magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

further evidence for inclination shallowing in red beds red beds and related shallow marine siliciclastic sediments within the extensional catalan neogene basins offer a unique opportunity to study the relationship between sediment anisotropy remanence acquisition and inclination shallowing on the basis of the dominant fabric features sedimentary facies are grouped into laminated massive and bioturbated the bulk susceptibility appears to be dominated by paramagnetic phyllosilicates and clay minerals the magnetic susceptibility fabric is dominantly foliated while the degree of anisotropy is very variable and is dependent on the facies type strong foliation dominates in the laminated sediments whereas weak anisotropy characterizes the bioturbated and massive mass flow facies moreover we observe a correlation between facies type and remanence inclination the undisrupted thinly laminated facies provides the most biased inclination values 20 as opposed to 60 of the expected geomagnetic field steeper inclinations are reported from bioturbated muds while the best record of the geomagnetic inclination is found in the weakly anisotropic mass flow deposits

late quaternary halimeda bioherms and aragonitic faecal pellet dominated sediments on the carbonate platform of the western continental shelf of india abstract the fifty fathom flat is a drowned carbonate platform located on the outer continental shelf of western india it is unique because the carbonate depositional environment prevailed until early holocene in spite of its proximity to a major siliciclastic tidal embayment study of the bathymetry seismic and sonograph data together with the petrology mineralogy and geochemistry of the carbonate sediments and limestones recovered from the platform resulted in a the discovery of late quaternary halimeda bioherms and b aragonitic faecal pellet dominated sediments echograms reveal buried pinnacles between 62 and 38 m and exposed 2 to 14 m high pinnacles between 65 and 85 m on the platform these pinnacles occur as narrow to broad mounds bioherms in seismic profiles and as linear ridges and massive structures in sonographs seismic profiles also show a prominent reflector about 25 m below the sea bed identified by boreholes as a late pleistocene erosion surface the platform consists of relict carbonate sands and limestones rounded and ovoid to ellipsoidal aragonite grains are predominant in the sediments sem studies indicate elongated parallel canals and crescent to hook shaped canals corresponding to faecal pellets occur in some grains algal grains and pellets enclosing algal fragments are also present the grains in the limestones are similar to those in the sediments halimeda and crustacean faecal pellets are abundant components with minor echinoid mollusc and benthic foraminifera the limestones are characterised by abundant pores dissolution cavities relic aragonite needles meniscus cements and drusy calcite fill the pores produced by dissolution of aragonite needles these features indicate vadose diagenesis the stable isotopes of the carbonate sands and limestones are almost identical and similar to isotope values of beachrocks of the red sea the limestones ages 8400 9200 yr b p lie within the same age limits 10 400 8300 yr b p as the surficial sediments it appears that fluvial sediments largely accumulate in the inner shelf and did not hinder carbonate growth on the platform lying on the outer shelf we presume that arid climate and nutrients brought by river runoff and shelf edge currents favoured the growth of algal bioherms dominated by halimeda between 13 700 and 8300 yr b p the absence of late holocene and modern carbonates on the platform may be due to the combined influence of climatic change upwelling and neo tectonic activity at about 9000 yr b p the bioherms dominated by halimeda produced abundant aragonite muds deposit feeding crustaceans and other invertebrates subsequently excreted these muds as faecal pellets the platform thus contains faecal pellet dominated sediments

surface currents in the caribbean sea as deduced from lagrangian observations the results of a satellite tracked drifting buoy experiment conducted in the caribbean sea from october 1975 through june 1976 are presented the buoy trajectories suggest that the aves rise beata ridge and nicaraguan rise are regions where considerable mesoscale variability occurs in the surface flow the variability is in the form of current deflections small diameter eddies ranging in size from 20 to 100 km large amplitude meanders with amplitudes greater than 150 km and large diameter eddies with diameters greater than 200 km which form on and near these rises and ridges in the central caribbean where the topographic relief is not as dramatic the flow is predominantly zonal intensifications in the surface flow occur off the coast of colombia south of the nicaraguan rise and south of the yucatan strait

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

to the islands born the research potential of submerged landscapes and human habitation sites from the islands of nw australia this paper focuses on the continental shelf of nw australia and on models for change in littoral and offshore environments of relevance to human occupation over the last 50 kyr major island groups occur on the shelf including the montebello and barrow islands and those of the dampier archipelago at lowest sea level around 22 ka these would have been uplands that then became progressively isolated by subsequent sea level rise by integrating archaeological and zooarchaeological records from excavations on these islands with the geology and geomorphology we interpret palaeoeconomic resource potential in relation to changing sea level and coastline morphology the preservation potential for submerged archaeological sites and features is also assessed current archaeological evidence from these offshore islands indicates that the submerged coastal landscape is likely to have been a potentially rich environment for resources and human occupation even at times of lowest sea level and regional aridity should any exploration of submerged archaeology be carried out in this region it is likely to be rewarding offering unique insights into late pleistocene coastal occupation

facies analysis and palaeoenvironmental interpretation of the late oligocene attard member lower coralline limestone formation malta the oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale so far few studies have considered the impact that these early reefs had on ramp development consequently this study aimed at presenting a high resolution analysis of the attard member of the lower coralline limestone formation late oligocene malta in order to decipher the internal and external factors controlling the architecture of a typical late oligocene platform excellent exposures of the lower coralline limestone formation occurring along continuous outcrops adjacent to the victoria lines fault reveal in detail the three dimensional distribution of the reef associated facies a total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp the inner ramp was characterized by a very high energy shallow water setting influenced by tide and wave processes this setting passed downslope into an inner ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals the middle ramp lithofacies were deposited in the oligophotic zone the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents compositional characteristics and facies distributions of the attard ramp are more similar to the miocene ramps than to those of the eocene an important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone this expansion may have commenced in the late oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies stacking pattern analysis shows that the depositional units parasequences at the study section are arranged into transgressive regressive facies cycles this cyclicity is superimposed on the overall regressive phase recorded by the attard succession furthermore a minor highstand correlated with the ru4 ch1 sequence and subsequent minor lowstand ch2 sequence have been recognized the biota assemblages of the attard member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions the apparent low capacity of corals to form wave resistant reef structures is considered to have been a significant factor affecting substrate stability at this time the resulting lack of resistant mid ramp reef frameworks left this zone exposed to wave and storm activity thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths

do red beds indicate paleoclimatic conditions a permian case study abstract terrestrial red beds have long been interpreted as desert deposits by comparison with modern red deserts more recently red beds have been interpreted as evidence of seasonally dry conditions and a permo triassic pangean monsoon red beds of cala viola sardinia are identified as paleosols and used to reconstruct late permian paleoclimatic conditions reconstruction of paleoenvironmental conditions based on the paleosols of the cala viola indicates warm humid conditions with no evidence of dry conditions as in a desert or of extreme seasonality as in a monsoon instead it is suggested that the red color of the paleosols is a result of former good drainage and that red color in general does not indicate specific paleoclimatic conditions

high energy inner shelf carbonate facies along a tide dominated non rimmed margin northwestern australia abstract high energy inner shelf carbonates occur within 1 to 2 km of the mainland along the southern part of the northwest shelf a non rimmed carbonate shelf along northwestern australia coarse grained sands and gravels are deposited on and adjacent to a chain of subtidal and intertidal rock platforms which lies oblique to regional ne sw directed tidal currents intertidal and subtidal sand sheets less than 50 cm thick predominate along the margins and interior of the chain and subtidal linear to sinuous sand bodies up to 10 m thick and 4 km in length form banner banks to the northeast of rock platforms and reefs other features include narrow fringing and patch coral reefs calcareous red algal reefs and intertidal through subtidal coral shingle ridges fleshy algae are abundant on open sand covered and bare rock platforms as well as forming a mantle on relict coral reef frameworks supratidal through subtidal sediments are comprised of skeletal molluscs foraminifera bryozoans corals calcareous red algae and rock fragments the latter derived from rock platforms calcareous chlorophytes e g penicillus and halimeda are not important sediment contributors in general sediments are not representative of the present day turbid and fleshy algal dominated subtidal environment the predominantly coarse grained lithofacies

a large coral reef in the pennsylvanian of ziyun county guizhou south china the substrate and initial colonization environment of reef building corals abstract studies of pennsylvanian reefs are mainly focused on algal reefs research on non algal framework reefs is relatively uncommon the bianping coral reef is the largest pennsylvanian framework reef in china the main builders of the reef were the rugose corals fomitchevella some paleontological phenomena indicate that the coral fomitchevella colonized a hard substrate which included encrusting tubular organisms microbial carbonate brachiopod biostromes ivanovia cf manchurica and antheria the presence of this hard substrate was the key factor in the initial colonization stage of reef building corals the hard substrate offered a stable habitat for fomitchevella during its colonization stage juveniles of fomitchevella attached to hard substrates while the adults were free living adult fomitchevella exhibit diameter increase in the skeleton the size transformation of fomitchevella corallites from small and thin to thick and long is very abrupt above the reef substrate fomitchevella has a fasciculate growth form in the bianping coral reef they are still in their growth position appearing as cross sections on bedding surfaces of the limestone fomitchevella initially colonized a hard substrate near the wave base with a moderate amount of wave action the sedimentation rates were low and the water was relatively clear during settlement fomitchevella was well adapted to mesotrophic conditions

the potential for discovery of new submerged archaeological sites near the dampier archipelago western australia abstract the islands of the dampier archipelago preserve a probable 30 000 year archaeological record that reflects the change from a continental to an island environment following post glacial sea level rise the geomorphological history of the dampier archipelago region in combination with preliminary hydrodynamic modelling of past tidal regimes provides the basis for a new model of how the shelf landscape may have developed between the last glacial maximum c 20 ka bp through the holocene marine transgression and up to the present day using first order geomorphological principles an assessment is made of the key late pleistocene and holocene sediment bodies that may preserve archaeological deposits we show that archaeology is most likely to be present in deposits associated with the early phases of inundation of the dampier archipelago dating from around 9 7 ka bp at this time relative sea levels were around 30 m to 15 m which was when coastal configuration was complex and the variety and scale of intertidal and shallow sub tidal environments wide in contrast we anticipate that coastal archaeology older than 12 ka bp when the post glacial sea levels were below 50 m will have been exposed to a phase of faster tidal currents on the continental shelf and hence eroded or poorly preserved our study aims to improve prospection for and later management of the as yet unknown submerged elements of west australia s rich archaeological heritage

the pilbara coast a riverine coastal plain in a tropical arid setting northwestern australia abstract the unifying feature of the pilbara coast of northwestern australia is that it is a sedimentary repository for a range of rivers that drains a high relief precambrian rocky hinterland and discharges sediments along a coastal plain which fronts a wave dominated environment in a tropical arid climate the combination of fluvial and shoreline accretion processes coastal cementation coastal erosion and ancestral landform architecture such as residual pleistocene limestone ridges and large outcrops of precambrian bedrock has produced a complex coastal system during the quaternary as a result the coast is dominated by active deltas beach dune shores inactive eroding parts of deltas and their barriers limestone barrier coasts bays associated with eroded limestone barriers and archipelago ria coasts quaternary sediments throughout the area while varied in their distribution and history at the smaller scales exhibit a recurring pattern of lithotopes and lithologies in the region there are three main quaternary suites pleistocene red siliciclastic sediments alluvium deltaic sediments and aeolian sand that form an inland zone pleistocene limestones that form local barriers and a holocene system within which are the sedimentary suites of deltas beach dunes tidal flats and tidal embayments

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

heterozoan carbonates in subtropical to tropical settings in the present and past water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments however parameters other than temperature also strongly influence ecosystems and consequently the carbonate grain associations in the resulting carbonate rock among those factors are biological evolution water energy substrate water chemistry light penetration trophic conditions co2 concentrations and mg ca ratios in the seawater increased nutrient levels in warm water settings for example lead to heterotrophic dominated associations that are characteristic of temperate to cool water carbonates failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past modern analogues of low latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm water carbonates careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate

sediment production and dispersal on foraminifera dominated early tertiary ramps the eocene el garia formation tunisia larger benthic foraminifera nummulites are common within eocene circumtethyan limestones despite their importance as sediment producers contradictions in the literature constrain current understanding about the location of the nummulites factory the el garia fm was deposited on a ramp with localized palaeohighs and whilst some authors suggested that the locus of nummulites production was in shallow water across the palaeohighs others concluded that production was significantly reduced over these palaeohighs and concentrated in the surrounding deeper 30 60 m water there are also marked dissimilarities between recent models in terms of the continuity correlation and resolution of depositional sequences to assess these models we integrate studies of the architecture and geometry of the el garia fm with detailed taphonomic biometric biofabric and palaeoecological characterization of nummulites tests we conclude that the highest rates of sediment production occurred in euphotic water over the palaeohighs and in nearshore environments nummulites on the palaeohighs were transported into the surrounding deeper water by oceanic and storm currents that swept the platform top producing a nummulitic sediment package that thickened and became increasingly fine grained and fragmented into outer ramp environments this transport exerted a major control on development of the ramp like geometries often seen at outcrop our findings question the validity of a recent sequence stratigraphic model that identifies decimetre scale milankovitch cycles even in largely allochthonous bio retextured mid outer ramp sediments our findings also suggest that the thin packages of el garia fm on the palaeohighs which have previously been interpreted as condensed sections that can be correlated with thicker more distal accumulations actually represent remnants of the sediment that was produced on the highs and exported into the basin

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

carbonate factory turnovers influenced by the monsoon xisha islands south china sea carbonate factory turnovers are the integrated result of climatic oceanographic and biological controls sedimentological geochemical and palaeontological data from core xk 1 drilled in the xisha islands south china sea reveal that photozoan to heterozoan carbonate factory turnovers during the early and middle miocene in isolated carbonate platforms were mainly caused by changes in the upwelling regime a heterozoan open bank association thrived in tropical shallow water under conditions of nutrient excess indicated by high ln cu ti and ln ba ti and by the increased abundances of rhodoliths and some foraminifer genera during the early and middle miocene episodes characterized by a heterozoan carbonate production correlate with times of east asian summer monsoon strengthening especially from 21 2 to 17 3 ma and from 15 6 to 11 6 ma this study thus provides new insights into the variability of shallow water carbonate deposition in monsoon influenced areas and also shows that carbonate factory turnovers can contribute to a better understanding of the monsoon9s evolution supplementary material biostratigraphic and magnetostratigraphic data coral content geochemical data and mineral compositions related to this study are available at https doi org 10 6084 m9 figshare c 4507070

high frequency cycles in the upper aptian carbonates of the organya basin ne spain high frequency cycles in upper aptian carbonates have been studied on the carbonate ramp of organya southeastern pyrenees the depositional area comprises a shallow marine to deeper marine transect a detailed facies model is developed subdividing the transect into an inner ramp area above fairweather wave base a mid ramp area between fairweather wave base and storm wave base and an outer ramp area below storm wave base

the red bed controversy revisited shape analysis of colorado plateau units suggests long magnetization times several triassic and earliest jurassic sedimentary units from the colorado plateau region have distributions of virtual geomagnetic poles vgps that are highly elongate along the path of apparent polar wander apw this suggests that the remanent magnetizations measured in these units were acquired over an extended period of time possibly approaching 35 m y and are not precisely coeval with the stratigraphic age of the rock comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock nor is it related to paleolatitude the rocks that yield elongate vgp distributions are dominantly red to brown mudstones and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization as suggested by larson et al j geophys res 87 1982 1081 and other authors however several superficially similar units from the colorado plateau have nearly circular vgp distributions the process by which remanence is acquired in clastic sedimentary rocks merits further study d 2002 elsevier science b v all rights reserved

mineralogy and grain size variations along two carbonate margin to basin transects pedro bank northern nicaragua rise abstract the analysis of nine periplatform cores in the surrounding of the pedro bank carbonate platform showed the presence of characteristic depositional environments during the last 300 ky this subdivision is based on mineralogical and grain size variations in space and time along the leeward downcurrent transect the fine sediment fraction 40 km aragonite is still the most abundant mineral but the percentage decreases while the pelagic carbonate mineral content lmc increases within the subordinate coarse fraction classes 63 m the very fine sand fraction dominates at proximal sites 20 km show a more bimodal distribution pattern in the coarse grain sizes with maximum percentages within the very fine and medium sand fraction indicating a mixed neritic pelagic signal during glacial lowstands in sea level a twofold division in the spatial distribution of the periplatform sediments is evident a proximal environment 28 km showing a strong dominance of the fine fraction 90 the increased coarse fraction percentage during glacials at proximal sites results from various interacting processes 1 lower input of fine neritic sediments 2 increased current winnowing and 3 redepositional processes at the upper slope during lowered sea level and the export of this material to proximal basinal sites at upcurrent sites the mineralogy displays a similar spatial evolution as seen along the downcurrent margin but with overall reduced percentages this is the result of the lower export potential of the platform against the main direction of the caribbean current the most important factor for neritic sediment dispersal in the study area the coarse fraction content is slightly higher during interglacials compared to the downcurrent margin during glacials a similar reduction in the percentage of the coarse fraction is evident in a spatial context this also substantiates the reduced sediment export and redeposition potential at the upcurrent margin

the lower cretaceous atzompa formation in south central mexico record of evolution from extensional backarc basin margin to carbonate platform abstract lower cretaceous depositional systems of southwestern oaxaquia in south central mexico were influenced by initiation of a continental arc on mainland mexico and subsequent accretion of the guerrero composite arc terrane to mainland mexico the atzompa formation defined herein which crops out in the sierra de tentzo constitutes a succession of conglomerate sandstone siltstone and limestone with early cretaceous fauna and detrital zircon maximum depositional ages that range 126 123 ma late barremian to early aptian the lower part of the atzompa records a transition from alluvial to deep lacustrine depositional environments suggesting the early stages of an extensional basin overlying deposits of anabranching axial fluvial systems that flowed to the ne se accumulated after a period of rapid subsidence in the tentzo basin also formerly undescribed fluvial facies grade up section to tidal deposits overlain in turn by a carbonate ramp succession that contains late barremian to early aptian fossils the ramp deposits of the uppermost atzompa formation are overlain on a sharp contact by basinal carbonates of early albian age the tentzo basin formed due to crustal extension of the overriding plate in a backarc setting was characterized by very high rates of sedimentation 3 6 mm yr during the early stages of basin formation rift initiation and rift climax and slower rates during the development of tidal systems and the carbonate ramp post rift stage regional and local subsidence took place in the backarc region of the zicapa magmatic arc which was established in the western margin of mexico by hauterivian time abrupt deepening following atzompa formation deposition is attributed to flexural subsidence related to collision of the guerrero composite volcanic terrane with the western margin of mexico following late aptian accretion of the guerrero terrane to oaxaquia the carbonate basin eventually shallowed to become a carbonate platform that faced the gulf of mexico

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

nutrient and temperature controls on modern carbonate production an example from the gulf of california mexico in addition to salinity and temperature nutrient concentrations in surface waters are known to have a significant impact on distribution of carbonate producing biota but have never been quantitatively evaluated against different temperatures along a latitudinal transect the western coast of the gulf of california mexico presents a natural laboratory for investigating the influence of oceanographic parameters such as salinity temperature and chlorophyll a a proxy for nutrients on the composition of a range of modern heterozoan and photozoan carbonate environments along a north south latitudinal gradient spanning the entire warm temperate realm 29 n 23 n chlorophyll a measured in situ at half hour resolution is highly variable throughout the year due to short term upwelling and increases significantly from the southern to northern gulf of california salinity in contrast fluctuates little and remains at an average of 35 from south to north carbonate production ranges from oligotrophic mesotrophic coral reef dominated shallow water areas minimum temperature 18 6 c through mesotrophic eutrophic red algal dominated inner shelf carbonate production in the central gulf minimum temperature 16 c and to molluscan bryozoan eutrophic inner to outer shelf environments minimum temperature 13 7 c the gulf of california data supplemented with oceanographic and compositional information from a database compiled from a spectrum of modern carbonate systems worldwide demonstrates the significance of nutrient control in the formation of heterozoan photozoan and transitional heterozoan photozoan carbonate systems and serves as a basis for more accurately interpreting fossil carbonates

rhodolith assemblages from the lower tortonian carbonate ramp of menorca spain environmental and paleoclimatic implications abstract lower tortonian distally steepened carbonate ramp of menorca mostly consists of foramol and rhodalgal facies deposited in inner middle ramp ramp slope and outer ramp settings red algae are abundant from the middle ramp to the lower part of the slope and their taxonomic assemblages are clearly related to the bathymetric position melobesioids percentage increases basinward passing from 55 8 in the middle ramp to 97 in the toe of slope mastophoroids are more abundant in the middle ramp 43 1 and decrease toward deeper paleoenvironments 1 2 lithophylloids and sporolithaceans appear as accessory components from the middle ramp to slope settings the percentage of melobesioids and mastophoroids observed in the middle ramp suggest that growth of the rhodoliths started in a water depth below 10 20 m the deepest occurrence of the rhodoliths is in the ramp slope environment where the dominance of melobesioids and the low percentage of shallower water subfamily suggests a water depth range of 70 to 100 m shape and structure of rhodoliths are indicative of high energy conditions in all ramp settings and they do not reflect a decrease in hydrodynamic energy related to water depth increase these conditions are related to the presence of unidirectional currents that produced cross bedded grainstones existing in the middle ramp ramp slope and at the toe of the ramp slope settings the high percentage of mastophoroids in the shallowest environments and the presence of lithoporella and sporolithon in the coralline assemblages suggests that carbonate production took place in tropical waters

detrital zircon u pb geochronology of southern guerrero and western mixteca arc successions southern mexico new insights for the tectonic evolution of southwestern north america during the late mesozoic late jurassic cretaceous arc related volcaniclastic rocks from the southern guerrero and western mixteca terranes of mexico were analyzed by u pb detrital zircon geochronology laser ablation multicollector inductively coupled plasma mass spectroscopy to place constraints on the depositional history and provenance of the rocks pre middle jurassic basement rocks and sandstone from the upper cretaceous mexcala formation were also analyzed to defi ne the origin and provenance of the prevolcanic substratum and the time of accretion of guerrero composite terrane sequences data from the taxco taxco viejo teloloapan and arcelia assemblages indicate that the youngest 129 141 ma zircon fraction in each sequence was derived from local volcanic sources whereas older populations ca 247 317 365 459 530 617 712 878 947 964 1112 1188 1350 1420 1842 1929 2126 2439 and 2709 3438 ma show sediment infl ux from varied sources most likely through grain recycling the major zircon clusters in these sequences match the populations recorded in the nearby acatlan complex in contrast the huetamo sample is dominated by lower cretaceous ca 126 ma zircons of local volcanic provenance and the zihuatanejo sample contains zircon clusters ca 259 ca 579 and ca 947 1162 ma comparable to major populations recorded in the underlying arteaga complex a sample from the middle triassic middle jurassic arteaga complex at tzitzio contains zircon clusters ca 202 247 ca 424 ca 600 ca 971 and ca 2877 ma consistent with an ultimate derivation from both north american and south american sources the sample from the las ollas suite contains comparable zircon populations ca 376 475 ca 575 ca 988 1141 and ca 2642 2724 ma and it is interpreted to be part of the prevolcanic basement in contrast the youngest zircon cluster ca 105 ma in the mexcala formation coincides with the major volcanic events in the taxco taxco viejo teloloapan and arcelia assemblages whereas the older clusters ca 600 ca 953 ca 1215 ca 1913 and ca 2656 2859 ma broadly match the major populations recorded in rocks from the acatlan complex these new data combined with available geochemical and isotopic data indicate that the taxco taxco viejo arc assemblage developed on continental crust the acatlan complex is the most plausible candidate the teloloapan and arcelia arc assemblages were developed on oceanic crust as offshore arcs facing the acatlan complex the zihuatanejo terrane assemblages were developed on the arteaga complex and evidence no infl uence from the acatlan complex this suggests that these assemblages were formed farther away or in a restricted basin the guerrero composite and mixteca arc successions are coeval with the alisitos arc of northern mexico and in part with the nevada and klamath ranges of the southwestern united states and with the arc series from the greater and lesser antilles and northern south america data indicate that during late mesozoic time southwestern north america was a site of intensive volcanism in a complex arc trench system similar to that of the east pacifi c our data are consistent with a diachronic accretion of the guerrero composite terrane sequences beginning during late cenomanian time with the amalgamation of the teloloapan and probably the arcelia assemblages and fi nishing at the end of cretaceous time with the accretion of the zihuatanejo terrane assemblages

faunal turnover and changing oceanography late palaeozoic warm to cool water carbonates sverdrup basin canadian arctic archipelago abstract carbonate sedimentary rocks in the fosheim hamilton sub basin at the eastern end of the sverdrup basin contain a particularly sensitive record of changing palaeoceanography and faunal turnover during the northward movement of pangaea through the late palaeozoic carboniferous and earliest permian inner ramp carbonates are a photozoan association of colonial corals fusulinids algae and ooids with sparse heterozoan elements crinoids brachiopods and bryozoans based on modern analogues these biotic components reflect warm water oligotrophic marine environments contemporaneous mid ramp and basin biotas are dominated by a heterozoan fauna solitary corals crinoids bryozoans and brachiopods that grew in cooler deeper waters that may have been beneath a thermocline initial cooling began in the latest sakmarian with an influx of cool deep water and climatic deterioration allowing cool and warm waters to mix disrupting the thermocline and producing a sub tropical marine biota whose remains were reworked by storms and re deposited as tempestites faunas are heterozoan crinoids bryozoans brachiopods and sponges with photozoan elements colonial corals and symbiont bearing fusulinids prominent only in inner ramp facies by late artinskian these photozoan components are only present in shallowest basin margin environments artinskian subtropical waters contained somewhat elevated trophic resources that together with mixing of photozoan and heterozoan elements in the subtropical inner ramp produced the greatest faunal diversity seen in the sverdrup basin the biota was completely boreal in nature and warm shallow waters with a warm refuge biota were now supplemented by cool waters derived from the panthalassa ocean profound change took place in the kungurian coincident with localised uplift during the melville disturbance and increased siliciclastic input neritic storm dominated sediments are wholly heterozoan brachiopods bryozoans crinoids and sponges with no remaining photozoan elements nutrient levels are elevated but not eutrophic as indicated by phosphate and glauconite extensive bioerosion numerous infaunal foraminifers and an impressive and diverse shelly biota that during times of high siliciclastic input was replaced by a profuse zoophycos infauna brachiopod and bryozoan faunas are distinctly boreal continued cooling relatively high nutrients as indicated by abundant glauconite and phosphate and decreased siliciclastic input eventually resulted in an abundant heterozoan biota brachiopods bryozoans crinoids and sponges with local gigantism but reduced brachiopod and bryozoan diversity these middle permian rocks may be the coolest water deposits in the sverdrup basin the faunas are still part of a boreal arctic province with strong ties to cool water faunas of uralian basin the succession ends with late permian biosiliceous spiculitic cherts containing accessory heterozoan components brachiopods bryozoans and few crinoids these sediments in part coeval with evaporites and terrestrial red beds on the eastern european platform are interpreted to reflect a combination of cool waters elevated nutrients and mid latitude storminess

platforms of the nicaraguan rise examples of the sensitivity of carbonate sedimentation to excess trophic resources the nicaraguan rise is an active tectonic structure in the western caribbean carbonate accumulation on its platforms has not kept pace with relative holocene sea level rise despite a tropical location remote from terrigenous sedimentation trophic resources apparently exceed levels favoring coral reef development because sponge algal communities dominate the drowning western platforms in contrast to mixed coral algal benthos on pedro bank and well developed coral reefs along the north coast of jamaica concentrations of biotic pigments in sea surface waters show a corresponding west east gradient oceanic waters flowing over the western banks carry nearly twice as much biotic pigment as oceanic waters north of jamaica sources enriching the western caribbean are terrestrial runoff upwelling off northern south america and topographic upwelling over the nicaraguan rise that relatively modest levels of trophic resources can suppress coral reef development holds important implications for understanding carbonate platform drownings in the geologic record

paleomagnetism of the cretaceous morelos and mezcala formations southern mexico a paleomagnetic study of platform facies carbonate rocks of the mid cretaceous morelos formation and deep water carbonate rocks of the overlying upper cretaceous mezcala formation sampled at zopilote canyon in guerrero state southern mexico indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the late cretaceous laramide orogeny the remanence carrier is interpreted to be magnetite although other mineral phases of high coercivity carry recent secondary overprints the overall mean is of dec 323 1j and inc 36 5j k 162 7 a95 2 7j n 18 sites 64 unfolding comparison with the north america reference direction indicates that this area has experienced a small yet statistically significant counterclockwise direction of 19 2f4 0j similar rotations are documented in other localities from southern mexico rotations are linked to mid tertiary deformation associated with the left lateral strike slip fault system that accommodated motion of the chortis and xolapa blocks d 2002 elsevier science b v all rights reserved

paleomagnetism of the moenkopi formation colorado plateau basis for long term model of acquisition of chemical remanent magnetism in red beds paleomagnetic studies of moenkopi samples from several sites on the colorado plateau including a complete 427 m section at sinbad valley western colorado reveal the following 1 the presence of both normal and reversed triassic components in many of the same samples at many localities 2 the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the sinbad valley section 3 a streaked pattern of the remanence directions in any one member in the sinbad valley section similar to that expected from polar wander 4 a lack of progressive change from the base upward and 5 the common intermediate remanence directions of weak intensity suggesting the presence of two or more nearly balanced anti parallel components throughout the formation the results of our investigation combined with previously reported petrographic and paleomagnetic studies of the moenkopi formation have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism crm acquired diagenetically over a geologically long time interval from the available data we have formulated the following model to explain how the crm was acquired through natural processes acting intermittently at geologically reasonable rates when initially deposited the moenkopi sediments consisted primarily of first cycle detritus containing a few percent of iron bearing accessory minerals such as biotite hornblende magnetite and ilmenite at some time after deposition under favorable subsurface conditions intrastratal alteration began involving breakdown of iron bearing silicate and oxide minerals partial replacement of some iron rich silicate grains by hematite martitization of magnetite growth of hematite pods and blebs in ilmenite and biotite and precipitation of crystalline hematite in both primary and secondary voids the authigenesis spanned at least two polarity events and probably continued over several tens of millions of years stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events any given lithologic unit tended to be dominated by one polarity the model given above seems to be capable of explaining not only those magnetic and petrographic features found in the moenkopi formation but also those of most other red beds

understanding the geological record of carbonate platform drowning across rifted tethyan margins examples from the lower jurassic of the apennines and sicily italy abstract in the geological record a drowning process is documented by various types of shallow water to pelagic or shallow water to mixed benthic pelagic carbonate transitions drowning unconformities are paraconformities to disconformities to angular unconformities and their drowning surfaces range from planar to highly irregular morphologies drowning successions display a mix of products of both the benthic and pelagic carbonate factories these successions can be also bounded by unconformities drowning unconformities and drowning successions are contrasted through a description and discussion of examples of escarpment bounded platforms from the jurassic of the northern apennines and subordinately from sicily and other tethyan sectors the areal distribution of drowning unconformities and successions is discussed with reference to a complex depositional system whose architecture was the product of the rift induced fragmentation of a regional hettangian carbonate megabank the examples in this study show drowning unconformities to be exclusive to intrabasinal highs while drowning successions are found both on highs and in hangingwall basins drowning unconformities with their long associated hiatuses and drowning successions are often seen to merge laterally into one another over very short distances on the same intrabasinal high the deposits of the drowning succession are sometimes missing on the top of the platform while they are found forming clinoforms along its flanks evidence that sediment could be permanently swept from the highs in these depositional systems at this stage

coastal forms and quaternary processes along the arid pilbara coast of northwestern australia abstract the pilbara coast is set in the most arid part of coastal australia this coast and coastal plain composed of riverine plains deltas tidal flats coastal dunes and limestone barriers relic deltas and archipelago ria shores is predominantly a terrigenous and carbonate sediment complex the coastal landforms therein formed during the quaternary through influence of ancestral landforms and fluvial and shoreline accretion coastal erosion and cementation the pre holocene features controlled development of holocene coastal forms and a variety of holocene depositional styles climate also plays a significant part in the coastal processes and in the generation of sedimentary products through high evaporation rates coupled with the limited rainfall cyclonic storms wind wind waves and the limited sediment delivery to the coastal zone this results in a distinctive range of stratigraphic sequences and chemical products as a result this arid coast is characterised by a range of features such as construction of arid zone deltas delta destruction and sediment redistribution during times of sediment depletion cyclone induced erosion and sedimentation mangroves and their associated sedimentary deposits evolution of coastal groundwater hypersalinity formation of salt flats and precipitation and cementation to form beachrocks high tidal crusts and gypsum precipitates the pilbara coastal stratigraphy and geomorphology suggests aridity was intricately involved in the sedimentation geomorphic evolution and pedogenic and diagenetic alteration of this coastal zone throughout the whole of the holocene and pleistocene

the western edge of the mediterranean pelagian platform a messinian mixed siliciclastic carbonate ramp in northern tunisia the marine messinian deposits of tunisia cover a narrow littoral strip some 300 km long between the northern bizerte and cap bon areas and the central eastern sahel region litho and biofacies analysis of six stratigraphic sections reveals the distinctive features of these deposits the lower messinian deposits are characterized by ubiquitous siliciclastics and abundant oolitic bioclastic limestones organized in an eastward facing ramp westward landward this ramp changes into coastal lagoons sometimes containing evaporites eastward the ramp passes to the reefal pelagian platform extending as far as lampedusa two main sedimentary cycles are distinguished 1 an early messinian siliciclastic retrogradational then oolitic bioclastic progradational cycle beni khiar formation and lower oued bel khedim formation 2 a late messinian brackish to continental cycle that probably accumulated in rapidly subsiding lagoons cued el bir formation and upper oued bel khedim formation the tunisian early messinian cycle is partly eustatically controlled but the late messinian cycle cannot be confidently correlated to other well known messinian series because of tectonic movements the lower messinian deposits of tunisia are also characterized by abundant suspension feeding organisms molluscs and bryozoans and rare corals calcareous algae echinoids and larger benthic foraminifers the proposed palaeoenvironmental model shows that the lower messinian ramp of tunisia was located on a current protected margin and subjected to continent derived sediment and nutrient supply eastward nutrient influx diminished and a shallow water isolated carbonate platform with coralgal facies developed between the western and the eastern mediterranean basins the main hydrological connection between these two basins occurred through a narrow seaway situated to the northeast of the pelagian platform south of sicily and malta

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

paleomagnetism of the moenkopi and chinle formations in central new mexico implications for the north american apparent polar wander path and triassic magnetostratigraphy in central new mexico red sedimentary rocks unconformably overlying permian carbonates of the san andres formation have been correlated with the early middle triassic moenkopi and late triassic chinle formations of the colorado plateau paleomagnetic samples from triassic sections exposed on basement cored uplifts along both the east and west side of the rio grande rift near albuquerque yield upon thermal and chemical demagnetization well defined high unblocking temperature dual polarity magnetizations carried by hematite the characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding parallel magnetization polarity zonation the moenkopi and lowermost chinle formations produced paleomagnetic poles respectively at 57 6 n 100 3 e n 36 sites k 74 1 a95 2 8 and 60 8 n 88 9 e n 17 sites k 130 3 a95 3 1 these data plus previously published and additional results from the underlying permian strata suggest that portions of central new mexico have experienced a small clockwise rotation i e less than 10 similar to that of the colorado plateau with respect to the north american craton the paleomagnetic directions of the chinle formation and related strata in eastern new mexico document about 12 great circle distance of rapid apparent polar wander during mid carnian to late norian times along a track which contains other cratonic poles of similar age we present a preliminary magnetic polarity time scale for the triassic that incorporates the present new mexico data and previously published data mostly form continental red bed sequences this magnetic polarity scale provides a basic framework which can be tested with future data from triassic sections where additional biostratigraphic control exists

genesis and dispersal of carbonate mud relative to late quaternary sea level change along a distally steepened carbonate ramp northwestern shelf western australia abstract accumulation of carbonate mud is minor on the northwest shelf western australia a broad 200 300 km wide distally steepened tropical carbonate ramp it is negligible along most of the inner ramp 0 to 50 m water depth owing to siliciclastic riverine input it is also negligible along the mid from 50 to 120 m ramp and where the entire ramp narrows to 100 km owing to constant reworking by storm waves and ocean swell in these areas carbonate mud is derived mostly from mechanical degradation of skeletal fragments but this does not include calcified remains of calcareous green algae e g halimeda although these plants grow locally only along the outer 120 to 200 m water depth ramp and slope is there any relatively abundant accumulation of carbonate mud here the sediment consists of modern pelagic foraminifer pteropod and nannoplantkon ooze and aragonitic needle rich 2 m micrite the latter having an age based on ams 14c of 19 ka distribution of aragonite micrite defines an extensive 5 0 104 km2 yet relatively narrow 40 km tract running 600 km along the platform margin crystal morphology mineralogy and isotope c o composition identify the micrite to be a seawater precipitate its production and seaward export having occurred during the last glacial maximum when lowered sea level transformed the otherwise broad platform into a narrow epicontinental shelf margin now the modern distal slope today this sediment body lies stranded in deep water about 200 300 km seaward of the shallow inner ramp the volume of micrite produced appears equal to the present day production potential of the great bahama bank rapid postglacial rise in sea level and change in platform geometry climate and oceanography were very influential factors in addition to water depth and area of submergence in the production and distribution of aragonite micrite along the northwest shelf in general 1 epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded across a platform and coastal clastics and absence of shoal barriers prevent development of protective lagoons and carbonate tidal flats 2 not all highstand carbonate systems are associated with significant amounts of micrite in shallow water environments nor do they necessarily export large volumes of micrite to the periplatform realm and 3 some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems

nutrient control of the global occurrence of isolated carbonate banks the growth and origin of the world s isolated carbonate banks are as yet not completely understood this paper presents a comparison of the areas depths and latitudinal distribution of the world s carbonate banks these are then compared with chemical parameters of seawater in the upper mixed zone of the oceans and at depth the results allow large scale inferences to be made on the interacting controls on bank growth and distribution the data support the hypothesis that nutrient rich waters commonly control bank depth and size

new approach for quantifying water depth applied to the enigma of drowning of carbonate platforms this research illustrates application of a fluid inclusion technique for quantifying water depth of ancient carbonate platforms jurassic limestones of monte kumeta italy were cemented with submarine calcite during a transition to carbonate platform termination the calcite cements contain fluid inclusions consisting of jurassic seawater and immiscible gas bubbles trapped during the growth and penecontemporaneous recrystallization of the cements crushing analysis indicates that gas bubbles are under pressures indicative of entrapment in water depths of 23 112 m assuming simple deepening and acknowledging chronostratigraphic errors rates of relative rise in sea level were initially less than 7 m m y followed by a rate of at least 33 m m y these slow rates are evidence that the platform s demise was caused by an environmental perturbation other than rapid sea level rise the facies transitions and regional studies indicate that the perturbation resulted from nutrient excess or eutrophication in shallow water followed by deepening into ephemeral dysoxic waters at depths perhaps as shallow as 23 m

relationship between median destructive field and remanent coercive forces for dispersed natural magnetite titanomagnetite and hematite summary remanent acquisition curves remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well defined grain size fractions between 5 and 250 m of natural magnetite titanomagnetite and hematite from these curves the remanent acquisition coercive force h cr the remanent coercive force hcr and the median destructive field of irm h i were determined theoretically these parameters should be the same for an assembly of non interacting homogeneously distributed randomly oriented single domain grains for a given hematite specimen h cr hcr and h i have about the same value in spite of the strong grain size dependence of these parameters for each specimen of magnetite and titanomagnetite the value of h cr is larger than hcr which again is larger than h i however the ratios h cr hcr and h i hcr appear to have a different constant value an interesting relationship which appears to hold for dispersed magnetite titanomagnetite or hematite grains between 5 and 250 m independently of grain size quantity and packing density of the magnetic material is

1300 km long late pleistocene holocene shelf edge barrier reef system along the western continental shelf of india occurrence and significance abstract a detailed analysis of echosounding and side scan sonar data collected from the western continental margin of india has revealed the presence of prominent shelf edge reefs concentrated mostly on the central and the southern parts their depth of occurrence varies between 85 and 136 m the reefs are 1 12 m high and 0 1 2 6 km wide av 700 m morphologically they may be classified into 1 simple and 2 complex types the former are single and broad or narrow av width 350 m while the latter are generally massive av width 950 m with several superimposed peaks sub bottom profiles indicate the presence of paleolagoons this reef system more than 1000 km long trends nnw sse i e parallel to subparallel to the present day shoreline it is surmised that coral algal reef growth commenced with the advent of the holocene transgression and favorable antecedent topography and continued until early holocene subsequently rapid sea level rise drowned the reefs the shelf edge reefs therefore are part of relict submerged barrier reef system and reflect late pleistocene early holocene shoreline

upper jurassic microsolenid biostromes of northern and central europe facies and depositional environment abstract microsolenid biostromes are a common and important reef type in many upper jurassic reefal sections comparative study of these biostromes in western europe allows the characterisation of this distinct biostrome type in terms of its taxonomic composition palaeoecology and sedimentary regime the biostromes are characterised by a coral community of relatively low generic diversity being dominated by the microsolenidae family of corals the coral skeletal biovolume is generally high and is overwhelmingly dominated by platy ecomorphotypes sedimentologically these biostromes developed in low energy marl and limestone regimes which were subject to low sedimentation rates and varying degrees of siliciclastic influx in contrast to the coral fauna the associated fauna shows a high generic diversity in particular a rich assemblage of echinoids and bivalves in addition the coral plates provide an ideal habitat for a well developed and strongly polarised encrusting and boring fauna top surfaces show limited post mortem encrustation by serpulids and cementing bivalves and are also bored by lithophagan bivalves the undersides of the plates show a highly developed and diverse cryptic fauna of serpulids thecidean brachiopods bryozoans calcareous sponges and sclerosponges it is concluded that the main control on the development of these biostromes was low light intensities though low energy level and low background sedimentation rates were also important it is suggested that relatively high nutrient levels may have also played a significant role the conditions for microsolenid biostrome development prevail in relatively deep water environments and hence these are the first coral dominated reefal structures to have developed in deeper water shallowing upward sequences the corals in these biostromes have adapted to these conditions by developing a strongly platy growth form but also reducing their growth rates substantially the microsolenids are particularly adapted to these environments in two ways 1 they secreted an extremely porous calcium carbonate skeleton which in environments where calcification rates are slow such as deep water environments is a distinct advantage 2 in response to the limited potential of phototrophy in these environments the microsolenids have developed a pennular septal structure which is suggested to be a means of improving heterotrophic feeding trophic analysis of the reef fauna reveals that the nutrient limitation model developed for present day coral reefs cannot be applied to these biostromes it is demonstrated that the microsolena and the present day agariciid leptoseris show a number of significant morphological and ecological similarities

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

influence of marginal highs on the accumulation of organic carbon along the continental slope off western india abstract twenty five surficial sediment samples were collected from the central western continental margin of india which consists of diverse physiographic entities continental slope shelf margin basin marginal high and the deep arabian basin at depths between 250 and 2050 m the sediments were studied to assess the relative importance of productivity oxygen minima grain size and bathymetry on organic carbon accumulation preservation the total organic carbon toc content in the study area varies from 0 70 to 5 86 highest values of organic carbon are recorded on the marginal highs 5 12 5 86 followed by shelf margin basin 3 53 4 22 and the continental slope 1 80 3 84 the organic carbon content is relatively low in the arabian basin 0 98 1 46 the sediments from marginal highs are characterized by high oc contents relative to upper slope at equivalent water depth and also the deeper shelf margin basin the elevated organic carbon concentrations on marginal high and upper slope are associated with coarse grained biogenic sediments while those in the shelf margin basin with fine grained silty clays the enrichment of organic carbon on the marginal highs is attributed to productivity related biogenic sediments and their faster burial it is surmised that diverse topographic features in the slope and the associated hydrodynamic processes also play an important role in the enrichment of organic carbon

magnetization acquisition in two mesozoic red sandstones abstract paleomagnetic samples have been collected from folds and other sedimentary structures produced by soft sediment deformation and from intra formational conglomerates at several locations of the chugwater formation of wyoming and the moenkopi formation of the colorado plateau both of triassic age most load casts and other small scale folds were magnetized after deformation slump folds were magnetized before or during folding two of the four conglomerates contain clasts that were magnetized before they were ripped up these observations indicate that most fine grained sandstones are magnetized after deposition but before burial by a meter or so of sediment some sandstones and finer grained samples can be magnetized very shortly after deposition if they are consolidated and dewatered by drying samples having more complicated magnetization histories can usually be identified by poor within sample grouping of remanence directions and by the behavior of individual samples during demagnetization

holocene turn on and evolution of the southern great barrier reef revisiting reef cores from the capricorn bunker group abstract extensive drilling of the great barrier reef gbr in the 70s and 80s illuminated the main factors controlling reef growth during the holocene however questions remain about 1 the precise nature and timing of reef turn on or initiation 2 whether consistent spatio temporal patterns occur in the bio sedimentologic response of the reef to holocene sea level rise then stability and 3 how these factors are expressed in the context of the different evolutionary states juvenile mature senile reefs combining 21 new c 14 ams and 146 existing re calibrated radiocarbon and u th ages we investigated the detailed spatial and temporal variations in sedimentary facies and coralgal assemblages in fifteen cores across four reefs wreck fairfax one tree and fitzroy from the southern gbr our newly defined facies and assemblages record distinct chronostratigraphic patterns in the cores displaying both lateral zonation across the different reefs and shallowing upwards sequences characterised by a transition from deep porites faviids to shallow acropora isopora coral types the revised reef accretion curves show a significant lag period ranging from 0 7 2 ka between flooding of the antecedent pleistocene substrate and holocene reef turn on this lag period and dominance of more environmentally tolerant early colonizers e g domal porites and faviids suggests initial conditions that were unfavourable for coral growth we contend that higher input of fine siliciclastic material from regional terrigenous sources exposure to hydrodynamic forces and colonisation in deeper waters are the main factors influencing initially reduced growth and development all four reefs record a time lag and we argue that the size and shape of the antecedent platform is most important in determining the duration between flooding and recolonisation of the holocene reef finally our study of capricorn bunker group holocene reefs suggests that the size and shape of the antecedent substrate has a greater impact on reef evolution and final evolutionary state mature vs senile than substrate depth alone

deepwater carbonate deposition in response to re flooding of carbonate bank and atoll tops at glacial terminations the late quaternary has experienced large glacial interglacial climatic variations and related 10 s to 100 meters high amplitude sea level fluctuations at milankovich frequencies from 10 s to 100 kyr during which carbonate platform tops have been exposed and re flooded in many occasions this study focuses on the accumulation of calci turbidites the aragonite onset sharp increase in fine sediments and their timing in deep basins adjacent to carbonate platforms a particular emphasis is developed on the occurrence of the first gravity flow event and aragonite onset sharp increase and their linkage to the initial re flooding of the platform tops during deglaciations three basins adjacent to isolated platforms in the bahamas the northern nicaragua rise and the gulf of papua were selected to represent pure carbonate versus mixed systems in quiescent versus tectonically active settings and various carbonate bank top morphologies ranging from atoll to relatively deeply and narrowly flooded flat top banks in spite of these differences each record illustrates a clear relationship between the timing of platform top re flooding and initiation of significant carbonate export by gravity flows and low density plumes into the surrounding basins the concept of re flooding window is introduced to characterize the prolific period of time during which bank and atoll tops are flooded enough to produce large export of bank derived aragonite and of calci turbidites in adjacent basins according to our datasets the main re flooding windows have occurred mainly on the last part of the sea level rise at each glacial termination t those periods being marked by some of the highest rates of sea level rise the analysis of a long piston core from the earthquakes prone walton basin northern nicaragua rise demonstrates that sea level not seismic activities played a major role as trigger mechanism for the initiation of gravity flows since the last four glacial interglacial transitions and supports the existence of an extra glacial termination during the early portion of the transition from mis 7 4 to mis 7 3 tiiia

impact of carbonate producing biota on platform architecture insights from miocene examples of the mediterranean region abstract different types of carbonate platforms formed in the mediterranean during the miocene low angle homoclinal types of ramp distally steepened ramps flat topped platforms and reef rimmed shelves the critical differences between these platforms result from differences in the capacities of the carbonate systems to accumulate sediments above hydrodynamic base level ecologically controlled accommodation the various depositional profiles and facies belt distributions resulted from the interplay between different sediment production and redistribution processes and the internal architectures resulted from the response of each type of platform to changes in accommodation heterogeneities driven by high frequency sea level cycles are maximized in platforms ruled by shallow water euphotic framework dominated production and minimized in low angle ramps where sediment resulted from aphotic and oligophotic carbonate production in the mediterranean region there is no direct relationship between type of platform and global temperature as shown by the coeval occurrence of different platforms in similar latitudinal settings although temperature is a key limiting factor other paleoceanographic factors such as trophic resources may have also been along with biological evolution important factors influencing different types of carbonate producing biotas

potentialities uncertainties and complexities in the response of coral reefs to future sea level rise coral islands formed of largely unconsolidated sands only a few metres above sea level are thought to be particularly vulnerable to sea level rise consequent upon global warming however scenarios which predict catastrophic flooding and loss of island area need reassessment particularly in the light of the continued downwards revision of projected rates of future sea level rise revised questions concern the interactions between reef growth and sea level change biophysical constraints on coral growth and the importance to reef systems of potential changes in the magnitude frequency and location of tropical cyclones and hurricanes it is clear that most reefs have the growth potential to meet even the highest of future sea level rise scenarios but too little is known about physiological and physical constraints to reef growth to adequately evaluate the importance of these two factors in constraining this potential at the present time future sea level rise in the tropical oceans and coral reef responses will take place against a backdrop of inter regional differences in holocene sea levels resulting from the varying interaction of eustatic and hydro isostatic processes these differences have generated varying constraints on the development of modern reefs and varying inherited topographies upon which future sea level changes will be superimposed these controls are particularly important in assessing differences in vulnerability to future sea level rise for reef islands in the pacific ocean and the caribbean sea

detrital remanent magnetization in hematite heavy mineral placer layers within channel sandstones of the morrison formation western united states have been shown to contain a very stable accurate record of the jurassic geomagnetic field these laminae are composed almost entirely of hematite grains in particular they are largely the hematite pseudomorph of magnetite martite the laminae record the correct geomagnetic field direction during a period of rapid apparent polar wander which suggests an early acquisition of the remanence several other remanence characteristics imply that the remanance is probably a detrital remanent magnetization drm 1 there is large directional dispersion in one lamina which had been heavily burrowed by organisms measurement of individual burrows shows that the burrowing disrupted and randomized the magnetization held in the martite grains 2 directions of laminae that were deposited on sloping surfaces are significantly deflected from the directions of the flat lying laminae 3 more than 50 of the martite grains have high temperature oxidation features suggesting that they may have been martite at the time of deposition these characteristics in particular the burrowing disturbance strongly suggest that the martite was deposited as martite and that the remanence is a drm as opposed to deposition of magnetite grains which were altered to hematite after deposition examination of recent weathering debris on igneous outcrops shows a substantial amount of martite on these outcrops this observation provides further evidence in support of the idea that hematite and in particular martite can be a deposited mineral in sedimentary rocks and thus can carry a drm

eustatic controls on carbonate facies in reservoirs and seals associated with mesozoic hydrocaron fields of the arabian gulf and the gulf of mexico abstract abundant subsurface data for mesozoic and cenozoic sections of the us gulf coast and the middle east make it possible to track the relationship of shelf carbonates and evaporites associated with minor amounts of clastics to eustasy our contention is that because the sedimentary deposition for both regions was in part controlled by gentle tectonic subsidence punctuated by eustatic variations the major hydrocarbon fields which occur here can be related to sea level behaviur at the time of deposition of the reservoir sections and their early diagenesis with the exception of chalks most of these carbonate hydrocarbon fields can be related to highstand system tracts and include the following 1 1 keep up reservoirs with sheet like geometry formed when carbonate accumulation matched sea level rise aggrading to form shoaling upward cycles during sea level highstands 2 2 give up and catch up reservoirs with lense like carbonate geometry formed on drowned shelves during and following rapid sea level rises often downslope from carbonate margins give up reservoirs occur when carbonate accumulation was unable to match sea level rise and catch up reservoirs form where carbonate accumulation was initially unable to keep pace with the sea level rise but then aggraded to sea level 3 3 reservoirs with the prograded discontinuous clinoform geometry of the platform margin formed during stillstands by carbonate accumulation that not only kept up with the sea level rise but also advanced in a seaward direction source rocks for these carbonate reservoirs are often formed during rapid sea level rises while the reservoir seals are usually transgressive shales dense transgressive limestones and or highstand evaporites

paleomagnetic evidence that the central block of salinia california is not a far traveled terrane new paleomagnetic results from late cretaceous 75 85 m y red beds on the central block of salinia indicate that salinia was located within 6 in latitude of its current cratonal north american position during the late cretaceous after correction for neogene san andreas fault transport the red beds formed as alluvial fan overbank deposits with hematite cement deposited directly on salinian granites in the la panza range paleomagnetic analysis shows two components of magnetization in the red beds a low blocking temperature present day overprint residing in goethite and a high blocking temperature 600 component residing in hematite the hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis the bedding corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions twenty three class i sites 95 20 have an average hematite direction with inclination 54 4 and declination 18 2 95 6 1 after structural correction these paleomagnetic data suggest that salinia resided at about 35 n latitude during the late cretaceous within 6 of its current location adjacent to cratonal north america by contrast a summary of paleomagnetic data from the peninsular ranges terrane and the sur obispo terrane which are currently outboard of salinia shows northward transport of these terranes of 12 to 22 relative to their current locations in north america since the cretaceous the offsets increase systematically away from the craton with the most outboard sur obispo terrane which is composed of accretionary prism and distal forearc material showing the largest degree of northward translation

rock magnetism of deformed upper triassic limestones from the lagonegro basin southern apennines italy the stable component of the natural remanent magnetization nrm of the upper triassic calcari con selce limestone formation at marsico nuovo lagonegro basin southern italy has an inclination which is about 45 too steep for the expected paleogeographic position in order to explain this abnormal magnetization direction several rock magnetic properties such as the dependence of susceptibility on temperature and alternating field frequency anisotropy of low field susceptibility anisotropy of anhysteretic remanent magnetization arm and characteristics of acquisition and thermal decay of isothermal remanent magnetization irm have been investigated strong inverse fabrics of both susceptibility and arm have been recognized which are due to magnetite in the superparamagnetic state on the one hand and in the stable single domain grain size on the other hand although internal deformation in the investigated rocks has not been reported in the geological literature this may be the cause of the observed anisotropies the anisotropy is probably responsible for the anomalous nrm directions of these limestones

evolution of the northern nicaragua rise during the oligocene miocene drowning by environmental factors abstract possible causes to explain platform drowning have been hotly debated by carbonate sedimentologists for more than a decade now in this paper we present multiple evidence to explain the drowning of a carbonate megabank that covered most of the modern northern nicaragua rise nnr during an interval spanning from late oligocene to early miocene by the interaction of several environmental factors the recovery during odp leg 165 of late oligocene to middle miocene sedimentary sequences in the sub seafloor of the modern channels and basin pedro channel and walton basin respectively that dissect the nnr site 1000 and south of the rise in the colombian basin site 999 combined with information from dredged rock samples allows us to explore in more detail the timing and possible mechanisms responsible for the drowning of the megabank and its relationship to miocene climate change the modern system of isolated banks and shelves dissected by a series of intervening seaways and basins on the nnr has evolved from a continuous shallow water carbonate megabank that extended from the honduras nicaraguan mainland to the modern island of jamaica available information suggests that this megabank broke apart and partially drowned in the late part of the late oligocene at around 27 ma and finally foundered during the late early miocene around 20 ma resulting in limited neritic coral growth in the areas where the modern isolated carbonate banks and shelves are occurring today available information also suggests that the southern and central parts of pedro channel were already a deep water area before the major episode of platform drowning and its formation predates the initiation of the caribbean current however after the partial drowning of the megabank the channel has become a major pathway for the caribbean current stratigraphic units identified in deep water carbonates sampled at odp sites 999 and 1000 help to constrain the environmental setting leading to the drowning of the banks changes in lithology and mass accumulation rates of both the carbonate and non carbonate fraction parallel stable isotope shifts and likely indicate regional changes in climate and circulation during the late oligocene middle miocene interval carbonate mass accumulation rates mars at site 999 suggest increased regional productivity during the early miocene terrigenous mars at both sites 999 and 1000 show a general increase from the burdigalian through the serravallian the temporal association among episodes of neritic platform deposition followed by increased productivity as identified by higher carbonate mars and positive excursion in carbon isotopes suggests that oceanographic changes such as local upwelling and nutrification have led to the partial drowning of the nnr megabank

paleoecology and sedimentary environment of the late devonian coral biostrome from the central devonian field russia the upper frasnian coral biostrome well exposed in the russkiy brod quarry central devonian field russia has been studied in detail with respect to paleoecology and sedimentary environment the biostrome formed by auloporid tabulates and solitary and colonial rugose corals originated in an offshore environment characterized by calmer periods with slow or halted sedimentation and more energetic periods when sedimentation rate increased the episodic higher sediment influx and stronger hydrodynamic regime are not only well expressed in the microfacies but also in the variability of colony integration of the rugose corals observed even within single coralla distinct development of constrictions rejuvenescences and deflection of growth directions in rugose corallites may also indicate unstable sedimentary conditions the latter features however may have in part also resulted from syn vivo biotic interaction with the associated auloporids both the facies and paleontological observations suggest that the biostrome originated by the colonization of deposited bioclasts by pioneering auloporids creating the framework for settlement of later generations of auloporids and rugose corals apart from abundant auloporids representing a single species and associated rugose corals the other encrusters are not numerous and poorly diversified represented by dominating foraminifers followed by single species of productid brachiopods stromatoporoids microconchids and cornulitids the coral associated macrobenthos has similarly a low diversity being represented by single species of spiriferid and rhynchonellid brachiopods and gastropods being developed in an offshore carbonate sedimentary system devoid of any organic rich deposits and characterized by extremely low abundance and diversity of suspension feeding organisms the biostrome is considered to have originated in a low productivity oligotrophic environment strictly dominated by heterozoan coral communities its development in a well oxygenated oligotrophic environment during the time when organic rich black kellwasser facies developed elsewhere additionally attests for multi causal scenarios for the frasnian famennian event during which other factors were responsible in different paleogeographic and facies settings

controls and development of late quaternary periplatform carbonate stratigraphy in walton basin northeastern nicaragua rise caribbean sea the late quaternary sedimentary record in periplatform oozes and muds deposited on the northern nicaragua rise results from the interplay of four main controls 1 input of pelagic carbonates and bank derived fine aragonite and magnesian calcite 2 input of siliciclastic sediments 3 dispersal and removal of sediments by the caribbean current and 4 partial seafloor dissolution of metastable carbonates high accumulation rates of the calcite coarse sediment fraction throughout the study area demonstrate that planktonic foraminiferal productivity peaked during interglacial stages neritic carbonate productivity also peaked during interglacial stages when bank and shelf tops along the nicaragua rise were submerged within the photic zone because the bank and shelf tops remained mostly exposed during glacial stages the surface area available for the neritic carbonate productivity was drastically reduced to a narrow band along the margins of pedro bank and the southern shelf of jamaica a large volume of siliciclastic sediments transported from the coastal area of south america and the eastern and western regions of jamaica were deposited during glacial stages within the eastern and western deep extensions of walton basin this finding contrasts with the input pattern of siliciclastic sediments in walton basin itself where the largest input of siliciclastic sediments occurred at each of the interglacial climatic optima corresponding to intervals of maximum sea level transgression finally cores in water depths exceeding 1100 m display during interglacial stages aragonite accumulation rates systematically lower than rates in cores from water depths shallower than 1100 m this indicates that some bank derived aragonite and magnesian calcite has been partially removed by seafloor dissolution in areas from water depths exceeding 1100 m results from this study on the northeastern nicaragua rise indicate that not only do offbank transport and water column saturation state influence the late quaternary record but also that the variable strength of the caribbean current and the proximity of sources for siliciclastic sediments have played a major role in the development of the late quaternary periplatform high resolution stratigraphy

microbial carbonates as indicators of environmental change and biotic crises in carbonate systems examples from the late devonian alberta basin canada microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the proterozoic we report here the varied roles that microbial carbonates played in late devonian carbonate platforms in the alberta basin canada we recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the frasnian famennian mass extinction detailed sequence stratigraphic analysis of two isolated platforms in the canadian rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation aggradation and backstepping and renewed progradation related to rates of second order and third order sea level change and basin infill the carbonate system was reorganized following annihilation of many carbonate producing biota during the frasnian famennian mass extinction microbial carbonates figure prominently in both frasnian platform development and the famennian recovery of the carbonate system following the frasnian famennian mass extinction during transgressive platform phases deeper water facies with a crinoid rugose coral and microbe dominated biota abruptly overlie prograding stromatoporoid framestones microbial carbonates consist of abundant girvinellid oncoids that nucleated around other fossil clasts the temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply microbes played a significant role in highstand reef margin facies where renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral stromatoporoid mounds microbially laminated carbonates also form stromatolitic mats in peritidal shallowing upward cycles and repetitive stromatolitic intervals commonly indicate sea level lowstands associated with the development of sequence boundaries stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats an end frasnian sea level lowstand exposed the isolated carbonate platforms transgression and mass extinction characterize the frasnian famennian boundary event and microbial carbonates occur at this horizon in western canada stromatolites oncoids and large scale microbial thrombolites appear to be

cretaceous history of pacific basin guyot reefs a reappraisal based on geothermal endo upwelling abstract the mid cretaceous histories origin growth death of algal rudist stromatoporoid reef communities located on many pacific basin guyots are complex and controversial these shallow water tropical communities originated on volcanic edifices extruded during the barremian albian grew upward during edifice subsidence transgression throughout the aptian albian and cenomanian and several of them died almost synchronously near the cenomanian turonian boundary during their periods of origin and growth we postulate that the reef ecosystems received dissolved oxygen by wave surge and nutrients by geothermal endo upwelling by this process oceanic waters of intermediate depth approx 500 1500 m were a drawn into the weathered and fractured volcanic summit and lower part of the older reef and driven upward through the porous framework by the remnant geothermal gradient of the volcanic foundation and b emerged at the reef surface to support the high metabolism of the living community the death of most of the reefs near the cenomanian turonian boundary approximately coincides with the most intense oceanic anoxic event oea in pacific ocean history during this oae the chemistry of the endo upwelled fluids arriving at the reef surface changed from nutrient oxygen rich to dysoxic anoxic toxic and killed the community additionally the combination of foundation subsidence and global transgression reduced aeration of interstitial reef waters by wave turbulence the post mortem history of the guyot summits consisted of a drowning to prevent revival of the reef communities b deposition of turonian cenozoic fossiliferous pelagic sediments in the former atoll lagoons and dead reef debris and c minor erosion submarine karstification of the dead reef frameworks

timing and distribution of calciturbidites around a deeply submerged carbonate platform in a seismically active setting pedro bank northern nicaragua rise caribbean sea the sedimentological study of thirteen sediment cores from the periplatform setting surrounding pedro bank northern nicaragua rise caribbean sea shows that during the last 300 ka turbidite deposition is controlled by at least four factors 1 late quaternary sea level fluctuations 2 prolific fine grained sediment production and export resulting in oversteepening of the upper slope environment 3 the proximity to the bank margin and 4 local slope and seafloor morphology the most intriguing finding of this study is the paucity of turbidites with only 101 turbidites in 13 cores in this tectonically active setting near the caribbean plate boundary throughout the last 300 ka the frequency of turbidite input during interglacial stages is three times higher than during glacial stages also it is obvious that changes in sea level influence the timing of turbidite deposition this is especially evident during the transgressions resulting in rapid renewed bank top flooding subsequent neritic sediment overproduction and offbank export the flooding event during each transgression is usually recorded by the onset of turbidite deposition at various sites along several platform to basin transects in down and upcurrent slope settings overall however more turbidites are deposited during the regressive rather than the transgressive phases in sea level probably as a result of sediment reorganisation on the slope resulting in slope failure five cores show highstand bundling of calciturbidites i e higher number of turbidites during highstands than during lowstands in sea level

tide influenced high energy environments and rhodolith associated carbonate deposition on the outer shelf and slope off the miyako islands southern ryukyu island arc japan sediments distributed off the miyako islands southern ryukyus japan include rhodoliths larger foraminifera cycloclypeus and or bryozoan dominated carbonates of low mud content on the outer insular shelf and planktonic foraminiferal mud free carbonate sand on the slopes sidescan sonograms show large scale 2 d subaqueous dunes in the deeper shelf sediments which suggest a nw se flow direction sea bottom photographs of the slope sediments to depths of 1000 m show small scale 2 d or 3 d ripples those sedimentologic features suggest the existence of relatively high speed currents on both the shelf and slope in the area current direction and velocity have been directly measured to clarify the origin and character of the current governing the area where the rhodoliths large foraminifera are distributed the results of measurements carried out at 14 stations at depth ranges of 14 to 799 m show that the direction and velocity of the currents on the shelf and slope change semi diurnally at most shelf stations the currents at high tide are to the north northwest i e from the pacific ocean to the east china sea conversely the currents at low tide are to the south southeast from the east china sea to the pacific ocean on the slope current movements are to the west at high tide and to the east at low tide it is concluded therefore that the currents on the shelf and slopes are tidally related current speed is small in protected areas and is high on the open shelf and shelf slopes the ryukyu island arc interrupts the tidal northwestern and southeastern flow this interaction probably develops substantial turbulence and topography induced upwelling and therefore forms a natural site for the formation of mud free and or coarse sediments with rhodoliths and large formainifera

sediment characteristics in reef areas influenced by eutrophication related alterations of benthic communities and bioerosion processes abstract this study investigated sediment deposits and suspended particulate matter in three reef areas of reunion island characterized by varying benthic communities and bioerosion processes as a result of different eutrophication levels the sediments were sampled using two different methods hand collecting directly on the reef bottom and sampling by traps this approach allowed a comparison between the composition of reefal bottom sediments and suspended particles in locations where algal communities prevailed over corals as a result of nutrification a shift from coral to coralline algae dominated sediments could be observed in addition a decrease in sediment production and a prevalence of very fine sand to mud sized grains over medium to fine sands existed in the nutrient enriched areas this grain size difference probably is caused by a decrease in grazing activity in the enriched areas high proportions of coralline algal debris and sponge spicules were specifically found in the sediments within areas receiving high nutrient input these two diagnostic components were abundant in both the suspended particulate matter and in the fine fractions

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

the importance of changing oceanography in controlling late quaternary carbonate sedimentation on a high energy tropical oceanic ramp north western australia the north west shelf is an ocean facing carbonate ramp that lies in a warm water setting adjacent to an arid hinterland of moderate to low relief the sea floor is strongly affected by cyclonic storms long period swells and large internal tides resulting in preferentially accumulating coarse grained sediments circulation is dominated by the south flowing low salinity leeuwin current upwelling associated with the indian ocean gyre seaward flowing saline bottom waters generated by seasonal evaporation and flashy fluvial discharge sediments are palimpsest a variable mixture of relict stranded and holocene grains relict intraclasts both skeletal and lithic interpreted as having formed during sea level highstands of marine isotope stages mis 3 and 4 are now localized to the mid ramp the most conspicuous stranded particles are ooids and peloids which 14c dating shows formed at 15 4 12 7 ka in somewhat saline waters during initial stages of post last glacial maximum lgm sea level rise it appears that initiation of leeuwin current flow with its relatively less saline but oceanic waters arrested ooid formation such that subsequent benthic holocene sediment is principally biofragmental with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore inner ramp deposits are a mixture of heterozoan and photozoan elements depositional facies reflect episodic environmental perturbation by riverine derived sediments and nutrients resulting in a mixed habitat of oligotrophic coral reefs and large benthic foraminifera and mesotrophic macroalgae and bryozoans indicators holocene mid ramp sediment is heterozoan in character but sparse most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation holocene outer ramp sediment is mainly pelagic veneering shallow water sediments of marine isotope stage 2 including lgm deposits phosphate accumulations at 200 m water depth suggest periodic upwelling or fe redox pumping whereas enhanced near surface productivity probably associated with the interaction between the leeuwin current and indian ocean surface water results in a linear ridge of pelagic sediment at 140 m water depth this ramp depositional system in an arid climate has important applications for the geological record inner ramp sediments can contain important heterozoan elements mid ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m saline outflow can arrest or dramatically slow mid ramp sedimentation mimicking maximum flooding intervals and outer ramp planktic productivity can generate locally important fine grained carbonate sediment bodies changing oceanography during sea level rise can profoundly affect sediment composition sedimentation rate and packaging

variations in relative abundances of goethite and hematite in bengal fan sediments climatic vs diagenetic signals abstract relative abundances of goethite and hematite in marine sediments have been increasingly used in reflectance spectrometry studies to infer the precipitation regime at the source area of detrital sediments this interpretation is contingent on the assumption that the ratio of these minerals is not modified by diagenetic processes our rock magnetic study of the 10 5 4 0 ma interval of the bengal fan sedimentary sequence indicates that this is not always the case we demonstrate that a statistical analysis of isothermal remanent magnetization acquisition curves provides an adequate estimate of the content of goethite and hematite in sediments we also identified distinctive biogenic and detrital components likely titano magnetite or maghemite in the studied samples the sediments of the bengal fan contain a significant change in relative abundance of goethite and hematite at 7 ma which is approximately coeval with a postulated climatic turnover in the sediment source region however systematic variations in the detrital and biogenic components as well as an increase in the total organic carbon content and the appearance of pyrite concretions at the same stratigraphic level suggest that this change in magnetic mineral content reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal when assigning climatic interpretations to changes in the relative abundance of goethite and hematite in marine sediments in future studies the possibility of diagenetic modification should be evaluated with rock magnetic methods alteration of ancient sediments can be detected and the severity of alteration can be estimated based on the presence and grain size variation of the ferrimagnetic fraction magnetite maghemite which is more sensitive to early reductive dissolution than high coercivity minerals goethite and hematite

similarities between planktonic and larger foraminiferal evolutionary trends through paleogene paleoceanographic changes abstract the paleogene evolutionary records of planktonic foraminifera and larger benthic foraminifera show parallel patterns in development that may in part reflect changes in nutrient flux to surface waters following the terminal cretaceous extinctions faunas were dominated by a few cosmopolitan opportunistic species more specialized k strategists in both groups diversified during the paleocene dominating into the early eocene consistent with he globally mild climatic regime and probable limited rates of nutrient flux to surface waters high latitude cooling intensified in the middle eocene promoting diversification of less specialized taxa characteristic of cooler and deeper waters higher latitudes meridional upwelling zones and boundary currents geographic ranges of warm water taxa diminished and fragmented yet many species survived thus global diversities during the middle eocene were the highest in the cenozoic as cooling and increased rates of nutrient flux further intensified in the late eocene extinction rates of warm water planktonic and larger benthic species exceeded origination rates of cooler water and less specialized forms so diversity dropped to intermediate levels where it remained through the late eocene and oligocene previous observations that planktonic foraminiferal faunas dominated by warm water taxa are often replaced by cool water faunas even though oxygen isotopes only recorded relatively minor cooling can be reconciled by the explanation that the biotic communities responded to changes in trophic resources and euphotic habitats that accompany the temperature reduction as well as to temperature change itself

sedimentology and acoustic mapping of modern rhodolith facies on a non tropical carbonate shelf gulf of california mexico abstract rhodolith facies are characterized by an abundance of free living coralline red algae and are a common feature of modern and ancient carbonate shelves worldwide rhodolith communities contribute significantly to the global calcium carbonate budget and fossil rhodoliths are commonly used to obtain paleoecologic and paleoclimatic information few attempts have been made to quantify the spatial extent of modern rhodolith facies despite their importance and common occurrence combining sediment sampling with acoustic facies mapping this study provides the first set of quantitative data on rhodolith facies distribution in the warm temperate southwestern gulf of california mexico though rhodoliths were the main carbonate producers other important calcareous biota were bivalves 19 bryozoans 13 and corals 6 based on cluster analysis biota were grouped into a rhodolith a rhodolith bivalve coral and a bivalve bryozoa biofacies the acoustic seafloor classification distinguished four acoustic facies which were confirmed by ground truthing an outer shelf silt facies a biogenic sand facies a peripheral rhodolith facies and a central rhodolith facies rhodolith facies covered approximately 40 of the surveyed seafloor 45 km2 results from quantitative analysis of sediment samples and acoustic mapping showed a significant correlation between grain size biofacies and acoustic seafloor facies we successfully applied an acoustic device to provide highly resolved continuous coverage of the seafloor and discriminate modern rhodolith facies from surrounding sediment this has important implications for quantifying rhodolith carbonate production in other regions as well as for ecological and conservation studies

marine biota response to clastic sediment influx a quantitative approach abstract there have been few quantitative studies of the effects of siliciclastic and volcaniclastic sediment influx on carbonate producing organisms from the geological record this study presents the results of quantitative field petrographic and acid digestion analyses of samples from three cenozoic shallow water mixed carbonate volcani siliciclastic depositional environments the miocene wonosari formation of south java the miocene batu putih limestone of east borneo and the eocene collsuspina limestone of northeast spain carbonate production in these systems is dominated by larger benthic foraminifera coralline algae and corals analyses of the quantity grainsize and type of volcani siliciclastic sediment within these mixed successions have allowed assessment of the influence of these factors on shallow water carbonate producers larger benthic foraminifera and coralline algae exhibit the greatest tolerance to siliciclastic sediment influx abundant up to 55 and 38 wt respectively at a range of grain sizes to coarse sand platy corals are also present within sediments containing a large proportion of insoluble material geologically averaged rates of clastic accumulation and their potential influences on carbonate producers inferred herein may be difficult to compare directly with the daily and seasonal fluctuations experienced by modern clastic influenced systems however this study may aid our understanding of the likely effects of increased clastic influx today commonly anthropogenically related to marine carbonate environments over longer time periods

carbonate platforms along the southeast bahamas hispaniola collision zone the southeast bahama banks have been colliding with the island of hispaniola along a restraining bend within the northern caribbean plate boundary zone since the late miocene a seamarc ii seismic reflection investigation was conducted along the deep water flanks of mouchoir bank and navidad bank to evaluate the response of these carbonate platforms to active plate tectonic collision overall our data provide evidence of large scale platform margin retreat which contrasts sharply with the prograding platform margins in the tectonically passive northwest bahamas north of silver bank there is a large 5 000 km2 deep water 3 000 4 000 m plateau that records the drowning and step back 50 km of an early cretaceous carbonate platform margin intact uneroded segments of this cretaceou platform margin may occur along the seaward edge of the deep water plateau bahama escarpment separated by large 20 km across erosional reentrants a coalescing system of broad up to 10 km shallow 200 m relief canyons akin to mountainous avalanche chutes funnel debris into the reentrants from the flank of silver bank the reentrants are interpreted as zones of structural weakness produced by passage of the atlantic spreading center along a transform plate boundary during the late jurassic early cretaceous drowning and step back appear to be part of a global response of carbonate platforms to a mid cretaceous tectonic pulse of ocean crust formation and relative sea level rise the southern margin of mouchoir bank has also been drowned and has stepped back by about 30 km this event occurred during the late tertiary prior to the mid pliocene and is attributed to tectonic tilting and subsidence resulting from oblique underthrusting of the southeast bahamas beneath hispaniola the southern margin of mouchoir bank is also characterized by large 20 km across amphitheater shaped scallops that expose tertiary shallow water limestones along their deep water flanks seaward of one scallop is a large 25 km wide 50 km long mass movement with flow lines preserved on the seafloor we interpret these scallops as catastrophic collapse structures triggered by large earthquakes generated along the collision zone since the late tertiary south of navidad and silver banks is a very complex seafloor morphology resulting in part from the accretion of a portion of the bahamas onto hispaniola there is also a series of e w oriented down to basin normal faults that have caused the platform margin to retreat extensional stress along this predominantly transpressive plate boundary may be a consequence of rotation of the nearby puerto rico block or lithospheric bending during oblique subduction this study provides an initial framework within which to view the actualistic response of carbonate platforms to plate tectonic collision

