Pore pressure measurements on IODP Exp308 sites

Overpressures measured with pore pressure penetrometers during Integrated Ocean Drilling Program (IODP) Expedition 308 reach 70% and 60% of the hydrostatic effective stress (View the MathML source) in the first 200 meters below sea floor (mbsf) at Sites U1322 and U1324, respectively, in the deepwater Gulf of Mexico, offshore Louisiana. High overpressures are present within low permeability mudstones where there have been multiple, very large, submarine landslides during the Pleistocene. Beneath 200 mbsf at Site U1324, pore pressures drop significantly: there are no submarine landslides in this mixture of mudstone, siltstone, and sandstone. The penetrometer measurements did not reach the in situ pressure at the end of the deployment. We used a soil model to determine that an extrapolation approach based on the inverse of square route of time (View the MathML source) requires much less decay time to achieve a desirable accuracy than an inverse time (1/t) extrapolation. Expedition 308 examined how rapid and asymmetric sedimentation above a permeable aquifer drives lateral fluid flow, extreme pore pressures, and submarine landslides. We interpret that the high overpressures observed are driven by rapid sedimentation of low permeability material from the ancestral Mississippi River. Reduced overpressure at depth at Site U1324 suggests lateral flow (drainage) whereas high overpressure at Site U1322 requires inflow from below: lateral flow in the underlying permeable aquifer provides one mechanism for these observations. High overpressure near the seafloor reduces slope stability and provides a mechanism for the large submarine landslides and low regional gradient (2°) offshore from the Mississippi delta.

Chemical profiles in sediment and basalt at DSDP Hole 74-525A

Forty sediment and four basement basalt samples from DSDP Hole 525A, Leg 74, as well as nine basalt samples from southern and offshore Brazil, were subjected to instrumental neutron activation analysis. Thirty-two major, minor, and trace elements were determined. The downcore element concentration profiles and regression analyses show that the rare earth elements (REE) are present in significant amounts in both the carbonate and noncarbonate phases in sediments; Sr is concentrated in the carbonate phase, and most of the other elements determined exist mainly in the noncarbonate phase. The calculated partition coefficients of the REE between the carbonate phase and the free ion concentrations in seawater are high and increase with decreasing REE ionic radii from 3.9 x 10**6 for La to 15 x 10**6 for Lu. Calculations show that the lanthanide concentrations in South Atlantic seawater have not been changed significantly over the past 70 Ma. The Ce anomaly observed in the carbonate phase is a redox indicator of ancient seawater. Study of the Ce anomaly reveals that seawater was anoxic over the Walvis Ridge during the late Campanian. As the gap between South America and West Africa widened and the Walvis Ridge subsided from late Campanian to late Paleocene times, the water circulation of the South Atlantic improved and achieved oxidation conditions about 54 Ma that are similar to present seawater redox conditions in the world oceans. The chemical compositions of the basement rocks correspond to alkalic basalts, not mid-ocean ridge basalts (MORBs). The results add more evidence to support the hypothesis that the Walvis Ridge was formed by a series of volcanos moving over a "hot spot" near the Mid-Atlantic Ridge. From the chemical composition and REE pattern, one 112 Ma old basalt on the Brazilian continental shelf has been identified as an early stage MORB. To date, this is the oldest oceanic tholeiite recovered from the South Atlantic. This direct evidence indicates that the continental split between South America and Africa commenced > 112 Ma.

Benthic foraminiferal genera in DSDP Hole 95-612

On the basis of lithologic, foraminiferal, seismostratigraphic, and downhole logging characteristics, we identified seven distinctive erosional unconformities at the contacts of the principal depositional sequences at Site 612 on the New Jersey Continental Slope (water depth 1404 m). These unconformities are present at the Campanian/Maestrichtian, lower Eocene/middle Eocene, middle Eocene/upper Eocene, upper Eocene/lower Oligocene, lower Oligocene/upper Miocene, Tortonian/Messinian, and upper Pliocene/upper Pleistocene contacts. The presence of coarse sand or redeposited intraclasts above six of the unconformities suggests downslope transport from the adjacent shelf by means of sediment gravity flows, which contributed in part to the erosion. Changes in the benthic foraminiferal assemblages across all but the Campanian/Maestrichtian contact indicate that significant changes in the seafloor environment, such as temperature and dissolved oxygen content, took place during the hiatuses. Comparison with modern analogous assemblages and application of a paleoslope model where possible, indicate that deposition took place in bathyal depths throughout the Late Cretaceous and Cenozoic at Site 612. An analysis of two-dimensional geometry and seismic fades changes of depositional sequences along U.S.G.S. multichannel seismic Line 25 suggests that Site 612 was an outer continental shelf location from the Campanian until the middle Eocene, when the shelf edge retreated 130 km landward, and Site 612 became a continental slope site. Following this, a prograding prism of terrigenous debris moved the shelf edge to near its present position by the end of the Miocene. Each unconformity identified can be traced widely on seismic reflection profiles and most have been identified from wells and outcrops on the coastal plain and other offshore basins of the U.S. Atlantic margin. Furthermore, their stratigraphic positions and equivalence to similar unconformities on the Goban Spur, in West Africa, New Zealand, Australia, and the Western Interior of the U.S. suggest that most contacts are correlative with the global unconformities and sea-level falls of the Vail depositional model.

Geochemistry of pore water and sediments offshore Nicaragua and Costa Rica

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 ?M. The kinetic constant (AA) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying AA = 1.3 * 10**-U9 mol of Si/cm**2/ s. This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

Permeability measurements on sheared and unsheared samples from different IODP Holes of Leg 316 and from IODP Hole 315-C0001E and ODP Hole 190-1174B

At subduction zones, the permeability of major fault zones influences pore pressure generation, controls fluid flow pathways and rates, and affects fault slip behavior and mechanical strength by mediating effective normal stress. Therefore, there is a need for detailed and systematic permeability measurements of natural materials from fault systems, particularly measurements that allow direct comparison between the permeability of sheared and unsheared samples from the same host rock or sediment. We conducted laboratory experiments to compare the permeability of sheared and uniaxially consolidated (unsheared) marine sediments sampled during IODP Expedition 316 and ODP Leg 190 to the Nankai Trough offshore Japan. These samples were retrieved from: (1) The décollement zone and incoming trench fill offshore Shikoku Island (the Muroto transect); (2) Slope sediments sampled offshore SW Honshu (the Kumano transect) ~ 25 km landward of the trench, including material overriden by a major out-of-sequence thrust fault, termed the "megasplay"; and (3) A region of diffuse thrust faulting near the toe of the accretionary prism along the Kumano transect. Our results show that shearing reduces fault-normal permeability by up to 1 order of magnitude, and this reduction is largest for shallow (< 500 mbsf) samples. Shearing-induced permeability reduction is smaller in samples from greater depth, where pre-existing fabric from compaction and lithification may be better developed. Our results indicate that localized shearing in fault zones should result in heterogeneous permeability in the uppermost few kilometers in accretionary prisms, which favors both the trapping of fluids beneath and within major faults, and the channeling of flow parallel to fault structure. These low permeabilities promote the development of elevated pore fluid pressures during accretion and underthrusting, and will also facilitate dynamic hydrologic processes within shear zones including dilatancy hardening and thermal pressurization.

Sedimentation across the Great Barrier Reef

The continental margin off northeast Australia, comprising the Great Barrier Reef (GBR) platform and Queensland Trough, is the largest tropical mixed siliciclastic/carbonate depositional system in existence. We describe a suite of 35 piston cores and two Ocean Drilling Program (ODP) sites from a 130*240 km rectangular area of the Queensland Trough, the slope and basin setting east of the central GBR platform. Oxygen isotope records, physical property (magnetic susceptibility and greyscale) logs, analyses of bulk carbonate content and radiocarbon ages at these locations are used to construct a high resolution stratigraphy. This information is used to quantify mass accumulation rates (MARs) for siliciclastic and carbonate sediments accumulating in the Queensland Trough over the last 31,000 years. For the slope, highest MARs of siliciclastic sediment occur during transgression (1.0 Million Tonnes per year; MT/yr), and lowest MARs of siliciclastic (<0.1 MT/yr) and carbonate (0.2 MT/yr) sediment occur during sea level lowstand. Carbonate MARs are similar to siliciclastic MARs for transgression and highstand (1.1-1.4 MT/yr). In contrast, for the basin, MARs of siliciclastic (0-0.1 MT/yr) and carbonate sediment (0.2-0.4 MT/yr) are continuously low, and within a factor of two, for lowstand, transgression, and highstand. Generic models for carbonate margins predict that maximum and minimum carbonate MARs on the slope will occur during highstand and lowstand, respectively. Conversely, most models for siliciclastic margins suggest maximum and minimum siliciclastic MARs will occur during lowstand and transgression, respectively. Although carbonate MARs in the Queensland Trough are similar to those predicted for carbonate depositional systems, siliciclastic MARs are the opposite. Given uniform siliciclastic MARs in the basin through time, we conclude that terrigenous material is stored on the shelf during sea level lowstand, and released to the slope during transgression as wave driven currents transport shelf sediment offshore.

Pollen analysis of sediment cores from the mouth of the river Congo to Walvis Bay and Lüderitz

The distribution of pollen in marine sediments is used to record vegetation changes over the past 30,000 years on the adjacent continent. A transect of marine pollen sequences from the mouth of the river Congo (~5°S) to Walvis Bay and Lüderitz (~25°S) shows vegetation changes in Congo, Angola and Namibia from the last glacial period into the Holocene. The comparison of pollen records from different latitudes provides information about the latitudinal shift of open forest and savannahs (Poaceae pollen), the extension of lowland forest (rain forest pollen) and Afromontane forest (Podocarpus pollen), and the position of the desert fringe (pollen of Caryophyllaceae, Chenopodiaceae and Amaranthaceae). High Cyperaceae pollen percentages in sediments from the last glacial period off the mouth of the river Congo suggest the presence of open swamps rather than savannah vegetation in the Congo Basin. Pollen from Restionaceae in combination with Stoebe-type pollen (probably from Elytropappus) indicates a possible northwards extension of winter rain vegetation during the last glacial period. The record of Rhizophora (mangrove) pollen is linked to erosion of the continental shelf and sea-level rise. Pollen influx is highest off river mouths (10-2000 grains year**-1 cm**-2), close to the coast (300-6000 grains year**-1 cm**-2), but is an order of magnitude lower at sites situated far from the continent (<10 grains year**-1 cm**-2).

Miocene through Pleistocene dinoflagellates from ODP Site 170-1039

Palynological data from offshore Costa Rica, allow us to investigate the relationship between dinoflagellate cyst assemblages and changes in regional oceanic primary productivity. From Miocene to Pleistocene, productivity at ODP Site 1039 was influenced by tectonic drift, as Site 1039 approached the continent, from the Equator to its current position at ~10°N. In addition, dinoflagellate abundance is modulated by regional productivity events, which modified primary productivity, as also indicated by available data on calcareous nannofossils, diatoms, TOC, and CaCO3 content. Five palynomorph intervals are defined. The early-late Miocene one, dominated by Batiacasphaera, represents relatively stable, productive oceanic conditions before the closure of the Indonesian and Panama Seaways. The late Miocene decrease in palynomorph recovery is related to the Carbonate Crash Event. The high abundance and diversity of the assemblages at the end of the late Miocene to early Pliocene indicate increased productivity related to the Global Biogenic Bloom, and a change in dominance from Batiacasphaera to Impagidinium to Nematosphaeropsis. The low abundance of the late Pliocene interval is related to El Niño-like conditions, and there is another change related to the disappearance of Batiacasphaera and dominance of Impagidinium, Nematosphaeropsis, and Operculodinium. The abundant Pleistocene assemblages represent increased marine productivity, and a high influx of continental palynomorphs and bissacate pollen, associated with the proximity of the Costa Rica Dome. Pleistocene dinoflagellates are characterized by Spiniferites and Selenopemphix, together with rare Impagidinium and Nematosphaeropsis.

Benthic foraminifera and sea level analyses from IODP Hole 310-M0005D

The course of sea-level fluctuations during Termination II (TII; the penultimate deglaciation), which is critical for understanding ice-sheet dynamics and suborbital climate variability, has yet to be established. This is partly because most shallow-water sequences encompassing TII were eroded during sea-level lowstands of the last glacial period or were deposited below the present sea level. Here we report a new sequence recording sea-level changes during TII in the Pleistocene sequence at Hole M0005D (water depth: 59.63 m below sea level [mbsl]) off Tahiti, French Polynesia, which was drilled during Integrated Ocean Drilling Program Expedition 310. Lithofacies variations and stratigraphic changes in the taxonomic composition, preservation states, and intraspecific test morphology of large benthic foraminifers indicate a deepening-upward sequence in the interval from Core 310-M0005D-26R (core depth: 134 mbsl) through -16R (core depth: 106 mbsl). Reconstruction of relative sea levels, based on paleodepth estimations using large benthic foraminifers, indicated a rise in sea level of about 90 m during this interval, suggesting its correlation with one of the terminations. Assuming that this rise in sea level corresponds to that during TII, after correcting for subsidence since the time of deposition, a highstand sea-level position would be 2 ± 15 m above present sea level (masl), which is generally consistent with highstand sea-level positions in MIS 5e (4 ± 2 masl). If this rise in sea level corresponds to that during older terminations, the subsidence-corrected highstand sea-level positions (30 ± 15 masl for Termination III and 54 ± 15 masl for Termination IV) are not consistent with reported ranges of interglacial sea-level highstands (-18 to 15 masl). Therefore, the studied interval likely records the rise in sea level and associated environmental changes during TII. In particular, the intervening cored materials between the two episodes of sea-level rise found in the studied interval might record the sea-level reversal event during TII. This conclusion is consistent with U/Th ages of around 133 ka, which were obtained from slightly diagenetically altered (i.e., < 1% calcite) in situ corals in the studied interval (Core 310-M0005D-20R [core depth: 118 mbsl]). This study also suggests that our inverse approach to correlate a stratigraphic interval with an approximate time frame could be useful as an independent check on the accuracy of uranium-series dating, which has been applied extensively to fossil corals in late Quaternary sea-level studies.

(Table 1) Trace element and age analysis from the Porites coral from IODP Exp 310

Climate responses and changes in marine environments during the last deglaciation have been controversial and few paleoceanographic data are available from the tropical South Pacific, though this region is crucial in the investigations of ocean-atmosphere interactions. Integrated Ocean Drilling Program Expedition 310 was conducted to establish the time course of the postglacial sea-level rise at Tahiti in the South Pacific. A principal objective of this expedition was to examine the variation of marine environments during the last deglaciation. As fossil Porites coral is ideal for assessing past marine environments, we selected only Porites specimens from the many coral specimens retrieved, examined them by XRD, and dated them by the 14C method. In all, we obtained 17 pristine Porites specimens composed of only aragonite with ages from 15 to 9 ka. Then, we measured Mg/Ca, Ba/Ca, and U/Ca ratios and Cd contents as proxies for upwelling and sea surface temperature. Higher Ba/Ca ratios and Cd content together with lower reconstructed SSTs using U/Ca ratios in the coral specimens between 12.6 and 9.8 cal ka compared to around 15 cal ka suggest that upwelling and/or entrainment of subsurface water into mixed layer was enhanced around Tahiti during this period. This finding is consistent with previous reports and supports the idea that the South Pacific was characterized by La Niña-like conditions at least from 12.6 to 9.8 cal ka.

Pb isotopes in individual sand-sized (>150 µm) ice-rafted feldspars deposited at subpolar North Atlantic Ocean Deep Sea Drilling Project Site 611 during the late Pliocene (Marine Isotope Stages, MIS, G6 through 100)

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39°N until 2.4 ±0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem,we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma.

Major components and major and trace elements from ODP Site 175-1085

We have analyzed 33 Pliocene bulk sediment samples from Ocean Drilling Program Site 1085 in the Cape Basin, located offshore of western Africa in the Angola-Benguela Current system, for 17 major and trace elements, and interpreted their associations and temporal variations in the context of an allied data set of CaCO3, opal, and Corg. We base our interpretations on elemental ratios, accumulation rates, inter-element correlations, and several multi-element statistical techniques. On the basis of qualitative assessment of downhole changes in the distributions of P and Ba, utilized as proxies of export production, we conclude that highs in bulk and biogenic accumulation that occur at 3.2 Ma, 3.0 Ma, 2.4 Ma, and 2.25 Ma were caused by increases in export production as well as terrigenous flux, and record a greater sequestering of organic matter during these time periods. Studies of refractory elements and other indicator proxies (SiO2, Al2O3, TiO2, Fe2O3, MgO, V, Cr, Sr, and Zr) strongly suggest that the terrigenous component of the bulk sediment is composed of two compositional end-members, one being 'basaltic' in composition and the other similar to an 'average shale'. The basaltic end-member comprises approximately 10-15% of the total bulk sediment and its presence is consistent with the local geology of source material in the drainage basin of the nearby Orange River. The increase in bulk accumulation at 2.4 Ma appears to reflect a greater relative increase in basaltic input than the relative increase in shale-type input. Although studies such as this cannot precisely identify the transport mechanisms of the different terrigenous components, these results are most consistent with variations in sea level (and associated changes in shelf geometry and fluvial input) being responsible for the changing depositional conditions along the Angolan Margin during this time period.

(Table 1) Different age analysis from IODP Site 310-M0018A

Deglacial reefs from Tahiti (IODP 310) feature a co-occurrence of zooxanthellate corals with microbialites that compose up to 80 vol% of the reef framework. The notion that microbialites tend to form in more nutrient-rich environments has previously led to the concept that such encrustations are considerably younger than the coral framework, and that they have formed in deeper storeys of the reef edifice, or that they represent severe disturbances of the reef ecosystem. As indicated by their repetitive interbedding with coralline red algae, the microbialites of this reef succession of Tahiti, however, formed immediately after coral growth under photic conditions. Clearly, the deglacial reef microbialites present in the IODP 310 cores did not follow disturbances such as drowning or suffocation by terrestrial material, and are not "disaster forms". Given that the corals and the microbialites developed in close spatial proximity, highly elevated nutrient levels caused by fluvial or groundwater transport from the volcanic hinterland are an unlikely cause for the exceptionally voluminous development of microbialites. That voluminous deglacial reef microbialites generally are restricted to volcanic islands, however, implies that moderately, and possibly episodically elevated nutrient levels favored this type of microbialite formation.

Geospatial analysis of groundwater depth for manual well drilling suitability in the Zinder Region, Niger

Manual and low-tech well drilling techniques have potential to assist in reaching the United Nations' millennium development goal for water in sub-Saharan Africa. This study used publicly available geospatial data in a regression tree analysis to predict groundwater depth in the Zinder region of Niger to identify suitable areas for manual well drilling. Regression trees were developed and tested on a database for 3681 wells in the Zinder region. A tree with 17 terminal leaves provided a range of ground water depth estimates that were appropriate for manual drilling, though much of the tree's complexity was associated with depths that were beyond manual methods. A natural log transformation of groundwater depth was tested to see if rescaling dataset variance would result in finer distinctions for regions of shallow groundwater. The RMSE for a log-transformed tree with only 10 terminal leaves was almost half that of the untransformed 17 leaf tree for groundwater depths less than 10 m. This analysis indicated important groundwater relationships for commonly available maps of geology, soils, elevation, and enhanced vegetation index from the MODIS satellite imaging system.

Geochemistry and kinetic models od Cretaceous samples

The source rock potential of Cretaceous organic rich whole rock samples from deep sea drilling project (DSDP) wells offshore southwestern Africa was investigated using bulk and quantitative pyrolysis techniques. The sample material was taken from organic rich intervals of Aptian, Albian and Turonian aged core samples from DSDP site 364 offshore Angola, DSDP well 530A north of the Walvis Ridge offshore Namibia, and DSDP well 361 offshore South Africa. The analytical program included TOC, Rock-Eval, pyrolysis GC, bulk kinetics and micro-scale sealed vessel pyrolysis (MSSV) experiments. The results were used to determine differences in the source rock petroleum type organofacies, petroleum composition, gas/oil ratio (GOR) and pressure-volume-temperature (PVT) behavior of hydrocarbons generated from these black shales for petroleum system modeling purposes. The investigated Aptian and Albian organic rich shales proved to contain excellent quality marine kerogens. The highest source rock potential was identified in sapropelic shales in DSDP well 364, containing very homogeneous Type II and organic sulfur rich Type IIS kerogen. They generate P-N-A low wax oils and low GOR sulfur rich oils, whereas Type III kerogen rich silty sandstones of DSDP well 361 show a potential for gas/condensate generation. Bulk kinetic experiments on these samples indicate that the organic sulfur contents influence kerogen transformation rates, Type IIS kerogen being the least stable. South of the Walvis Ridge, the Turonian contains predominantly a Type III kerogen. North of the Walvis Ridge, the Turonian black shales contain Type II kerogen and have the potential to generate P-N-A low and high wax oils, the latter with a high GOR at high maturity. Our results provide the first compositional kinetic description of Cretaceous organic rich black shales, and demonstrate the excellent source rock potential, especially of the Aptian-aged source rock, that has been recognized in a number of the South Atlantic offshore basins.