Clayey Limestone Research Articles

Very Low Grade Metamorphism of Upper Paleozoic–Lower Mesozoic Sedimentary Rocks Related to Burial and Thrusting in the Central Taurus Belt, Konya, Turkey

Metasedimentary and sedimentary rocks that represent the allochthonous Bolkardagi Unit crop out in the Central Taurus Belt. Devonian units include mainly slate, metadolomite, metadolomitic limestone, and metasandstone. Slates with slaty cleavage and chlorite-mica stacks are characterized by phyllosilicate, quartz, calcite, dolomite, feldspar, and goethite. Phyllosilicates consist of 2M1 and lesser amounts of 1M muscovite, IIb chlorite, pyrophyllite, paragonite. PM, C-V, C-S. rectorite, and dickite, and reflect conditions of the low epizone-anchizone. In the Carboniferous-Triassic units, limestone, clayey limestone, dolomitic limestone, marl, shale, and sandstone retaining primary textures are composed of calcite, dolomite, quartz,1Md illite, chlorite, and I-S, and locally smaller quantities of 2M1 muscovite, PM, paragonite, pyrophyllite, and rectorite. In contrast, the Triassic formation is made up of calcite, 1Md illite, I-S, kaolinite, smectite, chlorite, C-S, C-V, dolomite, and quartz. Textural and mineralogical data indicate that development of the diagenetic-metamorphic grade in the Central Taurus was related to sedimentary burial and thrusting; moreover, the metasedimentary rocks were metamorphosed in a typical anticlockwise P-T-t pathway in an extensional setting.

Geochemical evaluation of the Besikli oil field, Kahta,Adiyaman, Turkey

Mesozoic source rocks and crude oils from the Besikli oil field, Adiyaman, Turkey, havebeen characterized by a variety of organic geochemical techniques. The potential source rocksinvestigated include the Karababa (A, B, and C members) and the Karabogaz formations. TheKarababa-A member and the Karabogaz formation of clayey limestone and limestone areimportant source rocks in the basin. The crude oils examined in this study were obtained fromreservoir formations of the same age throughout the oil field. The source rock screening analysisindicates that the Karababa-A member and the Karabogaz formation have substantialoil-generation potential. The Karababa-B and C members have no source potential because theydo not contain sufficient amount of total organic matter content (0.12–0.24 TOC wt%). Selectedsource rock and oil samples were characterized in detail, and distributions and concentrations (both relative and absolute concentrations, of biomarkers at different oils were examined. Thebiomarker characteristics were combined with other geochemical and geological data to interpretthe sources, depositional environments, and the age of the oils. Oil-source rock correlations weremade between source rocks and crude oils based on qualitative and quantitative biomarkerdistributions. Petrographic descriptions and vitrinite reflectance measurements were used toevaluate petrographic properties of source rocks and maturity levels of the formations. Thesource rocks for the oil from the Karababa-A member and Karabogaz formation are not matureenough and therefore oil should have migrated from the outside of the study area.

The application of rock mass classification systems to underground excavation in weak limestone, Atatürk dam, Turkey

The Atatürk dam was built across the Firat River on clayey limestone. A grout curtain, providing impermeability in the left and right abutments, was done in grouting galleries. The well known rock mass classification systems for tunneling purposes [rock structure rating (RSR), rock mass rating (RMR) and rock mass quality index (Q)] were used to classify the rock mass along these galleries. Based on RSR, RMR and Q values, the rock masses in the galleries have been classified into three different classes. Correlation between the three classification systems is discussed and suggestions are made for using rockbolt, shotcrete with wiremesh and steel ribs for supporting the rock mass.

Mineralogical and genetic relationships between carbonate and sepiolitepalygorskite formations in the neogene lacustrine Konya basin, Turkey

Neogene (Upper Miocene-Pliocene) lacustrine sediments of northern Konya consist of conglomerate, sandstone, marl, mudstone, claystone, clayey limestone, and limestone. Limestones are white, beige-cream colored, fine-grained and contain remnants of plant roots. Brecciation, calcretion, mud cracks, and dissolution voids are common. Claystones are typical with white and green colors. Green claystone cropping out at the lower part of the sequence is alternated with mudstone and sandstone. White claystone alternating with carbonate units appears at the upper part of the sequence. Carbonate units are found as intercalated layers and lenses of conglomerate, sandstone, and mudstone.

