Contribution of the geomagnetism to the region of the massif Jbel Saghro geology,Eastern Anti-Atlas,Morocco

Our investigation relates to the interpretation of the magnetic data of Jbel Saghro, by aeromagnetic processing using Geosoft and ArcGis software. The data were obtained from digitizing the preexisting maps. From the magnetic field anomaly maps, different anomalies were identified with differences in intensities, sizes, and shapes and in being isolated or interfered. The directions are in general ENE-WSW, NNW-SSE, W-E, and NW-SE which are in good agreement with the general tectonic structure of the region. In order to correspond the anomalies to their natural sources and their geometrical configuration, it was necessary to calculate the residual polar. This avoids the presence of two lobes for the same source, what makes it possible to delimit the geological sources laterally. Thus, the attribution of each magnetic anomaly to its geological source becomes possible. Indeed, generally magnetic anomaly of strong intensities and big wavelengths is associated to very magnetized structures related to the Precambrian basement, and the negative a.m. is connected to structures of weak magnetization located in general in the cover. The application of the tilt angle filter allowed the localization of the outcrops of the inliers and new geophysical lineaments. On the other hand, the local maxima of the horizontal gradient made it possible to reveal the large structural features concerning the main tectonic events of the area.     

Two orogenies in the Meatiq area of the Central Eastern Desert,Egypt

The Pan-African basement exposed in the Meatiq area west of Quseir, Egypt, consists of an infracrustal basement overthrusted by a supracrustal cover. The infracrustal rocks were developed as a result of an old orogeny referred to as the Meatiqian orogeny where granite—gneiss, migmatitic gneisses and migmatized amphibolites were formed. The granite—gneiss represents a deformed granite pluton emplaced at 626±2 Ma, whereas the migmatitic gneisses and amphibolites are of mixed igneous and sedimentary parentage. In view of the data so far available, the nature of the Meatiqian orogeny could not be deciphered. In spite of the young isotopic ages, it is suggested that at least the metasedimentary gneisses represent older rocks in the stratigraphic sequence of the infracrustal basement.The supracrustal cover represents a part of an extensive ophiolitic mélange obducted onto the infracrustal basement during the next orogeny (Abu Ziran orogeny) which culminated at 613±2 Ma. An active continental margin-type regime can adequately explain the evolution of such a supracrustal cover. During obduction, the ophiolitic mélange and the upper 2 km thick part of the infracrustal basement were intensely deformed and metamorphosed under PT conditions of the greenschist—epidote amphibolite facies. The deformed infracrustal basement was converted into mylonitic—blastomylonitic rocks and schists composing five thrust sheets, and subsequently intruded by synkinematic granitoid sheets. Later, both the infracrustal basement and the overlying supracrustal cover were isostatically uplifted, subjected to complex shallow folding giving rise to the major Meatiq domal structure, and were intruded by a postkinematic adamellite pluton at 579±6 Ma.     

Processes reshaping the Nile delta promontories of Egypt: pre- and post-protection

Shoreline positions established from beach profile surveys combined with wave data are jointly analyzed, as a function of their contribution to coastal processes, to investigate the interaction between waves, shoreline orientation and coastal structures along the Nile delta promontories, Rosetta, Burullus and Damietta. Repeated beach profile surveys along the promontory sectors (64 km long in total) have been analyzed to determine rates of shoreline changes prior to construction (1971–1990) and after construction of protective structures (1990–2000). The behavior of coastline pre- and post-construction indicates that coastal erosion fronting protective structures has declined in the case of the seawalls at the tips of the Rosetta and Damietta promontories, or has been partially replaced by sand accumulation in the case of detached breakwaters at Baltim (east of Burullus promontory) and at Ras El Bar (west of the Damietta promontory). As a consequence, downdrift erosion has been initiated in local areas adjacent to these structures in the direction of longshore sediment transport. The 5-km-long seawall protecting the Rosetta promontory has stopped the dramatic erosion of this highly eroded area (formerly shoreward retreated 88 m/year), with adverse local erosion at its west and east ends, being 3 and 13 m/year, respectively. Similarly, the 6-km-long seawall built on the eastern tip of the Damietta Promontory, still under construction, has nearly stopped the severe erosion, which was formerly 10 m/year. The detached breakwaters at both Baltim and at Ras El Bar have accumulated sand at accretion rates of 37 and 14 m/year, respectively. This sand accumulation is associated with downdrift erosion of 25 and 13 m/year at Baltim and Ras El Bar, respectively. Results reaffirm that the original erosion/accretion patterns along the Nile delta promontories have been reshaped due to the massive protective structures built during the last decade. This reshaping along the examined promontories is generally controlled by the temporal variability in the intensity and reversibility of wave directions and associated longshore currents, coastline orientation and by the existing coastal protection structures.     

