ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00555.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00556.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe Oligo‐Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub‐environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa‐marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro‐magnesian minerals and calcic feldspar resulted in the release of ions into intra‐stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals.These include early hematite coatings on framework grains, pore‐lining zeolite and pore‐filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K‐feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit.A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K‐feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00557.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTDetailed studies of a new, complete Marl Slate core in South Yorkshire have provided information on isotopic (δ13C, δ18O, δ34S) and geochemical variations (trace elements and C/S ratio) which enable the formulation of a model for carbonate and sulphide precipitation in the Late Permian Zechstein Sea. Calcite and dolomite are intimately associated; the fine lamination, organic character and absence of benthos in the sediments are indicative of anoxic conditions. Lithologically the core can be divided into a lower, predominantly sapropelic Marl Slate (2 m) and an upper Transition Zone (0·65 m) of alternating sapropel and calcite‐rich and dolomite‐rich carbonates.C/S ratios are 2·22 for the Marl Slate and 1·72 for the Transition Zone respectively, both characteristic of anoxic environments. δ18O in the carbonates shows a large and systematic variation closely mirrored by variations in calcite/dolomite ratio. The results suggest a fractionation factor equivalent to a depletion of 3·8% for18O and 1·5% for13C in calcite. The δ34S values of pyrite are isotopically light (mean value = ‐ 32·7%) suggesting a fractionation factor for the Marl Slate of almost 44%, typical of anoxic basins.The results are related to stratification in the early Zechstein Sea. Calcite was precipitated in oxic upper layers above the halocline. Below the oxic/anoxic boundary framboidal pyrite was precipitated, resulting in lower sulphate concentration and elevated Mg/Ca ratio (due to calcite precipitation). As a result of this, dolomite formation occurred below the oxic/anoxic interface, within the anoxic water column and in bottom sediments. Variations in calcite/dolomite ratios, and isotopic variations, are thus explained by fluctuations in the relative level of the oxic/anoxic boundary in t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00558.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThe tube‐building polychaeteLanice conchilegaforms intertidal mounds in association with macroalgae. This assemblage produces structures that can record tidal and seasonal cycles in the stratigraphic record. They mark low‐tide levels because the assemblage occurs below neap low tide and many of the structures form when water drains off the assemblage.Mounds are created by disturbance of the sediment surface around aggregations of the assemblage and accumulation of sediments within the aggregations. The initial aggregations may be relict patches of adult worms, new clumps of juveniles or both. Juvenile worms and drift algae easily settle and survive in high‐density patches, whereas predators and waves more readily disturb low‐density patches. Algae and tubes extend through accumulated sediment and create a new, higher surface. Through this interactive process the high‐density patches increase height rapidly while the low‐density patches erode. Regardless of density, when the initial distribution is regular the surface remains flat, rising evenly or eroding evenly. Accumulation and erosion increases during spring tides and decreases during neap tides.Mound development also follows a seasonal pattern. The recruitment of juvenile worms in spring facilitates algal settlement and initiates mound buildup. The rate of tube‐building and algal growth increases in early summer as the erosive forces of storms decline. This leads to the greatest development of mounds in late summer just before the macroalgae begin to die. The loss of algae coincides with autumnal storms, and causes catastrophic erosion of the mounds into relict patches of dormant tube‐worms. Patches of tubes that survive the winter enhance spring recruitment and renewal of mound development.The shape of the mounds is often obscured during burial and preservation. Because of this, it may be difficult to distinguish the original form of ancient mounds without contemporaneous cementing organisms. But cycles of deposition and erosion are recorded inLanicetubes and other biogenic structures. These structures can be used in conjunction with physical structures to define tidal height, seasonality or current regime. In general however, the presence of tube aggregationsper seis not diagnostic of a specific hydrau

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00559.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTEven with careful petrographic and mineralogic characterization of marine limestones, intertidally and subtidally lithified rocks are often difficult to differentiate, thus hindering an accurate delineation of the diagenetic environment. Limestones from water depths of 6 to 8 m off Kuwait vary in petrographic character from oosparite and biosparite (in which the cement is entirely aragonite) to oomicrite and biomicrite (in which at least some of the cement is microcrystalline magnesian calcite). Carbon‐14 dates suggest that the oosparite may have lithified at depths shallower than at present (possibly intertidally) during a lower stand of sea‐level. In contrast the biosparite, oomicrite and biomicrite appear to be contemporaneous and to have lithified subtida

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00560.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTRecent carbonate sediments from Jamaican north coast fringing reefs were collected along three parallel traverses in the vicinity of Discovery Bay. Each traverse extended from near shore across the back reef, reef crest, and fore reef to a depth of 75 m. Relative abundances of the biotic constituents vary between sites, reflecting general patterns of reef community composition. The sediment is dominated by highly comminuted coral fragments (27·1% to 63·1%), plates of the calcareous green algaHalimeda(0·4% to 38·7%), coralline algae (4·7% to 16·2%) and the encrusting foraminiferanHomotrema rubrum(0·7% to 9·5%), with lesser amounts of other taxonomic groups (non‐encrusting foraminifera 1·3–5·5%; molluscs 1·4–7·0%; echinoderms 0·9–5·0%). Coral fragments, coralline algae and particles ofHomotrema rubrumdominate the sediments of the shallow portions of the fore reef (5–15 m), whereas plates ofHalimedaare most abundant in sediments from the back reef and deeper portions of the fore reef ( 24 m). Q‐mode cluster analysis, using sediment constituent data, resulted in the delineation of four reef biofacies over the depth ran

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00561.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTA Late Precambrian fluvial sandstone sequence in northern Norway is dominated by large‐scale cross‐sets that show eitherlenticularortabulargeometries in the streamwise sections. The lenticular sets interdigitate and in places show nearly symmetrical formsets. The tabular sets are in places solitary, but are mainly grouped in cosets. In both cross‐set types, the cross‐strata range fromconcave‐uptosigmoidalin shape, with the latter variety comprising subhorizontal to gently inclined topset strata (with parting lineation) that merge uninterruptedly downflow into the steeper (10–2°) foresets. Within the cross‐sets the geometry and dip azimuths of the foresets are conspicuously consistent, although the concave‐up and sigmoidal strata commonly alternate downcurrent.The cross‐strata characteristics suggest flood stage deposition from relatively high velocity steady currents heavily laden with suspended sand. Both cross‐set types are interpreted as representing bedforms generated by flow in the dune to upper‐stage plane‐bed transition. The lenticular cross‐sets probably represent periodic dunes, but it is far less clear whether the long bedforms represented by the tabular sets should be classified as dunes, or rather as solit

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00562.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTGlacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially‐induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass‐movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass‐movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00563.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTThere is little visible primary hydrodynamic lamination preserved in the Bridport‐Yeovil Sands as a result of intense bioturbation. Where lamination is present, it exhibits wave‐produced characteristics, although current ripple lamination is also found. The grain orientation of a variety of bioturbated and non‐bioturbated fine‐grained sandstones has been determined by measuring the magnetic susceptibility anisotropy. The magnetic fabric is of a primary style and preserves two lineation directions approximately 90° apart in azimuth. These lineation directions are interpreted as the result of grain long‐axis orientations produced by wave and current processes. The magnetic fabric is dominantly carried by a small proportion of paramagnetic minerals, thought to be largely detrital chlorite and micas. This magnetic fabric has been acquired by depositional alignment of the detrital phyllosilicates and by reorientation of the phyllosilicates during the early stages of compaction. The magnetic fabric of the intensely bioturbated sandstone is not significantly different in magnitude characteristics or in the preservation of lineation directions from that of the non‐bioturbat

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00564.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY