摘要:

ABSTRACTDiatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300‐725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300‐400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal‐A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal‐A between 600 and 700 m sub‐bottom depth. Inorganic opal‐A is rapidly transformed by crystal growth to opal‐CT. The result is formation of silica cemented mudstone and porcelanite beds.A regional acoustic reflector (called the bottom‐simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal‐A is transformed to opal‐CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub‐bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal‐A to opal‐CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°‐50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary.X‐ray diffractograms and SEM photographs show three silica phases, biogenic opal‐A, inorganic opal‐A’, and opal‐CT. These have crystallite sizes of 11‐16 A, 20‐27 A, and 40‐81 A, respectively, normal to 101. The d(101) reflection of opal‐CT decreases with depth of burial at DSDP Site 192. This occurs by solid‐state ordering and requires at least 700 m of burial.Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the al

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00307.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTHurricane washover fans from the Texas Gulf Coast exhibit large‐scale rhomboid bed forms developed on washover deposits of fine sand with varying shell content. Washover processes inferred from aerial photographs, storm characteristics, and physical settings suggest that these bed forms are the product of (1) storm surge flooding or (2) high wind shear stress.Multiple bed forms, including large‐scale rhombs, are responsible for sedimentary structures preserved in washover deposits. Proximal channels exhibit scour and fill sequences capped by mud drapes. Mid‐channel fan deposits also have scour bases marked by shell lags which are overlain by horizontal laminations and foreset and backset laminae. Distal fan sediments are relatively shell free and are interbedded with tidal flat deposits characterized by bioturbated, alternating sand and mud laminae.Rhomboidal patterns can form on the free surface of water in response to five processes: (1) wave interference from two externally independent sources, (2) wave interference from refraction of a single set of wave fronts, (3) standing oblique waves caused by bed roughness elements, (4) standing oblique waves formed at channel boundaries and channel transitions, and (5) wind stress. Geologically, standing oblique waves from unidirectional nearly supercritical flow is probably the most important process in rhomboid bed form develo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00308.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTThe aim of this work is to link physical sediment parameters to biological parameters by an equation of state, which describes how the given variables interact in biologically active deposits from accumulation areas, i.e. lake areas where fine material is being continuously deposited. In the model the following parameters are utilized: sediment depth, rate of deposition, degree of compaction, bulk density, water content, net biotransport, upward biotransport, downward biotransport, and substrate decomposition. The equation of state has been empirically tested with data from Lake Ekoln and Lake Vänern, Sweden. The model enables determinations of age frequency distributions for arbitrary sediment layers, and it has been shown that, for example, the sediment layer 12‐13 cm in Lake Ekoln has a median age of 15.3 years and that the deposits from the median year only constitutes about 15% of the total amount of material in this particular sediment layer. The spread due to bioturbation is considerable and the range at this sediment layer is 22 years. A mechanism to explain secondary lamination is introduced and discussed in the light of the results from the mod

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00309.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTA sedimentological study of Quaternary sediments from the northwestern part of the Barents Sea shows that their composition is controlled by the underlying Mesozoic bedrock and that very little sediment has been supplied from outside sources.The Quaternary sediments consist of Pleistocene glacial clays (moraines) and Holocene gravel, sand and mud, derived by erosion of the clay‐rich moraines, which again have been derived from underlying Mesozoic rocks.On the shallow Spitsbergen Bank (30‐100 m depth) we find a high energy facies of bioclastic carbonate sand and gravel and lag deposits of Mesozoic rock fragments from the underlying moraine.14C‐datings of the bioclastic carbonates (Molluscs and Barnacles) suggest that soft bottom conditions with Mya truncata prevailed in early Holocene time, succeeded by a hard bottom high energy environment with Barnacles in the last 2000‐3000 years. This may be due to a southward movement of the oceanic polar front into the Spitsbergen Bank due to colder climate in Late Holocene (subatlantic) time, which at present day produces strong bottom currents down to 100 m depth.On the Spitsbergen Bank carbonate sedimentation has succeeded glacial sedimentation as a result of withdrawal of clastic sediment supply in Holocene time and high organic productivity because of upwelling. A similar mechanism may have been operating during earlier glaciations, i.e. in Late Precambrian time to produce an association of glacial and carbonate sediments although the biological precipitation was different at that time. In Late Precambrian time precipitation or carbonate by algaes may have occurred in colder water on the shelves due to higher saturation of carbonate in the se

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00310.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTThe subdivision of thick sequences of turbidite sediments has been problematical because of the monotonous nature of the units. One method, of using detailed detrital petrography for a large number of specimens, has delineated variations with a sequence of Late Palaeozoic age in eastern Australia. The rocks occur within a single structural block and are all members of one sedimentary petrographic province. They have been subdivided into three stratigraphic units (Moombil Beds, Brooklana Beds and Coramba Beds) and greywackes from these units are quartz‐poor to quartz‐intermediate, feldspathic or volcanolithic types. Dacitic volcanism has provided most of the detritus and the contribution from non‐volcanic sources is small. The Coramba Beds are further subdivided into four petrographic units which are parallel to the stratigraphic boundaries. These lithostratigraphic units are based on the presence or absence of detrital hornblende, and the relative ratio of volcanic lithic fragments to feldspar. Vertical petrographic variations within the entire sequence indicate that although the acid volcanic source was predominant throughout the time of deposition, there is a noticeable increase in the contribution from intermediate‐volcanic, acid‐plutonic, low‐grade metamorphic and sedimentary sources towards the top of the sequence. Detrital hornblende is also present in the upper parts of t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00311.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTBands within the Chalk of Kansas made up of masses ofUintacrinus socialisshow an unusual preservation of crinoid ossicles: in contrast to their normal preservation in full relief as single large calcite crystals the ossicles are compressed and transformed to micrite.The micrite originated by a process different from the well‐known micritization by algal and fungal borings and subsequent cementation of the borings: it is the outcome of partial dissolution.Dissolution proceeded inside the sediment and preferentially attacked the echinoderms as the most soluble calcareous component of the chalk sediment. Later, the remains of theUintacrinuscrystals preferentially attracted syntaxial cement so that the layer changed to a hard band of limestone within the soft chalk.In addition to a second process of micritization the preservation ofUintacrinusdemonstrates (1) that the magnesium content of magnesian calcites survives the earliest stages of diagenesis within chalk, and (2) that a diagenetic comminution of large crystals (in an optical sense) to smaller ones is possible. Provided the ossicles of echinoderms are true single crystals (the knowledge in this field is summarized), this is an example of the often discussed ‘crystal diminuti

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00312.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTLarge‐scale circular (up to 250 m diameter) and rectilinear (up to 50 m across) depressions form regular ‘honeycomb’ patterns on pavements exhumed from below basalt flows in Carpentarian (1800‐1400 m.y.) sandstone of northern Australia. They are likened to soft‐sediment deformation structures produced experimentally in systems with reverse density gradients. The structures represent an intermediate scale between soft‐sediment deformation (small‐scale) and orogenic deformation

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00313.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTA new explanation of the sensitivity exhibited by Canadian clay sediments is outlined. The mechanism involves cementation of the microstructure and is based on the interaction of primary mineral particles with amorphous matter. The particles appear to be coated with amorphous matter and during sedimentation links develop as the particles come into contact. The speed of the sedimentation, the formation of cementitious links at low stresses and the post‐depositional rise in pH enhance the development of the high porosity structure

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00314.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY

摘要:

ABSTRACTA vertical rod sand trap has been constructed to acquire a volumetric measure of wind‐blown sand. The design is simple, construction cost is minimal, and collection efficiency is quite high. This device can be used effectively for short‐term monitoring of aeolian transp

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1978.tb00315.x

出版商:Blackwell Publishing Ltd    

年代:1978

数据来源: WILEY