Glacio-eustatic Sea-level Fluctuations Research Articles

LATE QUATERNARY GEOLOGICAL HISTORY OF RIO GRANGE DO SUL COASTAL PLAIN, SOUTHERN BRAZIL

The sedimentary deposits and geomorphic features preserved in the Rio Grande do Sul coastal plain, southern Brazil, represent a significant record of late Quaternary climatic changes with its associated glacio-eustatic sea-level fluctuations. The sediments of the coastal plain belong to two major depositional systems - an alluvial fan system developed along the inner part of the coastal plain and a barrier-lagoon complex with four distinctive transgressive-regressive cycles seaward. The alluvial fan sediments were derived from igneous and metamorphic rocks of the Precambrian shield and from sedimentary and volcanic rocks of the Parana Basin. Subsequently, they were reworked by four barrier- lagoon systems each representing a transgressive-regressive cycle. Each barrier probably originated at the landward limit of a transgression and was preserved due to regression of the shoreline forced by a glacio-eustatic sea-level fall. The four barrier-lagoon systems are believed to have formed during the last 400 ka assuming a correlation with the highstands represented by the last major peaks of the oxygen isotopic record. INTRODUCTION The Rio Grande do Sul coastal plain is an elongate (620 km) and wide (up to 100 km) physiographic province underlain by the Pelotas Basin. This basin, the southernmost of the Brazilian continental margin, has accumulated more than 10,000 m of mainly terrigenous sediments since its formation in the Early Cretaceous, when the South Atlantic began opening. The younger section of this sedimentary record is now exposed on the Rio Grande do Sul coastal plain and contains one of the most complete records of Brazilian coastal Quaternary sedimentation (Fig. 1). This paper summarizes knowledge about Rio Grande do Sul coastal plain sediments and suggests an evolutionary model for them during the Quaternary. PHYSICAL AND GEOLOGICAL SETTING OF THE STUDY AREA The coastal plain of Rio Grande do Sul has a nearly straight shoreline between about 29° S and 34° S latitude. Covering an area of about 33,000 km 2 , this large lowland embraces a great number of coastal water bodies, some of them of large dimensions, such as the Patos Lagoon with an area of 10,000 km 2 and Mirim Lagoon with an area of 3,770 km 2 . At the northern end of the coastal plain the adjacent highlands consist of Paleozoic and Mesozoic sedimentary and volcanic rocks of the Parana Basin that locally reach 1,000 m. At the southern section, igneous and metamorphic rocks of the Precambrian shield form lower highlands. At present, all sandy sediments eroded from these highlands and transported by rivers to the coast are trapped in the coastal lagoons and other backbarrier environments and none reaches the oceanic shoreline. The climate of the region is temperate, humid, with an even distribution of rain throughout the year, averaging around 1300 mm. The coastal plain is dominated by a bimodal high-energy wind re- gime. The dominant wind comes from NE and is more active in spring and summer months. The secondary W-SW wind becomes more important in the autumn-winter months. The coast of Rio Grande do Sul is a wave-dominated microtidal coast with semidiurnal tides with a mean range of about 0.5 m. The region is affected by swell waves approaching from SE that produce a net northerly alongshore transport of sediment. Besides the swell action, sea waves from E and NE and episodic storm waves from E and SE control erosional and depositional processes along the seashore. The adjacent continental shelf has an average width of 150 km and the shelf break is situated at a depth of about 170 m. Bottom sediments on the shelf are predominantly terrigenous elastics with some biodetrital concentrations that seems to be mostly relic. COASTAL PLAIN DEPOSITIONAL SYSTEMS Alluvial fan system This system includes fades formed through sediment gravity flows and alluvial processes along the inner part of the coastal plain. The sediments consist of mass flow deposits in proximal regions (mainly massive debris flow and colluvial slide deposits) grading seaward into waterlaid fades associated to braided channels with cross-stratified sandy and gravelly deposits. The composition and texture of the alluvial fan fades mainly reflects the nature (relief and composition) of their primary source area. To the north, the fades

LATE QUATERNARY GEOLOGICAL HISTORY OF RIO GRANGE DO SUL COASTAL PLAIN, SOUTHERN BRAZIL

The sedimentary deposits and geomorphic features preserved in the Rio Grande do Sul coastal plain, southern Brazil, represent a significant record of late Quaternary climatic changes with its associated glacio-eustatic sea-level fluctuations. The sediments of the coastal plain belong to two major depositional systems - an alluvial fan system developed along the inner part of the coastal plain and a barrier-lagoon complex with four distinctive transgressive-regressive cycles seaward. The alluvial fan sediments were derived from igneous and metamorphic rocks of the Precambrian shield and from sedimentary and volcanic rocks of the Paraná Basin. Subsequently, they were reworked by four barrierlagoon systems each representing a transgressive-regressive cycle. Each barrier probably originated at the landward limit of a transgression and was preserved due to regression of the shoreline forced by a glacio-eustatic sea-level fall. The four barrier-lagoon systems are believed to have formed during the last 400 ka assuming a correlation with the highstands represented by the last major peaks of the oxygen isotopic record.

Plio-Pleistocene cyclothems from Wanganui Basin, New Zealand: type locality for an astrochronologic time-scale, or template for recognizing ancient glacio-eustacy?

Wanganui Basin is a 200 200 km 2 ovoid sedimentary basin of Plio-Pleistocene age situated in a back-arc position in western North Island, New Zealand. The eastern edge of the basin has been subjected to smooth tectonic uplift since about 350 000 years BP, resulting in the exposure onland of young sediments which elsewhere are more usually located beneath the modern continental shelf. The ll of the basin comprises several kilometres of markedly cyclothemic sediment of Plio-Pleistocene age. Accurate environmental interpretation of the succession is greatly enhanced by the presence of abundant sedimentary structures, and fossil invertebrates whose conspecic descendants live in modern New Zealand seas. The maximum water depth attained during the deposition of most cyclothems is less than 100 m, i.e. less than the known 100{130 m magnitude of most Late Pleistocene sea-level fluctuations. The magnetostratigraphic boundaries and abundant tephra which occur within the section allow accurate international correlation. The 47 Wanganui cyclothems which correspond to the last 2.4 Ma (oxygen isotope stages 100{1) each correlate with the highstand part only of each glacial-interglacial isotope stage couplet, i.e. with oddnumbered isotope stages. Even-numbered glacial stages are represented by the unconformities which separate stratigraphically adjacent cyclothems, and which mark sealevel withdrawal and subaerial exposure of the inner part of the basin. Therefore, only about half of elapsed geological time is represented by sediment within shallowwater basin lls deposited during times of high-amplitude glacio-eustatic sea-level fluctuation. Such strata, including those at Wanganui, are unsuitable for use as type sections for time-scale intervals. Nonetheless, the cyclothemic sedimentary motifs described from Wanganui, and especially the known relationships there between shellbed type and systems tracts, provide invaluable insights for the interpretation of other Phanerozoic glacio-eustatic successions such as those of Permo-Carboniferous and Ordovician age.

Continental-scale sequence stratigraphy of the Namurian, Upper Carboniferous and its applications to reservoir prediction

The search for Upper Carboniferous reservoirs can be aided by the development of a chronostratigraphic framework combining detailed sedimentological information with a template using the diagnostic ammonoid-bearing marine bands from onshore analogues derived from European, American and Canadian basins. Analysing Namurian successions constrains key sand-prone intervals, which include the upper Kinderscoutian, lower Marsdenian and top Yeadonian, from a number of depositional settings. A controversial issue for the Upper Carboniferous is the relative importance of high frequency and high magnitude glacio–eustatic sea-level fluctuations as a driving mechanism in the development of basin fill over the controls exerted through the prevailing tectonic regime, climate and sediment supply. The recognition and characterization of time-equivalent sea-level rises and, with greater significance for hydrocarbon exploration, sea-level falls, from a number of European basins attests to their influence. Candidate reservoirs can be constrained at various temporal resolutions and may also be restricted geographically. Significant candidate reservoirs include multistorey fluvial incised valley fills and deepwater sand-rich successions. The most productive onshore reservoirs are coarse-grained/conglomeratic fluvial intervals located in the base of incised valleys where a significant proportion of the subsequent valley fill is fine-grained estuarine sediments. There is potential for large hydrocarbon reservoirs within sand-rich deep water successions but their occurrence in tectonically complex or deeply buried areas has thus far precluded their exploitation. A chronostratigraphic framework established for the UK and Ireland can be applied to European basins and, in a more limited sense, to North America, where marine horizons can be matched. The application of this framework to the subsurface lies in the recognition of key candidate reservoirs and their likely occurrence, both geographically and temporally.

Evolution and Distribution of Porosity Associated with Subaerial Exposure in Upper Paleozoic Platform Limestones, West Texas

Middle Pennsylvanian-Lower Permian limestones in the subsurface of west Texas were studied to determine how subaerial exposure and freshwater diagenesis (karstification) affected porosity distribution in meter-scale cycles. Approximately 87 depositional cycles are present in the gross reservoir interval (depths of 2600-3000 m), and each cycle is interpreted to represent a glacio-eustatic sea level fluctuation. Using recent radiometric age dating, average cycle duration is estimated at 160,000 yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals in the upper part of cycles that were subaerially exposed; however, many cycles that were subaerially exposed now lack porosity. Diagenesis and porosity development have distinct patterns related to duration of subaerial exposure. Four stages of porosity development are identified. (1) Very brief or no subaerial exposure (estimated at less than 5000 yr) caused little or no diagenetic change. (2) Brief to moderate subaerial exposure (estimated at 5000-50,000 yr) resulted in most primary pores being filled with calcite cement, and dissolution creating fine matrix pores (molds and intercrystalline pores). (3) Moderately long subaerial exposure (estimated at 50,000-130,000 yr) resulted in cements filling primary pores and some fine secondary pores, and dissolution creating small conduit pores (vugs, fractures, fissures). (4) Prolonged subaerial exposure (estimated at greater than 130,000 yr) resulted in most primary and secondary matrix pores being filled with calcite cement, but dissolution enlarged conduit pores (vugs, fractures, fissures). Present subsurface porosity in this field preferentially occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones in cycles subjected to brief subaerial exposure (stages 2 and 3). Matrix porosity (molds, intercrystalline pores) is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales. The distribution of porosity in the Southwest Andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification.

Strontium Isotope Ages of the Marine Merced Formation, near San Francisco, California

The strontium isotopic compositions of foraminifers (Elphidiella hannaiwere determined to refine the age of coastal sediments of the Merced Formation and to evaluate the relation between transgressive–regressive cycles and Pleistocene glacio-eustatic sea-level fluctuations. Relatively good age resolution (± 0.1 myr) is possible in the upper part of the section. The Sr isotope ages provide strong supporting evidence for the fission track ages of an ash layer at the top of the formation (0.45 myr), as well as recent age estimates for the mammoth fossil of ∼0.45 myr. Sr isotope ages in the upper part of the section also confirm that marine/nonmarine cyclic sedimentation in the Merced Formation is controlled by glacial–interglacial cycles. Good age resolution for the lower part of the section is not possible, due to little variation of the87Sr/86Sr ratio in global sea water between 2.4 and 4.3 myr. The age of the base of the formation is constrained only between 2.4 and 4.8 myr. Sediment accumulation rates in the upper portion of the Merced Formation (between 0 and 720 m) are significantly higher than those calculated for the basal portion (between 720 and 1670 m). The change in accumulation rate may be due to increased continental weathering rates and/or increased tectonic subsidence beginning about 0.8 myr ago.

Partitioning of transgressive deposits in the southeastern Yellow Sea: a sequence stratigraphic interpretation

The last glacio-eustatic sea-level fluctuation in the southeastern part of the Yellow Sea formed a number of unconformity-bounded sedimentary sequences. These sequences comprise: (1) regressive to transgressive incision-fills (Unit C); (2) transgressive sand ridges and swales (Unit B); and (3) transgressive to highstand muds (lower mud Unit A2, upper mud Unit A1). The sequence boundary between Unit C and the lower unit comprises erosional surface which can be traced for the entire study area. The even and sharp boundary between the incision-fills (Unit C) and the sand ridges (Unit B) represents a transgressive surface of erosion. The depositional boundary between the lower mud (Unit A2) and the upper mud (Unit A1) suggests a change in oceanographic conditions. The boundary is confined landward and correlated offshore with the top of Units B and C that is exposed on the seafloor. These transgressive deposits show partitioning, controlled by currents, tides, and sediment supply as well as physiography of the seafloor.

Foraminiferal Paleobathymetry and Mid-Cycle Architecture of Mid-Pleistocene Depositional Sequences, Wanganui Basin, New Zealand

Mid-Pleistocene strata in the Castlecliff section of New Zealand show a cyclic alternation of shoreline to shelf lithofacies. A typical cycle is bounded by erosional disconformities, and shows a threefold subdivision. The lower unit is characterized by a basal cross-bedded shell-rich gravel, which is overlain by shoreline-nearshore siltstones, and/or sandstones. The middle unit is disconformity bounded, and is characterized by condensed shellbed facies deposited in terrigenous, sediment starved, inner-shelf settings. The upper unit comprises relatively thick and monotonous successions of shelf siltstone. Castlecliff cycles are interpreted as depositional sequences, driven by glacio-eustatic sea-level fluctuation. The component facies successions represent transgressive systems tracts (TST), mid-cycle condensed shellbeds (MCS), and highstand systems tracts (HST), respectively. Six Castlecliffian foraminiferal associations have been defined, and their paleodepth significance determined by uniformitarian comparison with modern New Zealand faunas. The shallowest (shoreface-innermost shelf) faunas are confined to TST successions, while the deepest faunas (outer inner-shelf) are confined to HST successions. The stratigraphic position of maximum paleodepth, and therefore the position of the conceptual maximum flooding surface (MFS), varies. Sequences in which the maximum paleodepth occurs within the mid-cycle condensed shellbed closely correspond to ideal sequence models, in which conditions of maximum sediment starvation coincide with maximum paleodepth. Sequences showing maximum paleodepths in HST shelf siltstones indicate that the MFS may not necessarily be coincident with mid-cycle condensed facies or their associated disconformities. Sequences with maximum foraminiferal paleodepths spanning some or all of MCS-JST successions indicate that the MFS may be represented as an interval rather than a specific disconformity or stratal surface. These results suggest that the MFS concept, as a means to separate TST from HST strata, may be of little use in shoreline inner-shelf paleogeographic settings.

The Roaches and Ashover Grits: sequence stratigraphic interpretation of a ‘turbidite-fronted delta’ system

A single Upper Carboniferous fluvio-deltaic cycle (Namurian, R2b5) in the south part of the Pennine Basin of northern England has been reinterpreted using a sequence stratigraphic approach. In upward succession the deposits comprise basinal mudstones, localized thick density current deposits, delta slope deposits and delta-top sandstones, followed abruptly by basinal mudstones. Earlier interpretations linked these elements with a single, mainly regressive, cycle, referred to as a turbidite-fronted delta. Recent evidence for strong glacio-eustatic sea-level fluctuations in the Namurian suggests that it is unlikely that the previous simple model can explain the spatial arrangement of all the lithological units. This paper attempts to identify elements of the Exxon sequence stratigraphic model, in which changes of sea level are an essential part. The basinal mudstones represent deep-water deposition and imply a highstand of sea level. Thick density current deposits are now known to be localized close to the basinward limit of delta progradation, so cannot be seen as a ubiquitous component of the depositional system. They may form a detached fan, or a fan located at the foot of the delta slope. Slope siltstones include turbidite-like sandstones, but these are interbedded with tractional sandstones and Pelecypodichnus trace fossils, giving no clear indication of water depth. The delta-top sandstones, some coarse and pebbly, can in places be shown to consist of two parts separated by a significant erosion surface now regarded as a type 1 sequence boundary. The erosion surface is locally incised about 80 m into the delta deposits, forming a major palaeovalley. Giant cross-beds (foresets 20 m thick) forming part of the palaeovalley fill are restricted to the basinward end of this feature. A curve of relative sea level inferred for the R2b5 interval suggests a fourth-order cycle in which two sharp rises are separated by a gradual fall. Possible minor (fifth-order) rises and falls may be superimposed. © 1997 John Wiley & Sons, Ltd.

