Grand Cycles Research Articles

Paleoenvironmental and Carbon-Oxygen Isotope Record of Middle Cambrian Carbonates (La Laja Formation) in the Argentine Precordillera

Abstract The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the lower Paleozoic carbonate platform of the Argentina Precordillera. This is a key unit regarding the hypothesis of the Precordillera as a Laurentia-derived allochthonous terrane currently located in the south-central Andes. According to the faunal affinity and stratigraphic development of the thick Cambrian carbonate bank, the Argentine Precordillera would have been attached to Laurentia. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow subtidal unit is internally arranged into several Grand Cycles indicating a complex environmental mosaic, probably with local depocenters related to variable subsidence. This unit records the stabilization of the rifted margin of the Precordillera terrane, prior to the broadening of the carbonate sedimentation during the passive-margin drifting stage. A high-resolution δ13C and δ18O isotope study, in concert with a detailed paleoenvironmental analysis, was carried out to better understand both environmental and chronostratigraphic evolution of the La Laja Formation. Three δ13C positive excursions were recorded; the first one at the Glossopleura biozone within the Soldano Member, the second beginning at the base of the Rivadavia Member, and the third during deposition of the Las Torres Member. Comparisons with other Middle Cambrian curves, in the Precordillera and elsewhere (Rocky Mountains and Great Basin, U.S.A., the western Hunan Province in south China, and the Amadeus, Georgina, and Daly basins in Australia) suggest a global control on these excursions. Mechanisms to produce these positive excursions could be related to high bio-productivity and increased burial of Corg (organic carbon) produced by high nutrient influx to the ocean associated with a relative sea-level fall. Local environmental controls could have in part altered the original isotopic signal.

The second twentieth century: how the information revolution shapes business, states, and nations

Jean-Jacques Rosa offers an analysis of the grand cycle in social organization of the twentieth century, showing how the transformation in communication and information technology has led to the downfall of the old political and corporate hierarchies. He explains how today's explosion of freely available information is fueling the democratic free-market revolution and reveals its universal contemporary consequences.

Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman)

Oman records in its Neoproterozoic Nafun Group (Huqf Supergroup) an essentially complete, carbonate-rich Ediacaran succession. The Nafun Group overlies the presumed Marinoan rift-related Fiq Member (ca. 635 Ma) and ends just below the Precambrian–Cambrian boundary (542 Ma). Tectonic subsidence caused by thermal contraction following Fiq-aged rifting allowed the preservation of about 1 km of postrift stratigraphy, with no major stratigraphic breaks. The Nafun Group above the Marinoan cap carbonate (Hadash Formation) represents two siliciclastic to carbonate ‘grand cycles’, both initiated by significant transgressions: these cycles comprise the Masirah Bay/Khufai formations and the Shuram/Buah formations. The Khufai–Shuram boundary is associated with the start of a major carbon isotope perturbation. The uppermost ramp carbonates of the Khufai Formation record a smooth decrease in δ 13C from about +4‰ to values around 0, followed by two descending steps across which values plunge to a nadir of −12‰ in the overlying red siltstones and shales interbedded with thin limestones of the Shuram Formation. This fall in isotopic values is temporally rapid and coincident in both shallow and deep-water sections in the time span of a single parasequence. The δ 13C nadir is then followed by 50 million years of monotonic recovery. The ‘Shuram shift’ represents the largest δ 13C inorganic carbon negative excursion in Earth history. Although the snowball Earth theory links periods of depleted carbon isotopic ratios with periods of global glaciation, the non-glaciated context of the Shuram Formation suggests that the causal relationship between global glaciation and negative carbon isotopic excursions is non-unique. Although the precise mechanism driving this major perturbation of the carbon cycle remains enigmatic, the long-term remineralization of an isotopically depleted organic carbon reservoir in ocean water is a promising candidate.

