Biostratigraphic Methods Research Articles

UNIQUE CHARACTERISTICS OF THE STRATIGRAPHIC STUDIES OF THE EARLY PRECAMBRIAN. <br/> Paper 1. Particular qualities of traditional stratigraphic dismemberment

Despite the common approach to the study of stratigraphy of Cryptozoic (Precambrian) and Phanerozoic, the dismemberment of Early Precambrian possess a number of unique qualities that pose additional challenges to study. They are caused by the ever-present metamorphism and dislocations of Early Precambrian, the inability to apply bio-stratigraphic methods of analysis and other reasons. The use of isotopic geochronology can hardly compensate for the difficulties of analysis stemming from these difficulties. Because of this, the importance of stratotypes in the study of the stratigraphy of Early Precambrian is increased. The purpose of the study of stratotypes lies in determining both regional and global correlational indicators of stratons that were caused by the evolution of tectonic, paleo-climatic, and paleo-geographic conditions of their accumulation and consequent metamorphism. The paper underbines the structural and content differences applicable to the stratigraphy of Lower Precambrian and Phanerozoic, including the differences in local and global stratigraphic taxons. It also demonstrates the suitability of application "complexes" in the stratigraphic system of the Precambrian Ukrainian shield, as per the new "Stratigraphic Code of Ukraine".

Continental Upper Carboniferous red beds in the Variscan intermontane Saale Basin, central Germany: orbital forcing detected by wavelet analysis

Abstract Stratigraphical correlation within continental red beds is hampered by uniformity of sediments and the lack of fossils, such that classical lithostratigraphical and biostratigraphical methods often fail. For a drilled section of Upper Carboniferous non-marine sediments of the Variscan intermontane Saale Basin, almost 800 m in thickness, an alternative approach is proposed. Wavelet-based time-series analysis is used to identify the internal organization of the cyclicity, and to distinguish cycles of different magnitude and origin as being autocyclically, tectonically or climatically controlled. Based on this distinction, basin-wide correlations of fluvial red beds are possible using a combination of high-resolution stratigraphy, biostratigraphy and classical lithostratigraphy. The analyses of well Querfurt 1/64 suggest the presence of short and long eccentricity cycles, and a duration of 5–7 Ma if the grey facies at the base of the section is to be correlated with the Grillenberg Subformation sediments. This subformation is of Stephanian A or Barruelian age, respectively, such that the well Querfurt 1/64 exposes a nearly complete section of the Mansfeld Subgroup (complete Stephanian stage).

Record of carbonate preservation and the Mid‐Brunhes climatic shift from a seamount top with low sedimentation rates in the Central Indian Basin

In the present investigation, an age model of carbonate‐rich cores from a seamount top in the Central Indian Basin (CIB) was constructed using both isotopic (230Thexcess, AMS 14C, oxygen isotopes) and biostratigraphic methods. The chronologies using the two methods are in good agreement, yielding a record of the late Middle Pleistocene to the Pleistocene–Holocene transition (550 to 11.5 ka). The first appearance datum (FAD) of the radiolarian Buccinosphaera invaginata (180 ka) and coccolith Emiliania huxleyi (268 ka) and the last appearance datum (LAD) of the radiolarian Stylatractus universus (425 ka) were used. A monsoon‐induced productivity increase was inferred from carbonate, organic carbon and δ13C records in response to the Mid‐Brunhes Climatic Shift (MBCS), consistent with an increased global productivity. While the coccolith diversity increased, a decrease in coccolith productivity was found during the MBCS. At nearly the same time period, earlier records from the equatorial Indian Ocean, western Indian Ocean and eastern Africa have shown an increased productivity in response to the influence of westerlies and increased monsoon. The influence of easterlies from Australia and the intensification of aridity are evidenced by increased kaolinite content and clay‐sized sediments in response to the MBCS. An increased abundance of Globorotalia menardii and other resistant species beginning from marine isotope stage (MIS) 11 and the proliferation of coccolith Gephyrocapsa spp. indicate increased dissolution, which is consistent with the widespread global carbonate dissolution during this period. The relatively high carbonate dissolution during the transition period of MIS 3/2 and glacial to interglacial periods (MIS 6, 7 and 8) may be due to the enhanced flow of corrosive Antarctic Bottom Water (AABW) into the CIB.

Geodynamic formation settings of Ordovician and Devonian dike complexes in ophiolitic sections of the Southern Urals and Mugodzhary

The dike and volcanic complexes in the upper parts of the ophiolitic sections in the Paleozoides of the South Urals and Mugodzhary are Ordovician and Devonian in age. Two types of Ordovician complexes are distinguished by petrology and geochemistry. One of these types is characterized by a suprasubduction forearc formation setting and the second type developed in spreading basins in close proximity to island arcs. The Ordovician dikes formed in the setting of suprasubduction forearc spreading occur as blocks in the melange of the Sakmara Zone. Zircons from the plagiogranite associated with the dikes are dated at 456 ± 4 Ma. The Polyakovka dike complex in the north of the Cis-Sakmara-Voznesenka Zone is associated with basalts and cherts containing Ordovician conodonts. The dikes were probably formed during subduction of the spreading center; contributions of mantle-plume and subduction-related components are noted. Dike and volcanic complexes of Early-Middle Devonian age determined using isotopic and biostratigraphic methods are widespread. Two groups of complexes are distinguished by structural and geochemical features. The first group was formed in the setting of dispersed spreading in the second half of the Early Devonian. Boninites occur among the rocks of this group. The second group was formed in the setting of fast focused backarc spreading that developed up to the late Eifelian. Dike-in-dike suites close to the first group in composition cut through the Early Eifelian island-arc complexes in the frontal part of the arc. Zircons from the granitoid veins accompanying these dolerite dikes are dated at 391.9 ± 3 Ma (late Eifelian).

Probabilistic correlation of single stratigraphic samples: A generalized approach for biostratigraphic data

Existing quantitative methods for biostratigraphic dating and correlation commonly ignore one of the key strengths of the microfossil record—relative abundance data. In this study, we present a maximum likelihood-based biostratigraphic method that demonstrates how microfossil abundance can be used in the stratigraphic placement of isolated samples. Precise correlation and dating of isolated paleontological samples is not possible with current methods, which are primarily intended for the alignment of longer stratigraphic sequences. In contrast, the probabilistic approach provided by likelihood analysis results in sample age estimates with defined confidence intervals. Therefore, all the uncertainties inherent in our age assessment (resulting from small sample sizes, incomplete sampling, imperfect knowledge of stratigraphic distributions, lack of taxonomic resolution of biostratigraphic data, and underlying environmental, paleogeographic, and sedimentologic processes) are explicit in our results. We conclude with a field test of the method, from data collected from an oil well from the Catatumbo Basin, Colombia, illustrating the use of our approach in a real-world case study and highlighting how our method could be generalized to a wide range of stratigraphic problems.

Quaternary Palynostratigraphy of the Bulgarian Black Sea coastal area

The palynological method is one of the main biostratigraphic methods. Pollen analysis of marine sediments offers the possibility of obtaining long and presumably continuous records of coastal vegetation that are the source of plant microfossils found in the marine basins. Such sequences provide opportunity to develop complete and high-resolution records of terrestrial events. The stratigraphic subdivision of sediments from the western Black Sea area is based on qualitative interpretation of the pollen and spore assemblages, and the vertical and spatial distribution of selected indicator taxa

Upper Cretaceous inter-hemispheric correlation between the Southern Tethys and the Boreal: chemo- and biostratigraphy and paleoclimatic reconstructions from a new section in the Tethys Himalaya, S-Tibet

