Comparison of the Jurassic and Cretaceous sedimentary cycles of Somalia and Madagascar: implications for the Gondwana breakup

The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar.     

Unravelling the microfacies signatures of parasequences using computer-optimized similarity matrices

The microfacies of a Lower Cretaceous carbonate drillcore from Oman are characterized using optimizing matrices of Jaccard's similarity coefficients of community. Other than systems tract boundaries, there is no obvious evidence of individual parasequences in the core. However, diagnostic patterns in microfossil distribution identify environmental gradients recording changes in water depth. These gradients are used to define individual parasequences, parasequence sets, stacking patterns and key surfaces. The patterns suggest that deposition was controlled by successive fourth‐ to fifth‐order (high‐frequency) relative sea‐level cycles superimposed on an underlying third‐order relative sea‐level rise. Although the correlation of these depositional subunits to systematic changes in water depth and the rate of carbonate accumulation alone is not incontrovertible proof of such a sea‐level control, concurrent multiorder relative sea‐level cyclicity provides by far the most likely explanation. A microfacies deposited when the water depth was shallowing is characterized by a relay of microfossils with affinities that shallow upwards. Conversely, a microfacies that records a gradual increase in water depth has a relay of microfossils with affinities that deepen upwards. Microfacies characterized by an assemblage of microfossils with similar affinities record deposition when the benthic environmental conditions remained stable, either because of an equilibrium between shallow water carbonate deposition and rising sea level, or in deeper water where sediment composition was relatively insensitive to changes of water depth. Microfacies characterized by mixed affinity assemblages record syndepositional reworking. During periods of embedded multiorder sea‐level changes, individual parasequences within systems tracts are shown to record more complex environmental gradients than simply the repetition of successive shallowing‐up units as traditionally represented in carbonate sequence stratigraphic models. The microfacies of an individual parasequence may shallow up, or may record both deepening‐up and shallowing‐up depositional phases, as well as periods of sedimentation when benthic environments remained stable. Individual parasequence boundaries may be submarine or subaerial unconformities, or be conformable, as part of a predictable stratigraphic pattern related to the temporal position of an individual parasequence within the underlying third‐order cycle of relative sea‐level change. The hitherto ubiquitous use of assemblages to describe carbonate microfacies, coupled with the widespread use of the metre‐scale shallowing‐up template to identify parasequences, may have led to such complexities previously being overlooked.     

Prospects for interregional correlations using Wisconsin and Holocene aridity episodes, northern Gulf of Mexico coastal plain

Luminescence dating of extensive dune fields and associated eolian sandsheets provided a chronology of recently recognized Pleistocene and early Holocene dry climate episodes in the currently humid warm temperate northern-northeastern Gulf of Mexico region. Scattered parabolic dunes and clusters of intersecting parabolic dunes, along with elongated shore-transverse and shore-parallel dunes, developed. These landforms occur in a 390-km-long and 2- to 3-km-wide, semicontinuous belt in southeast Alabama and northwestern Florida. Dune elevations reach ± 22 m. Sangamon coastal barrier sectors were the primary source of the eolian sand. Deflation was coeval with early Wisconsin to mid-Holocene marine low sea-levels and associated distant shorelines. Early Holocene dune dates were synchronous, with indications of a hypsithermal dry interval in southeast Louisiana, the Yucatan, and the south Atlantic seaboard. Overlapping with dry episodes in Yucatan and the High Plains, Texas dunes and Louisiana and Texas prairie mounds, especially in the southwest Texas coast still dominated by dry climate, suggests intervals of early to late Holocene drought. The dates provide the basis for identifying and correlating Wisconsin, early, and late Holocene climate phases between currently semiarid and humid, coastal and interior areas. They contribute to future studies, including interregional paleoclimate modeling. Although Pleistocene coastal eolian deposition coincided with glaciation in the northern interior and with cooler temperatures of a reduced Gulf of Mexico, Holocene aridity phases may have been related to major variations in the position of high-pressure cells, storm tracks, and branches of the jet stream, and even to prolonged La Niña conditions.     

Cosmogenic nuclide chronology of pre-last glacial maximum moraines at Lago Buenos Aires, 46°S, Argentina

At Lago Buenos Aires, Argentina, ¹⁰Be, ²⁶Al, and ⁴⁰Ar/³⁹Ar ages range from 190,000 to 109,000 yr for two moraines deposited prior to the last glaciation, 23,000–16,000 yr ago. Two approaches, maximum boulder ages assuming no erosion, and the average age of all boulders and an erosion rate of 1.4 mm/10³ yr, both yield a common estimate age of 150,000–140,000 yr for the two moraines. The erosion rate estimate derives from ¹⁰Be and ²⁶Al concentrations in old erratics, deposited on moraines that are >760,000 yr old on the basis of interbedded ⁴⁰Ar/³⁹Ar dated lavas. The new cosmogenic ages indicate that a major glaciation during marine oxygen isotope stage 6 occurred in the mid-latitude Andes. The next five youngest moraines correspond to stage 2. There is no preserved record of a glacial advance during stage 4. The distribution of dated boulders and their ages suggest that at least one major glaciation occurred between 760,000 and >200,000 yr ago. The mid-latitude Patagonian glacial record, which is well preserved because of low erosion rates, indicates that during the last two glacial cycles major glaciations in the southern Andes have been in phase with growth and decay of Northern Hemisphere ice sheets, especially at the 100,000 yr periodicity. Thus, glacial maxima are global in nature and are ultimately paced by small changes in Northern Hemisphere insolation.     

The Cenozoic Lower Rhine Basin – rifting, sedimentation, and cyclic stratigraphy

In the Cenozoic, the Lower Rhine Basin formed as a rift at the southeastern terminus of the Dutch German Central Graben, while the Rhenish Massif was uplifted. The study focusses on the marginal marine and fluvial fill of the Lower Rhine Basin. A basin model is developed. Support for this study was given by extensive industry outcrop and well data, by new stratigraphical and sedimentological observations. The ingression and subsequent regression of the Cenozoic North Sea is analysed using the concept of base level cyclicity. As the geohistory of the basin was complex, a subsidence curve is constructed. Furthermore, an attempt is made to trace the simultaneous uplift of the Rhenish Massif.     

A Multi-cycle Climatic Fluctuation Record of the Last Interglacial Period:Typical Stratigraphic Section in the Salawusu River Valley on the Ordos Plateau,China

The Last Interglacial Period strata in the Milanggouwan section in the Salawusu River valley on the Ordos Plateau, China, have 8.5 sedimentary cycles composed alternately of eolian dune sands, fluvio-lacustrine facies and paleosols. Based on comprehensive analyses on the distribution of magnetic susceptibility and CaCO3 and paleo-ecology indicated by fossils in the region, it is considered that the sedimentation cycles resulted from dry-cold and warm-humid climate fluctuations. Magnetic susceptibility values and CaCO3 contents in stratigraphic sectors Ⅰ, Ⅲ, Ⅴ and Ⅱ, Ⅳ basically respectively present peaks and low vales, and the former three can in time correlate with MIS5a, MIS5c and MIS5e successively and the latter two with MIS5b and MIS5d. In addition, some horizons of eolian dune sands and the low vales of their magnetic susceptibility and CaCO3 are also correlated with 6 periods of cooling events indicated by the higher content of foraminifer Neogloboquadrina pachyderma (S.) documented in the V29-191 drill in the North Atlantic and the cold events recorded by δ^18O in the ice cores in GRIP, especially with 9 periods of dust events in Chinese Loess Plateau.     

An unusual fluvial to tidal transition in the mesoarchean Moodies Group, South Africa: A response to high tidal range and active tectonics

The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels.

The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.     

图的圈和路剖分问题

设G是一个顶点数为n的图，k为任意正整数且k≤n，证明了如果图G中任何一对不相邻顶点的最大度至少为n-k＋1/2，则G能剖分成k个子图Hi,1≤i≤k，其中Hi是圈或路；如果G是2-连通图，σ2（G）=min{dG（x）＋dG（y）x，Y∈y（G），x≠y，d（x，y）=2}≥n-k，G也能剖分成k个子图Hi，1≤i≤k，其中Hi是圈或路。     

不同初始饱水状态红砂岩冻融损伤差异性研究

借助于红砂岩冻融循环CT扫描试验，从扫描层面内不同的感兴趣区域内CT均值数大小及其变化以及CT图像，对4种典型的不同初始饱水状态红砂岩进行冻融循环条件下损伤扩展差异性研究。研究结果表明，对于初始损伤相同的岩石，初始饱水状态将决定冻融循环对其损伤扩展的影响程度。