A sedimentary prelude to Marinoan glaciation, Cryogenian (Middle Neoproterozoic) Keele Formation, Mackenzie Mountains, northwestern Canada

Continental margin deposition between the Sturtian- and Marinoan-aged global glacial periods is represented in NW Canada by a siliciclastic-to-carbonate grand cycle. The Keele Formation, a 300–600 m thick carbonate and siliciclastic deposit, comprises the upper part of this interglacial cycle. Sedimentary facies in the lower, carbonate-dominated member of the Keele span a broad spectrum of depositional environments, from below storm-wave-base to peritidal to continental, and are disposed across a relatively narrow paleogeographic belt. Lateral arrangement of facies and the gradual stratigraphic transitions between them, suggest deposition in a ramp setting; local slump structures and occasional debrites in deep-water facies imply that the ramp was distally steepened. Fluvial facies are concentrated at a single locality, indicating that terrigenous-clastic sediment was derived from a point source or sources. Most strata of the carbonates are “Bahamian” in character and similar to Phanerozoic warm-water carbonates in that they contain abundant calcite ooids of interpreted original aragonitic mineralogy and have well-developed muddy peritidal facies. Nevertheless, close juxtaposition of deep- and shallow-water facies in Keele rocks indicates high-amplitude sea-level change during deposition, on the order of ∼100 m, suggesting accumulation in a climatic icehouse period. This interpretation implies that although the globe possessed polar and high-altitude glaciers at this time, sea-surface temperature, at least at tropical latitudes, warmed dramatically between Neoproterozoic glacial episodes. The onset of Marinoan glaciation here is heralded by an abrupt shift to nearshore marine siliciclastic sedimentary rocks. These deposits form the upper member of the Keele Formation (informally called the Keele clastic wedge) and directly underlie Ice Brook Formation diamictites. Carbonates are rare and are restricted to mudstones and flat-pebble conglomerates that lack any indication of tropical conditions.

Facies and sequence stratigraphy of two Cambrian grand cycles: implications for Cambrian sea level and origin of grand cycles

ABSTRACT The Pika, Arctomys and Waterfowl formations comprise two cycles as originally defined by Aitken (1966, 1978, 1981). A detailed across-strike facies stratigraphic reconstruction based on measured outcrop sections and subsurface data reveal that the Pika and Waterfowl Formations consist of five transgressive-regressive sequences of subtidal facies and tidal flat facies each up to 100 m thick and some 105 to 106 years duration. The bulk of the Arctomys Formation consists of terrestrial to marginal marine playa deposits, correlates with an unconformity in the adjacent subsurface, and was deposited during a sea level low stand. The upper third of the Arctomys Formation records a single minor marine transgressive-regressive cycle. Transgressive facies can extend eastward onto the craton up to 1000 km from the platform to slope transition and record local relative increases in sea level of various magnitudes. Maximum transgressive or regressive phases do not necessarily correspond with the bottoms or tops of grand cycles. A sea level curve derived from our facies stratigraphy is different than the curve proposed by Bond et al. (1989) from R2 analysis, and also different to the sealevel curve of Montanez and Osleger (1993) for the southern Great Basin. End_Page 478------------------------

Effects of Grand-Cycle Cessation on the Diagenesis of Upper Cambrian Carbonate Deposits in the Southern Appalachians, U.S.A.

ABSTRACT The vertical transition from the mainly subtidal alternating shale and carbonate units (or grand cycles) of the Conasauga Group (Middle to Upper Cambrian) to the peritidal dolostone of the Knox Group (Upper Cambrian to Lower Ordovician) marks a major change in the early Paleozoic passive-margin sedimentation of the southern Appalachians. The grand cycles represent an interplay between intrashelf shale basin and carbonate-platform deposition. The end of grand-cycle deposition occurred in response to carbonate platform progradation over the infilled intrashelf basin and is associated with a prominent change in diagenetic patterns observed in the uppermost Conasauga Group strata--the Maynardville Formation. The Maynardville records upward shallowing from subtidal shale and limestone into peritidal dolostone. This change from a subtidal into a peritidal depositional regime influenced the early diagenesis of the Maynardville and, in conjunction with the changes in the regional facies distribution at the end of grand-cycle deposition, extended its influence upon the burial diagenesis of these deposits. The subtidal deposits of the Maynardville contain a variety of calcite cements. Dolomite is not abundant in these deposits, where it occurs mainly as a fine-crystalline ferroan phase associated with argillaceous layers and pressure-dissolution features. The local source for this dolomite during burial was provided by expulsion of pore fluids from interbedded shale deposits, diagenesis of clay minerals, and pressure dissolution of carbonates. Ferruginous coatings on hardgrounds in the subtidal deposits provided an additional local source of iron for the formation of replacement saddle dolomite. The subtidal deposits also contain saddle dolomite cement associated with Mississippi Valley-type (MVT) minerals, which indicate the involvement of externally derived fluids during late burial diagenesis of these strata. The peritidal deposits, on the other hand, are extensively dolomitized. Fine-crystalline dolomite most likely represents early diagenetic replacement of tidal-flat carbonate sediment. Coarse-crystalline replacement dolomite formed during burial by recrystallization of early dolomite and dolomitization by warm burial brines. Fenestrae, desiccation cracks, and evaporite-dissolution voids are occluded with dolomite cement, which is commonly complexly zoned. Zoned dolomite precipitated during shallow to intermediate burial and is postdated by saddle dolomite cement. Saddle dolomite in pore centers, dissolutional voids, and tectonic fractures formed during late burial from warm fluids associated with the migration of MVT brines. The effects of changes in the depositional setting on diagenesis are reflected in similar early diagenesis between the subtidal Maynardville and the underlying Conasauga Group carbonate deposits, and between the peritidal deposits of the upper Maynardville and the rest of the overlying Knox Group. The depositional environments and early diagenesis also affected the burial diagenesis. The changes in regional facies distribution, caused by carbonate platform progradation at the end of grand-cycle deposition, resulted in a decrease in the proportion of siliciclastic components. This decrease was accompanied by a decline in the amount of ferroan carbonate in the peritidal deposits. The Maynardville is conformably overlain by carbonate deposits of the Knox Group. This allowed the Maynardville-Knox succession to behave as a single hydrologic unit for the migration of burial fluids in response to the formation of the Ordovician foreland (Sevier) basin to the southeast during the conversion from a passive into a convergent continental margin. The Nolichucky Shale separates the Maynardville-Knox succession from the carbonate deposits of the underlying Conasauga Group, which were, in contrast, affected by the migration of burial fluids from the Cambrian intrashelf shale basin to the west.