A new, 430 m long and mostly continuous Upper Cretaceous section from southern hemisphere low paleolatitudes in the Tethys Himalaya (Guru, Tibet) is presented. The lithology, microfacies and fossil contents of the Guru section indicate a continuous shallowing trend from open oceanic conditions at the slope during the Turonian to shallow marine inner shelf environments in the Maastrichtian, interrupted by short periods of stagnation or slight deepening. Strong variations in sedimentation rates and contents of carbonate, quartz, organic carbon and sulfur appear to reflect a combination of regional and global processes. On regional scale, the patterns of varying elastic sediment supply seem to be related to the fast northward drift of the Indian plate from the temperate climate belt in the Turonian and Coniacian, crossing the arid zone during the Santonian and early Campanian, and passing into the tropical humid belt in the late Campanian. On global scale, similar sedimentary changes as in Guru with a transition from clay-rich Turonian sediments to Coniacian carbonates are found in other regions of the world, suggesting an additional influence of global oceanic and climatic factors. Intervals of omission, erosion and reworking in upper Coniacian and lower Campanian sediments of the Guru section appear to coincide with global sea-level lowstands. Well preserved planktic foraminifera occur over most of the Guru section and provide good biostratigraphic control for correlation of stable carbon and oxygen isotopes with data from the reference section for the boreal and Maastrichtian carbon isotope fluctuations can be related to cyclic variations in carbonate showing detailed correlation of bulk sediment carbon isotopes and comparable trends in oxygen isotopes. The real white chalk in northern Germany at Lagerdorf-Kronsmoor and with the English Chalk reference section, content at Lagerdorf-Kronsmoor which are thought to be driven by long eccentricity. This indicates orbital forcing of both carbonate accumulation and carbon isotope signature, most likely mediated by global sea-level changes. The possibility of detailed delta C-13 correlation from boreal sections of the northern hemisphere to a low-latitude section of the southern hemisphere allows for global correlation with an accuracy not achieved by biostratigraphic methods so far. It further enables linking of microfossil and macrofossil biozones, facilitating precise comparison of shallow and deep water sections, which is essential for the improvement of our understanding of timing, causes and effects of climatic and oceanographic processes.

Campanian Maastrichtian carbon isotope stratigraphy: shelf-ocean correlation between the European shelf sea and the tropical Pacific Ocean

The long-term climate cooling during Campanian - Maastrichtian times is not well understood to date, especially because of the uncertainty introduced by low temporal resolution of biostratigraphy and the pronounced provincialism between tropical and temperate taxa. Two new high-resolution carbon isotope records derived from the boreal shelf-sea section at Lagerdorf-Kronsmoor-Hemmoor, northern Germany and the tropical Pacific at Deep Sea Drilling Project Site 305, Shatsky Rise, reduce these uncertainties. The records can be correlated with an accuracy not achieved by biostratigraphic methods so far. Distinct carbon isotope events in the late Campanian and the early Maastrichtian can be identified at both localities suggesting to represent global carbon cycle perturbations. Especially, the negative carbon isotope excursion in the early Maastrichtian, a pronounced feature of open-ocean records from the Pacific and Southern oceans, is recognized for the first time at a shelf-sea locality related to the North Atlantic Ocean. Furthermore, two short-term positive excursions are identified as superimposed signals to this event. The improved stratigraphy provides the unique opportunity to recognize leads and lags between the carbon cycle and ocean circulation of different marine settings and ecosystems, leading to a better understanding of their causes and effects.

On the biostratigraphy of accretionary complexes in the Far East (critical review of several papers)

Being a relatively simple geological discipline, biostratigraphy requires accuracy and sequential logic. The incorrect use of the biostratigraphic method and inaccurate interpretation of biostratigraphic data lead to a distorted understanding of regional tectonics. Frequently occurring errors are analyzed using three recently published works on biostratigraphy of accretionary complexes in the Russian Far East as an example.

