bluuebunny/crossref_metadata_2025_split huggingface dataset:
The geometry and petrogenesis of hydrothermal dolomites at Navan, Ireland
Authors: BRAITHWAITE C. J. R. , RIZZI GIANCARLO 
Published: November 2003

ABSTRACTThe dolomites at Navan, Ireland, formed in Courceyan peritidal and shallow‐shelf limestones. The dolomite body has a plume‐like geometry, cross‐cutting both lithological boundaries and diagenetic barriers generated by sea‐floor cementation and emergence. The dolomitizing fluids rose parallel to major faults to diffuse laterally through the succession, controlled by variations in permeability that reflect both facies variation on deposition and pre‐dolomitization diagenesis.Cathodoluminescent zones reveal three principal stages of dolomite emplacement, separated by dissolution surfaces, with each stage reflecting several changes in the character of dolomitizing solutions. The predominance of dull zones indicates burial rather than surface conditions. The dolomites formed some time after burial in response to an areally limited hydrothermally‐driven flow. Isotope values (σ18O of — 6σ6 to — 10.4%δ and σ13C of — 0σ2 to +2σ5%δ PDB), and fluid inclusion data, suggest that these fluids had compositions similar to those of Carboniferous seawater. However, these became hotter with time, with temperature increasing from 60 to 160δC.The Navan dolomites are closely associated with Europe's largest zinc‐lead deposit. The distribution of the ores follows the same trend as that of the dolomites and paragenetic relationships indicate that dolomitization and mineralization were temporally and genetically related.
Late Miocene Halimeda alga‐microbial segment reefs in the marginal Mediterranean Sorbas Basin, Spain
Authors: MARTÍN JOSÉ M. , BRAGA JUAN C. , RIDING ROBERT 
Published: November 2003

ABSTRACTA ∼6 Ma Messinian (late Miocene) Bioherm Unit on the southern slope of the Sorbas Basin, SE Spain, contains numerous biotically diverse lensoid patch reefs that formed on a shelf to basin slope during a cycle of relative sea‐level change. Halimeda reefs are the largest and most complex of the patch reefs and are divisible into core, cap, and flank facies. On the upper and midslope they are up to 40 m thick and 400 m long. They become smaller downslope. The core consists of jumbled Halimeda segments, released by spontaneous disaggregation of the alga. The segments were stabilized close to their sites of growth and rapidly lithified by micritic and peloidal microbial crusts. Residual cavities were further veneered by isopachous marine cements. Flank facies, consisting of bedded packstones to rudstones, form wedge‐shaped units lateral to the mounds. Cap facies consist of bioclastic calcarenites/calcirudites and microbial carbonates. Synsedimentary lithification assisted rapid accretion and inhibited off‐mound export of sediment. Allochthonous reef‐derived blocks on the mid‐slope reflect penecontemporaneous rigidity of the Halimeda bioherms.Proximal Porites coral frame patch reefs associated with calcarenites were located near the shelf margin during the initial lowstand stage. Halimeda segment reefs associated with calcarenites and silty marls developed on the midslope and bivalve‐bryozoan‐serpulid reefs formed on the lower slope in silty marls with occasional turbidites. During the transgressive stage, coral patch reefs near the shelfbreak were overgrown by Halimeda. During highstand progradation, cap facies spread basinward as a sheet connecting many of the midslope patch reefs.These ancient analogues differ from most modern Halimeda reefs in being discrete laterally restricted patch reefs, surrounded by marly sediment, and located on a slope. They are, however, broadly comparable in biota, thickness, and depositional depth. Intense early lithification by microbial crusts and marine cements is an important feature of these Messinian segment reefs. It has not been reported from modern examples.
Hurricane control on shelf‐edge‐reef architecture around Grand Cayman
Authors: BLANCHON PAUL , JONES BRIAN 
Published: November 2003

ABSTRACTRimming the outer shelf of Grand Cayman is a submerged, 87 km long shelf‐edge reef that rises to within 12 m of mean sea level. It consists of an array of coral‐armoured buttresses aligned perpendicular to shore and separated by steep‐sided sediment‐floored canyons. Individual buttresses have a diverse coral‐dominated biota and consist of three architectural elements: a shield‐like front wall colonized by platy corals, a dome‐shaped crown colonized by head corals, and a shoreward‐projecting spur covered by varying amounts of branching coral. Buttresses are commonly fronted by coral pinnacles that, in some areas, have amalgamated with buttress walls to produce pinnacle‐and‐arch structures.As margin orientation changes, shelf‐edge‐reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves. Along margins exposed to fully developed storm waves, shelf‐edge‐reef buttresses are deep, have large amplitudes, and are dominated by robust head corals. These characteristics are consistent with hurricane‐induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows. Along margins impacted by fetch‐limited storm waves, reef buttresses are shallower, have intermediate‐amplitudes, and have a significantly higher proportion of branching corals. These characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves. Along margins fully protected from storm waves, the buttresses‐canyon architecture of the shelf‐edge reef breaks down producing a series of shallow, undulating, branching‐coral‐dominated ridges that merge laterally into an unbroken belt of coral. These characteristics correspond with negligible amounts of pruning and flushing during hurricanes.In addition to differences between margins, local intra‐marginal changes in shelf‐edge reef architecture are consistent with changes in the angle of hurricane‐wave approach. Open sections of the shelf‐edge reef, which face directly into storm waves, are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs. By contrast, on more sheltered, obliquely orientated sections, storm‐waves sweep debris along and off shelf producing little or no spur development. Instead, the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles.Over time, repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns. Vertical accretion is limited by the effective depth of storm‐wave fragmentation. Once this hurricane‐accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth, amalgamation, and infilling. Consequently, the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure — a pattern widely recognized in ancient reefs.
Early diagenesis in Pleistocene coral reefs, southern Sinai, Egypt: response to tectonics, sea‐level and climate
Authors: STRASSER ANDRÉ , STROHMENGER CHRISTIAN 
Published: November 2003

ABSTRACTThe uplifted Pleistocene terraces along the coast of southern Sinai exhibit a well developed reef system formed during isotope stage 9, and a younger one formed during isotope stage 5. An intermediate reef corresponding to isotope stage 7 occurs only as an erosional relic in the study area. The sediments comprise reefal framestones, peri‐reefal facies, coral rubble, and siliciclastic‐dominated beach and aeolian facies. The compositional and textural complexity of the sediments leads to a highly variable spatial distribution of diagenetic features. However, the geometric relationships and elemental analyses allow a reconstruction of the general diagenetic evolution: during the major eustatic sea‐level highstand of isotope stage 9, the Older Reef was constructed and cemented with aragonite and high‐Mg calcite. Climate was probably semiarid with some rainy periods which permitted the installation of ephemeral freshwater lenses, especially during the minor sea‐level lowstand within isotope stage 9. In these lenses, and during the subsequent major sea‐level lowstand, some freshwater dissolution occurred. The highstand during isotope stage 7 led to the construction of the Intermediate Reef. In the Older Reef, some high‐Mg calcite precipitated at that time. Dolomite cement formed either in marine interstitial waters modified by some freshwater input, or in a hypersaline context. Phreatic‐meteoric low‐Mg calcite cement covers, and partly replaces, previous marine cements and dolomite, but is still attributed to the major highstand of isotope stage 7 when freshwater lenses could develop during minor sea‐level lowstands. The subsequent major sea‐level lowstand was dominated by an arid climate, and only a little freshwater corrosion occurred. The Younger Reef formed during the major highstand of isotope stage 5. Aragonite and high‐Mg calcite cements, as well as some dolomite, are common within the reef, whereas freshwater cements are limited to beach and aeolian facies. Due to tectonic uplift, only the lower part of the Older Reef was reflooded during isotope stage 5, and only some aragonite crystals precipitated on top of dolomite or low‐Mg calcite. The interrelationships between tectonics, sea‐level variations of different orders, and climatic changes thus had a profound impact on the diagenetic history of these reef systems.
Facies distribution and retreat of Middle Triassic platform margin, Guizhou province, south China
Authors: ENOS PAUL , JIAYONG WEI , YANGJI YAN 
Published: November 2003

ABSTRACTThe southern margin of the vast Yangtze platform in central Guizhou Province, China, retreated during the Anisian (early Middle Triassic) by shedding skeletal debris and boundstone blocks into the margin of the adjacent basin. Anisian platform deposits are shoaling‐up cycles that commonly terminated in subaerial exposure. Platform‐margin facies are obscured by massive dolomitization and mechanical erosion. Distal basin deposits are terrigenous mudstone and siltstone. At the basin margin a wedge of mixed carbonate and terrigenous rocks consists of (A) thin‐bedded dolostone and limestone, (B) lime breccia with thin‐bedded mudstone, and (C) lime mudstone with breccia. Blocks within the breccias indicate that the shelf margin contained extensive boundstone formed by ‘Tubiphytes,’encrusting organisms, and early marine cement. Interspersed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin.The profile of the shelf margin from detailed mapping indicates 1.7–2.7 km of platform‐margin retreat during deposition of a basin‐margin wedge 250 m thick. Intertonguing of various basin‐margin facies reflects alternating minor episodes of advance and retreat of the margin. Near‐parallelism of the tongues suggests low relief at the platform margin. An upward stratigraphic progression to more distal, carbonate‐free, terrigenous basin facies indicates a cessation of carbonate production on the platform owing to emergence during early Anisian time. Retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick, the largest observed dimensions. A re‐entrant in the margin 7 km wide and 10 km deep could also reflect collapse. Retreat occurred along 175 km of the platform margin. The lack of platform‐margin facies along this front suggests 3–7 km of retreat and a total area of 875 km2.The Anisian platform‐margin retreat in Guizhou is one of relatively few examples of platform‐margin retreat in the geological record.
Deglaciation sequences in the Permo‐Carboniferous Karoo and Kalahari basins of southern Africa: a tool in the analysis of cyclic glaciomarine basin fills
Authors: VISSER JOHAN N. J. 
Published: November 2003

ABSTRACTThe Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice‐rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles.The massive diamictites formed as aprons and fans in front of the ice‐grounding line, whereas the stratified diamictites represent more distal debris‐flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes.Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter‐term fluctuation of the ice margin is present within the major advance‐retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea‐level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono‐eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer‐term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.
Possible seismic origin of molar tooth structures in Neoproterozoic carbonate ramp deposits, north China
Authors: FAIRCHILD IAN J. , EINSEL GERHARD , SONG TIANRUI 
Published: November 2003

ABSTRACTThe Xinmincun Formation forms the uppermost unit of a thick Neoproterozoic section which accumulated near the east margin of the North China Block and is overlain by two thin nearshore to continental formations below fossiliferous Lower Cambrian sediments. Although tectonically deformed, sedimentary structures are preserved undeformed on cleavage‐parallel surfaces, and an 80 m section has been reconstructed by correlation across minor folds and faults in the Golden Stone beach area, 50 km NE of the city of Dalian, southern Liaoning province.The measured section shows 65 m of storm‐dominated deposits, consisting of alternations of micrites and sharp‐based graded intraclastic grainstone beds (tempestites), some with rudaceous, commonly erosional or guttered, bases. The top 15 m of section shows three alternations of similar subtidal lithofacies with partly to completely dolomitized peritidal deposits (laminated, sometimes fenestral and desiccated, micrite beds, and intraclastic rudite and grainstone beds). Tempestite beds become thinner and less abundant upwards towards a muddy upper shoreface zone. This environment was characterized in part by the occurrence of micrite with thin or streaky lamination (probable storm‐resuspended sediment), interbedded tempestites, numerous erosion surfaces, and evidence of liquidization and sediment slumping into hollows.Molar tooth structures are widespread in micrite beds of the Xinmincun Formation and are present in lesser abundance in tempestites and liquefied channel‐fills. Originally the structures were sub‐vertical cracks, 1–20 cm long, tapering upwards and downwards. Subsequently they became filled with microspar cement and buckled rigidly during compaction of surrounding sediment during burial. Evidence of repeated episodes of cracking, presence of brecciated cracks and localization of cracking within beds, together with variable degree of development and variation of preferential alignment in plan indicates a mechanical origin. Crack generation may have been by seismic surface waves generated by movement along faults defining either the basin's margins or its internal structure, or possibly by wave action during storms. A seismic origin for molar tooth structure is consistent with other Neoproterozoic occurrences. Their preferential occurrence in Precambrian deposits arises from the relative rigidity of micritic sediment at this time related to lack of bioturbation, incipient cementation and possibly microbial binding.
Early Cambrian braid‐delta deposits, MacKenzie Mountains, north‐western Canada
Authors: MACNAUGHTON ROBERT B. , DALRYMPLE ROBERT W. , NARBONNE GUY M. 
Published: November 2003

ABSTRACTLatest Neoproterozoic to earliest Cambrian strata in north‐western Canada provide an example of a pre‐vegetation braid‐delta depositional system. Depositional environments represented in the succession include braided fluvial and braid‐delta distributary channels, aeolian dune fields and interdistributary lagoons/bays, as well as mouth bar, beach to shoreface, and prodelta to distal shelf settings. Three formations have been investigated: the Ingta Formation formed in wave‐dominated nearshore to offshore shelf environments with little or no apparent deltaic influence, whereas the overlying Backbone Ranges and Vampire formations contain an extensive record of braid‐delta deposits ranging from braidplain to distal prodelta facies. On the braid‐plain, river channels reached widths of up to several kilometres. Such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial‐dominated mouth‐bar deposition with lesser wave influence; wave‐dominated deltaic successions are rare in the succession. Interdeltaic areas were characterized by wave‐dominated prograding shorelines. Interdistributary lagoons probably formed primarily in abandoned distributary channels. Delta‐front/prodelta deposits are silt‐rich and contain abundant soft‐sediment deformation, including slumps. The deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants. This is illustrated in the dominance of braided fluvial deposition and of silt‐rich offshore facies that may have resulted from enhanced aeolian transport of loess. The non‐actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies. However, the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures, while the hypothesized latest Proterozoic‐earliest Cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession.
Effect of bacteria on the elemental composition of early diagenetic siderite: implications for palaeoenvironmental interpretations
Authors: MORTIMER R. J. G. , COLEMAN M. L. , RAE J. E. 
Published: November 2003

ABSTRACTMany ancient early diagenetic siderite concretions are thought to have been precipitated by microbially‐mediated reactions, and their trace element chemistry has been used to infer palaeoenvironments of formation (e.g. marine versus freshwater). In this study, pure cultures of the microorganism Geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures (18–40°C). Magnesium and calcium, in three different ratios (3:1, as in seawater, 1:1, and 1:3 as in freshwater), or manganese were added to some cultures at each incubation temperature to study trace element incorporation. The siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite. However, this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated, as is assumed in palaeoenvironmental interpretations. Instead, manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity, which itself is dependent upon the nutritional status of the microorganisms. Magnesium incorporation was found to be rate‐independent, and at high Mg/Ca ratios, it caused inhibition of calcium incorporation. In short, the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations.
Spillover models for axial rivers in regions of continental extension: the Rio Mimbres and Rio Grande in the southern Rio Grande rift, USA
Authors: MACK GREG H. , LOVE DAVID W. , SEAGER WILLIAM R. 
Published: November 2003

ABSTRACTThe Pliocene‐early Pleistocene history of the ancestral Rio Grande and Quaternary history of the Rio Mimbres in the southern Rio Grande rift, New Mexico, illustrate how axial rivers may alternately spill into and subsequently abandon extensional basins. Three types of spillover basins are recognized, based on the angle at which the axial river enters the basin and whether it descends the hanging wall dip slope or footwall scarp to reach the basin floor. In the Mimbres basin type, the axial river enters and flows through the spillover basin nearly parallel to the footwall scarp, resulting in a narrow belt of basin‐axis‐parallel channel sand bodies located near the footwall scarp. In contrast, an axial river may enter a spillover basin at a high angle to its axis, either descending the hanging wall dip slope (Columbus basin type) or footwall scarp (Tularosa basin type), and construct a fluvial fan, consisting of radiating distributary channels orientated nearly perpendicular to the basin axis. Faulting exerts significant control on river spillover by creating the topographic gaps through which the axial river moves and by terminating spillover by subsequently uplifting or tilting the gap. Spillover may also be autocyclic in origin as a result of aggradation to the level of a pre‐existing gap, headward erosion creating and/or intersecting a gap, or simple river avulsion upstream of a gap. Predicting facies architecture in the three types of spillover basins is critical to successful subsurface exploration for hydrocarbon reservoirs, groundwater aquifers or placer mineral deposits.
Contrasting flooding histories of Mississippian carbonate platforms revealed by marine alteration effects in palaeosols
Authors: WRIGHT V. PAUL , VANSTONE SIMON D. , MARSHALL JAMES D. 
Published: November 2003

ABSTRACTPalaeosols associated with exposure surfaces in Mississippian platform carbonate sequences in Britain invariably show evidence for later alteration by sea water. These alteration effects can be attributed to flooding of the emergent platforms during transgressions that terminated exposure surface development. A study of 230 palaeosol profiles representing 60 stratigraphic levels has revealed a two fold division of these marine hydromorphic effects. Palaeosols in ramp sequences (Chadian‐Arundian stages) are capped by ferroan dolomite horizons with carbonized rootlets, pyrite and thin coals. The ferroan dolomites exhibit δ13C and δ18O values indicative of formation in brackish waters. These are interpreted as coastal marshes that developed landward of a transgressive shoreline. Younger Asbian‐Brigantian palaeosols lack these dolomites but have been extensively pyritised. The pyrite also developed through marine hydromorphic alteration but flooding was relatively instantaneous over the flat topped platforms. These differences in flooding history reflect both different platform configuration and more rapid transgressions during the Asbian‐Brigantian, likely a result of glacio‐eustatic effects. Flooding characteristics of the Asbian‐Brigantian platforms differ from those associated with late Cainozoic examples, apparently because complete platform rims were not developed. Similar mineralogical alteration effects are likely to be common in other platform sequences in the geological record, but have not been documented.