摘要:

ABSTRACTDominantly coarse‐grained, shallow‐marine, metasedimentary rocks of the Early Proterozoic Uncompahgre Group (UG) record periods of shoaling and drowning on different temporal scales that are attributed to episodic long‐term oscillations in relative sea‐level with superimposed shorter duration excursions in relative sea‐level. Long‐term events are probably tectonic whereas short‐term events are eustatic.The 2–5 km thick Uncompahgre Group consists of 250–600 m thick, dominantly coarse‐grained quartzite units (Q1–Q4) and 200–300 m thick mudstone/pelite units (P1–P5). Five depositional systems comprise the Uncompahgre Group. The outer shelf system (OSS) is composed of Bouma‐type beds and intercalated mudstones that are transitional vertically to parallel‐laminated to wave‐rippled sandstones and hummocky cross‐stratified sandstones of the inner shelf system (ISS). Trough cross‐stratified sandstones comprise the shoreface system (SHS). The tidal inner shelf/shoreface system (TIS/SHS) consists of a complex interlayering of cross‐bedded sandstones, thin‐bedded conglomerates, mudstones and rippled sandstones. Trough cross‐bedded pebbly sandstones and thin‐ to thick‐bedded conglomerates represent the alluvial system (ALLS).Depositional systems in the UG are associated in transgressive and highstand‐systems tracts that make up four sequences (1 to 4). Sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities. The basal Q1–P1 unit (lower sequence 1) consists of ALLS to TIS/ SHS to ISS comprising a transgressive systems tract. A maximum marine incursion is reflected by deposition of OSS facies in stratigraphic units P1–P2. Shoaling in the transition from P2 to the uppermedial portion of Q2 (OSS—ISS—SHS to a thick TIS/SHS—ALLS) records the highstand systems tract of upper sequence 1. A subtle disconformity/paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract. The drowning to shoaling pattern is replicated in sequence 2 (upper Q2 to P3 to upper medial Q3); sequence 3 (upper Q3 to P4 to upper‐medial Q4); and an incomplete sequence 4 (upper Q4 through P5). Thinner shoaling intervals of OSS—ISS—SHS in P3 and in lower Q2, Q3 and Q4 represent parasequences.Sequences of 107years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin. Parasequences record shorter duration temporal controls of c. 104to 105years related to eustatic oscillations. As a consequence of shoaling and aggradation/ progradation in the highstand systems tract, TIS/SHS and ALLS overlie and are temporally separated from OSS to ISS to SHS. This transition records filling of the basin to sea‐level leading to a shelf geometry that was conducive to tidal amplification. A composite relative sea‐level curve integrating long‐term pulsatory subsidence and short‐term eus

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00955.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Cow Head Group is an Early Palaeozoic base‐of‐slope sediment apron composed of carbonate and shale. Whereas coarse‐grained conglomerate and calcarenite are readily interpreted as debris‐flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre‐ to centimetre‐thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate‐shale sequences (GCSS) deposited from waning carbonate turbidites and fall‐out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin.The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments.These attributes characterize not only fine‐grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00956.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTWell to poorly preserved sandstone surfaces with glacial grooves, longitudinal ridges, bulbous bedforms and large lodged clasts occur sporadically at the base of the Dwyka Formation along the western margin of the Karoo Basin. The bedforms developed when ice overrode a thin (0·1–2·0m thick) subaqueous icemarginal apron formed primarily during periods of ice front retreat. Bergstone mud and rain‐out diamicton blanketed the glacial bedforms.The subglacial bedforms formed by (i) the lateral movement of water‐saturated sediment into low‐pressure zones, caused by crevasses and cavities at the base of the ice; (ii) the presence of areas of higher strength substrate, due to variations in bed lithology and porewater dissipation; and (iii) sediment flowage into low‐pressure zones on the leeside of obstacles formed in areas of higher strength substrate due to dissipation of pore‐water pressures and sediment compaction. The preservation of the bedforms, with their delicate slump fans, is attributed to separation of the glacier sole from the substrate during a sudden rise in sea‐level. A series of dynamic ice‐marginal events, including feedback relationships between sea‐level oscillations, isostatic responses, ice‐margin fluctuations, ice‐margin type and the type of substrate, controlled the deposition of the basal sedimentary sequence and the formation of the associated glacial bedforms. The presence of a complex combination of glacier‐related formative and depositional processes may have consequences for past interpreta

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00957.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTChemical sediments are common and diverse in the c. 3500 Myr old North Pole chert‐barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi‐permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea‐shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be inco