Gas bubble and expansion crack origin of "molar-tooth" calcite structures in the middle Proterozoic Belt Supergroup, western Montana

Molar-tooth structures in the Belt Supergroup are complexes of interconnecting, thin sheets and small spheroids, composed of uniform, blocky 5-15 mm calcite crystals that cut fine-grained clayey carbonate layers. Bauerman (1885) likened the network of sheets to the corrugated surface of an elephant molar tooth. Molar-tooth structure described here comes from the Chamberlain and Helena formations of the Middle Proterozoic Belt Supergroup, but the structure occurs in other silty, clayey limestone and dolomite of the Belt and in Middle and Late Proterozoic rocks elsewhere in the world. Previous investigators have applied the term to several kinds of structures and attributed them to a variety of physical, biological, and chemical processes. We adhere to Bauerman's (1885) original citation of and limit our definition to blobs, vertical ribbons, and horizontal ribbons (O'Connor 1967, 1972; Eby 1977). Observations of molar-tooth samples suggest to us that ribbons formed as open voids through which gas and water flowed before being filled by fine, equant sparry calcite crystals. We tested the gas hypothesis by experimenting with mixtures of plaster of Paris, clay, water, sugar, and yeast in glass aquaria. Yeast metabolized the sugar, producing CO 2 gas that first formed bubbles in the mud slurry, nearly identical in shape to Belt blobs. The bubbles rose to the surface and pulled water behind them, stiffening the mud. We later sealed off the mud surface with plaster. The confined gas within the stiffened mud produced vertical and horizontal expansion cracks that were nearly identical in form to vertical and horizontal ribbon shapes. Finally, we precipitated fine equant crystals of calcium carbonate, identical to calcite, by mixing solutions of Na 2 CO 3 and CaCl 2 . The close replication of blobs and ribbons by biogenic gas convincingly explains the origin of the blob and ribbon forms. This explanation satisfies all the constraints of the data. We propose that some ribbons in the Chamberlain and Helena formations formed as gas cracks in water-saturated mud within a meter of the depositional surface, and were partially filled with fine calcite spar before compaction. Other cracks in the mud were first compacted and later filled by calcite. Still other structures probably formed deeper in the sediment column. Patterns of structure are commonly repeated within sedimentary cycles up to tens of meters thick, indicating that they may have formed during periods of lowstand between depositional cycles. Biogenic gas generated in Belt sediment probably included H 2 S, as evidenced by microcrystalline pyrite in calcite, CO 2 , and methane. Rapid calcite precipitation in the gas voids is probably necessary for preservation and may be a secular feature of the Middle and Late Proterozoic.

An Autochton, Recent Lateritic Deposit in Eastern Tourids: The Buyukbelen (Farasa-Yahyali-Kayseri, Turkey) Iron Occurrences

Buynkbelen laterites at 2.5 km southeast of Farasa (Yahyali-Kayseri) village in the west of eastern Tourids are located at the southwest of Attepe (Mansurlu-Feke-Adana) area being the most important hydrothermal iron province of Turkey. In the area, Upper Cretaceous aged Cavdarusagi olisthostrome and Pozanti-Farasa peridotite nappe belonging to Aladag ophiolite sequence, and transgressively overlying Miocene aged conglomerates (Zebil Formation) crop out (Kupeli, 1996). Cavdarusagi olisthostrome contains mainly carbonate and lesser ophiolitic rock blocks and fragments in a matrix of serpentinite, diabase, chert intercalated pelagic limestone-marl-shale, chert and radiolarite, all of which show lateral and vertical transition to each other. Laterites studied are composed of semi-autochton, a blanket of 5 to 15 m thick lateritic iron crust including also karstic pockets, and surrounding red coloured, thin (0.5-1 m) soil cover. These laterites are observed as an autochton recent weathering zone above chert intercalated clayey limestones of the Cavdarusagi olisthostrome. In the weathering zone, hematite and goethite levels are distinguishable. Hematite part is 5 to 10 cm thick as thin bands exhibiting short lateral continuation. The weathering zone is characterised mainly bay yellowish brown coloured, thick goethite level. In addition to hematite and goethite, reddish rust coloured limonite is also common in the north of lateritic crust where is heavily karstic. Karstic cavities range from a few decimetres to 7 m in diameter. Continuing karst formation within ore mass and Dept. of Geol. Selguk University, Konya 42079, Turkey

Carbon and oxygen isotope record across Pridoli to Givetian stage boundaries in the Barrandian basin (Czech Republic)

Abstract Carbon and oxygen isotope composition and the chemistry of limestones, clayey limestones and shales were studied across stage boundaries from Pridoli to Givetian in the Barrandian basin. No substantial isotopic shifts were found at the Pragian–Emsian and Emsian–Eifelian boundaries. A gradual shift in δ13C values from −2.6 to +3.6‰ was observed in Pridoli and Lochkovian limestones. This shift is inferred to have been related to a combination of higher productivity, increased deposition of organic matter and shallowing of the basin. The global Kacak–otomari event just below the Eifelian–Givetian boundary is marked by an abrupt change in carbon isotopic values from about 1 to 3‰ in pelagic carbonates. No δ13C shift was found in shallow-water carbonates at the Eifelian–Givetian boundary. High δ18O values up to −1.3‰ in Pragian, Emsian and early Eifelian limestones indicate that the Barrandian basin was relatively isolated from the world ocean during those intervals. Lower δ18O values up to −7.5‰ in late Eifelian limestones may indicate the end of isolation.

Clastic petrofacies, provenance and organic facies of the Bozbel Formation (Lutetian) in the Eastern Sivas Basin (Turkey)

The Bozbel Formation (Lutetian) of the eastern part of the Sivas Basin, central Anatolia, Turkey, displays a range of lithofacies which vary greatly in thickness. The formation consists of conglomerates, sandstones, mudstones and shales, marls, limestones, and sandy limestones, clayey limestones, tuffites, and volcanic breccias. The aim of this study is to determine the clastic petrofacies of the sandstones and to describe the characteristics of the organic facies and source rocks present in the Bozbel formation. The sandstones can be classified essentially as arkoses, lithic arkoses and feldspathic litharenites, cement-rich and containing minor amounts of olivine, pyroxene, chlorite, biotite, muscovite and opaque minerals. These sands were derived from magmatic arc and mixed provenances. The source-rock characters of the finer-grained Bozbel rocks and their organic facies have been determined by means of total organic carbon (T.O.C.) analyses, rock-eval pyrolysis, vitrinite reflectance data and clay mineral paragenesis, together with illite crystallinity values. The organic matter content is dominated by type 3 kerogen. Illite crystallinity indices and vitrinite reflectance data run parallel with each other and are consistent with the catagenetic stage of organic matter evolution. The Bozbel Formation in the south Hafik region is appropriately mature to yield hydrocarbons, but because of the generally low content of organic matter it is unlikely to have generated petroleum but is known to be gas prone.

Are there turbidites in the Silurian/Devonian boundary stratotype? (Klonk near Suchomasty, Barrandian, Czechoslovakia)

A discussion arose in 1977 regarding the nature of the Silurian/Devonian boundary bed at Klonk, and the beds below and above it. Present revision of the stratotype sequence found that most clayey limestones display a multiple and composed rhythmic arrangement of laminae. Deposition of pelagic particles, effects of traction bottom currents, and turbidite inputs are distinguishable, however, the latter are rare. Semilithified surfaces and hardgrounds were found. The boundary bed No. 20 consists of several laminated rhythms. The Devonian base, marked by first occurrences ofMonograptus uniformis corresponds to a semilithified surface, a break in deposition for several tens to hundreds of years, and a change in direction of bottom currents. A moderately rippled set at the Devonian base is only about 1 cm thick and passes again into the horizontal laminated rhythms. The deposition of the boundary bed lasted about 1.2 to 2.0 Ka. It cannot be explained as a turbidite.

Основы литостратиграфии нижнемелового отдела в Болгарии

The Lower Cretaceous deposits are widespread in North Bulgaria: in the Fore-Balkan, the Transitional Zone, and the Moesian Platform. The scheme of the formal lithostratigraphic units includes 37 Formations and has been elaborated for all this territory on the basis of outcrops and numerous boreholes. Four Groups (West-Balkan Carbonatic Group, Central-Balkan Flysch Group, Vraca Urgonian Group and Loveč Urgonian Group) consist of 19 Formations, and the other 18 Formations are independent. A total of 15 Members has been introduced within 9 of the Formations. The composition and interrelations of tbe lithostratigraphic units reflect the inhomogeneities and the different tendencies of the development of tile Early Cretaceous basin. The most expressive facies boundary in the west-east direction is the so-called Jablanica Line. West of it, carbonatic and marly deposits predominate in the Fore-Balkan. The carbonatic Formations belong to the West-Balkan Carbonatic Group and the Vraca Urgonian Group. The Salaš Formation is built up of clayey limestones and marls, and is widespread in the West Fore-Balkan, as well as in the western part of .the Moesian Platform. The asymmetry of the basin with a dry land as source region to the south of it, is clearly outlined to the east of the Jablanica Line, within: the Central and East Fore-Balkan and the adjacent Transitional Zone. This character of the basin controlled a facies differentiation with development of predominantly terrigeneous units in the south, and of carbonatic and marly units, to the north. This difference is expressed at its best at the lower levels of the Lower Cretaceous where units of the Central-Balkan Flysch Group are interfingering with units of the West-Balkan Carbonatic Group. The lateral transition takes place within the Transitional Zone. At higher stratigrapic levels (Hauterivian - Aptian) the asymmetry is expressed into a horizontal transition of terrigenous non-flysch deposits (Kamchija Formation and Roman Formation) with predominantly marly deposits (Gorna-Orjahovica and Trâmbeš Formations) to the north. A specific feature is the presence of Urgonian deposits in the Transitional Zone (Loveč Urgonian Group). At both sides of its development area, fans of the terrigeneous deposits of the Roman Formation are traced towards north. They are bounded to the Iskar (at the west) and Etar (to the east) depressions. Urgonian limestones occur also within the Moesian Platform in the area of Ruse (Ruse Formation). The highest (Albian) levels of the Lower Cretaceous section have been observed only in northwest Bulgaria. They are represented by the marls of the Sumer Formation, the glauconitic predominantly sandy deposits of Malo Peštene Formation, and the marls of Rabiša Formation.

Albian ammonite biostratigraphy in Northwest Bulgaria

The Albian Stage in Northwest Bulgaria is of more restricted occurrence compared to the other Lower Cretaceous Stages. It consists of marls interbedded by clayey limestones and by less common sandstones (Sumer Formation) and glauconitic sandstones, siltstones and marls (Malo Peštene Formation). The Albian sediments are of considerable thickness without any hiatuses and condensations in the sections. The deposits contain diverse macrofaunas dominated by ammonites. This paper describes the ammonite and lithological successions in 15 Albian sections. The Albian is considered here to include the L. (L.) tardefurcata Zone from its base up to the S. dispar Zone. The three Albian substages are characterized and new evidence is reported on the Albian ammonite successions in Northwest Bulgaria. It has made it possible to supplement and to further subdivide the zonations proposed by previous authors. Part of the zones where possible, have been subdivided into subzones. The following biostratigraphic units are characterized in ascending order: L. (L.) tardefurcata Zone with L. (L.) tenuicostata Subzone and L. (N.) regularis Subzone; D. mammillatum Zone with P. (I.) steinmanni Subzone; H. dentatus Zone with L. lyelli Subzone and H. spathi Subzone; E. loricatus Zone with A. intermedius Subzone; E. lautus Zone; M. (M.) inflatum Zone with H. orbignyi Subzone, H. varicosum Subzone and C. auritus Subzone; S . dispar Zone. An attempt is made to correlate them with units already established in other parts of the Mediterranean Region.