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

facies architecture and high resolution sequence stratigraphy of an upper cretaceous platform margin succession southern central pyrenees spain abstract excellent exposures of upper cretaceous santonian carbonate platforms on the northern flank of sant corneli anticline southern central pyrenees provide detailed information of facies architecture in both depositional strike and dip directions basic accretional units are differentiated by facies contrast across mappable surfaces these surfaces do not show clear evidence of subaerial erosion and are correlated basinward with bedding planes across which there are subtle changes in skeletal composition two types of basic accretional units have been identified based on bedding patterns internal facies architecture and skeletal composition 1 rudist buildups consist of a rudist and coral belt at the platform margin passing landward into a slender hippuritid lithosome locally overlain by a bioclastic blanket that passes basinward into bioclastic apron like clinobeds and into fine grained packstone wackestone 2 calcarenite wedges consist of yellow brown benthic foraminifer rich grainstones to grain dominated packstones with scattered rudist shells and small coral colonies passing basinward into mud dominated packstones to wackestones with variable siliciclastic content quartz sand to silt and clay rudist buildups and calcarenite wedges alternate although not rhythmically these changes in platform skeletal composition reflect changes in the dominant type of carbonate producing biota independently of the changes in accommodation both types of basic accretional units rudist buildups and calcarenite wedges form simple sequences and parasequences according to internal lithofacies arrangement and inferred sea level cyclicity cycles or paracycles high frequency sea level cyclicity fits in the milankovitch frequency band long term changes in accommodation governing aggradation progradation and backstepping of basic sequences and parasequences reflect tectonic influence rather than long term changes in eustatic sea level

sand and rhodolith gravel entrainment on the mid to outer shelf under a western boundary current fraser island continental shelf eastern australia abstract carbonate sediments on the east australian continental shelf off fraser island represent a transition from the warm water tropical carbonate deposits in the north i e the great barrier reef to cool water temperate carbonate deposits in the south on the outer shelf in 40 140 m water depth these deposits are characterised by pebble to cobble sized rhodoliths current meter data obtained from this area demonstrates that near bed southward flowing currents of up to 130 cm s occur episodically to depths of over 70 m these current events may last for several days and are probably related to intrusions of the east australian current a western boundary current onto the shelf flume experiments on different size classes of rhodoliths suggests that the threshold speed referenced to 100 cm above the bed u 100 cr for 10 mm diameter rhodoliths is of the order of 45 cm s whereas u 100 cr for 50 mm diameter rhodoliths is of the order of 80 cm s therefore the shelf currents measured are competent to initiate rhodolith movement percentage exceedence of these threshold speeds ranges from 1 to 10 surface waves also generate nearbed oscillatory flows which when combined with the steady current are considered to be important in initiating movement of the rhodoliths

cryptic encrusting communities in a middle devonian mesophotic paleoenvironment of the holy cross mountains poland abstract the middle devonian givetian chaetetid sponges and alveolitid tabulate corals that inhabited mesophotic paleoenvironments of the southern shelf of laurussia holy cross mountains poland have been studied with respect to encrustation patterns these epibiotic communities are characterized by modest diversity and high dominance and show distinct polarization with respect to the exposed and cryptic sides of the host s skeletons the most abundant davidsoniid brachiopods and microconchids show strong preferences toward the lower cryptic sides while auloporid tabulate corals dominate on the exposed sides of the hosts the remaining and much rarer epibionts cornulitids and rugose corals show no clear preferences in some shallow water silurian paleoenvironments auloporids are known to dominate the cryptic sides of the hosts thus considering the auloporid tabulates as photosymbiotic corals their absence or rarity in the middle devonian cryptic habitats might have resulted from insufficient irradiance levels in otherwise depleted light conditions as a result the available space and lack or lower competition for the area with faster growing colonial auloporids enabled solitary encrusters to colonize the cryptic niches the scarce bioerosion traces and rather low diversity of epibionts in our case may indicate that these mesophotic reefs developed in a nutrient poor oligotrophic habitats the davidsoniid dominated cryptic encrusting communities were widespread in some paleoenvironments during the middle devonian being known from both laurussia and south china craton however such mesophotic cryptic communities characterized by distinct polarization in comparison to open surface environments contrast with other givetian deep water aphotic cryptic communities reported so far where such polarization is hardly visible

chemical remanent magnetization due to deep burial diagenesis in oolitic hematite bearing ironstones of alabama abstract oolitic hematite bearing ironstones of the silurian red mountain formation of alabama are shown to carry a single component remanence stable enough to have survived major folding of probable permian age nevertheless the remanence direction ten sites yielding a paleopole at 38 0 n 132 4 e with dm 3 6 dp 1 9 its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the pennsylvanian some 130 ma after deposition this remanence is likely a chemical remanent magnetization crm acquired during diagenesis induced by heating due to deep burial under a pennsylvanian clastic wedge two possible mechanisms for acquisition of crm during deep burial diagenesis are considered in hypothesis i the oolitic hematite transformed from original geothite when heated to about 80 c acquiring crm in hypothesis ii the oolitic hematite originated from ferrihydrite and was too fine grained to acquire stable crm until heat raised the solubility of hematite allowing grain growth hypothesis i explains the timing of remanence acquisition better but there is some evidence that oolitic goethites may be stable to considerably more than 80 c hypothesis ii has some difficulty explaining preliminary paleomagnetic results from oolitic hematite bearing ironstones of the silurian clinton group new york state we prefer hypothesis i but both hypotheses remain plausible both hypotheses warn that continental red beds may also acquire crm during diagenesis induced by deep burial heating long after deposition but before folding

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

ecological control of sedimentary accommodation evolution from a carbonate ramp to rimmed shelf upper miocene balearic islands abstract stratal patterns and facies architecture develop in response to sediment flux and the available space for sediment accumulation accommodation in siliciclastic systems these two parameters are independent in carbonates however they are highly interdependent 1 sediment input production highly depends on biological systems and consequently on intrabasinal conditions nutrients temperature salinity etc 2 sediment dispersal depends on the production loci and on the interaction between the amount and type of sediment being produced and the hydraulic energy additionally biological processes binding baffling framework construction and even cementation frequently modify it 3 accommodation is not an independent factor with respect to sediment input type efficiency and area of the carbonate factory depend on sea level changes and sea floor morphology the base level for sediment to accumulate accommodation depends on the type and locus of sediment being produced the upper miocene carbonate platforms of the balearic islands illustrate how changes in intrabasinal environmental conditions nutrients and or temperature produce changes in stratal patterns and facies architecture if they affect the biological system during early tortonian carbonate production occurred both in the shallow euphotic zone foramol and in the deeper oligophotic zone rhodalgal originating a distally steepened ramp during late tortonian and early messinian carbonate production mainly occurred in the euphotic zone where framework producing biota coral reefs formed a rimmed platform although deposited under similar conditions of high frequency sea level fluctuations these two types of platforms exhibit different internal facies architecture and distribution of heterogeneities because of differences in base level in carbonates base level for sediment accumulation depends on the type and amount of sediment being produced on the production loci and on the processes controlling sediment dispersal base level for the loose foramol rhodalgal sediment associations of the ramp was controlled by wave and storm base and the associated currents whereas base level for the framework dominated reef complex was sea level furthermore the increase on effective accommodation space to allow the reef rimmed shelf to prograde onto the distally steepened ramp both platforms being depositional sequences resulted from an ecological change rather than significant relative sea level change namely by the shift from grain to framework producing biota the euphotic framework producing biota was able to fill the accommodation space existing between wave and storm base level and sea level that was not previously filled by the loose bioclasts

the influence of terrestrial run off on marine biotic communities an example from a thrust top carbonate ramp upper pennsylvanian foreland basin picos de europa nw spain article i nfo high diversity marine biotic communities characterize the lower kasimovian upper pennsylvanian carbonates of the las llacerias formation in the andara massif picos de europa province cantabrian mountains spain the carbonates accumulated on a small thrust top ramp system within the shallow low energy and euphotic realms biotic communities were characterized by abundant photozoans calcareous algae and cyanobacteria and heterozoans including filter and suspension feeders calcareous and siliceous sponges bryozoans brachiopods crinoids diverse other metazoans rugose solitary and colonial corals tabulate corals bivalves gastropods and echinoids benthic foraminifers including very abundant small incrusting tubiforms and conspicuous swarms of cement filled spheres calcispheres or penella small sized buildups of chaetetid sponges syringoporids branching rugose corals anthracoporella and phylloid algae commonly developed in these shallow water environments meanwhile mud mounds formed at greater water depths mostly below the lower limit of the phothic zone the occurrence of such diverse biotic communities and the grain composition of the shallow water deposits not recorded in the platform top successions of the antecedent bashkirian moscovian carbonate shelf valdeteja and picos de europa formations are interpreted as linked to increased nutrient levels mesotrophic to mildly eutrophic conditions in the actual sea surface water the nutrients are inferred to have resulted from land derived fluxes from the rivers draining the adjacent growing variscan orogenic chain nutrients were subsequently transported into the narrowed marine shallowing foreland basin located in the eastern side of pangea within the palaeoequatorial humid belt comparison to the pennsylvanian auernig group in the carnic alps allows the discrimination between the influence of land derived versus upwelling derived nutrients providing a useful tool in the interpretation of biotic assemblages in the fossil rock record

environmental factors influencing skeletal grain sediment associations a critical review of miocene examples from the western mediterranean the presence of foramol rhodalgal and bryomol skeletal grain associations in ancient shallow marine limestones is commonly interpreted as evidence for non tropical palaeoclimate despite temperature being only one of several factors influencing skeletal grain associations such interpretations neglect the multitude of factors other than temperature that influence carbonate producing biota these include nutrients water energy water transparency depth of the sea floor salinity oxygen ca2 and co2 concentrations mg ca ratio alkalinity substrate requirements competitive displacement as well as biological and evolutionary trends this uniformitarian approach also disregards the probability that conditions of present day biological systems may not be representative of past conditions of analogous systems here the importance of considering these other factors is illustrated through two examples of carbonate platforms in the western mediterranean these platforms are dominated by foramol rhodalgal and bryomol associations of miocene age in spite of having formed in tropical conditions the platforms discussed are 1 the lower tortonian ramp on menorca balearic islands and 2 the lower middle miocene ramps of the central apennines italy evidence for tropical conditions in the mediterranean during the period of growth of these platforms is provided by species of red algae and larger foraminifera by data from coeval continental basins and by global oxygen isotope data

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

oyster shells as recorders of short term oscillations of salinity and temperature during deposition of coral bioherms and reefs in the miocene lorca basin se spain abstract we use 18 o and 13 c of oyster shells inferred seawater 18 o w and temperature tolerance ranges of modern zooxanthellate coral communities to constrain and model paleo salinities of marginal marine waters of the miocene lorca basin spain salinities ranged from 22 to 32 psu during growth of coral bioherms and carpets of the soriana and parrilla formations and from 35 to 39 psu during deposition of the hondo formation reef complex the periods with lower modeled salinities correspond with deltaic and alluvial fan sedimentation in the area the modeling is supported by 87 sr 86 sr data and provides new insights into the evolution of the local depositional settings departures of sr isotope values from the global oceanic curve occur within the intervals with the lowest modeled salinities and the non oceanic 87 sr 86 sr preserved in individual oyster shells was probably sourced from uplifted aquitanian burdigalian limestones 87 sr 86 sr values that plot along the global ocean sr isotope curve for the late miocene indicate a tortonian age for the upper part of the soriana formation and the parrilla formation and a messinian age for the hondo formation more generally we demonstrate how the chemical analyses of oyster shells provide additional information regarding short term oscillations of salinity and temperature of seawater beyond what can be obtained by traditional paleoecological methods