Thin Sediment Research Articles

The New Britain trench and 149° embayment, Western Solomon Sea

The western New Britain Trench contains relatively thin sediment fill in the east, compared to the west where a sequence of thick turbidites is ponded behind a basement high in the trench axis, The trench trends toward Huon Gulf, but intersects the Trobriand Trench at an acute angle at the 149° Embayment, where both trenches end. Seismic structure west of the trench is incoherent, related to incipient collision of the Indian-Australia Plate and the South Bismarck Plate. The collision suture is marked by the Markham Canyon, continuous in its upper reaches with the Ramu-Markham Fault Zone on shore.

Bottom effects on matched‐field localization performance in the North Pacific

PACIFIC ECHO was an experiment conducted jointly by the Naval Research Laboratory (NRL) and Defence Research Establishment, Pacific (DREP) in May–June 1986. Vertical array measurements of a 15‐Hz cw signal projected by the NRL MK‐VI source, towed at a depth of 100 m, were taken on the 675‐m‐long DREP array at both thinly (< 50 m) and thickly (> 500 m) sedimented areas. The array was located in separate cases in the direct path and the shadow zone, and the top sensor was at a depth of approximately 350 m. Matched field processing between theoretical and measured fields was applied to this data set in order to perform range‐depth localization. Both a linear (Bartlett) and a nonlinear, minimum‐variance (Capon) estimator were employed using theoretical fields generated by a normal modes program. The bottom model plays an important role in the shadow zone where acoustic energy is received predominantly via bottom interacting paths. The thin sediment site, in turn, is especially interesting because of the interaction with the elastic subbottom. The effect of differences in measurement site (thin or thick sediment), source‐received separation, and processor estimator on performance of the matched field processor are discussed.

An iridium anomaly in the Middle‐Lower Jurassic of the Venetian Region, northern Italy

A significant iridium enrichment has been found in a Jurassic marine sequence, about 180 million years old, outcropping in the Alps of the Venetian region. The maximum iridium concentration of 3.2 ± 0.2 ng·g−1 occurs in a brown crust 2–3 mm thick, containing essentially iron hydroxide. This crust, characterized by a nearly total absence of detritic components, lies on top of upper Lias limestones, and is overlaid with Bajocian‐Bathonian limestones. The origin of the Ir anomaly is not clearly understood yet but could be explained by either a drop in the sedimentation rate resulting in the concentration within a thin sediment layer of the normally infalling cosmic dust and/or by an increase of cosmic material infall following an asteroid or comet impact(s).

The evolution of low-temperature convection cells near spreading centers; a mechanism for the formation of the Galapagos mounds and similar manganese deposits

The Galapagos mounds occur in two fields south of the Galapagos spreading center over crust which is between 500,000 and 900,000 years old. Detailed investigations of composition and chronology of these mounds (e.g., Honnorez et al., 1981; Lalou et al., 1984) have shown that their deposition started in 300,000-year-old sediments with the formation of hydrothermal sediments and ended with the recent formation of manganiferous crust. On the basis of numerical models the flow distribution in hydrothermal convection cells near active spreading centers is studied here as a possible formation mechanism for these and similar deposits.The flow at an active spreading center is dominated by a strong convection cell associated with the intrusion process at the ridge axis. This cell is stationary relative to the ride axis. All the other convection cells which form off-axis because of the heat input from the cooling plate are not stationary but move in general with the moving plate away from the ridge axis. As a consequence of this movement, a transition zone exists between the on-axis and off-axis convection where new off-axis cells are formed once the distance between stationary and moving cells has become too large. The beginning of low-temperature convection occurs thus at some distance from the ridge axis (ca. 10 km) in older crust which has in general a thin sediment cover. This convection can be related to the formation of the hydrothermal sediments in the Galapagos mounds.When the spreading rate is relatively low and the rate of fluid flow in the off-axis cells is high, merging of convection cells in this transition zone can occur. The merging episodes cause strong pulses of low-temperature hydrothermal convection, which are not related to off-axis intrusive events. As a segment of the crust moves through the transition zone, it can be exposed to several of these merging episodes. The evolution of off-axis convection cells near an active spreading center provides thus a mechanism which can cause one or more pulses of short-lived (ca. 10,000 years), low-temperature ( approximately 200 degrees C) convection in a segment of the crust. This activity has the characteristics necessary for the deposition of manganese crust or lenses of massive manganese ore associated with marine sediments. Calculated intervals between onset of convection and merging episodes are in good agreement with observations from the Galapagos mounds. Flow rates associated with the merging episodes are sufficient to explain the formation of sizable Mn lenses such as those found associated with the Appenine ophiolites and the Nicoya ophiolite.