2 1/2 Proposals to Save Social Security

1. Introduction In the grand cycle seems to govern the security systems of the nations of the world, it may once again be the United States' turn to move. Chile famously has adopted compulsory individual retirement accounts, Australia has moved to a kind of compulsory 401(k) superannuation plan, Switzerland to a system of compulsory occupational pensions, and Sweden to a system of compulsory notional accounts, where individual contributions are recorded by the government and accumulate as individual entitlements do not correspond to private sector investments. Canada, facing roughly the same type of shortfall as the United States, simply raised payroll taxes with a plan for direct investment by the pension authority in the stock market has not yet been broadly controversial. Interesting attempts (e.g., Robson 1996) to advance Chilean-type solutions did not find much political traction. But in the United States, the trustees of the major public pension plan, Old-Age and Survivors Insurance and Disability Insurance (OASDI), have been forecasting for about 15 years (essentially since immediately after the implementation of the last reforms were to save Social Security) the plan as structured could not meet its obligations through a 75-year forecast window. But there has been no legislative correction. There certainly have been a number of proposals: indeed the 1994-1996 Advisory Council on Social Security came up with three. Here we review a newer proposal presented in book form: The Real Deal: The History and Future of Social Security by Sylvester J. Schieber and John B. Shoven. We also review a second book True Security: Rethinking American Social Insurance by Michael J. Graetz and Jerry L. Mashaw. This book is also about saving security, but the problem is thought of as not so much about OASDI financial projections but instead (p. 5) that the long-term political support for insurance could come unglued unless the effectiveness of current insurance, broadly defined, is improved. Finally, a third book, Social Security: The Phony Crisis by Dean Baker and Mark Weisbrot, focuses on discussions of OASDI solvency (although it discusses aspects of health insurance as well) but argues current action to restructure OASDI is not required to save it. Hence we have only counted this in our title as one-half a proposal: no pejorative connotation is intended. We begin in section 2 with a brief annotated outline of each of the three books. We put most of our emphasis on OASDI issues, as do Schieber-Shoven and Baker-Weisbrot; as noted Graetz-Mashaw covers a very wide range of insurance programs. In section 3 we make some more general observations. A central point is the reminder a positive economics approach cannot choose among these or any proposals: to a large extent these proposals are different simply because their authors have different values. Section 4 concludes. 2. Overviews The Real Deal: Schieber and Shoven This is a substantial and worthwhile book. Schieber and Shoven have devoted several chapters to a history of United States Social Security from its inception. They then discuss current problems in depth, provide a discussion of a number of recent reform proposals, and give a significant proposal of their own. According to their history, right from the beginning the central issue facing OASDI was to fund or not to fund, is, whether there would be a fund to cover the public pension promises being made to current workers (and hence a long delay before any significant benefits would be paid) or whether instead there would be a reliance on subsequent generations to cover the promises in the future (and hence transfers could begin relatively quickly). The initial plan signed into law by Roosevelt in 1935 was a funded one. But by the 1939 amendments, this was changed to a largely unfunded plan by a coalition of liberals who wanted to alleviate current poverty among the elderly and conservatives who argued the accumulation of a trust fund would lead to untoward government intervention in the economy through social investment. …

Carbonate Deposition and Sequence Stratigraphy of the Terminal Cambrian Grand Cycle in the Southern Appalachians, U.S.A.

ABSTRACT The cessation of shale and carbonate deposition of the Conasauga Group grand cycles (Middle to Upper Cambrian) and the establishment of the widespread peritidal carbonate deposition of the Knox Group (Upper Cambrian to Lower Ordovician) represent a prominent change in sedimentation along the early Paleozoic passive continental margin in the southern Appalachians. To evaluate the causes for this change, this study focuses on the Maynardville Formation, which is the uppermost carbonate unit of the Conasauga Group. The Maynardville consists of: (1) a lower subtidal facies succession, which is underlain by the Nolichucky Shale and resembles the rest of the Conasauga Group carbonate deposits; and (2) an upper peritidal facies succession, which is conformably overlain by similar peritidal deposits of the Copper Ridge Dolomite (Knox Group). Deposition of shale and subtidal carbonate took place in deep-ramp (Nolichucky) to shallow-ramp, subtidal-shoal, and lagoonal settings (lower Maynardville). The carbonate ramp was westward sloping toward the Conasauga intrashelf shale basin. To the east, the ramp was linked to a broad, semiarid carbonate tidal flat encompassing a variety of peritidal environments (upper Maynardville and Copper Ridge). The Maynardville is a shallowing-upward succession that formed by carbonate platform aggradation and basinward progradation. The transition between the Maynardville and the Copper Ridge Dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand-size detritus. This interval is interpreted as a sequence boundary correlative with the craton-wide late Steptoean (Dresbachian-Franconian or Sauk II-Sauk III) unconformity. The change from a Conasauga to a Knox depositional style may be related to distinct stages in passive-margin evolution. The end of grand-cycle deposition in the early Late Cambrian is coincident with the cessation of tectonic activity along extensional features (an intracratonic graben and other fault systems), and marks the transition into a mature-passive-margin setting. The mature margin was characterized by decreased rates of thermal subsidence, which, coupled with the infilling of the Conasauga intrashelf basin, favored shallow-water carbonate deposition. The final stabilization of the margin is reflected in the deposition of the thick peritidal carbonate strata of the Knox Group.

Past and Present in Dryden's "Fables"

a more general revival of interest in political history on the part of literary scholars. Recent studies have pointed out many levels of partisan expression in the works of Dryden's final decade, when the old poet, cast out of the laureate's post and living by his wits, contrived to communicate his Jacobite principles by devices ranging from subtle mistranslation to overt defiance.' I have embraced this mode of interpretation by tracing the political aspects of Dryden's translations of Juvenal and Persius and by arguing for the presence of Jacobite innuendo in King Arthur.2 My topic here, however, is history in a larger sense. Always aware of parallels between past and present events, which he dramatized in the biblical and mythical allusions of all of his court panegyrics and in the sustained typological narrative of Absalom andAchitophel, Dryden in his final years took comfort in imagining history moving in grand cycles. Without necessarily claiming deep belief in typology or in the cyclical nature of history, he selected for translation and included in his original poems a number of passages in which the end of a life, a century, or an age resembles its beginning. In Fables, these patterns include the poet's own life and poetic career, the politically tumultuous century just coming to its end, and the whole of Western literary history.

Carbonate Deposition and Sequence Stratigraphy of the Terminal Cambrian Grand Cycle in the Southern Appalachians, U.S.A.

Carbonate Deposition and Sequence Stratigraphy of the Terminal Cambrian Grand Cycle in the Southern Appalachians, U.S.A.

Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time–scale and the long–term behaviour of the planets

During the Late Triassic and Early Jurassic the Newark rift basin of the northeastern US accumulated in excess 5 km of continental, mostly lacustrine strata that show a profound cyclicity previousl...

