Lowstand Deposits Research Articles

ABSTRACT: Variable Lowstand Deposition and Tectonic Implications along the Louisiana Continental Margin during QuaternaryNeogene

ABSTRACT: Variable Lowstand Deposition and Tectonic Implications along the Louisiana Continental Margin during QuaternaryNeogene

Marine Sand Resources Offshore Israel

The continental margin of northern Sinai and Israel, up to Haifa Bay, is the northeastern limb of the submarine Nile Delta Cone. It is made up predominantly of clastics from the Nile and its predecessors. The continental shelf and coastal plain of Israel are built of a series of shore-parallel ridges composed of carbonate-cemented quartz sandstone (locally named kurkar), a lithification product of windblown sands that were piled up into dunes during the Pleistocene. The drop in global sea level and regression during the last glacial period exposed the continental shelf to subaerial erosion and created a widespread regional erosional unconformity which is expressed as a prominent seismic reflector at the top of the kurkar layers. The subsequent Holocene transgression abraded much of the westernmost kurkar ridges, drowned their cores, and covered the previous lowstand deposits with marine sands, which were in turn covered by a sequence of sub-Recent clayey silts. The Mediterranean coasts of Sinai and Israel are part of the Nile littoral cell. Since the building of the Aswan dams the sand supplied to Israel's coastal system is derived mainly from erosion of the Nile Delta and from sands offshore Egypt that are stirred up by storm waves. The sands are transported by longshore and offshore currents along the coasts of northern Sinai and Israel. Their volume gradually declines northward with distance from their Nile source. The longshore transport terminates in Haifa Bay where some sand is trapped, and the test escapes to deeper water by bottom currents and through submarine canyons, thus denying Nile-derived sand supply to the 40-km-long Akko-Rosh Haniqra shelf. The sand balance along Israel's coastal zone is a product of natural processes and human intervention. Losses due to the outgoing longshore transport, seaward escape, and landward wind transport exceed the natural gains from the incoming longshore transport and the abrasion of the coastal cliffs. The deficit is aggravated by the construction of (1) seaward-projecting structures that trap sands on the upstream side and (2) offshore detached breakwaters that trap sands between themselves and the coast. The negative sand balance is manifested by the removal of sand from the seabed and the consequent exposure of archaeological remains that were hitherto protected by it. The sediments that escape seaward from the longshore transport system form a 2.5- to 4-km-wide sandy apron adjacent to the shore that extends to where the water is 30–40 m deep. The apron's slope (0.5–0.8°) is steeper than the theoretical equilibrium slope for the median grain-size diameter in this zone (0.1–0.3 mm). The beach sands and the apron's surficial sands are well sorted. Their grain size decreases with distance from shore, from 0.2–0.3 mm nearshore to 0.11–0.16 mm by the drowned ridge. The coarse-grained fraction consists of skeletal debris (commonly 5–12% carbonate matter) and wave-milled kurkar grains (locally named zifzif). In deeper water, the basal sands underlying the fine-grained sediment cover consist of 1- to 30-cm layers whose composition ranges from silty sands to various types of sands (fine, medium, coarse, and gravelly) to zifzif. For the most part, they contain large amounts of skeletal debris (20–60%) and small fragments of kurkar. Two types of kurkar rock were encountered offshore: a well-sorted, fine- to medium-grained (0.074–0.300 mm) lithified dune sand with variable amounts of carbonate cement, ranging from hard rock of low permeability to loose sand; and a porous sandstone made up predominantly of algal grains and skeletal debris (calcarenite).

Depositional response to Quaternary fourth-order sea-level fluctuations on the Latium margin (Tyrrhenian Sea, Italy)

Abstract More than 10 000 km of high-resolution seismic profiles permit detailed study of six fourth-order seismic stratigraphic sequences deposited during the last million years on the Latium continental shelf, Tyrrhenian Sea, Italy. Sedimentation occurred on a relatively young passive margin characterized by a narrow and relatively steep shelf where sediment storage capacity in adjacent subaerial basins was limited. The Late Pleistocene deposits are locally tilted and eroded to different levels along the sea-floor so that carefully placed seafloor gravity cores help constrain the age of the seismic sequences by the dating of micro-fauna. Correlation with a deep well located on the coast constrains the basal third-order sequence boundary on the shelf. The seismic and a limited core database make it possible to: (1) detail variability in the architecture, stratal patterns and bounding surfaces of the sequences across the shelf and adjacent continental slope; (2) define a hierarchy of seismic units and their bounding surfaces; (3) make a correlation with the published oxygen-isotope curves; (4) develop a detailed stratigraphic framework and model for the fourth-order sequences deposited during last 0.8 Ma; (5) define the effects of the long-lasting eustatic falls on margin sedimentation; (6) recognize volumetric partitioning of sedimentation between systems tracts. The seismo-stratigraphic expression of the third- and fourth-order sequence boundaries varies greatly from the inner to the outer part of the margin. Where subsidence allowed the preservation of lowstand systems tract (LST) deposits on the shelf, they are bounded by erosional unconformities interpreted to reflect fourth-order glacioeustatically-driven cycles. Relatively thin (<10 m) lens-shaped bodies mark the transition from the unconformities to their correlative conformities and are interpreted to have been deposited during the eustatic minimum. The deposits bounded between correlative conformities show an upward loss in acoustic transparency thought to indicate upward-coarsening and regression within the sequences. Downdip on the continental slope, sequence boundaries are concordant surfaces correlative with unconformities on the shelf. However, these surfaces are locally scoured by channellized features, interpreted to record slope erosion related to the discharge of river bedload during lowstands. There is marked asymmetry and volumetric partitioning between systems tracts; most of the Late Quaternary deposits that comprise the Latium continental margin are interpreted to have formed during forced regression and lowstand. Offshore of the northern and central Latium shelves forced regressive and lowstand deposits account for some 1000 km 3 of shelf and slope deposition during the last eustatic cycle. In contrast, sediments attributed to the transgressive and highstand systems tracts account for approximately 37 km 3 . High-frequency, high-amplitude asymmetric sea-level changes driven mainly by glacioeustasy are interpreted to have controlled deposition. Following classic three-fold sequence stratigraphic models, the unconformities created by shelf subaerial exposure and erosion represent sequence boundaries at the base of depositional sequences. However, if as is the case of the Latium margin during the Late Pleistocene, where a continental margin is formed almost exclusively of forced regressive deposits, each sequence basal boundary will paradoxically be situated above the forced regressive deposits that are deposited as the subaerial exposure surface forms, i.e. above the whole of its own depositional sequence. In this respect, the incorporation of a fourth forced regressive or falling stage systems tracts specific to times of base-level fall would help avoid this inconsistency.

Processes of sedimentation on a lacustrine border-fault margin; interpretation of cores from Lake Malawi, East Africa

Large lakes are attractive research sites because they provide ideal spatial scales for understanding the complex relationships between facies distributions and processes of sedimentation. In this paper, we describe deepwater facies and associated environments of deposition, based primarily on cores, from the tectonically active South Rukuru sublacustrine fan system of Lake Malawi, east Africa, where river discharge is high, offshore gradients are steep, and gravity-driven processes are common. The coring program was tied to a simultaneous seismic investigation, which is discussed in a companion paper (Soreghan et al. 1999). All facies, except the hemipelagic drape, contain widespread evidence of gravity-driven sedimentation processes, with turbidity currents being responsible for most of the sediment transport and deposition. Multiple fining-up turbidite units dominate the sandy facies of the canyons, channels, mouth-of-canyon fans, and basin-plain fill environments, and thin distal turbidites are present in cores through most of the muddier facies. Angular gravel and coarse sand are common not only throughout the canyons of the sloping relay ramp region but also as far offshore as the main axial channel and its depocenter, the basin-plain fill facies. Turbidites range in thickness from 0.25 to 2.0 m, are usually stacked, and can be recognized by well-defined Bouma sequences. A hemipelagic drape of highly laminated to structureless mud, which forms an uneven blanket over much of the lake bottom, is commonly interspersed with turbidites. Because lake levels can change rapidly as a result of climatic variations, the rate of sediment mobilization is extremely fast relative to marine systems. Vertical excursions in lake level of 100 m over only a few hundred years may lessen the distinctions between highstand and lowstand deposits. The border-fault setting allows coarse sediment to be delivered to the steep offshore, even during periods of lake high-stand. However, during periods of falling and low lake level it is likely that downcutting and erosion of highstand deltas increase the magnitude and intensity of turbidite deposition in the mouth-of-canyon fan and in the basin-plain fill environments.

Calcareous Upper Pleistocene Lowstand Deposits Indicate Shelf/Slope Tectonic Instability

Calcareous deposits, including shell, hash, and oolites, have been recorded in Upper Pleistocene lowstand deposits seaward of major rivers such as the Mississippi and the Ganges. These calcareous sediments indicate a major reduction or cessation of terrigenous deposition during lowstand/glacial maximum. Extensive high-energy sediments, including coarse sands and gravels, can bracket thin, low-energy, fine-grained calcareous deposits within lowstand sequences. These various sequences may indicate up to three orders of magnitude of variations in deposition rates. Summarizing glacial lake outbursts/rapid stream rerouting suggests that during final deglaciation there could have been massive outflows at multiple decade to century intervals. Continental glacial oscillations throughout the entire glacial maxima could also have produced massive, short-term floods and deposition rates. Such swings in deposition rates must impact on geologic processes within the underlying sedimentary wedge. Processes impacted include (but are not limited to) compacting, faulting, dewatering, diapiric motion, diagenesis, and heat generation.

Surficial Sediments along the Inner Continental Shelf of Maine

Through 10 years of support from the Minerals Management Service Association of American State Geologists' Continental Margins Program we have mapped along the Maine coast, seaward to the 100 m isobath. In all, 1,773 bottom sample stations were occupied, 3,358 km of side-scan sonar and 5,011 km of seismic reflection profiles were gathered. On the basis of these data, a surficial sediment map was created for the Maine inner continental shelf during the Year 8 project, and cores and seismic data were collected to evaluate sand thickness during Years 9 and 10. Sand covers only 8 % of the Maine shelf, and is concentrated seaward of beaches off southern Maine in water depths less than 60 m. Sand occurs in three depositional settings: (1) in shoreface deposits connected dynamically to contemporary beaches; (2) in submerged deltas associated with lower sea-level positions; and (3) in submerged lowstand shoreline positions between 50 and 60 m. Seismic profiles over the shoreface off Saco Bay, Wells Embayment, and off the Kennebec River mouth each imaged a wedge-shaped acoustic unit which tapered off between 20 and 30 m. Cores determined that this was sand that was underlain by a variable but thin (commonly 1 m) deposit of estuarine muddy sand and a thick deposit of glacial-marine mud. Off Saco Bay, more than 55 million m3 of sand exists in the shoreface, compared with about 22 million m3 on the adjacent beach and dunes. Seaward of the Kennebec River, a large delta deposited between 13 ka and the present time holds more than 300 million m3 of sand and gravel. The best sorted sand is on the surface nearshore, with increasing amounts of gravel offshore and mud beneath the surficial sand sheet. Bedforms indicate that the surficial sand is moved by waves to at least 55 m depth. Seaward of the Penobscot River, no significant sand or gravel was encountered. Muddy estuarine sediments overlie muddy glacial-marine sediment throughout the area offshore area of this river. No satisfactory explanation is offered for lack of a sandy delta seaward of Maine's largest river. Lowstand-shoreline deposits were cored in many places in Saco Bay and off the Kennebec River mouth. Datable materials from cores indicated that the lowstand occurred around 10.5 ka off the Kennebec. Cores did not penetrate glacial-marine sediment in the lowstand deposits, and seismic profiles were ambiguous about the vertical extent of sand in these units. For these reasons, no total thickness of sand was determined from the lowstand deposits, but given the area of the surficial sand, the volume is probably in the hundreds of millions of cubic meters.

Cenozoic global sea level, sequences, and the New Jersey Transect: Results From coastal plain and continental slope drilling

The New Jersey Sea Level Transect was designed to evaluate the relationships among global sea level (eustatic) change, unconformity‐bounded sequences, and variations in subsidence, sediment supply, and climate on a passive continental margin. By sampling and dating Cenozoic strata from coastal plain and continental slope locations, we show that sequence boundaries correlate (within ±0.5 myr) regionally (onshore‐offshore) and interregionally (New Jersey‐Alabama‐Bahamas), implicating a global cause. Sequence boundaries correlate with δ18O increases for at least the past 42 myr, consistent with an ice volume (glacioeustatic) control, although a causal relationship is not required because of uncertainties in ages and correlations. Evidence for a causal connection is provided by preliminary Miocene data from slope Site 904 that directly link δ18O increases with sequence boundaries. We conclude that variation in the size of ice sheets has been a primary control on the formation of sequence boundaries since ∼42 Ma. We speculate that prior to this, the growth and decay of small ice sheets caused small‐amplitude sea level changes (<20 m) in this supposedly ice‐free world because Eocene sequence boundaries also appear to correlate with minor δ18O increases. Subsidence estimates (backstripping) indicate amplitudes of short‐term (million‐year scale) lowerings that are consistent with estimates derived from δ18O studies (25–50 m in the Oligocene‐middle Miocene and 10–20 m in the Eocene) and a long‐term lowering of 150–200 m over the past 65 myr, consistent with estimates derived from volume changes on mid‐ocean ridges. Although our results are consistent with the general number and timing of Paleocene to middle Miocene sequences published by workers at Exxon Production Research Company, our estimates of sea level amplitudes are substantially lower than theirs. Lithofacies patterns within sequences follow repetitive, predictable patterns: (1) coastal plain sequences consist of basal transgressive sands overlain by regressive highstand silts and quartz sands; and (2) although slope lithofacies variations are subdued, reworked sediments constitute lowstand deposits, causing the strongest, most extensive seismic reflections. Despite a primary eustatic control on sequence boundaries, New Jersey sequences were also influenced by changes in tectonics, sediment supply, and climate. During the early to middle Eocene, low siliciclastic and high pelagic input associated with warm climates resulted in widespread carbonate deposition and thin sequences. Late middle Eocene and earliest Oligocene cooling events curtailed carbonate deposition in the coastal plain and slope, respectively, resulting in a switch to siliciclastic sedimentation. In onshore areas, Oligocene sequences are thin owing to low siliciclastic and pelagic input, and their distribution is patchy, reflecting migration or progradation of depocenters; in contrast, Miocene onshore sequences are thicker, reflecting increased sediment supply, and they are more complete downdip owing to simple tectonics. We conclude that the New Jersey margin provides a natural laboratory for unraveling complex interactions of eustasy, tectonics, changes in sediment supply, and climate change.

Sequence stratigraphy of a sandstone‐rich Permian glacial succession, Fitzroy Trough, Canning Basin, Western Australia

The Canning Basin in northwest Australia contains hydrocarbon‐bearing Permo‐Carboniferous glacial successions, one of a number of such units that occur widely distributed across Gondwana. Up to 2.5 km of clastic sediment eroded by Permian ice sheets from the adjacent Precambrian craton was trapped in the Fitzroy Trough in the northeast part of the Canning Basin. These sedimentary rocks are between 60 and 80% fine to medium sandstone. During relative sea‐level lowstands, regionally extensive erosion surfaces with a relief of several hundred metres developed. Lowstand and early transgressive deposits consisting of sandstone and finer grained elastics that infill topographic lows were deposited on these erosion surfaces. Later transgressive deposits consist of glaciomarine mudstones and rain‐out diamictites, subaqueous outwash fans and deltaic deposits, composed mostly of sandstone. Alternatively, in some situations, the sequences consist entirely of aggrading sandy fluvial braidplain deposits. Some relative highstands may have resulted from isostatic loading of the basin by the ice sheet, so that highstand systems tracts in the Grant Group may correlate with relative lowstands in other basins. Englacial and proglacial lake outbursts deposited extensive sheets of massive, relatively well‐sorted sandstone in all systems tracts. The hydrology of these catastrophic floods meant that much fine sediment was washed out of the basin. This process, combined with the nature of the Precambrian basement adjacent to the Canning Basin produced the high proportion of sandstone in the Grant Group depositional system. Sequence architecture within the Grant Group was determined by interaction of eustasy with tectonic subsidence and sediment supply. Depositional space was created by thermal subsidence in response to Permian extension (which began at approximately 295 Ma) which probably also resulted in uplift of the source area on the Pilbara Craton. In general, sediment supply overwhelmed available depositional space. Glacial sediments produced before the initiation of thermal subsidence would have bypassed the Fitzroy Trough entirely. Late Palaeozoic glacial successions preserved in basins in Oman and Brazil, which lay at the opposite extremes of the Gondwana Plate, have remarkably similar architecture to those of the Canning Basin suggesting similar depositional controls and large‐scale tectonic linkages.

Erosional offshore sand ridges and lowstand shorefaces; examples from tide- and wave-dominated environments of France

ABSTRACT Offshore sand are described from many continental shelves in the world, as well as in the stratigraphic record, where they commonly are productive reservoirs. Some of these ancient sand bodies, initially interpreted as sand ridges, are now reinterpreted as lowstand shoreface deposits. Modern sand bodies, in contrast, have received relatively little attention with regard to reinterpretation of their origin, largely because of the lack of information about their internal structure. Improved techniques in acquisition and processing of very high-resolution seismic profiles, along with some shallow cores, allow us to reconstruct the architecture of sand bodies from the Celtic Sea (tide-dominated) and the Gulf of Lions (wave-dominated) shelves of France, in water depths of 100-170 m. In both areas, our investigations demonstrate that these particular sand consist mainly of lowstand deposits (estuarine/deltaic systems, sharp-based shorefaces), reworked during transgressions. In the Celtic Sea, intense erosion by combined waves and tidal currents resulted in the shaping of shore-oblique ridges by cannibalization of older lowstand deposits. In the Gulf of Lions, the shore-parallel orientation of the lowstand shorefaces has been preserved, leaving an ancestral sand body with reworked (transgressive) surface deposits. Understanding the architecture and distribution of offshore sand requires taking into consideration not only the effects of relative sea-level changes and sediment supply, but also the role of hydrodynamical processes. The erosional sand we describe represent a new category of outer-shelf sand bodies, a combination of the autocyclic examples described by Houbolt (1968) in the southern North Sea and the allocyclic lowstand shorefaces mainly described in the stratigraphic record of the Western Interior Seaway of North America. Our findings have applications for predicting the geometry of ancestral sand and their orientation and position with respect to paleo-shorelines. The magnitude of erosional processes also implies that a large amount of shelf sediment (mainly sand) was transferred to the adjacent deep-sea floor during the early transgression.

Upper Quaternary strata of the upper continental slope, Northeast Gulf of Mexico; sequence stratigraphic model for a terrigenous shelf edge

ABSTRACT The shallow subsurface of the upper slope, northeastern Gulf of Mexico, shows abrupt changes in depositional environments, sequence stratigraphy, and sedimentation rates reflecting Pleistocene to Holocene glacioeustasy. Stratigraphic change is recorded in lithologic, sedimentologic, biostratigraphic, and oxygen isotopic data from a nearly continuous 245.5 m core from Viosca Knoll Block 774, a gamma-ray log of the borehole, and high-resolution seismic-reflection data. Mud with minor sand at the base of the boring (245.5 to ~ 136 m) was deposited during several sea-level cycles corresponding to isotope stages 13 to 8, and includes a thick, probable stage 10 unit. A thick, coarsening-upward deltaic mud and sand at ~ 136-59.9 m likely was deposited during oxygen isotope stage 8 glaciation. The section upward from ~ 59.9 m contains: a deltaic silt and sand probably deposited during stage 8; a transgressive sand shoal recording the stage 8-7 transition; highstand mud and sand of stage 7; a relatively thin stage 6 lowstand mud and sand; stage 5 highstand mud and sand; stage 4-2 mud and sand; and Holocene sandy silt. Almost all contacts between systems tracts are transitional in core. Inferred lowstand deposits are lithologically and sedimentologically variable, reflecting degree of delta progradation and subsequent erosion, and consist of sand and (or) mud. Thicker, coarser intervals at the site represent shelf-margin deltas likely deposited during falling sea level. Falling sea level and lowstand sediment is characterized by mostly middle to inner neritic benthic foraminifera, cool-temperature planktonic foraminifera, and high oxygen isotopic values of the planktonic foraminifera Globigerinoides ruber. Transgressive and highstand systems tracts are thin. Rising-sea-level sections and highstand condensed intervals consist of clay or silt with carbonate concretions and with numerous pyritized microfaunal tests and small burrows. Condensed sections also contain abundant microfauna, mostly warm-temperature planktonic foraminifera, outer neritic to upper bathyal benthic foraminifera, and transitional to interglacial 18O values of Globigerinoides ruber. Some sandy turbidites appear to be interbedded with highstand mud.

Rates of delta progradation during highstands: consequences for timing of deposition in deep-marine systems

Estimated times required for 24 modern river systems to form a shelf-edge delta range from 8.5 ka to 116.5 ka, depending on fluvial sediment supply, delta width, shelf volume and shelf transport rates. These values indicate that transport of sand into deep-marine systems is likely to be significant during third-order highstands of relative sea-level. Factors such as shelf transport dynamics may slow delta progradation while submarine canyons cutting the shelf may reduce the time before deep-marine deposition occurs. Interpreting ancient sand-rich deep-marine strata as lowstand deposits without sufficient palaeogeographic information may not therefore always be appropriate.

Sedimentological and diagenetic controls on Cambro-Ordovician reservoir quality in the southern Hassi Messaoud area (Saharan Platform, Algeria)

Abstract The Cambro-Ordovician reservoirs of the Hassi Messaoud area comprise quartzitic sandstones, which rest unconformably on granitic basement and are capped by the Hercynian unconformity. Two sequence stratigraphic cycles are identified: a lower cycle of lowstand, transgressive and highstand deposits, and an upper cycle in which only lowstand deposits are preserved below the Hercynian unconformity. Sandstone of the greates thickness and highest porosity and permeability occur within the lowstand deposits of the two sequences. Petrographic and scanning electron microscope studies were conducted in two wells in the southern Hassi Messaoud area on five sandstone units, to which the following chronostratigraphic terms are applied, from base to top: Ra + Ri lithozones (which contain the bulk of the reserves), Alternance Zone, El Atchane Sandstones and Hamra Quartzite. Reservoir quality is found to bear a strong relationship to clay content and mineralogy. All reservoirs are characterized by an extensive quartz cementation, which reduces porosities to generally below 10%. Permeability is related to clay content and type, with effective permeabilities generally limited to the quartz- and kaolinite-rich Ra lithozone. Permeability is lowest in reservoirs rich in fibrous illite and/or chlorite, such as the Hamra Quartzite. As porosity shows a less direct relationship to matrix mineralogy, there is a poor statistical relationship observed between porosity and permeability. Comparative diagenetic studies carried out within both the oil-bearing and the water-bearing parts of the reservoirs have determined that all the secondary processes occurred under freely operating diagenesis, pre-dating oil emplacement in the structure.

SEQUENCE STRATIGRAPHIC INTERPRETATION OF CARBONATE WIRELINE LOG MOTIFS: AN EXAMPLE FROM THE NORTH WEST SHELF OF AUSTRALIA

An integrated biostratigraphic, wireline and seismic sequence stratigraphic study has been conducted to constrain the timing and evolution of Late Cretaceous to Tertiary depocentres along the North West Shelf of Australia. During this study a model for the sequence stratigraphic interpretation of wireline logs in this carbonate-dominated regime has been developed.A series of readily identifiable, lowstand clastic deposits interspersed within the predominantly carbonate passive margin section of the North West Shelf provide well-defined correctable events with which to divide the section. Biostratigraphic data have indicated the presence of missing section at the base of these clastic deposits and their shelfal equivalents. These events have been correlated to define sequence boundaries that are represented on wireline log data by a sharp increase in the gamma signature. Lowstand systems tracts exhibit an irregular sonic and upwardly increasing gamma signature. Transgressive systems tracts show characteristically upward-decreasing gamma and sonic profiles. Maximum flooding surfaces have been identified as the point of cleanest carbonate sedimentation represented by gamma minima on wireline logs. Log motifs exhibiting little character have been interpreted as highstand systems tracts. On seismic these sequence stratigraphic events are represented by stratal geometries that would be expected for these systems tracts.The model has enabled the definition of a higher resolution chronostratigraphic framework for the Mid Cretaceous to Recent section of the North West Shelf than has previously been possible. Forty basin-wide events have been identified from the biostratigraphic and wireline log analysis, thirty of which can be tied throughout the Barrow, Dampier and Roebuck basins.

Abstract: Sequence Stratigraphic Framework and Depositional Variations of Early Miocene Lowstand Systems Tracts, Southwestern Louisiana

Abstract Facies of Miocene lowstand systems tracts comprise a large portion of the remaining exploration potential for deep untested objectives I southwest Louisiana. This complex and highly variable section is contained within 22 major depositional sequences deposited from 21.9 to 7 million years ago; each sequence ranges in duration from 0.5 to 1.5 million years. Regional well-log, seismic, and biostratigraphic correlation and mapping of the sequence stratigraphic framework defines the occurrence and extent of lowstand facies that filled intra-slope basins across south Louisiana's coastal zone. From oldest to youngest, the sequence encompass the Cristellaria R through Cristellaria K biozones. Within onshore and state water areas, 14 sequences contain regionally correlative lowstand systems tract components. Almagamated, massive turbidite facies of basin floor fan complexes are not recognized within intra-slope basins in association with rapid depositional cycles. Deep-water fan and channeled levee facies of slope fan complexes and deltaic facies of prograding complexes are well-developed throughout the early and middle Miocene section. Late Miocene lowstand deposits are best developed in the offshore trend. Syndepositional fault movement and salt withdrawal controlled thickness trends, facies patterns, sandstone content, and the position and trend of the shelf-slope break. Sandstone-rich basin floor fans, slope fans, and prograding deltaic complexes are best developed in intraslope basins where paleobathymetric confinement and accommodation were greatest. End_of_Record - Last_Page 349-------

Sequence stratigraphy of Holocene incised-valley fills and coastalevolution in the Gulf of Cádiz (southern Spain)

This first sedimentary interpretation of two incised-valley fills in the Gulf of Cadiz (southern Spain), which accumulated during the last fourth-order eustatic cycle in response to fluvial incision, changes of sea level, and correlative deposition, relates the filling of the estuarine basins and their barriers with four regional progradation phases, H1 to H4. The cases studied are the wave-dominated Guadalete, and the mixed, tide and wave-dominated Odiel-Tinto estuaries. The sequence boundary is a type-1 surface produced during the lowstand of the Last Glacial period ca. 18 000 14C yr BP. No fluvial lowstand deposits were found in the area. Due to rapid transgression the valley fills consist of transgressive and highstand sediments. The maximum landward advance of the estuarine barriers occurred ca. 6500–6000 14C yr BP during the maximum of the Flandrian transgression, but there is no evidence of sea level rising appreciably above the present. A large part of the estuaries was filled during H1 (ca. 6500–4400 14C yr BP) but ravinement by shifting tidal inlets destroyed most of the coeval barriers. During the H2 phase (ca. 4200–2550 14C yr BP) sedimentation was favoured by arid conditions and concentrated in the axial estuarine zones and the barriers. Between H2 and H3 prevailing winds changed from W to WSW, increasing spit growth to the east and south-east. Progradation of bay-head deltas and flood-plains during H3 (ca. 2300–800 14C yr BP) and H4 (500 yr ago to the present) further reduced the accommodation space in the largely-filled valleys, and sediment by-passed the estuaries and accumulated in the estuarine barriers as fast-growing spits. Arid conditions and increasing human activity have caused rapid coastal modifications.

Contemporaneity of component units of the lowstand systems tract: An example from the Pleistocene Kazusa forearc basin, Boso Peninsula, Japan

The lowstand systems tract of the standard sequence-stratigraphic model consists of three major component units—in ascending order, the basin-floor fan, slope fan, and lowstand wedge. However, there are no modern or ancient examples of a lowstand systems tract that contain each of the three component units from the base to top. Three-dimensional outcrop analysis and detailed chronostratigraphic correlation of the lowstand deposits that infilled the Pleistocene Kazusa forearc basin exposed on the Boso Peninsula of Japan reveal that the three component units of the lowstand systems tract are largely age equivalent. Their development was controlled much more by geographic setting, in conjunction with spatial variation in sediment supply and basin subsidence, than by the position of relative sea level. It is concluded that the lowstand systems tract of the sequence-stratigraphic model is a theoretical concept that is not accurately reflected in the stratigraphic relationships shown in the present field case.

Biostratigraphy, sedimentology and sequence stratigraphy of the Tournaisian–Viséan transitional strata in South China (Guangxi)

The biostratigraphy and sedimentological evolution of the Tournaisian–Viséan (T–V) transitional strata in South China (Guangxi) have been investigated. The sediments were deposited on a carbonate platform and in slope and basinal environments. In the T–V transitional strata, six foraminiferal associations have been distinguished which allow correlation between the shallow and deep water deposits. A careful examination of the evolutionary stages of the foraminifer Eoparastaffella provides a more accurate criterion for the definition of the T–V boundary, but does not significantly modify the historical one. The distinction of two morphotypes is based on the elevation of the last whorl and the peripheral outline. Tournaisian specimens of Eoparastaffella have a well rounded periphery (morphotype 1) contrasting with the subangular periphery of younger Viséan specimens (morphotype 2). A coefficient can be deduced from simple biometric measurements for more precisely defining the T–V boundary. The sequence stratigraphy of the T–V strata in South China has been reconstructed by combining biostratigraphical and sedimentological data. It allowed the correlation of the T–V transitional strata between the platform area and the slope and basinal locations. Late Tournaisian strata were deposited during a highstand systems tract. Near the end of the Tournaisian, a major drop in relative sea-level led to the development of an unconformity in the platform area. Lowstand deposits formed during latest Tournaisian time in the basin where a detailed biostratigraphic framework has been devised. Sediments deposited during the ensuing transgressive systems tract overlie the late Tournaisian highstand sediments in the platform area and the latest Tournaisian lowstand deposits in the basin. A major drop in relative sea-level near the end of the Tournaisian has been recognized worldwide. Therefore, the possibility of using the sequence stratigraphy of the T–V strata in South China for worldwide correlations should be investigated. © 1997 John Wiley & Sons, Ltd.

Platform‐to‐basin correlation by high‐resolution sequence stratigraphy and cyclostratigraphy (Berriasian, Switzerland and France)

ABSTRACTBased on detailed analyses of facies evolution and stacking pattern of Berriasian carbonate‐dominated sections in the Swiss Jura Mountains, the Swiss Ultrahelvetic and the French Vocontian Trough, a high‐resolution platform‐to‐basin correlation is proposed. Biostratigraphical tie points are furnished by ammonites, dinoflagellates, calpionellids, and ostracod‐charophyte assemblages. The hierarchical stacking of small‐scale depositional sequences reflects Milankovitch cyclicity: sequences corresponding to the 20‐, 100‐, and, locally, 400‐ka orbital cycles can be identified. Elementary (20 ka) sequences on the platform generally consist of one bed of shallow subtidal to intertidal, high‐ or low‐energy carbonate facies, whereas on the slope and in the basin they are commonly developed as limestone‐marl couplets. These elementary sequences group into small‐scale composite sequences reflecting the first orbital eccentricity cycle (100 ka), which in turn build up large‐scale (3rd‐order) composite sequences. One 3rd‐order sequence has been analysed in detail: according to the cyclostratigraphic interpretation, it took ≃2 Myr to form, which is in accordance with the duration of the corresponding ammonite subzones. Sequence‐stratigraphic and cyclostratigraphic platform‐to‐basin correlation shows that for about 900 ka the platform was exposed or only partly flooded, whereas on the slope and in the basin, lowstand deposits with channel fills and slumps accumulated. With rising sea level, accommodation space gradually increased on the platform and a thickening‐upward sequential pattern with transgressive facies developed, while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds. This situation lasted about 700 ka. The following 300 ka were characterized by sediment starvation and increased bioturbation on the platform, and by more marly, transgressive sediments on the slope and in the basin. The maximum‐flooding phase is more or less isochronous on the platform and in the basin, although the surface with the best‐developed maximum‐flooding features may be displaced by one or two small‐scale composite sequences because of superimposed high‐frequency sea‐level fluctuations, and/or local variations in substrate morphology and sediment distribution. Third‐order highstand conditions prevailed for only about 100 ka. The combination of cyclostratigraphy and high‐resolution sequence stratigraphy, constrained by good biostratigraphy, is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another.

Oligocene sequence stratigraphy and basin development in the Danish North Sea sector based on log interpretations

Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark. Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals. The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats.

Distribution of sedimentary organic matter in a mixed carbonate‐siliciclastic platform environment: Oxfordian of the Swiss Jura Mountains

ABSTRACTOutcrop sections from the Swiss Jura, consisting of carbonate‐siliciclastic deposits spanning the Middle‐Late Oxfordian boundary, provide a palaeogeographical cross‐section ranging from coastal to shallow platform and intra‐platform basin environments. Using a sedimentological and a 3rd order sequence stratigraphic framework based on those sections, the distribution of sedimentary organic matter (palynofacies) has been spatially studied in relation to sequence stratigraphy.The main factors influencing the spatial variations of sedimentary organic matter are the proximity of land, organic productivity, level of biodegradation and hydrodynamic conditions of the palaeoenvironment. These factors determine the distribution of the land‐derived, relatively allochthonous organic constituents (phytoclasts, pollen and spores), and marine, relatively autochthonous constituents (phytoplankton and foraminifera linings).Five main palynofacies parameters appear as good indicators of proximal‐distal trends: the relative proportion of total phytoclasts; the proportion of fresh, translucent fragments among these phytoclasts; the ratio of marine to continental palynomorphs; and the relative amount and species diversity of dinoflagellate cysts. In lowstand deposits, high‐energy hydrodynamic conditions may affect these trends on the shallow platform, where storms tend to increase the proportion of the marine organic fraction landwards. These five palynofacies parameters are largely related to water depth. Therefore, when compared with the established 3rd order sequence stratigraphic framework, their stratigraphic variations indicate relative sea‐level changes or, alternatively, proximality changes. Parameters indicative of more distal conditions increase in the transgressive and early highstand, and decrease in the late highstand. The opposite occurs for parameters indicative of more proximal conditions. Foraminifera linings seem to be particularly concentrated near the shelf edge and on the slope. Finally, bisaccate pollen may display cyclicity associated with alternating humid and dry periods. This palaeoclimatic signature may be related to orbital cyclicity.