Volcanism of the Palaeoproterozoic Bushveld Large Igneous Province : the Rooiberg Group, Kaapvaal Craton, South Africa

The volcanic rocks of the Rooiberg Group represent the uppermost unit in the Palaeoproterozoic Transvaal Supergroup and form one of the largest provinces of silicic volcanic rocks in the world. Although stratigraphically associated with the Transvaal basin-fill, the Rooiberg Group is petrogenetically linked with the larger Bushveld magmatic event for which emplacement was preceded by the extrusion of the vast Rooiberg lava flows in the northern part of the Kaapvaal Craton. Like many silicic-dominated Large Igneous Provinces (LIPs), which are increasingly recognised in the rock record, the Rooiberg Group volcanics are intracontinental, subaerial, and are dominated by voluminous lava flows. Originally, the Rooiberg Group is inferred to have covered an area of more than 200,000 km2 of which, after erosion, an area of 50,000–67,000 km2 remains. The lava flows form a stratigraphic succession up to 6 km thick, and are divided into four formations in ascending order: Dullstroom, Damwal, Kwaggasnek and Schrikkloof. Due to a scarcity of reliable geochronological data, the temporal span of the Rooiberg Group is poorly understood. The Rooiberg Group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 ◦C. Minor explosive eruptions are represented by pyroclastic rocks, and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes. The rocks are essentially undeformed and have not been buried so that their original textures are well preserved. The Bushveld Complex and the associated Rooiberg Group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume. This thorough review of the geochronology, physical volcanology, and geochemistry of the Rooiberg Group enables construction of a geodynamic model.The University of Pretoria and the National Research Foundation of South Africa.http://www.elsevier.com/locate/precamre

Forced regressive wedge in the Mesoproterozoic Koldaha Shale, Vindhyan basin, Son Valley, central India

The present paper highlights the sequence development within the Mesoproterozoic Koldaha Shale Member of the Kheinjua Formation, Vindhyan Supergroup which records the occurrence of a forced regressive wedge and associated discontinuity surfaces at the base of the wedge. Nine lithofacies have been identified within the study area that are grouped into three lithofacies associations varying in depositional setting from outer shelf, through shoreface-foreshore-beach to continental braidplain. The outer shelf sediments are aggradational to slightly progradational representing highstand systems tract. The rapidly progradational, wedge-shaped shoreface to foreshore-beach succession occurs sharply or erosively above the outer shelf sediments and is bounded by a regressive surface of marine erosion (RSME) at the base and by a subaerial unconformity at the top. This, along with its downstepping trajectory, supports deposition of this sedimentary wedge during falling sea level. A laterally extensive soft sediment deformation zone occurs at the base of the wedge. The forced regressive wedge is incised by fluvial braidplain deposits that rest on an erosive surface representing a sequence boundary. The thin braidplain deposits are the product of aggradation during a subsequent early rise in relative sea level, and thus, they are inferred to represent a lowstand systems tract. The constituent architectural elements that characterize the braidplain deposits are downstream accretion elements and small channel elements. Further landward, the base and top of the shoreface wedge merge to form an unconformity across deposits that rest directly on the outer shelf sediments. The identification of forced regressive wedges has significant economic importance in view of the potential occurrence of hydrocarbons within the Proterozoic formations.UGC (F.PSW-010/11-12 (ERO) dated 08.08.2011).http://www.elsevier.com/locate/marpetgeo2017-03-31hb201

Petrography and geochemistry of the Middle Siwalik sandstones (tertiary) in understanding the provenance of sub-Himalayan sediments in the Lish River Valley, West Bengal, India

A petrography–geochemistry-based evaluation of the provenance of the sandstones of the Tertiary Middle Siwalik Subgroup in the Lish River Valley, West Bengal, is presented. The framework grains in the sandstones suggest mixing of sediments from spatially separated gneissic, quartzitic and phyllitic source rocks. Modal values of different framework minerals suggest that recycled sediments in an orogenic setting were deposited in the Middle Siwalik basin in the area. The major and trace element ratios suggest dominantly felsic input and mixing with subordinate basic material in an upper continental crustal setup. The major and trace element data also indicate that rocks of a passive margin setting acted as the source to the sediments. The present paper postulates that the Middle Siwalik sediments were derived from pre-Himalayan gneissic and metabasic rocks of an erstwhile passive margin setting and presently forming the Higher and Lesser Himalaya, respectively.University of Pretoria and National Research Foundation of South Africa.http://link.springer.com/journal/125172017-02-28hb2016Geolog

First- and second-order global sequence stratigraphic correlations and accommodation charts for the Kaapvaal, Karelian, São Francisco (-Congo) and Slave cratons : an introduction

This work provides a pilot study of global sequence stratigraphic correlation for the Precambrian for five chosen cratons. Detailed chronostratigraphic charts summarize the supracrustal geological evolution of each craton, and are in the form of adapted Wheeler diagrams to enable estimation of first- and secondorder sequence stratigraphy for the cratons. Evidence within the Precambrian sedimentary record for events of apparent global significance is examined, across several preserved Precambrian cratons, utilising chronological data, inferred geodynamic and basin evolutionary histories, palaeosols, erosional hiatuses, and interpreted chemical, biochemical, palaeobiological, palaeoatmospheric and palaeoclimatic changes. The adapted Wheeler diagrams attempt to reflect time within hiatuses as well as within depositional sequences, in accord with the distinctly punctuated nature of the global stratigraphic record. The supercontinent cycle is examined for its antiquity and its application to Precambrian cratons, and a commentary is given on an emerging “conventional view” of the Precambrian wherein supercontinentality is seen as a global phenomenon by the Neoarchaean already (or alternatively only by ca. 2.0 Ga), on the nature of the “Great Oxidation Event” at ca. 2.4e2.3 Ga and possibly concomitant widespread glacial events at approximately the same time period. It is hoped that the present pilot study will stimulate an examination of accommodation changes over time for all ancient cratons, thus enabling a more comprehensive assessment of global correlations and high-order (first- and second-order) accommodation changes. This might lead to an improved appreciation of the inherent complexity of the individual facets making up the currently developing “conventional view” of Precambrian geological evolution.http://www.elsevier.com/locate/marpetgeonf201

Palaeoseismicity in relation to basin tectonics as revealed from soft-sediment deformation structures of the Lower Triassic Panchet formation, Raniganj basin (Damodar valley), eastern India

The Raniganj basin in the Damodar valley of eastern India is located within the riftogenic Gondwana Master-Basin. The fluvio-lacustrine deposits of the Lower Triassic Panchet formation of the Damodar valley in the study area preserve various soft-sediment deformation structures such as slump folds, convolute laminae, flame structures, dish-and-pillar structures, sandstone dykes, pseudonodules and syn-sedimentary faults. Although such soft-sediment deformation structures maybe formed by various processes, in the present area the association of these structures, their relation to the adjacent sedimentary rocks and the tectonic and depositional setting of the formation suggest that these structures are seismogenic. Movements along the basin margin and the intra-basinal faults and resultant seismicity with moderate magnitude (2–5 on Richter scale) are thought to have been responsible for the soft-sediment deformations.http://www.ias.ac.in/jess/nf201

Neoproterozoic substrate condition vis-à-vis microbial mat structure and its implications : Sonia Sandstone, Rajasthan, India

The present study deals with the Neoproterozoic mat-infested substrate sculptured by inferred bioturbations, excellently preserved within the ca. 600 Ma Sonia Sandstone, northwest India. The bioturbations are horizontal, being preserved on the bed-surfaces and have been classified into linear grooves and discoidal structures. The linear grooves are of two types, one group represents a straight to meandering pattern with raised sand ridges on either side, while the other group represents a mostly meandering pattern with changing width along the length, without raised ridges. Both types of grooves resemble trails of advanced organisms. The discoidal structures have five internal lobes emerging from the centre and merging with the outer ring and may represent body impressions of an organism; alternatively may represent variants of Ediacara or other soft bodied organisms. The interaction of the substrate with these possible bioturbators has great significance in establishing the evolutionary history of the substrate and resulting bioturbations during the Proterozoic. Microbial mats possibly provided oxygen and nutrients to the organisms and played an important role in their preservation. The study also tries to extrapolate the life style of these trace-makers. All these traces may raise speculation about the onset-time of some higher order organisms.DST Fast track project (Young Scientist Scheme, No: SR/FTP/ES-03/2011).CAS (Phase V), UPE-II programme of Jadavpur University. National Research Foundation and University of Pretoria, South Africa.http://www.elsevier.com/locate/jseaes2016-07-31hb201

Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup, eastern Himalaya, Darjiling district, India

The Himalayan fold-and-thrust belt has propagated from its Tibetan hinterland to the southern foreland since ~55 Ma. The Siwalik sediments (~20 – 2 Ma) were deposited in the frontal Himalayan foreland basin and subsequently became part of the thrust belt since ~ 12 Ma. Restoration of the deformed section of the Middle Siwalik sequence reveals that the sequence is ~325 m thick. Sedimentary facies analysis of the Middle Siwalik rocks points to the deposition of the Middle Siwalik sediments in an alluvial fan setup that was affected by uplift and foreland-ward propagation of Greater and Lesser Himalayan thrusts. Soft-sediment deformation structures preserved in the Middle Siwalik sequence in the Darjiling Himalaya are interpreted to have formed by sediment liquefaction resulting from increased pore-water pressure probably due to strong seismic shaking. Soft-sediment structures such as convolute lamination, flame structures, and various kinds of deformed cross-stratification are thus recognized as palaeoseismic in origin. This is the first report of seismites from the Siwalik succession of Darjiling Himalaya which indicates just like other sectors of Siwalik foreland basin and the present-day Gangetic foreland basin that the Siwalik sediments of this sector responded to seismicity.The National Research Foundation, South Africa and the University of Pretoria.http://www.springerlink.com/content/0016-7622/nf201

Investigation on the possible control of grain size on development of ‘mud cracks’: a preliminary experimental study of recent Gangetic alluvial samples, West Bengal, India

The so-called mud cracks, possibly the most common deformation feature in sediment, are evident in almost all dried-up exposed sediment surfaces. Several experimental attempts have so far been made to explain the evolution of mud cracks. However, most of the experiments were carried out with bentonite clay or other artificial substances in controlled heating condition. Therefore, those experiments could not or were not performed to elucidate the control of grain size on crack development although it has been observed that cracks develop in almost all types of soil. The present work is an attempt to understand the control of sediment grain size in development and evolution of these cracks. This experimental study was carried out in near-natural condition. Sediment samples of three different grain-size fractions ranging from sub-sand size to very coarse sand size were used for the experiment. The 10-day-long experiment with watering and drying of the samples under sunlight shows that the time gap between watering and appearance of cracks, smoothness of crack walls, number of cracks and also number of polygons with cracks as their arms are dependent on grain size of the sediment.http://link.springer.com/journal/125172022-01-15hj2021Geolog