摘要:

ABSTRACTThe modern Eel River shelf and analogous Pleistocene Rio Dell Formation in northern California provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence, and thereby enables further understanding of muddy‐shelf processes. The modern shelf is the site of accumulation of a thick deposit of Holocene mud. Both large‐scale sediment distribution patterns and small‐scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation, even on a high‐energy, ‘storm‐dominated’ coast. The major factors in preservation of this flood ‘signature’ are the cohesive behaviour of fine‐grained sediments and episodically rapid rates of sediment input.The Rio Dell Formation includes approximately 400 m of mostly fine‐grained shelf deposits that accumulated offshore from a palaeo‐Eel River mouth. The shelf sediments comprise four depositional sequences. Sequence 1 records progradation from outer to inner shelf depths. Facies trends closely resemble across‐shelf trends on the modern shelf, suggesting that processes were similar. Detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf.Muddy facies of the Rio Dell sequence are characterized by bioturbated, clayey silts, interbedded with event layers of several types. Clay‐rich silt layers are interpreted as flood deposits and physically stratified, coarse‐silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms. Sediment‐transport calculations and consideration of grain‐size distributions of bioturbated sediments, which form the bulk of the Rio Dell sequence, suggest that these sediments are the result of biological homogenization of the fine‐grained flood deposits and of the coarser‐grained storm deposits. The results of this study in general indicate that fine‐grained shelf deposits do preserve a distinguishable, if s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00602.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTShales constitute more than 60% of the world's sediments, yet while facies models for sandstones and carbonates are at a high level of sophistication, the study of shales has clearly lagged behind. In the mid‐Proterozoic Newland Formation six major shale facies types, deposited in nearshore to basinal environments, are distinguished on the basis of bedding characteristics, textural features, and the proportions of silt, clay and carbonate. Textural features of these shale types are related to sedimentary environments as deduced from associated lithologies. The shales are undisturbed by bioturbation, and their textural and sedimentary characteristics reflect subaqueous growth of microbial mats, erosion and deposition by storms, deposition of flocculated vs. dispersed clays, continuous slow background sedimentation, winnowing by waves or currents, and subaerial exposur

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00603.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSandy turbidites, grain flows, conglomeratic mass‐flows and oxide‐facies iron formation are present in the late Archaean Beardmore‐Geraldton terrain, a metasedimentary belt which extends for at least 80 km in an E‐W direction. The marine portion of this basin contains four lithofacies associations (LA): (1) Thinbedded, iron formation‐clastic sediment association. This association represents a continuum of deposit types containing iron formation; subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand/silt. (2) Thin‐bedded, turbidite‐dominated association. These sediments consist mostly of silt/sand beds which either show no vertical trends, or thin and fine upwards over a few metres. (3) Medium‐bedded, turbidite‐dominated association. Most of these sediments are medium to coarsegrained, vertically unstructured sand sequences with occasional structured intervals. (4) Thick‐bedded association. This is dominated by poorly graded sands up to 7–8 m in thickness. Sand beds are characterised by a thin basal zone of coarse sand and pebbles, a large central interval containing a mixture of medium and coarse sand, and a thin upper zone of fine sand/silt.The overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars. We infer a nearshore break in slope, locally with large channels (LA 4) extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans (structured portions of LA 3). However, many deltaic surfaces probably were not tapped by major channels, but merged downslope into a submarine ramp. Sediment was transported across the ramp by slump events and sheet‐like grain flows (unstructured portions of LA 3). Iron formation and LA 2 sediments probably accumulated both in upper‐mid ramp areas with low sediment delivery rates, and distal to fan‐ramp successions.As major streams on the braid plain changed position, associated submarine channels and slump‐fed ramp deposits also would have shifted laterally. This produced overlap of different facies associations in both the fan and ramp environments, which may explain why observed vertical trends in bedding are limited to several metres. We suggest that on narrow, active Archaean cratonic margins, additional non‐regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity, and seismic activity associated with the arc, also contributed to the lack of vertically structured sequences. Turbidite sequences in such environments in general may contain important contributions from both submarine ramp

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00604.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Permian White Rim Sandstone of the Canyonlands National Park, Utah, contains a wide variety of sedimentary structures and features that largely result from stages in erg migration and marine influence on an erg margin. Three spatially distinct lithological and depositional facies are recognized and can be distinguished as informal units within the formation. The aeolian dune facies is composed predominantly of fine‐grained cross‐stratified sandstone of the White Rim erg. This facies is the most widespread and comprises the bulk of the formation. Within the aeolian dune facies are small subfacies that represent interdune deposits. A sheet sand facies, composed of parallel‐bedded sandstone, makes up a significant part of the lowest part of the White Rim Formation. This facies appears to have been the precursor or leading (progradational) edge to the main erg system. The final facies is a reworked or veneer facies of rippled to disturbed sandstone that is localized in its extent. It is restricted to the upper few metres of the formation and is transitional in some places to the Triassic Moenkopi Formation. This veneer facies contains many structures which indicate marine reworking as well as periods of desiccation or subaerial exposure.Some previous interpretations of the White Rim Sandstone have tended to classify the whole formation as one depositional setting. It is clear that at the margin of a sand sea, as shown in the White Rim Sandstone, there are transitional facies due to the interactions with other environments. Additionally, variation in the stratigraphic relationships of facies can be related to stages of erg migration. Erg margin deposits preceded central erg development. Erg initiation occurred during a probable relative sea level low. Sea level influence is recorded at the top of the formation because erg termination accompanied a relative sea level high with cut‐off of sand supply. Transgression of the Permian Kaibab Sea over the White Rim erg was probably the main process in preservation of original dune topographic relief. Sea level fluctuations also may have affected distribution of facies and the complexities of structures at the erg margin. Subsequent fluvial reworking of the veneer facies may have obliterated Late Permian features during lowest Triassic Moenkopi dep

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00605.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSeveral types of aeolian deposits have been recognized in Kuwait: (a) smooth sand sheets that resemble desert floor sand, (b) immobile sands that include rugged vegetated sand sheets and wadi fill deposits, and (c) mobile sands that form active sand sheets and sand dunes. Simple size frequency curves illustrate the genetic relationship between the various aeolian sediment types. The four size parameters, namely, mean size, sorting, skewness and kurtosis, were calculated. Scatter plot diagrams of sorting versus mean size and sorting versus kurtosis are effective in differentiating smooth sand sheet deposits from dune sands. Active sand sheet deposits can also be recognized because they are usually located between the two end members–smooth sand sheets and dune sands. Size parameters change with location regardless of their types. Coarsening and positive skewness usually increase downwind. Mineralogical and textural characteristics of the aeolian deposits in Kuwait revealed that they are mostly derived from the lower Mesopotamian muddy flood plain deposits, the sand fraction of the Al‐Dibdibba gravelly deposits and the disintegrated material from calcretic and gypcretic duricrusts. Distribution of depositional and deflational areas indicates that the northern desert of Kuwait is characterized by a positive sand budget, whereas the southern desert has a negative sand bud

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00606.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTObservations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms.The major arms of the dune studied are oriented NE‐SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms.Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documente

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00607.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTAbundant sand‐sized mud aggregates in the Cooper and Diamantina Rivers, Lake Eyre Basin, Australia are attributed to bedload transport of aggregates formed in deeply‐cracked floodplain soils. The conditions required for formation of pedogenic mud aggregates are: (i) abundant clay containing at least minor swelling clay, and (ii) a climate with at least seasonally hot dry periods. The worldwide distribution of these soils (Vertisols) suggests that a significant amount of mud is transported as pedogenic aggregates by modern rivers.Ancient analogues in which mud aggregates and Vertisol profiles have been recognized are the Jurassic East Berlin Formation (Connecticut, USA) and the Carboniferous Maringouin Formation (New Brunswick and Nova Scotia, Canada). The dominant red mudstones of these formations are interpreted as mainly bedload sediments deposited by sheet floods in semi‐arid palaeoclimates. The Triassic Hawkesbury Sandstone (NSW, Australia) also contains sand‐sized mudstone aggregates, thought to be pedogenic, but its paleosol and other facies point to formation in a wetter palaeoclimate. The indications are that bedload transport of mud as pedogenic aggregates was as significant a process in ancient rivers as it is at

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00608.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTFifteen lithified paleosols, closely spaced in vertical sequence, occur in the top 90 m of the late Albian Boulder Creek Formation in the foothills of northeastern British Columbia, Canada. The paleosols have well‐developed profiles 0·5 to 1·5 m thick, including A, B and C horizons. The paleosols are characterized by their grey colour, cutans, vertical roots, peds, spherulitic siderite and absence of sedimentary structures. The paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events. These unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain. The climate was humid to subhumid. Overall, the environment in which these soils developed was of low relief and subject to little erosion. The water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris. Cumulatively, the paleosols in this interval may represent 150000 yr of non‐depos

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00609.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTStromatolite biostromes and bioherms in the lower two units of the Late Proterozoic Loves Creek Member of the Bitter Springs Formation represent shallowing upward and deepening upward sequences. In the central unit stromatolite form is governed by relative position in an asymmetric shallowing upward sequence. Ooid and/or peloid‐intraclast grainstones and small, irregular bulbous and columnar stromatolites characterize the basal, transgressive portion of cycles. Domal, columnar and stratiform stromatolites comprise the bulk of the cycle. These forms accreted in a gradually shallowing epeiric sea. Domal stromatolites predominate in the deeper parts of cycles. Here synoptic relief gradually increases upwards. Columnar and stratiform stromatolites predominate in the shallower parts of cycles, where synoptic relief rapidly diminishes upwards. In thin‐bedded dolo‐mudstones at the tops of cycles the co‐occurrence of desiccation cracks, tepee structures, scalloped dissolution surfaces, gypsum moulds and anhydrite nodule pseudomorphs provides evidence for subaerial exposure. In contrast, stromatolites in a unit at the base of the Loves Creek Member accreted during a gradual rise in sealevel. Stratiform, columnar and domal stromatolitic building blocks of the shallowing upward cycle are present in this deepening sequence, but only the lower half of the shallowing upward cycle is represented. Synoptic relief of the stromatolitic laminae gradually increases upward throughout the basal stromatolitic unit. Recognition of a deepening upward stromatolite sequence at the base of the Loves Creek Member, and a disconformity surface between this sequence and the underlying Gillen Member, permits palaeoenvironmental re‐interpretation of the Loves Creek Member as a single ‘large scale’ se

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00610.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow‐mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes.The framework of these reefs was principally the product of micro‐organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is aRenalcisboundstone that lacks archaeocyathans. Within these, abundant upward‐directed thalii ofRenalcisformed a framework that trapped fine‐grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis‐bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundantEpiphyton, but may include archaeocyathans, and the microbial microfossilsGirvanellaandRenalcisas well as cryptomicrobial clotted micrite. In this type of reef, frame‐building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth‐framework cavities, as well as rare metre‐sized caverns. Within these spaces,Epiphytonand, less commonlyRenalcis,encrusted framework elements, fine‐grained sediments accumulated, and pervasive sea‐floor cements were precipitated.Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan‐rich reefs, as well as deep‐water stromatactis‐bearing mud‐mounds. TheEpiphyton‐Girvanella‐archaeocyathan frameworks and stromatactis‐bearing boundstones, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb00611.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY