A mineral survey of the Cretaceous of the Benue Valley,Nigeria

The economic importance of the Cretaceous sequence of the Benue Valley is highlighted. The linear sedimentary basin or trough, as sometimes described, is underlain essentially by marine Cretaceous rocks, starting from the Middle Albian, and ranging to the Maastrichtian. There is, so far, no evidence of the occurrence of younger rocks in the Upper and Middle Benue area, though in the Lower Benue and the Niger Delta regions, Palaeocene to Recent formations overlie the older formations.The depositional processes and the tectonics associated with this period caused the emplacement of an accumulation of a number of important minerals of sedimentary and igneous origins, including lead-zinc, barytes, limestone and coal. There are indications of the possible occurrence of evaporites as evidenced by brine-springs associated with collapse features and fracture systems related to diapiric dome structures. The use of the minerals in the Nigerian economy is discussed. There are also favourable prospects for uranium and hydrocarbons. The immediate future will usher in a period of intensive exploration work aimed at evaluating these resources.     

Organic geochemical evaluation of the oil/gas-generative potential of organic matter in Cretaceous strata from the Lower Benue Trough,Nigeria

An analytical procedure involving Rock-Eval pyrolysis of whole-rocks was adopted on fresh outcrop samples covering the three lithostratigraphic units in the Afikpo Basin of the Lower Benue Trough. Three petroleum systems are present in the Cretaceous delta frame: the Asu-River Group, the Eze-Aku Group and proto-Niger Delta sequences. The Afikpo Basin has been correlated to three petroleum systems in the Lower Congo Basin, Niger Delta and the Anambra Basin. The organic geochemistry of the shales, carbonaceous mudstones and coal beds show relatively moderate to high total organic carbon contents. The best potential hydrocarbon source rocks are the Eze-Aku Group and proto-Niger Delta shales, carbonaceous mudstones and coal beds where maturation was attained. The high total or-ganic contents, thermal maturity and terrigenous characters of the Asu-River Group, Eze-Aku Group and proto-Niger Delta sediments, suggest the presence of a large amount of natural gas with a small quantity of oil accumulation. Variations in source rock facies were observed from one lithostratigraphic unit to another, and initial HI values as a function of TOC were proposed for each lithostratigraphic unit. The results also show that TOC, HI, OI, S2 and Tmax vary from older to younger rocks. The Tmax values discriminate the rocks into immature and mature source rocks. Source rocks with high Tmax suggest high geothermal gradient/or recycled organic matter. Also high Tmax and S2 yield indicate late and post maturity. Recycled organic matter is characterized by low Tmax. The principal source rocks for gas in the Afikpo Basin are the Eze-Aku Group and proto-Niger Delta beds deltaic systems, consisting mainly of III to IV kerogens with a subordinate amount of type II organic matter. Based on the obtained results, it is concluded that the Cretaceous shales, carbonaceous mudstones and coals in the Afikpo Basin of the Lower Benue Trough are capable of generating and expelling hydrocarbons in the case of sufficient maturity.     

Influence of tectonics and palaeoenvironment on late cretaceous clay sedimentation in the upper benue trough,Nigeria

The Late Cretaceous sedimentary sequence in the Upper Benue Trough consists of seven formations. Outcrop samples from the type localities of these units were analysed by X-ray diffraction for their day mineral content. The composition and distribution of the clay minerals are influenced by tectonics, relief and water depth. Three main depositional phases are recognizable on the basis of clay mineral assemblages, two of which are associated with tectonic movements. The first phase, characterized by the presence of chlorite, illite, kaolinite and mixed-layer clays is common to Albian-Cenomanian sediments. This phase is succeeded by mixed-layer clays and smectite-rich, Turonian-Early Santonian sediments. The re-occurrence of chlorite and illite in association with kaolinite and mixed-layer clays in the Post-Santonian sediments constitutes the third depositional phase. These clay mineral assemblages are indicative of two periods of tectonism (Albian and Late Santonian) and an intervening Turonian-Early Santonian period of quiescence in the Upper Benue Trough.     

Brittle deformation in the Afikpo Basin （Southeast Nigeria）： Evidence for a terminal Cretaceous extensional regime in the Lower Benue Trough, Nigeria

This paper deals with the regional and structural framework of the Cretaceous rocks in the Afikpo Basin located in the southeastern part of the Lower Benue Trough. Results from regional tectonics are presented together with those of the microtectonic analysis of microfaults in the Owutu-Afikpo-Adadama area in the basin. The Owutu-Afikpo-Adadama ridge at the north-central part of the basin marks the boundary between the Late Cenomanian-Turonian-Conianian sediments and the Campanian-Maastrichtian sandstones. This ridge trends N45°E on average and is faulted in three main directions, namely: (1) N-S normal faults; (2) NE-SW strike-slip faults; and (3) NW-SE strike-slip faults. The faulted rocks along these brittle discontinuities are mainly cataclastics with internal fracture cleavage and sigmoidal quartz mosaics that are reminiscent of extensional deformation. The cataclasites often bear slickenside striations.     

Geochemical characterization of Cretaceous sandstones from the Southern Benue Trough,Nigeria

Geochemical studies of sandstones from the three lithostratigraphic successions in the southern Benue Trough of Nigeria were undertaken for a geochemical characterization of the sandstones, and to assess their strati-graphic and source evolution.Major and trace elements data were obtained from outcrop sandstone samples. The SIO2/Al2O2, Fe2O3/K2O ra-tios and CaO contents have been used to characterize the Cretaceous sandstones into Al-rich and high and low Fe2O3/K2O ratio sandstones. Results indicate that there are geochemical features that display stratigraphic trends across the succession from the Asu River Group, Eze-Aku Group to the proto-Niger Delta succession which may imply a discontinuous evolution from different source terrains of Precambrian and Mesozoic ages that supplied the sediments. The Asu River Group sandstones have lower SiO2, Al2O3, Fe2O3 and higher MgO; the Eze-Aku sand-stones have higher TiO2, CaO, alkalis and lower MgO while the proto-Niger Delta sandstones have higher SiO2, Al2O3, Fe2O3 and lower alkalis and CaO. These discontinuities signify the influence of tectonic impulses that af-fected the southern Benue Trough during the Cretaceous time. Changes in ratios of TiO2/Al2O3, Fe2O3, Cr and Zr suggest an increasingly mafic contribution to the depositional basin with time. The chemical index of alteration in-creases with time, possibly suggesting that a more intense weathering regime in the hinterland developed with time.     

Fluid inclusion and stable isotope studies of Pb-Zn-fluorite-barite mineralization in the lower and middle Benue Trough,Nigeria

Lead-zinc-fluorite-barite veins in the lower and middle Benue Trough (Nigeria) are located within the Lower Cretaceous (Albian) carbonaceous shales, limestones, and arkosic sandstones of this intracontinental rift structure. The veins in the lower Benue Through consist of sphalerite + galena+marcasite ± chalcopyrite ± barite in a gangue of siderite and quartz hosted by carbonaceous shales, whereas in the middle Benue Trough, fluorite, barite, quartz, and similar sulfide minerals are hosted by limestone and sandstone. Fluid inclusion temperatures in vein minerals range from 95°C to 200°C (without pressure corrections) and salinities range from 14 to 24 equiv. wt% NaCl. Oxygen isotope compositions of limestone wall rocks (middle Benue) have been lowered from premineralization ¹⁸O values of about 25 per mil to approximately 16 per mil. Fluid in equilibrium with vein calcite has a calculated ¹⁸O of +2.6 per mil at 130°C. The ⁸⁷Sr/⁸⁶Sr ratio of this calcite (0.71497) suggests that strontium and calcium had a considerably more radiogenic source than the Cretaceous limestone or evaporite did (⁸⁷Sr/⁸⁶Sr=0.7073–0.7078). Observed strontium data, lead isotope compositions of galena, and REE patterns in fluorite suggest that the Lower Paleozoic basement rocks in the trough or their weathered equivalents are likely sources for the Benue Trough ore components. Sulfur isotope data suggest that the sulfur was probably contributed from the Cretaceous evaporites in the trough.Our data favor a basinal brine source for the ore-forming fluid. Fluid criculation probably resulted from high geothermal gradients accompanying continental rifting. Brine interaction with the clastic, carbonate, and evaporite rocks led to metal and sulfur leaching and later deposition in fractures accompanying the Cenomanian deformation and uplift in the Benue Trough.     

Paleocurrent and sedimentological studies in the Benue Trough,Nigeria

Paleocurrent analysis indicates an easterly and a westerly source, respectively, for the Cretaceous and Tertiary sediments in the middle and upper Benue Trough. Petrographic differentiation of the pre-Santonian and post-Santonian sediments is based on their feldspar contents. Petrographic evidence suggests that the Tertiary sediments are polycycled; this fact, coupled with the inferred westerly source, suggests that the present limits of the trough are not the paleolimits. Thus the rift boundary faults are probably further westward and eastward of the present trough margins.     

Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf Syncline (Upper Benue Trough), northeast Nigeria

The Peta Gulf Syncline (Upper Benue Trough, northeast Nigeria) is a fault-bounded pull-apart sub-basin. The boundary faults are mainly northeast-southwest-trending en echelon strike-slip faults, truncated along their lengths by normal and tear faults with stepovers. The eastern marginal faults underwent rotation during sedimentation, whereas the steeply dipping western marginal faults were inactive.The Peta Gulf Sub-basin is filled by the Bima Sandstone Formation (Lower Cretaceous) which has three siliciclastic members: (i) B₁: medial fan coarse-grained to microconglomeratic sandstones; (ii) B₂: full fluvial medium-grained sandstones with minimal fines; and (iii) B₃: lacustrine and flood basin deposits comprising alternating fine-grained sandstones and siltstones/claystones. Sediment supply was from east to west and facies changes show a general fining in this direction. B₃ offers the most favourable environment/lithology for U concentration.The only significant U occurrence in the Peta Gulf Syncline is the Zona U anomaly, which occurs within transitional B₂-B₃ brecciated sandstones with wall rock alterations zones. The mineralised zone has elevated SiO₂, Fe, As, Ba and W levels but is depleted in the alkalis, Zr, Rb and Sr. This chemical zonation supports the epigenetic orgin of this anomaly.     

Wrench structural deformation in Ras Al Hilal-Al Athrun area, NE Libya: a new contribution in Northern Al Jabal Al Akhdar Belt

Al Jabal Al Akhdar is a NE/SW- to ENE/WSW-trending mobile part in Northern Cyrenaica province and is considered a large sedimentary belt in northeast Libya. Ras Al Hilal-Al Athrun area is situated in the northern part of this belt and is covered by Upper Cretaceous–Tertiary sedimentary successions with small outcrops of Quaternary deposits. Unmappable and very restricted thin layers of Palaeocene rocks are also encountered, but still under debate whether they are formed in situ or represent allochthonous remnants of Palaeocene age. The Upper Cretaceous rocks form low-lying to unmappable exposures and occupy the core of a major WSW-plunging anticline. To the west, south, and southeast, they are flanked by high-relief Eocene, Oligocene, and Lower Miocene rocks. Detailed structural analyses indicated structural inversion during Late Cretaceous–Miocene times in response to a right lateral compressional shear. The structural pattern is themed by the development of an E–W major shear zone that confines inside a system of wrench tectonics proceeded elsewhere by transpression. The deformation within this system revealed three phases of consistent ductile and brittle structures (D₁, D₂, and D₃) conformable with three main tectonic stages during Late Cretaceous, Eocene, and Oligocene–Early Miocene times. Quaternary deposits, however, showed at a local scale some of brittle structures accommodated with such deformation and thus reflect the continuity of wrenching post-the Miocene. D₁ deformation is manifested, in Late Cretaceous, via pure wrenching to convergent wrenching and formation of common E- to ENE-plunging folds. These folds are minor, tight, overturned, upright, and recumbent. They are accompanied with WNW–ESE to E–W dextral and N–S sinistral strike-slip faults, reverse to thrust faults and pop-up or flower structures. D₂ deformation initiated at the end of Lutetian (Middle Eocene) by wrenching and elsewhere transpression then enhanced by the development of minor ENE–WSW to E–W asymmetric, close, and, rarely, recumbent folds as well as rejuvenation of the Late Cretaceous strike-slip faults and formation of minor NNW–SSE normal faults. At the end of Eocene, D₂ led to localization of the movement within E–W major shear zone, formation of the early stage of the WSW-plunging Ras Al Hilal major anticline, preservation of the contemporaneity (at a major scale) between the synthetic WNW–ESE to E–W and ENE–WSW strike-slip faults and antithetic N–S strike-slip faults, and continuity of the NW–SE normal faults. D₃ deformation is continued, during the Oligocene-Early Miocene, with the appearance of a spectacular feature of the major anticline and reactivation along the E–W shear zone and the preexisting faults. Estimating stress directions assumed an acted principal horizontal stress from the NNW (N33°W) direction.     

鄂西白垩纪远安盆地的变形带构造

鄂西白垩纪远安盆地中大量发育变形带及其密切相关的断层,主要分为两期,早期为带有正断性质的北北东向组变形带及简单的正断层,形成在同沉积或早成岩阶段,而晚期为带走滑性质的北-南向组和北东向组变形带及复活断层,形成在白垩纪以后.根据实测的共轭变形带和断层滑动数据,古应力分析的结果表明,早期为北西西-南东东向拉张的伸展状态,而...     

鄂西远安白垩纪盆地的宏观构造变形机制

白垩纪远安盆地是江汉盆地西北缘的次一级地堑构造,发育有多种类型的低温宏观构造变形机制,包括变形带、断层、节理、脉体和缝合线。除断层外,其它构造变形机制的分布明显受不同岩性地层或单层厚度的控制。节理主要发育在盆地南、北两边上白垩统跑马岗组中至厚层粉砂岩与泥岩互层里,缝合线和方解石脉只形成在盆地西缘上白垩统罗镜滩组砾岩里,变形带只出现在盆地南部上白垩统红花套组块状砂岩里。主要根据变形构造交切或限制关系,本文将这里的宏观变形构造划分为四期,并恢复出各期的构造应力场。前两期属于拉张成因,出现在晚白垩世裂谷阶段,第一期的拉张方向为近EW向,第二期拉张方向NWW-SEE向;后两期属于挤压成因,出现在晚白垩世以后的后裂谷阶段。相应的水平最大主压应力分别呈NNE-SSW向和NWW-SEE向,具有顺时针旋转的趋势。     

Interpretation of high resolution aeromagnetic data over southern Benue Trough,southeastern Nigeria

High resolution airborne magnetic data of parts of the southern Benue Trough were digitally processed and analyzed in order to estimate the depth of magnetic sources and to map the distribution and orientation of subsurface structural features. Enhancement techniques applied include, reduction to pole/equator (RTP/RTE), first and second vertical derivatives, horizontal gradients and analytic signal. Results from these procedures show that at least 40% of the sedimentary basin contain shallow (<200 m) magmatic bodies, which in most cases are intermediate to mafic intrusive and hyperbysal rocks, and may occur as sills, dikes or batholiths. Magnetic lineaments with a predominant NE–SW trend appear to be more densely distributed around the basement rocks of the Oban Hills and metamorphosed rocks around the Workum Hills. 3D standard Euler deconvolution and Source Parameter Imaging (SPI ^TM) techniques were employed for depth estimation. Results from the two methods show similar depth estimates. The maximum depth to basement values for 3D Euler and SPI are 4.40 and 4.85 km with mean depths of 0.42 and 0.37 km, respectively. Results of 2D modelling of magnetic profiles drawn perpendicular to major anomalies in the study area reveal the existence of deep seated faults which may have controlled the emplacement of intrusive bodies in the basin. The abundance of intrusive bodies in the study area renders this part of the southern Nigerian sedimentary basins unattractive for petroleum exploration. However, the area possesses high potential for large accumulation of base metal mineralization.     

青藏高原东北缘祁连山地区古地震记录与区域构造意义

青藏高原东北缘的地质地貌特征与基本构造格架受阿尔金左行走滑断裂、东昆仑左行走滑断裂和海源左行走滑断裂控制,在青藏高原的碰撞造山过程中地震作用贯穿始终.2011至2012年,作者对青藏高原东北缘东起靖远(海原断裂东段)西至昌马盆地(阿尔金断裂东南)的软沉积物变形(震积岩)进行的系统调研表明,青藏高原东北缘祁连山地区从三叠纪至第四纪各时代(侏罗纪以外)的地层中,均发现丰富的古地震纪录(震积岩).代表性震积岩发现点和层位有:靖远宝积山上三叠统、靖远糜滩乡下白垩统、昌马盆地下白垩统以及赤金堡和酒泉磁窑口附近的第四系.主要的震积岩类型有:液化脉、负载、球-枕构造、液化卷曲、层间微断裂等.根据区域地质资料分析,这些震积岩均分布于区域性大断裂附近.本文简单讨论了主要震积岩发现点与其相邻的区域构造(地震)活动之间的关系,根据靖远附近上三叠统普遍发育震积岩,提出海原断裂带的初始活动时间很可能就在晚三叠世,与青藏高原东缘龙门山地区印支期的造山地震同时期.     

西非海岸盆地群形成演化及深水油气田发育特征

西非海岸盆地群的形成演化受非洲板块构造演化和大西洋张裂的控制,总体上可以分为前裂谷阶段、裂谷阶段和后裂谷阶段。沿岸15个主要盆地从北往南可细分为5段,分别为北段盐盆、中段盆地、尼日尔三角洲、中南段盐盆、南段盆地。深水油气田主要发育于后裂谷阶段的上白垩统和第三系海退层序中,以油田为主。在尼日尔三角洲和中南段盐盆的下刚果盆地,所发现的油气田个数及可采储量均比较集中,个数占西非深水油气田总个数比例分别为55%和29%,占总可采储量的比例分别为48%和41%。烃源岩主要有裂谷阶段下白垩统的湖相烃源岩,后裂谷阶段上白垩统、第三系的海相烃源岩。绝大多数储层为浊积岩储层,类型以深水水道砂岩为主,其次为席状砂储层和砂质碎屑流储层。圈闭类型主要为构造-地层复合圈闭,其次为构造圈闭,另外还有地层-岩性圈闭,其所占比例较小。盐构造和泥岩塑性变形构造总体上控制了西非深水区大多数复合圈闭和构造圈闭的形成。     

Structural Mapping of the Bentong‐Raub Suture Zone Using PALSAR Remote Sensing Data,Peninsular Malaysia: Implications for Sediment‐hosted/Orogenic Gold Mineral Systems Exploration

The Bentong‐Raub Suture Zone (BRSZ) of Peninsular Malaysia is one of the major structural zones in Sundaland, Southeast Asia. It forms the boundary between the Gondwana‐derived Sibumasu terrane in the west and Sukhothai Arc in the east. The BRSZ is genetically related to the sediment‐hosted/orogenic gold deposits associated with the major lineaments in the Central Gold Belt of Peninsular Malaysia. In this investigation, the Phased Array type L‐band Synthetic Aperture Radar (PALSAR) satellite remote sensing data were used to map major geological structures in Peninsular Malaysia and provide detailed characterization of lineaments and curvilinear structures in the BRSZ, as well as their implication for sediment‐hosted/orogenic gold exploration in tropical environments. Major structural lineaments such as the Bentong‐Raub Suture Zone (BRSZ) and Lebir Fault Zone, ductile deformation related to crustal shortening, brittle disjunctive structures (faults and fractures) and collisional mountain range (Main Range granites) were detected and mapped at regional scale using PALSAR ScanSAR data. The major geological structure directions of the BRSZ were N–S, NNE–SSW, NE–SW and NW–SE, which derived from directional filtering analysis to PALSAR fine and polarimetric data. The pervasive array of N–S faults in the Central Gold Belt and surrounding terrain is mainly linked to the N–S trending of the Suture Zone. N–S striking lineaments are often cut by younger NE–SW and NW–SE‐trending lineaments. Gold mineralized trend lineaments are associated with the intersection of N–S, NE–SW, NNW–SSE and ESE–WNW faults and curvilinear features in shearing and alteration zones. Compressional tectonic structures such as the NW–SE trending thrust, ENE–WSW oriented faults in mylonite and phyllite, recumbent folds and asymmetric anticlines in argillite are high potential zones for gold prospecting in the Central Gold Belt. Three generations of folding events in Peninsular Malaysia have been recognized from remote sensing structural interpretation. Consequently, PALSAR satellite remote sensing data is a useful tool for mapping major geological structural features and detailed structural analysis of fault systems and deformation areas with high potential for sediment‐hosted/orogenic gold deposits and polymetallic vein‐type mineralization along margins of Precambrian blocks, especially for inaccessible regions in tropical environments.     

Quaternary deformation around the Palo Negro area,Pampa Norte,Argentina

The Pampa Norte region is a great plain characterized by low slopes and accumulation of hundreds of meters of thick loose sediments. A high morphostructure denominated San Guillermo block stands out in the central plain, the Tostado–Selva scarp forming its western boundary. It is located in an intraplate setting characterized by low tectonic activity. However recent uplift can be inferred by means of terrain analysis and the sedimentology of the Palo Negro Fm. Pond deposits (Palo Negro Fm.) observed in the scarp suggest topographic inversion during the Late Quaternary in the Palo Negro area. The morphology indicates that the deformation was widely distributed, forming a gentle (ca. 5 m amplitude and 13 km wavelength) asymmetric fold. Low sinuosity lineaments located in the base of the scarp, coincident with knick points in the topographic profile, can be interpreted as the projection of tip-lines by high-angle fault-propagation. This geometry is compatible with reverse kinematics on blind faults.A deformation style with reactivation of pre-existing faults is consistent with structural observations. Seismic reflectors suggest the presence of Cretaceous high-angle normal faults, and the orientation of lineaments is similar to the orientation of the graben systems and transverse accommodation zones originated during the opening of the South Atlantic Ocean.One OSL dating of 67.4 ± 5.1 kyr B.P (from Palo Negro Fm. supposed as deposited on a flat plain floor) and a height difference of 9.5 m measured in the Tostado–Selva scarp account for an averaged uplift rate of 0.14 mm/year.     

烃浓度盖层封闭天然气的有效性及其研究意义

本文在烃浓度盖层封闭机理及其形成地质条件分析的基础上，通过烃浓度盖层封闭能力形成时期的确定，利用其与气源岩排气门限的匹配关系，探讨了烃浓度盖层封闭天然气的有效性。烃浓度盖层封闭能力形成时期只有早于或相近于气源岩排气门限时，才能有效地封闭气源岩排出呈分子扩散相运移的天然气；否则其封闭的有效性变差。并以松辽盆地三肇凹陷青山口组烃浓度盖层对下伏侏罗系气源岩天然气封闭的有效性为例进行分析，阐明其在天然气远景资源评价中的重要意义。     

Basement controls on fault development in the Penola Trough,Otway Basin,and implications for fault-bounded hydrocarbon traps

The Penola Trough is an intensely faulted northwest – southeast-trending half-graben structure. It is bound to the south by the major listric Hungerford/Kalangadoo Fault system. Several large prominent faults observed in the Penola Trough show offset of basement at depth. These basement-rooted faults have exerted significant controls on the geometry of smaller intra-rift faults throughout the entire structural history of the area. Faulting of the basement was initiated during the initial rift event of the Late Jurassic – Early Cretaceous. Faulting first propagated through a pre-existing basement fabric oblique to the north – south extension direction prevalent during this time. This resulted in the formation of the Hungerford/Kalangadoo and St George Faults with a northwest – southeast and north-northeast – south-southwest trend, respectively. A series of east – west-trending basement faults subsequently initiated perpendicular to the north – south extension direction as extensional strain increased in magnitude. Significant displacement along these basement-rooted faults throughout the initial rift event was associated with the formation of a complex set of intra-rift faults. These intra-rift faults exhibit a broadly east – west orientation consistent with the interpreted north – south extensional direction. However, this east – west orientation locally deviates to a more northwest – southeast direction near the oblique-trending St George Fault, attributed to stress perturbation effects. Many of the intra-rift faults die out prior to the end of the Early Cretaceous initial rift event while displacement on basement faults continued throughout. Faulting activity during the Late Cretaceous post-rift fault event was almost exclusively localised onto basement faults, despite a significant change in extension direction to northeast – southwest. A high-density, en échelon array of northwest – southeast-trending fault segments formed directly above the St George Fault and the large east – west-trending basement faults contemporaneously reactivated. Seismic variance data show that post-rift fault segments that are hard-linked to the St George Fault at depth have propagated through near-surface units. Non-basement-linked post-rift fault segments that lie away from the St George basement have not. This suggests that recent fault activity has continued to occur preferentially along basement faults up to relatively recent times, which has significant implications for fault seal integrity in the area. This is empirically validated by our structural analysis of fault-dependent hydrocarbon traps in the area, which shows that partially breached or breached hydrocarbon columns are associated with basement faults, whereas unbreached hydrocarbon columns are not.     

New insights on the tectonic and paleogeographic evolution of the central Atlasic domain of Tunisia

Geological and geoseismic profiles and well data gathered with field observations from the Atlasic Chain in central Tunisia highlight folded structures, tectonic events, and significant faults. These events controlled basin formation and evolution during successive Mesozoic extensional phases, followed by the tectonic inversion during the Atlasic Orogeny known on a Tethyan scale. The Cretaceous extension is well recorded through deposition, which supplied the normal faults system and influenced sediment distribution and regional subsidence. The major event is the normal slip of the principle inherited fault during the Cretaceous subsidence. The northwestern blocks, which are north of the faults of Mrhila–Trozza–Cherichira and Ballouta and west of the NS axis, correspond to continually subsiding areas of the Upper Cretaceous series. Subsequent faulting reactivated compressional structures such as strike-slips, reverse, and thrust faults during the Tertiary Orogeny which largely affected the Tunisian Atlasic domain. Geological profiles point out the evidence of the Upper Cretaceous emersion of the central Tunisia domain and lateral thickness variation of the series from Jurassic to Quaternary, unconformities, and halokinesis movement.     

PALEOGEOLOGIC RECONSTRUCTIONS OF THE RUSSIAN PLATFORM

A belt of low-angle normal (or detachment) faults ～250 km long extends from the northern end of the Salton Trough, California to southern Laguna Salada, Baja California, Mexico. The detachment system is divided into two principal segments. The northern segment, here termed the “west Salton detachment system,” comprises top-to-the-east detachment faults along the eastern Peninsular Ranges that root under the Salton Trough. The southern segment, here termed the Laguna Salada detachment system, comprises top-to-the-west detachment faults in northeastern Baja California and the Yuha Desert region of the southwesternmost Salton Trough. Detachments of that system root under Laguna Salada and the Peninsular Ranges of northern Baja California. Both of these systems experienced a major episode of activity in late Miocene to Pleistocene time, synchronous with deposition of the Imperial and Palm Spring formations, and the Laguna Salada detachment system may still be active. Thus, their activity temporally overlapped, partly or completely, with activity on dextral faults of the San Andreas boundary between the Pacific and North American plates, and with accretion of new transitional crust. Some of the detachment faults in the northern segment may have had mid-Miocene normal slip and/or Cretaceous thrust or normal slip as well, although compelling evidence for either is lacking. These detachment faults are distinctly younger than detachments east of the San Andreas fault, which generally ceased activity by middle or late Miocene time and are overlapped by marine or lacustrine rocks (Bouse Formation); these units are equivalent in age to the syntectonic strata of the Salton Trough but are much thinner and essentially undeformed.