Relative Sea-level Fluctuations Research Articles

C-, O- and Sr-isotope stratigraphy across the Lower–Middle Cambrian transition of the Cantabrian Zone (Spain) and the Montagne Noire (France), West Gondwana

C-, O- and Sr-isotopes have been analysed for three sections of the Cantabrian Zone (northern Spain) and the southern Montagne Noire (France) that cross the Lower–Middle Cambrian transition, or Bilbilian–Leonian transition in its Iberian sense. The base of the Leonian (Middle Cambrian) is regionally defined by the first appearance of the trilobite Acadoparadoxides mureroensis, which is well constrained in the Cantabrian Zone but absent in the Montagne Noire. In the Bilbilian parts of the three sections, nearshore to peritidal strata are capped by erosive unconformities. The succeeding Leonian relative sea-level rise is recorded in similar fashion in both environments, leading to the deposition of aggradational bioclastic limestones and/or transgressive limestone/shale couplets (the so-called griotte facies) overlain by offshore shales, which relate to platform drowning and coastal smothering during the Leonian–Caesaraugustan transition. Bilbilian–Leonian carbonates show a long-term trend to higher δ 13C values that was punctuated by a succession of positive excursions. In Crémenes–Valdoré (Cantabrian Zone) and Ferrals-les-Montagnes (Montagne Noire), two Bilbilian δ 13C shifts (an older uncertain peak at + 0.1‰/+ 0.01‰ and a younger certain shift at + 0.6‰/+ 1.7‰), and two Leonian shifts (peaking at + 1.1‰/+ 1.2‰ and + 1.3‰, respectively) are apparent; at Genestosa (Cantabrian Zone) a fifth early-middle Caesaraugustan shift is identified (peaking at + 2.1‰). These positive excursions and increasing trend are similar to published trends from the Lower–Middle Cambrian transition in Siberia and Australia, although in those areas only two δ 13C excursions have been reported so far. The marked negative δ 13C excursion reported from the Lower–Middle Cambrian boundary of Laurentia and China is not recorded in our dataset. Oxygen isotope and trace element (Ca, Mn and Sr) data indicate that our δ 13C trends are unlikely to be diagenetic features, while the lack of correlation with relative sea-level fluctuations, facies changes, or benthic community dynamics means that they could be of potential for global stratigraphic correlation. Sr isotope compositions for the Cantabrian Zone succession exhibit consistent values upsection, which suggests that seawater 87Sr/ 86Sr remained around 0.7089–0.7090. Our more limited dataset for the Montagne Noire region confirms this value, and both are consistent with published constraints from Laurentia for seawater 87Sr/ 86Sr across the Lower–Middle Cambrian boundary. The only other 87Sr/ 86Sr study to cover the boundary interval reported significantly lower 87Sr/ 86Sr values ∼0.7087–0.7088, indicating perhaps that the Lower–Middle Cambrian boundary in Siberia lies stratigraphically below its equivalent levels in Europe and Laurentia. However, we consider it more likely that these Siberian samples have been diagenetically altered to less radiogenic values.

Assessing δ13C and C/N ratios from organic material in archived cores as Holocene sea level and palaeoenvironmental indicators in the Humber Estuary, UK

Organic δ 13C and C/N analyses of estuarine deposits provide proxies for changes in the source of organic matter, which can be driven by fluctuations in relative sea level, river discharge, and catchment disturbance. Here we present the results of a comprehensive vegetation and sediment δ 13C and C/N survey of Welwich Marsh (outer Humber Estuary, UK), together with high-resolution δ 13C and C/N analyses of Holocene cores collected nine years previously from the Humber Estuary and the Lincolnshire Marshes, Eastern England, UK. The contemporary intertidal δ 13C and C/N dataset shows a gradual increase in surface sediment δ 13C with decreasing marsh height and suggests that δ 13C is controlled by the degree of tidal inundation and thus reflects organic matter source. However, sediment C/N ratios are less sensitive to tidal changes and the recent introduction of C 4 salt-marsh species complicates the contemporary analogue. The Holocene δ 13C and C/N records are in general agreement with existing microfossil data and provide additional palaeoenvironmental information. This includes support for an estuary-wide expansion of marine conditions from c. 3.3 ka cal. yr BP, followed by a contraction of marine conditions after c. 2.7 ka cal. yr BP, and evidence for an increase in delivery of terrigenous organic matter to the inner estuary in the late Holocene. Bulk organic δ 13C and C/N analysis is shown to be a reliable and independent indicator of coastal environmental change and is therefore a complementary technique to the more commonly used microfossil approach. However, this study also shows that in some circumstances the technique may be compromised when applied to sediments from cores that have been stored for a period of time.

Oncoid growth and distribution controlled by sea-level fluctuations and climate (Late Oxfordian, Swiss Jura Mountains)

Abundant lagoonal oncoids occur in the Late Oxfordian Hauptmumienbank Member of the Swiss Jura Mountains. Four oncoid types are observed in the studied sections and classified according to the oncoid surface morphology, the structure and composition of the cortex, and the texture and fauna of the encasing sediment. Micrite-dominated oncoids (types 1 and 2) have a smooth surface. Type 1 has a rather homogeneous cortex and occurs in moderate-energy environments. Type 2 presents continuous or discontinuous micritic laminae. It is associated with a low-diversity fauna and occurs in high-energy facies. Bacinella and Lithocodium oncoids (types 3 and 4) display a lobate surface. They are dominated by microencrusters (Bacinella irregularis and Lithocodium aggregatum) and are found in low-energy facies. The stratigraphic and spatial distribution of these oncoid types shows a correlation with the sequence-stratigraphic evolution of the studied interval, and thus with relative sea-level fluctuations. It can be shown that these sea-level fluctuations were controlled by orbital cycles with 100- and 20-kyr periodicities. At the scale of 100- and 20-kyr sequences, types 1 and 2 oncoids are preferentially found around sequence boundaries and in transgressive deposits, while types 3 and 4 oncoids are preferentially found around maximum floodings and in highstand deposits. This implies that changes of water energy and water depth were direct controlling factors. Discrepancies in oncoid distribution point to additional controlling factors. Platform morphology defines the distribution and type of the lagoon where the oncoids flourished. A low accumulation rate is required for oncoid growth. Additionally, humidity changes in the hinterland act on the terrigenous influx, which modifies water transparency and trophic level and thus plays a role in the biotic composition and diversity in the oncoid cortex.

Relative sea level changes and regional tectonic evolution of seven coastal areas in NW Greece since the mid-Holocene

This study presents relative sea level (RSL) curves for seven coastal areas in Akarnania and the northwestern Peloponnese (NW Greece) since the mid-Holocene. RSL fluctuations are deduced from 48 14C-AMS dated sedimentological sea level markers from 27 vibracores drilled in near-coast geological archives as well as from six geoarchaeological sea level indicators of known ages. Seven palaeo sea level curves including uncertainty bands are reconstructed for a coastal zone spanning a distance of 150 km. Considerable intra-regional differences in sea level evolution exist. These differences are mainly due to tectonic reasons. In general, RSL in northwestern Greece has never been higher than today. Rates of local sea level rise were highest until 5500–5000 cal BC (up to 12.3 m/ka) and lowest during 4000–500 cal BC (0.2–1.4 m/ka). During the past 2500 or so years, RSL has accelerated anew (0.7–2.7 m/ka). Calculating differences between local mean sea level curves provides quantitative information on intra-regional differences of tectonic activity. The coastal plains of Palairos and Elis show signs of uplift, whereas the Mytikas and Boukka plains are strongly subsiding. Compared to other areas of the eastern Mediterranean, northwestern Greece has been subject to significant net long-term subsidence. Regional tectonic events (RTEs) were detected for the time around 4000, 2500, 500 and 250 cal BC as well as around 250 and 1250 cal AD. RTEs are characterized by changes of uplift/subsidence rates or by the redirection of local tectonic movements. The question if some of the RTEs were of a supra-regional nature is still open. From a geodynamic point of view, the results presented show that Akarnania's southwestern fringe is being downwarped while the tectonic block as a whole is moving towards the southwest. Strongest subsidence rates are observed for central Akarnania. At Akarnania's fringes, subsidence is reduced by the influence of strong uplift of adjacent areas such as around Preveza and the northern Peloponnese.