Tethyan–Mediterranean organic carbon‐rich sediments from Mesozoic black shales to sapropels

AbstractThe Jurassic to Holocene record of black shale deposition in the Tethys‐Mediterranean region is unrivalled by that of any other ocean basin, either in land sections or drill cores. The term ‘black shale’ is used here broadly for sediments with elevated organic carbon concentrations (> 1%), including the Pliocene to Recent sapropels. Most of the black shales are devoid of benthonic organisms, are laminated, and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic. The Tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales. These records have been central in understanding climate control on ocean dynamics and biogeochemical cycles. The Mesozoic black shales were deposited within well‐defined time envelopes of around 0·5 to 1 Myr. These black shales, which were confined to certain chronostratigraphic intervals in the Jurassic and Cretaceous, were recognized as expressions of global Oceanic Anoxic Events in the mid‐1970s and subsequently named after prominent researchers (Bonarelli, Selli, Goguel). The black shale episodes were dated by biostratigraphic methods and by high‐resolution chemostratigraphy and cyclostratigraphy. Mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate. Research into younger (Pliocene to Pleistocene) sedimentary cycles (including black shales, termed sapropels) exposed in land sections, or found in pelagic and hemipelagic marine sediment cores of Late Quaternary age, started in the 1950s. Main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes, palaeoclimate evolution of the circum‐Mediterranean area and biogeochemical dynamics in anoxic basins. After 50 years of intensive research, the Tethyan and Mediterranean black shales remain subjects of fascination in Earth Science. Tracing the origins and building upon recent progress, the current hypotheses on their formation are reviewed here. It is a panorama of complex interplays between global and regional tectonics, climate dynamics during both Ice House and Greenhouse states of global and regional climate, oceanographic responses to these climate changes, and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity.

Weichselian sediment succession at Ruunaa, Finland, indicating a Mid‐Weichselian ice‐free interval in eastern Fennoscandia

Core material obtained from three boreholes was studied from the Ruunaa area, Finnish northern Karelia, in order to reveal the Late Pleistocene environmental history of eastern Finland. Conventional litho‐ and biostratigraphical methods were used and suitable sediment samples were dated using quartz optically stimulated luminescence. The results indicate that two till units were deposited by two separate Weichselian ice advances into the area. The first advance took place prior to 52 kyr ago, most likely during the early Middle Weichselian, while the second is thought to have taken place during the Late Weichselian after 25 kyr ago. The sediment succession at Ruunaa therefore indicates a Middle Weichselian ice‐free period around 50–25 kyr ago in the eastern part of Fennoscandia.

Two Species of Zosterophyllum from South China and Dating of the Xujiachong Formation with a Biostratigraphic Method

Abstract: Zosterophyllum longa sp. nov. is reported from the Lower Devonian Pingyipu Formation, Jiangyou District, northern Sichuan of China. This new plant has vertically elongate sporangia differing from all known species of Zosterophyllum, demonstrating morphological variations in Zosterophyllophytina. New materials of Zosterophyllum yunnanicum from the Lower Devonian Xujiachong Formation, Qujing District, eastern Yunnan of China show that fertile axes of this species are anisotomous or pseudomonopodial or K‐shaped in branching. Sometimes fertile lateral axes are very short and bear an axillary axis at the dividing point. Distinct thickening occurs along the whole margin of a sporangium, with sporangial lobes undeveloped. The evolution of Zosterophyllum is tentatively discussed. A biostratigraphic method of Gerrienne and Streel is introduced and applied to dating of the Lower Devonian Xujiachong Formation, which is considered as of the late Pragian‐early Emsian.

AGE OF ESTONIAN KUKERSITE OIL SHALE – MIDDLE OR LATE ORDOVICIAN?

Modern principles of stratigraphy and introduction of a new international time scale for the Ordovician Period have changed the view on regional stratigraphy. Seven global stages and three series allow for a more universal application of the Ordovician time scale, but in some cases the newly defined units differ from their former usage. The regional successions can only be dated by comparing them with global type sections, which for the Upper Ordovician is located in Scania, southern Sweden. Using biostratigraphical methods, we provide some new insights into correlation between the international standard and the Uhaku and Kukruse regional stages in Estonia. The Kukruse Stage and the deposits of kukersite oil shale, which have commonly been treated as middle part of Middle Ordovician, must, in fact, be considered as Upper Ordovician.

Opportunities and Challenges of a Highly Resolved Geological Timescale

The advent of greatly improved radiometric dating techniques with lower uncertainties, the development of new dating and correlation techniques, including vastly expanded quantitative biostratigraphic methods, and the possibility of reliable extension of orbital cyclostratigraphy into the Paleozoic all promise a great improvement in the ability of geologists to construct high-resolution temporal frameworks far deeper into the past. Such techniques have already allowed the generation of a high-resolution temporal framework for the Ediacaran-Cambrian radiation of metazoa, helped greatly narrow the duration of the great Permo-Triassic mass extinction and eliminated several hypothesized causes, and narrowed the duration of the oceanic anoxic event at the Cenomanian-Turonian (Late Cretaceous boundary). Temporal resolution of 100 kyr (0.02%) or even better into the early Paleozoic now seems likely, opening a host of new questions for reliable investigation. Further exploiting the possibilities of these techniques will require paleontologists to improve methods to integrate these disparate techniques, improve our understanding of the analysis of evolutionary rates, and confront the challenges of settings where geochronologic resolution may be greater than paleontologic resolution.

Detection of Late Cretaceous eustatic signatures using quantitative biostratigraphy

Research Article| July 01, 2006 Detection of Late Cretaceous eustatic signatures using quantitative biostratigraphy James S. Crampton; James S. Crampton 1GNS Science, P.O. Box 30-368, Lower Hutt, New Zealand Search for other works by this author on: GSW Google Scholar Poul Schiøler; Poul Schiøler 2The Geological Survey of Denmark and Greenland, Ø. Voldgade 10, DK-1350, Copenhagen K., Denmark Search for other works by this author on: GSW Google Scholar Lucia Roncaglia Lucia Roncaglia 3Danish Oil and Natural Gas A/S, Agern Alle 24-26, DK-2970 Hørsholm, Denmark Search for other works by this author on: GSW Google Scholar GSA Bulletin (2006) 118 (7-8): 975–990. https://doi.org/10.1130/B25826.1 Article history received: 26 Mar 2005 rev-recd: 13 Dec 2005 accepted: 10 Mar 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James S. Crampton, Poul Schiøler, Lucia Roncaglia; Detection of Late Cretaceous eustatic signatures using quantitative biostratigraphy. GSA Bulletin 2006;; 118 (7-8): 975–990. doi: https://doi.org/10.1130/B25826.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This study tested the potential for high-resolution quantitative biostratigraphic methods to distinguish tectonic from supposedly eustatic sequences. The study employed automated multidimensional graphic correlation using constrained optimization (program CONOP). Our analysis was based on 15 onshore sections in eastern New Zealand spanning the Coniacian to Maastrichtian stages (Upper Cretaceous), and 398 well-constrained lowest- or highest-occurrence records derived from 245 dinoflagellate species. The age of the resulting composite section was calibrated using six dated bioevents; two of these events were tied to geochemical or paleomagnetic datums. Following calibration, the composite section had an average relative temporal resolution of less than 130 k.y. through the study interval, although the succession of events was not dated to this level of precision in an absolute sense. Although the CONOP composite will not resolve unconformities or condensed intervals that occur across all studied sections, such “global” unconformities can be identified using clusters of events that are placed at a single composite level. Fifteen major event clusters were identified, 10 of which coincided closely with sequence boundaries identified in the Northern Hemisphere; the probability of this coincidence arising by chance alone is ∼8%. Although these results must be regarded as provisional, they suggest that high-resolution quantitative stratigraphy can provide a potent tool for the identification and correlation of stratigraphic sequences. Furthermore, our findings add significant support for recent studies that have argued for the presence of Late Cretaceous eustasy. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Holocene forest dynamics and human impact in southeastern Estonia

