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.     

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.     

Liquid hydrocarbon source-rock potential of mid-Cretaceous coals and coal measures in the Middle Benue Trough of Nigeria

Received: 23 March 1999 / Accepted: 25 August 1999     

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.     

Heavy metal concentrations in soils and plants in the vicinity of Arufu lead-zinc mine,Middle Benue Trough,Nigeria

This study focused on the influence of base metal mining on heavy metal levels in soils and plants in the vicinity of Arufu lead-zinc mine, Nigeria. Soil samples (0-15 cm depth) and plant samples were collected from cul-tivated farmlands in and around the mine, the unmineralized site and a nearby forest (the control site). The samples were analyzed for heavy metals (Fe, Zn, Mn, Cu, Pb, Cr and Cd) by Atomic Absorption Spectrophotometry (AAS). The physical properties of soils (pH and LOI) were also measured. Results showed that soils from cultivated farm-lands have neutral pH values (6.5-7.5), and low organic matter contents (10%). Levels of Zn, Pb and Cd in culti-vated soils were higher than the concentrations obtained from the control site. These heavy metals are most probably sourced from mining and agricultural activities in the study area. Heavy metal concentrations measured in plant parts decreased in the order of rice leavescassava tuberspeelings. In the same plant species, metal levels decreased in the order of ZnFeMnCuPbCrCd. Most heavy metals were found in plant parts at average concentrations normally observed in plants grown in uncontaminated soil, however, elevated concentrations of Pb and Cd were found in a few cassava samples close to the mine dump. A stepwise linear regression analysis identified soil metal contents, pH and LOI as some of the factors influencing soil-plant metal uptake.     

Further evidence of marine Cenomanian,lower Turonian and Maastrichtian in the Upper Benue basin of Nigeria (West Africa)

Field investigations in the Upper Benue rift basin of Nigeria highlight the necessity for a review of the Cenomanian-Turonian biostratigraphy of the area. Three ammonite zones of Vascoceras bulbosum (Reyment), Paravascoceras costatum (Reyment) and Pseudotissotia (Bauchioceras) nigeriensis (Woods) have been recognized at sections exposed at the quarry of the cement factory near the village of Ashaka. The sediments within the Vascoceras bulbosum zone also contain specimens of Exogyra olisiponensis (Sharpe), Kanabiceras septemseriatum (Cragin) and Epengonoceras dumbli (Cragin). These are all indicative of an uppermost Cenomanian age and reveal that at least the lower portion of the Gongila Formation as exposed at Ashaka is faunistically equivalent to the Pindiga Formation as exposed in the type section at Pindiga.The shales in the Gombe region in which Libycoceras ismaeli (Zittel) was found are probably part of the Maastrichtian Fika shales outcropping extensively in the northern part of the area. The discovery of Cyclolithes nov. sp. within Gombe sandstones at Dakiti near Kumo confirm that they are at least partially marine in origin and Maastrichtian in age. This could be considered as an additional evidence in favour of the presence of marine waters in the Upper Benue during the last part of the Cretaceous.     

Modern and late Quaternary clay mineral distribution in the area of the SE Mediterranean Sea

The present-day clay mineral distribution in the southeastern Levantine Sea and its borderlands reveals a complex pattern of different sources and distribution paths. Smectite dominates the suspended load of the Nile River and of rivers in the Near East. Illite sources are dust-bearing winds from the Sahara and southwestern Europe. Kaolinite is prevalent in rivers of the Sinai, in Egyptian wadis, and in Saharan dust. A high-resolution sediment core from the southeastern Levantine Sea spanning the last 27 ka shows that all these sources contributed during the late Quaternary and that the Nile River played a very important role in the supply of clay. Nile influence was reduced during the glacial period but was higher during the African Humid Period. In contrast to the sharp beginning and end of the African Humid Period recorded in West African records (15 and 5.5 ka), our data show a more transitional pattern and slightly lower Nile River discharge rates not starting until 4 ka. The similarity of the smectite concentrations with fluctuations in sea-surface temperatures of the tropical western Indian Ocean indicates a close relationship between the Indian Ocean climate system and the discharge of the Nile River.     

Diagenesis, palaeoclimate and tectono-sedimentary influences on clay mineralogy and stable isotopes from Upper Cretaceous marine successions of the Basque-Cantabrian Basin (N Spain)

Clay mineralogy and whole-rock stable isotopes (δ¹⁸O and δ¹³C) of Upper Cretaceous marly sediments on the Basque-Cantabrian Basin have been integrated to determine the main effects of diagenesis, palaeoclimate and tectono-sedimentary factors in sections belonging to deep- (Barrika) and platform-marine (Isla de Castro, Villamartín and Olazagutía) settings.The mean values for the clay assemblages and δ¹⁸O exhibit notable differences among the sections, partially explainable by the influence of diagenesis. The Barrika sediments, with more diagenetically advanced illite-smectite (I-S) mixed-layer (R1, 70% illite), authigenic chlorite, and low δ¹⁸O (−4.05‰ PDB), experienced higher diagenetic grade than Isla de Castro and Olazagutía, which have R0 I-S (20% illite) and heavier δ¹⁸O. Villamartín was also affected by higher diagenesis than Isla de Castro and Olazagutía, given the occurrence of R1 I-S (60% illite) and low δ¹⁸O (−4.11‰ PDB). However, the absence of other clays in Villamartín (e.g. authigenic chlorite) is indicative of less diagenetic grade than Barrika. These results show the useful integration of clay mineralogy and stable isotopes to detect different diagenetic grades in distinct marine successions of the same basin.Despite being influenced by diagenesis, the clay mineralogy partially preserves its inherited signature. This allows detection of major contents of I-S and mica, and minor kaolinite, interpreted as indicative of warm palaeoclimatic conditions. High kaolinite content in Villamartín and absence of kaolinite in Isla de Castro, though, are considered to be a product of neither diagenesis nor palaeoclimatic influences. Instead, tectono-sedimentary causes, related to unsuitable conditions for clay formation and transport from the local source areas, contributed to original clay differences. The inferred effects of diagenesis, palaeoclimate and tectono-sedimentary factors make this work important to show the potentially great variety of controls on the clay mineralogy of marine sections, which are often uncritically treated in studies concerning the Late Cretaceous.     

Climatic and sea‐level control of Jurassic (Pliensbachian) clay mineral sedimentation in the Cardigan Bay Basin,Llanbedr (Mochras Farm) borehole,Wales

Early Jurassic climate is characterized by alternating cold and warm periods highlighted by studies based notably on oxygen isotopes measured on belemnite guards and other marine invertebrate shells. These climatic changes include changes in the hydrological cycle, and consequently weathering and runoff conditions. In order to clarify the erosion and weathering conditions during the Pliensbachian, this study determined the mineralogical composition of the clay fraction of 132 samples taken from the entire stage drilled in the Llanbedr (Mochras Farm) borehole (Cardigan Bay Basin). The clay mineral assemblages are composed of various proportions of chlorite, illite, illite/smectite mixed‐layers (R1 I–S), smectite and kaolinite, with possibly occasional traces of berthierine. The occurrence of abundant smectite indicates that the maximum burial temperature never exceeded 70°C. Consequently, clay minerals are considered mainly detrital, and their fluctuations likely reflect environmental changes. The variations in the proportions of smectite and kaolinite are opposite to each other. Kaolinite is particularly abundant at the base of the jamesoni Zone, in part coinciding with the δ¹³C negative excursion corresponding to the Sinemurian/Pliensbachian Boundary Event, and through the davoei Zone, whilst smectite is abundant in the upper part of jamesoni and base of ibex zones and through the subnodosus/gibbosus subzones of the margaritatus Zone. The kaolinite‐rich intervals reflect an intensification of hydrolysis and an acceleration of the hydrological cycle, while the smectite‐rich intervals indicate a more arid climate. The spinatum Zone is characterized by a distinct clay assemblage with abundant primary minerals, R1 I–S, kaolinite reworked from previously deposited sediments or from Palaeozoic rocks, and probably berthierine originating from contemporaneous ironstone‐generating environments of shallower waters. This mineralogical change by the end of the Pliensbachian likely reflects a transition from a dominant chemical weathering to a deeper physical erosion of the continent, probably related to a significant sea‐level fall consistent with a glacio‐eustatic origin.     

Basaltic magmatism related to the early stages of rifting along the Benue Trough: The Obudu dolerites of south-east Nigeria

Dolerites of the Obudu Plateau of south-eastern Nigeria occur predominantly as dykes cutting Precambrian basement gneisses. Minor dolerite sills have also been mapped. The dolerites are dominantly olivine tholeiites and are geochemically of the oceanic basalt type. They were emplaced in a within-plate tectonic setting and yield a ⁴⁰Ar/³⁹Ar plateau age of 140.5 ± 0.7 Ma, which is interpreted as the age of their emplacement. This age indicates that the Obudu dolerites are the products of basic magmatism related to the early stages of rifting along the Benue Trough of Nigeria. The Obudu dolerites are comparable with Mesozoic tholeiitic dykes of the northern Amazon craton, whose intrusion preceded the break-up of Africa and South America.     

Characteristics of clay minerals in sediments of Hemudu area,Zhejiang, China in Holocene and their environmental significance

The Ningshao Plain experienced sea-level fluctuation and climate change in the Holocene and gave birth to a Neolithic Civilization, the Hemudu Culture (about 5000 BC). Using XRD method, this paper studied the clay mineralogy of sediments in core HMD-1301 from Hemudu Site area in Ningshao Plain. Results show that the clay mineral assemblages in all samples are quite consistent. They are dominated by illite (about 57%) and smectite (about 26%), and a small amount of kaolinite (about 6%) and chlorite (about 9%). In addition, illite, chlorite, and kaolinite are originated mainly from eolian loesses. Smectite is resulted mainly from chemical weathering of local volcanic rocks outcropped. Illite crystallinity indicates the climatic characteristics of the source region, and illite chemistry index reflects the climate nature of the sedimentary area. It is our understanding that water medium properties in a sedimentary environment affect clay mineral assemblage and the distribution, based on which were cognized the Holocene transgression-regression events in the Hemudu Site area. These mineral indices are well comparable with geochemical indicators in identifying the transgression-regression events. We believe that these events affected the ancient civilization of the ancient residential communities at the time.©2019 China Geology Editorial Office.     

新疆库车凹陷侏罗系多粒级黏土矿物分析方法及其对沉积-成岩环境的指示

本文选取库车坳陷侏罗系阿合组—齐古组的碎屑岩样品,利用Stokes原理,通过同步跟进与离心分析相结合的多粒级提取的方式提取不同粒径的黏土矿物,包括〉20μm;20μm～5μm;5μm～2μm;2μm～0．2μm和〈0．2μm。不同粒径组分的黏土矿物呈现出不同的型貌特征,在20～2μm范围内的样品中,自生黏土矿物少见,大量可见碎屑来源的黏土矿物,由于经过了长期的机械搬运,难见完整的晶型。在〈2μm的样品中大量存在自生黏土矿物,大量成集合体,同时也存在碎屑蚀变成因的黏土矿物。通过XRD方法定量分析了所分离出的不同粒径黏土矿物的相对含量,黏土粒径范围内的黏土矿物相对含量呈现些规律变化。碎屑伊利石含量有随着粒级的减小而减少的趋势,而自生伊利石和伊蒙混层只在2～0．2μm及0,2μm以下粒级才出现,并且集中在0．2μm以下粒级范围内;在古气候,沉积环境和母岩特性及分布基本一致的情况下,高岭石的分布有随着岩性的变粗呈现升高的趋势,而伊利石呈负相关关系。碎屑岩中黏土矿物的形貌特征表明,黏土矿物有陆源碎屑来源与自生两种成因,应用Decoform软件对0．2μm以下黏土矿物中不同成因的伊利石进行了计算,并就存在的比例与粒径的关系进行了讨论。黏土矿物组合揭示侏罗纪沉积环境由酸性环境向微碱性环境过渡,早侏罗世和中侏罗世早期为气候潮湿时期,高岭石的含量明显高于侏罗世晚期地层,恰克马克期到齐古期高岭石含量开始出现减小的趋势。从恰克马克期开始,气候缓慢向干旱变化,高龄石含量降低,而伊利石的平均含量明显增加,反映黏土矿物组合主要受沉积环境和成岩作用共同影响。     

鄂尔多斯盆地北部砂岩铀矿床粘土矿物与氧化带成因关系研究

对鄂尔多斯盆地北部铀成矿带产铀层位中侏罗统直罗组砂岩进行系统采样和分析,以探讨砂岩中粘土矿物特征与氧化带成因之间的关系。采用X‐射线粉晶衍射、电子探针分析等现代分析手段,测定粘土矿物相对含量、微观形貌特征。结果表明：研究区古层间氧化的演变序列为砂体原始灰色→古氧化作用（红色或黄色）→碳酸盐化（残留了部分红色）→后期还原作用（绿色）,对应的岩石蚀变期次为早期氧化酸性蚀变→氧化期后弱碱性蚀变→晚期还原弱碱性蚀变;早期氧化砂岩在还原性气体作用下,Fe³⁺还原转化为Fe²⁺,高岭石在有Fe²⁺存在的条件下会向绿泥石转化,这一反应在碎屑颗粒表面,形成许多极细小的针叶状绿泥石,从而使古层间氧化亚带呈现特征的绿色。     

Process of slip-surface development and formation of slip-surface clay in landslides in Tertiary volcanic rocks, Japan

Once landslide movement has been initiated, the sliding mass tends to move toward a more stable position. However, landslides that have occurred in some Tertiary volcanic rocks in Japan show recurrent movement. A possible reason for this behavior is that the mineral composition and properties of slip-surface materials results in better ‘lubrication’ along the slip surface as movement continues. In order to determine the causes, five landslides in Japanese Tertiary volcanics with different movement histories were studied. Based on geological investigation, the results presented in this paper were obtained by studying the development process and formation of slip surfaces and clays in landslides in these volcanics.

For rocks of similar types and mineralogies the clay content of slip-surface materials can be directly related to the displacement and history of slide movement. This study indicates that the type of slip surface ranges from: (1) striation type (showing only striations on bedrock without clay), to (2) brecciated type (clayey breccia), (3) mylonite type (clay with breccia), and to (4) clay type (clay without breccia). If the slip surface occurs in tuff or tuffaceous materials and is subjected to increasing slide movement, the smectite content in the slip-surface clay increases, while the silicate mineral content decreases. Furthermore, the smectite content is observed to increase within the same landslide from the head to the toe portion of the landslide (direction of groundwater flow). The groundwater along the slip surface (above the impervious zone) contains an increased concentration of Ca⁺⁺ ions. As the concentration of HCO₃⁻ ions exceeds 40 mg/l, an environment is created that promotes the formation of smectite. This process indicates that there is an active ion exchange within the groundwater near the slip surface that forms smectite. Thus, the formation of smectite in the slip surface requires the presence of tuff and a sufficient concentration of HCO₃⁻ ions in the groundwater.

The study results presented here indicate that frictional resistance decreases as clay content increases. The reason for repeated movement of Japanese landslides in Tertiary tuffs or tuffaceous rocks at or in the immediate vicinity of the slip surface is attributed to: (1) an increase in clay materials along the slip surface as the slide movement is repeated and (2) the presence of groundwater that promotes the formation of smectite. As is commonly known, smectite is one of the clay minerals that has lowest frictional resistance.     

Changes in clay mineral content of tidal flat sediments resulting from dike construction along the Lower Saxony coast of the North Sea, Germany

The clay mineral content of the < 2 µm fraction in tidal flat sediments in the former Harle Bay on the Lower Saxony coast has changed considerably since the thirteenth century up to the present time. The proportion of smectite has decreased from 29 to 11 wt%, whereas the proportion of illite has increased by about 10 wt% and those of chlorite and kaolinite by about 3 wt% each. The chemical composition of the major clay mineral illite has also changed: Al₂O₃ has decreased by about 2 wt%, K₂O by about 1·5 wt% and SiO₂ by about 3 wt%. It can be concluded that the changes in the clay mineral content are mainly influenced by dike construction and sea level rise, modifying flow pattern, submarine morphology, sedimentation and tidal range. These processes cause smectite to settle less rapidly because of its small grain size and low density resulting from interlayer water, so that the sediment becomes impoverished in this mineral.     

Sequence stratigraphy in the upper cretaceous of the Basco-Cantabrian Basin (northern Spain)

The sedimentary cycles of the Cenomanian to Maastrichtian were investigated in the Basco-Cantabrian Basin (BCB) in northern Spain (Provinces of Alava, Vizcaya and Burgos). The depositional area was a distally steepened carbonate ramp which extended from Catalonia northwestwards to the Basque country. The investigated sediments range from calciturbidites and pelagic marls to marl-limestone alternations deposited on a distal carbonate ramp. Shallow marine limestones, marls and intertidal clastics and carbonates were deposited on the proximal part of the carbonate ramp. The establishment of a regional sequence analysis is based on the investigation of seismic profiles, well logs and outcrop sections. Examples of outcrop sections are interpreted in terms of sequence stratigraphy (unconformities of third- and second-order cycles, depositional geometries, systems tracts). The sequence stratigraphic interpretation of outcrop sections is based on facies analysis, interpretation of observed depositional geometries and correlation of unconformities and marine flooding surfaces through the basin. A biostratigraphic framework is established based on ammonites, inoceramids, planktonic and benthic foraminifera. As a result, a regional sequence stratigraphic cycle chart is presented and compared with published global cycle charts. The correlation of the regional cycle chart with published cycle charts is good. In the Cenomanian and Turonian, several sequence boundaries in the BCB are shifted by up to one biozone compared with the global chart. Some type 1 boundaries of the standard chart are only type 2 in the BCB. Important type 1 boundaries in the BCB are: top Geslinianum Zone with a 100 m lowstand wedge at the basis of the sequence (sequence boundary 92.2) ; base Petrocoriense Zone with a 250 m shallowing-upwards lowstand wedge at the basis (sequence boundary 89.2); and within the Syrtale Zone (sequence boundary 85.0).The Campanian-Maastrichtian sequence record is strongly disturbed by local compressive tectonics. Several sequences are recognizable and can be correlated with the global cycle chart. Correlation is hampered by the low biostratigraphic resolution in the western basin part. Subsidence analysis of several sections of the Upper Cretaceous of the BCB and its interpretation in the regional tectonic context leads to a discussion of the causes of the observed cyclicity. A regional eustatic curve is presented for the Upper Cretaceous of the BCB. Stage and substage names were used according Code-Committee (1977). Correspondence to: K.-U. Gräfe     

Colour origin of upper cretaceous pelagic red sediments within the Eastern Pontides, northeast Turkey

 Characteristic samples of Upper Cretaceous pelagic red sediments from different parts of the Eastern Pontides (NE Turkey) have been investigated by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy coupled with energy dispersive analyses (SEM/EDAX). The red sediments are composed of limestone and muddy limestone, and characterized by abundant planktonic foraminifers. Hematite content ranges from 0.5 to 3.0 wt.%. Electron microscope observations suggest that the hematite pigment has a diagenetic origin. The red colour is due to presence of hematite pigment, and indicates oxidizing conditions during early diagenesis in a relatively deep marine environment. Received: 4 January 1999 / Accepted: 8 July 1999     

Size and shape variation in the planktonic foraminiferHeterohelix striata (late cretaceous, southern Scandinavia)

The biserial planktonic foraminiferal species Heterohelix striata (Ehrenberg)has been analyzed by morphometric techniques from six sequential late Maestrichtian core samples from southern Sweden and one sample from the Daniaquarry of Denmark. The study is based on measurements of number of chambers, size of test (length, width, and thickness),marginal angle, and chamber sizes. In addition, prolocular and apertural sizes were analyzed in the Daniaquarry sample. The study includes analyses of the ontogenetic development of the tests, stratigraphic variation in size and shape, intrasample variability, and relationship between gross morphology of the test and prolocular and apertural dimensions.     

A new species of the genus Echinorhinus (Chondrichthyes,Echinorhiniformes) from the upper cretaceous of southern South America (Argentina-Chile)

We describe isolated shark teeth collected from levels of the Calafate Formation at the SE coast of the Argentino Lake, Calafate city, Santa Cruz province, Argentina (Atlantic Ocean), and from the Algarrobo coast at the Valparaíso Region in central Chile (Pacific Ocean). The teeth belong to a new species of the echinorhiniform genus Echinorhinus. Echinorhinus maremagnum n. sp. was a taxon distributed in both the southwestern Atlantic and the southeastern Pacific. This new taxon constitutes the oldest record of echinorhiniforms from South America and one of the few Mesozoic records at a worldwide scale.     

Utility of clay Minerals in the Determination of Sedimentary Transport Patterns in the Bay of Cadiz and the Adjoining Continental Shelf (SW-Spain)

The contents and the distribution of clay minerals in modern sediments of the Cadiz bay and the continental shelf have been studied aiming to establish the sedimentary exchange model and the pathways followed by the clay fraction between the bay and the adjoining continental shelf. The most abundant clay minerals in the muddy sediments consist mainly of illite, smectite, interstratified illite-smectite, kaolinite and chlorite. The application of factor analysis method (PCA) to clay minerals data are considered to be useful in the determination of sedimentary transport patterns. The data from clay mineral assemblages, and their distribution map, make possible to establish the fine sediments transport paths in the study area using clay mineral as natural tracers. This model of transport takes in consideration the facies distribution, the supplies sources and the way of fine sediments are incorporated to the marine environment.

Two flows paths have been established: the outflows coming from Cadiz bay and the littoral zones; and the inflows coming from external marine zones of the bay. These flows are controlled by tidal currents and the morphology of the coast. The action of surge and the marine currents, specially the Atlantic Surface Water flow, are also important in the transport of fine sediments coming from sources located to the north of the study zone.