摘要:

ABSTRACTA small mound, 4 × 23 m in size, on the fore‐reef slope of the Ancient Wall reef complex at Mount Haultain, Alberta, consists of fenestral, renalcid micrite and wackestone. Stromatoporoids are the most abundant skeletal component but do not form an organic framework. Renalcids, together with early micrite cementation, performed a supporting and stabilizing role but only rarely directly encrusted skeletal material. Many of the primary shelter voids, interparticle voids and fenestrae were filled with internal sediments and submarine micrite cements. A considerable proportion of the micrite matrix appears to be cement.The location and development of the bioherm was controlled by a local hard substrate and suitable environmental conditions. The mound developed on and around a large block within an underlying megabreccia bed soon after it was deposited in a debris flow on the fore‐reef slope. It formed near wave base in about 30–50 m water depth and was subject to gentle current action. Laminar stromatoporoids grew on the flanks and top of the mound, and fenestral, renalcid micrite and wackestone accumulated in the central part. Deepening water and increased rates of mud accumulation terminated further build‐up of the bioherm and it was buried by basin mud.Fenestral, renalcid micrite occurs in a number of Devonian and Lower Palaeozoic organic mounds. This study suggests that early, submarine cementation was a significant factor in the development and preservation of this distinctive l

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01682.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTSandstones of the uppermost Witteberg Group in the Cape fold belt of South Africa exhibit unusual and distinctive soft‐sediment deformation structures. These structures include folds, axial planar cleavage and micro‐fold lineations. Interfering fold patterns and intersecting sets of lineations are indicative of repeated deformation. The sandstones are immediately overlain by glacial and proglacial sediments of the Carboniferous Dwyka Group, indicating that the deformation was related to glaciation. Possible environments of deformation include: (a) subglacial dragging of unconsolidated material, (b) subaqueous slumping beyond the limit of floating ice, and (c) englacial deformation of material incorporated by freezing into the base of the glac

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01683.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTCombined field, sedimentological, mineralogical, isotopic and geochemical study of the Lower Cretaceous dolomites of Central Tunisia has demonstrated considerable diversity in origin. Environments of dolomite formation include deep phreatic, karst, lacustrine and evaporitic sabkha. All four groups of dolomite are composed of non‐stoichiometric and/or disordered crystals which are more or less rich in calcium and in iron. Petrographic fabrics are of three types: replacement, recrystallization and cementation. These three fabrics are proposed among the various criteria for the different environments of dolomitization. Average isotope (δ18O and δ13C) contents for these four dolomite groups range from −10·5 to +0·4%o(PDB) and −3·9 to + 3·7%orespectively. The distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity. Ferrous iron, also very common, is regarded as an indicator of relatively deep reducing conditions, mainly in meteoric groundwaters. Sodium distribution is related to inclusions within the dolomite, its distribution being relatively constant in all four groups; it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids. This study confirms that dolomitization may occur under widely different palaeoenvironments, either at the surface or during burial. With the exception of the sabkha environment, dolomitizing fluids seem to have been essential

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01684.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTThe occurrence of authigenic carbonates formed in three different environmental situations, within the continental Siwalik Group, has been used to compare the lithological and petrographic characters of the contrasted lithofacies. The three lithofacies are: (1) calcrete conglomerate, (2) case‐hardened conglomerate, (3) cornstone (pedogenic, nodular calcrete).The calcrete conglomerate facies laterally intertongues with the channel conglomerates. It consists of pisolites which are interpreted to have formed from carbonate‐rich spring waters emerging on to the gravelly substrate of dry, abandoned channels. The laminae characteristics of these pisolites are distinctly different from those of marine origin and also from comparable biogenic materials.Case‐hardened conglomerate occurs in the youngest part of the Siwalik stratigraphic column, in boulder conglomerates having limestone as the principal clast component. This lithofacies has resulted from cementation of the conglomerate through continued dissolution and re‐precipitation of calcite, by meteoric water, downwards from the surface. It displays a coarsely crystalline, sparry calcite cement with no evidence for displacive growth or replacement by calcite.Cornstones (nodular calcrete) occur in several sedimentary cycles of the Middle Siwalik Sub‐Group. These are immature and commonly associated with thinly‐bedded sandstones (levée) and red shales (overbank). This lithofacies is a result of concentration of carbonate through capillary action associated with pedogenic activity. Ooids developed in cornstone are essentially micritic in nature and usually composed of less than five indist

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01685.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTThe most common wave‐generated structures in the nearshore lacustrine sediments of the south‐east Shetland basin are cosets of undulatory and unidirectional ripple cross‐lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post‐vortex) ripples, and the unidirectional cross‐sets were formed by the migration of vortex (orbital) ripples at lower strengths.Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand‐starved substrate. Some lenses were reworked during periods of more prolonged wave activity.The Inman‐Komar plot of near‐bottom orbital diameter versus ripple spacing (λ= 0.80d0for smalld0, or λ= 0.65d0as modified by Miller&Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimumd0conditions) is utilized in this study since wave periods in lakes are small.The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the or

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01686.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTThe Bilelyeri Group comprises complexly deformed Mesozoic sedimentary rocks of continental‐margin affinities (Kumluca Zone). These are structurally intercalated between a coeval carbonate platform to the west (Bey Daǧlari Zone) and late Triassic ophiolitic rocks and sediments, interpreted as emplaced marginal oceanic crust, to the east (Gödene Zone). Four formations erected in the Bilelyeri Group record the later stages of continental rifting and the progressive development of part of a Mesozoic passive continental margin.The two late Triassic formations, the Telektaş Tepe and the Hatipalani Formations, are dominated by terrigenous clastic and calcareous clastic sediments, including large detached blocks of reef limestone. These rocks were laid down by mostly mass‐flow and turbidity‐flow into steep‐sided rift depressions. Organic reefs were constructed in bordering shallow seas while terrigenous clastic sediment was shed from exposed basement horsts. Thick sequences of mafic lavas were extruded (Norian) in axial parts of the rift zones, followed by a regional change to deposition of pelagicHalobia‐bearing limestone. This culminated in a major hiatus involving large‐scale sliding of shallow‐water limestones into deeper water.The Jurassic to early Cretaceous Dereköy Formation mostly consists of siltstones, radiolarian cherts and mudstones, intercalated with redeposited limestones and black shales. During this time parts of the margin were bordered by major offshore carbonate complexes constructed partly on basement fragments previously rifted off the parent continental areas. Black shales and reduced hemipelagic sediments were deposited in an elongate trough between the main platform and an offshore complex to the east. Some degree of margin reactivation in the early Cretaceous is indicated by renewed deposition of turbiditic sandstone and chloritic clays in some distal sequences. Strong relative enrichment of manganese in some horizons is attributed to offshore volcanic exhalations. Subsequent regional subsidence in the mid‐to late Cretaceous is suggested by a switch to predominantly calcareous, pelagic sedimentation on the adjacent platform and the offshore massifs as well as on the Bilelyeri margin.Tectonic disruption of the platform edge during the late Cretaceous is implied by major redeposition of shallow‐water shelf limestones in proximal Bilelyeri sequences. The Bilelyeri margin and the adjacent Gödene Zone were tectonically deformed in latest Cretaceous to early Tertiary time and were thrust over the adjacent Bey Daǧlari platform in the early Miocene.Viewed in an East Mediterranean perspective, the Bilelyeri sequences were part of a locally north‐south trending segment of a regionally east‐west margin to a substantial oceanic area further south. This segment apparently suffered significant strike‐slip deformation both during its construction and its later emplacement. Instructive comparisons can be made with other areas of the East Mediterranean, e

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01687.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTAgglutinated deep‐sea benthonic foraminifera with tests composed of phillipsite are documented and illustrated from a red clay sequence in a Giant Piston Core from the central North Pacific. Associated with the benthonic foraminifera are phillipsite crystal rosettes and aggregates of phiilipsite crystals. The benthonic foraminifera and phillipsite crystals constitute most of the>38 μm fraction, but have little effect upon the mean size of the red clay sedimen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01688.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTThe Namurian black shales studied have a good palaeontological control. Pyrite, illite and organic matter are higher in the marine shales, and siderite and kaolinite are higher in the non‐marine shales. The distribution of the diagenetic iron minerals is attributed to anionic activities controlled by salinity. The same control could account for the observed clay‐mineral distribution by differential flocculation. The major element geochemistry closely reflects the above mineral variation. Mn and Zr are higher in the non‐marine shales due to siderite and zircon respectively. Co, Cr, Rb, Y and Ba are not significantly different and a detrital source, mainly clay minerals, is suggested. In the marine shales the elements Pb, Cu, V, Ni, Sr and Zn are significantly higher. Using a discriminant function analysis the palaeosalinity groups are separated and the contribution of each element to the separation is calculated. The present element locations are thought, from correlation analysis, to be pyrite and organic matter. By analogy with experimental work and modern environments, the element enrichment is mainly attributed to reactions involving organic matter and oxyhydroxide material in environments in which salinity and slow rates of sedimentation were important factors. The element enrichment cannot be related directly to seawater concentrations, unlike some black shales, and the accumulation rates for Pb and Cu are thought to be unusually

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01689.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTAeolian processes may cause shoreward tilting of discoidal pebbles in a beach sequence. Subsequent covering by washover, back‐berm, plane‐laminated sand may give a spurious indication of palaeo‐sea position unless a regional sedimentological analysis is under

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01690.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY

摘要:

ABSTRACTShell aragonite from ammonites collected in the Upper Cretaceous of West Greenland was investigated by means of macroscopic/microscopic visual evaluation, analyses of calcite/aragonite ratios, carbon and oxygen isotopic compositions and Sr and Mg concentrations of shell carbonate and of amino acid compositions of organic matrices. The results are:(1) Material visually classified as well preserved may have suffered diagenetic modifications of mineralogical and chemical composition. (2) Of the chemical and mineralogical parameters studied, amino acid composition, calcite/aragonite ratios and magnesium concentrations were found to be most sensitive to post‐depositional modifications, while oxygen isotope composition and strontium concentrations showed detectable diagenetic modifications only after more pronounced alterations. (3) Based on the Mg/Ca ratios and calcite concentrations of the shell aragonite, a diagenetic classification has been proposed grouping the material into well preserved, moderately preserved and poorly preserved. (4) The chemical and mineralogical composition of the best preserved material suggests that the Upper Cretaceous ammonites had a shell composition similar to that of modernNautilusand other aragonite‐shelled mollu

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1981.tb01691.x

出版商:Blackwell Publishing Ltd    

年代:1981

数据来源: WILEY