Solar torsional oscillations and the grand activity cycle

Some consequences of a nonlinear coupling between magnetic field and rotation are studied within a solar type 2D dynamo model for a spherical convective shell. The magnetic feedback on the rotation law produces two main effects. First, the torsional oscillations are excited. Second, a long-term amplitude modulation of the dynamo cycles is produced. The latter may be identified with the grand cycle of solar activity. The dynamo model seems to be in accord with the phase relations between the torsional and magnetic activity oscillations observed in the 11-year cycle as well as in the 55-year grand cycle. It, however, fails to reproduce the observationally suggested global decreasing trend in the equatorial rotation rate.

A Late Cambrian positive carbon-isotope excursion in the Southern Appalachians; relation to biostratigraphy, sequence stratigraphy, environments of deposition, and diagenesis

ABSTRACT A positive carbon-isotope excursion is recorded within the Upper Cambrian sedimentary succession in the southern Appalachians that consists of the Nolichucky Shale, the Maynardville Formation, and the Copper Ridge Dolomite. The lower part of the succession contains Aphelaspis zone fauna (Early Steptoean). The extensively dolomitized and poorly fossiliferous nature of the upper part of the succession precludes any detailed biostratigraphic determinations. Correlation with similar positive carbon-isotope excursions in coeval successions elsewhere suggests that this excursion represents a perturbation in the global cycling of carbon. Comparison of excursions at different localities in North America provides a means for the application of carbon-isotope stratigraphy. In the southern Appalachians the excursion started during deposition of the upper Nolichucky Shale. Maximum 13C values (4 to 5o/oo PDB) are associated with the conformable interval at the Maynardville/Copper Ridge Dolomite transition, which has been interpreted as a correlative conformity in sequence-stratigraphic terms. The excursion ended during deposition of the lower Copper Ridge Dolomite. In western North America the excursion started at the base of the Pterocephaliid Biomere (near the base of the Aphelaspis Zone). This well-documented excursion ended prior to the end of the Pterocephaliid Biomere, with the maximum excursion at the Sauk II/Sauk III unconformity. This supports the correlation between Late Steptoean (Dresbachian/Franconian) sea-level fall and the sequence boundary at the end of Cambrian Grand Cycle deposition in the southern Appalachians. The cause of this carbon-isotope excursion remains unclear. The excursion most likely reflects the enhanced burial of organic carbon promoted by ocean stratification, a warm nonglacial climate, and a sea-level maximum during the early Late Cambrian. The onset of regression may have contributed to the maximum carbon-isotope excursion by enhancing sedimentation rates, and by increasing organic productivity because of increased nutrient availability. The removal of carbon from the ocean surface may have caused a decrease in pCO2 of the atmosphere. The resulting cooling episode could have triggered an oceanic overturn bringing 12C-enriched bottom waters to the surface, which in conjunction with oxidation of organic matter during the sea-level fall, ended the carbon-isotope excursion. Comparison of 13C and 18O values of matrix samples to the associated cement phases provides insights into the relationship between isotope variations and depositional and diagenetic environments. 13C values of peritidal dolomicrite define a rather smooth stratigraphic variation curve, whereas the values for subtidal micrite have significant scatter resulting from involvement of organic matter in diagenesis. Fibrous to bladed calcite cement from the subtidal deposits has 13C and 18O values comparable to the associated micrite, suggesting precipitation from marine water and similar diagenetic modifications. Meteoric diagenesis may be responsible for the depletion of 13C and 18O in equant calcite cement relative to the micrite. For saddle dolomite cement, the depletion of 18O and 13C values similar to those for the peritidal dolomicrite, are consistent with formation during burial at elevated temperatures in a rock-dominated system. This study demonstrates the potential of applying carbon-isotope stratigraphy, developed in highly fossiliferous successions, to stratigraphic intervals with poorly constrained biostratigraphy. Such studies require evaluation of the effects of depositional environments and diagenesis upon the preservation of marine isotope signatures.

Depositional Sequence Stratigraphy of Lower Cambrian Grand Cycles, Southern Great Basin, U.S.A.

The Lower Cambrian of the southern Great Basin records third-order cyclical alternation between siliciclastic- and carbonate-dominated sedimentation in a rapidly subsiding passive margin. These alternations, termed Grand Cycles, are a prominent feature of Cambrian stratigraphy on the Laurentian continent and have been the subject of study for more than 30 years. A sequence-stratigraphic analysis of Lower Cambrian Grand Cycles in the southern Great Basin indicates that the alternating siliciclastic- and carbonate-dominated depositional systems record cyclical changes in accommodation volume along the Laurentian passive margin. In outer ramp/platform settings, type-1 sequence boundaries coincide with the top of the carbonate half-cycle; in mid-ramp settings, the sequence boundaries lie within the siliciclastic half-cycle. Thick depositional sequences contain well-developed lowstand wedge, transgressive, and highstand systems tracts. Thinner depositional sequences lack a well-developed lowstand systems tract. At least five (and as many as seven) third-order cycles of accommodation change, superimposed on a long-term, second-order cycle, are preserved in the Lower Cambrian of the southern Great Basin. This study, like most studies, supports the view that Grand Cycles, and their associated depositional sequences, are a product of cyclical eustatic changes in sea level. However, this does not rule out the possibility that allogenic forcing resulting from changes in climate also played an important role. Periods of wet/cool climate may have promoted siliciclastic half-cycle development, whereas periods of dry/warm climate promoted carbonate half-cycles.

Sequence Sequence stratigraphy, sea-level changes and depositional systems in the Cambro-Ordovician of the North China carbonate platform

The Cambro-Ordovician strata in North China were deposited over a very extensive craton, extending some 1500 km east-west and 1000 km north-south. The dominantly shallow-water carbonate succession reaches up to 2000 m in thickness and two megasequences (transgressive-regressive cycles) can be distinguished: Lower Cambrian through Lower Ordovician strata, and Middle through Upper Ordovician strata, separated by a major palaeokarst. The first megasequence consists of nine sequences which are generally 50-150 m in thickness. The Lower Cambrian sediments consist of phosphorites and phosphatic sandstones, deposited during the flooding of the craton. Carbonates, mudrocks and evaporites were deposited in the Early Cambrian under an arid climate, laying the foundation for the subsequent long period of shallow-water carbonate deposition which lasted some 70 m.y. The Middle and lower Upper Cambrian sequences consist predominantly of mudrocks and storm deposits (‘tempestites’) in the lower part and oolitic grainstones and tidal-flat lime mudstones in the upper part; these represent outer-mid and mid-inner-ramp depositional systems, respectively, of the transgressive and highstand systems tracts. These sequences have strong similarities with the ‘Grand Cycles’ in the Cambro-Ordovician successions of North America. In the Upper Cambrian, there is a distinctive unit of glauconitic purple-red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence. In the Upper Cambrian, there was a phase of tilting of the North China Carbonate Platform to the north, and storm deposits, especially intraclastic conglomerates and hummocky cross-stratified grainstones-packstones, were very common at this time. Also common in upper Middle and lower Upper Cambrian strata are stromatolitic-thrombolitic bioherms, several metres in diameter. The upper Upper Cambrian through Lower Ordovician strata are dominantly fine-grained limestones and dolomites deposited in shallow-subtidal and inter- to supra-tidal environments on a low-energy epeiric-sea platform. This part of the succession is the regressive part of the first megasequence, so that overall the platform shows an evolution from platform initiation to platform foundation, to a ramp-depositional system and then an extensive epeiric platform. On a small scale, the succession is composed of metre-scale shallowing-upward cycles (parasequences) arranged into cycle sets of 10–30 m thick. The Middle-Upper Ordovician megasequence consists mostly of shallow-water carbonates with several thick evaporite units. Minor palaeokarsts and palaeosoils separate the sequences, which are composed of metre-scale cycles. Regional uplift affected the North China Platform in the Ordovician and sedimentation did not resume until the Carboniferous. There is a good correlation of the two transgressive-regressive megasequences described here with the global 2nd-order relative sea-level curve for the Cambro-Ordovician.

Facies and sequence controls on the appearance of the Cambrian biota in southwestern Mongolia: implications for the Precambrian–Cambrian boundary

AbstractNeoproterozoic–Cambrian rocks of the Zavkhan Basin (Govi-Altay, western Mongolia) comprise large-scale alternations of siliciclastic- and carbonate-dominated units (cf. ‘Grand Cycles’). Analysis of such depositional sequences near the base of the Cambrian confirms that the distribution of trace fossils, small shelly fossils and calcimicrobial structures was strongly controlled by ecology and taphonomy, corresponding to specific points in a sea-level cycle. Evolution of the Cambrian biota is thus viewed through aseries of narrow time windows, once only for each depositional cycle. Correlation of the Precambrian–Cambrian boundary level on the basis of the first appearance of thePhycodes pedumassemblage is also fraught with difficulty, since stratigraphic resolution may be limited to a single sea-level cycle(c. 1–5 Ma). It is suggested that, in many cases, basin analysis will need to be undertaken before this boundary can be drawn.

Cross‐platform architecture of a sequence boundary in mixed siliciclastic‐carbonate lithofacies, Middle Cambrian, southern Great Basin, USA

ABSTRACTStratigraphic analysis of mixed siliciclastic‐carbonate lithofacies within the Middle Cambrian Bonanza King Formation of the southern Great Basin reveals three distinct facies associations that record a range of depositional environments from semi‐arid tidal flats to deeper subtidal, restricted lagoons. Stratigraphic trends, cross‐platform facies variations and correlation of individual surfaces across 250 km of the study area suggest that these mixed lithofacies were deposited in three temporally distinct phases. (1) Extensive progradation of mixed peritidal environments culminated in a prolonged episode of subaerial exposure marked by an areally extensive intraclast breccia (0·5–1·2 m thick) that we interpret to be a major Type 1 sequence‐bounding disconformity. (2) Abrupt flooding of the exposed platform resulted in the deposition of mixed deeper subtidal lithofacies, including a condensed interval of fissile, fossiliferous shale. (3) Progressive shallowing and aggradational accumulation was accompanied by a decrease in siliciclastics and a shift to pure carbonate deposition. Deep‐water siliciclastics and megabreccias record deposition along the base‐of‐slope off the Middle Cambrian shelf‐edge, and are interpreted to represent lowstand deposits emplaced during the prolonged episode of subaerial exposure of the shallow shelf.The presence of fine siliciclastics in both peritidal facies and sharply overlying deeper subtidal facies of the study interval within the Bonanza King suggests a variable, but relatively continuous, influx of terrigenous material throughout an extended period of accommodation change, apparently asynchronous with respect to the predictive model of reciprocal sedimentation. We suggest that the primary siliciclastic source changed with relative sea‐level position. During lowered sea level, aeolian processes acting upon the unvegetated Cambrian craton transported fine siliciclastics onto peritidal and shallow‐subtidal environments. During higher sea level, coastal siliciclastic reservoirs supplied sediment that was transported for long distances by geostrophic currents flowing along the submerged platform.As opposed to many Cambro‐Ordovician grand cycles that are commonly interpreted to consist of a transgressive shaly half‐cycle grading upward into a regressive carbonate half‐cycle, the sequence boundary within this Middle Cambrian succession occurs within siliciclastic‐rich, mixed lithofacies rather than in adjoining purer carbonates, implying that some ‘grand cycles’ should not be considered synonymous with ‘sequences’. Interbasinal correlations of the Type 1 sequence boundary within the mixed unit are speculative, primarily because of the inherent imprecision of available trilobite biostratigraphy. However, there is evidence that an extended episode of subaerial exposure may have been continent‐wide during the Ehmaniella trilobite biochron.

Neoproterozoic of the Mackenzie Mountains, northwestern Canada

Neoproterozoic strata of the Mackenzie Mountains are more than 10 km thick, and comprise two supergroups of differing style. The Mackenzie Mountains Supergroup (4–6 km thick), an epicratonic succession of mainly shallow-water siliciclastic and carbonate strata, contains detrital zircons dated at 1080 Ma and is cut by dykes and plugs dated at 780-778 Ma. The overlying Windermere Supergroup (5–7 km thick) comprises basal rift deposits and glacial diamictites that are overlain by three, kilometre-scale, siliciclastic to carbonate “Grand Cycles” deposited on a passive margin. “Grand Cycles” in the eastern Mackenzie Mountains were deposited in a shallow-water setting, whereas contemporaneous cycles in the western Mackenzies were deposited on a continental slope; mapping of successive shelf-margin positions reveals southwestward progradation. The Windermere Supergroup is unconformably overlain by the Ingta Formation, which contains the Proterozoic-Cambrian boundary. Subtrilobite Cambrian strata are approximately 1.5 km thick, and are overlain by trilobite-bearing Lower Cambrian carbonates of the Sekwi Formation. Fossils are abundant and biostratigraphically useful in these Neoproterozoic strata. Acritarchs, carbonaceous megafossils, and microbial structures occur throughout the entire succession, but are especially well preserved in the Little Dal Group of the Mackenzie Mountains Supergroup. Edicara-type fossils first appear below the second Windermere glacial unit (Ice Brook diamictite), and range upwards through 2.5 km of strata to near the top of the Windermere; three distinctive assemblages corresponding to the Twitya, Sheepbed, and Blueflower formations can be recognized. The Neoproterozoic-Cambrian boundary occurs within the Ingta Formation, and is marked by an abrupt change from simple, subhorizontal burrows ( Planolites-Torrowangea Assemblage) to complex feeding burrows of the Phycodes Assemblage. Small shelly fossils of Nemakit-Daldyn aspect occur in the upper part of the Ingta Formation, slightly above the basal Cambrian boundary. The presence of two glacial diamictites in the Windermere and numerous sequence boundaries and flooding surfaces throughout the succession provides opportunities for regional and global event correlation. The palaeomagnetic record shows three hair-pin curves that should be recognizable in continents attached to Laurentia. Carbonate interbeds throughout the succession typically yield relatively unaltered carbon isotope ratios; C and Sr chemostratigraphy shows marked excursions that appear to be globally correlatable.

The interactions of sea-level change, terrigenous-sediment influx, and carbonate productivity as controls on Upper Cambrian Grand Cycles of western Newfoundland, Canada

The Upper Cambrian stratal record in western Newfoundland is complex and reflects the interplay and feedback between (1) changes in third-order sea level, (2) episodic incursion of terrigenous fines onto the paleoshelf, and (3) productivity levels of the ancient carbonate factory. Mixed carbonate and terrigenous clastic rocks of western Newfoundland are configured in large-scale, third-order stratal patterns called (such cycles are tens to hundreds of meters in stratigraphic thickness and appear to span millions of years). Grand Cycles are divided here into ribbon half-cycles (composed principally of ribbon rock) and oolitic half-cycles (dominated by oolite). Stratigraphic distribution of shale in these rocks, however, is not everywhere coincident with large- and small-scale patterns in carbonate lithofacies, and it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately, in part, from the dynamics of carbonate accumulation. Areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform, presumably in response to third-order changes in accommodation potential. Meter-scale cycles in carbonate lithofacies are of two types: peritidal cycles that fine upward, and shelf cycles that coarsen upward. Thick successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated (by a sequence boundary marked by peritidal cycles) from lowstand deposits comprising thick successions of ootite shelf cycles. The former mid-Grand Cycle transitions of previous workers are thereby reinterpreted in this study as sequence boundaries. The long-term position of sea level, and thus the degree of platform inundation, influenced the health of the platform carbonate factory. During times of low sea level, the platform was bathed in water more saline than open-ocean sea water, and it had a robust non- skeletal carbonate factory that was resilient to poisoning by episodic influx of terrigenous fines onto the shelf. In contrast, the carbonate factory during highstands was less resilient to terrigenous poisoning and was susceptible to localized drowning by the reduction in the rate of carbonate-sediment production. The thick shale units are therefore interpreted to represent the coincidence of (and interaction between) (1) incursion of terrigenous fines onto the ancient platform and (2) a weakened highstand carbonate factory unable to recover from this terrigenous poisoning.

The secular modulation of solar rotation from 1943 to 1992 and its time-delayed correlation with the 55-year grand cycle of the 11-year solar cycle

We devised a new method, which we call the running-segment method, to achieve high-resolution time series of indices of solar rotation for determining the latitude dependence of the differential rotation by a least-squares fitting of the daily translation of positions of sunspot groups during a fixed time segment of 11 years. The segment is moved by an amount of one year to determine the differential profile of the next point of time. Time of the determined rotation data is defined by an arithmetic mean of the beginning and ending years of the segment. The rotation underwent an acceleration from 1948 to 1974 and a deceleration from 1974 to 1987. We found that the time profile of the indexM, the angular momentum surface layer density defined by integration of the angular momentum volume density over the whole surface, follows almost exactly the time profile of the 11-year running mean of the yearly mean of the sunspot relative number with a delay time of about 20 years. The acceleration (deceleration) phase corresponds to the ascending (descending) phase of amplitude of the 11-year solar cycle of cycle 16 (19) to cycle 19 (20) with a delay time of about 20 years. The cycles 15–20 correspond to the 55-year grand cycle V of the 11-year cycle. The delay time of about 20 years agrees well with the delay time predicted by a nonlinear dynamo theory of the solar cycle for driving the 55-year modulation of the 11-year solar cycle. The agreement suggests that the Lorentz force of the magnetic field of the solar cycle during grand cycle V drives the solar rotation modulation from 1948 to 1987 and that the force needed about 20 years to modify the rotation during 1943–1992.