dolomitization of the oligocene miocene bluff formation on grand cayman british west indies the oligocene miocene bluff formation on grand cayman is formed of hard white finely crystalline dolostone in which the precursor textures and fossil microstructures are commonly preserved the d

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

the role of fungi in the diagenetic alteration of spar calcite fungi may play a far more important role in carbonate diagenesis than has been previously suspected this can be demonstrated by placing known species of fungi on crystals of iceland spar calcite such experiments show that fungi can 1 cause sparmicritization through dissolution 2 produce spiky calcite through dissolution 3 trap and bind loose particles of calcite and 4 promote the growth of new calcite crystals although not observed in this series of experiments fungi can bore into substrates and cause sparmicritization the activity of fungi involves a complex interplay of substrate destruction and construction without the intervention of vast quantities of fluid

the tindouf basin a marine refuge during the serpukhovian carboniferous mass extinction in the northwestern gondwana platform abstract several macrofaunas and microfossils of the carboniferous saharan basins have longer stratigraphic ranges than those of other basins in the western palaeotethys realm particularly in the tindouf basin morocco algeria foraminifers are particularly abundant and diverse in the serpukhovian and basal bashkirian compared to coeval basins and some taxa have longer ranges than in the neighbouring reggan and bechar basins although this effect is more marked compared to the western palaeotethyan assemblages in europe several rugose coral species are recorded from the early bashkirian that previously were considered to have disappeared in the serpukhovian the tindouf basin as one of the most western saharan basins in north africa shows the greatest stratigraphic ranges of taxa which diminish eastwards evidence for a mass extinction event during the serpukhovian in the tindouf basin has not been clearly recognized although a possible influence of glaciation is observed in the faunal diversity eustatic sea level changes were experienced in tindouf with the cyclic pattern of sedimentation but warm water ocean currents from the palaeoequator were able to maintain tropical conditions on the platform tectonics in the area led to emerging land masses and barriers and created a partly isolated basin in this sector of the western part of the sahara platform in northern gondwana the combination of those factors controlled the environmental conditions in the area allowing the persistence of the fauna for longer stratigraphic ranges than its equivalent counterparts in the western palaeotethys

genesis of terrestrial oncoids cayman islands british west indies terrestrial oncoids up to 4 3 mm long are common in sinkholes that penetrate the dolostones of the oligocene miocene bluff formation on grand cayman and cayman brac the vaguely laminated terrestrial oncoids which generally lack a nucleus are formed of detrital material micrite clays dolomite calcified filaments and spores and insect fragments the abundant diverse assay of microorganisms includes six different types of filaments and five different types of spores which can be attributed to fungi algae and or cyanobacteria thin mats of mucus are commonly associated with the microorganisms the microorganisms contribute to the formation of the terrestrial oncoids by i calcification of the filaments and spores ii trapping and binding detrital material and iii binding with their mucus detrital material to the surface of the oncoids identification of terrestrial oncoids relies on i the recognition of a microbial assemblage ii the demonstration that the microorganisms played an ac

boxwork and ferromanganese coatings in hypogenic caves an example from sima de la higuera cave murcia se spain abstract this paper examines the greyish blue deposits that were recently discovered in the lower levels of the sima de la higuera cave murcia se spain which occur as patinas over the walls and ceilings as well as coating boxwork formations their mineralogy was determined using xrd and micro raman spectroscopy while edx microanalysis was used to determine their elemental composition the mineralogical analyses revealed the presence of mn oxides todorokite and pyrolusite and fe with a low degree of crystallinity whereas edx microprobe showed elevated concentrations of mn 38 2 wt fe 15 2 wt and pb 8 1 wt the ferromanganese oxyhydroxides occur as botryoidal aggregates overlying blades of calcite that have a visibly sugary texture the speleogenetic model proposed describes 1 an initial phase of precipitation of hydrothermal calcite veins of hypogenic origin within the fissures of the host rock under phreatic conditions and 2 a subsequent vadose phase involving preferential corrosion of the carbonate host rock caused by lowering of the ph resulting from co 2 diffusion in condensed water and oxidation of fe and mn under aerobic conditions probably mediated by microorganisms it is this later phase that gave rise to the boxwork the boxwork of the sima de la higuera cave is a singular example of a formation that is generated by dissolution corrosion of the rock due to acidification caused by oxidation of iron and manganese

coral reefs and growth dynamics of a low angle carboniferous platform records from tianlin southern china abstract metazoan reef building receded globally during the carboniferous after the late devonian extinction events shallow water bioconstructions were generally small and scarce and corals played a minor role in their construction however in southern china one large 50 m high and three small 6 16 m high coral reefs dating from the late visean serpukhovian have recently been discovered in two localities in guangxi to understand the occurrence of these coral reefs and to constrain the factors that controlled their growth three sections around langping guangxi were measured and selected samples were analysed using petrography biostratigraphy and chemostratigraphy the depositional environment of these exceptional mississippian reefs was interpreted as a low angle platform 1 2 5 recording oligotrophic conditions and normal salinity the three small reefs have grown around the fair weather wave base whereas the large coral reef has grown in a deeper environment between the fair weather and storm wave base during carboniferous the global temporal reef distribution can be partly ascribed to paleoclimate changes however in southern china the growth of extended mississippian coral reefs is restricted to a short time window the late visean serpukhovian which coincides with high amplitude and high frequency sea level fluctuations most likely related to glacial pulses recent studies revealed that several rugose coral species survived the mid carboniferous glaciation event thanks to the persistence of warm water ocean currents maintaining tropical conditions on the platform due to the scarcity of extended coral reefs elsewhere it cannot be excluded that tianlin provided an ecological refuge for corals related to oceanic currents another potential favorable factor could be the mid ramp depositional setting the deep water platform areas could potentially provide more stable conditions less affected by global stress events however the deep reef refugia hypothesis remains debatable as only very few studies have explored its validity at the community level even if the causes of the occurrence of an extended coral reef in southern china cannot be fully constrained the existence of this large coral framework in the late visean serpukhovian provides additional evidence that coral communities did build reefs after the late devonian extinction events

a fossil hydrothermal deposit in the south pacific abstract hydrothermal sediments greenish granular nontronite black mn fragments were found in a core collected at 29 28 08 s 131 36 81 w in the south pacific the mnoxides consist of well crystallized todorokite and birnessite the nontronite is nearly al free the mn oxides as well as the nontronite are characterized by a very low ree and trace metal contents within the hydrothermal sediments authigenic quartz is common oxygen isotope geothermometry indicates that the quartz was formed under low temperature about 20 c hydrothermal conditions the mineralogy and chemistry of the hydrothermal sediments reflect varying eh values of the hydrothermal fluids ranging from slightly oxidizing mn oxides quartz to reducing nontronite the observed hydrothermal sediments are comparable to those described from mounds in the galapagos rift and similar settings on the mid atlantic ridge

biogenic structures and micrite in stalactites from grand cayman island british west indies stalactites from modern and old caves developed in the bluff formation of grand cayman island contain laminae and bulbous masses of micrite intercalated with sparry calcite the micrite as well as some of the sparry calcite around it contains small up to 25 m long ovate to spherical bodies that have a high concentration of either manganese or iron such bodies may be of bacterial origin the micrite contains numerous calcified filaments that are probably of algal origin calcification of the filaments occurred either during life or shortly after death of the algae the algae played an important role in trapping and binding the micrite furthermore the algae may be directly or indirectly responsible for the formation of much of the micrite

geochemical evidence of saharan dust parent material for soils developed on quaternary limestones of caribbean and western atlantic islands abstract most previous workers have regarded the insoluble residues of high purity quaternary limestones coral reefs and oolites as the most important parent material for well developed clay rich soils on caribbean and western atlantic islands but this genetic mechanism requires unreasonable amounts of limestone solution in quaternary time other possible parent materials from external sources are volcanic ash from the lesser antilles island arc and saharan dust carried across the atlantic ocean on the northeast trade winds soils on quaternary coral terraces and carbonate eolianites on barbados jamaica the florida keys united states and new providence island bahamas were studied to determine which if either external source was important caribbean volcanic ashes and saharan dust can be clearly distinguished using ratios of relatively immobile elements al 2 o 3 tio 2 ti y ti zr and ti th comparison of these ratios in 25 soils where estimated ages range from 125 000 to about 870 000 yr shows that saharan dust is the most important parent material for soils on all islands these results indicate that the northeast trade winds have been an important component of the regional climatology for much of the quaterary saharan dust may also be an important parent material for caribbean island bauxites of much greater age

the role of microorganisms in phytokarst development on dolostones and limestones grand cayman british west indies phytokarst is spectacularly developed on the hard finely crystalline dolostones of the oligocene miocene bluff formation and to a lesser extent on the soft limestones of the pleistocene ironshore formation on grand cayman the black weathered surfaces which are a reflection of an organic coating contrast sharply with the white colour of the unaltered host rocks which lack an organic coating on the dolostones this organic coating is formed principally of fungal sporangia and spores associated with mucilage along with lesser numbers of algae and bacteria endolithic and epilithic filamentous microorganisms are rare on the limestones the organic coating is formed principally of endolithic and epilithic filaments and mucilage fungal spores are rare and no sporangia were found the limestones of the ironshore formation are characterized by an intensely bored zone beneath the weathering surface in contrast the dolostones of the bluff formation have relatively few endoliths penetrating from the w

protolith relations of the gravina belt and yukon tanana terrane in central southeastern alaska metamorphic rocks west of the coast mountains batholith in central southeastern alaska are divided into the gravina belt taku terrane and newly defined ruth assemblage the ruth assemblage comprises metapelite quartzose metaclastic strata quartzite marble felsic metatuff mafic metavolcanic rocks and orthogneiss depositional and emplacement ages of 367 pm 10 ma and 345 pm 13 ma inferred from discordant u pb zircon analyses on felsic metatuff and granodioritic orthogneiss respectively require that at least portions of the ruth assemblage be late devonian and early mississippian in age the assemblage is similar in age and protolith to and thus correlated with the yukon tanana terrane the gravina belt is characterized by upper jurassic and lower cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the alexander terrane metamorphic rocks that structurally underlie the taku terrane and ruth assemblage are included in this assemblage trace

thermobarometric constraints on the structural evolution of the coast mountains batholith central southeastern alaska thermobarometric data from amphibolite facies metamorphic rocks west of the coast mountains batholith provide important constraints on the structural evolution of the mid cretaceous sumdum fanshaw

relict pleistocene calcareous tufa of the chlupacova sluj cave the bohemian karst czech republic a petrographic and geochemical record of hydrologically driven cave evolution abstract the tufas of the presently shallow chlupacova sluj cave document an unusual history of deeper to shallower hydrologic processes in cave evolution the wider geological context of the cave along with fluid inclusion and stable isotope c and o analyses of calcite from tectonic veins cutting through its wallrock are evidence of the origin of the cave in the deeper subsurface under the influence of ascending 25 70 c warm saline 2 0 22 3 wt eq nacl waters driven by regional hypogene processes the formation of calcite veins which occurred at 700 400 ka was probably coeval with the early stages of cave development however two generations of tufa formed in the later pleistocene during the mature shallow subsurface stage of cave development when daylight and cold meteoric waters penetrated the cave through ceiling windows and tectonic fractures the first generation tufa which consists of cauliflower like aggregates of hollow outward radiating carbonate tubules covered with isopachous banded calcite cements is interpreted as ancient bryophyte tufa that was deposited about 620 140 530 110 ka the presence of microscopic biomorphic fabrics resembling micro stromatolites calcareous algal chambers calcite crystals with cloudy cores and clear sparitic rims and specific biomarkers identified in tufa extracts suggest that in addition to mosses algae and bacteria may have also been instrumental in promoting the deposition of tufa tufa stable isotope c and o characteristics and fluid inclusion contents point to deposition of this first generation tufa from fresh water with a minor admixture of higher hydrocarbons at the surface temperature post dating the first generation tufa minute grains of exotic minerals e g stibnite gothic arch calcite gypsum mn minerals opaline silica and sparry calcite were precipitated in the tufa moulds and between carbonate tubules as cements these minerals suggest at least one event in which hydrothermal waters entered the cave from below after deposition of the first tufa from cool meteoric waters in this hydrothermal phase the cave may have been a hot spring the second generation tufa originated during the riss wurm interglacial at 121 2 1 107 2 6 ka following a period of intense water erosion that partly erased older spelean deposits from cave walls this tufa consists of highly porous friable crudely laminated carbonate sediments with floral and molluscan remains characteristic of common cold water karstic tufa this and the earlier tufa were in turn covered by later fine grained cave sediments contributing to the preservation of pleistocene tufa that records an alternation of deep and shallow waters in the cave over at least the last 500 000 years

u pb geochronology of late cretaceous and early tertiary plutons in the northern coast mountains batholith u pb geochronologic studies demonstrate that steeply dipping sheetlike tonalitic plutons along the western margin of the northern coast mountains batholith were emplaced between 83 and 57 perhaps 55 ma less elongate tonalitic granodioritic bodies in central portions of the batholith yield ages of 59 58 ma coeval with younger phases of the tonalitic sheets large granite granodiorite bodies in central and eastern portions of the batholith were emplaced at 51 48 ma trends in ages suggest that the tonalitic bodies generally become younger southeastward and that at the latitude of juneau plutonism migrated northeastward across the batholith at 0 9 km ma variations in the age shape location and degree of fabric development among the various plutons indicate that late cretaceous paleocene tonalitic bodies were emplaced into a steeply dipping dip slip shear zone that was active along the western margin of the batholith postkinematic eocene plutons were emplaced at shallow crustal levels inhe

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

geology of the western flank of the coast mountains between cape fanshaw and taku inlet southeastern alaska the western flank of the coast mountains batholith between cape fanshaw and taku inlet is underlain primarily by jura cretaceous strata of the gravina belt pre permian permian and triassic strata of the taku terrane and mid proterozoic to upper paleozoic rocks of continental margin affinity the continental margin rocks include mid proterozoic to lower paleozoic quartzite and marble of the tracy arm assemblage mid paleozoic metavolcanic and subordinate metasedimentary rocks of the endicott arm assemblage and upper paleozoic quartz rich metaturbidites and metaconglomerate of the port houghton assemblage we suggest that these three assemblages are correlative with components of the yukon tanana terrane which underlies a large region of yukon and eastern alaska rocks of the gravina belt taku terrane and yukon tanana terrane are juxtaposed along west vergent thrust faults of mid cretaceous age and are internally deformed and disrupted along latest cretaceous to early eocene dip slip and possibly strike slip shear zones these rocks and structures together with mid cretaceous to eocene plutons of the coast mountains batholith separate rocks of the alexander wrangellia terrane to the west from rocks of the stikine terrane to the east mid cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the alexander wrangellia terrane from central alaska to northwestern washington the continental margin rocks in the northern coast mountains may be part of the in situ north american margin that has been overthrust by the stikine and adjacent terranes alternatively these rocks may have 1 rifted from and then returned to the north american margin 2 moved 800 km along left lateral or right lateral faults from elsewhere along the north american margin or 3 originated near a continent other than north america

waulsortian like mounds of the mississippian pekisko formation northwestern alberta petrographic and chemical attributes abstract the tournasian age pekisko carbonates in the normandville field northwestern alberta form waulsortian like bryozoan crinoid mounds that developed in fairly deep low energy cool water systems close to the ramp margin three main depositional environments occur 1 crinoidal apron with wackestone grainstone and floatstone facies 2 mound flank with grainstone wackestone packstone and floatstone facies dipping 35 and 3 bryozoan mound core composed of rudstone and floatstone facies with fenestrate bryozoa minor crinoids and carbonate mud local highs due to fault bounded blocks created from the collapse of the devonian peace river high may have controlled the location of mound nucleation diagenesis of the bryozoan crinoid mounds included calcite cementation compaction dolomitization silicification and hydrocarbon emplacement events the mound core facies contains submarine fascicular optic calcite and bladed prismatic calcite cements and later ferroan brightly luminescent pore filling blocky spar cement the crinoid apron facies contains syntaxial cement associated with crinoids and the ferroan blocky spar cement the mounds are dominantly limestone however in one well dolomite dominates the lower section four types of dolomite have been identified partial replacive chemical compaction related pervasive dolomite and saddle dolomite cement all dolomites are non stoichiometric caco3 mole 56 6 62 6 the partial zoned replacive dolomite replaces micrite and syntaxial rim calcite in mound flank and crinoid apron facies the chemical compaction related dolomite is found along dissolution seams and stylolites and has similar cl characteristics to the replacive dolomite the pervasive dolomite is fabric destructive and has dull cores and bright rims in cl saddle dolomite 0 15 mm has brightly luminescent concentric zoning and occurs in vugs and fossil pore spaces chemical and isotopic analysis of the bryozoan crinoid mounds indicate that the original marine signatures in micrite early cements some crinoids and brachiopods have been preserved however carbon isotopic values for some crinoids matrix and dolomite show more positive values compared to known mississippian carbonate values recrystallization during shallow burial has reset the oxygen isotopic composition of some crinoids and micrite oxygen and carbon isotopic compositions of most dolomites overlap with altered crinoids and early calcite cements however saddle dolomites have lighter 18o values similar to saddle dolomites from the devonian wabamun group in this area the isotopic variations in later ferroan calcite cements show an inverted j trend possibly due to variable amounts of water rock interaction while the sr isotopic ratio of submarine calcite cement coincides with that of mississippian seawater the later ferroan calcite cement is more radiogenic indicating a different source of fluids

rugose coral biostromes in the late visean mississippian of nw ireland bioevents on an extensive carbonate platform abstract the extensive upper visean asbian platform carbonates in nw ireland bricklieve limestone fm glencar limestone fm and dartry limestone fm contain distinctive rugose coral biostromes which are dominated by different species of the genus siphonodendron these are in stratigraphic sequence pauciradiale biostrome oldest martini biostrome and several junceum biostromes youngest they represent bioevents caused by special short lasting ecological conditions and can be used as approximately synchronous horizons to correlate within the region the pauciradiale biostrome is the thickest laterally most persistent and most variable in facies and biotic composition of all biostromes it formed on a tectonically influenced platform with a landward seaward zonation from northwest to southeast mainly above storm wave base and below fair weather wave base the northwestern streedagh facies is characterized by the presence of clusters of large sheet like colonies of s pauciradiale the intermediate o donnell s rock facies is unique for the predominance of the fasciculate genus solenodendron the southeastern bricklieve facies represents the amalgamation of autochthonous and allochthonous coral debris and bioclastic debris with localized small patches of coral boundstone mass occurrences of fasciculate rugose corals re appear in the martini biostrome this biostrome developed in a shallower water setting just above fair weather wave base on a levelled carbonate platform the junceum biostromes are thinner decimetre to some metres thick and less persistent they formed in deeper water mostly below storm wave base except for the composite 2nd junceum biostrome of the bricklieve mountains which records a period of shallowing according to facies and coral morphologies which were compared with modern scleractinian growth forms sea level variation exerted one of the strong controls on the rise and decline of the biostromes the pauciradiale biostrome formed during an extended shallowing upward cycle in a depth interval leading to the climax of siphonodendron pauciradiale the cycle ended with the development of the martini biostrome in shallow water its demise is probably caused by drowning during the late asbian sea level rise during that time slice the junceum biostromes flourished in deeper water on the platform coral growth ceased almost completely after formation of the junceum biostromes during the falling sea level of the latest asbian siliciclastic input and resulting turbidity as well as turbulence formed a complex cascade of ecological constraints in addition to the local tectonic influences they combined to result in the depth factor which controlled the distribution of predominating coral populations and the succession of the different biostromes in the asbian of nw ireland

petrographic and geochemical features of sinkhole filling deposits associated with an erosional unconformity on grand cayman abstract on grand cayman exposures of dolostones belonging to the cayman formation miocene represent an erosional unconformity that has been developing since the late pliocene 3 6 ma sinkholes that developed during this time have remained open or become partly to fully filled with various combinations of rootcrete breccias loose limestone and dolostone lithoclasts and white red and orange limestones these sinkhole filling deposits have different geochemical attributes to the neogene and pleistocene marine carbonates that form the bedrock of the island the deposits in the sinkholes formed in response to the variations of sea level climate exposure and vegetation that developed during the period when the erosional unconformity was developing the rootcrete oncoids red and orange limestones are terrestrial in origin whereas the limestone and dolostone lithoclasts and white limestones are derived from marine deposits on the erosional unconformity intense root activity led to the formation of rootcrete and terrestrial oncoids but also selectively blackening reworked marine carbonates the red and orange limestone matrices which formed under more arid conditions contrast with the other sinkhole filling deposits that formed during periods when the climate was semi arid to humid the distinctive ree signatures of the sinkhole filling deposits which are different from those of the bedrock limestones and dolostones can probably be attributed to trace amount of terra rossa and or airborne saharan derived dust that are present in those deposits

debate about ironstone has solute supply been surficial weathering hydrothermal convection or exhalation of deep fluids ironstone is any chemical sedimentary rock with 15 fe an iron formation is a stratigraphic unit which is composed largely of ironstone the solutes which have precipitated to become ironstone have dissolved from the earth s surface from the upper crust e g the basaltic layer of oceanic crust or from deeper within the earth genetic modellers generally choose between surficial weathering e g soil formation and hydrothermal fluids which have convected through the upper kilometre of oceanic crust most genetic modellers attribute cherty laminated iron formations to hydrothermal convection and noncherty oolitic iron formations to surficial weathering however both types of iron formations are attributable to the exhalation of fluids from a source region too deep for convection of seawater evidence for a deep source of ferriferous fluids comes from a comparison of ancient ironstone with modern ferriferous sediment in coastal venezuela a deep source origin for ironstone has wide ranging implications for the origins of other chemical sedimentary ores e g phosphorite manganostone bedded magnesite sedimentary uranium ore various karst filling ores and even petroleum preliminary study of a modern oolitic iron deposit described herein suggests that the source of iron and silica to iron formations may have been even deeper than envisioned within most hydrothermal convection models

manganese precipitates in the karst terrain of grand cayman british west indies ten morphologically distinct forms of mn precipitates probably birnessite occur in a wide variety of paleokarst features that characterize dolostones of the bluff formation on grand cayman these

waulsortian type bioherm development and response to sea level fluctuations upper visean of bechar basin western algeria abstract an exceptionally well developed and well preserved upper visean succession rich in sponge bryozoan crinoid mounds in the foreland carboniferous bechar basin northwestern algerian sahara is described and discussed the succession is composed of recurrent facies mosaics forming individual superposed members an ideal member is made up of two distinct fades assemblages the lower assemblage forms the actual mounds and is composed of sponge bafflestone wackestone at the base overlain by massive sponge fenestellid bafflestone wackestone and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone in contrast to massiveness of the mounds the upper facies assemblage is composed of well bedded crinoid packstone grainstone and ooid grainstone with local rugose coral and algal foraminiferal banks on the basis of benthic assemblages and nature of substrate seven bathymetric zones are defined and indicate that each member of the bioherm rich formations is a shallowing upward parasequence controlled by an asymmetrical transgressive regressive cycle the local curve of relative sea level for the late visean shows 13 cycles interpreted as fourth order eustatic sea level changes each cycle averaging half a million years in duration third order cycles could not be identified the platform model proposed is a distally steepened ramp 15 20 km wide that developed in a rapidly subsiding foreland basin lateral distribution of facies on the ramp depended strongly on local tectonic setting whereas vertical development was controlled by sea level fluctuation the deep water mud rich sponge bryozoan crinoid mounds developed in 100 150 m water depths during phases of sea level highstand whereas deposition of shallow water facies occurred on top of the mounds in less than 70 80 m water depths during regressive phases the bechar mounds share similarities with the classical lower carboniferous waulsortian mounds but they differ in two aspects the abundance of large sponges which is unique and their vertical zonation wherein basal facies consist of sponge dominated assemblages the proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the bechar basin because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

caymanite a cavity filling deposit in the oligocene miocene bluff formation of the cayman islands caymanite is a laminated multicoloured white red black dolostone that fills or partly fills cavities in the bluff formation of the cayman islands the first phase of caymanite formation occurred after deposition lithification and karsting of the oligocene cayman member the second phase of caymanite formation occurred after joints had developed in the middle miocene pedro castle member caymanite deposition predated dolomitization of the bluff formation 2 5 ma ago caymanite is formed of mudstones wackestone packstones and grainstones allochems include foraminifera red algae gastropods bivalves and grains of microcrystalline dolostone sedimentary structures include planar laminations graded bedding mound shaped laminations desiccation cracks and geopetal fabrics original depositional dips ranged from 0 to 60 although caymanite originated as a limestone dolomitization did not destroy the original sedimentary fabrics or structures the sediments that formed caymanite were derived from

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

mn and fe rich black travertine shrubs bacterially and nanobacterially induced precipitates abstract black shrubs i e arborescent forms are present within hot water travertine deposits from morocco the shrubs are commonly a few centimeters high and laterally grouped along individual laminae other than in mineral composition the black shrubs closely resemble the calcite shrubs from other hot water travertine deposits e g bagni di tivoli italy and mammoth hot springs yellowstone national park they are composed of a variety of minerals that are mn rich and also have a significant fe oxide component similar mn and fe rich shrub like forms have been described from desert varnish deep sea mn nodules and marine carbonates frutexites sem analyses show that bacterial bodies are densely packed within the mn rich black travertine shrubs whereas no bacterial bodies are evident within the enclosing aragonite and calcite laminae the bacterial bodies range in shape and size from rods commonly 0 8 m long by 0 1 m in diameter to spherical forms commonly 0 3 m in diameter that form chains up to 1 m long to nanobacterial spheres 150 to 200 nm in diameter that make up densely packed chains commonly 15 m in length it is well documented within the microbiological literature that not only do bacteria have the ability to induce the precipitation of mn and fe they can concentrate mn in mineral precipitates from waters with very low to essentially undetectable concentrations of mn the black mn rich shrubs within the moroccan travertines other hot water travertines and related features such as desert varnish deep sea mn nodules and frutexites are postulated to also be the result of bacterially induced precipitation

single grain heavy mineral provenance of garnet and amphibole in the surveyor fan and precursor sediments on the gulf of alaska abyssal plain implications for climate tectonic interactions in the st elias orogen abstract the st elias orogen formed as a result of the northwestward drift of the yakutat terrane along and final collision with the alaska margin in the miocene its exhumation coincided with changing glacial conditions that are considered to have strongly interacted with mountain building processes a significant part of the record of these tectonic climatic interactions is stored in sediments on the gulf of alaska abyssal plain including the surveyor fan our study examines temporal provenance changes of miocene through pleistocene sediments of the surveyor fan gulf of alaska to constrain the dynamics of exhumation and mass transfer from the evolving st elias orogen to the adjacent surveyor deep sea fan we present single grain geochemical data of amphibole and garnet and 40 ar 39 ar cooling ages of biotite and amphibole together with point counting data of heavy minerals from sands and silts from two sites in the distal and proximal fan drilled by iodp expedition 341 in 2013 sites u1417 and u1418 respectively a shift in heavy mineral composition during the miocene predating the onset of glaciation points to a tectonically induced change in erosion centers and sediment transport probably caused by the rise of the st elias mountains garnet and amphibole data suggest the chugach metamorphic complex is the main sediment source implying input to the surveyor fan from sources relatively far in the north during the miocene changing provenance signals from the miocene to pliocene suggest rising input from the lower grade metamorphic areas at the southern flanks of the orogen indicating the advance of glaciers to the tidewater line providing material from this flanking region higher input from the chugach metamorphic complex in all pleistocene sediments suggests the northern hemisphere glaciation at the plio pleistocene boundary caused erosion and sediment yield from the interior of the orogen climatic changes at the mid pleistocene transition did not cause significant changes in the provenance signal

intrinsic versus extrinsic controls on the development of calcite dendrite bushes shuzhishi spring rehai geothermal area tengchong yunnan province china abstract in the rehai geothermal area located near tengchong there is an old succession of crystalline calcite that formed from a spring that is no longer active the thin bedded succession exposed on the south bank of zaotang river is formed of three dimensional dendrite bushes that are up to 6 cm high and 3 cm in diameter with multiple levels of branching bedding is defined by color which ranges from white to gray to almost black and locally accentuated by differential weathering that highlights the branching motif of the dendrites the succession developed through repeated tripartite growth cycles that involved phase i that was characterized by rapid vertical growth of the dendrite bushes with ever increasing branching phase ii that developed once growth of the dendrites had almost or totally ceased and involved an initial phase of etching that was followed by the precipitation of various secondary minerals sheet calcite trigonal calcite crystals hexagonal calcite crystals hexagonal plates formed of ca and p mn precipitates si mg reticulate coatings opal ct lepispheres on the branches of the calcite dendrites and phase iii that involved deposition of detrital quartz feldspar clay and calcite on top of the dendrite bushes the tripartite growth cycle is attributed primarily to aperiodic cycles in the co 2 content of the spring water that was controlled by subsurface igneous activity rather than climatic controls high co 2 coupled with rapid co 2 degassing triggered growth of the dendrite bushes as co 2 levels waned saturation levels in the spring water decreased and calcite dendrite growth ceased and precipitation of the secondary minerals took place possibly in the microcosms of microbial mats deposition of the detrital sediment was probably related to surface runoff that was triggered by periods of high rainfall critically this study shows that intrinsic factors rather than extrinsic factors e g climate were the prime control on the development of the tripartite growth cycle

open and filled karst features on the cayman islands implications for the recognition of paleokarst the dolostones of the oligocene miocene bluff formation on the cayman islands are characterized by well developed surficial interface and subsurface karst features that are open or filled with speleothems and sediment some of the open caves show evidence of a minimum of two stages of development related to dissolution in the phreatic zone and the vadose zone the development of speleothemic formations e g stalactites stalagmites columns is limited in some caves but extensive in others some of the caves have limited amounts of sediment on their floors in these caves the speleothems and sediments are still formed of caco3 and thus contrast sharply with the dolostone of the bluff formation in which they occur some of the joints sinkholes and caves are filled with breccia caymanite terra rossa terra rossa breccia pisolitic limestone and speleothems although filled such karst features are analogous to the open karst features that occur elsewhere on the islands the filling of joints sinkh