Quaternary evolution of the Gulf of İzmit (NW Turkey): a sedimentary basin under control of the North Anatolian Fault Zone

The Quaternary evolution of the Gulf of Izmit, situated on the tectonically active North Anatolian Fault Zone (NAFZ), was investigated using seismic reflection, paleontologic, and sediment textural data. On the basis of seismic stratigraphic and sedimentologic-paleontologic interpretations, four depositional units were distinguished within the Plio-Quaternary sequence of the Gulf of Izmit. According to these data, Plio-Quaternary deposits supplied from the northern terrestrial area started to accumulate during a progradational phase, in a south-facing half-graben. A coarse-grained sedimentary unit prograding into the gulf from the south since 200 ka b.p. indicates a dramatic variation in the evolution of the gulf, with the initiation of a new strike-slip fault of the NAFZ and a corresponding uplift of the Armutlu Peninsula in the south of the gulf. During the evolution of this fault from a wide shear zone consisting of right-stepped strike-slip faults and pull-apart basins to a localized principal fault zone, sediments were deposited under the influence of northerly prograding terrestrial and shallow-marine conditions due to relative sea-level fluctuations in the Marmara Sea. During this period, the Gulf of Izmit was invaded mainly by Mediterranean and partly by Black Sea waters. In the latest glacial period, shallow areas in the gulf became subaerially exposed, whereas the central and western sub-basins of the gulf turned into lakes. The present evolution of the Gulf of Izmit is controlled by the after effects of the new rupture of the NAFZ and the estuarine nature of the gulf environment.

Depositional framework and sequence stratigraphic aspects of the Coniacian–Santonian mixed siliciclastic/carbonate Matulla sediments in Nezzazat and Ekma blocks, Gulf of Suez, Egypt

Superb outcrops of mixed siliciclastic/carbonate rocks mark the Coniacian–Santonian Matulla Formation exposed in Nezzazat and Ekma blocks, west central Sinai. They are built up of various lithofacies that reflect minor fluctuations in relative sea-level from lower intertidal to slightly deep subtidal settings. Relying on the facies characteristics and stratal geometries, the siliciclastic rocks are divided into seven depositional facies, including beach foreshore laminated sands, upper shoreface cross-bedded sandstone, lower shoreface massive bioturbated and wave-rippled sandstones, shallow subtidal siltstone and deep subtidal shale/claystone. The carbonate rocks comprise lower intertidal lime-mudstone, floatstone and dolostone, shallow subtidal skeletal shoal of oyster rudstone/bioclastic grainstone, and shoal margin packstone. Oolitic grain-ironstone and ferribands are partially intervened the facies types. Deposition has taken place under varied conditions of restricted, partly open marine circulation, low to high wave energy and normal to raised salinity during alternating periods of abundant and ceased clastic supply. The facies types are arranged into asymmetric upward-shallowing cycles that record multiple small-scale transgressive–regressive events. Lime-mudstone and sandstone normally terminate the regressive events. Four sequence boundaries marking regional relative sea-level falls divide the Matulla Formation into three stratigraphic units. These boundaries are Turonian/Coniacian, intra-Coniacian, Coniacian/Santonian and Santonian/Campanian. They do not fit with those sequence boundaries proposed in Haq et al.’s global eustatic curves (1988) except for the sea-level fall associated with the intra-Coniacian boundary. Other sequence boundaries have resulted from regional tectonic impact of the Syrian Arc Fold System that has been initiated in north Egypt during the Latest Turonian–Coniacian. These boundaries enclose three well-defined 3rd order depositional sequences; their enclosing shallowing-upward cycles (i.e. parasequences) record the 4th order relative sea-level fluctuations. 34 and 20 parasequence sets, in the order of a few meters to tens of meters thick, mark the Matulla sequences in Nezzazat and Ekma blocks respectively. Each sequence shows an initial phase of rapid sea-level rise with retrogradational sets, followed by lowering sea-level and progradation/aggradation of the parasequence sets. The transgressive deposits display predominance of deep subtidal lagoonal facies, while highstand deposits show an increase in siliciclastic and carbonate facies with the progressive decrease of lagoonal facies. The sedimentary patterns and environments suggest that the regional, partly eustatic sea-level (i.e. intra-Coniacian) changes controlled the overall architecture of the sequence distribution, whereas changes in the clastic input controlled the variations in facies associations within each depositional sequence.

Comparison of Interglacial Warm Events since the Marine Oxygen Isotope Stage 11

Large numbers of oxygen isotopic curves of benthic foraminiferal tests from deep-sea sediment cores have been published. The curves are well-established reliable proxies for past climate and relative sea level fluctuations. In order to understand possible trends in the future climate, a precise identification of warmest events in the past interglacial records becomes a necessity. In this review, we have compared nine hitherto published high-resolution oxygen isotopic records of the last 420 thousand years in order to understand the intensity of the past warm events during interglacial periods. The rating of the intensity of the interglacial events as depicted by the oxygen isotopic variability is as follows ; Marine Isotope Stage 5.5>9.3>11.3>1>7.5. This rating of interglacial warming is closely comparable with the standard oxygen isotope curve of deep-sea sediment cores and also to the hydrogen isotope curve of the EPICA Dome C ice core from the Antarctica. The remarkably high sea level during the warmest interval within MIS 5.5 reached about 7±4m above the sea level during MIS 1, and even possibly above the present-day sea level. The MIS 11.3 periods is distinctive as the longest warm period among the last five interglacial periods. This observation clearly suggests that detailed studies of MIS 5.5 and 11.3 are essential for the prediction of the future environment of the Earth under the global warming.

To evaluation of new methodical approaches in the sequence stratigraphy analysis

Despite great practical achievements, sequence stratigraphy failed to elaborate until now the reliable technique for establishing relative sea-level fluctuations (RSLF). The method proposed recently by Yu.A. Volozh with colleagues for determining the RSLF represents one of interesting but unconvincing attempts in this respect. Use of their method for fundamental and practical purposes requires prudence because of potential errors.

Tidal rhythmites of the Marais Vernier Seine estuary, France and their implications for relative sea-level

The Marais Vernier, located in the upper reaches of the Seine River Estuary (northern France), preserves a thick Holocene succession of laminated detrital deposits and peats. The spectral analysis of one laminated deposit suggests that tidal processes were largely responsible for supplying sediments to this system. Two periods (16 and 32 laminae per cycle) were identified. These cycles can be related to either both the synodic neap–spring cycle (the so-called “fortnightly cycle”) and the anomalistic monthly cycle or both the semiannual and annual tidal cycles. The recognition of tidal influence on these deposits provides insight into the likelihood of some relative sea-level fluctuations previously inferred from these sediments. The depositional rates calculated for the laminated deposits, based on the recognition of these tidal cycles, suggest that the time interval required to deposit the sediments was far shorter than is implied by considering rates based on regional sea-level rise alone.

A Mediterranean Holocene restricted coastal lagoon under arid climate: Case of the sedimentary record of Sabkha Boujmel (SE Tunisia)

The Holocene sedimentary record of Sabkha Boujmel (SE Tunisia) is expressed by a shallowing-upward carbonate lagoon–tidal flat cycle (2.3 m thick) unconformably overlying continental silt–sandy sediment, Late Würmian in age. The sedimentary package of this cycle starts with transgressive marginal shallow marine (intertidal to subtidal) bioclastic sands grading upwards to black mudstone, rich in organic matter (T.O.C. up to 1.3%) deposited within a lagoon protected from the sea by Upper Pleistocene lithified sand spits. The uppermost part of the cycle is represented by oobioclastic carbonate sands covered with dead biodegraded microbial mats and/or reddish sands of aeolian origin deposited in intertidal to supratidal environments. The facies arrangement, particularly the spatial distribution of the ancient and the more recent microbial mats, records the progressive infilling of the lagoon as well as the progradation of the shoreline during the last 2000 years. The organic-rich facies which provide an age varying between 4130 and 6800 yr B.P. were deposited when the Boujmel lagoon started to be progressively separated from the Mediterranean Sea. The main factors controlling the facies and the thickness variation are the local topographic sea-floor irregularities most likely controlled by the inheritance morphology resulting from an important fluviatile digging that occurred during the last glacial maximum, the relative sea-level fluctuations, the hydro-isostatic rebound and the climate.

Evolução sedimentar miocênica nos estados do Pará e Maranhão

Miocene deposits, lithostratigraphically designated as the Pirabas and Barreiras formations, are well exposed in northeastern State of Pará and along the coastline of the State of Maranhão, where they are represented by a variety of sedimentary facies ranging from limestones to siliciclastics. The majority of the limestones and mixed limestone to siliciclastic deposits were formed in environments ranging from marine transitional to shelf. These deposits are internally organized into three stratigraphic successions defined by regional discontinuity surfaces. Analysis of these surfaces, combined with facies characteristics and stratal patterns, suggests an incised valley estuarine system with discontinuous deposition due to multiple episodes of high-frequency relative sea level fluctuations superimposed on an overall transgression to highstand. The establishment and evolution of this incised valley estuarine system occurred after an interval of up to 40 Ma of non-deposition and/or erosion, and probably resulted from eustatic fluctuations combined with tectonic reactivation. Based on the results obtained for the Miocene deposits exposed in the states of Pará and Maranhão, it is proposed that studies combining facies analysis, ichnology, stratal architecture, and mapping of key stratigraphic surfaces might be profitably applied to similar deposits elsewhere in Brazil.

Syn-kinematic palaeogeographic evolution of the West European Platform: correlation with Alpine plate collision and foreland deformation

AbstractSequence stratigraphic correlations indicate that intermittent changes of the kinematic far-field stress-field regimes, and the associated geodynamic re-organisations at the plate-tectonic contacts of the African, Apulian, Iberian and European plates, affected the Tertiary palaeogeographic evolution of the West European Platform through a combination of intra-plate tectonics and fluctuations of relative sea level. A temporal sequence of first-order stages in structural, palaeotopographic and palaeohydrographic development of the platform can be distinguished from the Paleocene onwards. These formative stages are closely linked to major plate-boundary events involving the development of the Pyrenean and Alpine orogens, and can be traced throughout the West European Platform.

Clay Mineral Analysis of Sediments in the Changjiang Delta Plain and Its Application to the Late Quaternary Variations of Sea Level and Sediment Provenance

Abstract Clay mineral analysis was conducted on the fine-grained sediments of a 20-meter core of Late Quaternary deposit in the south-central portion of the modern Changjiang delta plain to understand and characterize paleoenvironmental depositional conditions. Four clay mineral suites (Zones I to IV) were recognized in the core. The lowest level (Zone I, Late Pleistocene) of kaolinite (48%) and illite (45%) is representative of a pedogenic formation of alluvial environment. The next upper suite (Zone II, Early Holocene) of kaolinite (44%) and chlorite (22%) is related to an erosional coastal environment associated with an encroaching sea level. The suite (Zone III, Early to mid-Holocene) overlying Zone II incorporates the addition of smectite (6%), the abundance of which is closely linked to the topographic change and the relative sea level fluctuation in the study area. The uppermost suite (Zone IV, Late Holocene) of illite (53%) and smectite (7%) accumulated during the period of major deltaic progradat...

Santonian–Campanian planktonic foraminifera in the New Jersey coastal plain and their distribution related to the relative sea-level changes

The Upper Cretaceous (Santonian–Campanian) sediments of the New Jersey coastal plain subsurface yielded rich planktonic foraminiferal assemblages at certain stratigraphic levels in three wells: Ocean Drilling Program (ODP) Leg 174AX at Bass River and Ancora sites and a United States Geological Survey borehole in Freehold. Relative sea-level fluctuations can be recognized by following the presence or absence and amounts of two groups of planktonic foraminifers, the shallow water and deep water faunas. The former consists of globular-chambered species, while the complicated tests showing keels, heavy ornamentation and elaborated umbilical structures are confined to the latter. The Merchantville (late Santonian) and Marshalltown (early late Campanian) formations, which represent transgressive system tracts, show the richest and most diverse planktonic foraminiferal assemblages. In the highstand system tracts, represented by the Woodbury (early Campanian), lower Englishtown (middle Campanian), Wenonah (late Campanian), and Mount Laurel (late Campanian) respectively, a gradual decrease in the planktonic foraminiferal diversity and richness was observed. Planktonic foraminiferal peaks of lower amplitude are recorded in the proximity of the minor flooding surfaces. A five-fold planktonic foraminiferal zonal framework for the Santonian-Campanian stratigraphic interval is proposed. Paleobathymetry strongly influenced the planktonic foraminiferal distribution and biostratigraphic resolution.

The paradox of the global standard Late Ordovician–Early Silurian sea level curve: Evidence from conodont community analysis from both Canadian Arctic and Appalachian margins

Relative sea level fluctuations in the Late Ordovician–Early Silurian were largely driven by Gondwana deglaciation, three possible short-lived ice readvances, and the closure of the Iapetus Ocean. This study establishes and compares sea level events on the Canadian Arctic and Appalachian margins of Laurentia based on the pattern of conodont communities, and demonstrates that sea level events were not synchronous on the two margins. Conodont data were compared for a) the Arctic margin from the Cape Phillips Formation (Richmondian, Upper Ordovician through lower Sheinwoodian, Lower Silurian), Cornwallis Island, Canadian Arctic Islands, and b) the Appalachian margin from the upper Ellis Bay, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations (Richmondian, Upper Ordovician, Rhuddanian, Aeronian, and most of Telychian, Lower Silurian), Anticosti Island, Quebec. The pattern of Late Ordovician–Early Silurian conodont communities is established based on three cluster analyses on conodonts from Cornwallis Island (4967 specimens representing 54 species from 77 samples, Cape Phillips Formation) and Anticosti Island (1980 specimens representing 21 species from 25 samples, upper Ellis Bay Formation; 24,839 specimens representing 42 species from 123 samples, Becscie, Merrimack, Gun River, Jupiter and Chicotte formations). Overall, the sea level curves inferred from the distribution of the conodont communities from Cornwallis and Anticosti exhibit gradual and rapid oscillating patterns, respectively. Specifically, during the time interval of the latest Ordovician and the Early Silurian (Rhuddanian), the sea level behaved differently in these two regions: 1) during the latest Ordovician, conodont community changes show the different patterns in the two regions, which reflects a transgression on Cornwallis, but a regression on Anticosti; 2) during the earliest Rhuddanian, similar conodont communities arose in both regions, but they indicate a regression on Cornwallis and a transgression on Anticosti; 3) in the early Rhuddanian, these communities were replaced by a deeper water community on Cornwallis and a shallower water community on Anticosti, which suggest a transgression on Cornwallis and a regression on Anticosti; 4) a highstand drove out almost all conodonts from Anticosti by the end of Rhuddanian, which is not seen on Cornwallis; 5) a deep-water conodont community remained almost unchanged on Cornwallis from the late Rhuddanian to the early Aeronian, whereas a shallow water conodont community returned to Anticosti in the early Aeronian. However, the sea level dropped on both Cornwallis and Anticosti during the late Telychian, and the sea level curves show a similar pattern. The difference in the sea level pattern during the late Richmondian, Rhuddanian and Aeronian on both the northern and southern margins of Laurentia reflected by conodont communities is supported by the studies on Quaternary glaciation/deglaciation events, where the melting of an ice sheet was accompanied by sea level change but with variable regional isostatic effects that do not generate a uniform global eustatic signal.

Sequence biostratigraphy and paleoenvironmental reconstruction in the Early Eocene Figols Group of the Tremp–Graus Basin (south-central Pyrenees, Spain)

The dinoflagellate cyst, palynofacies and foraminiferal records from the Early Eocene Figols Group of the Spanish Pyrenees exhibit major changes associated with relative sea-level fluctuations and variations in efficiency of the fluvial systems. The mixed siliciclastic–carbonate marine succession exposed in the Merli–Esdolomada area was deposited on the shelf in a tectonically active basin, under oxic conditions, in shallow to moderately deep marine waters (close to the lower boundary of the photic zone) influenced by river discharge from adjacent emerged lands. Sedimentary cycles are mainly recorded by fine-grained deposits. The paleoenvironmental interpretation of palynological and calcareous microfaunal records allows the definition of depositional sequences and enables to trace basinward surfaces identified in more proximal settings by means of sedimentological facies analysis. At the same time, the solid regional stratigraphic framework already available for the Figols Group allows new insights on the paleoecology of extinct taxa. The Merli–Esdolomada section (ME) spans two 3rd order sequences. The upper sequence reflects more marginal marine conditions than the lower one, thus pointing to an overall shallowing trend with time. In the lower sequence the maximum flooding interval is characterised by a rich and diverse microfauna, highest relative abundances of marine phytoplankton and typically neritic dinoflagellate cyst (dinocyst) assemblages dominated by Spiniferites and Cordosphaeridium. The final phase of the highstand systems tract, in proximity of the main sequence boundary, is characterised by a decrease in abundance and diversity of dinocysts, with the dominance of the lagoonal genus Polysphaeridium, and by decreased microfaunal diversity (discorbids, miliolids, larger foraminifers). Palynological records permit the reconstruction of activation and deactivation phases of the fluvial systems, variations in runoff from the hinterland, nutrient delivery to the sea and productivity in surface waters of the Tremp–Graus Basin. The overall abundance of Spiniferites and the scarcity of peridinioid cysts, Lingulodinium, Pediastrum and Botryococcus algae are interpreted to indicate oligotrophic conditions during the deposition of the investigated succession. Recurrent intervals where the temporary disappearance of Spiniferites corresponds to highest relative abundances of Operculodinium and herbaceous debris are the main evidence for river plumes and denote periods of enhanced fluvial discharge reflecting a Milankovitch-type cyclicity. The Rotalia group, which shows abundance peaks of Cuvillierina spp. in the shallower intervals, is often associated with these episodes, thus suggesting for this taxon more tolerance to turbidity than larger foraminifera. Within the maximum flooding zone, a Thalassiphora patula acme corresponding to the local disappearance of Homotryblium is deemed to record the distal expression of a fluvial activation which triggered water salinity stratification. Despite broad morphological similarities, Homotryblium exhibits more cosmopolitan preferences than Polysphaeridium, which is instead confirmed as a lagoonal euryhaline taxon. The increase of Spinizonocolpites pollen toward the top of the Figols Group records the northwestward migration of Nypa mangrove-palms from southern Europe during the mid Early Eocene.

Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline.

Major geodynamic and sedimentary constraints on the chronostratigraphic correlation of the lower-middle Cambrian transition in the western Mediterranean region

The sedimentary rocks of the lower-middle Cambrian transition in the western Mediterranean region have recorded a superposition of extensive tectonic, volcanic, epeirogenic, and eustatic events that led to a complex sequence framework that, in some cases, makes detailed chronostratigraphic correlations difficult. This paper summarizes and updates the relationships between event stratigraphy, fluctuations of relative sea level, setting of major stratigraphic discontinuities, unconformities, and condensed levels, and succession of benthic community replacements displayed by outcrops located in the Iberian Peninsula (Iberian, Cantabrian and Cordoba platforms), Moroccan Atlas (Souss Basin), and the Montagne-Noire and Sardinian platforms. The resulting mosaic of inter-related geodynamic processes is correlated by trilobite, archaeocyath-and acritarch-based chronostratigraphic scales, taking as reference for the base of the Middle Cambrian in West Gondwana the immigration of paradoxidid trilobites.

Hydrological changes related to the variability of tropical South American climate from the Cabo Frio lagoonal system (Brazil) during the last 5000 years

From west to east, the Fluminense coastline, composed of two series of lagoons, presents an aridity gradient induced by a local upwelling that is controlled by trade winds coming from the south Atlantic. The physico-chemical conditions of the lagoons are controlled by the duration of the dry season. They are characterized by hypersaline conditions with organic-carbonate sedimentation in the upwelling-influenced area, and by hyposaline conditions with organic-detrital sedimentation out of the upwelling-influenced area. On a core collected in the Brejo do Espinho hypersaline lagoon, diatom analysis supported by sedimentological data revealed a varying high water level from 4500 to 2200 14C yr BP induced by successive freshwater inputs in to the lagoon. Two major dry events occurred during this period, probably related to relative sea-level fluctuations. At 2200 14C yr BP, a drastic change occurred, as indicated by a carbonate nodule, carbonate-dominated sediments and polyto hypersaline diatoms, all suggesting arid conditions. The Brejo do Espinho record suggests that before 2200 14C yr BP, the central part of the Brazilian coast was mostly subjected to south to southeasterly winds and weaker trade winds, whereas after 2200 14C yr BP, trade winds dominated, implying drier conditions at the easternmost part of the Fluminense coastline. These results would also suggest a late Holocene increase in El Ninlo activity (after 2200 14C yr BP), consistent with several El Nifio records from western South America.

Sea level changes versus hydrothermal diagenesis: Origin of Triassic carbonate platform cycles in the Dolomites, Italy: Discussion

In his recent paper, Blendinger (2004) presents a new and unconventional hypothesis for the formation of the Triassic Latemar carbonate cycles. He assigned the cycles to intermittent hydrothermal influence alternating with normal marine deposition. Fluids with a composition similar to normal seawater were forced by elevated heat flow from an underlying hydrothermal field to circulate to the seafloor. These fluids are said to produce stratiform diagenetic features including tepees, and they favoured early dolomitization and the growth of cyanobacterial mats. This interpretation deviates strongly from the conventional models where these cycles are considered to be the result of relative sea-level fluctuations in a shallow-marine to subaerially exposed environment (Hardie et al., 1986, Goldhammer et al., 1987, Goldhammer et al., 1990 R.K. Goldhammer, P.A. Dunn and L.A. Hardie, Depositional cycles, composite sea-level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing: examples from Alpine Triassic platform carbonates, Geol. Soc. Amer. Bull. 102 (1990), pp. 535–562.Goldhammer et al., 1990, Goldhammer et al., 1993, Brack et al., 1996, Mundil et al., 1996, Egenhoff et al., 1999, Preto et al., 2001, Mundil et al., 2003 and Zuhlke et al., 2003). We welcome this entirely new approach, as it further stimulates a discussion on the validity of criteria used to detect subaerial exposure from carbonate facies. Furthermore, it highlights the importance of microbial growth and cementation processes for the development and the geometrical maintenance of post-extinction carbonate platforms such as the Latemar.