Stratigraphic architecture and evolution of a deep-water slope channel-levee and overbank apron: The Upper Miocene Upper Mount Messenger Formation,Taranaki Basin

This paper re-examines the Upper Miocene Upper Mount Messenger Formation, Taranaki Basin, to characterize its architecture and interpret its environmental evolution. Analysis of stratal architecture, lithofacies distributions, and paleotransport directions over the 250 m thick formation shows the outcrops provide a nearly dip parallel section displaying the lateral relationships between contemporaneous channel-levee and overbank depositional environments. At least five 30–40 m thick upward fining units are recognized in the north-central parts of the outcrop and are interpreted as large-scale overbank avulsion cycles. Each unit consists of thick- to medium-bedded predominantly planar laminated sandstone turbidites at the base that fine upward into thin- to very thin-bedded, planar laminated and ripple cross-laminated mud-rich turbidites. The units are traceable laterally over a distance exceeding 3 km where they are cut by channels that show basal mudstone draped by medium- to thin-bedded sandstone, and onlapped by thick-bedded planar laminated sandstone at the margin. The channels are separated by tapered packages of medium- to thin-bedded turbidites containing climbing-ripple cross-lamination interpreted as levees. The individual channel-levee and overbank avulsion cycles formed through four stages: 1) a channel avulsion spread sand into the overbank as an unconfined splay, 2) preferential scouring in one area of the splay led to development of a channel with small levees that prograded across the splay, 3) a deep incision followed by abandonment of the channel deposited a mud lining. Alternatively, the mud lining was formed during the first stage as the downdip portion of the channel was abandoned. 4) The channel filled at first by thick-bedded planar laminated and then by climbing-ripple cross-laminated sand. At this time, the growth of constructional levees progressively limited sand into the overbank. Ratios of Bouma division thicknesses calculated over a stratigraphic interval present a new method to distinguish deep-water depositional environments.     

Studies of a mangrove basin,Tuff Crater,New Zealand: III. The flux of organic and inorganic particulate matter

Water samples were collected at 15 min intervals over 11 tidal cycles from a tidal creek draining the mangrove-covered basin of Tuff Crater, Auckland, New Zealand. The samples were filtered and total suspended sediment (TSS), inorganic suspended sediment (ISS) and organic suspended sediment (OSS) were determined. Variation in TSS was high, the concentration varied over a tidal cycle and during over-bank flows concentrations were lowest at or near slack high water. Covariance between TSS concentrations and velocity and discharge meant that the calculation of particulate matter flux over tidal hemicycles was particularly dependent on the method of estimating tidal flux. The hypsometrically-based volumetric method was found to be inappropriate, predicting a positive budget (import) more often than observed. Instead particulate matter budgets were calculated by means of the velocity-area method and indicate a net export of TSS, ISS and OSS. Floating macrodetritus was observed on both the flood and ebb tides, but a net export was found on the two tides monitored. It is considered that on an annual basis floating macrodetritus export accounts for less than 2% of the detrital production in the basin, and organic suspended sediment export from the basin is less than 3 kg C ha^?1 day^?1 and is below the rate of detrital production. It is implied that a proportion of the organic detritus produced in this basin is degraded and recycled in situ.     

Stratigraphic evolution of a submarine channel-lobe complex system in a narrow fairway within the Magallanes foreland basin, Cerro Toro Formation, southern Chile

This study integrates newly acquired stratigraphic data, geologic mapping, and paleocurrent data to constrain the stratigraphic evolution of the oldest channel-lobe complex in the Upper Cretaceous Cerro Toro Formation in the Silla Syncline area of the Magallanes Basin, termed the Pehoe member. The Pehoe member ranges in thickness from 60 m in the north to at least 410 m farther down system and comprises three separate divisions (A, B, and C). A lower conglomerate unit and an upper one, termed Pehoe A and C divisions respectively, represent the fill of major incised submarine channels or channel complexes. These are separated by stratified sandstone of the Pehoe B division, representing a weakly confined lobe complex, either transient or terminal.The integration of new data with observations from previous studies reveal that the three main coarse-grained conglomerate and sandstone members in the Cerro Toro Formation in the Silla Syncline include at least seven distinct submarine channels or channel complexes and two major lobe complexes. The thinning and disappearance of these units along the eastern limb of the syncline reflect confinement of the flows to a narrow trough or mini-basin bounded to the east by a topographic high. This confinement resulted in unidirectional paleocurrents to the south and southeast in all deposits. Changes in depositional geometries are interpreted as reflecting changes in sediment supply and relative confinement. Submarine channels were from 700 m to 3.5 km wide and occupied a fairway that was 4-5 km wide. Flows moving south and southeast in this mini-basin probably crossed the eastern topographic high south of the present exposures and joined those moving southward along the axis of the foreland basin at least 16 km to the east.     

Sedimentary elements,evolutions and controlling factors of the Miocene channel system: a case study of the deep-water Taranaki Basin in New Zealand

Deep-water channel systems are important petroleum reservoirs, and many have been discovered worldwide.Understanding deep-water channel sedimentary elements and evolution is helpful for deep-sea petroleum exploration and development. Based on high-resolution 3D seismic data, the Miocene channel system in the deep-water Taranaki Basin, New Zealand, was analyzed by using seismic interpretation techniques such as interlayer attribute extraction and strata slicing. The channel system was divided int...     

Sedimentary characteristics and seismic geomorphology of gravity-flow channels in a rift basin: Oligocene Shahejie Formation,Qinan Slope,Huanghua Depression of Bohai Bay Basin,China

Deep-water gravity-flow sandstones are important hydrocarbon exploration and production targets in the Bohai Bay Basin, a Paleogene intra-continental rift basin in eastern China. In this paper, the seismic-sedimentology techniques are used to characterize, in plan view, the temporal and spatial evolution of a gravity-flow-channel complex of the Paleogene Shahejie Formation (Es) on the Qinan faulted-monoslope (Qinan Slope), Bohai Bay Basin. The results show that two or three gravity-flow channels, 9–12 km long and 0.5–2 km wide, were successively developed in later Es (Es1z–Es1s). The channels initially experienced westward migration and then shifted eastward. The corresponding wireline logs of the channel-fill sequences mainly present blocky-shaped or bell-like configurations, whereas their seismic profile features are characterized by strong amplitude reflections, such as U-shaped, plate-like, spindle-shaped and lenticular configurations.The syndepositional activity of three normal faults, i.e., the Nandagang Fault to the northwest, the Zhangbei Fault to the northeast and the Zhaobei Fault to the east led to gradient changes of the Qinan Slope, which have controlled the plan morphology (width, curvature, and bifurcation) of the gravity-flow channels. In the medium-late period of Es1z, triggered by intensive faulting on the three faults, the gradient of the Qinan Slope was steepened abruptly, resulting in an increase of flow velocity and erosion amplitude to underlying deposits. As a result, channels exhibiting narrow and straight configurations in plan view were formed. During the stage of early Es1z and Es1s, tectonic activity intensity was relatively low and the gradient of the Qinan Slope was gentle, so channels with great width and curvature were bifurcated and merged downstream.Comparison of the faulting amplitude of the three syndepositional faults suggests that the Nandagang and Zhaobei faults were inversely strengthened in the Es1z and Es1s. The Nandagang Fault to the west was found to be more active than the Zhaobei Fault to the east in the Es1z stage. This condition was reversed in Es1s. For that reason, the channels migrated to the west in the Es1z stage and then went back to the east during Es1s.Core analysis shows that the channel fills are mainly composed of sandy-debrites, slumps and turbidites. Among them, sandy debrites dominate deposition in terms of reservoir volume and hydrocarbon potential. These units primarily consist of sandstones and gravel-bearing sandstones, with bed thicknesses ranging from 10 to 40 m, an average porosity of 11% and a permeability of 25 mD. Being mostly encased in organic-rich dark mudstones, these sandy debrites are significant hydrocarbon exploration targets.The results of this study are not only useful to the hydrocarbon exploration and development planning for the Qinan Slope, but also helpful when considering other faulted-depressions in the Bohai Bay Basin and other intra-continent rifted basins around the world, particularly in terms of gravity-flow hydrocarbon exploration and research.     

Sediment mounds and other sedimentary features related to hydrate occurrences in a columnar seismic blanking zone of the Ulleung Basin, East Sea, Korea

A mound related to a cold vent in a columnar seismic blanking zone (CSBZ) was formed around site UBGH1-10 in the central Ulleung Basin (2077 m water depth), East Sea, Korea. The mound is 300–400 m wide and 2–3 m high according to multi-beam bathymetry, 2–7 kHz sub-bottom profiler data, and multi-channel reflection seismic data. Seafloor topography and characteristics were investigated using a remotely operated vehicle (ROV) around site UBGH1-10, which is located near the northern part of the mound. The origin of the mound was investigated through lithology, mineralogy, hydrate occurrence, and sedimentary features using dive cores, piston cores, and a deep-drilling core. The CSBZ extends to ∼265 ms two-way traveltime (TWT) below the seafloor within a mass-transport deposit (MTD) unit. Gas hydrate was entirely contained 6–141 m below the seafloor (mbsf) within hemipelagic deposits intercalated with a fine-grained turbidite (HTD) unit, characteristically associated with high resistivity values at site UBGH1-10. The hydrate is commonly characterized by veins, nodules, and massive types, and is found within muddy sediments as a fracture-filling type. Methane has been produced by microbial reduction of CO₂, as indicated by C₁/C₂₊, δ¹³CCH4, and δD₄CH analyses. The bowl-shaped hydrate cap revealed at 20–45 ms TWT below the seafloor has very high resistivity and high salinity, suggesting rapid and recent gas hydrate formation. The origin of the sediment mound is interpreted as a topographic high formed by the expansion associated with the formation of the gas hydrate cap above the CSBZ. The lower sedimentation rate of the mound sediments may be due to local enhancement of bottom currents by topographic effects. In addition, no evidence of gas bubbles, chemosynthetic communities, or bacterial mats was observed in the mound, suggesting an inactive cold vent.     

Fracture characteristics and stress directions in the quaternary sediments of the Ulleung Basin,East Sea: Based on resistivity image log data

Abstract

Geophysical evidence indicating the presence of gas hydrate has been found in the Ulleung Basin, which lies off the east coast of the Korean Peninsula; however, hydrate distribution in the basin is not well understood. Logging-while-drilling data for 13 sites in the Ulleung Basin, East Sea, were obtained to investigate the distribution pattern of gas hydrate. Most of the sites yielded log data indicating the presence of gas hydrate. Prominent fractures (both resistive and conductive fractures) were clearly identified on the resistivity borehole images, particularly at seismic chimney sites. Resistive fractures, which contain large amounts of gas hydrate, are prominent in the seismic chimney sites. The strike and dip of each fracture was calculated and displayed on a stereographic plot and rosette diagram. From the fracture orientations on the stereographic plots, the maximum horizontal stress is NW–SE, reflecting the regional stress regime around the Ulleung Basin, although the fracture orientations are broadly distributed, indicating that the fracture pattern is not well-ordered on the rosette diagram. The fracture dips are between 36.46° and 63.66°; the range of dip azimuths is 0.94°–359°, and exhibit little change with depth. The dip azimuths are generally westerly to southwesterly.     

Seismic geomorphology and depositional system of delta and terminal fan: A case study of the Neogene Shawan Formation in the Chepaizi Uplift,Junggar Basin,China

The mainpurpose of this article is to demonstrate the utility of stratal slice images for exploring the sequence stratigraphy and sedimentology of complex depositional systems. A seismic sedimentological study was performed to map sediment dispersal characteristics of the Neogene Shawan Formation in the Chepaizi Uplift of the Junggar Basin, China. The Chepaizi Uplift is developed on the Carboniferous igneous rock basement that lies at the western boundary of the Junggar Basin. The data sources primarily include lithology, well-logging and seismic data. In the main target strata, the Neogene Shawan Formation can be divided into three fourth-order sequences (SQN₁s₁, SQN₁s₂, and SQN₁s₃), and the sequence SQN₁s₁ is subdivided into three fifth-order sequences (SQN₁s₁¹, SQN₁s₁², and SQN₁s₁³). Based on the established fine-sequence stratigraphic framework, the sedimentary facies types have been identified, they are shallow braided-river deltas, fan deltas, littoral and sublittoral lakes, braided rivers, and terminal fans. Then, stratal slices have been used to clearly depict the boundaries of sedimentary facies. Accurate results have been obtained that characterize braided river channels, terminal fans, littoral and sublittoral lake beaches, and subaqueous distributary channels in the braided-river delta front. Additionally, this seismic sedimentology study reflects variations in source area and evolution history.     

Palynofacies,organic geochemistry and depositional environment of the Tartan Formation (Late Paleocene), a potential source rock in the Great South Basin,New Zealand

Detailed palynofacies analysis of sidewall core samples taken from below, within and above the Tartan Formation (Thanetian, Late Paleocene, 58.7–55.8 Ma), a potential source rock in the epeiric Great South Basin, shows that the formation is characterised by very high percentages of degraded brown phytoclasts, rare marine algae and amorphous organic matter and thereby represents a mix of terrestrial and marine kerogen. The results indicate that the formation was deposited in a marginally marine (hyposaline), proximal environment under bottom conditions that varied from anoxic to oxic along a nearshore–offshore transect. Samples from the upper part of the underlying Wickliffe Formation indicate deposition in a marginal to normal marine, proximal environment under anoxic to oxic bottom environments. The lower part of the overlying Laing Formation was deposited in an open marine, relatively distal setting under anoxic to oxic bottom environments.     

Structural patterns of the late Mesozoic crustal detachment system in the Raoyang Sag,Bohai Bay Basin,eastern China: New insights from 3D seismic data

Understanding the late Mesozoic tectonic origin and structural patterns of the Bohai Bay Basin (BBB) is of growing importance for its buried-hill petroleum exploration. This study revealed, based on 3D seismic and borehole data, that the Raoyang Sag in the Jizhong Depression, western BBB, was dominated by a crustal detachment system during the late Jurassic to the early Cretaceous. It was characterized by the WNW-dipping low-angle detachment fault F₀ (namely Cangxi Fault), the structural dome cored by Archean Basement rocks at the footwall and supradetachment basins on the hanging wall. We suggested the late Mesozoic volcanic materials and coarse deposits accumulated in the supradetachment basins had locally diminished the petroleum prospect, but the syndetachment deformation and denudation had contributed to it by means of promoting petroleum migration from source rocks to reservoirs and improving porosity and permeability of the reservoir rocks by karstification and structural fracturing.     

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin,East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.