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1.
Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of Ma and Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of Ma for the upper Gamohaan Formation and for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.  相似文献   

2.
The Campbell Group carbonates were deposited in a basin transgressing from southwest to the east and to northeast. At the southwestern margin of the Kaapvaal craton, in Griqualand West, mainly tidal flat carbonates and shallow marine deposits are found to be followed by subtidal carbonates marking the slow transgression. North of the Griquatown Fault, where the center of the Griqualand West sub-basin developed, continuous subsidence matched by stromatolitic carbonate growth, led to accumulation of the thickest carbonate pile (> 2000 m). There, the largest limestone deposits of South Africa are located. The deposit at Lime Acres and mining operations of the PPC Lime Ltd. are described and discussed in respect to diagenetic processes that permitted limestone preservation.  相似文献   

3.
The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.  相似文献   

4.
The Palaeoproterozoic Transvaal Supergroup floor to the Bushveld complex comprises protobasinal successions overlain by the Black Reef Formation, Chuniespoort Group and the uppermost Pretoria Group. The protobasinal successions comprise predominantly mafic lavas and pyroclastic rocks, immature alluvial-fluvial braidplain deposits and finer-grained basinal rocks. These thick, laterally restricted protobasinal sequences reflect either strike-slip or small extensional basins formed during the impactogenal rifting and southeasterly-directed tectonic escape, which accompanied collision of the Zimbabwe and Kaapvaal cratons during Ventersdorp times. The erosively-based sheet sandstones of the succeeding Black Reef Formation reflect northwand-directed compression in the south of the basin. Thermal subsidence along the Ventersdorp Supergroup and Transvaal protobasinal fault systems led to shallow epeiric marine deposition of the sheet-like Chuniespoort Group carbonate-BIF platform succession. After an estimated 80 Ma hiatus, characterized by uplift and karstic weathering of the Chuniespoort dolomites, slower thermal subsidence is thought to have formed the Pretoria Group basin. Widespread, closed basin alluvial fan, fluvial braidplain and lacustrine sedimentation, as well as laterally extensive, subaerial andesitic volcanism (Rooihoogte to Strubenkop Formations), gave way to a marine transgression, which laid down the tuffaceous mudrocks, relatively mature sandstones and subordinate subaqueous volcanic rocks of the succeeding Daspoort, Silverton and Magaliesberg Formations. Poorly preserved post-Magaliesberg formations in the Upper Pretoria Group point to possible compressive deformation and concomitant rapid deposition of largely feldspathic detritus within smaller closed basins.  相似文献   

5.
The Griquatown Fault Zone (GFZ) is a major target for Pb-Zn exploration in South Africa. The sedimentary, structural and thermal history of the fault zone are evaluated. The fault zone experienced a synsedimentary period of activity between 2550 and 2500 Ma and a major post-Postmasburg Group (less than 2223 Ma) episode of mainly vertical movements. Possible source rocks for generation of metalliferous brines are abundant along the southwestern margin of the Kaapvaal craton and shales southwest of the GFZ are time correlative to a thick peritidal stromatolitic carbonate sequence in the northeast. Fluids driven by compaction and orogenic pressure migrated across the GFZ , via the carbonates, towards the east. Metamorphic overprint south of the GFZ, based on illite crystallinity, and fluid inclusions north of the fault zone are above the oil window. Metamorphic peaks south of the fault zone are at 1750 Ma and 1213 Ma and the intensity of metamorphic overprint decreases from west to east. Because of high temperatures of metamorphic and orogenic overprint and possible remobilisation of fluids, Mississippi-Valley-Type (MVT) ore deposits are unlikely to be found within the Griquatown Fault Zone, but are expected, rather, to the northeast of it. Higher temperature, remobilised vein-related deposits could, however, occur in the GFZ itself. Received: 11 June 1996 / Accepted: 7 January 1997  相似文献   

6.
C and Sr isotope compositions of carbonate rocks from the intracontinental São Francisco basin can track ocean connections and restriction. The lower three formations of the Bambuí Group can be grouped into three chemostratigraphic intervals (CI), recording different evolution stages of the basin. Lowermost CI-1 comprises the basal cap carbonates of the Sete Lagoas Formation displaying an initial C negative excursion, followed by a coeval C and Sr positive excursions (δ13C values from − 5 to 0‰ and 87Sr/86Sr ratios from 0.7074 to 0.7082) in 10 m of stratigraphic record. It marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression. CI-2 comprises carbonates of the middle portion of the Sete Lagoas Formation with δ13C values around 0‰ and 87Sr/86Sr ratios around 0.7082 that matches those observed worldwide for the Late Ediacaran. It records the onset of a Gondwana sea pathway connecting several epicontinental basins, allowing migration of index-fossil Cloudina sp. Uppermost CI-3 starts after a major positive excursion in the δ13C values reaching + 16‰ and a steepened decrease of 87Sr/86Sr ratios to 0.7075 which are lower than those expected for the Ediacaran-Cambrian boundary. This interval comprises the upper Sete Lagoas, Serra de Santa Helena and Lagoa do Jacaré formations and records the end of the connection of the São Francisco basin to the Gondwana sea pathway setting a restricted epeiric sea. Restriction was probably caused by Late Ediacaran uplifting of orogenic belts surrounding the basin. Other West Gondwana Cloudina bearing units also display the same mismatch in the Sr isotope ratios, suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations.  相似文献   

7.
The Hammamat molasse sediments of the Eastern Desert of Egypt were deposited in isolated basins formed during an initial stage of orogen parallel N–S extension (650–580 Ma) in the Neoproterozoic time. Supply of sediments to the molasse basins began after the eruption of Dokhan volcanics (602–593 Ma), exhumation of core complexes (650–550 Ma), and intrusion of late tectonic granites (610–550 Ma). The late Neoproterozoic structures in the molasse sediments include: (1) NNW-directed thrusts due to NNW–SSE shortening (650–640 Ma), indicated by the presence of NE-, ENE-, and WSW-trending folds and NNW-directed thrusts. (2) SW- and NE-directed thrusts due to ENE–WSW constriction during oblique convergence and arc accretion at around 640–620 Ma. Many of the map- and mesoscopic-scale NW-trending folds in the core complexes, the molasse sediments, and the Neoproterozoic nappes in the Eastern Desert are related to this event. Sinistral shearing along the Najd Fault System (650–540 Ma) resulted in the development of subvertical foliation, subhorizontal stretching lineation, and NW-trending tight folds overprinting earlier folds. Stretched pebbles are oriented NW–SE and WNW–ESE in the molasse basins localized within the Najd Fault System, and NE–SW in the basins outside the influence zone of this NW-trending fault system. Strain estimated using pebbles from nine molasse basins indicate that the amount of strain differs from one basin to another and from one place to another within the same basin. Weak tectonic strain (Rs = 2.16–2.24) is obtained from post-orogenic basins; moderate strains are reported from foreland basins (Rs = 2.37–3.18), whereas moderate to high tectonic strains are recorded from the intermontane basins (Rs = 2.40–4.36). The obtained tectonic strain and K values indicate that the flattening strain prevails in the post-orogenic and foreland basins, whereas as both constrictional and flattening strains are recorded in intermontane basins. Strain variation from one basin to another and within the individual basin is attributed to presence of thrust and sinistral shear zones. Away from the deformed zones, the pebbles show no significant stretching. Two phases of thrusting and an episode of transpressional sinistral shearing are the latest structure features of the East African orogeny in the Arabian–Nubian Shield.  相似文献   

8.
 Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambrian basement in the interior and mid-Palaeozoic strata along the palaeo-Pacific margin. Small sea-board Pacific basins form an exception in having a Carboniferous to Early Permian fill overlying Devonian metasediments and intrusives. The Late Palaeozoic geographic and tectonic changes in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian to Early Carboniferous period (up to 330 Ma) accretion of microplates along the Patagonian margin of Gondwana resulted in the evolution of the Pacific basins. Thermal uplift of the Gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 Ma. At the end of this period the Gondwana Ice Sheet was well established over the uplands. The period 260–300 Ma evidenced the release of the Gondwana heat and thermal subsidence caused widespread basin formation. Late Carboniferous transpressive strike-slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellsworth-Central Transantarctic Mountains) in which thick glacial deposits accumulated, formed inboard of the palaeo-Pacific margin. In the continental interior the formation of Zambesi-type rift and extensional strike-slip basins were controlled by large mega-shear systems, whereas rare intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to northwest-directed subduction along the palaeo-Pacific margin. The orogenic cycle between 240 and 260 Ma resulted in the formation of the Gondwana fold belt and overall north–south crustal shortening with strike-slip motions and regional uplift within the interior. The Gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-Tethys shear) split the supercontinent in the Permo-Triassic into a West and an East Gondwana. By a slight clockwise rotation of East Gondwana a supradetachment basin formed along the Tethyan margin and northward displacement of Madagascar, West Falkland and the Gondwana fold belt occurred relative to a southward motion of Africa. Received: 2 October 1995 / Accepted: 28 May 1996  相似文献   

9.
黔南Ladinian-Carnian期海侵与碳酸盐岩台地演化   总被引：2，自引：0，他引：2       下载免费PDF全文
位于扬子地块西南缘的黔南地区在中三叠世晚期(相当于Ladinian—Carnian期)出现了一次大规模的海侵，使从晚元古代至中三叠世末期(约从850～236Ma)以浅水相碳酸盐岩沉积为主的扬子碳酸盐岩台地被海水淹没，形成了一套深水相的瘤状暗色灰泥岩和代表最大海泛面的黑色页岩沉积，之后扬子台地被以陆相碎屑岩为主的沉积所覆盖，从此结束了其长达约630my的碳酸盐岩台地生长演化历史。研究表明，黔西南地区Ladinian—Carnian期海侵与全球海平面变化同步，是一次全球性的海平面变化。  相似文献   

10.
Summary  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. Received May 7, 2000;/revised version accepted January 15, 2001  相似文献   

11.
This study uses Sr isotope chemostratigraphy to place constraints on the depositional age of carbonate rocks from the Tuva-Mongolian microcontinent. The age of carbonate rocks of the Irkut Formation (87Sr/86Sr initial ratio equal to 0.70480–0.70485) is determined to be older than 1250 Ma, whereas carbonates of the Zabit (0.70706–0.70727 and 0.70828–0.70848) and Agarin Gol (0.70725–0.70743) formations were deposited in the interval 630–560 Ma.  相似文献   

12.
The Kuh-e-Surmeh carbonate-hosted zinc-lead deposit, located within the Simply Folded Belt of the Zagros Mountains in southwestern Iran, is an orogen-related Mississippi Valley type deposit originally formed in the foreland Thrust Belt of the Zagros Mountains. Structural and textural observations indicate that ore deposition took place as open-space fillings in brecciated carbonate rock and as internal sediments consisting of fine-grained ore minerals interlayered with carbonates. The preferred genetic model for the concentration of the ore metals is that of dewatering of the Zard-Kuh basin due to regional tectonic compaction tectonism and expulsion of basin-derived fluids into the highly porous and brecciated dolomitized rocks of the Dalan Formation. The metals precipitated from dense basinal brine (15 wt% equiv. NaCl) at low temperatures (less than 200 °C), typically within strata of a Late Paleozoic carbonate platform. Received: 21 July 1998 / Accepted: 20 August 1999  相似文献   

13.
Following terrane amalgamation of early oceanic lithosphere, the southern and central parts of the Kaapvaal Craton were a coherent unit by 3.1 Ga. Juxta-position of the northern and western granitoid-greenstone terranes including the Murchison Island Arc was the result of terrane accretion that started at 3.1 Ga. The culmination of these events was the collision of the Kaapvaal Craton, the pre-cratonic Zimbabwe block and the Central Zone to generate the Limpopo granulite gneiss terrane. Coeval with these orogenic events the central Kaapvaal Craton underwent extension to accommodate the development of the Dominion, Witwatersrand/Pongola and Ventersdorp basins. The craton scale Thabazimbi-Murchison Lineament development during the 3.1 Ga accretion event and continued to influence the tectonic evolution of the Kaapvaal block throughout the period under review as indicated by the syn-sedimentary tectonics of the > 2.64 Ga Wolkberg Group, overlying Black Reef Formation and the Transvaal Sequence. The Transvaal and Griqualand West basins developed in the Late Archaean (> 2.55 Ga) with basin dynamics influenced by far field stresses related to the Limpopo Orogeny. During this period the Thabazimbi-Murchison Lineament lay close to the northern margin of the depository. Reactivation of the Lineament between 2.4 and 2.2 Ga resulted in inversion of the Transvaal Basin and formation of the northward verging Mhlapitsi fold and thrust belt. The half-graben setting envisaged for the deposition of the Pretoria Group was influenced by the Thabazimbi-Murchison Lineament as was the emplacement and subsequent deformation of the Bushveld Complex.  相似文献   

14.
Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42 Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ13C values ranging from −1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22–2.1 Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ13C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15 Ga, as indicated by the 2146 ± 7 Ma U–Pb age of dacites occurring at the top of the unit. Negative δ13C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085 ± 5 Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45–2.22 Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.  相似文献   

15.
Second- and third-order fault-bounded Precambrian basins frequently host deposits of the sedimentary massive sulphide group. Three-dimensional geometric modelling of the thickness of preserved basin-fill successions of the Transvaal Supergroup, using DATAMINE software, and residual gravity modelling of the contemporary basement floor, help delineate areas of exploration potential in this unit. Two main depositional axes are tentatively identified for the basal volcano-sedimentary protobasinal Transvaal successions. A sheet-like geometry was indicated for the succeeding Black Reef sandstones and Chuniespoort Group chemical sedimentary rocks. The uppermost Pretoria Group thickness model delineates eastern and western second-order basins separated by a central submerged palaeohigh. A similar isopach pattern is noted for the thick shales of the Silverton Formation in this group, with, in addition, a well-defined third-order basin in the northwest of the western second-order basin. The residual gravity model indicates two linear palaeovalleys adjacent to this western basin, one coincident with one of the axes inferred for the protobasinal rocks. The fault-bounded second- and third-order basins and depositional axes postulated here are consistent with known geological data and suggested sedimentation models. Cumulative distortions implicit in the DATAMINE computer modelling technique are reduced when the method is applied on the basin-wide scale, enabling identification of regional exploration target areas rather than immediate prospecting targets. Received: 14 August 1996 / Accepted: 13 March 1997  相似文献   

16.
Supriya Roy 《Earth》2006,77(4):273-305
The concentration of manganese in solution and its precipitation in inorganic systems are primarily redox-controlled, guided by several Earth processes most of which were tectonically induced. The Early Archean atmosphere-hydrosphere system was extremely O2-deficient. Thus, the very high mantle heat flux producing superplumes, severe outgassing and high-temperature hydrothermal activity introduced substantial Mn2+ in anoxic oceans but prevented its precipitation. During the Late Archean, centered at ca. 2.75 Ga, the introduction of Photosystem II and decrease of the oxygen sinks led to a limited buildup of surface O2-content locally, initiating modest deposition of manganese in shallow basin-margin oxygenated niches (e.g., deposits in India and Brazil). Rapid burial of organic matter, decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the Archean-Proterozoic transition. Concurrently, a massive drawdown of atmospheric CO2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused Paleoproterozoic glaciations (2.45-2.22 Ga). The spectacular sedimentary manganese deposits (at ca. 2.4 Ga) of Transvaal Supergroup, South Africa, were formed by oxidation of hydrothermally derived Mn2+ transferred from a stratified ocean to the continental shelf by transgression. Episodes of increased burial rate of organic matter during ca. 2.4 and 2.06 Ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere. Black shale-hosted Mn carbonate deposits in the Birimian sequence (ca. 2.3-2.0 Ga), West Africa, its equivalents in South America and those in the Francevillian sequence (ca. 2.2-2.1 Ga), Gabon are correlatable to this period. Tectonically forced doming-up, attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric CO2 drawdown causing glaciation on the Neoproterozoic Rodinia supercontinent. Tectonic rifting and mantle outgassing led to deglaciation. Dissolved Mn2+ and Fe2+ concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation. During the Sturtian glacial-interglacial event (ca. 750-700 Ma), interstratified Mn oxide and BIF deposits of Damara sequence, Namibia, was formed. The Varangian (≡ Marinoan; ca. 600 Ma) cryogenic event produced Mn oxide and BIF deposits at Urucum, Jacadigo Group, Brazil. The Datangpo interglacial sequence, South China (Liantuo-Nantuo ≡ Varangian event) contains black shale-hosted Mn carbonate deposits. The Early Paleozoic witnessed several glacioeustatic sea level changes producing small Mn carbonate deposits of Tiantaishan (Early Cambrian) and Taojiang (Mid-Ordovician) in black shale sequences, China, and the major Mn oxide-carbonate deposits of Karadzhal-type, Central Kazakhstan (Late Devonian). The Mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans. During the Early Jurassic OAE, organic-rich sediments host many Mn carbonate deposits in Europe (e.g., Úrkút, Hungary) in black shale sequences. The Late Jurassic giant Mn Carbonate deposit at Molango, Mexico, was also genetically related to sea level change. Mn carbonates were always derived from Mn oxyhydroxides during early diagenesis. Large Mn oxide deposits of Cretaceous age at Groote Eylandt, Australia and Imini-Tasdremt, Morocco, were also formed during transgression-regression in greenhouse climate. The Early Oligocene giant Mn oxide-carbonate deposit of Chiatura (Georgia) and Nikopol (Ukraine) were developed in a similar situation. Thereafter, manganese sedimentation was entirely shifted to the deep seafloor and since ca. 15 Ma B.P. was climatically controlled (glaciation-deglaciation) assisted by oxygenated polar bottom currents (AABW, NADW). The changes in climate and the sea level were mainly tectonically forced.  相似文献   

17.
 Aquifers in the arid alluvial basins of the southwestern U.S. are recharged predominantly by infiltration from streams and playas within the basins and by water entering along the margins of the basins. The Tucson basin of southeastern Arizona is such a basin. The Santa Catalina Mountains form the northern boundary of this basin and receive more than twice as much precipitation (ca. 700 mm/year) as does the basin itself (ca. 300 mm/year). In this study environmental isotopes were employed to investigate the migration of precipitation basinward through shallow joints and fractures. Water samples were obtained from springs and runoff in the Santa Catalina Mountains and from wells in the foothills of the Santa Catalina Mountains. Stable isotopes (δD and δ18O) and thermonuclear-bomb-produced tritium enabled qualitative characterization of flow paths and flow velocities. Stable-isotope measurements show no direct altitude effect. Tritium values indicate that although a few springs and wells discharge pre-bomb water, most springs discharge waters from the 1960s or later. Received, February 1997 · Revised, September 1997 · Accepted, September 1997  相似文献   

18.
柴达木盆地石炭系沉积相及其与烃源岩的关系   总被引：5，自引：0，他引：5  
在实地观测柴达木盆地石炭系野外露头剖面岩性和生物特征的基础上,详细研究了柴达木盆地石炭系的沉积相类型及特征,探讨了石炭纪不同阶段的沉积演化以及沉积相对烃源岩发育的控制作用。柴达木盆地石炭系主要发育陆表海,可划分为三大沉积相:滨岸相、碳酸盐岩台地相、浅海陆棚相;六个亚相:浅滩亚相、潮坪亚相、沼泽亚相、开阔台地亚相、局限台地亚相、内陆棚亚相。石炭纪沉积演化经历了早石炭世两次海侵和晚石炭世的持续海侵,沉积环境由滨海向浅海过渡,以海陆交互沉积环境为主,大面积的海进使得北部山前局部地区出现地层超覆现象。沉积相控制了烃源岩的分布:柴达木盆地东北缘石炭系的潮坪亚相、浅海陆棚亚相广泛发育泥岩、炭质泥岩及碳酸盐岩,可作为有利的生烃源岩;柴西南缘下石炭统的局限台地亚相是碳酸盐台地上局部相对闭塞的地带,由于水体相对较深,水动能较低,沉积物中有机质丰富,也是有利的生烃源岩发育区。   相似文献   

19.
This paper models the physico-chemical conditions of a Neoarchæan to Palæoproterozoic marine basin in which the sedimentary sequence of BIF, Fe and Mn ores of the Lake Superior-type formed. The model is based on Eh-pH diagram stability fields for Fe, silica and Mn solubilities (taken from the literature) and on field observations of the lithological sequences. BIF formation took place in epicontinental marine basins with free access to the ocean. The main Fe source for BIF formation was ocean enriched with about 6–10 ppm ferrous Fe of hydrothermal geochemical affinity. Land-derived Fe influxes into the BIF-forming basins certainly contributed, but the lack of clastic sedimentation precludes estimation of element budgets. The main silica source for formation of chert layers is sea water. If silica was precipitated by evaporation, the silica concentration of the BIF-forming sea must have been close to saturation (15–20 ppm). Biogenic silica concentration from a possible silica undersaturated sea may not be excluded. These inferred BIF-forming conditions fit the global occurrence of Lake Superior-type BIF in general, whereas special sedimentary environments were probably responsible for the formation of highly enriched laminated Fe ore at the Maremane Dome and in the Sishen-Kathu mining district in Griqualand West, and for the FeMn ores in the Kalahari field. Formation of laminated Fe ore in the Maremane Dome and in the Sishen-Kathu areas were restricted to local deeps within the BIF basins, caused by karst collapse in the underlying Campbellrand dolomites. In such deeps, increased pH values relative to the normal BIF-forming sea caused sufficiently increased silica solubility, resulting in the almost exclusive sedimentation of colloidal Fe precipitates.In the Kalahari field, the BIF sedimentation pile became silica-depleted when approaching the Mn layers. This was genetically controlled by the increased pH of sea water and increased silica solubility. Under such increased pH conditions, Mn oxides become stable for precipitation if minimum Mn activity is achieved in the sedimentary basin. The sedimentation sequence of low silica BIF - kutnahoritic BIF - jacobsitic BIF - braunitic Mn ore can be explained, using combined Eh-pH diagrams, as reflecting a precipitation path of increasing redox potential in a pH environment slightly above 9. These conditions were achieved by closing the access of the basin to the open ocean, resulting in the reduction of water level by evaporation and thereby increasing salinity and pH. Precipitation of low silica BIF followed and, in the presence of sufficient Mn activity with increasing Eh in the precipitating water stratum, deposition of the Mn mineral associations occurred.  相似文献   

20.
华北地台中元古界主要由环潮坪石英砂岩、浅海碳酸盐岩和浅海—泻湖相暗色页岩3种沉积相组合构成,以陆表海浅水碳酸盐岩占主导。碳酸盐岩中除含有丰富的微古植物、宏观藻类和微生物建隆外,还发育大量的微生物成因构造(MISS)和微生物诱发的碳酸盐沉淀(MMCP)。微生物席和MISS构造在高于庄组上部(约1.6 Ga)和雾迷山组下部(约1.45 Ga)碳酸盐岩中尤为发育,表明活跃的微生物活动和高有机质产量。在石化微生物席中,发现有丝状、球状细菌化石和草莓状黄铁矿;围岩中发现有针状文石、花瓣状重晶石、放射状菱铁矿、铁白云石和葡萄状碳酸盐胶结物等多种自生碳酸盐矿物,指示甲烷厌氧氧化(AOM)导致的自生碳酸盐沉淀。中元古代的温暖气候和海洋分层、缺氧、硫化条件有利于微生物的高生产量和高有机质埋藏率。气隆构造和核形石状碳酸盐结核反映浅埋藏条件下活跃的成烷作用和甲烷排放,围岩和MMCP中富沥青质。华北地台中元古界富微生物席碳酸盐岩有良好的生烃潜力,有可能形成重要的烃源岩。据微生物席、MISS构造及MMCP的研究,初步估算华北地台中元古代碳酸盐岩的概略生烃潜力约为10×108t石油当量。  相似文献   

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