Litho-, chrono- and biostratigraphical methods were applied in the examination of a 9.9 m thick sequence of laminated sediments in Verijarv, a lake in southeast Estonia. The vegetation history, which covers at least the time from 10300 cal b.p. to the present, was inferred from the core, which was taken from the deepest part of the lake and correlated with the studied and dated pollen diagrams nearby. Reconstruction of the past vegetation dynamics is based on pollen percentage, accumulation rate and human impact diagrams. During the Holocene two main shifts in vegetation dynamics occurred, the first one at about 7700 cal b.p. when the pine-birch forest was replaced by deciduous mixed forest, and the second at about 3200 cal b.p. marked by the regeneration of pine-birch-spruce forest. A catastrophic forest fire at about 3450 cal b.p. opened up the landscape and gave rise to erosion and the sedimentation of a thick clayey layer. The steep decline in the Alnus curve between 1500–1650 cal b.p. coincides with the start of the continuous Secale curve and evidence of extensive land-use.

BIOSTRAT: a program for simulating the stratigraphic occurrence of fossils

BIOSTRAT is a program for simulating the stratigraphic occurrence of marine fossils in single outcrops or in cross-sections through a sedimentary basin. It is primarily for use as a forward model, but could be used to inverse model a particular outcrop or basin. BIOSTRAT combines a random-branching model of evolution with a Gaussian ecological model of species with respect to water depth. Several modules are included that facilitate using the output of the sedimentary basin model, STRATA. BIOSTRAT has applications in the testing of biostratigraphic methods, for exploring how evolutionary events get filtered by the stratigraphic record, for testing paleoecological statistical methods, and for understanding how patterns of morphologic change in fossil species are distorted by the processes of sediment accumulation.

Magnetostratigraphy of Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin, Colorado

The Denver Basin is a Laramide foreland basin that filled with synorogenic sediment shed from the rising Rocky Mountains from the end of the Cretaceous through the Eocene. This sedimentary sequence contains a rich and diverse biota that is difficult to correlate because of the low relief and poor exposures characteristic of this region. This study has correlated the stratigraphic sequence and fossil localities using a combination of three techniques. Magnetostratigraphy has proven to be an effective way to date these rocks as they contain a measurable and interpretable reversal sequence that can be correlated to the latest Cretaceous through Tertiary geomagnetic polarity time scale (GPTS). The palynostratigraphy of the region is well known and the rocks contain multiple levels that yield palynomorphs. Volcanic ashes found in both the Cretaceous and Tertiary units can be dated using the 40Ar/39 Ar isotopic dating method. Each technique has its advantages and disadvantages, but in combination, these three chrono- and biostratigraphic methods have the potential to date with a high level of precision virtually every fossil locality sampled in the Denver Basin. To effectively date the exposures from across the entire basin, a reference or benchmark section had to be established against which the biostratigraphic zonation could be directly correlated to the chronology. Due to the paucity of long surface outcrops, drilled wells were the only way to obtain a continuous rock sequence from which a reference section could be constructed. Two cores, one drilled at Castle Pines on the western margin of the basin, and another at Kiowa in the central part of the basin, provided a continuous rock sequence from the top of the Maastrichtian Pierre Shale to the Eocene rocks of the D2 synorogenic sequence. The magnetostratigraphic study of these cores established a reversal sequence that could be correlated to the GPTS ranging from polarity chron 31 through to chron 24. The palynostratigraphy yielded a zonation ranging from the Aquilapollenites striatus Interval Zone through to the early Eocene, and accurately placed the Cretaceous-Tertiary (K-T) boundary in each core. Isotopic ages were obtained from the Maastrichtian, early Paleocene, and early Eocene parts of the section, and allow us to independently confirm the calibration of the units to the time scale. With this chronostratigraphic framework in place, the individual fossil-bearing localities and surface sections from across the entire basin can be correlated and dated to a precision that is comparable to the calibrating isotopic ages. The measured sedimentation rates vary across this asymmetric basin, with higher rates in the western, proximal part of the basin, and a pronounced increase in sedimentation rate across the K-T boundary. Two separate packages of strata, separated by a regional unconformity between the D1 and D2 synorogenic sequences, were dated. The Maastrichtian through Paleocene sequence that encompasses the Pierre Shale, Fox Hills Sandstone, Laramie Formation, and D1 strata dates from about 69 to 64 Ma. The overlying D2 synorogenic strata are poorly constrained and date from about 54 Ma. The reversal pattern of the D2 sequence varies across the basin, which indicates that sedimentary hiatuses, probably caused by tectonically quiet intervals along the mountain front, were followed by differential subsidence across the basin as sedimentation resumed at different times across the 100 km (60 mi) breadth of the basin.

Lateglacial and Early Holocene environments inferred from a tufa deposit at Fyledalen, S. Sweden

A reinvestigation of a tufa deposit in Skåne, including a number of biostratigraphical methods and chronology was carried out. The tufa was formed in a paludal environment during the time period c. 9,500 to 8,000 BP. This period is characterised by exceptional low lake levels and high summer temperatures in southern Sweden. This may suggest that optimal conditions for tufa formation at the site coincided with warm and dry climate conditions, a low groundwater table, and high evaporation. During periods with colder conditions and/or higher groundwater table peat was deposited. The changes in the local environment at and around the site were inferred from the biostratigraphical records of pollen, plant macrofossils, molluscs, and insects. The results demonstrate a development from a dwarf shrub tundra during the Late-glacial, which turned to an open birch/pine woodland around 10,000 BP. At c. 9,000 BP hazel and birch were probably the dominant tree species in the woodland. The mollusc record suggests that the woodland remained relatively open until c. 8,500 BP when tree taxa such as alder, oak and elm became established in the Fyledalen valley. The site remained a marsh throughout the studied period, however, the results indicate a succession of different marsh plant communities.

Pleistocene stratigraphy in the Dellen region, central Sweden

Robertsson, A.‐M., Svedlund, J.‐O., Andrén, T. & Sundh, M. 1997 (September): Pleistocene stratigraphy in the Dellen region, central Sweden. Boreas, Vol. 26, pp. 237–260. Oslo. ISSN 0300–9483.The Pleistocene stratigraphy in the Dellen region, central Sweden was studied using field observations made during mapping of Quaternary deposits and fabric analyses in excavated sections. The lithostratigraphy was also studied by seismic refraction measurements, analyses of grain‐size distribution and organic carbon content. Biostratigraphical methods applied were pollen and diatom analyses. A general outline of the Pleistocene stratigraphy in the area is presented. Three different till beds are identified, the lowermost suggested to have been deposited during the Saalian glaciation and the other two during the Weichselian glaciation. According to the interpretation of the stratigraphy, it is questioned whether the first Weichselian ice sheet did in fact reach the Dellen area. A clayey sediment sequence at Norra Sannas accumulated during an interglacial, probably the Eemian. Most of the interglacial vegetation succession is reflected in the identified pollen flora. An initial phase with a light‐demanding forest of Belula and Pinus was followed by immigration of Alnus, Picea and scattered occurrences of Corylus. A freshwater diatom flora was identified dominated by plankton taxa, e.g. Aulacoseira italica, A. distans and Cyclotella spp. In the lower part of the sequence a brackish‐marine flora was registered, representing accumulation in a bay of the Eemian Sea. Fine‐grained sediments at the Sundson and Vastansjd sites are interpreted as rebedded Eemian sediments according to the pollen flora. An (Early Weichselian) interstadial age is suggested for sediments found at Bjuraker. Dating by the 14C‐ and OSL methods was carried out on the interglacial and interstadial sediments, respectively. The ages range from approximately 19000 to 92000 BP. Correlation of interglacial vegetation history with central Finland and other areas is discussed.