中・上部更新統の地蔵堂層にみいだされた氷河性海水準変動による約２万年または約４万年周期の堆積シーケンス

中・上部更新統の地蔵堂層にみいだされた氷河性海水準変動による約２万年または約４万年周期の堆積シーケンス

Recurring global sea-level changes recorded in shelf deposits near the G/M polarity transition, Wanganui Basin, New Zealand: Implications for redefining the Pliocene-Pleistocene boundary

A well exposed 1 km-thick association of late Pliocene and early Pleistocene (ca. 2.6-1.7 Ma) strata in Wanganui Basin, New Zealand, contains a progradational stack of twenty, 5th order (41 ka duration) depositional sequences that accumulated in shelf to shoreface paleoenvironments in response to global glacioeustatic sea level fluctuations. The sequences are correlated with δ 18 O Isotope Stages 100-58, and each 41 ka glacial/interglacial stage couplet is represented by an individual sequence comprising transgressive (TST), highstand (HST) and regressive (RST) systems tracts that can be readily related to generic sequence stratigraphic models. In general, deposition of each of the sequences occurred during odd numbered (interglacial) isotope stages and the sequence boundaries developed during the maxima of even numbered (glacial) isotope stages. The key palaeomagnetic tiepoints used for pinning the isotope record to the section are the Gauss/Matuyama boundary (2.60 Ma in Stage 104) and the top of Olduvai Subchron (1.77 Ma in Stage 63). A deeply incised sequence boundary, with up to 30 m of relief, at the base of Te Rimu Sand, represents the first widespread subaerial exposure in Wanganui Basin during the Matuyama Chron; it is correlated with the high amplitude positive (glacial) δ 18 O shift in Isotope Stage 100, which is considered to mark the initiation of major northern hemisphere continental glaciations at ca. 2.52 Ma. The overlying Hautawa Shellbed, containing the subantarctic bivalve Chlamys delicatula , defines the base of the Nukumaruan Stage, and represents the earliest faunal evidence of climatic cooling in New Zealand Pliocene-Pleistocene marine sequences. The sequence boundary at the base of Hautawa Shellbed is correlated with Isotope Stage 98 (ca. 2.48 Ma). Traditionally, the Pliocene-Pleistocene boundary in New Zealand was located at the base of Hautawa Shellbed, but Beu and Edwards (1984) showed that the boundary, as defined in the Vrica stratotype (astronomically calibrated age ca. 1.81 Ma), actually lies in the upper part of the Nukumaruan Stage. The Hautawa Shellbed, which lies closer to the Gauss/Matuyama palaeomagnetic transition (2.60 Ma), is therefore much older than the currently defined Pliocene-Pleistocene boundary. From a New Zealand perspective, a more logical position for the Pliocene-Pleistocene boundary would be close to or just above the Gauss/Matuyama polarity transition. Such a position would take advantage of the boundaries proximity to a readily identifiable polarity reversal, and lie close to the time of initiation and development of major northern hemisphere ice sheets.

Coupled two-dimensional diagenetic and sedimentological modeling of carbonate platform evolution

We have used a new coupled two-dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio-eustatic sea-level fluctuations. The model employs variable hydrological zones, internally defined from platform exposure and climate, to provide a framework within which diagenetic processes are simulated at specified rates. Simulations show a clear stacked sequence of diagenetic zones, which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin. However, spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading. The interaction between high amplitude sea-level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods.

