Deformation processes in the shallow levels of an accretionary prism: The Mesozoic Torlesse Terrane, South Island, New Zealand

Abstract The deformation style of the Torlesse Terrane along the southern Kaikoura coast, South Island, New Zealand, records shallow level deformation processes within an accretionary prism during the Early Cretaceous. The beds exhibit complicated structural features resulting from multistage deformations in a lithological unit, that were intimately related with the dewatering and lithification of terrigenous sediments. The earliest phase of deformation throughout the transect studied was the development of pinch-and-swell structures and boudinage fabrics due to layer-parallel extension while the beds were poorly consolidated. This was followed locally by mesoscopic tight to close recumbent folding. The beds are cut locally by two phases of mudstone intrusions. The earlier phase was initiated by 'in situ' fluidization of mudstone layers, whereas the later phase represented intrusion of siliceous claystone probably derived from an overpressured decollement. Minor faults at high-angles to bedding by layer-normal shortening then disrupted the beds throughout the transect. The deformation was followed by formation of meso- and macroscopic scale open to gentle folds by layer-parallel shortening. Kilometer-scale differential stratal rotations were produced during the final main tectonic phase that occurred in association with post-accretion Neogene regional disturbance.     

Earthquake-controlled event deposits and its tectonic significance from the Middle Permian Wandrawandian Siltstone in the Sydney Basin,Australia

Earthquake is a disaster event resulting from rapid and intensive crustal vibration caused by fault activity, volcanic eruption, or block dilapidation. Heezen and Ewing[1] and Heezen and Dyke[2] were the first to note earthquake-related mass movement and associated deposits in connection to the turbidity currents and submarine slumps triggered by the Grand Bank Earthquake in 1929. Seilacher[3] defined redeposited sedimentary beds, disturbed and modified by earth- quakes, as seismite. Since t…     

Subduction-channel model of prism accretion,melange formation,sediment subduction,and subduction erosion at convergent plate margins: 2. Implications and discussion

The subduction-channel model predicts five geologically and geophysically distinctive types of convergent plate margin. They differ primarily in the proportions of incoming sediment and upflowing melange offscraped and underplated near the toe of the overriding block. The model predicts complex patterns of uplift or subsidence of the forearc region due to differential underplating or subduction erosion and to initiation or cessation of melange upflow. It shows how subduction speed, sediment input, and pressure gradient along the channel govern the general distribution and magnitude of subduction-zone earthquakes. It can predict the upward trajectories of exotic blueschist or serpentinized peridotite blocks that become entrained in the upwelling melange; and it suggests mechanisms by which much larger tracts of coherent blueschist can be raised to shallower levels. It predicts characteristic structural and metamorphic histories for geologic units accreted during steady-state subduction, showing for example, that offscraped materials undergo subhorizontally-directed compression during final dewatering and accretion, whereas underplated ones undergo large simple shear. It gives the maximum depths reached by upwelled melange, which bear significantly on the metamorphic changes observed in them. It predicts large-scale melange diapirism in certain rare cases, in which the normally adverse pressure gradient is reversed in a short reach of the channel, as at Barbados Island. Finally, it explains why pelagic sediments are seldom found in accretionary complexes and gives insight into the factors controlling what portion of the incoming sediment, and the¹⁰Be and other tracers it carries, will reach the region of volcanic-arc magmagenesis.     

Radiolarian‐based study on the fabric and the formation process of the Early Cretaceous mélange near Zhongba,Yarlung–Tsangpo Suture Zone,southern Tibet

The Yarlung–Tsangpo Suture Zone (YTSZ), as the southernmost and youngest among the sutures that subdivides the Tibetan Plateau into several east–west trending blocks, marks where the Neo‐Tethys was consumed as the Indian continent moved northward and collided against the Eurasian continent. Mélanges in the YTSZ represent the remnants of the oceanic plate through subduction and collision. Mélanges are characterized by a highly sheared volcanoclastic or siliceous mudstone matrix including blocks of chert, claystone, and basalt. Detailed radiolarian analyses are conducted on the mélange near Zhongba County. Macroscopic, mesoscopic, and microscopic observations are combined in order to elucidate the relationships among age, lithology, and structure of blocks in the mélange. Reconstructed ocean plate stratigraphy includes Lower Jurassic limestone within the chert sequence accumulated at a depth near the CCD (Unit 2), Upper Jurassic thin‐bedded chert interbedded with claystone deposited in the wide ocean basin (Unit 3), and Lower Cretaceous chert with siliceous mudstone (Units 4 and 5), representing the middle parts of ocean plate stratigraphy. The results highlight the fabric of brecciated chert on mesoscopic scale, which is thought to be due to localized overpressure. The formation of mesoscopic and microscopic block‐in‐matrix fabrics in the mélange is proposed for the chert and siliceous mudstone bearing different extents of consolidation and competence during the progressive deformation of accreted sediments at shallow‐level subduction.     

