Limestone Horizons Research Articles

Paleoenvironmental significance of Messinian post-evaporitic lacustrine carbonates in the northern Apennines, Italy

The post-evaporitic Messinian in the northern sector of the Adriatic foredeep is characterized by the occurrence of abundant terrigenous deposits which have been subdivided into two allostratigraphic units (p-ev 1, p-ev 2) separated by an erosional unconformity of regional importance. The p-ev 1 consists of basal complex of resedimented evaporites overlain by monotonous succession of mudstone/siltstone alternations (thin bedded turbidites) with an overall coarsening and shallowing upwards trend. It is possibly related to the rapid basin infill after the end of the salinity crisis. The upper unit (p-ev 2) shows, in the most marginal areas, a clear cyclical stacking pattern made up of three fining-up sequences with fan-delta conglomerates at the base of each cycle. Here, a peculiar feature of the p-ev 2 deposits is the occurrence of three distinct laminated limestone horizons (termed colombacci) interbedded with fine grained, varved mudstones. These limestones are abiotic, despite the occurrence of abundant Ostracod assemblages (Lago–Mare fauna) in the interbedding pelites, and are interpreted as inorganically precipitated, deposited in a relatively deep-water environment, possibly anoxic and related to permanent stratification of water mass events during periods of lake level maxima. The combination of petrographical and geochemical observations on carbonate layers reveals that they correspond to events characterized by significant variations of environmental conditions. However, δ 18O (−5.30/+0.2) and δ 13C (−9.28/+1.68) signatures suggest that the precipitation of low-Mg calcite often occurred in a non-marine environment and that meteoric water input had a considerable influence on late Messinian sedimentation. Moreover, the limestones have a uniform 87Sr/ 86Sr composition (0.70864–0.70875) in the range of other Lago–Mare sediments from elsewhere in the Mediterranean, suggesting that some of the major Mediterranean basins were likely interconnected during the Lago–Mare phase. A strongly reduced rate of hydrological exchange with the ocean is also possible, but the possibility of marine influence during this stage is not completely excluded.

The youngest African clymeniids (Ammonoidea, Late Devonian) – failed survivors of the Hangenberg Event

A co-occurrence of the clymeniid Postclymenia evoluta Schmidt, 1924 and the goniatite Acutimitoceras hilarum Korn, 2002 is reported from the Anti-Atlas of Morocco. Both species occur in the same limestone horizon within the Acutimitoceras prorsum Zone, that has yielded an exclusive conodont fauna of the Upper praesulcata Zone (latest Devonian, above the Hangenberg Black Shale). This record is firm evidence that some clymeniids survived the global Hangenberg Event, but soon later became extinct without descendants.

Post-evaporitic restricted deposition in the Middle Miocene Chokrakian-Karaganian of East Crimea (Ukraine)

In the Middle Miocene of East Crimea, gypsum evaporites formed in a shallow basin from mixed seawater–nonmarine waters are overlain by marl, siltstone and claystone which contain a few horizons of stromatolitic limestone. The thickness and abundance of the stromatolitic horizons increase up the section. In the siliciclastic portion of the section, a very poor and taxonomically impoverished assemblage of benthic foraminifers ( Jadammina, Nonion, Haynesina, Astrononion and Eponides) is recorded. It is typical for a shallow water marsh or lagoon environment with a lowered salinity. Accordingly, the brackish conditions prevailing during gypsum precipitation in East Crimea continued afterwards, although at the end of evaporite deposition the basin became desiccated and then it was rapidly reflooded by brackish water. The impoverished biota and the occurrence of microbialites in the Ptashkino section indicate extremely unfavourable conditions for most living organisms during the deposition of both the terrigenous and carbonate beds. The water salinity thus was probably not only lowered but also anomalous in composition if compared to normal marine water. The occurrence of carbonate stromatolitic horizons is probably related to the periodic shallowing of the basin caused by a drop of the lake water table driven by climatic factors. The resulted changes allowed for a growth of the bizarre microbial-serpulid communities that gave birth to most of the stromatolites. In Karaganian time, the Eastern Paratethys was a huge lake isolated from the Tethys. This lake responded to any climatic fluctuation that in turn might lead to water level oscillations. In humid climate periods, the lake could be open with a surface outflow and, when it was drier, it could be a closed system without surface outflow. Gypsum evaporites and stromatolitic carbonates are clear evidence of strong evaporation in a dry climate that probably induced water level fall in the whole basin. However, all the time the environmental conditions were predominantly brackish, even during gypsum precipitation as suggested by the chemical composition of fluid inclusions in gypsum. Similar conditions may be expected in other evaporite-hosted, predominantly brackish basins, such as some Messinian basins of the eastern Mediterranean, where some evaporites were deposited in the oligohaline to mesohaline conditions typical of the Lago Mare deposits.

Middle Miocene seasonal temperature changes in the Styrian basin, Austria, as recorded by the isotopic composition of pectinid and brachiopod shells

Using the isotope record on pectinid and brachiopod shells, we reconstructed Middle Miocene palaeotemperatures for a shelf environment in the Styrian basin, Austria. Bivalve shells of Macrochlamys sp., collected from different horizons of the Lower Badenian Leitha limestone, show δ 18O values in the range of −3 to 0‰ (PeeDee Belemnite). The large intrashell variability of 3‰ indicates significant seawater temperature fluctuation. The results suggest that despite the high geographical latitude (ca. 40°N), the climate in southern Austria was warm (subtropical) with a pronounced seasonality at that time. In contrast, the δ 18O isotopic composition of a pectinid ( Flabellipecten sp.) and an indeterminate terebratulid brachiopod, from the siliciclastic deposits overlying the Leitha limestone, ranges between 0 and −1‰. This indicates cooler mean annual temperatures and reduced seasonal variations. A 39Ar/ 40Ar age on fresh volcanic biotites shows a value of 14.2±0.1 Ma. The tuffs containing the biotites are located below the layer containing the terebratulid and the Flabellipecten sp. The stable isotope and radiogenic data suggest that the drop in temperature is in direct relationship with the East Antarctic Ice Sheet expansion, which started at ca. 14 Ma. Due to the variation in temperature, the subtropical fauna in the Retznei quarry disappeared during the Early Miocene and the biogenic limestones were replaced by clastic sediments. The δ 18O isotopic profiles from three pectinids collected from the Leitha limestone indicate that these grew within ca. 1.5 years. Growth interruptions occurred during the warm season because during this period the pectinids very likely used their energy for the growth and maturation of gonads. Carbon isotopic compositions vary between 0 and 2‰. Statistical tests show that for some of the analyzed shells, the cyclicity of the δ 13C profiles may be explained by the temperature-dependent fractionation between air CO 2 and dissolved bicarbonate.

A recent sponge, Pleroma aotea Kelly ("Order” Lithistida: Family Pleromidae), in the late Eocene Ototara Limestone of Otago, New Zealand

Numerous remarkably well‐preserved lithistid sponges recovered from the late Eocene‐early Oligocene Ototara Limestone at Kakanui, North Otago, represent the first sponge body fossils to be described from the New Zealand Cainozoic. The sponges are scattered throughout a 1–3‐m‐thick volcaniclastic limestone horizon immediately overlying the Kakanui Mineral Breccia. The fossils are now solid calcite, the former siliceous skeleton having been replaced during diagenesis. The Kakanui sponges are the only known body fossils remaining from an extremely diverse sponge fauna which formed a major component of the benthos in the Kakanui‐Oamaru region in late Eocene‐early Oligocene times (c. 35–33 Ma). The sponge body fossils are described and compared with living sponges that have a similar flattened globose morphology. The fossils are morphologically indistinguishable from the living lithistid sponge Pleroma aotea Kelly ("Order” Lithistida: Family Pleromidae) from deep‐water seamounts and banks off north‐eastern New Zealand, and are significant in that they represent a stratigraphic range for the species of more than 35 million years. The present‐day distribution of P. aotea, limited to silica‐rich deeper waters, is in marked contrast to the relatively shallow warm water volcanic environments occupied during the Palaeogene. This restriction, and that of related lithistid sponges to silica‐rich deeper waters off northern New Zealand, is paralleled in other demosponges and several calcareous invertebrate groups such as barnacles, bryozoans, and crinoids.

Crustal structure and thermal anomalies of the Dunedin Region, South Island, New Zealand

Seven geophysical data sets are used to investigate a transect along the southeast coast of South Island, New Zealand. The specific focus of this study is the Dunedin volcanic center, which last produced volcanics at the surface 13–10 Myr ago. Wide‐angle reflection/refraction seismic data along a two‐dimensional profile reveal a low‐velocity lower crust and mantle beneath the Dunedin volcanic center. The low‐velocity lower crust coincides with a highly reflective region on a nearby multichannel seismic line and may represent a hot, fluid‐rich region of the crust. In addition, high mantle helium ratios measured in the Dunedin region suggest a current or recent mantle‐melting event. High heat flow recorded in the Dunedin region is consistent with a hot body emplaced in the midcrust ∼10 Myr ago (Miocene) whose heat is just reaching the surface today. Uplift of an Oligocene limestone horizon in the Canterbury basin can be explained by a buoyant load beneath the Dunedin volcanic center and low flexural rigidity of the lithosphere beneath the volcanic center during the Miocene. We interpret the data as revealing two separate thermal events beneath the Dunedin volcanic center, one during the Miocene, when volcanism was last occurring at the surface, and the other occurring currently. Active volcanism associated with the current mantle‐melting event has yet to reach the surface.

Metamorphism of the Permo-Triassic Cape Fold Belt and its basement, South Africa

The Permo-Triassic Cape Fold Belt around the southern tip of Africa consists of a thick sequence of Palaezoic siliciclastic sedimentary and pre-Cape basement rocks believed to be of Pan-African age. Both the basement rocks and the supracrustal rocks of the Cape Supergroup display only low metamorphic grades. Application of chlorite, chlorite-chloritoid Fe-Mg exchange, and calcite-graphite carbon isotope geothermometry to rocks from the unconformable contact between pre-Cape basement and the Cape Supergroup made it possible to distinguish pre-Cape and syn-Cape metamorphic overprints. During Pan-African metamorphism temperatures of up to middle greenschist facies conditions (around 400 °C) were reached, whereas lowermost greenschist facies conditions (around 300 °C) were not exceeded during the 220–290 Ma Cape orogeny. In the past, most if not all of the pre-Cape basement rocks, which form the Pan-African Saldania Belt, were considered to be of Neoproterozoic age. A hiatus of about 100 °C observed between two adjacent limestone horizons that previously had been grouped together into a single formation at the bottom of the allegedly Neoproterozoic Kango Group indicates that almost all of this group is syn- to post-orogenic with respect to the Pan-African orogeny. A revision of the stratigraphy of the Kango Group is therefore suggested. Only its lowermost member is truly Pan-African and probably related to about 620–740 Ma post-Sturtian cap carbonates in other Pan-African belts of southern Africa. The remainder of the Kango Group reflects the successive development of two stages of orogen-related intra-continental basins: The older stage led to a typical syn-orogenic foreland basin related to tectonic loading in the Gariep and Damara orogenic belts further north(west) between 570 and 540 Ma; the younger is believed to have formed either a further foreland basin or an intra-orogen pull-apart basin caused by later tectonic loading in the Ross orogenic belt and its continuation into the southern Saldania Belt between 510 and 480 Ma.

Trilobites of the Hagastrand Member (T yen Formation, lowermost Arenig) from the Oslo Region, Norway. Part I: Asaphidae

This is part one of a two-part description of the trilobite fauna of the Hagastrand Member of the Tøyen Formation. The Tøyen Formation (Lower Arenig) of the Oslo Region consists of poorly fossiliferous grey shales and black graptolitic shale. In the basal part of the Formation (i.e. Hagastrand Member) in the Eiker-Sandsvær, Modum and Oslo districts, limestone horizons contain trilobites of the Megistaspis (Paramegistaspis) planilimbata Zone. These beds were formerly assigned to the underlying Bjørkåsholmen Formation (former Ceratopyge Limestone). The studied trilobites and conodonts indicate a hiatus spanning the uppermost Tremadoc Megistaspis (Ekeraspis) armata Zone in the Eiker-Sandsvær area. The M. (E.) armata Zone is present in the deeper parts of the basin (in Oslo city), where some of the grey shales also contain graptolites. Limestones of the Hagastrand Member are considered to have been deposited on local highs, surrounded by more muddy substrates. They contain a trilobite fauna distinct from the underlying Ceratopyge-fauna. The present paper describes the 10 asaphids, including Niobe (Proxiniobe) longicauda n. subgen., n. sp. and Gog n. sp. The remaining 12 non-asaphid trilobite species found are described in the second part of this monograph (Hoel in press), where the localities studied are also described.

On the occurrence of foraminiferans in the lower Cambrian of the Llangynog Inlier, South Wales

Microfossils obtained from dissolution of a limestone horizon from the lower Cambrian Allt y Shed Sandstone of the Llangynog Inlier, Carmarthenshire, South Wales, are identified as agglutinated foraminiferans. Two genera and three species are represented and are figured. These microfossils add further to the scanty record of Cambrian foraminiferans, but do not resolve the problem of the precise age of the Allt y Shed Sandstone.

Manganese deposits in Turkey: Distribution, types and tectonic setting

Manganese deposits in Turkey are divided into four main groups on the basis of their age, host rocks and processes of formation. The first is hydrothermal–hydrogenetic and rarely diagenetic-type manganese deposits which are chiefly associated with radiolarian chert series. These deposits occur as lenses and small ore beds within ophiolitic melanges zones of Paleo-Tethyan, Karakaya and Neo-Tethyan (North Anatolian and South Anatolian) suture belts. The second is black shale-hosted manganese deposits that is found in black shale, calcareous shale and calciturbiditic limestone horizons of the passive margin sediments of Late Cretaceous age in the western Taurides. These deposits generally are metamorphosed to greenschist facies and include Mn carbonate–silicate and oxide minerals. The third is volcanic arc-hosted ores along the Black Sea margin which occur in different types of metasomatic, hydrothermal and stratabound settings. Rock associations include Upper Cretaceous dacitic tuff, reddish limestone, marl and hemipelagic claystone. The fourth is Oligocene-hosted ores in the Thrace Basin. These deposits are similar to those of the Paratethyan Oligocene belt which occur on the eastern and northern margin of Black Sea and includes the Chiatura (Georgia), Nikopol (Ukraine) and Varna (Bulgaria), respectively. The Oligocene Mn deposits of the Thrace Basin were formed by diagenetic replacement processes during a marine regression.

Foraminifera from the lower Eocene Sakesar Limestone at Nammal, western Salt Range, northern Pakistan

Study of the Sakesar Limestone of the lower Eocene age in the Nammal Gorge in western Salt Range is carried out. During the field investigation eight samples were collected from an 80 ft. thick limestone horizon, at equal interval. A new Alveolinid, foraminifera assemblage is identified.

Biocénoses à triopsidés(Crustacea, Branchiopoda) du Permien du bassin de Lodève (France)

Triopsids (Crustacea, Branchiopoda), which are abundant in the “Saxonian” facies of the Salagou Formation of the Lodève Basin and dated to the Thuringian, Saxonian or Artinskian by different workers, are described here. From thousands of carapace and thoraco-abdominal fragments and more rarely from intact specimens, we identify and describe two new species: Triops cancriformis permiensis nov. ssp. and Lepidurus occitaniacus nov. sp., which are morphologically similar to modern species. On the basis of the ecological preferences of modern triopsids and the sedimentological characteristics of their environment in Permian times, we confirm the occurrence of shallow, temporary expanses of water during the deposition of the Salagou Formation. These pools were either neutral or slightly alkaline and were scattered across a playa type environment and endured with the same characteristics during deposition of the formation which is more than 1000 m thick in the south of the basin. Aridity that prevailed during this long period of sedimentation had been suggested earlier from the study of sedimentological data (dessication cracks, rubefaction) and of Isopodichnus ichnites attributed to notostracean triopsids. This is now confirmed by the discovery of large numbers of these crustaceans in crescent-shaped megaripples from the base to the top of the Salagou Formation. The organism fragments are located in minor sequence limestone horizons within the sandstone body. The fossiliferous strata formed during restrictive stages of the living environment, essentially from moulting. Triopsids and Isopodichnus,, viewed as their trace fossilw, are not found together in the same strata showing that the fossilisation conditions for the organisms and their tracks differed.

Preliminary Geological Map and Description of the Himalaya-Karakorum Junction in Chogo Lungma to Turmik Area (Baltistan, Northern Pakistan)

Some 800 km2 of the area lying between the Hararnosh Spur of the Himalaya and the Southern Karakorum Complex, centered on the Chogo Lungma glacier system, is presented on a preliminary geological map, after three expeditions. It includes three major units from SW to NE: the High Himalayan Nanga Parbat - Haramosh Crystallines (HHC), the Ladakh Paleo Island Arc, and the Karakorum Metamorphic Complex intruded by granitoids. The three units are thrust to the south. The High-Himalayan ortho- and para-gneisses are metamorphosed to granulite facies and migmatised. The Ladakh Arc is made up of two groups of formation, the Askore Amphibolite to the SW and the Greenstone Complex to the NE, separated by a screen of serpentinized ultramafics. It is intruded by numerous granitoids including a leuco-trondhjemite body cutting the Himalaya-Ladakh contact. The rnetasedimentary and metavolcanic sequences of the Greenstone Complex include limestone horizons that have yielded post-Valanginian fossils. The Karakorum Metamorphic Complex includes orthogneiss domes of granitoids intruded into the surrounding metasedimentary formations. A syenitic dome of granitoid at Hemasil is apparently syntectonic. Two phases of deformation are observed in Karakorum and Ladakh and seem to correspond, but with different grades of metamorphism. The late doming structures are characteristic of the Karakorum. The Himalaya-Karakorum contact varies from south to north, from a normal fault to a strike-slip shear zone, and finally a thrust fault. The original Ladakh­ Karakorum contact (Shyok Suture) is folded and reactivated by late brittle thrusting, possibly the latest large-scale deformation of the region. The closure of the Ladakh-Kohistan back-arc basin followed by the collision of India is mostly overprinted by the recent structures.

Early Devonian conodonts from a limestone horizon in the Caballos Novaculite, Marathon Uplift, west Texas

Early Devonian (Lochkovian; eurekaensis Zone) conodonts occur in discontinuous limestone beds in the Caballos Novaculite at five localities in the northwestern half of the Marathon uplift, west Texas. Similar conodont faunas at all five localities indicate that the limestone beds lie at one biostratigraphic horizon within the Caballos. The upper novaculite member directly overlies the limestone horizon at one locality, giving the upper novaculite a maximum age of Lochkovian. The limestone beds are dominantly skeletal calcarenites that represent shallow-water carbonate material transported into a deeper water setting by gravity processes. Restriction of the limestones to the northwestern margin of the uplift and provenance of reworked clasts and redeposited Ordovician and Silurian conodonts suggest a North American shelf source and are evidence of the close proximity of some strata exposed in the Marathon uplift to North America in the Early Devonian. Icriodus gravesi n. sp. is described from the limestone fauna.

Conodonts from the lower Ordovician at Hunneberg, south-central Sweden

AbstractSix sections through lower Ordovician shale and limestone at Hunneberg have been sampled for conodonts. Forty-nine limestone horizons and 34 shale surface samples yielded over 40000 conodont elements. The degree of biostratigraphic resolution permitted further subdivision of the post-Tremadoc Paroistodus proteus Zone into four successive intervals. Preliminary studies of coeval rocks in other areas of Sweden show that this subdivision applies elsewhere as well. Comparison with published sections from, for example, Newfoundland, Estonia and Kazakhstan indicates that the fourfold subdivision of the P. proteus Zone proposed here could significantly increase the correlation accuracy, even internationally, within this critical part of the Ordovician. Graptolites and trilobites found together with the conodonts at Hunneberg indicated that the new subdivision could help to correlate biostratigraphic units based on these fossil groups. Thus the two lower intervals of the P. proteus Zone correspond to the M. (E.) armata trilobite Zone, and the upper two are approximately equal to the M. (V.) planilimbata Zone. The base of the T. phyllograptoides graptolite Zone at Hunneberg lies close to the boundary between the upper middle and uppermost intervals of the P. proteus Zone, but the graptolitic chronozonal boundary may be lower. The overlying Prioniodus elegans Zone is mainly represented by graptolitic shale at Hunneberg, but conodont faunas have been found in it, and co-occurrences of conodonts and graptolites have been noted.

Paleoecology of Upper Eifelian and Lower Givetian coral limestones in the northwestern Sauerland (Devonian; Rhenish Massif)

The prevailing sandy/silty lower part of the Middle Devonian in the northwestern Sauerland includes two coral limestone horizons, which contain a rich fauna of corals, stromatoporoids, and calcareous algae. The Ihmert-Formation is subdvided into three parts. The older coral limestone horizon is the Grunewiese-Member of the Ihmert-Formation (uppermost Eifelian), the younger is in the Bredenbruch-Member of the Unterhonsel-Formation (lower Lower Givetian). Conclusions about the environmental constraints are drawn from the sedimentology and the fossil content of the coral limestones. Predominant biostromes are built between storm wave base and normal wave base. Only the few bioherms grew above the normal wave base. These coral limestones were deposited in a tropical or subtropical normal marine environment in the shallow euphotic zone. Among the reef-builders epoecism is very frequent, and until now this phenomenon has not been investigated in detail. Fragile rugose and tabulate corals lived as commensals with stromatoporoids. Some other aspects of paleoecology are concisely presented.

The marl-limestone rhythmites from the Lower Kimmeridgian (Platynota Zone) of the central Prebetic and their relationship with variations in orbital parameters

The origin of the rhythmic marl-limestone bedding recognized in Lower Kimmeridgian (Platynota Zone) succesions of the central Prebetic (Southern Spain) is interpreted. A detailed analysis was carried out on the basis of field observations and mathematical procedures of two sections characterized by the variable alternation of marly and limestone horizons. The study primarily reveals signs of a possible secondary imprint in the stratification. However, we conclude that this imprint cannot have been sufficiently important to cause the recorded stratification or significantly alter the existing primary bedding. The mathematical procedure applied to the thicknesses of the beds reveals the existence of different orders of cyclicity that are not recognizable in the field. We were able to date these cycles by detailed biostratigraphy and the application of Fourier/spectral analysis. The close relationship between the timing calculated for the marl-limestone rhythms recognized in the Lower Kimmeridgian (Platynota Zone) successions and the periodicity assumed for Milankovitch cycles has been shown for the first time in the Prebetic Zone and leads us to accept the influence of orbital variations on sedimentation in the sector that was studied. We have also been able to recognize the influence of local factors which determined differences in the depositional organization in the sections studied.

