Lithological Cycles Research Articles

North Atlantic Drift Sediments Constrain Eocene Tidal Dissipation and the Evolution of the Earth‐Moon System

AbstractCyclostratigraphy and astrochronology are now at the forefront of geologic timekeeping. While this technique heavily relies on the accuracy of astronomical calculations, solar system chaos limits how far back astronomical calculations can be performed with confidence. High‐resolution paleoclimate records with Milankovitch imprints now allow reversing the traditional cyclostratigraphic approach: Middle Eocene drift sediments from Newfoundland Ridge are well‐suited for this purpose, due to high sedimentation rates and distinct lithological cycles. Per contra, the stratigraphies of Integrated Ocean Drilling Program Sites U1408–U1410 are highly complex with several hiatuses. Here, we built a two‐site composite and constructed a conservative age‐depth model to provide a reliable chronology for this rhythmic, highly resolved (<1 kyr) sedimentary archive. Astronomical components (g‐terms and precession constant) are extracted from proxy time‐series using two different techniques, producing consistent results. We find astronomical frequencies up to 4% lower than reported in astronomical solution La04. This solution, however, was smoothed over 20‐Myr intervals, and our results therefore provide constraints on g‐term variability on shorter, million‐year timescales. We also report first evidence that the g4–g3 “grand eccentricity cycle” may have had a 1.2‐Myr period around 41 Ma, contrary to its 2.4‐Myr periodicity today. Our median precession constant estimate (51.28 ± 0.56″/year) confirms earlier indicators of a relatively low rate of tidal dissipation in the Paleogene. Newfoundland Ridge drift sediments thus enable a reliable reconstruction of astronomical components at the limit of validity of current astronomical calculations, extracted from geologic data, providing a new target for the next generation of astronomical calculations.

Holocene sea-level rise and coastal aquifer interactions: Triggering mechanisms for environmental change and impacts on human settlement patterns at Dor, Israel

Future sea-level rise is expected to affect coastal aquifers and environments and have significant impacts on coastal communities. Here, we describe the impact of early to late Holocene sea-level variations on the coastal environment and human settlements of the Carmel Coast, Israel. One of these ancient communities, Tel Dor, was settled initially during the Late Pottery Neolithic (ca. 7 ka) on a wetland surface and then abandoned for ca. 1.5 ka before resettlement occurred on the adjacent aeolianite ridge from the Middle Bronze Age to the Crusader period (4.3–0.9 cal Ka BP). For the first time, high resolution chrono-bio-chemo-stratigraphy of sediment cores collected landward of the current shoreline at Dor are presented capturing a record of poorly sorted sand mixed with marine shells, well sorted aeolian sand and silty clay deposits. The record represents a series of brackish-freshwater wetlands formed in the coastal area of Dor between ca. 15 to 7 ka in response to relative sea-level rise and resulting rise of the coastal aquifers. After 7 ka, due to rising sea level and a transgressing shoreline, sand largely derived from the Nile Delta, reached the coast including the coastal wetland. Landward from the current shoreline, the period between ca. 7 to 4 ka is represented by alternating sand-silt facies consisting of reworked marine shells and brackish-fresh water biota. These lithological cycles reflect fluctuations between coastal and wetland environments governed by the response of the coastal aquifer to sea-level rise. Rapid sea-level rise led to a rise in the groundwater table and inundation of the area around Tel Dor, while periods with slower rates of sea-level rise resulted in coastal sand deposition. The settlement gap at Dor between 7 and 5.6 ka possibly reflects the behavioral response of the coastal settlers to sea-level fluctuation, and sediment depositional variation instigating aquifer inundation coastal marsh development and mobilization of sand bodies. This study provides a record of beach profile build-up along the Mediterranean coast of Israel and serves as an example of how sea-level rise affect unconfined coastal aquifers and the formation of wetlands due to rising water tables in low elevation coasts. Coastal inundation is a long-term risk factor for densely populated low-lying coastal regions that require a proactive approach for solving cascading impacts of sea-level rise.

Spatial and temporal cycle variations in the Eocene Lacustrine Green River Formation, Piceance Creek Basin, Colorado

Lithological cycles in the lacustrine Green River Formation of the Piceance Creek Basin, Colorado, vary spatially and temporally, and are challenging to correlate across the basin. In the center of the basin, lacustrine deposits contain bedded evaporites and kerogen-rich carbonate mudstone, whereas basin margin deposits contain alluvial sandstone, kerogen-poor mudstone, stromatolite, and mud cracks. Because coarsening upward cycles identified at the basin center differ significantly from cycles at the basin margin, exactly how cycle boundaries extend from one location in the basin to another is commonly unclear. In this investigation, eleven lithofacies are identified based on rock composition, sedimentary textures, physical sedimentary structures, biogenic structures, and chemical features. Lithofacies are grouped into four facies associations: (1) Alluvial Facies Association; (2) Shoreline Facies Association; (3) Shallow Lake Facies Association; and (4) Deep Lake Facies Association. Lithological cycles in the Green River Formation vary temporally and spatially, with each stratigraphic section preserving a record of deposition in a particular area of the basin as the Piceance lake evolved. To address the temporal and spatial variation of the cycles, multiple “type” cycles are described for each location in the study area. Facies associations and temporal stacking of cycles suggest that the Piceance lake was initially a deep, freshwater lake, following the Long Point transgression, as lake level rose above the Douglas Creek Arch to fill both the Uinta and Piceance Creek basins. Lake levels then stabilized but later began a period of increased fluctuation. Following basin closure, the Piceance lake became hypersaline under evaporative concentration conditions, precipitating nahcolite and halite at the basin center, with exposure of mudflats at the basin margin. The lake would remain this way until the Mahogany transgression deepened and diluted the lake. Together, facies associations and “type” cycles described in the Green River Formation lacustrine deposits reveal a record of lateral and vertical facies changes that occurred in the Piceance lake during its evolution.

Gypsum lithofacies and stratigraphic architecture of a Messinian marginal basin (Piedmont Basin, NW Italy)

During the Messinian salinity crisis (5.97–5.33 Ma) the Mediterranean was transformed into the youngest salt giant in Earth history. In marginal Mediterranean subbasins, the first stage of the crisis is recorded by up to 16 lithological cycles made up of shale/gypsum couplets (Primary Lower Gypsum unit, PLG), making a downslope transition to organic-rich shales. Gypsum comprises a wide array of lithofacies, including bottom-grown gypsum, cumulate deposits (i.e., resulting from gypsum nucleation in the water column and subsequent deposition on the seafloor) and the branching selenite. The latter appeared synchronously in many sections of the Mediterranean in the 6th PLG cycle (i.e., at about 5.87 Ma). To investigate the still enigmatic stratigraphic architecture of the lateral lithofacies transition from the shallower basin margin to the depocenter and the paleoenvironmental modifications driving the development of the branching selenite lithofacies, we carried out sedimentological, petrographic and geochemical analyses of the different types of gypsum lithofacies exposed along a margin-to-depocenter transect in the Piedmont Basin (NW Italy), corresponding to the northernmost Mediterranean subbasin. The results of this study suggest that the lateral and vertical distribution of gypsum lithofacies mostly reflects the position of an oxycline with respect to the seafloor. Bottom grown gypsum (cycles PLG1–PLG5) was deposited on a temporarily oxygenated seafloor above the oxycline in the marginal part of the basin. In the case of cumulate deposits (laminar gypsum and gypsiferous mudstones, cycles PLG6–PLG15), gypsum nucleated within the water column at the oxycline and was eventually deposited on an anoxic seafloor. The branching selenite lithofacies corresponds to a syngenetic product formed below the seafloor at the expenses of cumulate gypsum. Deposition of cumulate gypsum and formation of the branching selenite were controlled by the balance between the rate of sulfate reduction in bottom waters and nucleation of gypsum in the water column. The appearance of cumulate gypsum and the branching selenite from the 6th PLG cycle upwards reflects the intensification of water column stratification in response to increased isolation of the basin.

Microbial carbonate sequence architecture and depositional environments of Member IV of the Late Ediacaran Dengying Formation, Gaoshiti–Moxi area, Sichuan Basin, Southwest China

The recent discovery of the Anyue gas field, with huge reserves, has great potential for ultradeep gas production in carbonate reservoirs. Previous exploration and exploitation of Member IV of the Dengying Formation in the Gaoshiti–Moxi area deposited during the Late Ediacaran have provided useful insight into palaeo‐gas reservoirs, and geological significance of the Precambrian microbial carbonate strata. In this study, the depositional sequence architecture has been established and typical environments from Member IV of the Dengying Formation in the Gaoshiti–Moxi area have been reconstructed through detailed sedimentological analysis of cores, petrophysical data, well‐logging data and seismic interpretation. One third‐order sequence was identified, which is subdivided into seven fourth‐order sequences and 18–19 fifth‐order sequences. The third‐order sequence is bounded by denudation surfaces or onlap surfaces, and composed of transgressive system tracts and highstand system tracts. The fourth‐order sequences are bounded by instantaneously exposed punctuated surfaces or lithology and lithofacies transition surfaces, featured by specific lithological cycles, such as ‘lower microcrystalline dolomite, middle intraclastic dolomite and sparry thrombolite, and upper intraclastic dolomite’, ‘middle–lower sparry thrombolite and upper microcrystalline dolomite and sparry thrombolite’, ‘lower micritic thrombolite, middle micritic dolomite, and upper sparry thrombolite and laminites’, and ‘lower microcrystalline dolomite, middle intraclastic dolomite and argillaceous or micritic dolomite, and upper intraclastic dolomite’. The sequence characteristics reveal that periodic high‐frequency sea‐level change and palaeogeomorphology are controlling factors affecting sequence development, which lay a significant foundation for reservoir development. During the depositional period, a tidal flat on a restricted carbonate platform is well‐developed, allowing the identification of 16 microfacies, 13 microfacies associations, and 6 seismic facies. Inter‐lower tidal flats are widely developed with band‐shaped mound–bank complex, microbial mound, and shoal on the platform margin. On the inner platform, patchy microbial mounds, mound–flat combinations, and inter‐shoal/microbial mounds are developed in the Gaoshiti area, whereas patchy shoals and shoal–flat combinations are developed in the Moxi area. During the early depositional period of Member IV of the Dengying Formation, the microbial mounds and shoals were developed on the platform margin. The dolomitic flat and inter‐shoal/microbial mounds are mainly developed on the inner platform with little microbial mound and shoal deposition. During the late depositional period of Member IV of the Dengying Formation, large‐scale microbial mounds and shoals are sustainably and stably developed on the platform margin; enlarged dolomitic flat, patchy microbial mounds, and patchy shoals are predominantly developed in the platform interior with inter‐shoal/microbial mounds dying out.

Lacustrine paleoshoreline determination under established sequence stratigraphic framework and its controls on sand bodies and hydrocarbon accumulations

Abstract The lacustrine palaeoshoreline is the intersection between lake level and ancient land in geological time, and it marks the boundary between onshore deposits and subaqueous deposits. Identifying lacustrine paleoshorelines is the process of reconstructing past depositional environments. The study establishes the sequence stratigraphic framework of the Ordos Yanchang Formation based on a comprehensive analysis of seismic and well log data. The hierarchical lithological cycles is determined through Fourier transform and band-pass filtering analysis of Gramma Ray data. Subsequently, the lacustrine paleoshoreline is determined using the shoreline hierarchical control method under the established sequence stratigraphic framework, with additional evidence from cores, well logs and cutting and seismic sections. Lacustrine paleoshorelines has controlling effects on the sand bodies and hydrocarbon accumulations. For one thing, different types of paleoshoreline control the genesis of sand bodies, and the location of paleoshoreline in sag basin controls sand-body distribution. Furthermore, the migration of lacustrine paleoshoreline in sag basin indicates stacking patterns of sand bodies, and hydrocarbon accumulations.

Stratigraphy of early to middle Eocene hyperthermals from Possagno (Southern Alps, Italy) and comparison with global carbon isotope records

The late early Eocene to middle Eocene ~51–45 Million years ago (Ma) time interval in the middle bathyal, pelagic/hemipelagic succession of the western Tethys Possagno section in the Carcoselle quarry (Southern Alps, northeastern Italy), contains several episodes of negative carbon isotope excursions (CIEs) and concomitant dissolution of carbonates. Comparison with previously published carbon isotope records from deep-sea successions allows the identification of long-term trends and short-term events in our record, which provides a sound chemostratigraphic basis for correlation against the robust bio- and magnetostratigraphic scheme available for the studied succession and additional evidence of the global significance of the long-term trend and superposed perturbations tracked changes in δ13C values.Spectral analysis indicates that CIEs and associated lithological cycles are paced by orbital forcing, similar to what previously observed on the same interval in other deep-sea successions. The identification of astronomically forced geochemical cycles allows us to develop an orbitally tuned age model allowing to test the astrochronology of the ~56.0 Ma to ~47.5 Ma time interval.

Paleoclimatic cycles, sea level history and sequence stratigraphic elements in eocene–oligocene sediments of BIMOL-1 well northern Niger Delta Basin, Nigeria

Lithofacies succession and palynomorph data trends in BIMOL-1 well in the north-western Niger Delta Basin have been investigated in order to unravel paleoclimatic influence on paleo-sealevel change and facies evolution. Eight lithologic cycles composed of sand and shale were identified. Miospore speciation revealed forty two miospore form species and ten dinoflagellate cyst species. Miospore and dinocyst trends revealed six climate driven sea level cycles that influenced sedimentation and facies distribution and characteristics. Miospore age determination of the succession revealed L-Eocene to L-Oligocene age range. Nine wet and eight dry climate driven transgressive and regressive events were identified, corresponding to sea level rise and to sea level fall correspondingly. Dry climate occasioned continental progradation that generated thick sand intervals, while wet climate triggered sea level rise, generating thinner sand bodies as transgressive sand reservoirs. Seven high stands (HSTs), eight transgressive (TSTs) and eight lowstand (LSTs) systems tracts, distributed within nine sequences were identified. Candidate MFSs, the 50.0 Ma, 48.9 Ma, 46.1 Ma, 43.2 Ma, 41.0 Ma, 34.0 Ma MFSs were identified. Candidate SBs identified include the 50.7 Ma, 48.4 Ma, 47.2 Ma, 44.4 Ma, 42.7 Ma, 40.1 Ma and 32.4 Ma SBs. Early Rupelian sequence boundaries were identified. Erosion/non-deposition of the Priabonian and parts of the Bartonian stage were revealed that inferred erosion/non-deposition of about 7 Ma of sediments in the well area. A synthesis of results of the evaluated proxies revealed that Paleoclimate-driven sea level and paleovegetation trends acted as key facies generators in the well area.Keywords: Paleoclimateic cycles, climate driven sea level, Niger Delta, climatic index

Sequence subdivision and development characteristics of low-accommodation non-marine basins: a case study of the Yanchang Formation in the Ordos Basin

ABSTRACTThe third-order sequences of the Yanchang Formation in the Ordos Basin were determined by a comprehensive analysis of surface outcrops, core, logging and seismic data. Natural gamma curves were studied by the band-pass method. The hidden stratigraphic sequence and six lithological cycles were extracted. By examining sequence boundaries and lithological cycles, we identified six third-order (including SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6) and three second-order sequences in the Yanchang Formation. The sequence stratigraphy of the Ordos Basin was found to have the following special development characteristics: (1) the third-order sequence growths were totally stable, and the thickness of the Ordos Basin was relatively stable; (2) thick sand bodies did not form in lowstand systems because of underdeveloped slope breaks; (3) the third-order sequences had diverse features at different sedimentary periods; (4) the distribution of the depositional systems had various characteristics in different tectonic belts, and the entire sequence had the filling characteristics of multiprovenances and multicycles. The low-accommodation Ordos Basin in the Upper Triassic was limited by slow subsidence, gentle basement paleorelief, stable tectonic settings and sufficient sediment supply. The characteristics of the basin included stable developed sequences, limited preserved lowstand systems tracts, multiple depocentres and subsiding centres, development of many delta systems in the north and south of the basin controlled by the gradient of the basement paleorelief, and laterally stretching sediments. Results further indicated that the controlling factors of the third-order stratigraphic sequences of Yanchang Formation in the low-accommodation lacustrine Ordos Basin included sediment supply (provenance), climate and lake-level fluctuation. Overall, our case study indicated that the development characteristics and controlling factors of sequence stratigraphy in a low-accommodation lake basin provided a comprehensive method of establishing the sequence stratigraphic framework of a low-accommodation non-marine basin. Our results also predicted that the SQ3, SQ4 and SQ5 of the Qiaochuan and Zhidan areas favoured oil and gas prospecting of the Yanchang Formation.

The Barremian and Aptian stepwise development of the ‘Oceanic Anoxic Event 1a’ (OAE 1a) crisis: Integrated benthic and planktic high-resolution palaeoecology along the Gorgo a Cerbara stratotype section (Umbria–Marche Basin, Italy)

The Barremian and Aptian were times of global plate reconfiguration and profound changes in the ocean–climate system, culminating in the worldwide deposition of lower Aptian black shale layers (OAE 1a). Based on high-resolution lithostratigraphic and micropaleontological analyses, precursor conditions and timing of the anoxia are here reconstructed along a 33.06 m thick section in the proposed Barremian/Aptian boundary GSSP stratotype at Gorgo a Cerbara (Umbria–Marche Basin, Italy). A non-uniform history of benthic foraminiferal diversification is interrupted by the Selli Level (= OAE 1a) and by three turnover points for both benthic and planktic organisms, each highlighted by prominent breaks in lithology and outcrop morphology. The first two points, at ~ 1.1 Myr and ~ 20–50 kyr prior to the onset of the OAE 1a, correspond respectively to the nannoconid (= bathypelagic calcareous nannofossils) ‘decline’ and ‘crisis’ events, and separate: (a) pelagic limestone/chert-claystone lithological cycles of the uppermost Maiolica Formation, showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic Barremian benthic and planktic assemblage, dominated by k-selected nannoconids and Rhizammina; (b) greenish-grey cherty marls of the lowermost Marne a Fucoidi Formation, associated to declining sedimentation rates and eutrophic early Aptian assemblages dominated by radiolarians and lituolid benthic foraminifera; and (c) anoxic radiolarites and shales of the carbonate-free Selli Level. High magnitude and frequency assemblage fluctuations occur between the nannoconid crisis (~ 25 cm below the lowermost OAE 1a black shale layer) and the Selli Level base, with suggestions of bottom-water acidification. At ~ 800 kyr after the OAE 1a end, a third turnover point in the basal reddish member of the Marne a Fucoidi highlights ‘middle’ Aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera (Hedbergella spp.) and appearances of new benthic species, marking a shift towards better oxygenation. OAE 1a oxygen and pelagic carbonate factory crises are here suggested to have been step-wise and linked to global warming conditions.

The transition on North America from the warm humid Pliocene to the glaciated Quaternary traced by eolian dust deposition at a benchmark North Atlantic Ocean drill site

We present Plio-Pleistocene records of sediment color, %CaCO3, foraminifer fragmentation, benthic carbon isotopes (δ13C) and radiogenic isotopes (Sr, Nd, Pb) of the terrigenous component from IODP Site U1313, a reoccupation of benchmark subtropical North Atlantic Ocean DSDP Site 607. We show that (inter)glacial cycles in sediment color and %CaCO3 pre-date major northern hemisphere glaciation and are unambiguously and consistently correlated to benthic oxygen isotopes back to 3.3 million years ago (Ma) and intermittently so probably back to the Miocene/Pliocene boundary. We show these lithological cycles to be driven by enhanced glacial fluxes of terrigenous material (eolian dust), not carbonate dissolution (the classic interpretation). Our radiogenic isotope data indicate a North American source for this dust (∼3.3–2.4 Ma) in keeping with the interpreted source of terrestrial plant wax-derived biomarkers deposited at Site U1313. Yet our data indicate a mid latitude provenance regardless of (inter)glacial state, a finding that is inconsistent with the biomarker-inferred importance of glaciogenic mechanisms of dust production and transport. Moreover, we find that the relation between the biomarker and lithogenic components of dust accumulation is distinctly non-linear. Both records show a jump in glacial rates of accumulation from Marine Isotope Stage, MIS, G6 (2.72 Ma) onwards but the amplitude of this signal is about 3–8 times greater for biomarkers than for dust and particularly extreme during MIS 100 (2.52 Ma). We conclude that North America shifted abruptly to a distinctly more arid glacial regime from MIS G6, but major shifts in glacial North American vegetation biomes and regional wind fields (exacerbated by the growth of a large Laurentide Ice Sheet during MIS 100) likely explain amplification of this signal in the biomarker records. Our findings are consistent with wetter-than-modern reconstructions of North American continental climate under the warm high CO2 conditions of the Early Pliocene but contrast with most model predictions for the response of the hydrological cycle to anthropogenic warming over the coming 50 years (poleward expansion of the subtropical dry zones).

Lower Maastrichtian cyclostratigraphy of the Bidart section (Basque Country, SW France): A remarkable record of precessional forcing

Cyclostratigraphic analysis of the Maastrichtian limestone–marl alternations of Bidart (SW France) allows the hypothesis of orbital control on lithological cycles to be evaluated. Magnetic susceptibility (MS), oxygen and carbon isotope measurements, sampled at a high resolution, are analyzed using various cyclostratigraphic tools. A statistically significant orbital signal is detected, with a remarkable record of the precession corresponding to the limestone–marl couplets. This well expressed orbital forcing allows the building of a relative cyclostratigraphic time scale for the MS and δ13C records based on the 100kyr eccentricity cycle. The total duration of the section is estimated at 1.44±0.22Myr. Correlation based on calcareous nannofossil biostratigraphy and comparison of the scaled Bidart δ13C record to the astronomically calibrated δ13C signal of ODP Hole 762C shows that the studied section extends from −71.5 to −70Ma, covering the upper part of Chron C32n.1n and 2/3 of Chron C31r. Oxygen isotope data suggest a 2°C cooling of sea-surface temperatures during the studied interval. When placed on the long-term δ18O trend of the Bidart section, this interval is here recognized as the onset of the early Maastrichtian cooling event. With its excellent record of the precessional forcing, the Bidart section, along with other sections of the Basque Country, is a useful tool for the refinement of the Maastrichtian timescale.

Precession‐scale cyclicity in the fluvial lower Eocene Willwood Formation of the Bighorn Basin, Wyoming (USA)

AbstractLittle is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change.

Dynamic sedimentary conditions during periods of enhanced sequestration of organic carbon in the central southern Tethys at the onset of the Cenozoic global cooling

Stagnant bottom-water conditions (e.g., low and stable redox potential, long-water residence time) is an assumption commonly used to explain the preservation and burial of high amounts of organic carbon (Corg) in marine sediments. Rather than stagnant conditions, the evidence presented here from north-central Tunisia supports dynamic conditions during formation of variably Corg-rich, outermost shelf carbonates of the early–middle Eocene. The dynamic conditions are inferred by the deposition of four distinct lithofacies in this outermost shelf setting. Shedding of carbonate (i.e., mud and fragmented bioclasts) from the shallower source areas controlled the distribution of all lithofacies, with higher amounts of transported benthic debris occurring in the most proximal lithofacies and vice versa. This carbonate shedding also controlled the deposition of three orders of lithological cycles, from limestone/marly limestone couplets grading to cycles made up of groups of couplets. Bottom-water redox potential varied in intensity throughout this depositional setting, with moderate oxygen depletion (suboxic conditions) in the southern sector of north-central Tunisia and much higher oxygenation in the northern area. Evidence for suboxic bottom waters in the southern sector (higher Corg contents) is provided by higher trace metal (Cu, Ni, Zn, Cr, Mo, U and V) enrichments than in the northern area. Regionally heterogeneous primary productivity of surface waters is suggested to have caused a higher Corg burial flux in the southern sector compared to the north, a situation interpreted to have been related to varying upwelling patterns due to the effects of regional palaeogeography and the dominant wind patterns.The deposition of the studied Corg-rich carbonates spanned part of the calcareous nannofossil Zones NP13 to NP14 (~50–48m.y. ago) and coincided with the initiation of the Cenozoic global cooling subsequent to the early Eocene climatic optimum (EECO) (~52–50m.y. ago). An implication is that an increased Corg burial in north-central Tunisia could have been part of a major event sequestering atmospheric CO2 in marine sediments that caused climatic cooling immediately after the EECO. This implication, however, is difficult to reconcile with the data available elsewhere. Other than our Tunisian carbonates, major Corg sequestration in marine sediments of the early–middle Eocene transition are poorly documented globally. If this scenario is confirmed, other negative feedbacks, such as enhanced continental weathering, increased terrestrial carbon stock, decreased CO2 outgassing and/or changes in ocean circulation, would have been more influential to the onset of the Cenozoic global cooling.

Is chemical zonation in plutonic rocks driven by changes in source magma composition or shallow-crustal differentiation?

Lithologic and magnetic-susceptibility mapping of the western Half Dome Granodiorite of the Tuolumne Intrusive Suite of California reveals seven km-scale lithologic cycles, each of which is bounded by a sharp west-dipping contact that is subparallel to the external contact of the pluton. Crosscutting relations indicate that the cycles become younger to the east. Each cycle is inferred to have been a zone of partial melt in which an eastern melt-depleted base grades westward to a melt-rich top now preserved as a leucocratic facies of the Half Dome Granodiorite. Sharp contacts between cycles may record freezing episodes when the rate of heat input into the growing pluton dropped below that required to maintain interstitial melt. Thus, the interstitial melt zone migrated with time and its size at any given time need not have differed greatly from the ∼1 km thickness of the cycles. Cycles occur on the outer, older margins of the suite, and disappear toward the interior, younger intrusions. Inward disappearance of cycles likely reflects thermal maturation of the system such that melt was continuously present until the final migration of the solidus through the intrusive suite. Although the cycles span the compositional range from granodiorite to leucogranite, trace-element trends preserved in the cycles differ dramatically from those of both the Tuolumne Intrusive Suite and other Cretaceous zoned plutons of the eastern Sierra Nevada batholith. We suggest that (1) the compositional variations of the intrusive suite and the batholith reflect a signal from the source of the magmas, and (2) the geochemistry within the km-scale cycles reflects in situ crystal/liquid separation.

Milankovitch and sub-Milankovitch cycles of the early Triassic Daye Formation, South China and their geochronological and paleoclimatic implications

The mass extinction at the end of Permian was followed by a prolonged recovery process with multiple phases of devastation–restoration of marine ecosystems in Early Triassic. The time framework for the Early Triassic geological, biological and geochemical events is traditionally established by conodont biostratigraphy, but the absolute duration of conodont biozones are not well constrained. In this study, a rock magnetic cyclostratigraphy, based on high-resolution analysis (2440 samples) of magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) intensity variations, was developed for the 55.1-m-thick, Early Triassic Lower Daye Formation at the Daxiakou section, Hubei province in South China. The Lower Daye Formation shows exceptionally well-preserved lithological cycles with alternating thinly-bedded mudstone, marls and limestone, which are closely tracked by the MS and ARM variations. Power spectral, wavelet and amplitude modulation (AM) analyses of the ARM and MS series reveal strong evidence for the presence of Milankovitch to sub-Milankovitch frequencies dominated by precession index signal and 4–5ka cycles. Cycles expressed by variations in MS and ARM were likely controlled by changes in the input of fine-grained detrital magnetite, which in turn may have been driven by astronomically induced changes in monsoon intensity in the equatorial eastern Paleotethys during the Early Triassic greenhouse period.On the basis of the 100-ka tuning results, the astronomically constrained duration of the Induan stage is 1.16Ma, with the Griesbachian and Dienerian substages of 490ka and 670ka, respectively. The new astronomical time scale also provides time constraints for the conodont and bivalve biozones and the carbonate carbon isotope (δ13C) records of the Lower Triassic Daye Formation. Time constraints for the conodont biozones include 34ka for Hindeodus parvus, 24ka for Isarcicella staschei–I. isarcica, 366ka for Neogondolella planata–Ng. carinata, 66ka for Neogondolella discreta, 255ka for Neospathodus kummeli and 415ka for Neospathodus dieneri. The duration for the negative δ13C shift from high δ13C values near the Indun/Olenekian boundary to the lowest value in the Early Smithian is estimated as 430ka. Global comparison indicates that Milankovitch and 4–5ka sub-Milankovitch forcing depositional rhythms may have been common in tropical and sub-tropical carbonate platforms during the Early and Middle Triassic time. The ultimate control on the 4–5ka cycles may have been millennial-scale fluctuations in solar insolation.

Paleosols in Late Moscovian (Carboniferous) Marine Carbonates of the East European Craton Revealing "Great Calcimagnesian Plain" Paleolandscapes

Abstract Late Paleozoic strata commonly include lithologic cycles (“cyclothems”), driven by sea-level fluctuations that repeatedly flooded and exposed broad expanses of epeiric shelves. Such successions, commonly with multiple unconformities, are well documented in Permo-Pennsylvanian sedimentary successions, but remain less understood in the East European Craton (EEC), part of the Euramerican continental shelf rim. This study documents unconformities and paleosols from the Podolskian to Myachkovian (middle Pennsylvanian) epeiric-carbonate succession of the EEC and explores the non-actualistic soil types, landscapes, and the character and density of vegetation in this Pennsylvanian calcimagnesian dryland environment. Thin (mostly < 1 m) paleosols developed at seven successive unconformities consist of an upper terrestrial clay layer or “topclay”, a crust of beta calcrete beneath the topclay, and a weakly karsted substrate limestone with rare rhizocretions. Solution pits and small (< 3 m deep) karst sinkholes are rare. Nontruncated topclays commonly consist of two layers: in situ clayey epipedons and resedimented terrestrial clays. The latter are interpreted as playa sediments. The Myachkovian unconformities of the northernmost localities studied, near Arkhangelsk, lack beta calcrete and root traces, and the limestone beneath these unconformities is sepiolitized. In Podolskian paleosols of the southern Moscow Basin, the topclays are predominantly palygorskitic. The Myachkovian paleosol clays are smectitic–illitic with occasional chlorite admixture. The majority of late Moscovian paleosols surveyed formally conform to lithic (rendollic) haplocalcids within the aridisol class or rendzic calcic leptosols. A detailed study of a representative Podolskian paleo-pedon reveals development of shallow soil carbonate, low alumina/bases and Ba/Sr ratios, enhanced Mn and Sr, presence of soil gypsum and opal, and a characteristic peak in magnetic susceptibility, all suggesting a semiarid to arid pedogenic environment. The palygorskite clay of this paleo-pedon retains 1.1–1.5% of connate organic matter in the form of covalently bound organomineral complexes, and is fulvate-dominated resembling organic matter from extant dryland soils. The palygorskitic composition of Podolskian topclays and shallow (< 30 cm) pedogenic carbonates is interpreted to reflect hot, well-drained semidesert conditions with precipitation less than 300 mm/yr. The transition from palygorskitic to smectitic soil clays across the Podolskian–Myachkovian boundary, apparently deeper soil carbonate, and an increase in the degree of karstification record a general increase in humidity such that annual precipitation may have exceeded 300–400 mm. The available data reconstruct the “great calcimagnesian plain” landscapes formed during low sea level, supposedly glacial phases, across tens of thousands of square kilometers of the EEC. This non-actualistic system apparently lacked a fluvial network and was covered by uniform soils and playas. The primary productivity of this early (310–305 Ma old) dryland ecosystem inferred from high TOC and rhizogenic beta calcretes was not markedly different from present-day steppe or savanna grasslands and semideserts. In the northernmost outcrops surveyed, near Arkhangelsk, drier conditions probably limited colonization by higher plants. The available data open a “hidden chapter” in the Paleozoic evolution of terrestrial ecosystems which is not recorded by paleobotany and urges modifications in the concept of low-productivity character of pre-Miocene dry landscapes.

Platinum-group minerals and tellurides from the PGE-bearing Xinjie layered intrusion in the Emeishan Large Igneous Province, SW China

The Xinjie layered intrusion is one of a number of major ultramafic-mafic bodies hosting Fe-Ti-V deposits and Cu-Ni-PGE sulfide deposits in the Pan-Xi (Panzhihua-Xichang) area of the Sichuan Province, SW China. The Xinjie ultramafic-mafic layered intrusion, genetically related to the Permian plume-related Emeishan flood basalts, consists of three lithological cycles, each representing a sequence from ultramafic to mafic-felsic composition. The basal part of the intrusion is composed of three lithological units, namely, the Marginal Unit (MU), Peridotite Unit (PeU) and Pyroxenite Unit (PyU). In the present study, three major PGE-mineralised Cu-Ni sulfide layers were discovered within the Marginal and Pyroxenite Units. The major base-metal sulfides (BMS) comprise chalcopyrite, pyrrhotite, and pentlandite. Detailed microscopic and microprobe analyses revealed the presence of the sperrylite and Pd-Pt-Bi-Te minerals (merenskyite, moncheite, and michenerite). These PGMs are commonly associated with the BMS, or magnetite coexisting with BMS in the PGE-enriched layers. The 1:1 substitution between Pt and Pd, as well as between Te and Bi, confirms the complete solid-solution series between moncheite and merenskyite. The textural association of the PGMs with BMS and Fe-Ti oxides (magnetite) suggests that the PGMs may have crystallised slightly later than the hosting magnetite and BMS. The formation of magnetite may have played an important role in producing the sulfur-saturated melt and the PGEs thus concentrated in the sulfide liquid during the crystallisation history. It is therefore suggested that the Cu-Ni-PGE-bearing layers in the basal part of the Xinjie intrusion were generated by magma evolution processes.

Marine microplankton and calcareous nannofossil palaeoecology of the Toarcian stratotype

In the Bifrons to Aalensis Ammonite Zones of the Vrines section (Toarcian stratotype), woody phytoclasts, nonsaccate pollen grains, and marine assemblages (dinoflagellate cysts, acritarchs, and foraminiferal linings) dominate the palynofacies. The dinoflagellate cyst assemblage is cosmopolitan with minor Boreal influences, characterized by relatively high quantities of Micrhystridium, Baltisphaeridium, Mendicodinium spinosum, Nannoceratopsis, and the Parvocysta suite, dominating in turn the marine assemblages. Marine assemblage compositions, both dinoflagellate cysts and acritarchs, and calcareous nannofossil abundances are different in marl and limestone lithotypes of the Vrines section. Calcareous nannofossils are generally more abundant in marls than in limestones, they display however a cyclic pattern of semiquantitative abundances in phase with lithological cycles. Although a diagenetic overprint cannot be completely excluded to explain such a difference, it seems likely that these dissimilarities are in part primary, the results of variations in terms of proximality–distality, and climatic fluctuations. A mean duration of 117.6 Kyr per marl–limestone alternation, and the stacking of four marl–limestone alternations for 470.6 Kyr, suggest a control by the Earth's two orbital eccentricity cycles. It is likely that the palaeoenvironmental conditions, which influenced the formation of marl–limestone alternations, also controlled the variations in marine phytoplankton assemblages.

Quantification of deep-time orbital forcing by average spectral misfit

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon well-constrained radiometric age data.