砾岩储层地震波传播方程:三重孔隙结构模型

针对砾岩储层的砂、砾、泥三重孔隙结构特征,本文分析砾岩孔隙区域、砂岩孔隙区域以及泥岩孔隙区域相互之间的孔隙流体流动机制,将静态的砾岩骨架本构方程与动态的孔隙流体运动方程联立,提出了复杂砾岩储层的弹性波传播理论方程.采用实测砾岩储层参数,在算例中与双重孔隙介质理论进行对比分析,验证了本文理论方程的合理性;基于三重孔隙介质模型,分析不同储层环境下纵波的传播特征,结果显示:随流体黏滞系数增大,在衰减-频率轴坐标系中,砾与砂、砂与泥孔隙区域间局域流导致的两个衰减峰向低频端移动,而Biot全局流导致的衰减峰向高频端移动;嵌入体尺寸及背景相介质渗透率的变化,主要影响纵波速度频散曲线沿频率轴左、右平移,不影响波速低频、高频极限幅值;嵌入体含量及孔隙度的变化改变了岩石干骨架的弹性、密度参数,不仅影响速度频散曲线沿频率轴平移,而且影响其上、下限幅值;砾包砂包泥三重孔隙介质模型所预测的衰减曲线中,低频段"第一个衰减峰"主要由砾岩孔隙区域与砂岩孔隙区域之间的局域流导致,中间频段"第二个衰减峰"主要由砂岩孔隙区域与泥岩孔隙区域之间的局域流导致,超声频段"第三个衰减峰"由Biot全局流导致.对慢纵波传播特征的分析显示,砂岩骨架(局部孔隙度较大)内部的宏观孔隙流体流动造成的耗散明显强于砾岩与泥岩骨架.     

过零点个数地震属性理论诠释及应用

在地震解释与储层预测中,地震属性扮演着重要角色.在砂砾岩储层研究过程中,常规属性效果并不理想,无法满足实际应用的需要.而过零点个数,虽是一种不被人经常使用的地震属性,但我们发现在砂砾岩体油藏预测中应用效果良好.为此,研究过零点个数地震属性的定义与计算方法,讨论子波主频、砂岩厚度、泥岩夹层和地层韵律性等与它的关系,通过模型测试与实际资料应用对其做较全面的诠释.研究表明:(1)对于单一界面地震记录,过零点个数不随子波主频和波长的变化而变化;(2)砂岩厚度与过零点个数呈正相关,厚砂岩的过零点个数要比薄砂岩大;(3)砂体间存在的泥岩夹层,厚度变化对过零点个数影响较小,但它会影响波组的形态;(4)不规则沉积与均匀沉积相比,过零点个数属性值要大,因此,此属性比较适合多期次发育的砂砾岩体或其他薄互层结构的储层预测.总的来说,过零点个数地震属性能够有效地识别砂砾岩体发育区,可为此类储层预测提供了一种新的优势属性.     

Characteristics and sedimentary processes of lamina-controlled sand-particle imbricate structure in deposits on Lingshan Island,Qingdao, China

Imbricate structure is a common sedimentary structure and is well developed in sandy sediments.Here,we report a laminacontrolled fine-sand-particle imbricate structure in a set of very fine grained sedimentary rocks(fine sandstone,siltstone,and mudstone)at the Dockyard and Qianceng Cliff areas of Lingshan Island,Qingdao,Shandong,China.Sets of up to 300 laminae are found in stratigraphic profiles in these areas.The laminae are generally less than 1 mm thick,with most being 0.3–0.4 mm thick and the thinnest being0.1 mm.The dip angle of the imbricate structure varies widely,between 0°and 90°,with an average dip angle of about 40°–50°,which is higher than that of imbricate riverbed gravels(about 34°).The dip angle is a function of the shape and sorting of the particles,as well as the hydrodynamic conditions under which these fine-grained sediments were deposited.Several profiles show well-developed multiscale,soft-sediment deformation structures.Flute casts,load casts,and groove casts are also common.Fragments of carbon remains occur widely and commonly constitute stringers several millimeters thick and up to 10 cm long,together with fine clasts.Vitrain lenticles are also common.Based on the imbricate structure of the Lingshan Island deposits,it is shown that in addition to paleocurrent analysis,the imbricate structure can be used to infer information about fluid properties,transport characteristics,and sedimentary processes of the depositional environment.A near-bottom underflow,either of authigenic origin or derived from cold river water in winter in a delta interdistributary bay or delta-front environment,is inferred to have provided the hydrodynamic setting in which this imbricate structure formed.The imbricate structure,together with other sedimentary structures and features,shows that the sedimentary rocks on Lingshan Island were deposited in an inland,shallow-water environment,such as a delta,and not in a deep-water or submarine continental-slope environment.     

Composition and distribution of seabed and suspended sediments in north and central Torres Strait,Australia

Widespread seagrass dieback in central Torres Strait, Australia has been anecdotally linked to the delivery of vast quantities of terrigenous sediments from New Guinea. The composition and distribution, and sedimentological and geochemical properties, of seabed and suspended sediments in north and central Torres Strait have been determined to investigate this issue. In northern Torres Strait, next to Saibai Island, seabed sediments comprise poorly sorted, muddy, mixed calcareous–siliciclastic sand. Seabed sediments in this region are dominated by aluminosilicate (terrigenous) phases. In central Torres Strait, next to Turnagain Island, seabed and suspended sediments comprise moderately sorted coarse to medium carbonate sand. Seabed sediments in this region are dominated by carbonate and magnesium (marine) phases. Mean Cu/Al ratios for seabed sediments next to Saibai Island are 0.01, and are similar to those found in New Guinea south coastal sediments by previous workers. Mean Cu/Al ratios for seabed sediments next to Turnagain Island are 0.02, indicating an enrichment of Cu in central Torres Strait. This enrichment comes from an exogenous biogenic source, principally from foraminifers and molluscs. We could not uniquely trace terrigenous sediments from New Guinea to Turnagain Island in central Torres Strait. If sediments are a factor in the widespread seagrass dieback in central Torres Strait, then our data suggest these are marine-derived sediments sourced from resuspension and advection from the immediate shelf areas and not terrigenous sediments dispersed from New Guinea rivers. This finding is consistent with outputs from recently developed regional hydrodynamic and sediment transport models.     

Application of common reflection angle migration for imaging deformation structures in an inner accretionary wedge,Nankai Trough,Japan

To better image deformation structures within the inner accretionary wedge of the Nankai Trough, Japan, we apply common reflection angle migration to a legacy two-dimensional seismic data set acquired with a 6 km streamer cable. In this region, many seismic surveys have been conducted to study the seismogenic zone related to plate subduction. However, the details of the accreted sediments beneath the Kumano forearc basin are still unclear due to the poor quality of seismic images caused by multiple reflections, highly attenuated signals, and possibly complex geological structures. Generating common image gathers in the subsurface local angle domain rather than the surface offset domain is more advantageous for imaging geological structures that involve complex wave paths and poor illumination. By applying this method, previously unseen structures are revealed in the thick accreted sediments. The newly imaged geometric features of reflectors, such as the folds in the shallow part of the section and the deep reflectors with stepwise discontinuities, imply deformation structures with multiple thrust faults. The reflections within the deep accreted sediments (approximately 5 km) are mainly mapped to far angles (30°–50°) in the common reflection angles, which correspond to the recorded offset distances greater than 4.5 km. This result indicates that the far offset/angle information is critical to image the deformation structures at depth. The new depth image from the common reflection angle migration provides seismic evidence of multiple thrust faults and their relationship with the megathrust fault that is essential for understanding the structure and evolution of the Nankai Trough seismogenic zone.     

Erosion and deposition by debris flows at Mt Thomas,North Canterbury,New Zealand

The grass-covered slopes on the southern flank of Mt Thomas, an upfaulted block of highly sheared sandstone and argillite 40 km NW of Christchurch, New Zealand, are presently undergoing severe erosion by a combination of mass-wasting processes. Gully erosion, soil slips, and debris flows have carved out a number of steep, deeply incised ravines, from which coarse debris is transported (primarily by debris flows) to alluvial fans below. Geologic and historical evidence indicates that debris flows have been episodically active here for at least the last 20,000 years and have been the dominant process in fan building. This demonstrates that catastrophic geomorphic processes, rather than processes acting at relatively uniform rates, can be dominant in humid-temperate areas as well as in arid and semi-arid regions. In April 1978, debris flows were triggered in one of two unstable ravines in the Bullock Creek catchment by a moderate intensity, long duration rainstorm with a return period in excess of 20 years. Surges of fluid debris, moving at velocities up to 5 m/s, transported a dense slurry of gravel, sand, and mud up to 3·5 km over a vertical fall of 600 m. Deposition on the alluvial fan occurred when the flows left the confines of an entrenched fan-head channel and spread out as a 0·16 km² sheet averaging 1·2 m thick. In all, 195,000 m³ were deposited, roughly a third of that being reworked sediments from the head of the fan. Sediment yield from this one event would be equivalent to several thousand years worth of erosion at average sediment discharge rates for small South Island mountain catchments. Samples of viscous fluid debris during surges contained up to 84 per cent solids, composed of 70 per cent gravel, 20 per cent silt, and 4 per cent clay. Fluid density of the material ranged between 1·95 and 2·13 g/cm³, and it was extremely poorly sorted. Between surges the fluid was less viscous, less dense, and unable to carry gravel in suspension. Severe fan-head entrenchment of the stream channel (approximately 10 m in less than 24 hours) was accomplished by the erosive action of the surges. Tectonic uplift of the Mt Thomas block and the weak, crushed condition of the bedrock appear to be ultimately responsible for the catastropic erosion of slopes in the Bullock Creek catchment. However, forest clearing within the last few centuries appears to have greatly increased the rate of mass wasting and gully erosion on these slopes.     

Intertidal subsidence and collapse features on wave‐exposed,drift‐aligned sandy beaches subject to Amazon mud: Cayenne,French Guiana

Short‐lived collapse features affecting the intertidal sandy profile were observed on mud‐rich beaches in Cayenne. They were formed in packets of beach sand accumulating over thick (several metres) fluid to under‐consolidated foreshore mud, which adjusts to this sand loading by seaward and longshore migration of fluid mud and subsequent in situ mud dewatering, thus generating subsidence and deformation of the overlying sand sheet. These sandy collapse features required active longshore sand transport and were washed out by wave processes at each rising tide. They finally disappeared permanently as adjustment of the underlying mud enabled stabilization of the beach profile. Copyright © 2006 John Wiley & Sons, Ltd.     

Metamorphism and metamorphic K–Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

Abstract A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction‐related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite‐pumpellyite and pumpellyite‐actinolite. From the apparent interplanar spacing d₀₀₂ data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite‐actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K–Ar ages are reported for magnetic separates and <2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).     

Occurrence conditions of hyperpycnal flows, and their significance for organic-matter sedimentation in a Holocene estuary, Niigata Plain, Central Japan

River floods influence sedimentary environments and ecosystems from the terrestrial to the deep-marine. This study documents the occurrence conditions of hyperpycnal flows generated by river floods and related organic-matter sedimentation for Holocene sediments of the Niigata Plain, Central Japan, based on detailed sedimentary facies, total sulfur and total organic carbon content, diatom assemblages and organic-matter composition. Holocene sediments of the Niigata Plain consist of sand, mud and gravel that were deposited in estuarine and fluvial systems during a sea-level rise (15 000–6800 years BP) and stillstand (after 6800 years BP) following the Last Glacial Maximum. Hyperpycnites are present in the upper part of the estuarine lagoon sediments. The depositional age is considered to be about 5000 years BP. The hyperpycnites comprise two successions of a top fining-up unit and a basal coarsening-up unit, and include abundant terrigenous organic matter and freshwater diatoms. A large volume of freshwater is inferred to have flowed into the lagoon during deposition of the upper part of the lagoon sediments. In consequence, hyperpycnal flows may have readily formed in the lagoon, because the halocline was weak. The hyperpycnal flows also produced a layer of concentrated terrigenous organic matter in the uppermost part of the hyperpycnites. The abundant organic matter on the estuarine floor is inferred to have produced anoxic bottom conditions owing to oxidative decomposition by benthic bacteria.     

Interactions between andesitic magma and poorly consolidated sediments: Examples in the neogene shirahama group, South Izu, Japan

The Upper Miocene-Lower Pliocene Shirahama Group of south Izu, Japan, is a sequence of volcanielastic shallow-sea sediments and subaqueous lava flows. It is gently warped with indistinct preferred orientation and is cross-cut by many andesitic intrusive bodies which range in size from less than a meter to several hundred meters. The intrusive bodies exhibit various shapes and weak preferred orientation, and often have hyaloclastic and peperitic textures along their peripheries. Hydrothermally altered and chaotically disturbed zones of host rocks also occur along the contacts. The areas where the intrusive bodies are abundant are almost coincident with the areas where the more inclined beds and synsedimentary faults are developed. The intrusive bodies dragged the surrounding beds with or without synsedimentary faults. Some intrusions occurred along the faults. Conjugate sets of the faults indicate that they were produced under unstable stress conditions. These relationships between the beds and the intrusive bodies are assumed to have originated mostly by interaction between hot magma and poorly consolidated wet sediments at a shallow depth beneath the sea floor. When the intrusions took place, the stress conditions in the beds were irregular and unstable, judging from the shapes and orientation of the intrusive bodies, and also from the deformation characteristics of the enclosing beds. Some of the intrusive bodies are probably feeder dikes which supplied lave to the Shirahama Group.     

The basic instrusions at Chirha and Tikwa,Shahdol district,M.P.

While mapping parts of the Shahdol district, the author for the first time noticed a dolerite dyke at Tikwa and a basalt sill at Chirha intrusive into the Gondwanas (Supra-Barakars). Though the dolerite dyke shows no noteworthy features in the field, the sill encloses xenoliths of Supra-Barakar sandstone in its basal portion. The present paper gives an account of the mode of occurrence, petrography and age relationship of these intrusions. The presence of abundant xenocrysts of quartz in the basalt sill leads the author to term it a quartz-xenocryst-basalt. The occurrence of Deccan Trap flows in the close proximity of the intrusions suggest that they may be related to the period of Deccan Trap activity.     

Geological constraints for tectonic models of the alpha ridge

Initial interpretations of the CESAR geological samples are re-examined in light of new data from the Alpha Ridge and circum-Arctic region. A composite stratigraphy for the CESAR and Fletcher Island (T-3) pre-Neogene cores shows a sequence of Campanian-Maastrichtian organic-rich terrigenous mud overlain by Maastrichtian-Eocene biosiliceous marine deposits with a low organic content, terminating in volcanoclastic mudstone of Late Eocene age. CESAR core 6 contains a transition zone in which biosiliceous sediment is replaced by volcanoclastic and terrigenous sediment of Paleocene-Eocene age. Palynomorphs provide a Late Eocene age for the volcanic outcrop dredged from Northern Alpha Ridge. Textural and geochemical studies of laminated biosiliceous sediments were made with special techniques for quantitative analyses of very small samples (1–10 mg) and particle sizes of less than 5 microns. Results show that the laminated sediments were deposited very slowly in an oxidizing environment. Laminae in CESAR core 6 mainly reflect cyclical variations in the formation and/or accumulation of particulate iron, probably due to periodic hydrothermal venting. Absence of detrital sediment, sparsity of pyroclastic material and lack of diagenetic alteration of the biogenic sediments suggest that the eastern Alpha Ridge was not an area of major tectonic activity during the Eurekan Orogeny, from ca. 80-40 Ma.     

闽南沿海晚更新世地层的初步研究

本文对闽南沿海晚更新世沉积的分布特征和主要的地层剖面进行了描述,并根据野外沉积与地貌观察及~(14)、微体古生物与孢粉分析资料,对上述地层与沉积的年代、成因与古气候状况进行了分析与讨论     

Magnetic anisotropy of clayey and silty members of tertiary flysch from the Silesian and Skole nappes (Outer Carpathians)

AMS studies earlier carried out on the flysch sediments from the Outer Western Carpathians focused on the sandstone members deposited from turbidity currents. The main conclusion of these studies was that the dominant features of the AMS fabric developed during sedimentation and the tectonic overprint was weak. In recent years a large number of claystone (deposited also from turbidity currents, but in calm water) localities were sampled from the Krosno beds of the Silesian nappe, primarily for paleomagnetic study. Nevertheless, the AMS of the samples was also measured. The magnetic fabrics of the claystones were dominated by foliation, but lineations were also expressed and attributed to deformation. The question arose how different are the reactions of the claystone and the siltstone beds to deformation.     

Lateral variations in the lithology and organic chemistry of a black shale sequence on the Mesoarchean seafloor affected by hydrothermal processes: The Dixon Island Formation of the coastal Pilbara Terrane,Western Australia

The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia is a 3.2 Ga volcanic–sedimentary sequence influenced by syndepositional hydrothermal activity formed in an island‐arc setting. We documented lateral variations in stratigraphy, hydrothermal alteration, and biological activity recorded in the sedimentary rocks (over several kilometers), with the aim of identifying areas of biological activity and related small‐scale structures. The Dixon Island Formation comprises volcaniclastics, black chert, and iron‐rich chert within seven tectonic blocks. Based on detailed geological mapping, stratigraphic columns, carbon isotope composition, and organic carbon (C_org) content, we found lateral (>5 km) variations in stratigraphy and carbon isotope compositions in a black chert sequence above the Mesoarchean seafloor with hydrothermal activity. Two felsic tuff layers are used as stratigraphic marker beds within a black chert sequence, which was deposited on altered volcanic rocks. The black chert sequence in each tectonic block is 10–20 m thick. Thickness variations reflect topographical undulations in the paleo‐ocean floor due to faulting. Early‐stage normal faults indicate extensional conditions after hydrothermal activity. Black chert beds in the topographically subsided area contain higher C_org contents (about 0.4 wt%) than in areas around the depression (<0.1 wt%). Carbon isotope compositions for the black chert vary from ?40 to ?25‰, which are similar to values obtained for a black chert vein within the komatiite–rhyolite tuff sequence (underlying the black chert sequence). Those for other rock types in the Dixon Island Formation are ?33 to ?15‰. Results indicate that deformation occurred soon after the final stages of hydrothermal activity. After this early‐stage deformation, organic‐rich sediments were deposited over an area several kilometers across. The organic‐rich sediments indicate stagnant anoxic conditions that resulted in the deposition of siliceous and organic matter from hydrothermal vein systems. When hydrothermal activity terminated, normal faulting occurred and organic matter was deposited from the sea surface and silica from the seafloor.     

Buruanga peninsula and Antique Range: Two contrasting terranes in Northwest Panay,Philippines featuring an arc–continent collision zone

Alternating chert–clastic sequences juxtaposed with limestone blocks, which are units typical of accretionary complexes, constitute the Buruanga peninsula. New lithostratigraphic units are proposed in this study: the Unidos Formation (Jurassic chert sequence), the Saboncogon Formation (Jurassic siliceous mudstone–terrigenous mudstone and quartz‐rich sandstone), the Gibon Formation (Jurassic(?) bedded pelagic limestone), the Libertad Metamorphics (Jurassic–Cretaceous slate, phyllite, and schist) and the Buruanga Formation (Pliocene–Pleistocene reefal limestone). The first three sedimentary sequences in the Buruanga peninsula show close affinity with the ocean plate stratigraphy of the North Palawan terrane in Busuanga Island: Lower–Middle Jurassic chert sequences overlain by Middle–Upper Jurassic clastics, juxtaposed with pelagic limestone. Moreover, the JR5–JR6 (Callovian to Oxfordian) siliceous mudstone of the Saboncogon Formation in the Buruanga peninsula correlates with the JR5–JR6 siliceous mudstone of the Guinlo Formation in the Middle Busuanga Belt. These findings suggest that the Buruanga peninsula may be part of the North Palawan terrane. The rocks of the Buruanga peninsula completely differ from the Middle Miocene basaltic to andesitic pyroclastic and lava flow deposits with reefal limestone and arkosic sandstone of the Antique Range. Thus, the previously suggested boundary between the Palawan microcontinental block and the Philippine Mobile Belt in the central Philippines, which is the suture zone between the Buruanga peninsula and the Antique Range, is confirmed. This boundary is similarly considered as the collision zone between them.