The raw 2D multichannel seismic reflection data presented in this report were acquired during expedition SO301 north of the Rodrigues Triple Junction. Data collection targeted the Central Indian Ridge, including the 25°S Oceanic Core Complex (OCC) and the Kairei hydrothermal field, to investigate crustal structure and faulting processes at a slow spreading ridge. A total of 29 seismic lines were recorded using an 8 km, 756-channel streamer and an air gun source array. The dataset covers approximately 1871 line kilometers and includes profiles across ridge segments, the OCC, and conjugate flanks. The data are provided in raw SEG-D format with associated navigation and standardized metadata. Detailed information on the acquisition can be found in the SO301 cruise report (https://doi.org/10.48433/cr_so301). This technical report provides further information on the acquisition of the 2D multichannel seismic reflection data set.

Our research project is dedicated to the development of a comprehensive model for analysing the distribution, dimensions, and evolution of Pleistocene tunnel valleys and their deposits in northern Germany and adjacent areas. The primary objective is to leverage these findings to assess the likelihood of future tunnel-valley formation, with potential implications for the long-term (over the next 1 million years) safety of a radioactive waste repository.To achieve our goal, we are relying on a 3D seismic dataset. Previously, the mapping of tunnel valleys on land is primarily based on 2D seismic and boreholes, which unfortunately do not provide the required accuracy. Therefore, we have opted to utilize a marine seismic dataset. This 3D seismic dataset 'GeoBasis3D' was acquired by the BGR in 2021.The 3D seismic dataset is situated within the German Exclusive Economic Zone (EEZ) in the 'Entenschnabel' area. In this region, two intersecting tunnel valleys exist, with one located above the 'Belinda' salt dome. The interpretation of the tunnel-valley base based on the seismic data, and we will observe the influence of the crestal faults above the salt dome on the genesis and filling of the tunnel valley. The filling of the tunnel valleys will be described in terms of seismic facies. Different sedimentary processes can be interpreted from the seismic data. The deepest parts of the tunnel valley are directly filled, and the valley widens above. Some slumping can be detected along the steep slopes of the tunnel valley. Different phases of sedimentation can be observed within the tunnel valley, including both glacifluvial and glacilacustrine phases with parallel and homogenous reflectors. Since there are no available geological cores for the Quaternary in the area of the seismic surveys, we will have to rely on cores from Danish North Sea for the lithostratigraphic description of the sediments and for their chronological classification.Our aim is to analyse sediment facies to draw conclusions about the backfilling process and repeated erosion phases. This will enable us to compare the findings with the development of onshore tunnel valleys in the next step. The tunnel valleys are a type of glacial erosion that can reach depths of up to 600 meters above sea level in northern Germany. They can have an impact on the long-term safety of a repository, which is required by law to be located at a minimum depth of 300 meters below ground level.

In June 2021, a novel Danish national carbon capture and storage strategy was ratified by the Danish Parliament, and this was followed by the initiation of the project ‘CCS2022–2024’, led by the Geological Survey of Denmark and Greenland. In collaboration with other institutions, we acquired and interpreted new 2D seismic data between 2022 to 2024 to investigate and mature eight sites for potential subsurface storage of CO2 in Danish onshore and offshore areas. This Bulletin contains a series of papers that present important results of the work. In this introduction paper, we provide an overview of seismic acquisitions and the interpretation of seismic data together with existing deep wells. The study sites selected are large subsurface structures located in onshore Jylland, Sjælland and Lolland and offshore Denmark in the eastern North Sea. The onshore targets are the Gassum, Havnsø, Rødby, Stenlille and Thorning structures, while the offshore sites comprise the Inez, Jammerbugt and Lisa structures. The project work comprises a series of reports regarding extensive seismic acquisition, processing and interpretation of the new and pre-existing seismic data as well as other publications emanating from the project. This Bulletin and the technical reports present an improved understanding of the formation, composition and geometry of the investigated structures. The studies include the mapping of the reservoir and seal formations, identification of principal faults, interpretation of the stratigraphic and structural development, reservoir and seal characterisation and estimates of the static storage capacity. Hence, this research provides a significant step forward concerning characterisation of the geology and maturation of the potential storage sites. In addition, it has inspired new ideas, including an updated regional stratigraphic interpretation of the Triassic succession of the Danish Basin and correlation with adjacent basins.

ABSTRACT Seismic data from the North Sea commonly show vertical acoustic blanking (VAB) often interpreted as fluid conduits with implications for Quaternary development. The robustness of this interpretation has long been controversial as the infill of tunnel valleys can also cause vertical blanking. Using 2D and 3D seismic data and sediment echosounder data from the German North Sea, we investigate VAB to determine a geological or imaging origin of these anomalies. We detected multiple VAB occurrences throughout the North Sea. 3D data from the Ducks Beak (‘Entenschnabel’) reveal a correlation of VAB with bright spots in incised channels directly below the seafloor. Large source–receiver distances allow imaging the subsurface below the channel without signal penetrating through it (undershooting). This method removes the blanking. Energy absorption by shallow biogenic gas trapped within the channels explains the observed VAB. Hence, the blanking represents an imaging artifact, highlighting the need for careful seismic processing with sufficient offset before interpreting such anomalies as fluid pathways. The channels belong to a postglacial channel system related to the now submerged lowlands of Doggerland. This work demonstrates the usability of mapping VAB to detect shallow features for paleo‐landscape reconstruction and identification of shallow gas for hazard assessments, for example.

In 1964, exploration drilling in the German Sector of the North Sea hit a gas pocket at ∼2900 m depth below the seafloor and triggered a blowout, which formed a 550 m-wide and up to 38 m deep seafloor crater now known as Figge Maar. Although seafloor craters formed by fluid flow are very common structures, little is known about their formation dynamics. Here, we present 2D reflection seismic, sediment echosounder, and multibeam echosounder data from three geoscientific surveys of the Figge Maar blowout crater, which are used to reconstruct its formation. Reflection seismic data support a scenario in which overpressured gas ascended first through the lower part of the borehole and then migrated along steeply inclined strata and faults towards the seafloor. The focused discharge of gas at the seafloor removed up to 4.8 Mt of sediments in the following weeks of vigorous venting. Eyewitness accounts document that the initial phase of crater formation was characterized by the eruptive expulsion of fluids and sediments cutting deep into the substrate. This was followed by a prolonged phase of sediment fluidization and redistribution widening the crater. After fluid discharge ceased, the Figge Maar acted as a sediment trap reducing the crater depth to ∼12 m relative to the surrounding seafloor in 2018, which corresponds to an average sedimentation rate of ∼22,000 m 3 /yr between 1995 and 2018. Hydroacoustic and geochemical data indicate that the Figge Maar nowadays emits primarily biogenic methane, predominantly during low tide. The formation of Figge Maar illustrates hazards related to the formation of secondary fluid pathways, which can bypass safety measures at the wellhead and are thus difficult to control.

The Olga Basin is a Paleozoic and Mesozoic sedimentary basin in the northern Barents Sea. Gas seepage at the basin's margins indicate active but so far poorly understood petroleum systems. Despite the consensus that late Cenozoic uplift and erosion must have strongly influenced petroleum systems in this region, the amount of erosion is controversially discussed. To decipher the influence of erosion, we studied petroleum generation from potential Paleozoic and Mesozoic source rocks based on two basin and petroleum systems models. Three scenarios were calculated for each model with Cenozoic erosion amounts of 500 m, 1000 m and 1800 m, respectively. Petroleum has been generated in all scenarios from the Carboniferous to Triassic source rocks, but Jurassic or younger source rocks remain immature even for the highest amount of erosion. Maturity estimates from bound gas measurements of seafloor sediments in the Olga Basin indicate that the migrated gas originates from an oil window mature source rock (vitrinite reflectance between 0.65% and 1.1%), which fits well to the calculated maturity of Upper Permian to Triassic potential source rocks. Our petroleum systems modelling data indicate that main hydrocarbon generation from a Carboniferous source rock occurred during the Triassic and generation from Upper Permian to Mid-Triassic source rocks occurred during the Cretaceous to Mid-Paleogene period. Different erosion scenarios had no influence on petroleum generation from a Carboniferous source rock, whereas modelled decreasing erosion from 1800 m to 500 m suppresses petroleum generation from Triassic source rocks by c. 60%. Petroleum might have charged closure structures at different stratigraphic levels (Carboniferous-Jurassic) mapped in the northeastern Barents Sea. Gas seepage, indicated by enhanced bound gas concentrations and the occurrence of pockmarks and flares, at the basin's margins can be explained by subcropping Jurassic shales to the south and intense faulting to the north.

In the southern part of the Norwegian Barents Sea extensive hydrocarbon exploration and drilling has led to several oil and gas discoveries, yet little is known on the petroleum systems and potential hydrocarbon reservoirs in the northern Norwegian Barents Sea. If hydrocarbons generated by thermogenic processes deep in the subsurface migrate to the surface, traces of these hydrocarbons can remain in the near-surface sediments and provide indirect information on potential petroleum systems. Near-surface hydrocarbon prospecting is a method frequently used in hydrocarbon exploration of frontier areas, where no direct geochemical information from drilling is available. To improve knowledge of subsurface structures and the evolution of potential petroleum systems in the northern Barents Sea, seismic data and near-surface sediment samples were collected. Analysis of bound hydrocarbon gases extracted from these sediments revealed concentrations significantly above background in areas along the Hornsund-Knollega Fault Complex as well as near margins of the Olga Basin. Generally, the compositional and stable carbon isotope signatures of bound gases indicate thermogenic origins from source rocks of oil window maturity for near-surface gases with anomalous high concentrations, whereas for sediments with low concentration of bound gases, mixture of thermogenic and microbial gas is indicated. Amount and composition of bound gas extracted from source rock samples from Spitsbergen indicate that contribution of transported material may have influenced the bound gas in near-surface marine sediment of the Barents Sea in areas where presence of mature organic matter is indicated. In the Knolegga Fault Complex near the western Barents Sea margin high concentrations of thermogenic gas in near-surface sediments are associated with fault-bound basins and most likely originate from the Paleocene-Eocene Torsk Formation. In the Olga Basin higher bound gas concentrations occur near the southern border of the basin corresponding to sub-cropping Late Jurassic Early Cretaceous shales, whereas elevated concentrations in the northern Olga Basin are associated with reactivated faults, reaching close to the surface. Sediments above the center of the basin show significantly lower bound gas concentrations. These observations indicate that the Jurassic shales act as a regional seal for hydrocarbons and that reactivated faults at the basin margin represent pathways for migration to the surface. 1D basin and petroleum systems modeling for the Olga Basin indicates that Early to Middle Triassic sediments reached oil window maturity and represent the most likely source for thermogenic near-surface gas in that area.

Numerous studies have addressed various aspects of the East African Rift system (EARS) but surprisingly few the offshore continuation of the south-eastern branch of the rift into the Mozambique Channel. Here, we present new evidence for neotectonic deformation derived from modern seismic reflection data and sup- ported by additional geophysical data. The Kerimbas Graben offshore northern Mozambique is the most prominent manifestation of sub-recent extensional deformation. The seismic reflection data reveals that recent normal faulting often utilizes preexisting, deeply buried half-graben structures which likely are related to the formation of the Somali Basin. The ca. 30 km wide and ca. 150 km long symmetric graben is in a stage where the linkage of scattered normal faults already did happen, resulting in increased displacement and accommodation of most of the extension across the basin. However, deep earthquakes below the rift indicate a strong and still preserved lithospheric mantle. Extension is becoming diffuse where an onshore suture, subdividing the northern from the southern metamorphic basement onshore Mozambique, is closest to the offshore rift. It appears likely that this suture is the origin for the variation in rifting style, indicating that mantle fabric resulting from a Cambrian collision has been preserved as mechanical anisotropy of the lithospheric mantle. Further south the rift focuses in an about 30 km wide half- graben. An important finding is that the entire offshore branch of the EARS lacks significant volcanism. Along the off- shore EARS there are only negligible indications for recent volcanism in the reflection seismic data such as sills and dikes. Apparently the Comoros mantle plume (French and Romanowicz, 2015) has a very minor influence on the progressive extensional deformation along the northern Mozambique continental margin, leading eventually to breakup sometimes in the future. Combining structural with earthquake data reveals that the magma-poor offshore rift is in a stage where mainly the lithospheric mantle is extended but not yet broken.

AbstractThe Nares Strait is a waterway separating NW Greenland and North America. The nature of the Nares Strait has been subject of discussion for decades, especially if it represents a transform fault that compensated the opening of the Baffin Bay in the Paleogene as Alfred Wegener supposed in 1912. The Kane Basin in the central part of Nares Strait provides an opportunity to cross the proposed fault. Geophysical data were acquired in 2001 and 2010, including among others multichannel and wide‐angle seismic data. The eastern part of the Kane Basin is characterized by a solid platform most likely representing a continuation of the Paleoproterozoic Inglefield‐Mobile‐Belt (Greenland). In the western part, a sedimentary basin with northwestward tilted and eroded layers of Cretaceous age can be resolved. The transition between those two units shows the plate boundary between Greenland and North America and can be considered as a relic of the Wegener Fault.

Messinian evaporites of locally more than 3‐km thickness occupy the subduction zone between Cyprus and Eratosthenes Seamount. Based on a dense grid of seismic reflection profiles, we report on compressional salt tectonics and its impact on the Late Miocene to Quaternary structural evolution of the Cyprus subduction zone. Results show that evaporites have experienced significant post‐Messinian shortening along the plate boundary. Shortening has initiated allochthonous salt advance between Cyprus and Eratosthenes Seamount, representing an excellent example of salt which efficiently escapes subduction and accretion. Further east, between Eratosthenes Seamount and the Hecataeus Rise, evaporites were compressionally inflated without having advanced across post‐Messinian strata. Such differences in the magnitude of salt tectonic shortening may reflect a predominately north–south oriented post‐Messinian convergence direction, raising the possibility of a later coupling between the motion of Cyprus and Anatolia than previously thought. Along the area bordered by Cyprus and Eratosthenes Seamount a prominent step in the seafloor represents the northern boundary of a controversially debated semi‐circular depression. Coinciding with the southern edge of the salt sheet, this bathymetric feature is suggested to have formed as a consequence of compressional salt inflation and seamount‐directed salt advance. Topographic lows on top of highly deformed evaporites are locally filled by up to 700 m of late Messinian sediments. The uppermost 200 m of these sediments were drilled in the course of ODP Leg 160 and interpreted to represent Lago Mare‐type deposits (Robertson, Tectonophysics , 1998d, 298 , 63‐82). Lago Mare deposits are spatially restricted to the western part of the subduction zone, pinching out towards the east whereas presumably continuing into the Herodotus Basin further west. We suggest a sea level control on late Messinian Lago Mare sedimentation, facilitating sediment delivery into basinal areas whereas inhibiting Lago Mare deposition into the desiccated Levant Basin. Locally, early salt deformation is believed to have provided additional accommodation space for Lago Mare sedimentation, resulting in the presently observed minibasin‐like geometry.

