摘要:

ABSTRACTA computer code using sequential fragmentation/transport theory was used to deconvolute and characterize a large grain‐size data set taken from the AD 79 Vesuvio deposits. The results allow us to interpret transport and deposition processes. Four principal morphological classes of grain‐size spectra were recognized in the AD 79 deposits:1unimodal distributions with coarse modes and very good sorting;2polymodal distributions in which relative fractions of each subpopulation are considerably variable;3polymodal distributions, but with one mode greatly prevailing over the other ones;4flat spectra in which a large number of size classes show the same loading.Because different eruptive, transport and deposition conditions may have operated on pyroclasts which occur in the same bed, we have assigned grain‐size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the textures of the beds. The fragmentation/transport processes considered here occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high‐concentration flows (as a fluidized system or one with an extremely high sedimentation rate).Variation in strength and position of modes throughout the entire vertical section of AD 79 products illustrates changes in transport and deposition processes with time. Size spectra from Vesuvio quantitatively demonstrate contemporaneous deposition from fall and surge mechanisms as well as contributions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flow deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the top. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margins. This study supports the model of continuous modification in loadings of several discrete subpopulations during deposition from a single explosiv

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01511.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Riverine Plain of south‐eastern Australia is the result of prolonged Cenozoic fluvial activity. Single thread, anabranching and distributary channels and floodplains, and associated aeolian dunes, characterize the uppermost sequences. Based on detailed interpretations of Late Quaternary fluvial sedimentation and surficial stratigraphy for this 77 000‐km2basin, earlier ‘prior stream’ and ‘ancestral stream’ models of fluvial deposition, deduced from limited stratigraphic and chronological evidence, are replaced with aggradational palaeochannel and migrational palaeochannel models. Thermoluminescence dating reveals four distinct phases of palaeochannel activity between 105 and 12 ka; the first (Coleambally phase) late in Oxygen Isotope Stage 5, the second (Kerarbury phase) in Stage 3, the third (Gum Creek phase) before and the fourth (Yanco phase) after the Last Glacial Maximum (LGM) in Stage 2. The first three of these phases were characterized by mixed‐load laterally migrating sinuous palaeochannels with occasional transitions to a straighter bedload‐dominated mode, and vice versa. The first two phases concluded with a bedload‐dominated episode resulting in aggradational palaeochannels on the surface of the Plain, and the third phase (prior to the LGM) did also in its downstream reaches. The phase following the LGM was characterized entirely by large mixed‐load sinuous migrational palaeochannels. These exhibited no terminating bedload episode, because the onset of Holocene climates reduced the size of the flood peaks, greatly diminished the supply of bedload from the upper catchments and resulted in streams evolving to their present highly sinuous suspended load form. The result is a complex stratigraphic architecture consisting of vertically and laterally accreted units extending over hundreds of kilometres in the form of channel‐sand stringers, sand sheets and derivative aeolian dunes partially or wholly enca

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01512.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThroughout most of the Phanerozoic, reef rigidity resulted as much, or more, from early lithification by microbial carbonates and biologically induced cements (non‐enzymatic carbonates) than from biological encrustation of, or by, large, enzymatically secreted metazoan skeletons. Reef framework is divided into four categories: (1) skeletal metazoan; (2) non‐skeletal microbialite (stromatolite and thrombolite); (3) calcimicrobe; and (4) biocementstone, in which small or delicate organisms serve as scaffolds for rigid cement crusts. The last three categories are dominated by non‐enzymatic carbonates. Skeletal framework and non‐skeletal microbialite framework were the most abundant framework types through the Phanerozoic. The composition and abundance of skeletal framework was controlled largely by mass extinction events, but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship. Microbialite framework was abundant throughout the Palaeozoic and early Mesozoic, but declined after the Jurassic. Calcimicrobe framework was important during the Cambrian‐Early Ordovician and Devonian and biocementstone framework was important from the late Mississippian to the Late Triassic.The Phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large, skeletal ‘reef builders’, or with a variety of physicochemical parameters, including sea‐level history, Wilson Cycle or global climate cycles. Because non‐enzymatic carbonates result from induction by non‐obligate calcifiers, and not enzymatic precipitation by obligate calcifiers, the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals. Changes inpCO2, Ca/Mg ratios, cation concentrations and temperature may have affected the abundance of non‐enzymatic carbonates and, hence, reefs, independently from the effects of these same parameters and mass extinction events on skeletal reef biota. The decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the Jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the Jurassic. Reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently. Hence, reef history was regulated not just by skeletal reef biota, but by parameters governing non

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01513.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTDuring the upper Pleistocene the Central Altiplano of Bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations. Development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands, discontinuously covering the 300‐km‐long and 100‐km‐wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of Uyuni. Distribution, size and shape of the bioherms are diverse due to various factors, e.g. the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth. On larger palaeoterraces, the build‐ups coalesced to form platform‐like carbonate accumulations. Although the morphologies closely resemble those induced by cyanobacteria, they were predominantly constructed by other plant communities, probably dominated by filamentous green algae. Cyanobacterial communities flourished in association with these plants, but they did not contribute significantly to the architecture of the bioherms; they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build‐ups. A prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects. The build‐ups located between 3660 and 3680 m elevation display up to three major parts: (1) a massive inner core formed during an early Minchin highstand, before 40 ka; (2) a large peripheral envelope deposited at about 40 ka (late Minchin) and (3) a thinner outermost crust formed during a late glacial event. Lake level dropped during interlacustrine stages, sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system, i.e. the present‐d

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01514.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA ground‐penetrating radar survey of aeolian dunes in the Al Liwa area of Abu Dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures. The interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles. NNW— SSE‐orientated radar profiles, approximately parallel to the prevailing wind direction, show two sets of dipping reflectors which are interpreted as sets of cross‐stratification and second‐ and third‐order bounding surfaces. Radar profiles orientated WSW—ENE across the prevailing wind direction are dominated by concave‐up reflectors which are interpreted as trough‐shaped scours and sets of trough cross‐stratification produced by oblique progradation of barchanoid dunes. Nested troughs, with small sets of trough cross‐stratification within larger troughs, may be due to reactivation following wind reversal, or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes. Convex‐upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs. Overall there is a tendency for the larger second‐order bounding surfaces to dip downwind, which confirms Brookfield's ideas of the relative migratio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01515.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Zechstein Basin of Poland was an area of widespread cyclical deposition of carbonates and evaporites during Late Permian time. The Zechstein shelves, along both the northern and the southern margins of the basin, were sites of shallow‐water sedimentation during the formation of the Main Dolomite and Platy Dolomite, two widespread carbonate units. These units consist of oolitic, peloidal, skeletal, micritic and evaporitic carbonates formed in depositional settings ranging from open marine to coastal (lagoonal, sabkha and salina). Although originally deposited as limestones, the Main Dolomite and Platy Dolomite are inferred to have been completely replaced by dolomite through very early stage (essentially penecontemporaneous) reflux of hypersaline brines.The dolomites of the two basin margins, however, have very different petrographic and isotopic characteristics. Many northern shelf dolomites show early stage calcitization (dedolomitization) and even, in some cases, evidence of a subsequent redolomitization event. These northern shelf samples also have a broad range of carbon and oxygen isotopic ratios (up to 12%0for oxygen). Samples from the southern shelf, on the other hand, are petrographically much simpler; they do not show complex calcitization and redolomitization patterns. Likewise, their isotopic values are much more tightly clustered, with only about a 5%0range of oxygen isotopic ratios.The differences between dolomites of the same age from the northern and southern margins are best explained by regional variations in river water influx during episodic exposure events associated with regional or global sea‐level fluctuations. The distribution of clastic terrigenous materials and palaeokarstic features indicate that areas of the northern shelf had extensive river input, an influx largely lacking on the southern shelf. Early formed dolomites appear to have been calcitized during sea‐level lowstands through the infiltration of meteoric fluids into the evaporitic dolomites created during the previous highstand. In some cases, redolomitization occurred when meteoric fluids were again replaced by hypersaline brines during subsequent sea‐level highstands.Although repeated sea‐level fluctuations are clearly evident in these strata, it is likely that associated climatic changes (rainfall variations) also played a role in forming these complex diagenetic patterns. Age‐equivalent strata from Texas and New Mexico (from sites at much lower palaeolatitudes) show no such alteration patterns; samples from Greenland (slightly higher palaeolatitudes) show even more intense diagenetic alteration during depositional cycles. Thus, the examination of patterns of diagenesis may be useful in interpreting ancient, palaeolatitudinally sensitive clima

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01516.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTSalt usually accumulates in shallow ephemeral brines as simple flat beds. However, in permanent brines deepening in solar evaporation ponds in the southern Dead Sea, salt accumulation is complicated by the growth of isolated vegetable‐like salt structures and networks of polygonal salt walls. These walls divide large salt ponds, which are easily homogenized by wind, into thousands of small compartments that are less easy to mix. Instead, brines in the compartments stratify when a surficial layer about 10 cm thick supersaturates and floats above the cumulate floors on brines that are merely saturated.Salt reef growth in the southern Dead Sea has previously been attributed to mixing of ions common to brines already in the ponds and those pumped in from the northern basin (with or without subsurface brines rising through the pond floors). A new factor is emphasized here that, whatever the origins of the brines, salt reefs remain emergent by epitaxis, thein situgrowth of crystalline substrates already in the surficial supersaturated layer.Epitaxis can be invisible in crystal clear brines and prolongs the obvious crystallization season by replacing sedimentation of grains nucleated and grown on the brine surface as the dominant mechanism of deposition. Salt reefs develop botryoidal overhangs that can merge into salt platforms that roof over deep brines. Salt shallows on the reef platforms and around the pond shores are both characterized by the deposition of the thin flat beds with vertical palisade textures well known from other ephemeral brines. Salt reefs are interpreted as tepee structures which have grown by epitaxis as fast as saturated brines have deepened around them. Epitaxis may also account for the rapid deposition of thick beds of pure salt in rifts that open to oceans.Local histories of salt reef compartmentalization can be read from the shapes of reefs that record their relative rates of growth and drowning. Rather than diminish evaporation area, damp emergent salt reefs act as giant transpirative pumps that accelerate salt crystallization and reconfigure the evaporation ponds into areas smaller than thought necessary by chemical engineers. The natural end of reef formation may be when shallow brines on hollow reef platforms become ephemeral on solid salt flats. Former salt reefs are likely to be distinguishable in the undeformed geological record by their botryoidal layers of nonvertical chevron crystal

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01517.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Book Review in This ArticleOrbital Forcing Timescales and Cyclostratigraphy. By M. R. House and A. S. GaleLate Cenozoic Lava Dams in the Western Grand Canyon. By Kenneth Hamblin

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01518.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01519.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01520.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY