Maximum Transgression Research Articles

Dinoflagellate assemblages and sea-level fluctuations in the Maastrichtian of southwest Georgia

Analysis of Maastrichtian dinoflagellate assemblages from the upper Ripley and Providence Formations in two cores from southwestern Georgia show changes that can be related to fluctuations in relative sea-level and in depositionla environments. Sediments in the USGS Ft. Gaines Core were deposited in transitional zone to nearshore and estuarine environments close to a major fluvial source, whereas co-eval sediments in the USGS Albany Core were deposited in middle to outer shelf environments, lateraly displaced away from any fluvial source. Lower Maastrichtian assemblages in the Ft. Gaines Core contain common Senegalinium obscurum and Cerodinium pannuceum. These point to overall transgressive conditions in the lower Providence, whereas decreases in percent marine phytoplankton indicate individual progradational episodes. The upper Providence of the Ft. Gaines Core shows a marked decrease in dinoflagellate species abundance and percent and absolute abundances of marine phytoplankton, with a concurrent increase in relative abundance of Achomosphaera-Spiniferites cysts. These indicators to a relative regression and shoreward movement of facies through the upper Maastrichtian. Lower Maastrichtian assemblages in the more seaward Albany Core are dominated by Chatangiella? robusta, Exochosphaeridium bifidum, and Cerodinium panneceum, and have a very high percent marine phytoplankton signal. These are interpreted to represent transgressive conditions with no progradation of the shoreline. The upper Providence shows high species abundance, increasing dominance by Achomosphaera-Spiniferites cysts, individual abundance peaks of several different species, and a decrease in percent marine phytoplankton, indicating an overall relative sea-level regression through the upper Maastrichtian. Comparison with dinoflagellate assemblages from Maryland suggest that high abundances of Areoligera cysts at about the lower/upper Maastrichtian boundary may reflect the level of maximum transgression in the middle of the Maastrichtian. Three new species are described: Florentinia perforata, Yolkinigymnium expansum, and Palambages trilicius.

Palaeokarst‐influenced depositional and diagenetic patterns in Upper Permian carbonates and evaporites, Karstryggen area, central East Greenland

ABSTRACTThe Karstryggen area of eastern Greenland represents the western edge of sedimentation in the Jameson Land Basin, an arm of the northern Zechstein seaway. Upper Permian strata of this area were deposited as two major sequences. The first marine incursion transgressed largely peneplaned Lower Permian strata and deposited thin, paralic conglomerates, sandstones and shales (the Huledal Formation) followed by a thick package of carbonates and evaporites (the Karstryggen Formation). Although the Karstryggen Formation represents the transgressive maximum of this sequence, it contains only marginal or restricted marine strata, including micritic, stromatolitic and peloidal carbonates and thick, but localized, bedded gypsum deposits. These lithofacies indicate that relatively arid climates prevailed in this basin, as in most of the Zechstein region.A major regression, associated with a change to a more humid climate, terminated Karstryggen sedimentation. Pre‐existing evaporites and carbonates underwent diagenetic alteration, including widespread calcitization and dissolution of gypsum. More importantly, topographic relief in excess of 120 m was generated by fluvial drainage systems and karstic sinkholes.A second marine incursion, accompanied by a return to a semi‐arid climate, drowned this high relief topography, producing a complex sequence of strata (the Wegener Halvø Formation) in which sedimentation was greatly influenced by the rugged underlying terrain. Marine cemented algal‐molluscan grainstones draped pre‐existing palaeotopography during the initial stages of flooding. Continued drowning led to differential sedimentation on ‘highs’ and in ‘lows’. Oolitic and bryozoan‐brachiopod grainstones formed as shoals on the crests of most prominences, whereas shales, conglomeratic debris flows, evaporites, or oolitic turbidites were deposited in the lows. More restricted sedimentation took place in the westernmost areas which lay closest to the mainland shoreline and were situated to the west of a palaeotopographic ridge. There, oolitic, stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions.Three subcycles mark smaller scale relative changes of sea level that occurred during deposition of the Wegener Halvø Formation; they are delimited by regional surfaces with moderate relief (5–20 m) developed during subaerial exposure. Widespread diagenetic changes, including leaching of aragonitic grains, dissolution/collapse brecciation of evaporites and meteoric calcite cementation, occurred in association with these smaller scale sequence boundaries, again reflecting climatic oscillations.Relative sea level fluctuations, coupled with regional climate changes, played a dominant role in determining both depositional and diagenetic relations in these strata. These features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern Zechstein Basin and Barents Shelf, and may have economic significance for the localization of hydrocarbons.

Taphonomic expressions of sedimentary hiatuses: field observations on bioclastic concentrations and sequence anatomy in low, moderate and high subsidence settings

Field studies of post-Palaeozoic siliciclastic records reveal a strong concordance between different types of bioclastic concentrations and discontinuity surfaces within third-order sequences (≈ 1 My duration), supporting the use of taphonomic criteria in establishing the relative magnitudes of sedimentary hiatuses. Comparison of records across a spectrum of subsidence rates, however (from ≤ 10 m to > 1 km/My), shows that, along with appreciable changes in sequence anatomy, the nature of surface-mantling bioclastic concentrations also changes. The most significant surfaces (second- and third-order sequence boundaries, surfaces or intervals of maximum transgression, transgressive surfaces) tend to be either bare or mantled with taphonomically complex hiatal and lag concentrations. These were more consistently encountered in low subsidence than in moderate subsidence records. In high subsidence records, major surfaces were more often mantled by composite or event concentrations, if they were bioclastic at all. In all subsidence settings, comparatively minor surfaces (parasequence boundaries, bed set boundaries and bedding planes) were bare or mantled with relatively simple event and composite concentrations. Although all fossil assemblages are biased taphonomically to some degree, relative degrees of bias should almost certainly vary among discontinuities as a general rule, suggesting specific adjustments in sampling strategies for evolutionary studies.

Kystes de dinoflagellés et palynofaciès: indicateurs des variations bathymétriques dans le bassin de Guercif (Maroc) au Jurassique (bathonien terminal—Oxfordien basal)

The basin of Guercif (northeast Morocco) is a large depression at the African margin of the Tethys, filled with Jurassic sediments and discordant Tertiary sediments. The boring MSD1 carried out on in the western subsident part of the basin, has reached the Upper Bajocian at 3600 m. Cores from between 2850 and 1650 m have been studied. The sediments, composed of alternating silty clays, sometimes calcareous, and fine sandstones, are dated at Upper Bathonian ( Ctenidodinium sellwoodii-Ctenidodinium combazii assemblage) to Lower Oxfordian ( Wanaea thysanota-Systematophora areolata assemblage). Lithofacies and palynofacies indicate sedimentation-types ranging from coastal to inner neritic shallow shelf. The continental influx has always been important. The maximum transgression is suggested by the abundance of disaccate pollen, chitinous foraminifers, dinoflagellate cysts with a wide specific diversity, and by low percentages of vascular tissues. Non-marine phases are characterized by the abundance of land-plant tissues, spores of pteridophytes, Classopollis pollen and the absence of dinoflagellate cysts and foraminifers. Three transgressive phases took place in the basin of Guercif from the Upper Bathonian to the Lower Oxfordian. The major marine transgression occurred during the end of the Upper Callovian as suggested by indications of aridity.

Prospectivity of Northern Barinas Area Through Modern Seismic Data

The area of study is located to north of the Merida arch, in the Barinas-Apure basin. The geologic section comprises early Cretaceous (Albiense) to Quaternary deposits. The Cretaceous sequence lies unconformably over the metamorphic basement; the lower section of Navay Formation, of Cretaceous age, is consider to be the source rock, and it is associated with maximum transgression (91.5 mma). In Barinas, this represent the lateral equivalent of La Luna Formation of the Maracaibo Lake. The Gobernador and Paguey Formations (later middle Eocene), overlie unconformably, the Cretaceous rocks. The lower section of the later formation is associated with maximum transgressive cycles as a result of the collision of the South American and Caribbean plates. Former interpretation of analogic seismic data showed small structures that were less evident towards the deeper part of the basin. Recently, Corpoven acquired 2350 km of excellent digital seismic data with a coverage of 3000%. This has allowed us to subdivide the area in three hydrocarbons prospective zones:(1) Northern Barinas Zone: Characterized by faults and structures very similar to other prospective fields of the area. (2) South Andean Slope: This was affected by the Andean orogenesis. In this zone we find the same sedimentary column, butmore » the structures are bigger than the former producing. (3) Southern Guanate Zone: This zone shows light faulting and abundant acoustic anomalies probably associated to facies changes.« less

Chronostratigraphy of the Cretaceous-Oligocene Unconformity in Northern Monagas, Eastern Venezuela

Palynological analyses of several subsurface sections in the autochthonous block in Northern Monagas, eastern Venezuela, show a Cretaceous-Oligocene unconformity. The Maastrichtian to Paleocene sandstones of the San Juan Formation unconformably underlie the Oligocene to Miocene sandstones of the Merecure Formation. Log analyses indicate two stratigraphic sequences within San Juan and two more within Merecure, and integration with paleontological data indicates changes which can be correlated with Haq et al.'s (1988) Sequence Chronostratigraphic Charts as follows: (1) The basal San Juan sandstones represent the late Maastrichtian (68 Ma) drop of seal level and the UZA-4.5/TA-1.1 Sequence Boundary (SB). The Maastrichtian or Maastrichtian to Paleocene San Juan sandstones represent either the TA-1.1 and the TA-1.2 Third-Order Cycles. (2) The late Oligocene sequence overlying the unconformity, represents the TB-1.3 Cycle starting at 26.5 Ma. Within this lower part of the Merecure sediments, an increase in the abundance and diversity of marine palynomorphs represents a maximum transgression correlatable with the 26 Ma Condensed Section contained in the Tb-1.3 Cycle. (3) The upper part of Merecure represents the Tb-1.4 Cycle starting at 25.5 Ma. Within this cycle, the lowermost (Oligocene) sandstones contain almost exclusively terrestrial palynomorphs, while the (Miocene) alternating sands and shales of themore » upper part contain diverse marine palynomorphs indicating a change from Low Stand Wedge to Transgressive deposits. This change is correlated with the Oligocene/Miocene limit at 25.2 Ma.« less

Organic Facies and Systems Tracts: Implications for Source Rock Preservation and Prediction

Sequence stratigraphy is concerned with making predictions about reservoirs ahead of the drill, however, little attention has been paid to the configuration of organic-rich facies of source rock quality. We suggest that preservation of source rock type facies in clastic systems is mutually exclusive and time successive. The main database is a collection of cores and other samples through the Holocene Rhone delta. The early Holocene Transgressive Systems Tract (TST) contains five levels of channelization. The most significant peat bed is located immediately landward of the shoreline of maximum transgression (SMT). The Highstand Systems Tract (HST) consists of two parasequences, containing mostly laterally continuous strandplain complexes without peat. In addition to sufficient accommodation space, an important control on formation of fresh-water peats and organic-rich shelf muds is availability of river-induced nutrients. Peat quality, however, is best without riverine clastics. In a delta plain, a balance between these two controls may be reached when river-fed nutrients are trapped there indirectly. The potential for such a condition arises in a TST setting. On the shelf, eutrophication of marine habitats is also controlled by river-fed nutrients, but excess river clastics are detrimental to marine source rock quality. A balance between these two controls more » may be reached in HST settings where fine-grained riverine clastics are forced onto the shelf rather than in the delta plain. In this case, nutrient supply to the shelf results in large quantities of marine biomass. This biomass becomes sufficiently concentrated due to moderate fine-grained riverine sedimentation which guarantees burial and preservation. Thus, varying river-water and nutrient supply in TST and HST settings seems to control large-scale preservation patterns of both continental and marine organics. This hypothesis suggests further potential for using sequence stratigraphy for source rock occurrence. « less

The Far East Hydrocarbon Habitat: The Charge Perspective

From a hydrocarbon exploration point of view, the Far East is typically the realm of Tertiary basins with youthful prospects. Nearly all of the oil and gas in these basins is terrestrial in origin and the current cycle of tectonic development began in late Mesozoic-early Tertiary history during which much of what is now east and southeast Asia consisted of a large land area comprised of microcontinental blocks welded together by fold belts. In the Paleogene, this continental area was subjected to back-arc extension and collapse as a consequence of complex plate readjustments. Subsidence took place in fault-bounded (graben) depressions throughout the area and widespread lacustrine environments were established. The middle to late Tertiary history of these basins was dictated by their proximity to the open ocean and by the extent of crustal subsidence, but follows a transgressive-regressive cycle that gives rise to the following groups of plays: (1) early Tertiary transgressive clastics, basically oil prone, (2) Miocene carbonates of the maximum transgression, gas prone, and (3) late Tertiary regressive clastics, oil and gas prone. The mechanisms of generation and migration in Far East Tertiary basins were poorly understood and the type and volumes of hydrocarbons trapped often appear tomore » be unpredictable. At present, three basic depositional realms are envisaged for source rock accumulation: (1) lacustrine grabens ([open quotes]deep lakes[close quotes]) that contain rich algal-SOM type I-II source rocks and are oil prone, (2) paralic coal and coaly shale swamps, developed in a variety of onshore deltaic environments; charge from these source rocks is variable, and (3) coastal and marine, especially related to deltas where terrestrial organic material is introduced, accumulates, and is bacterially degraded and preserved. The distribution and types of hydrocarbons produced from these source rocks and their relations to hydrocarbon habitat are reviewed.« less

Facies Relationships and Systems Tracts in the Late Holocene Mississippi Delta Plain

ABSTRACT Facies relationships in abandoned Holocene Mississippi Delta complexes are characteristic of both retrogradational transgressive systems tracts (TST) and progradational highstand systems tracts (HST). In the Barataria interlobe basin, delta-plain facies of the early Holocene Maringouin/Teche delta complex (TST), which accumulated from 7500 to 6000 yr BP, are overlain by a lagoonal facies 1-2 m thick (MFS) that accumulated during the maximum flooding event from 6000 to 3500 yr BP. Wave reworking transformed the distributary sands of retrogradational delta complexes into stratigraphically backstepping shoreline sand bodies. The most landward of these shorelines, the Teche shoreline, overlies the MFS and is, by definition, the shoreline of maximum transgression (SMT) (Penland et al. 1987 , 1987b; Kosters 1989). Relatively thick peats of high organic content, dating from 2400 to 1100 yr BP, are located immediately landward of this shoreline. Younger delta lobes, rapidly prograding since 1100 yr BP, have shifted the coastline seaward of the Teche shoreline, and form the first progradational HST parasequence. Thin, organic-poor salt marsh sediments are accumulating within this parasequence landward of the present shoreline. Rising relative sea level provides increased accommodation space while fresh water may be held within the delta plain, creating conditions of both groundwater and nutrients favorable to accumulation of high-quality organic facies of this type. In a subsequent progradational setting, stable relative sea level results in less accommodation space landward of the shoreline, while fresh water and nutrients are discharged into the Gulf of Mexico, forcing formation of brackish and salt marsh environments, unfavorable to accumulation of high-quality organic facies. These hypotheses may help explain the variability of some littoral high-quality coals vs. carbonaceous shales in the rock record.

Holocene evolution of an estuary on a tectonically rising coast: the Pakarae River locality, eastern North Island, New Zealand

Estuarine and beach deposits in the vicinity of the present coastline at Pakarae River record the infilling of an estuary and subsequent development of a sequence of seven marine terraces during Holocene time. At the maximum of the last glaciation about 18,000 years ago the shoreline at the ancestral Pakarae River was approximately 20 km east of the present shoreline. By about 9000 years BP the sea had transgressed across most of that coastal plain to lie within a few hundred metres of the base of the present coastal hills. Seventeen radiocarbon ages from estuarine deposits record the overall rise in post-glacial sea level, but in the period c. 9500-7000 yrs BP there are reversals to the overall rising trend. Between 9500 and 8500 yrs BP there appears to have been a eustatic fall in sea level of at least 4 m. This observation is supported by data from several other localities around New Zealand. Maximum transgression occurred about 6500–7000 yrs BP when the sea reached the base of hillslopes and an extensive estuary existed behind a barrier bar. Since that time the barrier bar disappeared, probably due to stranding in an uplift event, and the coastline advanced progressively outward toward its present position. Coastal progradation (sea level regression) and subsequent erosion have occurred in association with episodic large earthquakes at about 6700, 5400, 3910, 2450, 1570, 1000 and 600 yrs BP. The present distribution of terraces has been influenced by coastal erosion, which has removed all trace of some terraces from some areas, and river erosion has modified the marine terraces near the river.

Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation

ABSTRACTDetailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non‐marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross‐beds, bidirectional cross‐beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits.In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non‐marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.

O SUBGRUPO ITARARÉ (PERMOCARBONÍFERO) NA REGIÃO DO MÉDIO TIETÊ, ESTADO DE SÃO PAULO

The sediments of the Permo-Carboniferous Itarare Subgroup of the Tiete river middle valley, State of Sao Paulo, Brazil were studied in outcrops and cores from wells drilled up to 310 m for groundwater, in order to get a general picture of their stratigraphic relationships. The Subgroup in this valley may be subdivided into the Itu and Capivari, testifying a transgressive cycle starting with the non-marine Itu Formation, and followed by the marine Capivari Formation. The marine transgression maximum resulted in the deposits of the Tatui Formation belonging to the Permian Guata Subgroup. A paleogeographic model is developed for the Capivari Formation in the Tiete river middle valley, on basis of outcrops and eight wells throughly cored down to 310 m. A continuous offshore marine environment occupied the areas located south and west of Capivari and Rafard (SP) towns. Submarine debris flows, coming from northeast, encroached upon this offshore environment in at least two channels, one of them reaching present day urban Capivari.

Paraglacial barrier—lagoon development in the late pleistocene Baltic Ice Lake, southwestern Baltic

Marine geological investigations including shallow seismic, sedimentological and stratigraphical studies were carried out in Fakse Bay, southwestern Baltic. Seismic sequence analytical methods were used, supplemented by detailed lithological descriptions, pollen analysis and 14C dating of core material. Detailed correlations between sediment facies units and seismic sequences resulted in the establishment of four depositional sequences: I. Glacial related deposits, II. lagoon/pond and freshwater coastal deposits, III. Lake-bog-lake deposits, and IV. Marine, (sub)littoral deposits. Most attention has been paid to Sequences II and III, showing the development of a transgressive freshwater barrier beach ridge and a following regression. Pollen spectra indicate Allerød and Younger Dryas Chronozone ages (11,500-10,000 14C yrs B.P.) for the development of a sheltered lagoon/pond basin behind the transgressive freshwater barrier beach ridge. The freshwater coastal deposits are interpreted as the southwesternmost marginal deposits of the Baltic Ice Lake and represent the transgression maximum. This transgression maximum reached a level of about 13 m below the present sea level. The final drainage of the Baltic Ice Lake resulted in the development of a sheltered basin behind the former beach ridge. A fluctuating groundwater level here favoured the deposition of alternating lake gyttja and bog peat deposits in the central part of the basin. Pollen analysis and 14C dating reveal that the lake deposits correspond to high groundwater levels associated with the Ancylus Lake (9000 yrs B.P.) and the Littorina Sea (7000 yrs B.P.) transgressions, while the bog sediments represent the low water stands in the early Holocene.

Lower Palaeogene stratigraphy of the northern North Sea

The Lower Palaeogene stratigraphy of the northern North Sea is described using wireline log and biostratigraphic data. At group and formation level a close similarity can be shown with the stratigraphy defined in the Outer Moray Firth area of the central North Sea. A two-fold subdivision into the Montrose and Moray Groups is recognized. The Montrose Group consists of the Maureen and Lista Formations while the succeeding Moray Group contains the Sele, Balder, Dornoch and Beauly Formations. The Maureen and Lista Formations are dominantly argillaceous units that contain poorly defined sandstones; the most laterally extensive of these is the Heimdal Member developed within the Lista Formation. The Dornoch and Beauly Formations are shelfal sandstone units that pass eastwards and northwards into the age equivalent, dominantly argillaceous Sele and Balder Formations. The Sele Formation contains the Hermod Member while the Balder Formation contains the Odin Member. These are massively developed sandstone units of very limited extent. The Lower Palaeogene succession contains a number of high gamma log markers that are often associated with influxes of microfossils and microflora. These are interpreted as condensed claystone horizons representing pauses in marine sedimentation during episodes of maximum transgression. They can be correlated with similar marker horizons described from the Outer Moray Firth area, giving further confidence that the stratigraphy forms a regional framework that can be applied over the whole North Sea area.

Le biostrome à Crassostrea gasar (Bivalvia) de l'Holocène du Sine-Saloum (Sénégal); Données nouvelles et interprétation écostratigraphique

A biostrome of Crassostrea gasar Dautz was discovered by the authors in 1989 near the village of Bassoul (Senegal) into the present mangroves environment of the Sine-Saloum marine estuary. Following a new campaign it was cartographied (total superficies 10 ha), thickness measured (0,65 m) and age clarified: it begins at −5440 years B.P. and stops suddenly at −4850. A hole allows to note that the oysters are developped in situ in the sandy clayed sediment; they are bivalved, large and thick. First, they are settling down flat, then revolve on one's side, and next stand up to become vertical in the end. Three samples of left valves, during this bioconstruction development phases, allowed the measurement of four morphologic parameters (length, width, dimension of the ligament area, maximal thickness); their bivariate analyses have been studied and interpreted according to the biostrome evolution. The micropaleontologic study (foraminifera, ostracoda, palynology) indicates a marine environment, polyhalin and not hyperhalin as in the present, in a nearly mangroves biotope with herbaceous areas and only one fern (Schizeaceae), and with fluviatil water inflows. These data are in accordance with the humid climate of this middle Holocene period, at the maximum of transgression. Under the biostrome, black peaty clays (0,40 m) correspond to organic muds of classic mangrove (basis of the hole −6590 years B.P.). A temporary balance during about 600 years was realised by the synchronism of this two phenomenon, climatic (pluvial period), and eustatic (transgression), allowing the life of the oysters populations in the form of a biostrome, and not only fixed on the mangroves roots as at present. But its unique localization, comparable to a stable shoal, remains to be explicited. In the time, this episod follows the one of Anadara senilis L. terraces and takes place in a regional context of the Nouakchottian stage (−6800 to −4200 B.P.) whose characters are like this clarified and complemented. Theath of the biostrome had been sudden, owning to a contemporaneous and speedy fall of the marine level and of the raininess: this abrupt event forecasts the approach of the Tafolian, aride and regressive period. A chemical corrosion occurs afterwards because the recent putting up of a new mangrove, with acide soll, dissolving the horizontal bed top on several centimeters.

Late Pleistocene-Holocene Sequence Stratigraphy of the Mississippi-Alabama Shelf, Northeastern Gulf of Mexico: Implications for Global Change

ABSTRACT Sequence stratigraphy is the study of rock relationships within a framework of repetitive, unconformity-bounded depositional sequences and provides a methodology by which to study and understand stratigraphic, lithofacies, and paleogeographic relationships among the strata within a depositional basin. The primary premise of sequence stratigraphy is that stratal stacking patterns and lithofacies distributions are related to cycles of transgression and regression that result from cyclical changes in relative sea level; relative sea level changes, in turn, result from the combined effects of eustatic sea level changes, basin subsidence, sediment yield, and climatic conditions. While the concepts of sequence analysis have been applied in numerous studies of the ancient rock record, research into the applicability of this approach to the modern record is generally lacking. An understanding of the sequence stratigraphic setting and history of modern depositional systems, when combined with data from ancient analogs to which sequence analysis has been applied, may have predictive value in determining possible scenarios for change in modern coastal and offshore areas and the rates of such change. A major late Pleistocene sea level fall that exposed the continental shelf in the northern Gulf of Mexico area is well documented. During this lowstand, the exposed continental shelf was subjected to erosion and stream valley entrenchment, which resulted in an incised, type 1 unconformity surface. This surface forms the base of the late Pleistocene-Holocene sequence in the study area. Sediment bypassed the exposed shelf through these valley systems and accumulated as slope and basin lowstand systems tract deposits. In the northern central Gulf, studies have shown that the Mississippi Canyon and Fan formed during this sea level phase by these processes. On the Mississippi-Alabama shelf, incised valleys formed on the type 1 surface are much smaller than the Mississippi Canyon to the west, but are numerous and readily delineated on seismic reflection records. Preliminary data show that these features are filled with generally coarse-grained siliciclastics that were deposited as incised valley fill associated with renewed transgression. Progressive marine transgression during the Holocene eventually flooded the entire continental shelf, depositing backstepping sets of transgressive systems tract parasequences. Rapid deepening led to sediment starvation on the shelf as active depositional loci were translated landward. Submarine hardbottoms consisting of surficial rock and shell rubble are common in the offshore in the northeastern Gulf of Mexico. The thickness of the Holocene section in this area and the presence of these hardbottoms indicate that the Mississippi-Alabama shelf has been sediment starved throughout most of its Holocene history. Preliminary assessment of these hardbottom features indicates that they represent, in part, condensed section associated with the transgressive phase of the late Pleistocene-Holocene sea level cycle. This condensed section is probably correlative to that reported by Boyd et al. (1989) in the north-central Gulf area. Subsequent to maximum transgression, relative sea level has been falling in the north-central Gulf, and the Mississippi delta system has been prograding into the basin as a highstand systems tract. Sediment yield from rivers in the northeastern Gulf is not as great as that of the Mississippi River. Sequence stratigraphic analysis of late Pleistocene-Holocene strata of the Mississippi-Alabama shelf can be used as a tool for establishing the anticipated naturally-controlled lithostratigraphic succession, sea level position and rate of change, and distribution of modern geomorphic features. The recognition of deviations from the anticipated sequence stratigraphic framework may allow for the determination of physical environmental changes which may be attributed to anthropogenic events. End_of_Record - Last_Page 849-------

Stable isotope studies and the hydrological regime of sabkhas in southern Kuwait, Arabian Gulf

The marine sabkhas in Al-Khiran, Kuwait were formed by sedimentary offlap and relative fall in sea level since the last Holocene transgressive maximum ∼ 4500–4700 years BP. In the absence of significant marine flooding of the coastal sabkhas today (due to the presence of chenier ridges), their hydrological regime is almost entirely controlled by sub-surface continental groundwater influx. This topographic-hydrologic framework is also reflected in the isotopic data, and to some extent in the chemistry of the groundwaters, where the major ions show a steady evaporative concentration from inland to coast and from coast to sabkha, with a “high” chloride plateau in the central parts of the marine sabkha. Monitoring of the groundwater table (GWT) over a three-year period indicates that: (a) when the GWT depths are < 1.2 m, the sabkha is in a state of depositional equilibrium; (b) GWT fluctuations (due to drawdown or recharge) exceed 0.5 m yr −1; and (c) calculations based on measured fall in GWT levels, porosities of the sediments and the volume of anhydrite formed in the sabkha suggest that in the last 4700 years since progradation commenced, only about 2–4 cm yr −1 (net) of groundwater is lost from the sabkha surfaces—a surprisingly low rate for this arid region. This is interpreted as due to recharge of the marine sabkha (hosted by marine sediments) by significant lateral flow of continental groundwater from the regional system—a process that maintains long-term stability of the sabkhas and without which deflationary processes would dominate. The oxygen and deuterium isotopic compositions of the marine and continental sabkha waters form a group ( δ 18 O=0 ± 2‰ ; δD = − 10 ± 7‰) isotopically distinct from local meteoric water and Pleistocene formation waters. They represent evaporation of the old formation waters, with a characteristic increase in the heavier isotopes. Only sabkha waters in areas of tidal flooding (the narrow creek areas) can be shown to be evaporated seawater. The δ 18 O and δD values of the residual brine increase to about +6‰ and +23‰, respectively, when they reverse and are arrested at the halite precipitation point. Only a minor amount of mixing between these two evaporated waters could take place. Isotopic analyses of sulphur and oxygen in dissolved sulphate of sabkha waters and precipitated mineral sulphate confirm a minor contribution of seawater sulphate ( δ 34 S = +19 ± 2‰ ; δ 18 O = + 11‰ ) in the tidal areas, but show that the bulk of the marine sabkha water sulphate ( δ 34 S = + 14 ± 2‰ ; δ 18O + 14 ± 1%.) and coexisting sulphate minerals is dominantly of continental origin. At Al-Khiran, Gulf of Sirte (Libya) and in Abu Dhabi (United Arab Emirates), δ 34S values of sabkha sulphate decrease to +14‰ on the landward side: close to the values between + 13 and + 16‰ for continental sabkhas and continental getch (gypsum). This pervasive homogeneous sulphate source may be recycled particulate sulphate distributed by major dust storms. These results have important implications in evaporite sedimentology.

Coseismic uplift of holocene marine terraces in the pakarae river area, Eastern North Island, New Zealand

Holocene marine terraces along 15 km of the northeastern coast of North Island record episodic tectonic uplift. A maximum of seven terraces are arranged in staircase fashion and lie about 20 km above the subduction interface between the Pacific and Australian plates. The highest (T1) corresponds with the maximum of the Holocene marine transgression about 6700 14C yr B.P. Younger terraces are marine abrasion platforms overlain by thin beach deposits. Radiocarbon ages of marine shells from beach deposits indicate that uplift above marine conditions occurred ca. 6700(T1), 5500(T2), 3900(T3), 2500(4), 1600(T5), 1000(T6), and slightly younger than 600(T7) yr B.P. Uplift probably occurred coseismically. The maximum late Holocene uplift rate in the study area is 8 mm/yr. Altitudinal distribution of terraces suggests deformation exists as a ca. 20-km elongate dome, broken at the southern end by the Pakarae fault, which trends across the dome. Rupture on this fault has accompanied the growth of the dome but is not responsible for it. Bathymetric profiling suggests that an active fault, parallel to and about 5 km offshore, is probably responsible for the episodic coastal uplift.