The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Summary. In the summer of 1984 an electrical survey using magnetometric off-shore electrical sounding (MOSES) was conducted at two sites in Middle Valley, part of the northern Juan de Fuca Ridge complex. MOSES has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer. Site 1, at 48°32N, 128°42W, is in the central part of the turbidite-filled basin. Using a two-layer model of conductive sediments overlying a fractured basalt basement, the sediment resistivity and thickness were found to be 0.82 ± 0.06 Ωm and 1800 ± 300 m, respectively. The basement resistivity, although not well constrained by the data is consistent with the results obtained at site 2. Site 2, located at 48°10N, 128°50W, has a thinner sediment layer, which appears to vary with position. The sediment conductivity—thickness product is the parameter determined by the data. If the sediment resistivity were the same as at site 1, the sediment thickness would be 140 ± 30 m to the SE of site 2, and 240 ± 55 m to the NW. The fractured basalt basement has a resistivity of 8.5 ± 3.4 Ωm and is at least 1000 m thick. Using temperature-corrected pore fluid resistivity, the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement. These values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment-filled basin and determined for layer 2A/B basalts in DSDP hole 504B, respectively.

Controlled-source electromagnetic sounding of the oceanic lithosphere

The attenuation of ionospheric signals in the frequency range 0.06–24 Hz by sea water effectively precludes using the magnetotel-luric method to study the electrical structure of the upper oceanic lithosphere. We have carried out a dipole–dipole electromagnetic sounding in the North Pacific by injecting electromagnetic signals into the ocean and sea bed. The crust at the site is 25 Myr old and has a thin sediment cover. The technique, similar to that used in earlier work1,2, involves dragging a horizontal dipole antenna along the sea floor. The electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10–65 km. As the ambient electric field is very small (varying from 10−18 V2 m−2 Hz−1 at 0.1 Hz to 10−24 V2 m2 Hz−1 above 1 Hz; ref. 3), the controlled-source signals could be easily monitored. Our data are consistent with a simple one-dimensional Earth model consisting of a 3–7-km-thick crustal layer of moderate conductivity (∼0.001 S m−1) underlain by a thicker region of very low conductivity (<2 × 10−5 S m−1). The results suggest an upper mantle water content of at most 0.1% by volume.

Vertical porewater advection in deep-ocean sediments, investigated by means of a general diagenetic model

Although vertical porewater advection is known to be an important transport mechanism in thin sediment sections close to spreading centres, it is not certain that the phenomenon is confined to such areas. Because of recent reports of vertical advection in deep-ocean areas remote from spreading centres, the possible influence of the phenomenon on diagenetically produced porewater profiles is of interest. The present paper uses the general diagenetic model of Jahnke et al. (1982) to investigate the steadystate and time-variant responses of a natural diagenetic system to advective forcing. Because of the complexity of the system, and present uncertainties in the appropriate values of important input parameters, the model indicates that it is not possible to detect porewater flow of ⪅ 15 cm year −1 using such an approach. In addition, the rapid response of the porewater system to imposed changes means that the effects of a sporadic advective event would not significantly perturb the equilibrium porewater profiles of the 25 cm section considered here for longer than a few years after the cessation of the event.

Sedimentary response to Mississippian tectonic activity at the east end of the 38th Parallel fracture zone

Carbonate fades of the Greenbrier Group establish the presence of an east-west-trending Mississippian uplift and record the timing of late Paleozoic tectonic movement in the east-central Appalachian basin at the east end of the 38th Parallel fracture zone. This fracture zone has been defined by previous workers from the alignment of igneous intrusive and volcanic rocks, ore mineralization zones, major fault systems, and Landsat lineaments across eastern and central North America. This fracture system is apparently associated with displacement along faults in the Precambrian basement rock. Movement along the fracture zone occurred periodically throughout the Phanerozoic. The lineament may be distinguished both geomorphically (on Landsat imagery and side-looking airborne radar) and stratigraphically. Mississippian tectonic activity along the eastern section of the fracture zone is indicated by anomalous thinning of sediments, facies patterns, and stratigraphic relations of uppermost Devonian to Upper Mississippian strata of the eastern Appalachian Plateau.

Seismic Stratigraphic Framework of Louisiana Continental Margin: Clues to Hydrocarbon Sourcing: ABSTRACT

ABSTRACT Interpretation of several thousands of miles of multi-channel seismic data indicates that large volumes of continental rise and basinal sediments have been continuously entrapped and are overriden by the basinward flow or creep of broad, extensive salt tongues which form the lower slope and Sigsbee Escarpment. Thick accumulations of lower Cretaceous to upper Miocene sediments are identified beneath the salt at least 60 km landward of the escarpment. Spotty seismic data suggest that the Cretaceous to Mid-Miocene succession extends up to 120 km landwards of the escarpment and beneath the present shelfbreak. Pleistocene-Holocene sequences of the Deep gulf can be traced up to 15 km landwards of the escarpment, where they are truncated by the base of the salt. Similar truncation of older deep gulf sequences by salt occurs progressively farther landward of the escarpment. Along the middle and lower slope, salt occurs primarily in broad and extensive salt tongues with thin sediment cover. In contrast, salt diapirs and ridges are more common below the outer shelf and upper slope. The outer shelf and upper slope is also characterized by deep basins with thick ponded sedimentary fills of probable Miocene to Holocene age. In several cases these basin fills appear to lie directly on top of the underlying Cretaceous-Early Tertiary marine sediments. Traverse faults and imbricate thrust slivers of salt and shallow salt diapirism and salt sills or plugs along the escarpment are similar to features along the compressional toe of large submarine slides. Along the Louisiana shelf, directly updip, large scale growth-faulting reflects a largely tensional regime associated with sediment loading. The Louisiana slope and rise exhibits a strong southward bulge. This bulge is considered to be indicative of the unstable nature of the margin and may represent an enormous slide along a decollement zone located at the base of salt. The onset of significant sediment loading on the shelf in Miocene to Pleistocene depocenters suggests that slope instability, growth faulting, salt, diapirism, and (finally) salt flow over the bathyal-abyssal sediments was probably initiated in late Miocene time. Recently, the case has been made that hydrocarbons found in seeps and reservoirs on the outer shelf and upper slope are generated from Miocene anoxic basins buried at depth. Anoxic slope basins are presently rather rare and their total sediment volume is quite small. It is unlikely that these anoxic basins were significantly more numerous in Miocene times or that their sediment volume was significantly greater. Therefore, the scenario of large allochthonous salt tongues, or flows, overriding flat-lying continental rise and basinal sediments high in organics suggests that much of the hydrocarbons found in shallow Plio-Pleistocene reservoirs along the shelf and upper slope may have been generated from the now deeply buried Cretaceous to early Tertiary marine sedimentary sequences since late Miocene-early Pliocene time, and that they migrated vertically upward relatively recently along post-sedimentary faults which cut the decollement. End_of_Record - Last_Page 71--------

Indonesian fluvial cassiterite placers and their genetic environment

Placer valley systems in the areas of Belitung and Singkep islands and in the Tujuh archipelago, Indonesia, are described with the use of geological and contour maps, longitudinal sections and grade distribution charts. The main cassiterite concentration (&gt;95% of all minable cassiterite) in these deposits rests directly on weathered bedrock. Cover-layer (Miencan) cassiterite concentrations occur at various levels, indicating continued upstream erosion and downstream sedimentation of cassiterite. The valley bottoms currently reach depths below mean sea level of ~50 m; greater depths, down to –100 m, are recorded on sonic sections and can be deduced from wildcat drill-holes downstream from the mined areas. All offshore and large parts of the onshore placers are palaeoplacers, which have been protected against erosion by marine, littoral and paludal sediments. The stratigraphical succession of this cover has been established in the Tujuh area, but a definite dating has not yet been achieved. The collected data indicate a concentration process on the bottoms of fluvial valleys, which at that stage contained only a thin sediment layer that was composed mainly of coarse, angular clastics and heavy minerals. The distribution of cassiterite on the valley floors is locally determined by morphology—at the lowest parts, in local depressions and behind obstacles—whereas on a larger scale it indicates a largely autochthonous or residual provenance, with maximum fluvial transportation distances of ~500 m. In contrast, many of the W Malaysian cassiterite placers are alluvial concentrations and found at various levels within the relatively fine-grained sedimentary valley fill.

Margins of the Southwest sub-basin of the South China Sea—A frontier exploration target?

The deep South China Sea Basin is about 500 km narrower (when measured along fracture zone directions) in the southwest sub-basin than in the eastern sub-basin. Reconstruction of the South China Sea Basin to predrift configurations thus creates an unacceptable 500-km overlap of continental or transitional crust in the western half of the proto-South China Basin. In order to maintain continuity through the plate-tectonic evolution of this basin, about 500 km of differential crustal extension must have occurred in the west in relation to that in the east. Virtually no deep-penetration seismic data exist across the northwest and southwest foundered and extended continent-transition crust. However, plate tectonics allows us to place constraints on the effective β even in the absence of direct information on crustal thickness, heat flow, or subsidence history. The problem here is essentially the inverse of that normally presented. Assuming various endmember models and knowing the effective β (i.e., extension), we can predict the cumulative subsidence (and hence maximum sediment thicknesses), the regional thermal history, and crustal thicknesses. The total continental plus transitional margin crust in the western sub-basin represents a differential stretching factor about twice that inferred for rifted margins of the eastern sub-basin. The actual mean stretching factor for the western sub-basin margins may be as high as β = 5 or 6. The anomalous crustal extension inferred for the western half of the basin leads us to speculate on the likelihood that numerous rift basins and their associated sediments may have been subjected to unusual thermal conditions that would be favorable for the early maturation of hydrocarbons. Although this region remains virtually unexplored, the foregoing analysis suggests it should perhaps be one focus for additional frontier exploration. This portion of the South China Sea margin (young, thin sediments) offers the ideal natural laboratory for the further testing of continental margin thermal subsidence models.

Structure of Shumagin Continental Margin, Western Gulf of Alaska: ABSTRACT

The Shumagin continental margin lies between Kodiak Island and Unimak Pass. The oldest rocks known to underlie the margin are highly deformed, deep-water turbidites of Late Cretaceous age (Shumagin Formation). These turbidites were intruded by Paleocene and early Eocene granodiorites. Paleogene sedimentary rocks of the Kodiak region may extend southwest to and underlie at least parts of the Shumagin margin but are not known in outcrop on shelf islands. The Cretaceous and Paleogene(?) rocks form acoustic basement on multichannel seismic reflection data, and are overlain by a basin fill of probable late Miocene and younger age. Mesozoic rocks of the Alaska Peninsula extend seaward only to the inferred location of the Border Ranges fault. The Shumagin margin is characterized by five major structural features or trends: (1) Shumagin basin, containing about 2.5 km of late Miocene and younger strata above acoustic basement; (2) Sanak basin, containing as much as 8 km of dominantly late Cenozoic strata in two subbasins separated by a basement high; (3) Cenozoic shelf-edge and upper-slope sedimentary wedges that are 3-4 km thick and possibly as thick as 6 km; (4) a midslope structural trend, Unimak ridge, that is characterized by numerous surface and subsurface structural highs; and (5) a 30-km wide accretionary complex at the base of the slope. A thin (less than 1-2 km) sediment cover of Miocene and younger age covers the continental shelf areas outside of Shumagin and Sanak basins. The tectonic history of the margin includes: (1) Late Cretaceous or early Tertiary removal of the seaward part of the Cretaceous Alaska Peninsula margin along the Border Ranges fault and accretion of the Shumagin Formation against the truncated margin; (2) Miocene uplift and erosion of the shelf; (3) middle or late Miocene uplift of Unimak ridge; and (4) late Miocene and younger subsidence and infilling of Sanak and Shumagin basins, and subduction-accretion along the Aleutian Trench. End_of_Article - Last_Page 657------------

A simple shallow water propagation model including shear wave effects

The Perkeris model has proved to be very useful in describing some features of acoustic propagation in shallow water, and as a simple test of ideas in normal mode theory. The basic model consists of a homogeneous layer of fluid overlying an infinite homogeneous fluid half-space of greater sound speed. Here we extend the Pekeris model to handle the case of a fluid overlying an elastic basement in which the shear speed is less than the (compressional) speed of sound in the fluid. This gives rise to leaky modes in which both the mode eigenfunctions and eigenvalues are complex. The model predictions are compared to some measured propagation losses for a shallow water site overlying a chalk bottom, where shear-wave conversion at the water-chalk interface causes large losses. The predictions of the simple model explain the very high losses measured at frequencies less than 200 Hz. At higher frequencies the sound speed profile and a thin sediment layer become important, but then an all-fluid normal mode model is in agreement with the measured results.

Transects of the ancient and modern continental margins of eastern Canada

Rocks of the west flank of the northern Appalachian Orogen (miogeocline) record the history of the late Precambrian-early Paleozoic passive continental margin of Eastern North America. The ancient margin was destroyed by ophiolite obduction and arc collision during the Ordovician Taconic Orogeny. The present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re-entrants of a previous orthogonal margin bounded by rifts and transforms. Four major terranes are recognized east of the miogeocline in Newfoundland and Nova Scotia. From west to east, these are the Dunnage, Gander, Avalon and Meguma. The Dunnage and Gander terranes were linked to the miogeocline during the Middle Ordovician Taconian Orogeny. The Avalon terrane arrived later, possibly during the mid-Paleozoic Acadian Orogeny. The Meguma terrane of southern Nova Scotia had docked with the Avalon terrane by Carboniferous time. The Dunnage terrane contains arc volcanics which lie above an ophiolitic substrate. The Gander terrane comprises a thick sequence of clastic sedimentary rocks, underlain by basement rocks with continental affinities. It has been interpreted as a continental margin, perhaps once on the eastern side of the Paleozoic Iapetus ocean. The Avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by Grenvillian basement. Its tectonic affinities are unclear. The Meguma terrane comprises a thick sequence of sediments, derived from the south-east. It is found only in southeastern Atlantic Canada. The boundaries between terranes are compressional in the west and steep, transcurrent faults in the east. The surface extent of the geological terranes is grossly correlative with deep structural zones, although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth. A striking feature of the deep crustal structure is a lower, high velocity crustal layer beneath the Dunnage and Gander terranes. The modern margin of Atlantic Canada developed by rifting and by transform motion between adjacent continents. Stretching and thinning of the lithosphere, and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust, are the most likely processes responsible for the evolution of the modern margin. These processes predict the observed deep sedimentary basins along the margin, the thinning of continental crust, and the high seismic velocities found within the ocean-continent transition zones. Rifting adjacent to Nova Scotia began in Late Triassic-Early Jurassic time between the present African and North American plates. These plate motions are also responsible for the major transform margin south of the Grand Banks. Separation between Iberia and the eastern Grand Banks occurred in mid-Cretaceous time, before the Late Cretaceous opening of the Labrador Sea. While the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust, the Grand Banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover. Numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment. Mimicry of Paleozoic promontories and re-entrants by modern rift and transform margin segments, the location of Mesozoic sedimentary basins on ancestral Appalachian structures, and the reactivation and propagation of major Precambrian and Paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins. The rift phase of both the ancient and modern passive margins is characterized by volcanism, mafic dike intrusion and by the development of basins filled with clastic sediments. The drift phase of both the ancient margin and the present Nova Scotia margin is marked by a change in sedimentary environment, such that carbonates replaced the rift phase clastic sediments. Two of the markers used to delineate the ancient ocean-continent transition zone; carbonate banks and steep gravity anomaly gradients, should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone. Furthermore, it is naive to view the ancient transition as simple and narrow, for the modern margins exhibits complex transition zones between 30 and 300 km wide. In general, the evolution of the ancient and modern passive margins appear to be remarkably similar. Predictably, closing the present Atlantic will mimic the evolution of the Appalachian Orogen.

Regional Setting of Niobrara Formation in Northern Great Plains

Natural gas is currently produced from the Upper Cretaceous Niobrara Formation in northeastern Colorado, northwestern Kansas, and several small fields in Nebraska. As a part of studies of low-permeability gas reservoirs in the northern Great Plains, the regional geologic setting of the Niobrara has been investigated in North Dakota, South Dakota, and Nebraska. In North Dakota and South Dakota, 2 Niobrara chalk tongues grade into calcareous shales, which in turn intertongue northward and westward with thick noncalcareous shales of the Pierre Shale. In Nebraska, 3 Niobrara chalk tongues and intervening calcareous shales thicken southward as a unit. An isopach map of the interval from the Niobrara base to the Ardmore Bentonite Bed in the overlying Pierre Shale shows an oblate area of thin sediments extending from southwest to northeast out of the Nebraska Panhandle and into central South Dakota. Erosion along the basal Niobrara unconformity is greatest in the area of thin strata and an unconformity is locally developed on the top of the Niobrara in that same area. Structural contours of the Ardmore Bentonite Bed suggest that the area of thin N obrara strata presently approximates the south flank of the Williston basin and north flank of the Denver and Kennedy basins. Chalk tongues are interpreted as low-angle shelf surfaces, known as carbonate ramps, which sloped gently to the northwest and southeast off a paleotectonic high. The paleotectonic high cut obliquely across the seaway and was close to the position of the Transcontinental arch that influenced Paleozoic sedimentation. As a result, the present-day stratigraphy and structural setting of the Niobrara are different north and south of the arch crest.

