滇东寒武系磷矿成因新见解——风暴成矿模式初探

近年来,随着风暴流和风暴岩的新理论、新概念的不断引入,在分析沉积环境、沉积成矿作用时,不断提出了许多新思路,也不断地冲击和修正以往的传统认识,作者于1986年随同成都地矿所寒武系磷矿专题组对滇东一带包括昆阳梅树村、海口、鸣矣河、晋宁王家湾、二街、八街、安宁白登、江川清水沟、宜良大滴水、寻甸先锋等矿区的磷矿进行了详细的研究,在下寒武统中谊村段及其相当的地层中发现了多层磷质风暴沉积物。本文拟概略介绍磷质风暴沉积物的标志、特征、剖面结构并初步探讨磷矿富集与风暴事件关系的沉积演化模式。     

元古界会理群凤山营组风暴流沉积特征

四川省会理县洪川桥地区,是会理群凤山营组广泛分布地区之一,出露面积达百余平方公里。在洪川桥凤山营组剖面的上部(洪剖119层—108层),发育一套约845m厚的风暴流沉积。现将其特征及形成机制作一初步分析,以供参考。一、岩性特征风暴流沉积物,主要为灰—深灰色粉砂质粉屑灰岩和部分泥质灰岩。有时两者呈薄的互层出现。一般层厚几毫米到几厘米,沿走向在数十厘米或几米内尖灭,形成一个个薄的透镜     

Braarudosphaera -rich sediments in the Turonian of the Bohemian Cretaceous Basin, Czech Republic

Braarudosphaera -rich sediments occur in the Turonian (nannofossil zones CC13 and UC9a) epicontinental marine sediments of the Bohemian Cretaceous Basin. This phenomenon is linked to the input of terrigenous material during deceleration of sea-level rise, or stillstand in sea level, which possibly both triggered the Braarudosphaera bigelowii bloom and reduced the diversity of the nannoplankton assemblages. The occurrence of Marthasterites furcatus and Lithastrinus septenarius probably reflects depositional conditions that were suitable for their preservation. Hence, the resultant limitations to their use as local zonal markers in the Bohemian Cretaceous Basin are discussed. The influence of the Tethyan Bioprovince is documented by the coincident first occurrences of L. septenarius and M. furcatus. The presence of the high-latitude species Thiersteinia ecclesiastica indicates penetration of Boreal biota into this depositional area within the Turonian.     

Sea level changes in the upper Aptian-lower/middle(?) Turonian sequence of Cauvery Basin,India – An ichnological perspective

Cauvery Basin, a pericratonic rift basin along the Eastern Continental Margin of India, evolved during the breakup of the Eastern Gondwanaland. It exposes both syn-rift and later post-rift passive margin deposits ranging from Barremian to Miocene. The Karai Formation, upper Aptian-lower/middle (?) Turonian represents the oldest passive margin in the Cauvery Basin. It is bounded at both contacts by major sequence boundaries viz. the break-up unconformity and the Turonian tilt event. The present communication deals with the ichnology of the Karai Formation and its integration with sedimentary facies and biostratigraphy to interpret the sea level changes during deposition. A traverse between the villages Karai and Kulakkalnattam was studied in detail for this purpose. Based on the lithological position, characters and internal grain size trends, the Karai Formation is sub-divided into four informal lithologic units; the lower three units, constitute a lithostratigraphic unit known in literature as the Gypsiferous Clay Member, while the uppermost, corresponds to the Sandy Clay Member. At the base, clays of the Karai Formation unconformably onlap onto the Precambrian basement or the fluvial syn-rift deposits across the break-up unconformity. Upper Aptian to middle Cenomanian, units I and II showing the distal Cruziana ichnofacies, deepening of the basin and a retrogradational stacking pattern represent a transgressive system tract (TST). This long phase of transgression is attributed to continuous accommodation created by the post-breakup thermal subsidence. The upper part of unit II (middle Cenomanian) shows condensation, with its top representing the maximum flooding surface (MFS). Upper Cenomanian to lower/middle (?) Turonian, units III and IV characterised by a shift from the distal Cruziana to the Skolithos ichnofacies, an initial aggradational and later deltaic, progradational stacking pattern resulting from a fall in the relative sea level and filling up of accommodation space represent the highstand system tract (HST). A further fall in the relative sea level led to the exposure, incision and erosion of the Karai Formation over which the younger transgressive sequence of the Trichinopoly Group was deposited with an angular unconformity.     

Nearshore to offshore facies and depositional history of the Ordovician Daylesford Limestone New South Wales

The Daylesford Limestone is the basal formation of the Ordovician Bowan Park Group of central western New South Wales. The formation contains four main limestone types and minor intercalated terrigenous beds. Limestones are: (1) grain‐stone, (2) grey skeletal wackestone and packstone, (3) dark grey burrowed wackestone and packstone, and (4) dark grey burrowed lime mudstone. Grainstone and grey skeletal wackstone and packstone are dominant in eastern sections; they are laterally equivalent to, and interfinger with, dark grey muddy limestones that dominate western sections. Lithoclasts are abundant in the grainstone but are absent from muddy sections to the west except in thin beds above disconformities. The rock types of the Daylesford Limestone also tend to occur sequentially above some disconformities; the full sequence is: grainstone (or grey skeletal wackestone and packstone) grading up into dark grey burrowed wackestone and packstone and thence into dark grey burrowed lime mudstone. Each sequence is probably trans‐gressive and reflects deepening water.

During deposition of the Daylesford Limestone, an area to the east was uplifted, providing lithoclasts to be reworked into the neighbouring depositional basin. Uplift also produced numerous regressions and subaerial disconformities. Facies patterns were essentially similar throughout the history of the formation. Grainstone accumulated in high‐energy nearshore environments adjacent to the uplifted area, and grey skeletal wackestone and packstone in low‐energy nearshore environments. Dark grey lime mudstone formed in offshore low‐energy environments to the west of the uplifted area; and dark grey wackestone and packstone in intermediate environments. In parts, burrowing organisms kept pace with sedimentation and locally mixed interbedded grainstone and muddy limestone.     

Medieval Dark Earth in an Active Alluvial Setting from the Uffizi Gallery Complex in Florence,Italy

Archaeological excavations at the Biblioteca Magliabechiana, which is part of the Uffizi gallery complex in Florence (Tuscany, central Italy), exposed Dark Earth deposits between alluvial sediments of the nearby Arno River. The term Dark Earth refers to thick, dark colored, and seemingly homogeneous deposits occurring in urban settings throughout Europe, broadly dated to the post‐Roman period. The stratigraphic sequence of the Biblioteca Magliabechiana was investigated by integrating geomorphological, chronological, and archaeological information with micromorphological and soil analytical data. This combination of approaches resulted in the identification of an early phase of Dark Earth formation (7th century A.D.) during which the area was characterized by the accumulation of manure or night soil in wet peri‐fluvial conditions. After a phase of abandonment, Dark Earth accretion resumed (10th to early 11th century A.D.), at this stage due primarily to the dumping of domestic waste. Subsequent progressive abandonment of the area was followed by the deposition of alluvium, probably during the major flood of November 4, 1177. The present study demonstrates that Dark Earth is the outcome of strong bioturbation and human reworking acting on anthropogenic deposits and possibly on alluvial sediments.     

Radiolaria,planktonic foraminifera,and stratigraphy of Turonian-lower Coniacian in the Biyuk-Karasu section,Crimea

The first data on the distribution of Radiolaria and planktonic Foraminifera in the section at Biyuk-Karasu River in central part of the Crimean Mountains, are presented. Based on the study of radiolarian findings, the upper Cretaceous deposits of Biyuk-Karasu section are subdivided into the following biostratigraphic units: Alievium superbum-Phaseliforma turovi (middle Turonian), Dactylodiscus longispinus-Patulibracchium (?) quadroastrum (upper Turonian), and Orbiculiforma quadrata-Patellula sp. B (Turonian-Coniacian boundary deposits). The stratigraphic interval of Alievium superbum-Phaseliforma turovi has been previously characterized by the complex of Alievium superbum-Phaseliforma sp. A (middle Turonian, Mt. Chuku section, SW of the Crimean Mountains, and middle Turonian, Mt. Ak, central part of the Crimean Mountains). Based on the study of Foraminifera findings, the following biostratigraphic subdivisions were identified: Whiteinella paradubia (lower-lower part of the middle Turonian), Marginotruncana pseudolinneiana (uppermost middle Turonian), and Marginotruncana coronata (upper Turonian). The complex of beds with Marginotruncana pseudolinneiana and Marginotruncana coronata are comparable to the deposits of zones of same name in the Crimean-Caucasian region.     

Phosphate-rich deposits associated with the Mio-Pliocene unconformity in south-east Australia

Phosphates are present on the surface of the Mio-Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south-east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south-east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south-eastern Australia.     

中国扬子地台西缘寒武纪风暴事件与磷矿沉积

中国扬子地台及邻区海域,在早寒武世至中寒武世早期,风暴活动频繁,地理位置由西向东,由南向北,地层层序自下而上三个组段发现有风暴岩和风暴沉积物,其中以下寒武统梅树村组的磷质风暴岩最典型。风暴活动冲击川滇岛陆及牛首山岛陆的内侧,从而使两个岛陆间的海面下降,造成外海海水填充并形成上升洋流。富磷海水随着上升洋流运移,为磷质沉积提供来源。因此,风暴和风暴流作用不仅形成和加速了上升洋流的循环和补给,同时又是磷矿簸选富集的物理驱动力。风暴流向海的流动和上升洋流通过的方向,沿着扬子地台西缘形成了我国重要的磷质富集带与工业磷矿分布区。     

Cartographie de corps stériles sous couverture quaternaire par méthode de résistivités électriques dans le gisement phosphaté de Sidi Chennane (Maroc)Sterile bodies mapping under Quaternary cover using resistivity-sounding method in the phosphatic bearing of Sidi Chennane (Morocco)

In the Ouled Abdoun sedimentary basin (Morocco), the phosphatic series is composed of regular interbedded phosphatic and marly limestone layers. Some phosphatic deposits in this basin show sterile bodies causing two kinds of problems: (1) as they are hard, compact and masked by a Quaternary cover, they disturb the exploitation in some yards and give bad reserve calculations; (2) even the use of wells and mechanical boreholes did not evidence their delimitation. Therefore, electric prospecting method has been used to evidence their geometrical shape. Petrographical and geometrical studies on these sterile bodies allowed us to choose the appropriate geophysical method to map them. The electrical resistivity survey that we used in the Sidi Chennane area shows that this technique is a good tool to contour these sterile bodies. This method is now considered as useful to the mining engineers to get round them during the exploitation. To cite this article: A. Kchikach et al., C. R. Geoscience 334 (2002) 379–386.     

Palynofacies and sea-level changes in the Upper Cretaceous of the Vocontian Basin, southeast France

Upper Cretaceous platform carbonates of the Vocontian Basin (southeastern France) have been investigated in a cross-section from the proximal deposits exposed in the lower Rhône Valley to the distal part of the basin in the Southern Subalpine Ranges north of Nice. The stratigraphic interval studied in detail spans the uppermost Turonian and Coniacian.Palynofacies patterns were used to detect eustatic signals at a third-order scale and are the tool for correlation of proximal and distal platform deposits. The organic constituents observed in the studied samples have been grouped into a continental fraction, including higher plant debris (phytoclasts) and sporomorphs, and a marine fraction with dinoflagellate cysts, acritarchs, prasinophytes, and foraminiferal test linings. The main factors influencing the stratigraphic and spatial distribution of land-derived, allochthonous, and marine, relatively autochthonous, organic particles are the proximity of land, the organic productivity, the degree of biodegradation and the hydrodynamic conditions of the depositional system. Palynofacies parameters used for the sequence stratigraphic interpretation are: (1) the ratio of continental to marine constituents (CONT/MAR ratio); (2) the ratio of opaque to translucent phytoclasts (OP/TR ratio); (3) the phytoclast particle size and shape; and (4) the relative proportion and species diversity of marine plankton. Ternary diagrams illustrating significant proximality changes were used to decipher transgressive-regressive trends within the succession.High amounts of translucent phytoclasts and decreasing values of the CONT/MAR ratio occur during the phase of relative sea-level rise in the upper Turonian. The stratigraphic interval of maximum flooding around the Turonian/Coniacian boundary is marked by the highest abundance and species diversity of dinoflagellate cysts, and by high percentages of opaque, equidimensional particles within the phytoclast group. The OP/TR ratio is still high within the lower Coniacian representing the early highstand deposits, whereas the relative abundance of marine constituents is again decreasing. Sedimentary organic matter of the upper Coniacian is dominated by large, blade-shaped, mainly opaque phytoclasts, which are a characteristic palynofacies signature of late highstand deposits.The present study demonstrates the high potential of palynofacies analysis in high-resolution stratigraphy and correlation of sedimentary series of shallow epeiric seas.     

乌裕尔河依安段沉积特征研究

以乌裕尔河依安段沉积体为研究对象,通过对乌裕尔河依安大桥剖面考察及对依安县境内乌裕尔河沉积体挖探槽、探坑,结合样品的粒度分析结果,详细研究了其沉积特征。研究表明,乌裕尔河依安段为细粒曲流河沉积体,在垂向上总共形成了3期沉积旋回,主要发育河道、河漫滩和天然堤3种沉积亚相以及河床滞留沉积、点沙坝、河漫滩和沼泽4种微相。结合粒度分析结果,乌裕尔河依安段沉积体在C-M图上主要发育三段:递变悬浮段、均匀悬浮段和悬浮滚动段,反映了乌裕尔河水动力变化情况及沉积特征。     

New data on microbiota of the Middle Volgian substage in the Loino Section (Kirov oblast)

The calcareous nannoplankton was established for the first time in Upper Volgian deposits of the Loino Section (Kirov oblast). Here, single coccolites of representatives of genus Watznaueria of wide stratigraphic and geographic distributions dominate. Representatives of genus Zuegrhabdotus are less common, whereas species of genus Polypodorhabdus are rare. The taxonomic composition of the calcareous nannoplankton assemblage does not allow us to distinguish the nannoplankton zone in the Loino Section, but it gives us an idea about its distribution at the Volgian Stage in the Russian Plate. As corresponding microfauna, foraminifers, assigned to the Lenticulina infravolgaensis–Saracenaria pravoslavlevi Zone, and ostracods were identified to substantiate the age of host deposits. The ostracod assemblage is assigned to the Macrodentina (Polydentina) subtriangularis Beds established on the Volga River left bank in deposits corresponding to the ammonite zones Panderi and Virgatus assigned to the middle Volgian substage. On the basis of ostracods, the warm-water well-aerating environment of the upper sublittoral (as deep as 50 m) was reconstructed.     

Anatomy of a transgressive lag: Panther Tongue Sandstone, Star Point Formation, central Utah

Abstract The Panther Tongue of the Star Point Formation in central Utah contains a variety of transgressive lag deposits that, when mapped regionally, show a sensitive dependence upon pre‐existing topography of the palaeoshoreline. The Panther Tongue consists of a coarsening‐upward sandstone wedge that prograded into the Western Interior Seaway during Late Cretaceous (Santonian) time. High‐resolution sequence stratigraphic analysis revealed that this member was deposited during the long‐distance (>50 km) regression and transgression of a delta into shallow‐marine environments, containing basal highstand, forced regression, lowstand and transgressive systems tracts. Based on grain size, clast composition, lateral extent and stratigraphic position, the coarse sandstones on top of the Panther Tongue were classified into four types: (1) simple; (2) dispersed; (3) oxidized; and (4) local lags. The simple lag is composed of dark grey coarse sandstone with oyster fragments and shark teeth. This lag is typically extensively bioturbated and massive. Laminated and cross‐bedded units are also common. This type of coarse sandstone is interpreted as a typical transgressive lag. The dispersed lag differs in that it contains abundant mud and commonly occurs as multiple beds in thick intervals of muddy sandstone. Mixing of bay/estuarine and shallow ‐ marine mud with simple lag sand may be responsible for deposition of this type of coarse sandstone. The oxidized lag is distinctive in its reddish colour with extensive bioturbation and is commonly overlain by a simple lag. The local lag is composed of thin‐bedded, dark grey, coarse sandstone, occurring locally between the mouth bar and distributary channel. The variation in types, grain size and bed thickness of the coarse‐grained lags was mainly controlled by antecedent topography as suggested by immediately underlying lithofacies. Relatively thick (≈30 cm) simple lags are present on top of mouth‐bar sandstones, whereas dispersed lags are common on top of the distributary channel sandstone and in bay/estuarine and shallow‐marine mudstones. Erosion of topographic highs (mouth bar) resulted in relatively thick accumulation of simple lags. In topographic low areas such as distributary channel, estuary, bay and shallow‐marine environments, fine‐grained muddy sands that were eroded from the nearby topographic highs were redeposited. Intermittent storm waves transported coarse sands both landward and seaward, forming a dispersed lag. The net effect was reworking of local topographic relief during overall transgression, forming an apparently planar transgressive surface of erosion.     

论华南喷流—沉积块状硫化物矿床

现代海底喷流－沉积硫化物矿床的发现极大地推动了海底热液成矿理论的发展,也大大地提高了对古代海底喷流块充化物矿术的研究水平。本文指出喷流－沉积是重要的成矿作用,提出喷流－沉积矿床是华南Ｃｕ、Ｐｂ、Ｚｎ、Ｓｎ、Ａｇ、Ａｕ等矿产资源的重要来源,形成了一批超大型矿床,并将华南许多曾被认为属夕卡岩矿床重新确认为喷流－沉积岩床。文章还论述了华南喷流－沉积块状硫化物矿床的特征、分类、时空分布及其成矿特点等问题,提出断裂拗陷带型喷流－沉积块状硫化物矿床是华南具有特色的类型,而陆相断陷盆地中喷流－沉积矿床值得进一步深入研究。     

A facies analysis of the Pucara Group (Norian to Toarcian carbonates,organic-rich shale and phosphate) of Central and Northern Peru

The Pucará Group contains an excellent record of marine uppermost Triassic and basal Jurassic sedimentation. New biostratigraphic and lithostratigraphic studies have led to a reassessment of the regional internal correlation of the group and thus to a new understanding of the facies relationships. There is a relatively complete record of sedimentation from late Norian to late early Jurassic, the only notable omissions apparently occurring in the latest Norian and at the Hettangian-Sinemurian boundary. The oldest constituent formation, the Chambará, which is locally of great thickness, consists largely of dark cherty limestones representing basinal conditions of sedimentation. The succeeding Aramachay Formation marks widespread recommencement of sedimentation in the Semicostatum Zone of the Sinemurian, with organic-rich shales passing up into phosphatic deposits. It is proposed that this event is the consequence of a significant eustatic rise of sea level. The record of sedimentation of the youngest, Condorsinga, formation is restricted to Central Peru and no other middle-upper Lias units are identified. Alternating marginal marine and shallow neritic conditions are inferred with high carbonate production rates and dolomitizing environments. The character of the sediments, particularly the phosphatic deposits, suggests a tectonically quiescent continental margin with unrestricted access to the palaeo-Pacific Ocean, in contrast to earlier palaeogeographic interpretations. As such the Pucará Group represents unusual conditions in the development of the Andean regime.     

Types of massive sulphide deposits in the Urals

Massive sulphide deposits in the Urals are found within volcanic and volcanic-sedimentary sequences of Ordovician to Middle Devonian ages. Four types of economic sulphide deposits have been recognized: Cyprus, Besshi, Urals and Baimak. The Cyprus-type copper sulphide deposits are hosted by mafic volcanites that occur in the basal parts of Palaeozoic volcanic sequences. The Besshi-type copper-zinc deposits are located within clastic sedimentary rocks intercalated with basalts and andesites. Zinc-copper deposits of the Urals-type are hosted by bimodal rhyolite-basalt assemblages, which occur at a higher stratigraphic level than those of Cyprus- and Besshi-types. The Baimak-type zinc-copper-barite deposits are associated with intrusive quartz porphyries which occur in the upper parts of bimodal volcanic successions. In addition there are some sulphide deposits of zinc-lead-barite and zinc-copper composition hosted by Ordovician terrigenous sequences which occur within depressions in Precambrian blocks. These types of sulphide deposits have been formed at various stages of divergence and convergence of the Earth's crust during the orogenic history of the Urals. Received: 27 June 1997 / Accepted: 14 May 1998     

北羌塘坳陷冬曲地区上三叠统巴贡组扇三角洲沉积特征

北羌塘坳陷冬曲地区上三叠统巴贡组为一套碎屑岩沉积,在垂向上具有两个向上变粗的充填序列。根据沉积体产出形态和叠置序列、岩石组合与结构、沉积构造及流体性质等沉积特征,确定该套沉积体为扇三角洲相沉积,进而可划分出扇头、扇中、扇尾3个亚相。发育底冲刷构造、粒序层理、平行层理、交错层理等,表明其流体性质既有重力流又有牵引流特征。通过该套扇三角洲沉积体的认识,对研究该区古构造背景、沉积环境及油气地质条件等具有重要意义。     

Evaporites through time: Tectonic,climatic and eustatic controls in marine and nonmarine deposits

Throughout geological time, evaporite sediments form by solar-driven concentration of a surface or nearsurface brine. Large, thick and extensive deposits dominated by rock-salt (mega-halite) or anhydrite (mega-sulfate) deposits tend to be marine evaporites and can be associated with extensive deposits of potash salts (mega-potash). Ancient marine evaporite deposition required particular climatic, eustatic or tectonic juxtapositions that have occurred a number of times in the past and will so again in the future. Ancient marine evaporites typically have poorly developed Quaternary counterparts in scale, thickness, tectonics and hydrology. When mega-evaporite settings were active within appropriate arid climatic and hydrological settings then huge volumes of seawater were drawn into the subsealevel evaporitic depressions. These systems were typical of regions where the evaporation rates of ocean waters were at their maximum, and so were centred on the past latitudinal equivalents of today's horse latitudes. But, like today's nonmarine evaporites, the location of marine Phanerozoic evaporites in zones of appropriate adiabatic aridity and continentality extended well into the equatorial belts.Exploited deposits of borate, sodium carbonate (soda-ash) and sodium sulfate (salt-cake) salts, along with evaporitic sediments hosting lithium-rich brines require continental–meteoric not marine-fed hydrologies. Plots of the world's Phanerozoic and Neoproterozoic evaporite deposits, using a GIS base, shows that Quaternary evaporite deposits are poor counterparts to the greater part of the world's Phanerozoic evaporite deposits. They are only directly relevant to same-scale continental hydrologies of the past and, as such, are used in this paper to better understand what is needed to create beds rich in salt-cake, soda-ash, borate and lithium salts. These deposits tend be Neogene and mostly occur in suprasealevel hydrographically-isolated (endorheic) continental intermontane and desert margin settings that are subject to the pluvial–interpluvial oscillations of Neogene ice-house climates. When compared to ancient marine evaporites, today's marine-fed subsealevel deposits tend to be small sea-edge deposits, their distribution and extent is limited by the current ice-house driven eustasy and a lack of appropriate hydrographically isolated subsealevel tectonic depressions.For the past forty years, Quaternary continental lacustrine deposit models have been applied to the interpretation of ancient marine evaporite basins without recognition of the time-limited nature of this type of comparison. Ancient mega-evaporite deposits (platform and/or basinwide deposits) require conditions of epeiric seaways (greenhouse climate) and/or continent–continent proximity. Basinwide evaporite deposition is facilitated by continent–continent proximity at the plate tectonic scale (Late stage E through stage B in the Wilson cycle). This creates an isostatic response where, in the appropriate arid climate belt, large portions of the collision suture belt or the incipient opening rift can be subsealevel, hydrographically isolated (a marine evaporite drawdown basin) and yet fed seawater by a combination of ongoing seepage and occasional marine overflow. Basinwide evaporite deposits can be classified by their tectonic setting into: convergent (collision basin), divergent (rift basin; prerift, synrift and postrift) and intracratonic settings.Ancient platform evaporites can be a subset of basinwide deposits, especially in intracratonic sag basins, or part of a widespread epeiric marine platform fill. In the latter case they tend to form mega-sulfate deposits and are associated with hydrographically isolated marine fed saltern and evaporitic mudflat systems in a greenhouse climatic setting. The lower amplitude 4 and 5th order marine eustatic cycles and the greater magnitude of marine freeboard during greenhouse climatic periods encourages deposition of marine platform mega-sulfates. Platform mega-evaporites in intracratonic settings are typically combinations of halite and sulfate beds.     

Geotechnical characteristics of some clays from south—western Nigeria: implications for industrial utilization

Clays in adequate quantity and of quality for industrial use occur in the Ifon area of Ondo State, Nigeria, in the form of alluvial deposits at Sobe, as sedimentary deposits at Non and Arimogija and as residual clays at Omialafara.

The alluvial clays from Sobe and the sedimentary clays from Ifon are highly plastic while the sedimentary clays from Arimogija are moderately plastic and the Omialafara residual clays only slightly plastic. The clays in general are kaolinitic in composition with minor amounts of illite.

The unconfined compressive strength characteristics of fired pellets of these various clays indicate that the reddish to greyish-brown alluvial clays at Sobe tend to have higher strength values than the dark grey clays of Arimogija. Porosity, firing shrinkage, clay mineralogy and colour after firing all indicate that the clays are very suitable for the manufacture of ceramic wares.