Depositional Environments and Facies Analysis of the Cherokee Group in West-Central Kansas

The Cherokee Group of early Desmoinesian Pennsylvanian age in west-central Kansas is comprised of a mixed siliciclastic and carbonate sequence. It was deposited in environments that are transitional from continental to marginal marine as the Hugoton Sea transgressed the Mississippian unconformity on to the Central Kansas uplift. Sandstones of the Cherokee Group are important oil reservoirs in west-central Kansas, but they are highly variable and difficult to predict. Core studies and subsurface analysis reveal two persistent and widespread limestone beds that form useful stratigraphic markers within the Cherokee. They provide a framework for facies analysis and regional mapping that may be useful as a predictive tool for oil exploration. Six basic lithofacies are interpreted from lithologies and sedimentary structures observed in cores obtained from four wells in eastern Ness County: (1) basal Pennsylvanian conglomerate, (2) fluvial sands, (3) fine-grained tidal flat deposits, (4) shallow-marine limestones, (5) shoreline sands and tidal channel sands, and (6) braided stream, sandy conglomerates. These facies are correlative with components of an ideal Kansas cyclothem. Two transgressive-regressive cycles are identified and maximum transgression is correlated with two widespread limestone beds. Following burial of the Mississippian karstic surface, deposition of peritidal sediments occurred on a uniform shallowmore » shelf, punctuated by periods of subaerial exposure and weathering. Clastics derived from the eroding Central Kansas uplift were probably supplied to the coastal plain by braided streams and reworked by coastal processes.« less

宮崎平野の完新世地形発達史

The Miyazaki Plain in southern Kyushu, Japan, is characterized with well-developed Holocene terraces. On the basis of the distribution and altitude of the terraces, they are subdivided into four groups: Shimotajima I, Shimotajima II, Shimotajima III, and Shimotajima IV terraces.This paper discusses the evolution of terraces, sea level change, associated environmental change and tectonic movements during the Holocene in the study area, based upon the field observation of outcrops and cored samples, 14C datings, archaeological remains, and paleontological analysis of mollusks. The results are as follows:1) Before 5, 000y.B.P.The sea level had risen rapidly since at least 9, 000y.B.P., and reached +4 to +5m (a.s.l.). Thick sandy or silty sediments beneath the Akahoya ash(K-Ah) which erupted from the Kikai caldera about 6, 300y.B.P. were deposited during the transgression. The shallow marine mollusks such as Tegillarca in the deposits indicate that tidal flats and bays were widely formed in the Miyazaki Plain. The Akahoya ash is covered with sandy marine sediments 2-3m in thickness, which might indicate that the sea level rose slightly after the ash fall. At the maximum transgression, the sea level rose to 7.0-7.5m above the present sea level around the Ishizaki River, judging from the height of the buried benches and the upper limits of the sandy marine deposits. The sandy deposits on the Akahoya ash include many mollusks such as Tellinemactra edentura, one of the typical tropical species which lived on the sandy bottom beneath the open sea. Their presence suggests that the bays were buried by sandy material which was carried by the river and the long-shore current and changed into the open sea with a sandy beach.2) 5, 000-3, 000y.B.P.The regression started about 5, 000y.B.P. While the shoreline retreated, many sandy ridges emerged on the Shimotajima I surface. Lagoons behind the sandy ridges gradually changed into swamps. After the sea level lowered to 5-4m (a.s.l.), it became stable or rose slightly, and the Shimotajima II surface was formed.3) After 3, 000y.B.P.Many sandy ridges on the Shimotajima II surfaces emerged as the regression began about 3, 000y.B.P. After the regression, the sea level became relatively stable, and the Shimotajima III surfaces were formed. The sea level was declining again around 1, 600y.B.P., and became adjacent to the present sea level, forming the Shimotajima IV surfaces.The altitudes of the Shimotajima I and II terraces show that the Miyazaki Plain has been uplifted, tilting northward during the Holocene. The uplift might be due to aseismic crustal movement, because there have been no historical earthquakes which caused emergence of the Miyazaki plain, and we can find no submarine active faults which could uplift the Miyazaki plain. The aseismic uplift would be related to the buoyancy of the Kyushu-Palau Ridge which is subducting under southern Kyushu.