An investigation of acoustic interaction with the ocean bottom from experimental time series generated by explosive sources

Received time series from explosive sources in a thin sediment environment are compared to simulated time series calculated by a ray theory model. The comparisons are made in various frequency bands to aid in the identification of sediment penetrating arrivals by taking advantage of the frequency dependence of the absorption of the sediment. For large ranges, most of the received energy refracts within the sediment. As the range decreases, the fraction of received energy due to reflections at the water-sediment interface increases. Discrepancies between the experimental and simulated time series will be presented and interpreted in terms of reflections from thin layers within the sediment and nonspecular basement reflections. [Work supported by Naval Ocean Research and Development Activity.]

Porosity and hydraulic properties of sediments from the Galapagos Spreading Center and their relation to hydrothermal circulation in the oceanic crust

Density and porosity of sediments were measured on DSDP legs 69 and 70 samples from the Galapagos spreading center. Permeability and the hydraulic impedance of each sediment layer were estimated from measured values of porosity. The gradients of porosity and density with depth where sediment layers are thin ( ≲50 m thick ) are anomalously high compared with those of other areas and with the upper part of thicker sediment layers in this area. A good correlation was found between the anomalous porosity and density gradients and the present‐day heat flow. We interpret these observations to suggest that these high gradients may be due to active hydrothermal circulation through a thin sediment cover, which is inhibited by a thicker sediment layer, and that the pattern of hydrothermal circulation may be essentially fixed with the moving plate. Hydraulic impedance of the sediment layer was estimated from the observed depth variation of porosity and was shown to increase rapidly with its thickness. Our interpretation that a threshold thickness of about 50 m would inhibit direct diffuse discharge or recharge of hydrothermal flow through the undisturbed sediment layer yields after an average permeability of the underlying basement layer of about 3 −6×10−14 m2 ( 30–60 mdarcies ).

Analysis of Hydrocarbon Potential of Outer Continental Shelf, Slope, and Rise of the Niger Delta, Nigeria, from Seismic and Geological Data: ABSTRACT

Whereas hydrocarbon potential studies of most parts of the Nigerian sedimentary regions have been documented, no similar assessment has yet been made for the outer continental shelf (OCS), slope and rise of the Niger delta. The shelf, slope, and rise of the delta are frontier areas in which no wells have been drilled to date. For this broad evaluation of hydrocarbon potential of the deep-water Niger delta, six published seismic sections were reviewed in conjunction with available geological information and geophysical data. On the basis of an assumed average velocity of 2.0 km/sec, the seismic data indicate a sedimentary thickness of about 2 km (1.3 mi) in the deeper portions of the rise. This thickness increases shoreward to more than 3 km (1.9 mi) at the foot of the continental slope and 5 to 7 km (3 to 4.3 mi) beneath the outer continental shelf. The distal relationship of the OCS, shelf, and rise to the sedimentary discharge of the Niger delta have made these areas of essentially deep-water marine sediments. With the high organic carbon content commonly associated with these areas, the thick shales in this distal deltaic environment would be very rich in kerogen. Coarse-grained clastics are identifiable on the seismic sections by their distinct and continuous reflection character. These potential reservoirs are common and widespread on the shelf and rise. The reservoir rocks occur as deep-sea fans, turbidites, canyon fills, and as onlap fills between diapiric intrusions, and are enclosed by shale, thus providing favorable conditions for the formation of stratigraphic traps. The diapirism in the slope and outer shelf provides avorable conditions for structural traps. The tectonic origin of the Niger delta area implies a history of initially high geothermal gradients which decreased with time as the margin moved farther away from the Mid-Atlantic Ridge. The geothermal gradient map of the Niger delta indicates higher values for the OCS relative to the onshore and shallow-water areas where rapid sedimentation has depressed the geothermal gradients. The general increase of geothermal gradients toward the mid-ocean ridges would also provide gradients much higher than the 1.8°F/100 ft of the shallow shelf and probably approaching the 3.0°F/100 ft of the Anambra basin region. These gradient ranges are sufficient to mature the kerogen of the oldest shale source rocks (more than 40 m.y.) and to generate hydrocarbons within even the thinnest (2 km 1.2 mi) sediment observed. In the areas of thin sediment, hydrocarbon potential would be appreciably increased in the vicinity of oceanic fracture zones where geothermal gradients are locally high. The hydrocarbon potential increases shoreward as sedimentary thickness increases and is very significant in the upper rise, the slope, and the OCS. The decline in giant field discoveries in the Niger delta is a pointer to the depletion of Nigeria's onshore and shallow-water oil reserves. The continuing increase in world oil demand in the face of dwindling reserves and the steady improvement in deep-water drilling technology combine to make future petroleum exploration and production economically promising in Nigeria's high-potential deep-water frontier. End_of_Article - Last_Page 416------------

Marine geology of the Hess Rise: 2. Basement morphology, sediment thickness, and structural geology

The basement morphology and sediment thickness of the Hess Rise, an oceanic plateau in the central North Pacific, have been mapped on the basis of seismic reflection profiles. The acoustic stratigraphy on and around the rise is correlated with the lithostratigraphy at Deep Sea Drilling Project sites 464, 310, 465, and 466. A total sediment isopach chart of the rise reveals small‐scale departures from the expected sedimentary pattern (thick sediment in shallow areas; thin sediment in deep areas). Sediment‐filled basement depressions result from mass transport; thin sediment (&lt;50 m) occurs on steep scarps, basement ridges, and areas affected by bottom currents. A pre‐Senonian sediment isopach chart shows a thickening from less than 50 m to more than 250 m of sediment from the northeast to the southwest. This trend seems explainable only in terms of the time‐transgressive nature of seafloor formed at a mid‐ocean ridge. The axial trend of the rise (N30°W) parallels nearby Mesozoic magnetic lineations and seems to be isochronous as deduced from the Deep Sea Drilling Project data. The Hess Rise began developing in late Aptian time along a segment of the Pacific‐Farallon Ridge. Important events in the history of the rise are late‐stage volcanism on the southern margin of the rise along the Mendocino Fracture Zone, tectonism and volcanism about 85 Ma that resulted in a major regional unconformity (reflector C), and another period of tectonism and volcanism between 65 and 43 Ma that coincided with the formation of the Emperor Seamounts and created structural benches on the western side of the rise. A significant change in the paleoenvironment that apparently occurred around the Paleogene‐Neogene boundary (∼25–20 Ma) caused pronounced changes in the depositional environment and resulted in another major regional unconformity (reflector A).

Wrench and thrust tectonics along a Mesozoic–Cenozoic continental margin: Antalya Complex, SW Turkey

The segment of the Antalya Complex W of Antalya Bay displays the results of latest Cretaceous to Miocene deformation on a previously passive Mesozoic continental margin. The 300 km 2 area detailed here includes parts of, from W to E, a Mesozoic carbonate platform, its bordering sediment margin and the adjacent marginal oceanic crust with sediment cover. The platform is folded on N–S axes and has been overthrust westward by its sediment margin. This thin sediment sequence has been detached from its basement in a spectacular W-verging imbricate thrust belt. Similar, though less organized, westward thrusting occurs in the thin sediment cover to the marginal oceanic crust, and may also affect the igneous basement itself. However, this part of the margin is dominated by an array of anastomosing subvertical screens of serpentinite up to several kilometres wide. These are thought to be cool, upward protrusions of the hydrated ultramafic substratum to the marginal crust into a then active, braided, strike-slip fault system. The fault strands now divide the intact marginal rocks into large lozenges, some of which have oblique and sigmoidal structures and steeply plunging folds. The thrust and wrench tectonics may have operated in concert in a regionally transpressive regime, or thrusting may have accompanied only the initiation of wrench tectonics. The local interplay of transpression and transtension was partly responsible for the present complexity of the area. Even more important was the original mechanical inhomogeneity of the margin, particularly the inclusion of rifted-off continental fragments in the marginal ocean crust and the strong basement/cover contrasts on the margin.