Geochemical evidence for a non-alkalic origin for the carbonatic bodies of Kibuye, Rwanda

The coarse-grained carbonatic bodies of Kibuye have been interpreted as magmatic carbonatites (spathic crystal from, uncomformable contacts, situation along the rift), metamorphozed limestones (occurrence within a sedimentary sequence containing a.o. limestone horizons) or limestones mobilized through magmatic activity (abundance of gabbro-doleritic dykes and sills in the vicinity). These bodies are made up of nearly pure carbonate, either calcic or calco-magnesian. They are R.E.E.-, Ba-and Sr-poor, thereby precluding any magmatic alkalic affinity. Their 87Sr/ 86Sr ratio of 0.71141 is likewise too high for a mantellic filiation. This ratio is however intermediate between those shown by the gabbro-dolerites and some dolomitic phyllites of the Burundian Sequence (Middle Proterozoic), which have respectively yielded 0.70991 and 0.71250 (when corrected for 1400 M.a. of 87Rb decay). The latter values, together with the 18O and 13C ratios, suggest that the origin of the carbonatic bodies must be searched for in a process where sedimentary dolomites have been remobilized, probably under the influence of the magmatic intrusions. Although situated in the vicinity of the Western Rift, as many other alkalic complexes, the carbonatics rocks of Kibuye are not representatives of this magmatic kindred.

Biological reconstruction of late ancient populations from Bulgarian lands

Manganese deposits in Turkey are divided into four main groups on the basis of their age, host rocks and processes of formation. The first is hydrothermal–hydrogenetic and rarely diagenetic-type manganese deposits which are chiefly associated with radiolarian chert series. These deposits occur as lenses and small ore beds within ophiolitic melanges zones of Paleo-Tethyan, Karakaya and Neo-Tethyan (North Anatolian and South Anatolian) suture belts. The second is black shale-hosted manganese deposits that is found in black shale, calcareous shale and calciturbiditic limestone horizons of the passive margin sediments of Late Cretaceous age in the western Taurides. These deposits generally are metamorphosed to greenschist facies and include Mn carbonate–silicate and oxide minerals. The third is volcanic arc-hosted ores along the Black Sea margin which occur in different types of metasomatic, hydrothermal and stratabound settings. Rock associations include Upper Cretaceous dacitic tuff, reddish limestone, marl and hemipelagic claystone. The fourth is Oligocene-hosted ores in the Thrace Basin. These deposits are similar to those of the Paratethyan Oligocene belt which occur on the eastern and northern margin of Black Sea and includes the Chiatura (Georgia), Nikopol (Ukraine) and Varna (Bulgaria), respectively. The Oligocene Mn deposits of the Thrace Basin were formed by diagenetic replacement processes during a marine regression.

FACIES DISTRIBUTION AND PETROLEUM GEOLOGY OF THE BOMBAY OFFSHORE BASIN, INDIA

In view of the recent oil and gas dicoveries in the Bombay Offshore basin, its detailed geology has been worked out in terms of its source rock and reservoir potential. An agewise, layer‐cake lithofacies analysis and five depositional model maps from Paleocene to Middle Miocene age, together with a number of paleotectonic sections has led to the reconstruction of a xeneralized depositional model of the basin as a whole. This proposed depositional model envisages the Bombay Offshore basin as a shelf‐to‐basin carbonate model. During Paleogene and Early Neogene time the clastic supply by the proto‐Narmada river from the NE resulted in delta progradation up to the Dahanu depression, which was a region of pro‐deltas and lagoons with a considerable thickness of finer clastics. Beyond the Dahanu depression, the Bombay High and Bassein‐Alibag‐Ratnagiri shelf remained open carbonate platforms, while the DCS area was the locale of shelf‐edge carbonate build‐up.The potential targets for exploration are the deltaic sandstone to the north in the Tapti area—particularly where the sandy facies interfingers with the shales and the porous limestone horizons in the Bombay Platform, Bassein‐Alibag‐Ratnagiri shelf and the DCS trend. The Dahanu depression is thought to be the main source rock area.