Abstract Channel horizons, delimiting a major Pliocene sedimentary prism in the central North Sea basin, have been investigated in the context of oceanographic, climatic and tectonic controls, using 3D seismic and well data from the UK and Norwegian sector. Aggrading channel structures, forming 700–2500 m wide and 75–150 m deep linear to arcuate troughs, superimpose the truncated distal part of the prism in the northern Central Graben. The origin of the aggradational channel and prism complex is related to differential deposition and erosion by contour currents that intensified as tectonic subsidence formed a deep marine basin (>600 m) with open connections to the Atlantic and Norwegian Sea. The largest channel troughs appear to be filled with consolidated sediments and emerge from areas where the Neogene strata is pierced by salt diapir chimneys. From Early Pleistocene ( c . 2.5 Ma) the channels were subject to progressive burial by rapid clinoform progradation. Based on the seismic observations, a depositional model is proposed that relates contourite channel development to fluid expulsion from salt diapir structures and fracture zones extending from lower Miocene strata. The sedimentary prism accumulated over a Late Miocene/Early Pliocene Unconformity marked by incised channels that are reminiscent of a northward diverging drainage system. This erosive low-stand development is probably related to late Alpine compression, which promoted uplift in the British Isles and the Channel region. The ensuing subsidence of the central North Sea, associated with concomitant uplift of the Norwegian–Danish Basin, generated the present southwestward dip of 0.5–0.8° of the basal unconformity. The Late Miocene compressional phase followed by rapid basin depending during Pliocene to early Pleistocene suggests that present concepts of North Sea basin development have to be re-evaluated.

Abstract The Cape Turnagain area is located on the inboard portion of the Hikurangi subduction margin, on the northern Wairarapa coast of the North Island of New Zealand. A 4.5 km long coastal section of sea cliffs of Mio-Pliocene sediments contains numerous tubular carbonate-rich concretions. Their morphology and petrographical observations suggest they were possibly formed by fluid flows of carbonate-rich water through a silty sediment. These tubular concretions could be fossil fluid expulsion structures similar to dewatering chimneys described offshore in New Zealand. The external diameter of the concretions observed in situ reaches 60 cm and internal canal up to 4 cm. Up to four canals are encountered in a single concretion. A positive relationship is observed between the chimney size and the number of canals or cumulative diameter of canals, suggesting that the size of the concretion is a function of the fluid which flowed through the plumbing network. The increased number of tubular concretions in upper Miocene siltstones compared to overlying Pliocene strata could be linked to a compressive event that caused overpressuring and the expulsion of fluids through the sediment pile.

Abstract Four-component ocean-bottom-cable (4-C OBC) seismic data acquired in deep water across the Gulf of Mexico were used to study near-sea-floor geologic characteristics of fluid-gas expulsion systems. Although these 4-C OBC data were acquired to evaluate oil and gas prospects far below the sea floor, the data have great value for studying near-sea-floor geology. The research results summarized here stress the importance of the converted-shear-wave (P-SV) mode extracted from 4-C OBC data. In deep water, the P-SV mode creates an image of near-sea-floor strata that has a spatial resolution an order of magnitude better than the resolution of compressional wave (P-P) data regardless of whether the P-P data are acquired with OBC technology or with conventional towed-cable seismic technology. This increased resolution allows the P-SV mode to define seismic sequences, seismic facies, small-throw faults, and small-scale structures that cannot be detected with P-P seismic data.

Abstract Intense faulting in the northern Gulf of Mexico slope province results from complex interactions between subsurface salt and the deposition of large volumes of sediment. Many of these faults provide pathways for subsurface fluids and gases to migrate from deep petroleum-generating zones to the modern seafloor. These migration pathways are concentrated along the margins of intraslope basins where they are directed by a spectrum of salt geometries. Both geological and biological responses are highly variable and dependent on rate of delivery as well as on fluid and gas composition. Qualitatively, rapid expulsions of gas-charged fluids (including fluidized sediment) result in the deposition of sediment sheets or, mud volcanoes. Both products of rapid expulsion vary greatly in scale. The sheet-like flows may be localized or extend over many square kilometers of the slope while mud volcanoes vary from < 1 m to several km in diameter. Hydrocarbons associated with rapid flux systems reflect little biodegradation during migration. Sediment samples from these seafloor expulsion areas frequently contain hydrocarbons that are remarkably similar to those that are produced from the parent deep subsurface reservoirs that are directly connected to the surface by faults. High accumulation rates, thin depositional units, and limited hydrocarbon storage capacity characterize sediments of rapid flux systems. Lucinid-vesycomyid clams and bacterial mats are the chemosynthetic communities that dominate in these settings. At the other end of the flux rate spectrum, slow hydrocarbon seepage results in lithification and mineralization of the seafloor. Microbial utilization of hydrocarbons promotes the precipitation of 13 C- depleted Ca-Mg carbonates as by-products. These products occur over the full depth range of the slope. Mounded carbonates can have relief of up to 30m, but mounds of 5-10m relief are most common. Mound-building carbonates represent mixed mineral phases of aragonite, Mg-calcite, and dolomite with Mg-calcite being the most common. Barite is another product that is precipitated from mineral-rich fluids that arrive at the seafloor in low-to-moderate seep rate settings. Hydrocarbons analyzed from these slow-flux settings are highly biodegraded and chemosynthetic organisms are generally limited to bacterial mats. Below water depth of approximately 500 m, intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. These environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux. However, this dynamic setting apparently has a constant supply of hydrocarbons to promote gas hydrate formation at the seafloor even though oceanic temperature variation (primarily on the upper slope) cause periodic shallow gas hydrate decomposition. In the northern Gulf, gas hydrates contain both thermogenic and biogenic gas. The presence of these deposits provides the unique set of conditions necessary to sustain dense and diverse chemosynthetic communities. The cross-slope variability of seafloor response to fluid and gas expulsion is not well known. However, present data indicate that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change downslope from isolated salt masses to canopy structures to nappes.