Литостратиграфия доприабоновых палеогеновых отложений из района, расположенного южнее г. Лыки Пловдивской области

Pre-Priabonian Paleogene sediments were found outside the confines of the East Rhodope Depression in the area between the villages of Lâkavica and Drjanovo. They are diverse in composition breccia-conglomerates, sandstones, limestones and less commonly clayey limestones to marls which alternate in the section of Lâki Group. The Group consists of three lithostratigraphic units: Staroselo Sandstone - Limestone Formation, Drjanovo Breccia - Conglomerate Formation and Pandursko Limestone Formation. The rocks of the three units are covered by the basal breccia-conglomerates of the Priabonian by a transgressive and unconformable contact. The units contain at different horizons abundant fossil flora and fauna which dominate in the upper parts of Drjanovo Formation. The occurrence of rocks of Lâki Group outside the East Rhodope Depression indicate that during the pre-Priabonian evolution the Paleogene sediments were of wider distribution in western direction. The character of the coarse-terrigenous sediments, alternating with sandstones and limestones, suggests that the rocks of Staroselo, Drjanovo and Pandursko Formations were deposited in a shallow basin in probably fresh-water to brackish environment.

Paléoécologie holocène des Marais du Rocher en Maillezais (Vendée), dans le Marais Poitevin (Littoral Atlantique - France)

This study allows the succession of palaeoenvironments in this marshland to be followed from approximately 7000 BP. The process of progressive emersion is particularly clearly documented. The succession of palaeoenvironments is as follows : — a coastal, sea-edge environment, with the shoreline following the limestone relief (first half of the armorican atlantic period), — a marine marshland, at first relatively landlocked, but later in closer contact with the sea (second half of the armorican atlantic period), — a lagoon environment, extending over more of the limestone as a result of the infilling of the marshland basin (5 700 to 3600 BP), — continued infilling, linked perhaps with a general marine regression, leading to a marshland environment of inland type. In situ peat deposits allow the level reached by the highest seas at this period to be determined (2 630 to 370 BP), — finally, an environment once more of coastal type, but disturbed by human intervention (building of dams) (after 370 BP). The deposits encountered here correspond to those described in the notice of the 1 : 50,000 geological map for Marans : marine alluvium attributed to the Flandrian (MFya : clay with Scrobicularia, green or blue, labelled "bri ancien"), resting on a Jurassic bedrock (J3c2 : clayey limestone of upper Callovian age), and covered by marshland sediments (FTz : peaty alluvium). This sequence is valid throughout the Maillezais area for land under 3 m above sea level.

Early Miocene Depositional Environments in the Northern Margin of the Mediterranean, Southwestern Anatolia: ABSTRACT

The continental and shallow marine clastics and carbonates exposed around the towns of Kale and Acipayam in southwestern Anatolia were investigated to interpret the depositional environments in the northern margin of the Mediterranean in terms of lithofacies and biozones. These deposits include Miogypsina intermedia and M. irregularis, indicating Burdigalian age when correlated with the same species in the different parts of the Tethys Sea and Indian-Pacific Oceans. The clastic lower part of the succession is characterized by sheet flow and braided-stream deposits of an alluvial-fan/fan-delta complex. Marine carbonates overlie these deposits, but in some places a transgressive lag deposit lies between the unconformity surface and the carbonates. The lag deposit unit corresponds to the Gastropoda biozone, including Ostrea, Terebralia, and Pecten. Four carbonate facies are recognized: (1) Clayey limestones with ahermatypic corals, ostracods, macrofossils, and foraminifers. This facies corresponds to the Textularia-Rotalia biozone. (2) Packstones and grainstones with abundant nearshore and some offshore foraminifers, corresponding to the Miliolidae biozone. (3) Packstones and wackestones with offshore foraminifers. This facies includes the Miogypsina irregularis-Miogypsina intermedia biozone. (4) Boundstones and very poorly sorted reef-talus conglomerates including hermatypic corals, foraminifers, and binding foraminifers. This facies is the coral biozone. These sediments define the northernmore » extent of the Tethys Sea in the investigated area during the Burdigalian. They were deposited in a shallow carbonate platform at the southern margin of the Anatolian mainland, which had a steep coast characterized by an alluvial-fan/fan-delta complex.« less

Depositional environment and coal petrology of citak (akhisar) lignite deposits, west turkey

The Miocene alluvial, fluvial and lacustrine deposits occurring to the east of Akhisar (West Turkey) contain an economically potential lignite layer. This Miocene sequence is divisible into five formations, of which the Yeniköy Formation contains an autochthonous lignite horizon in its uppermost part. The Yeniköy Formation consists of, in a broadly ascending order, cross-bedded sandstones, channel-fill conglomerates, algal limestones, mudstones and carbonaceous shales. The Küçükderment Formation, which overlies the Yeniköy Formation and is composed of calcareous shale, clayey limestone, mudstone and bituminous shale, is of apparently lacustrine origin. The lignite horizon changes in thickness from 0.5 to 5 m, and is variably interbedded with clay and mudstone. It has an average calorific value of 13419 kJ/kg. The average moisture, ash and sulphur contents are 20%, 14% and 3%, respectively. Petrographic analysis indicates that the lignite samples consistently contain large amounts (more than 90%) of huminite (humocollinite, humodetrinite) and small amounts of intertinite (fusunite, semifusunite) and liptinite (resinite, sporinite, alginite). Average huminite/vitrinite reflectance ranges from 0.34 to 0.38%. In the DIN coal classification the rank features indicate “dull brown coal”. Macerals in the lignites indicate that the coal-forming peat accumulated in a predominantly forest-moor swamp environment in which reducing and low pH conditions prevailed. Spore and pollen assemblages suggest that a subtropical and humid climate existed during deposition of the coal.

The geology of the Kalahari Gemsbok National Park

The floor rocks of the Kalahari Group are only known from boreholes. It consists of rocks of the Karoo Sequence with its associated intrusions of dolerite. The pre-Kalahari topography was dissected by rivers which drained in the direction of the present Botswana. The distribution of the oldest formation of the Kalahari Group, the Wessels Formation (clayey gravel), and the overlying Budin Formation (clay) was also determined from borehole records. The Karoo Sequence was probably the source of these rocks. The overlying Eden Formation (sandstone, grit and conglomerate) had a source which could yield much sand. All these formations were deposited under fluviatile conditions. The Mokalanen Formation (calcrete) and the Gordonia Formation (sand) indicate a change from a humid to an arid environment. The Lonely Formation (clayey diatomaceous limestone) was deposited in a lacustrine environment. It also indicates a higher rainfall in an otherwise arid period. The Goeboe Goeboe Formation consists of clay and sand in the pans and rivers.

Wetland impact assessment: Problems under the clean water act

Cenozoic sediments are spread across southeast and eastern Anatolia, including the Adıyaman–Malatya regions of southeast Turkey. This study presents the first detailed petrographic, micropaleontologic, and strontium isotope analyses of the Eocene marl, clayey limestone, and limestone units that outcrop unconformably above the Mesozoic Meydan and İspendere ophiolitic rocks in the Adıyaman and Malatya regions, respectively. These carbonate rocks contain abundant planktonic foraminifera, including Turborotalia frontosa, Subbotina senni, Subbotina eocaena, Acarinina bullbrooki, Guembelitrioides nuttalli, Globigerinatheka subconglobata, and G. curryi, and benthic taxa (Nummulites, Discocyclina, and Chapmanina).Two planktonic foraminiferal zones were defined: the Turborotalia frontosa zone representing the Early–Middle Eocene (Ypresian–Lutetian/Bartonian), and the Turborotalia possagnoensis zone representing the Middle Eocene (Lutetian–Bartonian). Strontium isotope ratios (87Sr/86Sr) calculated for four samples ranged between 0.707663 and 0.707784, indicating an age of 45.1–51.9 million years (Ypresian–Lutetian/Bartonian). The presence of echinoid thorns in the units indicates a shallow marine environment where carbonates developed on the seaward side of a reef. For the first time in the study region and southeast Anatolia, this study has revealed the stratigraphic position of the Nummulitic limestone of the Kırkgeçit Formation unconformably overlying the ophiolites. Therefore, based on strontium isotope analyses and planktonic foraminiferal micropaleontology, the Kırkgeçit Formation is estimated as Early–Middle Eocene (Ypresian–Lutetian/Bartonian) in age.

Radiolarian Preservation in Geologic Sequences: ABSTRACT

The following observations were noted during fieldwork in Tethyan regions. 1. Radiolarians are often preserved where organic matter is abundant, generally in reduced environments or microenvironments resulting from transgression or confined basins. Such an anaerobic environment preserves silica from dissolution. 2. Radiolarian localities are commonly restricted to small basins (e.g., the Gulf of California). 3. In limestones, radiolarians are commonly restricted to small nests preserved in pyrite. This restriction may be a result of their sedimentation within fecal pellets, reducing microenvironments. 4. Preservation is generally better in rocks rich in clays (clayey limestones, nonglassy cherts). The clays may produce a double effect: (a) to form protective varnish all around the shell, (b) to slow the opal-A to opal-CT transformation so the structure of the opal-CT is better organized and much less subject to subsequent dissolution. 5. Radiolarians in limestones are generally calcitized. However, in rich limestones where silica is not concentrated in nodules but is diffuse, radiolarians remain preserved in silica. This results from clays which limit (a) the opal-A to opal-CT transformation which occurs in a fluid state and (b) the migration of the silica fluid. End_of_Article - Last_Page 450------------

Limestone Response to Stress: Pressure Solution and Dolomitization

ABSTRACT Limestones that have been subjected to overburden or tectonic stress can be expected to have changed by some combination of deformation (plastic or brittle), neomorphism (as micrite conversion to microspar or aggrading pseudospar), and solution. Specific limestone units (beds, structures, or grains) may be preferentially resistant or responsive to change and with or without impurities (especially magnesium ions and clay particles) that will influence the character of change. Three styles of pressure solution response are recognized in Paleozoic limestones studied from Arizona, western Maryland, and southeast Kansas. Sutured-seam solution (common stylolites and grain-contact sutures) occurs in limestones that have structural resistance to stress and very little clay or platy silt content. Non-sutured seam solution (microstylolites, microstylolite swarms, and clay seams) occurs in limestones that have significant amounts of clay or platy silt. These limestones may be structurally resistant or responsive. Stress is accommodated both by solution and by lateral movement along microstylolite surfaces. Dolomite rhomb growth may or may not occur along microstylolite surfaces. Non-seam solution (pervasive solution thinning of a unit) occurs in clean limestones that have little or no structural resistance to stress. Dolomite growth is always observed to be associated with non-seam solution, but pervasive pressure solution without associated dolomitization should also be possible. Where clean, structurally resistant limestone units interface with responsive limestones or silty, clayey limestones, solution surfaces are not sutured. Nodular limestones are a characteristic product of non-sutured seam solution in silty, clayey limestones. Form and scale of fitted nodules may or may not be influenced by primary structure. The three forms of limestone pressure solution produce some types of finely crystalline stratigraphic dolomite, provide local sources of magnesium for dolomitization and calcite for porosity-filling cement, and severely distort primary sedimentary structures or create secondary structures.

Geology of the upper cretaceous and tertiary sediments of the Nigerian sector of the Iullemeden Basin (West-Africa)

Some of the best outcrops of the Upper Cretaceous and Tertiary sediments of the south-eastern sector of the Iullemeden basin (Fig.1) occur in the “Sokoto Embayment” around the Rima Valley of North-Western Nigeria. These sediments continue into the Niger Republic where they outcrop extensively in the Adar Doutchi. This paper results from field work in the area between Sokoto and Madaoua (1970, 1971) when the need for an accurate geographical definition as well as precise geological descriptions of type localities and type sections of the different formations was found necessary. Three Upper Cretaceous formations are described (Taloka, Dukamaje and Wurno). The Paleocene deposits have been subdivided into three lithologically distinct formations: two predominantly shaly and one clayey limestone. The Paleocene fauna, particularly the ostracods, bear close ressemblance to those described from the Gulf of Guinea (Reyment, 1965) and Libya (Barsotti, 1963). The Gwandu formation of the “Continental Terminal” consisting mainly of massive clays interbedded with coarse to medium grained sandstone, may be partly Eocene age.