Diagenetic and metamorphic history of the Umm Nar BIF,Eastern Desert,Egypt

The Umm Nar BIF was formed in a sedimentary environment. It is confined to an upper stratigraphic zone of pre-Pan-African metamorphosed shelf deposits. During the Pan-African deformational history, the BIF and the host metasediments were tectonically' overlain by ophiolitic melange succession. The metasediments and the mélange were subjected to a major folding phase and then thrust over the “Shaitian” sheared granite, prior to the intrusion of syn- to late- orogenic granitoids. The BIF is divisible into two main types: oxide-bands including magnetite and hematite, and oxide-silicate bands including magnetite, hematite and stilpnomelane. The associated gangues are quartz, calcite, epidote, garnet, plagioclase, graphite and muscovite. Rhythmic banding and lamination, cross-lamination and flaser structure are the most prominent primary features in the IF bands. The iron minerals and the associated gangue show a variety of textural aspects and microscopic interrelationships which indicate successive episodes of mineral accumulation and formation, involving deposition, recrystallization, blastic growths, overgrowths, replacement and deformations, during continuous burial and subsequent tectonic deformations. Editorial handling: DR     

The glauconite–Fe‐illite–Fe‐smectite problem: a critical review

Iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial. Our analysis of published data identifies end‐member compositions and mixtures and allows us to recognize controls of formation of different mineral species. The compositional fields of glaucony, Fe‐illite, Fe–Al smectites are determined in the M⁺/4Si vs. Fe/Sum of octahedral cations (M⁺ = interlayer charge). Solid solutions could exist between these phases. The Fe–Al and Fe‐rich clay minerals form two distinct solid solutions. The earliest phases to be formed are Fe–Al smectites or berthierine depending on the sedimentation rate. Reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments. The Fe is incorporated first in pyrite and then in silicates after oxidation. Potassium ions diffuse from the sea‐water–sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe‐illite or glauconite or a mineral assemblage (berthierine–nontronite) according to the available Al ion amounts in the microsystem. Mixed‐layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Despite the common belief of their value as palaeoenvironment indicators, these minerals can form in a variety of environments and over a period of millions of years during sediment burial.     

Rupture of an evaporating liquid bridge between two grains

The study examines rupture of evaporating liquid bridges between two glass spheres. Evolution of the bridge profile has been recorded with the use of high-speed camera. Geometrical characteristics of the bridge were then used to calculate evolution of the variables during the process: Laplace pressure, capillary force, and surface tension force. For the purpose of reference, the bridge evolution is followed also during kinematic extension. During both processes the diameter of the neck decreases, with an acceleration of about 1–2 ms before the rupture. Two distinct rupture modes are observed, depending on the bridge aspect ratio. After the rupture, the mass of liquid splits, forming two separate oscillating drops attached to the spheres, and a suspended satellite droplet. Just before the rupture, an increasing repulsive Laplace pressure, and decreasing negative surface tension force develop. Capillary force follows the trend of the surface tension force, with an accelerating decline. Duration of the whole process and liquid mass stabilization is from 10 to 60 ms.