offshore/prodelta to delta-front environments.

Description
The second facies assemblage comprises two sedimentary facies (F6 and F7; Table 2; Figure 5).Facies 6 is massive to laminated mudstone with centimetre-thick cross-laminated sandstone layers and no bioturbation (Figure 5A,C,D,E).One bed contains multiple vertical sand-filled cracks below a single bedding plane in F6 (Figure 5E).Facies 7 is unbioturbated, massive to trough cross-stratified sandstone with coal and mud rip-up clasts and pebbles overlying erosive basal surfaces (Figure 5A,B).

Interpretation
Facies 7 is interpreted as distributary channel fills that crosscut a delta plain (Buatois et al., 2012;Collins et al., 2018;Gugliotta et al., 2015).Mudstone beds of F6 represent either the floodplain, abandoned channel fills or mud banks flanking distributary channels in subaerial delta-plain environments.Minor flooding events on the delta plain are evidenced by thin sandstone laminae and ripple cross-laminated sandstone beds (Figure 5D; Buatois et al., 2012;Dillinger & George, 2019).Vertical cracks likely originated as desiccation cracks on the floodplain or mud banks, and were subsequently filled by sand during the next flood (Figure 5C,E; Tanner, 1998).
T A B L E 1 Nannofossils identified in samples from the Cholan Formation at the Zhuolan Grand Canyon (卓蘭大峽谷) along the Da'an River (大安溪), Taiwan (臺灣).

Sample DGC19-A DGC19-B DGC19-C
General information about the samples T A B L E 2 (Continued)

| Event beds in subaqueous deltaic environments
In subaqueous deltaic environments, several beds differ prominently from the rest of the stratigraphy and form three sedimentary facies that reflect event deposition (F8-F10; Table 2; Figures 6 and 7).Event beds are either unbioturbated or display vertical topdown burrows that originate from the top of the bed, or from within overlying beds.Vertical burrows are mostly Ophiomorpha, and more rarely Thalassinoides or Skolithos.

Description
The first category of event beds consist of cross-stratified and cross-laminated sandstone (F8-F9; Table 2; Figure 6).Facies 8 is made of two types of beds.The first bed type of F8 is sandstone with an erosive base that exhibits vertically aggrading symmetrical ripple cross-lamination (Figure 6A,B,C), and one example of this type of bed is eroded at the top by a channel form filled with laminated mudstone (Figure 6A).The second bed type of F8 displays HCS, wavy parallel and/or low-angle lamination; these beds occur within both sandstone-dominated (Figure 6D,E) and mudstone-dominated intervals (Figure 6F).In the second bed type of F8, symmetrical cross-lamination show wavelengths ranging from centimetre-scale (Figure 6F) to metre-scale (Figure 6D,E), and HCS is dominantly anisotropic.Metre-scale HCS may grade laterally into quasiplanar lamination (Figure 6D).Gutter casts rich in shells and coal fragments are occasionally associated with HCS beds (Figure 3D).Both types of F8 beds may display symmetrical ripple cross-lamination at their top (Figure 4F).Beds of F9 are erosive-based and unbioturbated trough cross-stratified sandstone with multiple reactivation surfaces (Figure 6D,E,G).Facies 9 displays foresets with no preferential orientation based on dip direction, and contains dispersed shell fragments, coal and mud clasts and pebbles (Figure 6D,E,G).

Interpretation
Beds of F8 reflect primarily the record of oscillatory and combined-flow processes acting during storms (Jelby et al., 2020;Myrow & Southard, 1996).Since TCs are the main type of storms affecting Taiwan (Dashtgard et al., 2020), storm beds in this study are generally considered as TC beds.Vertical aggradation of wave ripples suggests a relatively weak and steady oscillatory motion combined with a continuous sediment supply (Dumas & Arnott, 2006).Conditions favourable for the formation of aggrading wave ripples during a TC may be found at the periphery of the TC, where wind speeds are weaker, and oscillatory motion dominates over strong unidirectional currents on the sea floor (the reverse situation occurs closer to, but not within, the eye of a TC; Mitchell et al., 2005;Teague et al., 2007).The wavelength of oscillatory bedforms is directly linked to the wavelength of the waves that creates them and depends on water depth and sediment grain size (Perillo et al., 2014;Yang et al., 2006).Consequently, the wavelength of HCS can reflect wave size during TCs and other storms (Cummings et al., 2009;Myrow et al., 2002;Myrow & Southard, 1996), bathymetry and grain size.The along-bed shift from HCS to quasi-planar lamination is interpreted as evidence of the interaction between unsteady oscillatory motion and downwelling flow that is likely to occur during a storm (Jelby et al., 2020).Scour-based, massive to laminated mudstone beds that overlie TC beds are probably the products of hyperpycnal river outflows that take place during the waning phase of the TC (Figure 6A; Dashtgard et al., 2020;Jelby et al., 2020).Beds of F9 record evidence of multiple orientations of unidirectional storm flows based on the dip direction of foresets in trough cross-stratification.In addition to multiple foreset orientations, other features reflecting storm deposition include: the erosive base of beds; coeval deposition of sand, shells, coal clasts, mud clasts and pebbles; and the complete absence of bioturbation.Together these features support high-energy conditions in a shallowmarine setting with deposition of abundant terrestrial material.The bed characteristics of F9 closely match those proposed by Dashtgard et al. (2020) for recognising TC deposits in the palaeo-Taiwan Strait.Consequently, trough cross-stratified beds of F9 are also considered as TC beds.

Description
The second category of event beds in the Cholan Formation are characterised by soft-sediment deformation (SSD) structures (F10; Table 2; Figure 7).The first of two beds that constitute F10 shows decimetre-scale clasts of laminated sandstone 'floating' in a structureless siltstone matrix (Figure 7A,B) that resemble a matrixsupported conglomerate.The second bed that defines F10 is a tabular silty sandstone bed that exhibits metre-scale convolute bedding, which is interpreted as an expression of ball-and-pillow structures (Figure 7C,D).

Interpretation
The two types of beds that define F10 show evidence of liquefaction or fluidisation (Allen, 1982;Owen, 2003) and post-depositional deformation, and this is interpreted as follows.The density contrast of water-saturated sediment drives liquefaction and fluidised sediment mobilisation.This phenomenon is prone to occur in most subaqueous environments where either a large volume of sediment is rapidly deposited or when water-saturated sediment is disturbed and grain-to-grain bonds are broken (Allen, 1982;Owen, 2003).In this study, multiple examples of TC beds are identified, and most show evidence of rapid sedimentation.However, none of these beds contains SSD structures suggesting that rapid sedimentation during TCs did not result in sediment loading and deformation.The SSD beds documented in the Cholan Formation do not show evidence of slope movement (Figure 7) and this supports the interpretation of in-situ post-depositional deformation.The other observation consistent with post-depositional deformation is that the sandstone clasts (Figure 7A,B) are made of deformed laminated sandstone, suggesting that the sand was relatively consolidated prior to deformation.While it is not possible to entirely exclude the possibility that rapid sediment loading during TC caused SSD of underlying water-saturated mudstone, it is more likely that the frequent earthquakes that impact Taiwan shook and liquefied water-saturated sediment, which led to downslope movement of semi-competent beds, disruption of bedding and formation of convolute bedding and matrix-supported conglomerates.Incidentally, SSD structures in the Cholan Formation (Figure 7) are similar to deposits described elsewhere where earthquakeinduced liquefaction was interpreted as the triggering mechanism of deformation (Gibert et al., 2011;Gladkov et al., 2016;Martín-Chivelet et al., 2011;Sims, 2012).

| Palaeoenvironmental evolution of the Cholan Formation
The described Cholan Formation section records deposition in shallow-marine delta environments (Figures 2,3 and 4) that later evolved into subaerial delta-plain environments (Figures 2 and 5).The two nannofossil samples taken from shallow-marine strata (DGC19-A, DGC19-B; Figure 2) contain rare calcareous nannofossils (Table 1).The third sample (DGC19-C; Figure 2) was taken from the delta-plain facies and yielded only two nannofossil fragments.The paucity of nannofossils likely reflects the large amount of sediment that was continuously transported to the subaqueous delta and deposited at shallow water depths (0-30 m; see Nagel et al., 2013); these conditions were not favourable for coccolithophore colonisation nor preservation (Chen et al., 1977;Chi & Huang, 1981).The fragments of nannofossils found in DGC19-C possibly reflect sea water incursion into distributary channels, flooding of the delta plain, or reworked material from underlying marine strata.
Several studies have addressed the time associated with deposition of the Cholan Formation using biostratigraphy, magnetostratigraphy and/or astrochronology (Chen et al., 1977(Chen et al., , 2001;;Horng & Shea, 2007;Vaucher et al., 2021Vaucher et al., , 2023)).The average sedimentation rate calculated for the Cholan Formation exposed along the Houlong River (後龍溪; Figure 1B) is ca 110 cm kyr −1 (Chen et al., 1977;Vaucher et al., 2021).Assuming a similar rate for the Da'an River section, the duration associated with the deposition of the 135.5 m thick section is approximately 120 kyr.This estimate is consistent with previous sedimentation rates of 30-150 cm kyr −1 calculated at various stratigraphic positions along the Cholan Formation (Vaucher et al., 2023).Since no accurate time control exists for the Cholan Formation along the Da'an River, it is not trivial to disentangle eustatic versus supply-driven controls on sedimentation because both can produce similar sedimentary facies trends (Burgess & Prince, 2015).Tentative stratigraphic correlations have been proposed between exposures of the Cholan Formation and coeval strata (Chen et al., 2001;Lin et al., 2007;Nagel et al., 2013;Pan et al., 2015), and these correlations suggest two general proximal-distal trends: (i) from north-east to south-west, and (ii) from south-east to north-west.The north-east to south-west palaeoenvironmental trend reflects the development of the foredeep following oblique collision of the Luzon Arc and the Eurasian Plate, whereas the south-east to north-west trend is consistent with the strait margin (Castelltort et al., 2011;Lin & Watts, 2002;Nagel et al., 2013Nagel et al., , 2018)).However, the absence of a highresolution temporal framework and the high sedimentation rate together hinder precise regional correlations of strata that would enable development of a basin-wide sequence/genetic-stratigraphic framework.
Sedimentary basins characterised by high rates of sediment accumulation are more prone to record highfrequency allogenic fluctuations (i.e.sea level and sediment supply) that affect sedimentation.Observations made in the Cholan Formation along the Houlong River (north of the study location; Figure 1B) suggest that it is highly probable that both sea level and sediment supply affected deposition of the Cholan Formation along the Da'an River too, and that the resulting stratigraphy preserves evidence of orbitally driven sedimentary cycles (Vaucher et al., 2021(Vaucher et al., , 2023)).As demonstrated in other coeval sections, the high completeness of the Cholan Formation allows for astronomical tuning of its stratigraphic record (Vaucher et al., 2021(Vaucher et al., , 2023)), which is probably the only way to correlate sections of the Cholan Formation and obtain a precise depositional history and evolution for the WFB.This can be applicable to other basins with similar settings.

| Storm-flood-dominated deltas
The sedimentary facies of the Cholan Formation indicate that fluvial and storm processes acted together in shallow-marine environments of the WFB (Figures 2  through 7).Since its formation, the (palaeo-)Taiwan Strait (i.e. the WFB) has been repeatedly affected by TCs that cross over Taiwan first, then the Taiwan Strait, and finally  et al., 2020;Dashtgard et al., 2021a;Galewsky et al., 2006).Rapid increases in hinterland precipitation into short, steep rivers typically results in the of hyperpycnal discharge from such sediment-laden rivers, and this is the case for rivers in Taiwan (Figure 8; Milliman  , 2007, 2017;Mulder et al., 2003).The simultaneous delivery of sediment from river mouths and the action of storm waves that rework that sediment in coastal environments during TCs is referred to as 'storm flood' (Collins et al., 2017) or 'oceanic flood' (Wheatcroft, 2000).Stormflood deltas develop preferentially in tropical to subtropical climates and at the mouths of short rivers with small catchments that experience relatively frequent TCs or storms (Collins et al., 2017;Lin & Bhattacharya, 2021).
It is likely that the oceanic currents acting in the Taiwan Strait (Figure 1A) have impacted depositional dynamics since the formation of the strait (Nagel et al., 2018).Both alongshore and TC-induced currents remobilise sediment carried into storm-flood deltas during river floods.The persistent alongshore currents and the multiple rivers delivering sediment along the shoreline of west Taiwan prevent the formation of the typical loci of deposition expected in deltaic environments (Bhattacharya, 2006;Dillinger et al., 2022;van Yperen et al., 2020;Vaucher et al., 2020), and this results in multiple smaller delta deflected and merged into a single and larger delta on the west side of Taiwan (Figure 8); this is similar to straitmargin deltas elsewhere (Longhitano & Steel, 2017;Rossi et al., 2017).

A B D C
Gutter casts observed within the Cholan Formation formed through erosion by downwelling storm flows in shallow-marine environments (Figure 3D; Myrow, 1992).However, gutter casts in the Cholan Formation are not ubiquitous features as described in other storm-flood delta successions such as the Baram Delta (Borneo; Collins et al., 2017) or the Gallup system in the Cretaceous Western Interior Seaway (USA; Lin & Bhattacharya, 2021).The relatively small storm waves in the Taiwan Strait are a result of the strait being protected from the open Pacific Ocean by Taiwan, and this may explain the paucity of oscillatory structures and gutter casts in the Cholan Formation that are typical of other storm-flood deltas.Moreover, the dominance of current-generated sedimentary structures is interpreted as the result of strong alongshore currents in the Taiwan Strait, which may counteract and dampen oscillatory wave motion (Table 2; Figures 2, 3 and 6).The dominance of current-generated sedimentary structures, paucity of oscillatory structures and rare gutter casts should be analogous to other storm-flood-dominated deltas in seaways wherein alongshore currents dominate, and where there are multiple sediment sources fed by steep drainages and short river catchments.
The deposits that form in the studied shallow-marine strata of the palaeo-Taiwan Strait are best described as a storm-flood-dominated delta fed by multiple sediment sources (Figure 9).Following the initial proposition of 'oceanic flood' by Wheatcroft (2000), the storm-flood concept has only recently been introduced in the literature (Collins et al., 2017;Lin & Bhattacharya, 2021).In comparison with the Baram Delta (Collins et al., 2017) or the Gallup system (Lin & Bhattacharya, 2021), the type of storms affecting sedimentation in this study are TCs.Previous studies on palaeoenvironmental reconstructions of the Cholan Formation and coeval strata in Taiwan (Chen et al., 2001;Covey, 1984Covey, , 1986;;Nagel et al., 2013;Pan et al., 2015) did not integrate the storm-flood model.However, workers recognised concurrent river and wave processes acting on sedimentation (Dashtgard et al., 2020;Dashtgard et al., 2021a), and it is highly probable that other storm-flood delta successions (Figure 9) will be recognised both spatially and temporally in the WFB.

| Tropical cyclones: flux, motion and deposits
Tropical cyclone beds in the Cholan Formation preserve sedimentary structures that reflect a range of TC scenarios that affected the palaeo-Taiwan Strait (Table 2; Figures 6  and 9).First, inversely graded beds record hyperpycnal flows probably produced during TCs (Milliman et al., 2007;Milliman & Kao, 2005) but they could also reflect hyperpycnal discharge associated with elevated rainfall during other weather events (e.g.monsoon; Hage et al., 2019;Mulder et al., 2003).Consequently, hyperpycnites are not

A B
discussed here as characteristics of TCs.The response of Taiwan's drainage systems and wave energy generated in the adjacent Taiwan Strait is not consistent between TCs and depends on multiple factors.Tropical cyclones either pass over or near Taiwan (Figure 8), their intensity fluctuates, and the amount of precipitation varies (Agyakwah & Lin, 2021;Chen et al., 2018;Dashtgard et al., 2020;Milliman et al., 2007;Milliman & Kao, 2005).Between two consecutive TCs and depending on the alongshore and cross-shore position on the storm-flood delta, the amount of sediment received and the intensity of oscillatory and bottom currents will vary.Non-TC-induced currents acting in the strait also influence the preservation of TC beds.Dashtgard et al. (2020) described the archetypal sedimentary expression of TCs in the sedimentary record of the palaeo-Taiwan Strait, and the four stages of TCs were recognised in the TC bedsets: storm build-up, passage of the eyewall and eye, waning storm and relaxation (Mitchell et al., 2005;Teague et al., 2007).Here, none of the identified TC beds preserve evidence of all four stages.Trough cross-stratified sandstone (F9; Figure 6D,E,G) with coeval deposition of coal and mud clasts and pebbles probably record storm build-up and passage of the eyewall and eye.
Strong unidirectional currents affected the sea floor during these phases, and a high volume of mixed suspended and bedload sediment was brought into the storm-flood deltas and reworked (Dashtgard et al., 2020;Mitchell et al., 2005;Teague et al., 2007).Beds of F8 (Figure 6A,B,C,F) that display mainly oscillatory structures (aggrading wave ripples and HCS), likely reflect the waning and relaxation phases of TCs or record deposition on the periphery of TCs.During the waning and relaxation phases, oscillatory motion becomes more significant than unidirectional flows because of the decrease in storm intensity and associated slowdown in Conceptual model of a storm-flood-dominated delta system with multiple sediment sources and the associated delta plain developed along the coast of Taiwan during the Early Pleistocene.These environments are recognised from sedimentary facies in the Cholan Formation along the Da'an River (大安溪).The stratigraphic pattern of each sedimentary facies is shown.The cloud with the rain and the seismic icon symbolise the frequent tropical cyclones and earthquakes that affect Taiwan, respectively.The colour scheme and sedimentary structures of the small logs correspond to those of Figure 2. Fwwb: fair-weather wave base; swb: storm wave base.
unidirectional bottom currents (Jelby et al., 2020;Mitchell et al., 2005;Myrow & Southard, 1996;Teague et al., 2007).At the same time, sediment flux via river discharge remains high, and is responsible for the introduction of fluid muds that will later settle on the delta front.The fact that none of the described TC beds in the Cholan Formation along the Da'an River shows the four stages of a TC is likely due to differences in TC path, intensity, water depth and sediment supplied to the palaeo-Taiwan Strait.Herein this study supplements and lists different sedimentary signatures of TCs acting in a tectonically active strait where alongshore currents dominate in the shallowmarine realm, and multiple sources fed by steep drainages and short river catchments provided sediment.The complex depositional patterns described in TC beds in the Cholan Formation reflect hydrodynamic motion acting on the sea floor, which depends on: (i) storm conditions (i.e.wind speed, precipitation, duration) at different (palaeo) latitudes, (ii) position of the storm, (iii) basin physiography, (iv) basin margin type and (v) interaction with other coastal processes (Bhattacharya et al., 2020;Bhattacharya & MacEachern, 2009;Cheel & Leckie, 1992;Collins et al., 2017;Grundvåg et al., 2020;Jelby et al., 2020;Myrow & Southard, 1996;Sleveland et al., 2020;Snedden et al., 1988;Vaucher et al., 2017).

| Earthquake-triggered deformation?
Soft-sediment deformation structures are widely observed in ancient sedimentary successions (Avşar et al., 2016;Gladkov et al., 2016;Martín-Chivelet et al., 2011) and maybe interpreted as seismically induced structures by taking into consideration their geological context (Montenat et al., 2007;Owen et al., 2011;Sims, 2012).The high seismic activity associated with the uplift of Taiwan (Shin & Teng, 2001) triggered post-depositional deformation of compacted to weakly lithified beds of the storm-flood delta (Figure 7).The matrix-supported conglomerate and convolute structures in the Cholan Formation are suggested here as seismites recording earthquake-induced liquefaction (Allen, 1986;Owen, 2003;Sims, 2012).Fluvial sediment discharge during high rainfall events can result in rapid accumulation of thick sedimentary packages and the weight of this sediment mass could also result in liquefaction.However, thick TC beds identified in the Cholan Formation (Figure 6) do not show evidence of liquefaction suggesting that high sediment loads are not the mechanism responsible for forming SSD.In turn, it is likely that beds with similar SSD formed in other storm-flood-dominated deltas with high rates of sedimentation, and that were repeatedly subject to earthquakes.

| CONCLUSION
The Pleistocene Cholan Formation exposed along the Da'an River (大安溪) in the WFB of Taiwan has been analysed sedimentologically, ichnologically and biostratigraphically, and these data are used to refine the depositional model of this formation.The base of the Cholan Formation section records deposition in storm-flooddominated prodelta to delta-front environments, and these facies pass upwards into delta-plain facies.The storm-flood delta identified herein differs from other examples in that oscillatory-induced sedimentary structures (wave ripples and hummocky cross-stratification) and gutter casts are less common.Instead, sediment deposition in the storm-flood delta in the Cholan Formation was dominated by unidirectional traction flows in response to primarily shore-parallel currents operating in the palaeo-Taiwan Strait.
The storm-flood delta succession described here records multiple high-energy event beds that result from the action of TCs and earthquakes.Tropical cyclone beds are identified by (i) erosively based bedsets of trough crossstratified sandstone that show evidence of concurrent mud deposition (intercalated mud clasts and beds), (ii) sandstone showing evidence of aggrading wave ripples and (iii) HCS sandstone with rare gutter casts filled with coal and shell material and pebbles.These different types of sedimentary signatures reflect the different phases of TCs and help identify TC or storms in the rock record.Earthquake activity is recorded in soft-sediment deformation structures that show evidence of breakup of compacted to weakly cemented sandstone beds that sank into underlying water-saturated sediment during earthquakes.

CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.

F I G U R E 3
Photographs of shallow-marine facies.(A) Thinly laminated, brownish mudstone containing very fine light grey sandstone laminae (F1) that grades upwards into bioturbated mudstone (F2; 50 m; Figure2; B) Heterolithic interval with light grey wavy-parallel laminated sandstone interbedded with dark grey, massive to laminated mudstone (14 m; Figure2).Mudstone beds locally show erosive bases (white arrow).(C) Light grey sandstone-dominated heterolithics.Note low-angle cross-stratified sandstone with an erosive base and small coal clasts (cc) in the centre of the photograph (11.5 m; Figure2).(D) Gutter cast scoured into cross-stratified sandstone and filled with articulated and disarticulated shells and coal clasts.Note the similarity in the fill of the gutter cast and the overlying bed (white arrow; 1 m; Figure2).(E) Sandstone displaying symmetrical ripple cross-lamination (white arrow; 10.5 m; Figure2).(F) Mud drapes on combined-flow cross-lamination (29 m; Figure2).(G) Thick trough cross-stratified medium-grained sandstone (F5; 4 m; Figure2).Scale bar increments are 1 cm unless otherwise noted.Facies number is shown in the top right corner of each photograph.

F
Photographs of terrestrial facies in the Cholan Formation.(A) Overview of the stratigraphic pattern of delta-plain deposits composed of massive to trough cross-stratified sandstone (F7) interbedded with laminated mudstone (F6; 110 m; Figure 2).The position of B is highlighted by the dashed white box.(B) Erosive base of a trough cross-stratified sandstone bed that is rich in coal clasts, mud rip-up clasts and pebbles (white arrows).(C) Thick laminated mudstone interval with very fine-grained sandstone beds, laminae and desiccation cracks (128 m; Figure 2).The position of D and E are shown by the dashed white boxes.(D) Sharp-based, very fine-grained sandstone bed showing current-ripple cross-lamination within an interval of unbioturbated mudstone.(E) Desiccation cracks in laminated mudstone that are filled with massive, very fine-grained sandstone.Scale bar increments are 1 cm.Photographs of inferred tropical cyclone beds in the Cholan Formation.(A) Erosive-based and wedge-shaped very finegrained sandstone bed (F8) that truncates into laminated and wavy-bedded heterolithics (63 m; Figure 2).The top of the sandstone bed is scoured by a channel form filled with heterolithics and interpreted as formed via waning to post-storm hyperpycnal flow.(B) Lateral view of (A) exhibiting a variable quantity of muddy and organic remains (white arrows).(C) Lateral close-up of (A) showing that the sandstone bed consists mainly of vertically aggrading symmetrical cross-lamination.(D) Amalgamated fine-grained to medium-grained cross-stratified sandstone beds (F8, F9) interbedded within heterolithic beds (24 m; Figure 2).The position of E is demarcated by the white dashed box.(E) The lower bed of F9 shows multiple flow orientations based on trough cross-stratification foresets, and the upper bed (F8) displays low-angle and hummocky cross-stratification.Note that the hummocky cross-stratification exhibits a long wavelength.The position of G is shown by the white dashed box.(F) Wavy to quasi-planar-laminated fine-grained sandstone (F8) interbedded with mudstone (75 m; Figure 2).Scale bar increments are 1 cm.(G) Trough cross-stratification with mud and coal clasts, pebbles and shell remains on the foresets.etal.

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I G U R E 7 Photographs of earthquake-induced liquefaction (deformation) in the Cholan Formation.(A) and (B) show plastically deformed clasts of sandstone (yellow-grey), encased in structureless mudstone (dark grey; 64.5 m; Figure 2), thus corresponding to a matrixsupported conglomerate.(C) and (D) show a sandstone bed exhibiting convolute bedding at the bottom and a massive appearance towards the top (16 m; Figure 2).

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I G U R E 8 (A) Satellite image of Taiwan under fair-weather conditions.Note the dark colour of the seas and ocean around the island.(B) Satellite image of Taiwan 4 days after the passage of Typhoon Mindulle.The track on the TC is shown, and each TC symbol marks the position of the eye of the storm every 3 h.The lighter colour of the water surrounding Taiwan (mainly south and west part) reflects the distribution of a large suspended sediment plume exported by rivers during and following the tropical cyclone.The extent of bottomhugging hyperpycnal flows remains unknown (images source: https://earth obser vatory.nasa.gov/images/13444/ flood s-in-south ern-taiwan; Typhoon Mindulle track source: https://zoom.earth/storm s/mindu lle-2004/#map=daily).
This research was funded through Swiss National Science Foundation Postdoc.Mobility (P400P2_183946) and Postdoc.Mobility return (P5R5PN_202846) grants to R.V., a Natural Sciences and Engineering Research Council of Canada Discovery Grant (RGPIN-2019-04528) to S.E.D., and Ministry of Science and Technology (Taiwan) grant to L.L. (MOST 107-2116-M-002-011).L.L. acknowledges support from 'The Featured Areas Research Center Program' within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan.Yu-Yen Pan and Romy Ari Setiaji are warmly thanked for their valuable assistance in the field.We appreciate the constructive feedback from reviewers Sergio Longhitano and Mads E. Jelby and editors Paul Carling and Peter Swart, who helped to improve this manuscript.