Migrated hybrid turbidite-contourite channel-lobe complex of the late Eocene Rovuma Basin, East Africa

Analysis of 3 D seismic data and well log data from the Rovuma Basin in East Africa reveals the presence of a late Eocene channel-lobe complex on its slope.The first two channels,denoted as channel-1 and channel-2,are initiated within a topographic low on the slope but come to a premature end when they are blocked by a topographic high in the northwest region of the basin.New channels migrate southeastward from channel-1 to channel-6 due to the region’s sufficient sediment supply and stripping caused by bottom currents.The primary factors controlling the development of the channel complex include its initial paleo-topographic of seafloor,the property of gravity flows,the direction of the bottom current,and the stacking and expansion of its levees.The transition zone from channel to lobe can also be clearly identified from seismic sections by its pond-shaped structure.At a certain point,thest systems record a transiton from erosive features to sedimentary features,and record a transition from a confined environment to an open environment.Channels and lobes can be differentiated by their morphologies:thick slump-debris flows are partly developed under channel sand sheets,whereas these slump-debris flows are not very well developed in lobes.Well log responses also record different characteristics between channels and lobes.The interpreted shale volume throughout the main channel records a box-shaped curve,thereby implying that confined channel complexes record high energy currents and abundant sand supply,whereas the interpreted shale volume throughout the lobe records an upward-fining shape curve,thereby indicating the presence of a reduced-energy current in a relatively open environment.Within the Rovuma Basin of East Africa,the average width of the Rovuma shelf is less than 10 km,the width of the slope is only approximately 40 km,and the slope gradient is 2°–4°.Due to this steep slope gradient,the sand-rich top sheet within the channel also likely contributes to the straight feature of the channel system.It is currently unclear whether the bottom current has any effect on its sinuosity.

Global plate motion and earthquake activity

The global plate motion rates Ω are not uniform in time and space. The rotation rates were larger than 0. 545°/Ma for Cocos, Philippine Sea, Pacific, Nazca, Australia, India and Arabia plates, but smaller than 0. 315°/Ma. for other plates. Compared to 1997. 0, the Ω values of the three oceanic plates in 2000. 0 increased by,respectively,2.4% ,2. 1% and 41.7%, and the northward movement rates of the the India plate and western part of the Australia plate increased by 3.38 mm/a on the average. The spatial distribution of earthquakes was dependent on earthquake magnitude. Earthquakes of 5.0 ≤ Mw 〈 7.0 were located mainly in plate-margin zones and intra-plate crustal deformation zones joining the southern margin of Eurasia plate. Earthquakes of Mw≥7.0 concentrated basically in the circum-Pacific and South Asia zones, but hardly in midocean-ridge seismic zones. Earthquakes of Mw ≥ 8.0 were located only in the margin zones of the India, Aus- tralia,Pacific and Nazca plates orthogonal to the direction of plate motion. Compared with previous eighteen years, global earthquake activity enhanced obviously after 1994, especially after 2001. The Ω value of a plate was closely related to the activity of strong earthquakes. The largest earthquakes were located in the front-margin zones of plates having the largest Ω values. Energy released by strong earthquakes comes mainly from kinetic energy of the plates. Global seismicity enhancement was caused mainly by the acceleration of the three oceanic plates. Larger enhancement of global earthquake activity lagged behind the movement acceleration of the three oceanic plates by four years.

Seismic rock physical modelling for gas hydrate-bearing sediments

There are ambiguities and uncertainties in the recognition of gas hydrate seismic reflections and in quantitative predictions of physical information of natural gas hydrate reservoirs from seismic data. Rock physical modelling is a bridge that transforms the seismic information of geophysical observations into physical information, but traditional rock physics models lack descriptions of reservoir micro-structures and pore-filling materials. Considering the mineral compositions and pore microstructures of gas hydrates, we built rock physical models for load-bearing and pore-filling gas hydrate-bearing sediments,describe the mineral compositions, pore connectivity and pore shape using effective media theory, calculated the shear properties of pore-filling gas hydrates using Patchy saturation theory and Generalized Gassmann theory, and then revealed the quantitative relation between the elastic parameters and physical parameters for gas hydrate-bearing sediments. The numerical modelling results have shown that the ratios of P-wave and S-wave velocities decrease with hydrate saturation, the P-wave and S-wave velocities of load-bearing gas hydrate-bearing sediments are more sensitive to hydrate saturation, sensitivity is higher with narrower pores, and the ratios of the P-wave and S-wave velocities of pore-filling gas hydrate-bearing sediments are more sensitive to shear properties of hydrates at higher hydrate saturations. Theoretical analysis and practical application results showed that the rock physical models in this paper can be used to calculate the quantitative relation between macro elastic properties and micro physical properties of gas hydrate-bearing sediments, offer shear velocity information lacking in well logging, determine elastic parameters that have more effective indicating abilities, obtain physical parameters such as hydrate saturation and pore aspect ratios, and provide a theoretical basis and practical guidance for gas hydrate quantitative predictions.

SE Asian Cenozoic Larger Foraminifera:Taxonomic Questions,Apparent Radiation and Abrupt Extinctions

The Cenozoic larger foraminifera are important for biostratigraphy in SE Asia.This review examines the taxonomic inconsistencies of this group and especially the confusion over concepts of evolution,migration,radiation and extinction.It is proposed that from the Mediterranean to Indo-Pacific,Latest Eocene through Miocene,larger foraminifera populations were more homogenous than previously believed.Lineages show a slow morphological radiation interrupted by several extinctions.This pattern is superimposed on a long-term decline in larger foraminiferal abundance.The dating of the major biostratigraphic events are qualified.The main lineages of larger foraminifera can be classified by their evolutionary style.The classically“large”genera have highly peramorphic trends achieved through strong orthoselection,and these lineages were the most severely hit by a series of Mid-Eocene to Mid-Miocene extinction events.Other carbonate facies taxa that are traditionally grouped with the larger foraminifera are characterised by weak paramorphism,and these were much less affected by the extinction events.Some of these weakly peramorphic forms underwent Latest Miocene to recent radiation to and locally become rock-forming organisms.The three major and one minor larger foraminiferal extinction events since the Mid Eocene coincide with abrupt tectonic events across SE Asia.However,there are probably multiple causes for these extinctions,including global climatic changes.