Variations In Sedimentation Rate Research Articles

Volcanic ash layers facilitate high-resolution sequence stratigraphy at convergent plate margins: an example from the Plio-Pleistocene forearc basin fill in the Boso Peninsula, Japan

In this paper a method is presented for the definition of high-resolution siliciclastic depositional sequences in onshore outcrop belts in a forearc basin setting, and the development of high-frequency depositional sequences in the infill of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan, in terms of the interaction between forearc tectonics and glacio-eustasy, is discussed. Volcanic ash layers are commonly intercalated in siliciclastic successions at convergent plate margins and provide physically defined chronostratigraphic markers. The mapping of lateral termination of volcanic ash layers, together with the interpretation of depositional facies discontinuities, permits the identification of onlap, offlap and downlap stratal termination patterns in onshore outcrop belts. In particular, this type of analysis is useful to identify bounding surfaces in offshore muddy sediments. The relative abundance of intercalated volcanic ash layers and their associated pyroclastic/volcaniclastic fragments in marine siliciclastic successions is interpreted to correspond to temporal variation in the relative rate of supply of coarse-grained terrigenous sediments to a marine environment during a relative sea-level cycle. In particular, transgressive stages are characterized by the reduction of active supply of coarse-grained terrigenous sediments to a marine elastic environment and this results in the increase of relative abundance of intercalated volcanic ash layers and their associated volcanogenic lithofacies. This kind of sediment is hierarchically identified in response to the development of marine flooding surfaces, transgressive systems tracts and transgressive sequence sets. In general, boundaries of depositional sequences and their component building blocks are interpreted to provide a chronostratigraphic framework for local and regional correlations of siliciclastic successions. The mapping of volcanic ash layers can identify diachronous evolution of bounding surfaces and component building blocks of a high-frequency depositional sequence, in relation to spatial variations in sedimentation rates and basin subsidence which are superimposed by eustatic sea-level changes.

Condensation processes in shallow platforms

Sequence stratigraphy allows elucidation of the time-space relationships of deposits in sedimentary basins. However, the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings, nor as indicators of maximum regional transgressions. Condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes. Three separate processes of condensation are here distinguished: stratigraphic, sedimentary and taphonomic. These three categories of condensation can be used to analyze patterns of sediment distribution, and also to estimate lateral and vertical variations in sedimentation rates. Contrary to the usual interpretation, the maximum values of rate of sedimentation and degree of accommodation of sediments, as well as the highest relative sea level, occur in the lowest levels of the shallowing-upward sequences. The distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms.

Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo-Tokyo Bay, Japan

Palaeo-Tokyo Bay is a relic of the Plio-Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo-Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo-Tokyo Bay. Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non-deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south-eastern margin of palaeo-Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio-eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo-Tokyo Bay. The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.

Soil Erosion, Slope Management, and Ancient Terracing in the Maya Lowlands

A puzzling aspect of Prehispanic soil erosion and sedimentation in the Maya Lowlands is the variation noted between different regions. In the Petexbatún region of Guatemala, recent investigations indicate great variation in sedimentation rates between watersheds of different sizes. In some places, soil erosion was slowed by stone terraces, possibly in conjunction with other conservation methods. A review of the surficial geology and other data on terraces from across the Maya Lowlands indicates that variations in the form and distribution of ancient agricultural terracing was probably the result of environmental variability and population pressure largely during the Late Classic period.

Phosphorus accumulation rates in a Miocene low oxygen basin: The Monterey Formation (Pismo Basin), California

Phosphorus (P) limits oceanic productivity on long time scales, and therefore determining changes in oceanic P accumulation through time is important for modeling oceanic paleoproductivity. We determined P concentrations and accumulation rates at Shell Beach, California, a section of phosphatic-rich marine sedimentary rocks which includes the widespread Phosphatic and Siliceous Facies of the Monterey Formation. Phosphorus concentrations had wide ranges in the major lithologies found in these facies, with values in phosphatic shales ranging from 0.36 to 3.9 wt%, compared to 0.02 to 0.42 wt% in dolomitic and siliceous strata. Phosphorus accumulation rates (in μmol P cm −2 yr −1 for all values reported here) showed significant variability with both sample lithology and position in the section. Phosphorus accumulation rates were much higher in phosphatic shale strata (1.5–29) than in dolomitic and siliceous strata (0.1–3.1) in both facies. Furthermore, P accumulation rates were generally higher in phosphatic shale strata of the upper Siliceous Facies (3.1–29) compared to the lower Phosphatic Facies (1.5–14). Mean P accumulation rates, calculated using linear sedimentation rate models for each facies and observations of the relative amount of phosphatic shale, dolomite, and siliceous strata, are comparable between the portions of Phosphatic and Siliceous Facies studied. This indicates that the two facies, with their faunal and sedimentological characteristics considered to reflect major oceanic and climatic changes during the middle Miocene, are similar in terms of P burial. Total P accumulation rates are also comparable to other phosphogenic Miocene and modern environments. We modeled variable sedimentation rates to examine the sensitivity of mean P accumulation rates to the linear sedimentation rate assumption. Increasing shale sedimentation rates to values above those estimated by linear models yields higher mean P accumulation rates in both facies, though they remain comparable in terms of net P burial. Modeling also revealed that, for increased shale sedimentation rates in each facies, siliceous sedimentation must be about three times faster than dolomitic strata sedimentation.

Low-background gamma counting: applications for210Pb dating of sediments

Sediment cores from three lakes were dated with210Pb using a constant rate of supply (CRS) model. We used low-background gamma counting to measure naturally occurring levels of210Pb,226Ra, and137Cs in sediment samples because sample preparation is simple and non-destructive,226Ra activity provides a direct measure of supported210Pb activity for each sample analyzed, and137Cs activity may provide an independent age marker for the 1962–1963 peak in atmospheric fallout of this radionuclide. In one core supported210Pb activity was estimated equally well from226Ra activity of each sampling interval or from the mean total210Pb activity of constant activity samples at depth. Supported210Pb activity was constant with depth in this core. In a short freeze core, determining226Ra activity of every sample proved advantageous in estimating supported210Pb activity because supported210Pb activity could be estimated from210Pb measurements only at the deepest sampling interval. Supported210Pb activity estimated from226Ra activity also yielded more precise estimates of highly variable sedimentation rates. In the third core226Ra activity exceeded210Pb activity at the top of the core and varied 20 fold with depth. This high input of226Ra in disequilibrium with210Pb is attributed to recent erosion of radium-bearing materials in the drainage basin. These data invalidate the assumption that supported210Pb activity is constant in sediment cores and can be estimated from the mean total210Pb activity at depths where210Pb activity is constant. We recommend using gamma counting or another independent assay of226Ra to validate the assumption of constant supported210Pb activity in sediment cores if there is reason to expect that226Ra activity varies with depth.

Organic matter of the Mulhouse Basin, France: a synthesis

The lower Oligocene evaporite sequence of the Mulhouse Basin (France) contains organic matter-rich marl deposits. These marls display an overall cyclic variation of sedimentation rate, organic carbon content, hydrogen index and selected molecular parameters over a 30 m thick stratigraphic interval. The integration of all sedimentological and geochemical parameters has allowed the reconstruction and characterization of the paleoenvironment of deposition. The marls were deposited in a perennial lake that was at times connected to the sea. Two organic facies end members could be assigned to a lake stage with a marine connection and a lake stage that received dominantly continental water input. The overall stratigraphic variation in the organic matter content is interpreted to reflect the adaptation of the Oligocene flora to the changing paleoclimate and environmental conditions.

Sequence stratigraphy of an intrashelf basin carbonate ramp to rimmed platform transition: Maryville Limestone (Middle Cambrian), southern Appalachians

Middle and Upper Cambrian strata in the southern Appalachians consist of a sequence of six alternating limestone and shale formations (Conasauga Group). The Conasauga Group constitutes part of a thick pericratonic Cambro-Ordovician passive-margin sequence along the eastern North American continent. The Cambrian carbonate platform was flanked by a high-relief shelf margin toward the east, facing the open ocean, while to the west the carbonate platform sloped into an intrashelf basin. It is this western shelf margin that is the topic of the present study. Detailed lithofacies analysis of the Middle Cambrian Maryville Limestone along a shelf-to-basin depositional transect reveals that the shelf evolved from a gently basinward-sloping ramp to a rimmed platform fringed with steeper slopes. Cyanobacterial buildups ( Renalcis-Girvanella ) dominated the platform margin environments. Progradation of the platform occurred toward the craton. A process-oriented approach has allowed us to define the sequences, sequence boundaries, and the stacking pattern of the Maryville Limestone. The Maryville Limestone sequence consists of two depositional subsequences. The boundary between the two subsequences is not a sequence boundary, because it does not separate rocks deposited in different environmental regimes. The two subsequences within the Maryville sequence consist of a combination of aggradational, retrogradational, and progradational units (with respect to the platform interior). The stacking pattern recognized is the result of variations in sedimentation rate, subsidence, and eustatic sea-level change. Each of the dominantly carbonate units within the Conasauga represents this gradual transition from a ramp-like platform to basin transition into a rimmed platform. The transition between the Maryville Limestone (Middle Cambrian) and the overlying Nolichucky Shale (Late Cambrian) is a sequence boundary. This sequence boundary is both an exposure surface and a drowning unconformity, and it marks a distinct shift in the pattern of sedimentation. Shallow-water carbonate deposition terminated at the boundary followed by the onlap of deeper-water basinal siliciclastics over shallow carbonates. The approach presented here can serve as a useful analogue to other lower Paleozoic and possibly younger passive-margin sequences.

A critical re-evaluation of the Miocene/Pliocene boundary as defined in the Mediterranean

A critical re-evaluation of the Miocene/Pliocene (M/P) boundary as defined—informally—in the Mediterranean is presented. We show that ages reported earlier for this boundary (ranging from 4.81 to 4.93 Ma) do not imply any diachroneity of the basal Zanclean lithohorizon which reflects the Pliocene flooding of the Mediterranean and defines the M/P boundary. These ages most likely represent an artefact resulting from intraformational variations in sedimentation rate and inconsistent interpretation of coluor variations in terms of sedimentary cycles. The best estimate for the age of this boundary arrives at 4.86 Ma if the conventional geomagnetic polarity time scale (GPTS) of Berggren et al. [1] is used, and at 5.16 Ma if the new GPTS of Cande and Kent [2] is used. An age of 5.32 Ma is obtained if the astronomically calibrated time scale of Hilgen [3] is adhered to. Because of its presumably global paleoclimatological significance, the M/P boundary as currently defined in the Mediterranean is preferred to a redefinition of this boundary at the Gilbert/Chron 5 boundary in a continous open-marine sequence outside the Mediterranean. In addition, this redefinition would lead to unnecessary confusion in global chronostratigraphy.

Palaeomagnetic study and magnetostratigraphy of the Triassic Skagerrak Formation, Crawford Field, UK North Sea

Abstract A detailed palaeomagnetic study has been made of the Skagerrak Formation (Triassic) in two wells from the Crawford Field in the northern North Sea. In 9/28a-9 (unorientated) a well-defined magnetostratigraphy is present comprising up to four reversed and three normal zones. There are secondary magnetizations (Tertiary 8 Recent in age) particularly in areas which are oil stained. In 9/28a-13 (orientated), three normal and four reverse magnetozones can be isolated. Despite the existence of orientation data it is more difficult to establish a reliable Triassic magnetostratigraphy. This is partly due to the fact that the sandier sequence in this location is impregnated with oil and associated diagenetic effects have resulted in pervasive magnetic overprints which are difficult to separate from the original Triassic magnetization. The effects of oil emplacement included widespread dissolution and the precipitation of a suite of secondary iron minerals including pyrite and possible pyrrhotite or magnetite. These later phases carry a secondary component of magnetization which overprints the original Triassic characteristic remanence. Directions of secondary magnetization are consistent with oil emplacement in Tertiary times. In attempting to correlate the two wells a number of factors have to be taken into account, in particular variations in sedimentation rate and the lack of precision in the published Triassic polarity time scale. The results of this study strongly suggest that the cored sequences in the two wells are not time equivalent. However, the sequence in 9/28a-9 compares most closely with the Upper Triassic polarity time scale.

Temporal and spatial patterns of wetland sedimentation, West Tennessee

Dendrogeomorphic techniques were used to describe and interpret patterns of sedimentation rates at two forested wetland sites in West Tennessee. Fifty-five sampling stations were established along transects upstream and downstream from bridge structures, and 515 trees were examined for depth of sediment accretion and cored for age determination. Temporal variation in sedimentation rate may be related more to stream channelization and agricultural activity than to bridge and causeway construction. Sedimentation rates have increased substantially in the last 28 years, although channelized streams may have overall lower rates than unchannelized streams. Comparisons of sedimentation rates from deposition over artificial markers (short term) with those determined from tree-ring analysis (long-term) indicate that trends are similar where hydrogeomorphic conditions have not been altered substantially. No tendency for increased sedimentation upstream from bridges was observed. Deposition rates were inversely correlated with elevation and degree of ponding. Downstream deposition of sand splays appears to be related to flow constrictions and may be extensive. Mean overall rates of sedimentation (between 0.24 and 0.28 cm year −1), determined dendrogeomorphically, are comparable with other published rates.

A New Approach to Shale Compaction and Stratigraphic Restoration, Beaufort-Mackenzie Basin and Mackenzie Corridor, Northern Canada (1)

Despite a long history of study, porosity reduction in shales and mudstones is still poorly understood. Many published shale compaction curves from different sedimentary basins exist, but it is difficult to make comparisons because of variations in sediment composition, geologic age, tectonic history, and experimental technique. The Beaufort-Mackenzie basin provides a unique opportunity to study shale compaction because it contains shales with rather uniform physical and chemical properties, deposited under highly variable sedimentation rates. The sonic log provides a sensitive measure of porosity change with depth in the Beaufort-Mackenzie basin. A new porosity-sonic transit-time equation, calibrated using shale and mudstone core porosity measurements from this basin, yields an acoustic formation-factor exponent of 2.19 and a matrix transit time of 220 microseconds/m, which are believed to be valid for low TOC (<2%), noncalcareous shales. Log-determined shale compaction curves define distinctive compaction zones that correlate with sedimentation rate, pore pressure, and erosion. Normal shale compaction is linear below 500 m at 1%/100 m decrease in porosity for hydrostatic pressure conditions. In eroded areas, the amount of missing strata can be estimated by reference to the normal compaction curve. In compositionally heterogeneous shale sequences, a knowledge of shale composition and its effect on log response is essential to avoid introducing significant error into calculated compaction curves. Preliminary results from the Brackett basin imply that maximum burial is more important than maximum paleotemperature in reducing shale porosity. This suggests that shale porosity could be a useful temperature-insensitive paleodepth indicator for eroded sedimentary sequences.

Biological and evolutionary responses to transgressive-regressive cycles

Transgressive-regressive cycles involve environmental change and therefore, under the predictions of the theory of natural selection, biological response is expected. The nature, magnitude, and instrumentation of that response is, however, less well understood and difficult to predict. Generally the magnitude of biological response would be expected to be a direct function of the magnitude of the environmental change produced by alterations in sea level. However, this may not be the case. Moreover, the magnitude of environmental perturbation seen may itself not be a direct function of the magnitude, range, or even rate of sea level rise or fall.Biological responses to transgressive-regressive cycles are highly variable, yet may be empirically demonstrated. Although the reality of faunal changes can be observed, the precise forcing mechanism or mechanisms driving those changes may be hypothetical at best. The observed iterative morphological series seen in shallow water Jurassic ammonites, for example, appear to be produced by a complex interplay of species adaptation to changes in local habitat and response to immigration from oceanic realms, both of which are ultimately driven by relative sea level. Likewise, changes in diversity and species composition in Devonian shallow marine communities appear to be produced by the effect of variable sedimentation rates and shifts in the oxygen minimum zone, both of which also are related to changes in relative sea level.Published onlap-offlap sequence curves, as such, may offer very little to the paleobiologist interested in the evolutionary behavior of marine organisms. Additional geographic and areal data are required if any rigorous quantitative relationship between relative sea level and evolutionary biology is to be formulated. In the Devonian, for example, it can be demonstrated that a total lack of correlation exists between the evolutionary biology of major benthic marine organisms (brachiopods) and relative sea level as projected from onlap-offlap curves. It could be hypothesized that the rate of change of sea level is more important to organisms than relative sea level itself, though most hypotheses concerning the biological effect of sea level are explicitly framed in terms of relative sea level (usually invoking the species-area effect). Again, onlap-offlap curves alone offer little in testing such a rate hypothesis, as it can also be demonstrated that no correlation exists between the derivative of the Devonian relative sea level curve and the evolutionary biology of the Brachiopoda.

Magnetostratigraphy of the Jurassic-Cretaceous boundary interval—Tethyan and English faunal realms

Geomagnetic reversals and magnetic polarity chrons provide an important chronostratigraphic tool for global correlation. An integrated Tithonian-Berriasian biostratigraphic and magnetic polarity time scale for the Tethyan faunal realm for the Tithonian and Berriasian stages is compiled from 17 independent biomagnetostratigraphic sections. This time scale incorporates zones and first/last appearance datums from ammonites, calpionellids, calcareous nannofossils, and dinoflagellates. This database provides an estimate of the range of observed appearance datums or zonal boundaries relative to polarity chrons; such apparent “diachroneity” probably results from a combination of preservation of species and of paleontological methodology, rather than from migration. The lithologic transition from “Rosso Ammonitico” red marly limestone to “Maiolica” white limestone occurs at different times during the Tithonian among the various sections, ranging from polarity zone M22n (mid-Early Tithonian) in some Spanish “slope” and Italian basinal-facies sections to polarity zone M19n (mid-Late Tithonian) in the central Atlantic and some plateau-facies Italian sections. This widespread lithologic change is, therefore, probably a result of shifting local patterns of fertility overprinted on the main regional trend. Magnetostratigraphy from the Purbeck Limestone Formation in the English Boreal faunal realm was obtained from the “classic” section at Durlston Bay in Dorset. The Dorset section displays predominantly normal polarity with a minimum of three reversed-polarity zones, but distortion of the magnetic polarity pattern by variable rates of sedimentation in this marginal clastic environment coupled with lack of independent correlation methods currently precludes a unique correlation to the Tithonian-Berriasian magnetic polarity time scale. The Tithonian-Berriasian magnetic polarity time scale may eventually provide a global chronostratigraphic definition of the Jurassic-Cretaceous boundary.

The relationship between carbon, sulfur, and pyritic iron in the Amjhore Deposit, Bihar, India

Low C/S ratios have been generally observed in pyritiferous black shales of the lower Paleozoic and late Proterozoic (Berner and Raiswell, 1983). In addition to factors such as high rates of bacterial sulfate reduction which might have existed during these early periods due to the low oxygen content of ocean water, availability and quality of organic matter, or lack ofbioturbation and availability of reactive iron, low C/S ratios may also reflect low sedimentation rates. Three distinct situations within a single vertical stratigraphic column in the mid-Proterozoic Amjhore pyrite deposit reflect the effect of anoxic levels and sedimentation rates on pyrite formation. In the first (lower shale with a mean C/S ratio of 2.03), pyrite formation was diagenetic, whereas in the second (massive pyrite ore with a negligible carbon content) and the third (upper shale with a low C/S ratio of 0.27), pyrite formation took place under euxinic conditions. In environments where anoxic levels are high and the sedimentation rate low, low mean C/S ratios may occur. In such environments variations in total sulfate reduction per unit column of sediment deposited (resulting from variation in sedimentation rates or anoxic levels) are reflected in the slope of the carbon-sulfur plot by a greater slope. This may also result in lower intercepts on the sulfur axis. For upper shale samples the carbon-sulfur regression line has a very high slope of 3.73 and a low intercept on the sulfur axis of 0.21. Consistency of anoxic levels during deposition of upper shales is, however, indicated by the low standard deviation (0.051) of C/S data and a dominant control of the sedimentation rate on the sulfur content of the sediments is inferred. Sulfur isotope data on pyrite samples indicate an environment closed to SO (super -2) 4 which was initially open to H 2 S (or HS (super -) ) and later during deposition of upper shale became closed to it due to a limited supply of reactive iron. This confirms that the pyrite bed and upper shales were laid down in a restricted environment and substantiates the interpretation of the carbon-sulfur relationships observed in them.

A preliminary study of the comparative malting qualities of Sorghum bicolor and Sorghum guineensis

Malting qualities of Sorghum bicolor and Sorghum guineensis grown in the same field were studied. After the different sorghum grains were steeped in water (10 ± 2° C) with a six-hourly steep liquor change schedule, they were grown on a glass sheet lined with moist blotting paper for a period of 6 days. Germinated seedlings were kilned to a moisture content of 4%. Samples of sorghum ( S. bicolor and S. guineensis) were collected at three different stages (steeping, germination and malt) and subjected to analysis of sugar content, diastatic activity, protein content, viability, apparent extract, water extract and sedimentation rate. S. bicolor, which exhibited a significantly ( p < 0·05) high moisture content when compared with S. guineensis, also had a significantly ( p < 0·05) high protein content, malting loss, enzyme activity and acrospire length. However, no significant ( p > 0·05) difference was observed when the different sorghums were analysed for sugar and diastatic activity. Variations in sedimentation rates, moisture contents, protein contents, apparent extracts and cold and hot water extracts of the sorghum malts are discussed.

Sedimentology and Diagenesis of Resedimented and Rhythmically Bedded Chalks from the Eldfisk Field, North Sea Central Graben (1)

Chalk reservoirs in the Eldfisk field comprise autochthonous chalks of the Hod Formation, overlain by mixed allochthonous and autochthonous chalks of the Tor and Ekofisk Formations. Core material contains previously undescribed fabrics which are of importance in determining the sedimentological and diagenetic history of North Sea chalk reservoirs. Resedimentation within the Tor and Ekofisk Formations was dominated by slumping. Evidence of deposition from liquefied flows is scant. Sediment-filled fractures indicate early overpressuring of the sequence in response to burial beneath Tertiary mudrocks. Within the hydrocarbon zone, the Hod and Ekofisk Formations contain small-scale (about 7.5 cm) rhythmic alternations of oil-stained and unstained layers, related to variation in sedimentation rate and/or early diagenesis. Below the hydrocarbon zone, and in impermeable intervals within the hydrocarbon zone, small-scale alternations are replaced by decimeter-scale interbedding of clay-rich and clay-poor chalks more typical of autochthonous chalk facies and a product of more advanced chemical compaction. These fabrics suggest that porosity reduction is closely related to lower hydrocarbon saturation, in turn largely a function of clay content, early overpressuring, and structural position within the field.

A Technique for Disentangling Temporal Source and Sediment Variations from Radioactive Isotope Measurements with Depth

Radioactive isotope analyses are widely used in many aspects of geological studies - dominantly as a means of ascribing exact ages to strata and to provide an assessment of sedimentation rates. However, because of the complexity of sediment transport, deposition, and accumulation, as well as of the effects of compaction, consolidation, and diagenesis, and because of the intrinsic variation of isotopic concentrations at deposition, it is often quite difficult to provide a quantitative 'age profile,' relating sediment depth to age using procedures developed to date. This paper develops a quantitative procedure that permits the singular separation of effects due to source variations in time from those due to variations in sedimentation rate in observed profiles of any radioactive isotope concentration with subsurface depth. The procedure guarantees to keep depth to time conversion positive, guarantees to provide a positive source variation with time, guarantees a least squares fit to measured isotope data with depth, and operates with any radioactive isotope.

Peat in the “Niayes” of Senegal: depositional environment and Holocene evolution

The “Niayes” peat deposits north of Dakar, in Senegal, provide an unusual opportunity to study the continental and littoral detrital environment of the Holocene in West Africa. These organic deposits, that may attain a thickness of 10m, accumulated in Late Pleistocene interdune basins whose extend morphology depend closely upon the palaeohydrologic evolution of and the continental model for this zone during the Holocene. The present sub-Canarian climate of this region allows the preservation of an azonal vegetation of Guinean chorological affinity that is evidence of the wider development of now more southerly vegetation during the older Holocene. The nature of the sedimentary facies of these peatfields is closely related to the altitude of the basins of accumulation and the position of the fresh/salt water interface which conditions the recharge of the shallow aquifer. Thus, fresh-water and mangrove-swamp peats exist more or less closely associated according to the site. 14C age determinations gives ages for these deposits between 12000 B.P. and the present, and detailed palynological studies have shown that there were two periods of climatic optimum, one between 9000 and 7000 B.P. and one between 4000 and 2000 B.P. The highly variable rates of sedimentation (0.2–12,5 mm/y for the continental zones and 2 mm/y for the mangrove swamps) are related to the paleotopography of the water-table or to very local fluctuations of sea level. The evolution of the vegetal biomass, evaluated both qualitatively (relative representation of the various vegetation levels) and quantitatively (concentration of pollen per gram of dry sediment) during the course of the Holocene enables reconstruction of the complete climatic and hydrologic history of the region up to the dawn of the Present.

Geomagnetic secular variations (inclination) of high latitude fiord cores: eastern Canadian Arctic

Paleomagnetic measurements are reported from 11 piston cores, from the fiords and shelf of eastern Baffin Island, N.W.T., between latitudes 66 and 72 degrees north. The majority of the measurements arc from bioturbated, massive, or laminated mud, with some drop-stones and graded sand beds. Corrected radiocarbon dates on the acid-insoluble organic matter fraction, supplemented by AMS dates on in situbivalves, indicate that all cores extend into the early Holocene, and three extend into the latest Pleistocene Sedimentation rates averaged between 0.2m/ka and 1.4m/ka. Because of varying sedimentation rates, the depth scales are converted to 100 or 200 yr/sample time series. The results indicate a scries of geomagnetic secular oscillations with amplitudes in inclination of c. 10 degrees. A stacked record from four piston cores suggests seven major oscillations in inclination. Times when inclinations consistently exceeded 80° occurred c. 1,400, 4,500, and 8,000 B.P. The most characteristic oscillation occurred c.1,400 ± B.P., when inclinations were nearly vertical. Inclination errors are associated with gravity flows and/or with an increase in sand content, or changes in physical properties. In one core an interval of reverse polarity is attributed to a slump.