Sequence stratigraphy, seismic profiles, and cores of Pleistocene deposits on the Rhône continental shelf

High resolution seismic profiles on the Rhône deltaic shelf has shown the existence of a complex of Pleistocene superimposed prograding wedges interrupted by a major incised valley system cutting across the shelf. The individual wedges are interpreted, from internal geometry and coring data, as prograding shoreface deposits accumulated during periods of relative sea level lowering punctuated by small-scale relative sea level falls or forced regressions. They are related to 5th-order glacio-eustatic cycles and are stacked to form a composite 4th-order sequence. Radiocarbon dating (Tandetron AMS) of the upper wedge (to 40 ka BP) indicates a stratigraphic hiatus across the shelf corresponding to the glacial maximum lowstand at the end of the Würmian cycle (isotopic stage 2) during which sea level had dropped to 120 m below present sea level. Seaward tilting of the wedge boundaries indicate that the shelf has been subjected to differential subsidence that provides the necessary accommodation space for deposits to accumulate on the mid-outer shelf. The seaward tilting results from a cumulative effect of sediment and hydrostatic loading. Tectonism is also expressed by a westward shore-parallel tilting in the area east of the incised valleys. The amount of tilting, the stacking pattern of the wedges, and the location of incised valleys are structurally controlled, by the alpine orogen which dips westward under the Rhône margin. The tectonic component interacts with glacio-eustatic sea level fluctuations and accounts for the partition of the shelf into two areas with specific features separated by the Rhône incised valleys.

Lower Permian (Wolfcampian) Paleosol-Bearing Cycles of the U.S. Midcontinent: Evidence of Climatic Cyclicity

Lower Permian sedimentary cycles of the North American Midcontinent consist predominantly of very shallow marine and paralic facies and well-developed stacked paleosol profiles. Although recording glacio-eustatic sea level fluctuations, these cycles also contain evidence of cyclic climate change. This evidence includes the repeated carbonate-to-clastic facies pattern observed for meter-scale cycles, and the regionally consistent change from calcic to vertic paleosols within the variegated mudstones of most cyclothems. Climates are interpreted to have fluctuated from arid or semiarid conditions to seasonally wet/dry conditions during the course of a single cyclothem. Furthermore, within the Midcontinent, drier conditions appear to have characterized times of sea-level rise and highstand, whereas wetter more seasonal conditions characterized times of sea-level fall and lowstand. This relationship is interpreted to have resulted from variations in the intensity of a Pangean monsoon generated by glacial-interglacial cycles.

Geochemical analysis through the "transitional zone" of conodont faunal turnover in the Ordovician – Silurian boundary interval, Anticosti Island, Quebec

Geochemical analysis of the Ordovocian–Silurian boundary interval on Anticosti Island, Quebec, has revealed low Ir abundances for the lower Becscie Formation (0.005–0.039 ppb), comparable to those reported for the upper Ellis Bay Formation (0.005–0.058 ppb). When normalized as CaCO3-free values and ratioed to Al, a small amount of excess Ir (over background Ir levels) was observed in and on the upper surface of an oncolite platform bed in Member 7 (Laframboise) of the Ellis Bay Formation. Except for this bed, the abundances of Ir and other elements are relatively uniform throughout the section, but small enrichments are apparent in a few clay layers. As in other areas previously studied (South China, northwestern Canada, Scotland), the Ir maxima on Anticosti Island occur near the base of the graptolite persculptus Zone, and are attributable to condensation and slower sedimentation rates associated with the hardgrounds found within and on the upper surface of the oncolite platform bed. Although we cannot preclude a large extraterrestrial impact as the source for the Ir abundance maxima near the end of the Ordovician, which appear to be stratigraphically coeval and geographically widespread in the world, all of them are found to be associated with sedimentary condensation that may have resulted from glacio-eustatic sea-level fluctuations at the end of the Ordovician. From our data, we note that certain elements (e.g., Th, La, Lu, Sc, Cr, Ir, Fe, Al, Ca) show distinct abundance patterns in limestones, calcareous shales, and clays, apparently a result of increasing clay mineral contents.

Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin

The continental margin off the Niger Delta is undergoing deformation by gravity tectonism caused by rapid seaward sediment progradation. Three regional structural styles are recognized: (1) an upper extensional zone of listric growth faults beneath the outer shelf; (2) a translational zone of diapirs and shale ridges beneath the upper slope; and (3) a lower compressional zone of imbricated thrust structures (toe thrusts) beneath the lower slope and rise. Linked together on a regional scale, these styles suggest that large portions of this thick sediment prism are slowly moving downslope by gravity gliding or sliding, in a manner analogous to giant mass movements or mega-landslides. This deformation has created local intraslope basin up to 25 km wide, which are generally filled with thick (up to several kilometres) turbidites and mass transport deposits. Submarine canyons cut across these deformed zones and give rise on the lower slope to large, aggradational channel-levee systems which are distributaries for a few very large deep-sea fans. These fans and their channel-levee systems may extend up to hundreds of kilometres down the continental rise and are comparable in size with some of the largest modern fans such as the Amazon and Mississippi. Individual fan boundaries cannot yet be delineated; however, middle versus lower fan environments are recognizable on the basis of (1) channel morphology and size (e.g. large, leveed versus small, unleveed channels), (2) 3.5 kHz seismic facies (echo character) (e.g. well stratified versus prolonged echoes) and (3) sediment facies in piston cores (e.g. clays versus thick silt/sand turbidites). Off the easternmost edge of the Niger Delta, the submarine canyon-channel systems do not feed a deep-sea fan, but rather converge to feed a large, solitary channel-levee system, the Principé Channel, which extends 700 km across the rise to the Guinea Abyssal Plain. Transport and deposition of terrigenous sediments beyond the shelf edge have been accomplished mainly by (1) turbidity currents and (2) mass transport (slumps, debris flows) and have been controlled by fourth and fifth-order glacioeustatic sea-level fluctuations of 100 ka, or less, since at least the Miocene. During the Late Quaternary relatively short (≈10 000 year) periods of maximum sea-level rise during interglacial phases (such as the Holocene) shifted the locus of river sedimentation (i.e. deltas) landward across the wide shelf and temporarily shut off the terrigenous sediment supply to the deep sea. Pelagic sedimentation ensued and thin (<1 m) ‘condensed sections’ formed throughout the region. In contrast, the longer duration (≈90 000 year) periods of sea-level fall caused by glacials exposed the shelf; consequently, river deltas migrated seaward to the shelf edge and submarine canyon heads allowing large amounts of sediment to be continously transported to the slope, rise and deep-sea fans by turbidity currents and related mass flows. Deposition of these potentially sandy deep-water facies, coupled with their subsequent incorporation into intraslope basins and imbricate thrust structures through gravity tectonism, create the potential for significant hydrocarbon reservoirs within the deep-water portions of this continental margin.

Magnetostratigraphic chronology of late Miocene to early Pliocene biostratigraphic and oceanographic events in New Zealand

Successions of uplifted shallow marine sediments in New Zealand were among the first to provide evidence of latest Miocene climatic deterioration and were also among the first onshore successions where late Miocene to early Pliocene magnetostratigraphic records were established. A revised chronology of late Miocene to early Pliocene events has been determined from new magnetostratiigraphic results from two important successions at Blind River and Upton Brook, South Island, New Zealand (lat. 41°45′S, long. 174°05′E). Magnetostratigraphic results from Upton Brook are the fast from strata containing the Kapitean index mollusks and provide the first reliable estimate of the age of the Tongaporutuan-Kapitean boundary. A coherent chronology from Blind River and Upton Brook indicates that two synchronous low sea level and cool paleoclimatic events occurred in the late Miocene Kapitean Stage. This inference is supported from other well-known coeval successions in the New Zealand region. The revised New Zealand chronology supports recent determinations from the Mediterranean that indicate a two-stage Messinian salinity crisis and a 5.2-Ma age for the Miocene-Pliocene boundary. Data from New Zealand, the Mediterranean, and ocean cores around the world suggest that these latest Miocene events are related to glacio-eustatic sea level fluctuations. A 6.05-Ma age for the first occurrence datum of Globorotalia conomiozea indicates that significant diachroneity exists in the placement of this datum between the Southwest Pacific and the Mediterranean, confirming the observation that G. conomiazea is unsuitable for correlation between ocean basins.

Seismic imaging of Pleistocene deep-sea cyclothems: implications for sequence stratigraphy

SUMMARY Plate collision along the New Zealand Alpine Fault since the late Miocene has resulted in an abundant supply of temgenous sediment to an adjacent starved late Cretaceous rift-basin, the Bounty Trough. During the late Neogene, alternating pulses of temgenous (glacial) and biogenic (interglacial) sediment were deposited in the trough in the form of deep-sea cyclothems, in sympathy with glacio-eustatic sea-level fluctuations. Individual cyclothems ranging up to 15 m in thickness have been directly imaged by both 3.5 kHz and single channel airgun profiling. They are particularly conspicuous on the continental slope and in areas of the trough under the influence of overbank turbidite deposition from the Bounty Channel. On the abyssal Bounty Fan, approximately 50 cyclothems have been deposited since the late Pliocene, i.e. each cyclothem corresponds to two contiguous oxygen isotope stages. Each cyclothem also corresponds to the offshore conformity of a single 5th or 6th order seismic sequence, with the lower, temgenous portion marking a lowstand systems tract, and the upper, biopelagic part representing the transgressive and highstand systems tracts. Therefore, as a single continental margin sequence is traced seawards it passes from unconformity-bounded shelf and shelf-margin cyclothems, to slope sediments which include thick conformity-bounded cyclothems or autocyclic canyon deposits, to thin conformity-bounded deep-sea cyclothems on the distal rise, and finally to cryptically cyclic biopelagic sediment andor red clay on the abyssal plain.

Clastic sequences developed during late Quaternary glacio-eustatic sea-level fluctuations on a passive margin: Example from the inner continental shelf near Barnegat Inlet, New Jersey: Discussion and reply

Clastic sequences developed during late Quaternary glacio-eustatic sea-level fluctuations on a passive margin: Example from the inner continental shelf near Barnegat Inlet, New Jersey: Discussion and reply

Sedimentology of 40 000 year Milankovitch‐controlled cyclothems from central Hawke's Bay, New Zealand

ABSTRACTCyclothemic sedimentary rocks of the Plio‐Pleistocene Petane Group outcrop extensively in the Tangoio block of central Hawke's Bay, New Zealand. They are products of inner to mid‐shelf sedimentation and were deposited during glacio‐eustatic sea level fluctuations along the western margin of a shallow, pericontinental seaway located in a forearc setting. The succession consists of five laterally continuous cyclothems, each containing a fine grained interval of silt and a coarse grained interval of siliciclastic sand ± gravel or limestone. Five sedimentary facies assemblages comprising 20 separate facies have been recognized. Coarse grained intervals of cyclothems were deposited mostly during relative sea level lowstands and contain up to four facies assemblages: (1) a non‐marine assemblage (with three component facies, representing braided river and overbank environments); (2) an estuarine assemblage (with three component facies, representing tidal flat and mud‐dominated estuarine environments); (3) a siliciclastic shoreline assemblage (with six component facies, representing greywacke pebble beach, shoreface and inner shelf environments); and (4) a carbonate shelf assemblage (with four component facies, representing tide‐dominated, inshore and shallow marine environments). Fine grained intervals of cyclothems were deposited during sea level highstands when the Tangoio area was generally experiencing mid‐shelf sedimentation. This produced an offshore assemblage consisting of four component facies. The distribution of facies assemblages during relative sea level lowstands was dependent upon proximity to the shoreline, the type and rate of sediment supply to the basin, and shelf hydrodynamics. Carbonate shelf facies dominate coarse grained intervals in Cyclothems 3–5, but siliciclastic shoreline and non‐marine facies dominate in Cyclothems 1 and 2. The abrupt change from siliciclastic to carbonate sedimentation during relative sea level lowstand deposition is thought to have been induced by rapidly falling interglacial to glacial sea level accentuated by regional tectonic shoaling. This caused most of the terrigenous sediment supply to bypass the Tangoio area. Consequently, carbonate sediment accumulated in inshore and shallow marine settings. Facies assemblages rarely show lateral interdigitation, but are vertically stratified over the entire Tangoio block. Facies successions in each cyclothem preserve a record of relative sea level change during deposition of the Petane Group and are consistent with a Plio‐Pleistocene sea level change in eastern New Zealand of c. 75–150 m, i.e. approximately the magnitude suggested for Late Quaternary glacio‐eustatic sea level changes.