Platform Off Research Articles

Slope instabilities from echo-character mapping along the French Guiana transform margin and Demerara abyssal plain

The French Guiana transform margin and Demerara abyssal plain have been recently surveyed in the framework of the EXTRAPLAC French Program of extension of the continental shelf (Guyaplac survey, Ifremer-IFP–SHOM-IPEV). Based on the interpretation of some of the data collected during the Guyaplac survey (Simrad-EM12 multibeam bathymetric data, backscatter imagery, and 3.5 kHz profiles), the area can be divided into three morphostructural domains. (1) The western Guiana margin, including a part of the Demerara plateau, an important bathymetric relief prolonging the continental platform off Guiana and Surinam. This domain is bounded by (1a) the NW–SE trending northern border of the Demerara plateau which appears quite steep and corresponds to a transform segment of the margin, (1b) the N–S eastern border of the Demerara plateau which corresponds to a divergent segment of the margin. The Demerara plateau shows a segmented morphology, low slope gradients, and a very rough surface (ripples perpendicular to the slope direction). NNW–SSE structural steps seem to correspond to collapses of 100 km long blocs towards the east. Slumps initiate along these directions. The observed rough bathymetry seems to be related to creeping processes. At a greater scale (seismic data), this part of the margin has been totally destabilized (numerous imbricate transparent masses rooted at about 0.5 s.t.w.t.t. below seafloor). The NW–SE trending northern border of the Demerara plateau corresponds to a cliff-like continental slope, probably slightly smoother than other transform margins (Ghana/côte d'Ivoire margin). The N–S eastern border of Demerara plateau is characterized by numerous small-scale imbricate slumps. Some of these failures seem to be emplaced in the prolongation of the NNW–SSE structural steps identified on the Demerara plateau. (2) The eastern Guiana margin corresponds to a NW–SE oriented gullied transform margin segment. The associated continental slope is very steep and characterized by numerous imbricate slumps and related debris flows. Some undulated masses, probably corresponding to creeping sediments or to older mass-wasting events are still imprinted on bathymetry. This transform margin segment is nearly entirely destabilized and eroded. (3) The Demerara abyssal plain. This domain is characterized eastwards by channels belonging to the Amazon turbidite system and westwards, at the foot of Demerara continental slope, by sediment waves probably created by contour currents. To conclude, it seems that there is a strong relationship between the structure (transform and divergent segments) and the emplacement of recurrent slope instabilities. These are probably related to the steepness of the slopes but also to subsidence histories generating in some cases huge deep-seated collapses of the whole margin. Fluid ascents are common everywhere in the area, probably enhancing slope instability. Their origin is not constrained but the black shales or Cretaceous organic-rich layers could be good candidates.

Update to “Reconciliation of coarse mode sea‐salt aerosol particle size measurements and parameterizations at a subtropical ocean receptor site” regarding the use of aerodynamic particle sizers in marine environments

In the January 2006 issue of Journal of Geophysical Research, Reid et al. presented their findings regarding the sometimes large biases found in the measurement of the size and inferred mass concentration of coarse mode sea‐salt particles. This was done on the R/P Floating Instrument Platform (FLIP) off of the coast of Oahu, Hawaii, where long fetches of the clean marine environment could be studied. As part of this analysis, data from optical particle counters and TSI aerodynamic particle sizers (APS) (model 3320) were compared to filter mass and chemistry. It was found that a ground‐based APS 3320 gave superior performance on measurement of sea‐salt properties relative to other size spectrometers when previously published correction factors were used. However, a calibration study of APS instruments by Volckens and Peters (2005) was published while Reid et al. (2006) was in press. Volckens and Peters (2005) found significant differences in counting efficiency between wet and dry particles that included evidence for particle impaction inside the APS's acceleration nozzle. This finding does not impact Reid et al.'s (2006) key results or conclusions. However, it does bear relevance to specific comments made regarding what was suggested as appropriate use of APS‐type instruments. It also has bearing on some unexplained anomalous behavior in the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) wing‐mounted APS unit. In this short note we briefly review the APS performance bibliography, provide correction factors for size estimates presented in Reid et al. (2006), and discuss how the most current findings impact the study of coarse mode sea salt.

Tidal activity pattern and feeding behaviour of the ophiuroid Ophiocoma scolopendrina on a Kenyan reef flat

Ophiocoma scolopendrina exhibits a distinctive pattern of feeding activity on intertidal reef platforms off Kenya. With the first wave of the flooding tide, dense aggregations of these ophiuroids (up to 320 m−2) engage in a 1–2 min burst of surface-film feeding, vigorously sweeping the air-water interface and associated sea foam with the ventral surface of 2–4 arms. Suspension feeding (with arms extended in the water column) is the primary feeding mode throughout the rest of the tidal cycle (involving 25–65% of the population at a time), while bottom feeding (with arms extended along the substratum) is infrequent (<10%). Field experiments showed that surface-film feeding is regulated by water depth and can be triggered by suspended particles. This feeding mode appears to be an adaptation to the intertidal habitat, enabling the ophiuroids to exploit a nutrient-rich surface film during a temporal refuge (low tide) from fish predation. Dense populations of O. scolopendrina may represent an important trophic link between producers of particulate organic material and higher-level consumers in coral reef environments.

Effect of Hypoxia on the Distribution of Fishes Associated with a Petroleum Platform off Coastal Louisiana

AbstractA combination of hydroacoustics and visual survey techniques were used to estimate the abundance, size distribution, and species composition of fishes at a petroleum platform in the northern Gulf of Mexico. The effect of dissolved oxygen (DO) concentration on the density and distribution of fishes at this de facto artificial reef was examined. The techniques used permitted surveys of the entire water column, providing a more accurate reflection of the effect of a highly variable phenomenon (i.e., hypoxia) that may not be measured with traditional fisheries sampling equipment. Hypoxic conditions were observed during two of the five trips to the petroleum platform, South Timbalier 54. When hypoxia was present, fish density was dramatically higher in the strata immediately above the hypoxia. It was difficult to determine the exact impact of hypoxia on target strength (a measure of fish size), as the larger mean target strengths observed in the spring and summer may be due to seasonal factors and not to hypoxia alone. With the exception of the fall 1995 survey (which had significantly higher density and abundance estimates than other surveys), the estimated number of fishes did not vary among any of the sample periods. Even when 32% (15–22 m) of the water column was hypoxic (summer 1996), abundance estimates were slightly higher than in other survey periods without hypoxia (except fall 1995). While hypoxia reduced the availability of the water column to fishes, they concentrated where favorable DO levels were found. This included demersal species such as red snapper Lutjanus campechanus, which were found higher in the water column than expected.

Intercomparison of Sea-State and Zero-Crossing Parameters From the WACSIS Field Experiment and Interpretation Using Video Evidence

During the WACSIS field experiment, wave elevation time series data were collected over the period December 1997 to May 1998 on and near the Meetpost Nordwijk platform off the coast of the Netherlands from an EMI laser, a Saab radar, a Baylor Wave Staff, a Vlissingen step gauge, a Marex radar and a Directional Waverider. This paper reports and interprets, with the help of simultaneous dual video recordings of the ocean surface, an intercomparison of both single wave and sea state wave parameters.

TOPEX/Poseidon and Jason-1: Absolute Calibration in Bass Strait, Australia

Updated absolute calibration results from Bass Strait, Australia, are presented for the TOPEX/Poseidon (T/P) and Jason-1 altimeter missions. Data from an oceanographic mooring array and coastal tide gauge have been used in addition to the previously described episodic GPS buoy deployments. The results represent a significant improvement in absolute bias estimates for the Bass Strait site. The extended methodology has allowed comparison between the altimeter and in situ data on a cycle-by-cycle basis over the duration of the dedicated calibration phase (formation flight period) of the Jason-1 mission. In addition, it has allowed absolute bias results to be extended to include all cycles since the T/P launch, and all Jason-1 data up to cycle 60. Updated estimates and formal 1-sigma uncertainties of the absolute bias computed throughout the formation flight period are 0 ± 14 mm for T/P and +152 + 13 mm for Jason-1 (for the GDR POE orbits). When JPL GPS orbits are used for cycles 1 to 60, the Jason-1 bias estimate is 131 mm, virtually identical to the NASA estimate from the Harvest Platform off California calculated with the GPS orbits and not significantly different to the CNES estimate from Corsica. The inference of geographically correlated errors in the GDR POE orbits (estimated to be approximately 17 mm at Bass Strait) highlights the importance of maintaining globally distributed verification sites and makes it clear that further work is required to improve our understanding of the Jason-1 instrument and algorithm behavior.

Microbial dolomite crusts from the carbonate platform off western India

Abstract The occurrence of Late Pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western India is documented. Dolomite is the most predominant mineral in the crusts. In thin section, the crust consists of dolomitized microlaminae interspersed with detrital particles. Under scanning electron microscopy, these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin. Dolomite crystals encrust or overgrow the surfaces of the microbial filaments and/or cells; progressive mineralization obliterates their morphology. Well‐preserved microbial mats, sulphide minerals (pyrrhotite and marcasite) and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation. The crusts are similar to dolomite stromatolites, and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation. The dolomite is therefore primary and/or very early diagenetic in origin. The dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite. The results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites. The age of the crusts further suggests that the platform was situated at shallow subtidal depths during the Last Glacial Maximum.

Holocene lagoonal development in the isolated carbonate platforms off Belize

Thirty-one vibracores were taken in interior lagoons of Glovers Reef, Lighthouse Reef, and Turneffe Islands—three isolated carbonate platforms offshore Belize, Central America. Holocene facies successions overlying the Pleistocene limestone bedrock begin with soils, followed by mangrove peats, and marine carbonate sediments of lagoonal origin. The soils formed on top of subaerially exposed Pleistocene limestone before the Holocene transgression. Mangrove peats developed during initial flooding of the platforms (Glovers ca. 8.5 ky, Lighthouse ca. 7 ky, Turneffe ca. 6 ky BP). As water depths increased, reefs colonized platform margins, lagoonal circulation improved thereby promoting carbonate production. The basal lagoonal carbonate sediments are characterized by shell beds and/or Halimeda packstones–grainstones. Mollusk-dominated wackestones and packstones follow upsection in Glovers and Lighthouse Reefs. At present, open circulation prevails in Glovers and Lighthouse lagoons. In contrast, organic-rich Halimeda wackestones and packstones dominate the Turneffe Islands Holocene succession. The main lagoon area of Turneffe is enclosed by mangroves, and restricted circulation prevails. Factors that explain the differences in geomorphology, circulation, and facies are variations in depth of antecedent topography and degree of exposure to waves and currents. The thickness of Holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls. Holocene sedimentation rates average 0.6 m/ky, with highest rates in Turneffe (0.82 m/ky), followed by Lighthouse (0.53 m/ky), and Glovers (0.46 m/ky). Like in many other isolated carbonate platforms and atolls, lagoon floor sedimentation did not keep pace with rising sea level, leading to unfilled accommodation space. At present, Glovers has an 18 m deep lagoon, while Lighthouse and the main Turneffe lagoon are 8 m deep. It is unlikely that the lagoons will be completely filled during the Holocene sea level highstand cycle. This observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms.

Late Quaternary sea level and environmental changes from relic carbonate deposits of the western margin of India

Relic carbonate deposits along the western margin of India occur as dolomite crusts, aragonite sands (pelletal / oolitic) and aragonite-cemented limestones, oyster shells, corals, encrusted coralline algal and foraminiferal-dominated nodules. The petrology and mineralogy of the deposits indicate that except for aragonite sands and foraminiferal nodules, the others were formed in shallow marine conditions and serve as sea level indicators. Radiocarbon dates were measured for 62 relic deposits covering the entire margin. The age of these deposits on the continental shelf off Cape Comorin and Mangalore, between 110 and 18 m depth, ranges between 12, 61014C yr BP and 6,39014C yr BP. On the northwestern margin of India, especially on the carbonate platform (between 64 and 100 m), the age ranges from 17,250 to 6,73014C yr BP. The relic deposits of the Gulf of Kachchh at depths between 35 and 25 m are dated at 12,550–9,63014C yr BP. The age vs. depth plot of the relic deposits further indicates that the Gulf of Kachchh was inundated much early, atleast by 15 ka, after the Last Glacial Maximum, and was subjected to uplift and subsidence during the Holocene. The carbonate platform subsided during the early Holocene. Some of the relic deposits between Cape Comorin and Mangalore plot on or, closely follow the glacio-eustatic sea level curve. Despite abundant siliciclastic flux discharged by the Narmada and Tapti during the early Holocene, the platform off these rivers is largely devoid of this flux and carbonate sedimentation continued until 6,70014C yr BP. We suggest that the river-derived sediment flux diverted southwards under the influence of the SW monsoon current and, thereby, increased the turbidity on the shelf and slope southeast of the carbonate platform and facilitated the formation of deeper water foraminiferal nodules off Vengurla-Goa.

Submerged littoral sediments, beach ridges and wave-cut platforms off Troms, North Norway: Revisiting old questions

Reinterpretation of sea-floor morphology and modelling of Weichselian relative sea-level history on the continental shelf off Troms suggest that beach-like features and sandy basins are unlikely to have been formed primarily by littoral processes. Sea-floor sand transportation has probably been caused by bottom currents, which still flow today. Two seaward-dipping platforms, one at 50-110 m and one at 75-130 m depth on Sveinsgrunnen, Malangsgrunnen and Nordvestbanken, have previously been interpreted as wave-cut platforms. Their gradients are approximately parallel and 50 and 75 m deeper, respectively, than the extrapolated Late Weichselian North Andoy Shoreline (NAS). These platforms therefore seem to be older features, most likely formed mainly by glacial erosion and accumulation processes during the Pleistocene.

The effects of drilling muds on marine invertebrate larvae and adults

AbstractA series of laboratory experiments tested the effects of drilling muds from an active platform off southern California on larvae and adults of marine invertebrates. Red abalone (Haliotis rufescens) were used to determine effects of drilling muds on fertilization, early development, survivorship, and settlement, and experiments on adult brown cup corals (Paracyathus stearnsii) tested effects on adult survivorship, viability, and tissue loss. Exposures to drilling muds did not have an effect on abalone fertilization or early development. However, several exposures to drilling muds resulted in weak, but significant, positive effects of drilling muds on settlement of competent larvae. In contrast, settlement of red abalone larvae on natural coralline algal crusts decreased with increasing concentrations of drilling muds. This suggests that drilling muds affect either the abalone's ability to detect natural settlement inducers, or they affect the inducer itself. Exposure of brown cup corals to concentrations of drilling muds adversely impacted their survivorship and viability. These effects were likely caused by increased tissue mortality of the coral polyps.

Role of climate in partial drowning of the Queensland Plateau carbonate platform (northeastern Australia)

Ocean Drilling Program Leg 133 core data, samples, and geophysical data (logs and seismics) were analyzed to document late Miocene-early Pliocene partial drowning of the Queensland Plateau carbonate platform off Northeast Australia. The modern plateau consists of a mosaic of pinnacle reefs and larger (10 × 50 km) reefs representing relicts of early to middle Miocene buildups. Late Miocene-early Pliocene floatstones, packstones, and mudstones, rich in the larger benthic foraminifers Lepidocyclina and Cycloclypeus, drilled in a transect of sites across the drowned margin of a middle Miocene buildup show that the late Pliocene partial drowning of the platform was preceded by 4 Myr of neritic carbonate deposition without any reefs. The carbonate factory was unable to aggrade to sea level during this period as indicated by the lack of any shallowing trend in the succession. Monitoring of the Miocene to Recent neritic shedding pattern on the windward and leeward side of a pinnacle reef on the Queensland Plateau supports this interpretation. Shedding on the leeward side of the reef, which records periods of active reef growth, played a minor role during the late Miocene, increased during the Pliocene, and reached very high values during the late Pliocene and Pleistocene. Shedding on the windward side of the reef, which was active during lowstands of relative sea level, occurred during the late middle-early late Miocene and during the Pliocene-Quaternary. The data presented here, when combined with paleoceanographic data points to low surface water temperatures (17°–19°C) as a major factor which suppressed reef growth during the late Miocene-early Pliocene period.

Features of marine wind fields over New Zealand waters from ERS‐1 scatterometer data.

Abstract Wind data from the ERS‐1 scatterometer have been processed for New Zealand waters. These show spatial features of marine wind fields which have previously been difficult to resolve using conventional surface‐based measurements. Winds across the western access to Cook Strait, delineated by a corridor between Farewell Spit and western Taranaki, were analysed and profiles of wind stress extracted. These show characteristic structures for south‐easterly events in which the stress steadily increases from Farewell Spit towards the Taranaki Coast. In westerly or north‐westerly events the structure is more uniform. The mean stress across this corridor has been compared to that calculated from surface‐based measurements at Farewell Spit and the Maui‐A oil and gas production platform off Cape Egmont. The Farewell spit data lead to underestimates of the stress, which partially reconciles previous attempts to model wind‐driven currents off the west coast of the South Island from these data. In these the currents were underestimated. The Maui‐A data are unbiased in westerly events but give overestimates in southeasterly winds. An improved estimate of the mean stress can be derived from using a combination of wind data from these two stations.

Molluscan grazing and macroalgal zonation on a rocky intertidal platform at Perth, Western Australia

AbstractIntertidal limestone platforms off Perth show a characteristic pattern of algal zonation, with dense macroalgal beds nearshore bounded by a ‘barren zone’ along the seaward edge. Abalone (Haliotis roei) and several species of limpets and chitons are abundant in the barren zone, which is generally devoid of non‐coralline macroalgae. The relative importance of abalone versus limpets and chitons in limiting macroalgal abundance in the barren zone was evaluated by manipulating the presence of each group in a factorial experiment. Percentage algal cover was measured photographically in 0.25m2plots at 1–2 month intervals for 9 months. Mean algal cover (mainly the foliose green alga,Ulva rigida) was highest in plots where all grazers were excluded (77–99%), intermediate where only limpets and chitons were excluded (37–85%), and lowest where only abalone were excluded (4–30%) or where no grazers were excluded (2–19%). The effect of limpets and chitons accounted for 55–89% of the variance in total algal cover, whereas the effect of abalone generally accounted for &lt;10% of the variance. Similar results were obtained in terms of the biomass ofUlva rigidaat the end of the experiment.Haliotis roeiare relatively large and sedentary herbivores, feeding mainly on drift algae. Their effect on algal abundance was mediated both through pre‐emption of space, which might otherwise be colonized by algae, and by grazing around their home scar. Limpets and chitons are smaller than abalone, but were much more abundant. Intensive grazing of the reef surface by limpets and chitons precluded the establishment of non‐coralline macroalgae, even where abalone were absent.

Ultrastructure of sulphidogenic biofilms rich in sulphate-reducing bacteria causing corrosion in the offshore oil extraction platforms off Brazil's atlantic coast.

Microorganisms, especially sulphate-reducing bacteria (SRB), have long been implicated in metal corrosion in the petroleum industry. SRB can appear in planktonic or adherent forms, interacting with surfaces to produce thick, consortial biofilms. We carried out an ultrastructural investigation of biofilms and planktonic bacteria obtained from samplers on offshore platforms operated in southeast Brazil. A great variety of Gram-negative bacteria were observed. The surface coat and extracellular matrix of these cells stained strongly with ruthenium red, indicating their anionic character. Sessile bacteria were found to be enmeshed in an extensive extracellular matrix. The inner ultrastructure of sessile bacteria was characterized by vacuoles, inclusions and internal membranes. ESI analysis revealed the presence of corrosion products associated with the surface coat of these bacteria.

Noise control evaluation for control rooms and crew quarters on off-shore platforms

Noise sources on off-shore drilling and production platforms are similar to those found on-shore; i.e., engines, turbines, generators, gear boxes, pumps, compressors, etc. The major difference is they are located on a steel structure that transmits noise and vibration energy very efficiently. For those that live aboard, for a crew shift of as much as 3 weeks, their exposure time may not be limited to the 12-h daily work shift. Thus areas like control rooms, laboratories, bunk houses, dining, and recreation rooms should have lower noise levels, be located away from noise sources, and/or isolated from structureborne and airborne noise. This requires a different criteria and design approach. The criteria for this type of noise control problem is related to the speech interference and sleep interruption. Two case histories will be discussed that illustrate the scope of the problem, practical noise control solutions, and results. These include: (1) a semisubmersible combination drilling and production platform located in the Gulf of Mexico; and (2) smaller fixed leg flow station platforms off Indonesia.

Commensalism Between an Epizoic Limpet, Patelloida nigrosulcata, and Its Gastropod Hosts, Haliotis roei and Patella laticostata, on Intertidal Platforms off Perth, Western Australia

On intertidal limestone platforms off Perth and neighbouring islands, the limpet Patelloida nigrosulcata occurs only on the shells of living abalone (Haliotis roei) and other limpets (Patella laticostata). The incidence of commensalism varies among sites and between habitats within sites but is consistently high (&gt; 80%) among dense abalone populations along the seaward margin of platforms. There is usually only 1 limpet per shell, although 2 or 3 limpets occasionally co-occur. A strong positive relationship in size (shell length) between Patelloida nigrosulcata and H. roei indicates that limpets settle on juvenile abalone and grow at a rate proportional to the growth rate of their host. A similar size relationship exists between Patelloida nigrosulcata and Patella laticostata. Patelloida nigrosulcata forages diurnally from a home scar and markedly limits the growth of erect macroalgae on the host shell. Escape from intense interspecific competition with other molluscan grazers on the platforms may have been a strong selective force in the evolution of this commensal relationship.

The Early Cretaceous platform off northwest Florida: Controls on morphologic development of carbonate margins

An analysis of approximately 3500 km of multifold and 1000 km of singlefold seismic reflection data correlated with two exploration wells and other available geologic information records the morphologic development of the Early Cretaceous carbonate platform margin off northwest Florida. Early Cretaceous depositional patterns apparently controlled the concurrent formation of gently dipping (declivities < 10°) and steeply dipping (declivities > 10°) carbonate platform margin slopes in the vicinity of DeSoto Canyon, northeastern Gulf of Mexico. We suggest that the pronounced along-strike change in morphology reflects variations in platform width, input of terrestrial clastics and overall sediment availability to the platform's seaward margin slope. Southeast of the canyon, the Early Cretaceous slope bordered a relatively broad platform removed from terrigenous sediment input. Environmental conditions favored the uninterrupted development of skeletal and nonskeletal buildups along the paleo-shelf edge, which effectively prevented large quantities of shelf-derived clastic sediments from moving seaward. Consequently, as the carbonate platform grew and its height increased, the declivity of its seaward slope also increased, as predicted by models of long-term carbonate platform development. In contrast, northwest of the canyon, the slope bordered a narrower platform adjacent to a source of terrigenous clastics. More vigorous cross-platform transport of clastics and resultant adverse environmental conditions at the paleo-shelf edge inhibited consistent development of carbonate buildups, thereby creating only an intermittent sediment barrier to the adjacent slope. Resultant higher sedimentation rates on the slope caused it to aggrade and prograde along with the carbonate platform, allowing the margin slope to maintain its gentle morphology throughout the Early Cretaceous.

Non-tropical carbonate deposits on the modern New Zealand shelf

Extensive (ca. 50,000 km 2) shallow-marine platforms (< 250 m) off northern (34°S) and southern (48°S) New Zealand, and more local areas of shelf between, are blanketed by skeletal carbonate sediments > 70% CaCO 3), despite proximity to a tectonically active plate margin. In these regions the terrigenous sediment supply is presently low, and growth of epibenthos is fostered by firm substrates (rock, gravels, shells, seaweeds) and the generally energetic nature and high nutrient levels of open-shelf waters. Rapid transition into adjacent terrigenous-dominated facies is characteristic. Irrespective of water depth, the carbonates are coarse-grained and fragmental; carbonate mud is rare. Calcite dominates over aragonite. High-Mg calcite, widespread off northern New Zealand, is rare in the south. Skeletal material is dominated by bryozoans and bivalve molluscs, with significant local contributions from foraminifers, barnacles, calcareous red algae and echinoderms. The name bryomol is suggested for this distinctive temperate-region skeletal carbonate facies, which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component, known to be habitat-related. Bioerosion is an important mechanism of skeletal fragmentation and degradation. Many grains, especially aragontic bivalves, are infested by endolithic borers and have low preservation potential. Ages of skeletal material in the surficial deposits range from more than 20,000 years B.P. to modern, which is consistent with both low rates of carbonate production and sediment accumulation, and the wide range in preservation state of grains. Some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons, sand waves and sand sheets driven by storm-assisted tidal flows. Tracts of modern, palimpsest and relict carbonates can occur in juxtaposition. The facies characteristics of the New Zealand shelf carbonate deposits contrast significantly with those of the classical Bahaman-type carbonate model. However, they are similar to those reported from many other mid- to high-latitude carbonate shelves, and afford good analogues for most onland occurrences of New Zealand Cenozoic limestones.

Sedimentary provinces, and associated bedforms and benthos on the Scott Reef-Rowley Shoals platform off north-west Australia

The distribution of sediment types on the Scott Reef-Rowley Shoals platform, a marginal plateau adjoining the continental shelf of north-west Australia, has been investigated. Sediments in this region are predominately muds, with a high carbonate content. Sands, which are scarce, are mainly composed of both benthic and pelagic skeletal remains, with any gravel-sized material consisting of relict molluscan debris and, occasionally, relict coralline material. Correlation of sediment type with sea-bed characteristics as determined from photographs of the bottom at the same site showed significant relationships between mean grain size, and degree of sediment rippling and benthic activity. Similar correlations were found for the skewness measure of the analysis of sediment grain-size frequency. It is concluded that sediment type bears strong relationships with benthic activity and bedform morphology. Correlations between sediment provinces and the distribution of Australian scampi (Metanephrops sp.), a relatively new fishery in the region, are discussed.