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00958.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Late Proterozoic Pedro Leopoldo facies (Bambuí Group) in the vicinity of Belo Horizonte, Brazil, comprises alternating laminated microsparitic limestones (10–35 mm thick beds) and fibrous limestones (10–55 mm thick). The latter are composed of a mosaic of sparry calcite crystals. These irregularly crosscut rays and fans are composed of feathery precursor crystal bundles with squared‐off growth zones. Ghosts of an original fibrous mineral, hexagonal in cross‐section, are visible. The petrographic characteristics, very high strontium content and low magnesium content of the fibrous beds, as well as microspar beds, strongly argue for an original aragonitic mineralogy. The rays are interpreted as having formed by precipitation at the sediment‐water interface, whereas the micrite was precipitated from the water column prior to deposition on the sea floor. The lack of emergence features suggests widespread aragonite precipitation under persistently subtidal

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00959.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTOligocene oncoids from the lacustrine Süβwasserschichten of the Mainz Basin exhibit a pustular surface and a characteristic cavity on their undersides, which gave rise to their designation as ‘swallow nests’. Oncoids grew in situ, under low‐energy conditions during a pause in sedimentation. Three features indicate that they were never overturned: (1) oncoids can be placed in a stable position on a smooth surface; (2) laminae, which are mostly concentric around the entire oncoids, display polar thickenings on the upper side during all growth stages of oncoids; (3) the depression is always situated on the stable side of oncoids. Development of this cavity is favoured by unionid shells, in the stable position, serving as oncoid nuclei, but occurs also under flat nuclei. It is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions, on a fine‐grained substrate, and is considered as diagnostic of the in situ growth of oncoids.Oncoid cortices are mainly formed by aScytonema‐like cyanobacterium, whose carbonaceous organic remnants are occasionally preserved. Growth stages of individual oncoids were image‐processed and compared by means of shape parameters. Due to optimum growth conditions, cyanobacteria try to occupy the entire available space; hence, oncoids tend to achieve a spherical form even during in situ growth. Renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobac

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00960.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTThe Chaudfontaine ore deposit is composed mainly of barite and hosted in Frasnian shale and carbonate formations of Belgium. Thirteen sedimentary structures involving barite crystals are described and discussed. Many of these structures appear to have developed under gravity control. The role of mechanical and chemical ore apposition processes are detailed in each case.The study shows that two populations of barite were formed: (1) crystals which grew in brine above the water‐sediment interface and (2) crystals which developed in the sediment during early diagenesis. The two modes of formation have specific sizes, habits and structures. Both populations crystallized under the control of the prevailing physico‐chemical conditions, and the size and abundance of the barite crystals may be expressed as a function of the degree of barium supersaturation. High‐energy currents and unstable sediment caused previously deposited crystals to be reworked. Morphological similarities were found between Chaudfontaine barite and gypsum from evapo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00961.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTConcretions cemented mainly by siderite, Mg‐calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate‐reducing bacteria are active. δ13C values ranging from −3 to −11·8% (mean −5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ18O values ranged from −6·4% to + 0·8% (mean −1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to‐sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg‐calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the availabl

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00962.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTAn experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of ‘hump‐back’ bars and along the top of planar gravel sheets. Hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like‐sized and finer particles. For prescribed conditions, flat upper‐stage plane sand‐beds develop over the crestal location with pebbles rolling easily over the sandy bed. At the brinkpoint, flow separation ensures effective segregation of the gravel from the sand. Over the slip‐face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed‐form. Consequently, distinctive vertical assemblages of open‐work and closed contact framework gravels could be generated as another bar migrates over, and preserves, the initial structure.In respect to the observed mechanisms of sorting over the bars, a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper‐stage plane‐bed conditions on the crestal platform. The model then is used to ascertain whether the depositional environment ascribed to certain facies in the Bunter Pebble Beds, described in a recent publication, is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particl

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00963.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

ABSTRACTRecent Monte Carlo computer simulations have modelled the local, geometric mechanism by which mixed populations of differently sized particles order themselves when shaken, such that larger particles rise at the expense of the smaller. This process, called size segregation, is distinct from the mechanism of sifting where smaller particles fall between the interstices of larger particles. Size segregation is applied here to the sand grain populations found on the surfaces of dunes, which are involved in a sorting process driven by the wind. We present data from four barchan dunes located in the Eastern Province of Saudi Arabia. A simple model is used to demonstrate that size segregation, driven by saltation impacts, is energetically plausible at the wind velocities typically encountered on dune surfaces. Laboratory tests which support this model are also discussed.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1990.tb00964.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY