The Taoudéni Basin of the West African craton contains one of the few genuine terrestrial records of a Neoproterozoic ice age. In the Adrar region of Mauritania, an extensive permafrost landscape, ...lithified moraines (tillites) and striated pavements are draped by a thin, generally <5-m thick dolostone, which is lithologically and isotopically similar to other basal Ediacaran cap dolostones worldwide. In Adrar, the cap carbonate unit exhibits a complex depositional history with significant lateral facies variation related to the irregular post-glacial, topographic relief and the complex interplay between glacioeustasy and isostatic rebound. The Adrar cap carbonate package consists of one or two dolostone units, with an intervening siliciclastic package of up to 40
m thickness, and a laterally extensive, thin limestone bed that disconformably overlies the uppermost dolostone. The cap dolostone comprises mechanically laminated beds that are disrupted by fitted-brecciation, sheet cracking, tepee formation, karstic dissolution and chaotic vein networks of silica, calcite and barite. The overlying thin bed of limestone breccia comprises volcaniclastic and detrital debris and authigenic barite crystals, cemented and commonly replaced by marine calcite. The close association between relative sea-level changes, glacier retreat and cap dolostone deposition across the Taoudéni Basin implies that cap dolostones formed largely over a period of ∼10
4 years, the maximum interval over which isostatic rebound is likely to operate.
Barite has been reported from identical stratigraphic levels overlying terrestrial glacial deposits throughout NW Africa and formed locally during fluid mixing on, and in cavities beneath the seafloor during late stages of the post-glacial marine transgression. Samples of barite were collected from two distant localities in the Taoudéni Basin, in Mauritania and Mali.
87Sr/
86Sr ratios exhibit an unusually narrow range for barite that closely matches contemporaneous seawater
87Sr/
86Sr (0.7077–8). Barite δ
34S values range widely between 20‰ and 45‰ CDT, which indicates that sulphate derived from seawater and was subsequently modified by microbially mediated sulphate reduction. The consistent stratigraphic level and irregular distribution of barite deposits are consistent with a sedimentary exhalative origin for the barite, whereby Ba-rich fluids from shallow locations within the rock pile interacted with sulphate-bearing seawater; a hydrothermal origin for these fluids can be excluded. Isotopic constraints and the association of barite with terrestrial glacial deposits across the West African craton suggest that methane seepage from underlying permafrost may be one possible mechanism for Ba sequestration. The occurrence of seafloor barite precipitates at the contact between cap dolostones and overlying post-glacial limestones worldwide implies that changes in ocean composition, in particular increases in the sulphate content of ambient seawater provided an overriding control on barite mineralisation.
This paper provides the first general account of the facies associations that fill a Miocene fault bounded basin located in the central part of the Isparta Angle (Turkey), in order to define ...process-controlled sedimentary units reflecting environmental changes in a general tectonic and stratigraphic framework. The northern and western parts of the Köprüçay Basin are occupied by extensive conglomeratic successions belonging to three distinct alluvial fan–fan delta systems (AFD), which pass laterally into pelagic mudstones in the deeper part of the basin. In spite of syn- and post-sedimentary tectonics, most of the sedimentary units still display their original relationships. This allows definition and interpretation of the conspicuous facies changes in the clastic successions according to their position along complete sections across the basin. The bulk of the paper is thus devoted to the description of 11 sub-facies types that have been identified in the three AFDs, and their environmental interpretation. As a result, the origin and sedimentary infill of the Köprüçay Basin appears primarily controlled from Langhian to Tortonian by the N–S trending Kirkkavak Fault. After uplift and erosion of the western part of the basin, the final closure of the Isparta Angle resulted in a westward displacement of the Anatolian block during Upper Tortonian.
This paper attempts to describe the Neoproterozoic–Cambrian lithostratigraphic successions occurring on the West African craton and in the surrounding Pan-African fold belts, with special reference ...to glacial or glacially influenced deposits. It provides a brief synthesis of these terrains in order to propose inter-regional correlations, and to place the glacial events already described in the literature within the tectonic framework of this part of the world. Correlations are based on facies associations and isotopic databases, and supported by the occurrence of glacial deposits when these are ascribed to continental-scale glaciation. As expected, there is a diachronism of the main tectonic events around the craton when the mobile belts display a roughly similar overall facies trend reflecting the successive stages of the Pan-African orogenic cycle from rifting to collision. Contrary to most of the Neoproterozoic glacial strata elsewhere, which consist generally of marine diamictites preserved in marginal basins, West Africa displays the cratonic counterpart (tillites and associated terrestrial facies) deposited on exposed land surface by continental ice sheets. Lithostratigraphic correlations and a combination of relative dates on sedimentary rocks and on tectonic markers show that a major West African glaciation occurred between 630 and 610
Ma and can be correlated with the Marinoan ice age. This major climatic event is contemporaneous with the final stages of the Pan-African orogenic cycle. Under favourable climatic conditions (mid to high latitudes), the presence of surging reliefs at the rim of a wide cratonic platform may account for the development of the West African Marinoan ice sheet. The diachronism around the craton of collision-surrection events may also account for the occurrence of unrelated local mountain-type glaciation that could be mistaken with the craton-scale glaciation.
Paleontological study of 16 samples from four sections through the Silurian-Devonian marine deposits of the Hodh region establishes the development of the Middle Devonian and adds to our knowledge of ...the Paleozoic sequence in the West African platform. Above Silurian deposits locally dated by graptolites from the Llandovery, Devonian shales yield an abundant although low diversity benthic fauna, which is assigned to a Givetian age. The brachiopod fauna shows strong affinity with the NE Americas Realm. Two new species are described, Arcuaminetes deynouxi and Eleutherokomma monodi. The revision of the Devonian fauna of the Hodh area supports: 1) the apparent lack of Lower Devonian deposits; 2) the Givetian age of the Devonian succession; and 3) the strong eastern Americas Realm affinity of the fauna, as previously established for the Mauritanian Adrar area.
Facies and environmental setting of the Miocene coral reefs in the Late Cenozoic Antalya Basin are studied to contribute towards a better understanding of the time and space relationships of the reef ...development and the associated basin fill evolution in a tectonically active basin. The Antalya Basin is an extention–compression-related late post-orogenic basin that developed unconformably on a basement comprising a Mesozoic para-authocthonous carbonate platform overthrust by the Antalya Nappes and Alanya Massif metamorphics within the Isparta angle. The Late Cenozoic basin fill consists of thick Miocene to Recent clastic-dominated terrestrial and marine deposits with subordinate marine carbonates and extensive travertines. Late Miocene compressional deformation has resulted into three parts, referred as Aksu, Köprüçay and Manavgat sub-basins, bounded by north–south extending dextral Kırkkavak fault and the westward-verging Aksu thrust.
Coralgal reefs are common within the Miocene sequences and are represented by coral assemblages closely similar to that of the circum-Mediterranean fauna. They occur as massive, small, isolated, patch reefs that developed in two contrasting depositional systems (progradational coastal alluvial fan and/or fan-delta conglomerates and transgressive shelf carbonates) during Early–Middle Miocene and Late Miocene. The Early–Middle Miocene reefs are represented by rich and high-diversity hermatypic corals, mainly comprising
Tarbellastraea, Heliastraea, Favites, Favia, Acanthastraea, Porites, Caulastraea and
Stylophora with occasional presence of solitary (ahermatypic) corals,
Lithophyllia, Mussismilia and
Leptomusso, locally reflecting relative changes in the bathymetry. Densely packed, massive, domal and hemispherical growth forms bounded by coralline algae and encrusting foraminifera
Acervulina construct the reef framework. They occur in the fan-deltas and the transgressive open marine shelf carbonates of the Manavgat and the Köprüçay sub-basins. The Late Miocene reefs occur only in the Aksu sub-basin and are characterized by low-diversity hermatypic corals exclusively dominated by
Porites and
Tarbelastraea with minor
Siderastraea, Favites and
Platygyra. They developed on alluvial fan/fan-delta complexes and shallow marine shelf carbonates.
The Miocene coral reef growth and development in the Antalya Basin are characterized by large- to small-scale, transgressive–regressive reefal cycles which are closely related to the complex interaction of sporadic influxes of coarse terrigeneous clastics derived from the tectonically active basin margins and the related sea-level fluctuations.
The Late Cenozoic Antalya Basin developed unconformably on a foundered basement comprising Mesozoic autochthonous carbonate platform(s) overthrust by the Lycian Nappes, the Antalya Nappes and the ...Alanya Massif metamorphics within the Isparta Angle, southern Turkey. The present configuration of the basin consists of three distinct parts, referred herein as the Aksu, Köprüçay and Manavgat sub-basins, respectively, which are divided by the north-south-trending Kırkkavak Fault and the westward-verging Aksu Thrust. The Miocene fill of each sub-basin is characterized by thick accumulations of non-marine to marine clastics with locally developed coralgal reefs and reefal shelf carbonates. Based on lithostratigraphic and chronostratigraphic considerations, integrated with previously established data, the Miocene fill of the Antalya Basin is reorganized into nine formations and twelve members. A total of nineteen facies have been distinguished within this stratigraphic framework. The stratigraphic organization and the time and space relationships of these facies indicate contrasting styles of sedimentation characterized by several facies associations representing deposition in colluvial and alluvial fan/fan delta with coralgal reefs, reefal shallow carbonate shelf, base of fault-controlled fore reef slope and clastic open marine shelf environments in the tectonically active sub-basins. The coralgal reefs, which occur as small, isolated patch reefs developed on progradational alluvial fan/fan delta conglomerates, and the reefal shelf carbonates represent small to large scale, transgressive-regressive cycles which are closely associated with the complex interaction between sporadic influxes of coarse terrigeneous clastics derived from the tectonically active basin margins and/or related to the eustatic sea level changes during Late Burdigalian-Langhian and Late Tortonian-Messinian times. With regard to structural history, the Antalya Neogene basins exhibit contrasting behaviour according to their position within the Isparta Angle. West of Antalya, the Lycian Basin is linked to the eastwards advance of the overlying Lycian Nappes up to the Burdigalian; in the centre of the Isparta Angle, the Aksu and Köprüçay sub-basins are younger (Serravalian-Tortonian) and exhibit intense deformation, reflecting west-directed compressional events of Late Miocene to Lower Pliocene age. In contrast, the Manavgat sub-basin situated further east is only weakly deformed, and even farther east, the Ermenek and Mut basins are almost undeformed. Thus the evolution of the Neogene Antalya basins highlights the fundamental structural asymmetry of the Isparta Angle.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The recognition of ice‐related unconformities is of prime importance when dealing with the sequential architecture of pre‐Pleistocene glacial successions. Late Ordovician striated surfaces strikingly ...preserved in North Gondwana were long considered as abrasion surfaces at the sole of a grounded glacier overriding unlithified sediments, or having resulted from the scouring action of drifting icebergs. Field observations from Mauritania and Libya show that these striated surfaces require an alternative interpretation, which can be applied to similar surfaces described elsewhere, such as in Permo‐Carboniferous or Neoproterozoic glacial sequences. A new model is proposed that involves superimposed concurrent décollement planes within a subglacial brittle shear zone in unlithified sand beds. Shear zones in subglacial sediments are well known in Quaternary deposits but, to date, it has not been demonstrated that they may form striated surfaces occurring at continent scale with orientations consistent with palaeo ice‐flow reconstructions based on larger‐scale indicators.
The Neoproterozoic-age Mali Group of the southwestern Taoudeni Basin, NW Africa, represents, in the Walidiala Valley, a glaciogenic and post-glacial succession that brackets the Cryogenian-Ediacaran ...period boundary. At its base, debris flows and turbidite-like, sandy units of the Pelel Member pass upward into siltstone and shale of the Diagoma Member. These two units represent the progressive evolution from some portion of a fan delta fed by a nearby ice shelf to a more distal environment disturbed only by the occasional fallout from passing icebergs. The appearance of coarse-grained, cross-bedded sandstone beds and gravels of the overlying Tanague Member heralds a return to a shallower, fluvially influenced environment before abrupt transgression caps the glaciogenic succession. The transgressive unit consists of a regionally extensive, 2-7 m-thick, silty dolostone, the Bowal Member, which is isotopically and petrographically indistinguishable from ca. 635-Ma cap dolostone units elsewhere in NW Africa and worldwide. The Bowal Member comprises microcrystalline dolomite in turbidite-like depositional sheets disrupted by internal brecciation, fracturing and cementation by first chert and then dolomite. The stratigraphic succession in the Walidiala Valley closely resembles facies models relating to glacial retreat in a proximal glaciomarine environment affected by glacioeustasy. A large volcaniclastic debris flow has caused slumping and soft-sediment deformation within the cap dolostone of the Bowal Member. The widespread association of pyroclastic deposits with cap dolostone throughout the Taoudeni Basin implies that volcanism and deglaciation were roughly contemporaneous across a huge area. We consider that the volcaniclastic debris flow and soft-sediment deformation within the underlying Tanague Member were possibly triggered by seismic activity during deglaciation, caused by isostatic relaxation of the lithosphere. However, fitted brecciation of cap dolostone beds here and elsewhere in the world is more consistent with pervasive dolomite cementation.Original Abstract: Dans la vallee de Walidiala, au sud-ouest du bassin de Taoudeni, le groupe du Mali comprend une succession glaciaire et postglaciaire neoproterozoique se rapportant a la limite Cryogenien-Edicarien. Cette succession debute par des coulees de debris et turbidites sableuses formant l'unite de Pelel, laquelle passe vers le haut a des siltstones et argilites a blocs laches de l'unite de Diagoma. Ces deux unites marquent le passage d'un environnement de type lobe deltaique, proche d'une marge glaciaire, a un environnement plus distal, alimente notamment par des icebergs. Au-dessus, la presence de gres grossiers a stratifications obliques de l'unite de Tanague marque le retour a un environnement moins profond et sous influence fluviatile, avant la grande transgression regionale representee par un banc de 2 a 7 m de dolomies silteuses formant l'unite de Bowal. La composition isotopique et petrographique de cette unite carbonatee est identique a celle des cap dolostones qui coiffent les depots glaciaires neoproterozoiques ( 635 Ma) en Afrique de l'Ouest et ailleurs dans le monde. Il s'agit de dolomie microcristalline formant des lits turbiditiques localement brechifies, fractures, avec un ciment siliceux puis dolomitique. La succession stratigraphique de la vallee de Walidiala est tres proche des modeles proposes pour illustrer un retrait glaciaire en milieu marin proximal affecte par la glacio-isostasie. Un important ecoulement de debris volcanoclastiques a provoque glissements et deformations souples dans les bancs dolomitiques de l'unite de Bowal. L'association generalisee de depots pyroclastiques avec le depot des cap dolostones sur l'ensemble du bassin de Taoudeni suggere que les deux phenomenes sont contemporains et en relation avec les evenements tectoniques panafricains. On considere que l'ecoulement de debris et la brechification de l'unite de Bowal, ainsi que les deformations souples dans les gres de Tanague sous-jacents, ont pour origine une activite sismique liee a la relaxation isostatique de la lithosphere lors de la deglaciation
Lower to Middle Miocene rocks in the Karayün region of the Sivas Basin are represented by the Agilkaya and Egribucak Formations, forming a 4
km thick continuous section. Each formation is subdivided ...into three members, representing similar lithofacies. The lower members are made up of fluvial sheet-sandstone and red mudstone that migrated laterally on a flood basin within a semi-arid fan system. The middle members are composed of bedded to massive gypsum and red-green mudstone of a coastal and/or continental sabkha environment. They are intercalated with lagoonal dolomitic limestone and bituminous shale in the Agilkaya Formation and fluvial red-pink sandstone–red mudstone in the Egribucak Formation. The upper members are made up of shallow-marine fossiliferous mudstone and sandy limestone. Rapid vertical and horizontal facies changes in both formations reflect the locally subsiding nature of this molassic basin. The overall section can be also subdivided into three orders of fining-upward cycles. Type I cycles correspond to the Agilkaya and Egribucak Formations (each around 2
km thick) and indicate two long-term transgressive phases, controlled by tectono-eustatic factors. Type II cycles (140
m thick on average) are formed by alternations of fluvial sheet-sandstone and red mudstone capped by up to 50
m thick red flood basin mudstone. A 400
ka period Milankovitch cyclicity related to astronomical climatic fluctuations is suggested as a governing mechanism, although subsidence cannot be excluded. The existence of laterally persistent, fluvial sheet-sandstone overlain by red mudstone that forms fining-upward Type III cycles (10
m thick on average) is attributed to changes in subsidence rate. Rhythmic alternations between gypsum beds and red-green mudstone and/or dolomitic limestone and bituminous shale, observed in the middle members, might be related to minor periodical climatic changes rather than to tectonics.
