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Palaeoenvironmental approach to the geology, mineralogy and geochemistry of an Early Miocene alluvial-fan to cyclic shallow-lacustrine depositional system in the Aktoprak Basin (central Anatolia), Turkey
- A. Gürel, S. Kadir
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- Journal:
- Clay Minerals / Volume 45 / Issue 1 / March 2010
- Published online by Cambridge University Press:
- 09 July 2018, pp. 51-75
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The Aktoprak Basin, Turkey, provides important clues regarding regional palaeoclimatic changes which have not so far been elucidated. The purpose of the present study is to fill this gap using mineralogical, geochemical and isotopic characterization of cyclic sedimentation within Neogene sedimentary units. Early Miocene alluvial-fan to cyclic shallow-lacustrine sediments of the Aktoprak Basin consist of conglomerate, sandstone, mudstone, marl, limestone and palaeosol. The increasingly fine upward grain size and bed thicknesses suggest that the alluvial fan was fed by an ephemeral braided river. The cyclic sandflat–mudflat–shallow-lake succession is predominantly composed of small-scale sandstone-mudstone beds. Sandstone intervals in the sedimentary cycles are interpreted to be a result of sedimentation from shallow, ephemeral braided-river channels, the deposits having evolved under water-saturated conditions in response to climate changes. The mudstone intervals are interpreted as having been laid down via sedimentation during distal sheet floods which developed either under sub-aerial exposure or in a water-saturated environment. Palaeocurrent measurements indicate that the general transport direction in the Early Miocene was from east to west. The lacustrine sediments of the Aktoprak Basin consist of calcite + quartz + feldspar + serpentine coexisting mainly with smectite ± palygorskite ± illite ± chlorite. Illite and chlorite are either detrital, having originated from basement units, or the illite formed authigenically from smectite. Calcite and clay appear as cement-building conglomerate, sandstone and mudstone. The clay content decreases with increasing calcite and the presence of an inverse relationship between CaO and other oxides may correspond to an inverse relationship between calcite and other minerals. Furthermore, Al2O3, Fe2O3, TiO2, MgO, Ni, Co, Cu, Sr, Ba and Zr values in the smectite-bearing mudstone and sandstone are consistent with the chemical compositions of rocks in the surrounding area, mainly ophiolitic and volcanogenic associations. Micromorphologically, the development of smectite as a cement on leached feldspar, the occurrence of palygorskite fibres as bridges between crystals and rock fragments, as well as the presence of ‘edging’ smectite flakes suggest an authigenic origin. Field observations, mineralogy, geochemistry and the results of δ18O and δ13C isotopic determinations reveal that periodic palaeoclimatic changes were controlled by alternating wet and dry periods that resulted in a relative upward decrease in precipitation of authigenic smectite ± palygorskite within the sections, under the influence of geochemical and pH fluctuations in meteoric water within the shallow-lake environment.
Linking lacustrine cycles with syn-sedimentary tectonic episodes: an example from the Codó Formation (late Aptian), northeastern Brazil
- J. D. S. PAZ, D. F. ROSSETTI
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- Journal:
- Geological Magazine / Volume 142 / Issue 3 / May 2005
- Published online by Cambridge University Press:
- 16 June 2005, pp. 269-285
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The Codó Formation exposed in the eastern Grajaú Basin, northeastern Brazil, consists mostly of black shales, limestones and evaporites arranged into several shallowing-upward cycles formed by progradation of lake deposits. Three ranks of cycles are distinguished. The lower-rank cycles correspond to millimetric interbeddings of: bituminous black shales with evaporites, calcimudstones or peloidal wackestone–packstone; grey/green shale with calcimudstone, peloidal wackestone–packstone or ostracodal wackestone/grainstone; and ostracodal wackestone/grainstone and/or calcimudstones with cryptomicrobial mats and ooidal/pisoidal packstones. The intermediate-rank cycles average 1.7 m in thickness and are formed by complete and incomplete cycles. Complete cycles show a transition from central to intermediate and then to marginal facies associations and include two types: C1 cycle with central lake deposits consisting of evaporites and black shales; and C2 cycle with central lake deposits formed by grey/green shale. Complete cycles were produced by the upward gradation from central to marginal environments of the lake or saline pan–sabkha system. Incomplete cycles are those where at least one facies association is lacking, having been formed by successions either with central and intermediate facies associations (I1) or intermediate and marginal facies associations (I2). The higher-rank cycles are, on average, 5.2 m thick and consist of four depositional units that display shallowing-upward successions formed by complete and incomplete intermediate-rank cycles that vary their distribution upward in the section, and are bounded by sharp surfaces. While the lower-rank cycles display characteristics that reveal their seasonal signature, detailed sedimentological characterization and understanding of stratal stacking patterns related to the intermediate and higher-rank cycles support a genesis linked to syn-sedimentary tectonic activity. This is particularly suggested by the high facies variability, limited lateral extension, and frequent and random thickness changes of the intermediate-rank cycles. Additionally, the four higher-rank cycles recognized in the Codó Formation match with stratigraphic zones having different styles of soft-sediment deformation structures attributed to seismic activities. Therefore, the several episodes of lake shallowing recorded in the Codó Formation are linked to seismic pulses that alternated with sediment deposition. This process would have created significant changes in the lake water level and resulted in sharply bounded successions with upward gradation from deeper to relatively shallower facies associations.
Lithofacies and sedimentary cycles within the Late Dinantian (late Brigantian) of Fife and East Lothian: is a sequence stratigraphical approach valid?
- A. M. Kassi, J. A. Weir, J. McManus, M. A. E. Browne
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- Journal:
- Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 94 / Issue 2 / June 2003
- Published online by Cambridge University Press:
- 26 July 2007, pp. 95-113
- Print publication:
- June 2003
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The late Brigantian topmost parts of the Pathhead Formation (Aberlady Formation in East Lothian) and the succeeding Lower Limestone Formation crop out widely in Fife and East Lothian. The successions include nine deltaic, coastal floodplain and marine shelf cycles (cyclothems), of which the lowest examined terminates the Pathhead and Aberlady Formations and the remaining eight constitute the Lower Limestone Formation.
The cyclothems conform broadly to the ‘Yoredale” transgressive/regressive pattern in which a transgressive marine shelf phase is succeeded by delta progradation and terminates with a fluvial delta plain phase. Cycles may combine to form compound cyclothems up to more than 50 m thick, in which a basal, typically complete initial cycle of Yoredale pattern is succeeded by up to five base-absent minor cycles. These are thinner, more variable and less laterally persistent units in which the marine phase is weakly represented or absent.
Cyclothems reflect successive marine flooding events, possibly under eustatic control, succeeded by delta progradation and, ultimately, leading to extensive palaeosol formation, including coal seams. Sedimentation and palaeosol formation were partly controlled by fault-induced differential subsidence and are likely to have been related to autocyclic processes. Local uplift and subsidence associated with vulcanicity, as at Kinghorn and Elie, have led to thickening or thinning of sediments accumulated in a given time period.
Initial cycles initiate longer-period allocycles, corresponding broadly to third-order Exxon Production & Research (EPR) Type 1 sequences having a periodicity of around 1 Ma, within the Milankovitch orbital band. Two parasequences constitute each initial cycle: a lower, initiated on a marine flooding surface, and an upper, bounded by the base of the lowest thick sandstone in the cycle; cyclothem bases and sequence bases thus alternate. Parasequences and sequences are less well defined in minor cycles due to the problem of tracing the combined disconformity and soil profile of the underclay beyond the edge of channel sandstones. Minor cycles were controlled primarily by short-period autocyclic sedimentary and, or, tectonic processes, including delta-lobe switching and differential subsidence.
Although we have attempted to interpret the deposits of Fife and the Lothians in terms of sequence stratigraphy, we are not fully convinced that the patterns of associated changes widely recognised within the framework of sequence stratigraphy can be confidently applied in succesions in which autocyclic changes feature strongly in an area undergoing active basin subsidence associated with strike-slip faulting. There is no doubt that some of the cyclicity discerned in the late Brigantian successions of eastern Scotland was related to eustatic sea level changes, which gave rise to the widespread limestone platforms or marine bands. The formation of eight cyclothems within the 2·5–3·5 Ma of late Brigantian suggests a cyclicity of about 400 ka, which corresponds to the long period eccentricity cycles of Milankovitch rather than the 0·5–5·0 Ma of third-order EPR cycles.