The Cenozoic East African Rift System (EARS) extends from the Red Sea to Mozambique. Here we use seismic reflection and bathymetric data to investigate the tectonic evolution of the offshore branch of the EARS. The data indicate multiple and time transgressive neotectonic deformations along similar to 800 km of the continental margin of northern Mozambique. We observe a transition from a mature rift basin in the north to a juvenile fault zone in the south. The respective timing of deformation is derived from detailed seismic stratigraphy. In the north, a similar to 30 km wide and more than 150 km long, N-S striking symmetric graben initiated as half-graben in the late Miocene. Extension accelerated in the Pliocene, causing a continuous conjugate border fault and symmetric rift graben. Coevally, the rift started to propagate southward, which resulted in a present-day similar to 30 km wide half-graben, approximately 200 km farther south. Since the Pleistocene, the rift has continued to propagate another similar to 300 km, where the incipient rift is reflected by subrecent small-scale normal faulting. Estimates of the overall brittle extension of the matured rift range between 5 and 12 km, with an along-strike southward decrease of the extension rate. The offshore portion of the EARS evolves magma poor, similar to the onshore western branch. The structural evolution of the offshore EARS is suggested to be related to and controlled by differing inherited lithospheric fabrics. Preexisting fabrics may not only guide and focus extension but also control rift architecture.

The transition from continental rifting to seafloor spreading can be observed along the 2,000 km length of the Red Sea Rift system. Whereas the southern Red Sea shows seafloor spreading since 5 Ma and the central part gives evidence of a transitional stage, the northern Red Sea is thought to represent the latest stage of continental rifting. Ocean deeps along the rift axis are considered to be first seafloor spreading cells that will accrete sometime in the future to a continuous spreading axis. The northern Red Sea deeps are isolated structures often associated with single volcanic edifices in comparison with the further developed larger central Red Sea deeps where small spreading ridges are active. Our analysis of the northern Red Sea deeps showed that not all deeps can be related to initial seafloor spreading cells. Two types of ocean deeps were identified: (a) volcanic and tectonically impacted deeps that opened by a lateral tear of the Miocene evaporites (salt) and Plio-Quaternary overburden; (b) non-volcanic deeps built by subsidence of Plio-Quaternary sediments due to evaporite subrosion processes. These deeps develop as collapse structures. The volcanic deeps can be correlated with their positions in NW-SE-oriented segments of the Red Sea which are consequently termed volcanic segments. The N-S segment, linking the volcanically active NW-SE segments, is termed as "non-volcanic segment" as no volcanic activity is known, in agreement with the magnetic data that show no major anomalies. Accordingly, the deep that was analyzed in this segment is interpreted as a collapse-related structure. However, collapse-type ocean deeps are not limited to the non-volcanic segments as subrosion processes due to hydrothermal circulation are possible at any part of the axial depression. The combined interpretation of bathymetry and seismic reflection profiles gives further insight into lateral salt gliding. Salt rises are present where the salt flows above basement faults. The internal reflection characteristic of the salt changes laterally from reflection-free to stratified, which suggests significant salt deformation during the salt deposition. Acoustically transparent halite accumulated locally and evolving rim synclines were filled by stratified evaporite facies.

The Cyprus Arc in the Eastern Mediterranean represents the active collision front between the African and Eurasian (Anatolian) Plates. Along the Cyprus Arc, the Eratosthenes Seamount is believed to have been blocking the northward motion of the African Plate since the Late Pliocene-Early Pleistocene. Based on a dense grid of 2D reflection seismic profiles covering the Eratosthenes Seamount and western Levant Basin offshore Cyprus, new observations regarding the Cyprus Arc collision front at the triple transition zone Eratosthenes Seamount-Levant Basin-Hecataeus Rise are presented.The data show that the Levant Basin is filled with similar to 10 km of sediments of Early Mesozoic (probably Jurassic) to Plio-Quaternary age with only a localized deformation affecting the Miocene-Oligocene rock units. The sediments onlap directly against the steep eastern flank of the Eratosthenes Seamount to the west and the southern flank of the Hecataeus Rise to the north. The sediments show no deformation that could be associated with collision and are undeformed even very close to the two prominent structures. Pinching out of the Base Miocene reflector in the Levant Basin due to onlapping of the Middle Miocene reflector indicates uplift of the Eratosthenes Seamount and the Hecataeus Rise. In contrast to the Messinian Evaporites north of the Eratosthenes Seamount, the salt in the Levant Basin, even close to the Hecataeus Rise, is tectonically undeformed.It is proposed that the Eratosthenes Seamount, the western Levant Basin and the Hecataeus Rise act as one tectonic unit. This implies that the collision front is located north of this unit and that the Hecataeus Rise shields the sediments south of it from deformation associated with collision of the African and Anatolian Plates. (C) 2014 Elsevier B.V. All rights reserved.

Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.84.0 km s-1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 to 6.4 km s-1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.47.2 km s-1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle and lower crust with P-wave velocities from 5.3 to 7.0 km s-1. The upper and middle continental crust thin over a 120-km-wide continentocean transition zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.

Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic height (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found.The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal "normal" stable oxygen isotope values for the planktonic foraminifera Globigerinoides ruber near -1% which is indicative for carbonate formation in Red Sea surface water around 27 degrees C. However, below 182 cm in core 17008-1 highly variable 5 180 values for G. ruber between 0.26 and -10.68 parts per thousand, occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest 6180 values of -10.68% measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90 degrees C.Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0-170 cm, 370-415 cm, 69-136 cm), and allochthonous/thermally altered character (e.g. 2 and 4 corresponding to core depths 189-353 cm and 515-671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38-1.42% R-r.) and also natural coke particles were found. Similarly, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated T-max values up to 440 degrees C.An "oil-type" contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments.The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins. (C) 2010 Elsevier B.V. All rights reserved.

In the aftermath of the Mw 9.3 Indian Ocean earthquake and tsunami of 26 December 2004, which killed more than 250,000 people, numerous investigations have been commissioned near the epicenter offshore northern Sumatra to evaluate future earthquake and tsunami hazards. These projects have mapped seafloor morphology and imaged deep structures and faults in order to better understand the origin of megathrust earthquakes and tsunamis in the western portion of the Sunda Arc subduction system offshore northern Sumatra [e.g., Henstock et al., 2006].In contrast, the eastern part of the arc has received relatively little attention, even though it may be just as hazardous. Our geophysical data from the eastern Sunda Arc and the transition to the Banda Arc (Figure 1) provide evidence for recent tectonic activity and thus for a similar earthquake and tsunami risk.

Twenty multi-channel seismic lines in the southwest Sunda arc margin between Manna and west Java have been studied. This study depicts the structures and stratigraphy of the fore-arc basin since Late Paleogene in relation to the regional tectonic events. The paleomorphology of the Cretaceous continental margin persisted until the Oligocene and the paleoshelf margin of the continent extended north-westward off Sumatra. A residual basin filled with turbidite deposits developed just offshore of this margin. Subsequent fore-arc basin evolution was related to the slow down of subduction rate due to the collision of the Indian and Eurasian Plates in the Eocene. The rising Himalayan orogenic zone shed large amounts of sediment to the Indian Ocean and Sunda Trench beginning in the Late Paleogene which led to the growth of the accretionary prisms and the development of the Neogene fore-arc basin. At least two major structural events can be recognized in the fore-arc basin between Late Oligocene and Pliocene. Back thrust-faulting along the southern border of the fore-arc basin and initiation of the Cimandiri Fault Zone occurred in Late Oligocene, whereas the development of the Sumatra and Mentawai Fault Zones was initiated in Pliocene.Four Neogene sedimentary units can be recognized representing three main transgressive–regressive cycles and basin-fill deposits. The cycles resulted from a complex interplay of tectonically induced basin subsidence, eustatic sea level change and sediment supply related to volcanic activity that became abundant since late Middle Miocene. Turbidite deposition was common along and seaward of the basin slope during sea level lows in late Middle Miocene and Late Miocene. Sediment aggradation and progradation occurred during high sea level still stand and fall, respectively. Basin fills occurred mainly during the Pleistocene.