摘要:

ABSTRACTThe Middle Albian Ondarroa turbidite system is a coarse grained, deep water unit which outcrops in the north‐eastern part of the Basque‐Cantabrian region, west of the Pyrenees. It is about 18 km long and 7 km wide, and shows an unusual ‘L’shape resulting from both a direct morphotectonic confinement and the presence of nearby shallow water carbonate buildups.Eight main facies have been distinguished within this turbidite system: (1) clast‐supported conglomerates; (2) mud‐supported conglomerates; (3) slump deposits; (4) normally graded pebbly sandstones; (5) cross stratified sandstones; (6) interbedded graded sandstones and mudstones; (7) interbedded non‐graded sandstones and mudstones: and (8) mudstones. Inner system, middle system, outer system and basin plain divisions have been distinguished. The inner turbidite system is characterized by stacked channel fill conglomerates and lesser sandy turbidites and mudstones. The middle system consists of sandy and conglomeratic fining upwards sequences, normally several metres thick. The outer system has alternating non‐channellized sandstones and mudstones, without any predictable vertical arrangement. The basin plain is characterized by mudstone‐siltstone laminations and lesser, randomly occurring thin bedded sandy turbidites.Three main channel fills make up the inner turbidite system. Although all of them can be compared with the valley channel fills of the modern Mississippi Fan, and thus their bases can be interpreted as sequence boundaries, only the lowermost and the uppermost channel bases are documented as allocyclic boundaries. The Ondarroa turbidite system was deposited in an immature passive margin subjected to transtensional movements. It filled a composite pull apart depression with coarse clastics derived from a narrow platform to the north of the present outcrops which was invaded by fan deltas. A major pattern of sinistral strike‐slip faulting linked to the opening of the Bay of Biscay is invoked to explain the Ondarroa turbidite system appearance and its t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02002.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDiagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha‐related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precu

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02003.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA natural, pure quartz sand has been seeded with the bacteriumPseudomonas atlanticaand the fungusPenicillium chrysogenum,and angles of avalanche and repose have been measured under water using a laboratory clinometer. The lowest angles of avalanche occur in freshly packed clean sediment (control), with the seeded sediments having higher values. Among the latter, the lowest angles of repose occur in the bacterial seeded sediments, and the highest in the fungal seeded sediments. The largest differences between the angle of avalanche and angle of repose occur in the bacterial seeded and media control sediments. The smallest differences occur in the fungal seeded sediment. In most cases the second angle of avalanche is lower than the first angle of avalanche, whilst the second angle of repose is higher than the first angle of repose.The bacteria bind particles together with their extracellular polymeric material, while the fungus binds particles by holding them together with a network of hyphal filaments. In the bacterial seeded sediment growth is uniform over the sediment surface. In the fungal seeded sediment growth occurs as discrete colonies separated by bare sediment, and the fungal hyphae penetrate the sediment to a significant depth. On avalanching, the fungal colonies move down the slope with the hyphal filaments trailing behind them in the sediment. Overall, both the bacterium and the fungus increase slope stability. However, the fungal colonies maintain slope stability after avalanching more effectively than does the uniform bacterial growth. The results are discussed in relation to the wide range of biological effects that stabilize flat sediments and to laboratory and field studies on the stability of sediment slopes.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02004.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTExperimental results are reported concerning the nature of reflected flows generated when density currents are incident upon ramp‐type flow obstructions. The reflected flows are bores (moving hydraulic jumps that transport mass) with flow characteristics in common with either a group of solitary waves (weak Type A bores) or the original density current (strong Type C bores). Alternatively, the bore may have attributes in common with both of these end‐member forms (intermediate Type B bores). Bore strength is positively correlated with the ratio of reverse flow thickness to that of the residual tail of the forward flow. The largest values of this ratio occur when ‘proximal’reflections arrive at the steeper ramps. Measured particle paths in the bores indicate that natural examples will have the potential to transport and deposit sediment. Strong bores have velocity characteristics very similar to the original current and thus in nature the generated sequence of sedimentary structures will resemble those of the original depositing current. The train of solitary waves that make up a weak bore sequence exhibits a pulsating velocity profile at a point. Such flows may thus generate repeated sequences of structures separated by fine ‘drapes’that are distinguishable from the deposits of the original turbidity current. These conclusions are applied to examples of reflected turbidites described from the Palaeozoic to Quaternary sedimen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02005.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe La Casita Formation in north‐eastern Mexico represents a period of increased terrigenous clastic supply that is characteristic for the entire northern and western margins of the Gulf of Mexico Basin during latest Jurassic and earliest Cretaceous time. A prograding delta complex developed at the south‐eastern margin of the Coahuila Peninsula. Delta plain, delta front, prodelta and shelf areas show distinct lithofacies associations. Some of the most characteristic elements are fining upward and coarsening upward grain size trends, syndepositional deformational features, storm layers, phosphorites, fossiliferous carbonate concretions and an anaerobic biofacies.A possible break in sedimentation is inferred for parts of the Valanginian. This may be interpreted as a sequence boundary between second order cycles. The Kimmeridgian ‐ Berriasian cycle appears to correlate with the Cotton Valley cycle of the Texas Gulf

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02006.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Late Cretaceous to Early Eocene, dominantly micritic, Amuri Limestone Group (ALG) was deposited in an approximately NW trending trough, in eastern Marlborough, New Zealand. The ALG comprises: the Mead Hill Formation; the Teredo, Lower and Middle Limestone formations; and the Upper and Lower Marl formations. Chert and dolomite are concentrated in the Mead Hill Formation, which contains five of six recognized diagenetic zones: Zone I at the base of the ALG consists almost entirely of chert; Zone II consists solely of chert and dolomite; Zone III comprises chert and limestone; Zone IV is composed of chert plus dolomite; Zone V is a chertified mudstone; and the minor amounts of chert found in the Middle Limestone Formation comprise Zone VI. With the exception of Zones IV and V, chert decreases stratigraphically upwards and away from the basin centre. All the dolomites are composed of<1 mm diameter rhombohedra in discontinuous beds and lenses. Generally Ca‐rich, and non‐ to slightly ferroan, the dolomite contains approximately 500–900 ppm Mn and 200–400 ppm Sr. δ13C values average 1–2%PDBwith δ18O ratios of about ‐4%PDB. Mass balance calculations indicate that the Mg2+for dolomitization was derived from sea water. Sr, Fe and Mn concentrations are interpreted as indicating dolomite formation in the marine environment, with no influence from meteoric waters. The intimate association with pyrite implies dolomite formation in association with sulphate reduction, in the upper sediment column. δ18O data show that the bulk of the dolomite formed at temperatures below 50°C. All chert samples contain in excess of 90 wt% SiO2, about 1 wt% Al2O3and 1 wt% from losses on ignition. Generally all other major elements total less than 2 wt% oxide. δ18O values range from 26·8 to 29·0%SMOW. Chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate‐bound components from the site of chertification, and the effective dilution by SiO2of non‐carbonate‐bound insoluble residues. δ18O data indicate that chert formed in fluids of similar composition and temperature as the dolomite. The abundance of disseminated pyrite in cherts implies an association with sulphate reduction. Silica for chertification is thought to have initially come from dissolution of siliceous organisms. However, there is insufficient biogenic silica available to form the volumes of chert observed. It is suggested that the bulk of the silica came from SiO2‐rich pore waters generated by clay mineral reactions in the thick underlying mudstones. The ALG compacted down through these pore waters. Chert and dolomite nucleation are considered to have been penecontemporaneous. Dolomitization was initially probably the faster process, continuing as long as sulphate reduction prevailed and there was an adequate supply of Mg2+. The nucleation of chert, although initially slower (probably due to a relatively lower initial SiO2supply), continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix. The amount of chertification decreased progressively as SiO2supplies diminished, both stratigraphically upwards and

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02007.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTMillimetre to centimetre sized arborescent shrub‐like calcite precipitates are common constituents of hot water travertine shallow pool deposits of Quaternary age at Rapolano Terme, Tuscany, Italy. In the presently forming travertines, the shrubs consist of apparently random associations of (i) micrite aggregates and (ii) subhedral to euhedral rhombic spar crystal aggregates. In thin section, the micrite aggregates appear dark and the spar‐rhomb aggregates light, giving the shrubs a mottled appearance.Travertines are basically produced by CaCO3precipitation due to degassing and evaporation of the spring waters, although biological influence may also stimulate precipitation. The formation of masses of erect shrubs, rather than dense crystal crusts that form on slopes, is probably due to limited water flow in the pool environments.Microbes, including bacteria and diatoms, are important influences on shrub microfabric and external shape. The micrite aggregates are associated with bacteriform bodies, seen as tiny rods and spheres. The micrite precipitates around these bodies and in adjacent biofilm. Spar‐rhomb precipitation appears to be external to the biofilm, and may be related to the presence of diatoms which are locally closely associated with the spar‐rhombs, although an essentially inorganic origin, particularly for the more euhedral rhombs, cannot be ruled out.In the older Quaternary travertines, the original microfabric of the shrubs has been diagenetically altered. The original mottled appearance of the shrubs has become uniformly dark and micritic, and the evidence for the dual micritic and spar‐rhomb origin of the shrubs is obscured or destroyed. Spar‐micritization of the shrubs is probably due to abiotic, and locally biotic, dissolution. Previous studies did not recognize the diagenetic micritization and attributed shrub formation entirely to bacteri

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02008.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Tropic Shale and correlative Tununk Shale Member of the Mancos Shale accumulated during Cenomanian‐Turonian time, within prodeltaic environments near the western margin of the Western Interior Seaway of North America. Stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick, fine grained succession. The Tropic and Tununk shales were deposited during the Greenhorn second‐order sea level cycle, over a time span of about 2–5 million years. In southern Utah, six depositional sequences are superposed upon the record of this long term sea level change. The sequences developed during third‐order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences, arranged in retrogradational, aggradational and progradational parasequence sets.The Tropic Shale and Tununk Shale Member accumulated just basinward of the axis of maximum subsidence of a foreland basin. Stratal geometries and facies distribution patterns in the succession indicate that in southern Utah the Greenhorn cycle was tectonically controlled. During the Greenhorn transgression and highstand, rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area, in the more proximal foreland. At this time, hemipelagic facies accumulated at relatively slow rates in southern Utah and type 2 sequences developed during third‐order sea level cycles. In contrast, during the Greenhorn regression rates of thrust‐induced subsidence in the proximal foreland basin evidently slowed, and deltaic clinoforms prograded across the study area. At least one forced regression occurred in southern Utah at this time, and type 1 sequences developed.The formation of type 1 sequence boundaries in the upper part of the Tropic Shale and Tununk Shale Member points to episodes of base level fall and indicates that the six third‐order sea level cycles recorded in the succession were not the result of changes in sediment supply alone. The third‐order cycles may have been a consequence of episodic tectonism. The timing of these cycles, however, suggests that development of sequences and parasequences in the Tropic Shale and Tununk Shale Member may have been related to orbital forcing in the Milankovitch band. Glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency se

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02009.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Upper Permian Gröden Formation of the Northern Calcareous Alps (Austria) is composed of alluvial fan and playa lake sediments that were deposited in intramontane basins. A conspicuous feature of these redbeds is the abundance of magnesite in the form of nodules and discrete layers in mudstones as well as intergranular cement in sandstones. Sedimentological observations indicate that the bulk of these carbonates formed during early diagenesis and were probably syndepositional. Petrographically, most magnesites consist of micrite or, less commonly, microspar. An early non‐ferroan magnesite is post‐dated by later stage ferroan magnesites. Nodules consisting of recrystallized, sparry magnesite were observed only at one location. The general absence of relics of a non‐magnesite precursor mineral and the occurrence of shrinkage features suggest that the fine grained magnesites formed by transformation of a hydrated magnesium carbonate mineral, e.g. hydromagnesite. Carbon, oxygen, sulphur and strontium isotope ratios in conjunction with sedimentological criteria support a model of (hydro)magnesite precipitation in an inland playa lake system, which was fed by run‐off from the surrounding hinterland. The scarcity of evaporites and the dominance of magnesite over calcite and dolomite suggest that the playa lake brines were low in sulphate and had high Mg/Ca ratios. The source for the high magnesium concentrations is thought to be the weathering of Devonian dolostones and associated massive magnesite deposits in the catchm

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02010.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTSouth of the Caledonian Brabant‐Wales Massif a more than 200 m thick Tournaisian to Lower Visean replacive dolomite unit can be followed for several hundred kilometres from the Boulonnais (France) to Aachen (Germany). Field observations, of features such as karst cavities occurring at the top of the Lower Visean dolomite which are filled by Lower Visean crinoidal limestone, indicate that dolomitization and karstification took place during the Early Visean. This early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric. The minor element composition of the majority of the dolomites remains almost uniform throughout the entire studied area. Values for Fe, Mn, Na and Sr are normally in the range 700–4700 ppm, 15–400 ppm, 80–300 ppm and 50–200 ppm, respectively. The δ13C values (range‐0.72 to +5.31%o) mainly reflect the carbon isotopic composition of the precursor limestones. The δ18O values, in contrast, are highly variable: ranging from‐19.15 to +0.85%o. This rather large range of δ18O values is explained by multiple‐step re‐equilibration/recrystallization during progressive burial and subsequent uplift of the dolomites. These processes are also responsible for the high87Sr/86Sr values of the dolomites which range from about 0.7088 to 0.7098. They are distinctly more radiogenic than Lower Visean marine carbonates (0.7076–0.7078). Correlation, however, of δ18O values or87Sr/86Sr ratios with dolomite and/or cathodoluminescenec (CL) textures has not been very successful. This suggests that recrystallization may remain unrecognized if only petrographic techniques are used. Nevertheless, certain CL textures can be related to specific interactions with the ambient

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb02011.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY