alkane terpene and polycyclic aromatic hydrocarbon geochemistry of the mackenzie river and mackenzie shelf riverine contributions to beaufort sea coastal sediment abstract to study the largest source of river sediment to the arctic ocean we have collected suspended particulates from the mackenzie river in all seasons and sediments from the mackenzie shelf between the river mouth and the shelf edge these samples have been analyzed for alkanes triterpenes and polycyclic aromatic hydrocarbons pahs we found that naturally occurring hydrocarbons predominate in the river and on the shelf these hydrocarbons include biogenic alkanes and triterpenes with a higher plant peat origin diagenetic pahs from peat and plant detritus petrogenic alkanes triterpenes and pahs from oil seeps and or bitumens and combustion pahs that are likely relict in peat deposits because these components vary independently the season is found to strongly influence the concentration and composition of hydrocarbons in the mackenzie river while essentially the same pattern of alkanes diagenetic hopanes and alkyl pahs is observed in all river and most shelf sediment samples alkane and triterpene concentration variations are strongly linked to the relative amount of higher plant peat material polycyclic aromatic hydrocarbon molecular mass profiles also appear to be tied primarily to varying proportions of peat with an additional petrogenic component which is most likely associated with lithic material mobilized by the mackenzie river at freshet consistent with the general lack of alkyl pahs in peat the higher pahs found in the river are probably derived from forest and tundra fires a few anthropogenic pyrogenic compounds are manifest only at the shelf edge probably due to a weakening of the river influence we take this observation of pyrogenic pahs and the pronounced source differences between two sediment samples collected at the shelf edge as evidence of a transition from dominance by the mackenzie river to the geochemistry prevalent in arctic regions far removed from major rivers

oolitic sandbody depositional models and geometries mississippian of southwest britain implications for petroleum exploration in carbonate ramp settings abstract a 1000 m thick early mississippian carbonate supersequence the carboniferous limestone of southwest britain consists of three third order depositional sequences these comprise parasequences in various configurations and the whole forms a carbonate ramp stack within this framework five major oolitic carbonate sandbodies developed a castell coch limestone b stowe oolite c brofiscin oolite d gully oolite and e high tor limestone the depositional regime was storm and wave dominated throughout and the major sandbodies represent a range of progradational carbonate beaches barriers and detached subtidal shoals analysis of the three dimensional shapes and distribution of these five examples shows that they evolved to produce three major carbonate sandbody geometries a strings b sheets and c wedges these geometries are characterised using the five field examples and offered as a template which may assist in the exploration and reservoir modelling of petroleum rich high energy ramp systems progradation for up to 40 km of barrier islands stowe oolite and beach ridge plains gully oolite formation generated strings and thick sheets individually up to 10 20 m thick thin shoreface retreat carbonate packstone grainstone sheets up to 5 m thick high tor limestone developed during transgressions as veneers across flooding surfaces these are comparable with sheet sands developed in siliciclastic shelf depositional systems progradation for up to 30 km and vertical aggradation of shoreline detached oolite shoals castell coch limestone brofiscin oolite generated basinwards expanding or thinning wedges up to 30 m thick tectonically controlled stacking of strandplain sheets produced a composite carbonate sandbody up to 80 m thick gully oolite the intrinsic sedimentary and extrinsic eustacy tectonism climate factors which controlled these sandbody geometries are addressed establishing the positions of the sandbodies accurately within depositional sequences allows them to be located within inferred seismic sequence geometries and provides one possible solution to the difficult problem of predicting carbonate facies distribution in subtle stratigraphic plays in this ramp system the most homogeneous sandbodies up to 30 m grainstones with greatest reservoir facies potential are represented by shoal belt wedges potential grainstone reservoir facies in the prograding shorelines are limited to the upper parts of individual shoreface sequences max 10 m grainstones for shoreline carbonate sandbodies the greatest reservoir and stratigraphic trapping potential exists in the earliest ramp parasequences where enveloping offshore sediments are siliciclastic mudstones in later stages potential seals are likely to be less reliable low porosity outer ramp carbonates

the foreslope and toe of slope facies of the middle triassic latemar buildup dolomites northern italy abstract the latemar buildup was a circular carbonate buildup 4 km wide with a central platform core flat bedded interior platform and massive reef margin flanked on all sides by slope facies steeply dipping 30 35 foreslope breccias are present adjacent to the margin and flat lying graded grainstones at the toe of slope slope facies relate directly to depositional profile and slope angle the steeply dipping foreslopes consist of lobate breccia beds that are 25 m thick and a few tens of meters across and extend tens to hundreds of meters downslope some beds are conformably overlain by thinner 1 m thick beds of finer grained carbonate sediment the basal surfaces of the breccias are erosional and are anastamosing in both strike and dip views the breccia rains was derived principally from margin boundstones and deposited by rockfalls and avalanches planar clinoforms extend the entire height of the foreslope hundreds of meters and bracket depositional units clinothems tens of meters thick clinoforms appear to be shear surfaces formed during large slope failures avalanches graded grainstone beds less than 1 m thick are present at the nearly flat lying toe of slope these consist of redeposited shallow water platform interior and reef margin carbonate sands some with nodular limestone caps that are interpreted as proximal carbonate turbidites toe of slope breccias are the downdip extensions of foreslope breccias and pinch out abruptly basinward some slope depositional processes are related to high frequency fourth order and fifth order sea level changes toe of slope graded grainstones correspond to times of platform submergence in contrast foreslope breccia was deposited during both platform submergence and exposure the slope deposits do not record the high frequency cyclic rhythms identified in shallow water platform sections this is attributed to the nature of slope deposition downslope talus transport was episodic and localized graded grainstone beds reflect storm redeposition

distribution and transport of hydrocarbons in surface sediments of the alaskan outer continental shelf abstract surface sediments from the outer continental shelf of alaska were analyzed for hydrocarbons as part of an environmental survey sponsored by noaa blm sediments were collected from the proposed oil lease areas of beaufort sea southeastern bering sea norton sound navarin basin gulf of alaska kodiak shelf and lower cook inlet data on normal and branched alkanes and di and triterpenoids from capillary gas chromatography gc and gas chromatography mass spectrometry gc ms indicate that the area displays little evidence of petroleum hydrocarbons i e seeps except at a few isolated stations beaufort sea sediments have the highest hydrocarbon budget whereas kodiak shelf sediments have the lowest the molecular markers of the hydrocarbons are of a mixed marine autochthonous and terrestrial allochthonous origin norton sound and cook inlet sediments contain the highest levels of terrigenous lipids and kodiak shelf the lowest the abundance of alkenes implies that relatively low oxidizing conditions exist in some of the depositional areas studied suggesting that extraneous petroleum hydrocarbons introduced into these sediments would be expected to have a relatively long residence time the distribution of polynuclear aromatic hydrocarbons pah is complex and shows a pyrolytic origin the data on perylene strongly favor the hypothesis that it is performed from terrigenous precursors and transported into the marine environment with no apparent in situ generation polynuclear aromatic hydrocarbons of diagenetic origin are probably dominant in gulf of alaska and kodiak shelf sediments two stations north of kalgin island in lower cook inlet and one in southeastern bering sea show typical weathered petroleum distribution of n alkanes and triterpenoids probable pathways of transport and fate of petroleum hydrocarbons in case of an oil spill are also briefly discussed

the occurrence distribution and sources of polycycic aromatic hydrocarbons in the sediments of the georges river estuary australia concentrations of 13 polycylic aromatic hydrocarbons pah in sediment samples from the georges river estuary n s w have been determined by gas chromatography mass spectrometry gc ms the concentrations of individual pah were in the range of 0 1 5600 g kg m dry weight the average values of pah i i 1300 g kg l dry weight are well above those previously found in other industralized urban areas of australia the highest total pah concentrations in sediments are found in a section of the river which also has the highest concentrations of silt clay and organic matter indicating that the pah distribution may be controlled by sedimentation processes examination of the alkylated and non alkylated pah distributions in sediment extracts suggested that the main source of pah is from combustion products possibly associated with air particles with point source contributions of two ring pah from marinas and other boating activities at some sites

hydrocarbon geochemistry of the puget sound region iii polycyclic aromatic hydrocarbons in sediments abstract polycyclic aromatic hydrocarbon pah distributions and sources are characterized in 96 sediment samples from 24 210 pb dated cores collected at locations in the greater puget sound the highest pah concentrations are found within a few kilometers of several sources including industrial facilities in northern puget sound urban areas in central puget sound and river systems draining coal bearing strata regional patterns of combustion derived pah in surficial sediments indicate little atmospheric or waterborne exchange of pah between different regions of the sound significant subsurface maxima in combustion derived pah concentrations 210 pb dated at the 1950s occur only in sediment cores collected near urban centers perylene apparently derives from erosion of a terrestrial source with little or no evidence of in situ production at depth in sediment cores coal fragments are carriers of a characteristic suite of alkylated phenanthrene chrysene and picene derivatives concentrated near river mouths in central and southern puget sound

the latemar a flat topped steep fronted platform dominated by microbialites and synsedimentary cements abstract facies analyses on the latemar isolated carbonate platform suggest a higher proportion of microbial carbonates compared to literature this is evident especially in the margin upper slope zone where microbial boundstone associated with tubiphytes calcareous sponges botryoidal aragonite and radiaxial fibrous cements extended down to 250 m water depth the latemar flat topped high relief platform has been subdivided into five different facies belts inner and outermost platform margin upper and lower slope each facies belt represents subtidal environments except for the inner platform which is characterized by a peritidal facies association point counting analysis was carried out on ca 90 thin sections to quantify the different platform components microbialite and cements are most abundant they represent respectively ca 40 and ca 34 of the whole platform volume these results suggest that carbonate production at latemar occurred in a m factory microbial carbonates as well as early marine cements are most abundant on the upper slope margin and outermost platform although microbialites are found in all facies belts the presence of supratidal sedimentary facies restricted only to the inner platform and of a completely subtidal outermost platform suggests a new depositional profile for the latemar the central portion is occupied by the flat platform interior cyclically subaerially exposed facies of the subtidal outermost platform surround the inner platform and display a progressive increase in depth toward the margin the margin is submerged and essentially constituted by microbial and cement boundstone the slope is characterized by steep clinoforms 35 dominated by in situ microbialites in the upper middle portion 0 250 m depth evidently precipitated by non light dependent microbially influenced processes the lower slope is mostly detrital poor in platform top derived sediments and dominated by margin and upper slope derived rudstone and grainstone all these observations are in agreement with the slope shedding model observed in the pennsylvanian microbial margin in asturias n spain and allow the interpretation of the latemar as a flat topped steep fronted platform dominated by microbialites and cement boundstones

emplacement of the cuera and picos de europa imbricate system at the core of the iberian armorican arc cantabrian zone north spain new precisions concerning the timing of arc closure the final stages of the variscan orogeny mostly carboniferous in the western european variscan belt involved the development of the iberian armorican arc which is cored by the cantabrian zone nw iberia the cantabrian zone is the foreland of the western european variscan belt and it is interpreted to record the waning stages of the closure of the rheic ocean the distalmost tectonic units within the cantabrian zone the cuera unit and the picos de europa province were the last tectonic units emplaced at the core of the western european variscan belt orocline together they form an imbricate system and associated wedge top basins that are key to understanding the development of the orocline the emplacement of the cuera unit and the picos de europa province occurred in the latest pennsylvanian between moscovian and gzhelian times new detailed mapping together with stratigraphic sedimentologic and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the cantabrian variscan foreland fold and thrust belt most probably during the oroclinal bending of the sw european variscan belt that formed the iberian armorican arc the n s directed imbricate system was characterized by a shallow dip of the topographic surface 150 15 km and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying gondwana lithosphere and may reflect the closure of the iberian armorican arc

environmental control on granular clinoforms of ancient carbonate shelves the purpose of this paper is to document the influence of depositional environments on shallow water low relief clinoforms from the description of five ancient carbonate platforms the neoproterozoic namibia middle jurassic france lower cretaceous france upper cretaceous oman and miocene turkey these examples have been investigated on the basis of field observations the clinoforms are described with reference to geometric and compositional attributes declivity shape height sedimentary structures sediment fabric and components the results show great variability in stratal geometry declivity and facies distribution 1 depositional profiles vary from exponential to sigmoidal to oblique 2 maximal slope angles range from 3 to 25 most of them being grouped between 10 and 18 3 facies differentiation identified from lateral facies successions along beds and vertical facies successions through beds is pronounced to subtle this study documents linkages between depositional environments and clinoform attributes proximal shallow clinoforms display round edged exponential profiles sediment deposition has resulted from unidirectional currents in the upper convex section and storm generated oscillatory currents in the lower concave part the sediment fabric changes gradually along this type of clinoform there is little vertical facies differentiation through these clinobeds which have formed from a continuous amalgamation of deposits by contrast distal clinoforms shelf break distally steepened ramp settings yield a much broader spectrum of profiles and are generally shorter and steeper sedimentary structures in gravel sized deposits of the upper slope indicate pure traction by unidirectional currents conversely marks of oscillatory flows undular wavy top bounding surfaces of clinobeds are common in the lower slope intercalation of massive fine grained deposits suggests offshore transport of carbonate mud by suspension each distal clinobed represents a single flow event accordingly facies differentiation is weak laterally but may be pronounced through the clinobeds our study suggests that low relief forms of proximal shallow environments which contain coarse grained and photo independently produced debris record hydrodynamic equilibrium profiles whereas the higher relief forms of this setting rather reflect a high differential production rate of carbonate sediment with water depth the carbonate sediment of the distal clinobeds mainly derives from skeletal production by oligophotic and photo independent biota of the middle shelf ramp and upper portion of the clinoforms the contribution by in situ skeletal biota only becomes significant on the lower slope indicating that the distal submerged slopes of carbonate platforms are not organically but hydrodynamically generated our compilation shows that the slope angles of shallow marine low relief clinoforms do not simply correlate to the sediment grain size and fabric in contrast to what has been documented for the high linear slope profiles this difference stems from the depositional settings namely the involved transport mechanisms low relief clinoform accretion seems to be dominantly influenced by wave induced sediment transport in contrast to linear flanks of high relief clinoforms that build to the angle of repose and for which gravity is the primary transport process

dynamics of tethyan carbonate platforms tethyan carbonate platforms are relatively short lived depositional systems which were born developed died and sometime resurrected in a tectonically active area in contrast with pericratonic platforms which are generally huge in size hundreds of kilometers across and show wide facies belts very gentle slopes ramps and transitional boundaries tethyan carbonate platforms are smaller tens of kilometers and have steep slopes generally faulted boundaries and abrupt facies changes it appears clear that the overall control on these two contrasting systems is the geodynamic milieu relatively stable in the first case where eustatic oscillations and long term subsidence are probably the main dictators quite dynamic and aggressive in the second one where carbonate platforms developed through the entire geodynamic spectrum rifting transtension transpression and collision of the wilson cycle primary controls on tethyan carbonate platforms appear to be tectonics most important and eustatic sea level oscillations which determine their emplacement growth and decay their overall shape and architecture are a function of these factors secondary factors affecting tethyan carbonate platform evolution are physiography oceanographic conditions and biology as a function of geologic time especially across the mesozoic tertiary boundary facies and facies distribution and to a lesser extent their shape are controlled by thesemore variables less

progradation geometries of carbonate platforms examples from the triassic of the dolomites northern italy various types of progradation of triassic carbonate platforms are described from the dolomites of the southern alps the internal and external geometric relationships are exposed in spectacular natural sections and moreover their scale 500 1000 m of thickness is such that they can be compared with features found in seismic profiles the different types of progradation are controlled by a number of factors which normally interact with each other these factors include rate of basinal sedimentation rate of subsidence width of the platform depth of the surrounding basin and eustatic variations of sea level progradation is not a continuous process but episodic moments of massive debris input during which the platform advances alternate with long periods of negligible progradation during which basinal sediments accrete and onlap the toe of slope upper boundary relationships of the prograding platforms include offlap toplap and erosional truncation lower boundary relationships are horizontal climbing and descending progradations a variety of phenomena and circumstances have caused the cessation of progradation of the triassic platforms they include volcanism collapse of margins drowning rapid relative rise of sea level subaerial exposure relative fall of sea level and probably a natural decay of the system in the triassic of the dolomites two main progradation models can be put forward in the ladinian model progradation took place simultaneously with aggradation relative rise of sea level whereas the characteristic feature of the carnian model is toplap relative stillstand of sea level

deep water asymmetric cycles and progradation of carbonate platforms governed by high frequency eustatic oscillations triassic of the dolomites italy the basinal san cassiano formation triassic dolomites italy is interfingered with clinostratified megabreccia slope deposits of coeval carbonate platforms and to a large extent is composed of metre scale thickening coarsening upward cycles these asymmetrical cycles often representing bundles of five coarsening upward sequences are interpreted as platformbasin interactions governed by fourth and fifth order eustatic oscillations according to this model progradation of triassic platforms of the dolomites occurred mainly during fourthorder sea level lowstands

depositional facies and stratal geometry of an upper carboniferous prograding and aggrading high relief carbonate platform cantabrian mountains n spain seismic scale continuous exposures of an upper carboniferous bashkirian moscovian carbonate platform n spain provide detailed information about the lithofacies and stratal geometries quantified with differential global positioning system measurements of microbial boundstone dominated steep prograding and aggrading platform margins progradational and aggradational platform to slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable high relief depositional systems the bashkirian is characterized by rapid progradation at rates of 415 970 m my 1 characteristic outerplatform facies are high energy grainstones with coated intraclasts ooids and pisoids moderate energy algal skeletal grainstones to packstones and lower energy algal packstone and boundstone units the moscovian aggradational phase is characterized by aggradation rates of 108 m my 1 coated grain shoals are less common whereas crinoidal bars nucleated in well circulated settings below wave base boundstones form a belt 30 300 m wide at the platform break and interfinger inwards with massive algal skeletal wackestones mud rich banks the progradational phase has divergent outer platform strata with basinward dips of 12 to 2 steep clinoforms with dips of 20 28 are 650 750 m in relief and possibly sigmoidal to concave in the lower part the basinward dipping outer platform strata might be depositional for less than 6 consistent with lithofacies deepening seaward the basinward dip is attributed to the downward shift of upperslope boundstone forced by late highstand and relative sea level fall and to compaction induced differential subsidence during progradation the aggradational phase is characterized by horizontally layered platform strata clinoforms steepen to 30 45 reaching heights of 850 m and are planar to concave the evolution from progradation to aggradation at the bashkirian moscovian boundary is attributed to increased foreland basin subsidence and decreased boundstone accumulation rates progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top within the major phases aggradational progradational increments are produced by third to fourth order relative sea level

stratigraphic response of a carbonate platform to relative sea level changes broken ridge southeast indian ocean in contrast to terrigenous margins the stratigraphic response of carbonate platforms to relative sea level changes is influenced by carbonate productivity dissolution and diagenesis using seismic reflection and ocean drilling program odp drilling data from broken ridge we assess the importance of these processes in controlling the stratigraphic response of a carbonate platform to relative sea level changes prior to middle eocene rifting broken ridge and kerguelen heard plateau constituted a large carbonate platform odp drilling data from the kerguelen heard plateau leg 120 and broken ridge leg 121 reveal that the carbonate platform had two major episodes of tectonic uplift the early maastrichtian uplift was confined to the southern section of the kerguelen eard plateau sites 747 748 and gently tilted the platform toward the north the middle eocene rift induced uplift affected both broken ridge and the northern section of the kerguelen heard plateau sites 752 755 watergun seismic reflection and drilling data indicate the following stratigraphic response of the platform to relative sea level changes 1 prograding clinoforms consisting of carbonate sediments were deposited during a long term relative sea level rise submergence of the uplifted parts of the platform southern kerguelen heard plateau increased the area available for productivity allowing the carbonate sediment to prograde northward into the basin during the rise 2 onlapping sequences consisting of carbonate detritus were deposited in the basin during the middle eocene relative sea level fall the carbonate strata on broken ridge were uplifted to at least wavebase and eroded the detritus was transported northward toward the basin and onlaps the preexisting strata the approx mate volume balance between the sediment eroded from the crest of broken ridge and the onlapping sediments deposited along the northern slope suggests that mechanical erosion and transportation of carbonate sediment into the deep sea were important processes controlling the stratigraphy at broken ridge during the relative sea level fall we propose that the stratagraphic response of broken ridge to the middle eocene relative sea level fall is consistent with stratigraphic predictions based on terrigenous margins however during a relative sea level rise biogenic carbonate production may allow carbonate sediment to prograde basinward despite the fact that water depths are increasing stratigraphic models based on terrigenous margins do not account for the increase in sediment supply along carbonate margins during a relative sea level rise and thus require minor modification in order to predict the development of the prograding clinoforms

the meltwater hypothesis for subglacial bedforms this paper presents an historical and in places informal account of the meltwater hypothesis which invokes enormous outburst floods for the formation of subglacial bedforms it begins with a brief discussion of the difficulties of determining processes of formation for landforms which are not seen in formation analogy provides a solution to these difficulties analogy between erosional marks at the bases of turbidites and drumlins which were the starting point for this hypothesis rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins field tests on the sedimentology architecture and landform associations of drumlins in the livingstone lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial their form and genesis are discussed mainly with reference to sites at french river and wilton creek ontario though some remarkable bedrock erosional forms in antarctica support their regional extent initially in the meltwater hypothesis drumlins were thought to be cavity fills and erosional drumlins were recognized later this development is shown to be central to the realization that drumlin composition may be inferred from drumlin form the scale of drumlin fields measured at about 10 3 km 2 and the magnitude of the inferred floods require that the flood events were regional regional scale flood tracts in ontario quebec alberta and the northwest territories extending over 1000 km in length and several hundred kilometers in width support this suggestion floods had they occurred would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures the meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marksffluting rogen moraine hummocky terrain and transverse ridges recent work shows how these forms are best explained by the meltwater hypothesis the roles of water storage and release which underpin the theory of the meltwater hypothesis remain poorly understood r 2002 published by elsevier science ltd

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

reef fabrics biotic crusts and syndepositional cements of the latemar reef margin middle triassic northern italy the latemar reef buildup of the central dolomites northern italy provides a rare opportunity to examine an in place middle triassic upper anisian to lower ladinian platform margin that is not strongly deformed or dolomitized the margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms across this transition the depositional profile relates directly to a consistent lateral facies pattern 1 restricted biota grainstone of the platform interior 2 tubiphytes rich boundstone and 3 diverse biota grainstone that grades into 4 foreslope breccia beds the boundstone and diverse biota grainstone facies comprise the platform margin the boundstone facies consists of a framework of small 10 cm skeletal remains 10 by volume with associated biotic crusts internal sediments and syndepositional cements crusts and cements constitute most of the rock volume and created the boundstone fabric biotic crusts exhibit gravity defying geometries and range from a light grey structure grumeleuse rind to dark grey micritic laminae both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits indicating a syndepositional origin the diverse biota grainstone facies primarily consists of skeletal peloidal grainstone with a diverse open marine biotic assemblage in contrast to the restricted biota grainstones of the platform interior that have a low diversity restricted marine biota metre scale hexacoral boundstone and centimetre scale sponge boundstone and microbial boundstone occur as isolated patches tens to hundreds of metres apart within the diverse biota grainstone facies the depositional profile facies zonation and biotic constituents all indicate that the latemar buildup had a shallow water reef margin in contrast to previous interpretations that these were upper slope reefs the syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave resistant reef fabric

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

spatial geochemistry of a carboniferous platform margin to basin transect balancing environmental and diagenetic factors abstract conventional one dimensional chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time whereas this approach has shown to be of value when linking isobathymetric domains usually characterised by similar facies more caution is needed when correlations are extended across different physiographic settings and hence different facies belts here the spatial geochemical record of pennsylvanian platform margin to basin transects across a bathymetric range of about 800 m is documented and discussed in a process oriented context particularly the presence of layered palaeo water masses and their potential control on slope facies distribution and geochemical properties requires attention whereas carboniferous thermo and or chemo clines most likely affected depth related slope facies zonation it was facies change and hence variances in porosity permeability properties that controlled differential early burial diagenetic alteration specifically the lower slope related breccia facies is characterised by higher volumes of early burial carbonate cements this implies that these sediments entered the shallow burial domain with a considerable open pore space and gave way to an increased rock fluid ratio whereas the 13 c record is invariant with bathymetry the more diagenesis sensitive 18 o proxy records pronounced shifts observed across major facies boundaries from this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature it is differential rock fluid ratios that control the first order spatial shifts in 18 o composition these findings show that one dimensional chemostratigraphy will severely underestimate the complexity of three dimensional bathymetric data sets across platform margins this is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general

alkane and pah depositional history sources and fluxes in sediments from the fraser river basin and strait of georgia canada hydrocarbon data for dated sediment cores from fraser basin lakes and the strait of georgia in canada reveal a pristine pre industrial background that remains unaltered in modern sediments far from human activities these pristine sediments contain polycyclic aromatic hydrocarbons pahs from combustion natural forest and prairie fires and alkanes and pahs from weathering of soils containing vascular plant remains and natural petroleum hydrocarbons and algal production at other locations examined however these natural loadings have been augmented over the past century by hydrocarbons from petroleum the combustion of liquid oil gasoline and solid coal wood fuels and from processing of wood either directly from disturbed land and slash burning or indirectly through pulp mill effluent although contamination is clearest where there are nearby human activities alkane profiles pah distributions and alkane unresolved complex mixtures suggest a widespread contamination from petroleum combustion products from human activities can also be identified although with less source specificity than with the petroleum products regional emissions from anthropogenic combustion processes appear to support modern pah increases within the fraser river basin and there is little evidence of inputs from global emissions principal components analysis using the new centred log ratio normalisation procedure to normalise data discriminates well between pahs entering lakes from runoff or the atmosphere and pah formed diagenetically within lake sediments as well as between combustion and petroleum pahs

hydrocarbons in washington coastal sediments the sources and distributions of polycyclic aromatic hydrocarbons pah and aliphatic hydrocarbons are characterized in seventeen sediments from a highly river influenced sedimentary environment off the southwestern coast of washington the major hydrocarbons are land derived introduced as preformed compounds and display long term stability in sediment cores a series of pah of anthropogenic origin and two naturally derived compounds retene and perylene dominate the pah composition in these sediments plantwax n alkanes are the major aliphatic hydrocarbon constituents aliphatic hydrocarbons of marine origin pristane and a series of four acyclic multibranched c25 polyolefins are also observed in many sediments the concentrations of these marine derived hydrocarbons decrease to negligible levels rapidly with sediment depth from the sea sediment interface suggesting degradation in general the major land derived hydrocarbons are concentrated in the midshelf silt deposit which extends northwestward along the continental shelf from the columbia river mouth a quantitatively more minor natural series of phenanthrene homologs also of terrestrial origin is preferentially advected further offshore and deposited in continental slope sediments these distributions are consistent with recognized particle associations for these compounds and sediment dispersal processes in this coastal environment sediment core records suggest the present pattern of dispersal has persisted for at least the past century and possibly since the late pleistocene

facies geometry and growth phases of the valdorria carbonate platform pennsylvanian northern spain the 14 km wide valdorria outcrop pennsylvanian northern spain is one of the few examples of entirely exposed flat topped and high relief carbonate platforms reported in the fossil rock record laterally and vertically traceable stratal patterns expose three phases of growth phase i is a 430 m thick platform to slope succession that prograded over 6 km and is dated as early bashkirian akavasian askynbashian phase ii aggraded and prograded exhibiting 180 m thickness of cyclical platform top deposits dated as late bashkirian asatauian phase iii is a mound shape structure that developed over the platform top of phase ii as a new phase of platform nucleation it is 535 m thick and 2 km wide and dated as late bashkirian asatauian transition interval the observed changes of growth styles during platform evolution from a prograding to an aggrading prograding system and a rapid aggradational phase are inferred to be controlled by flexural subsidence in the active cantabrian foreland basin at the variscan orogenic front the metre scale shallowing upward cycles of the platform top are most probably due to glacioeustasy as evidenced by well recorded subaerial exposure surfaces superimposed on subtidal deposits and by a stratal pattern recurrent in a short interval of about 160 kyr observations of outcropping bashkirian cyclothems in an isolated carbonate system devoid of siliciclastic input are relevant for a better understanding of the impact of high frequency sea level fluctuations on the carbonate factory moreover progradation of the platform margin during phase i reaches a rate of 2500 m myr and 1810 m myr during phase ii rates that are high when compared to other pennsylvanian examples the aggradation rate of 447 m myr calculated for late bashkirian transition interval phases i and ii uncorrected for compaction missing beats and erosion is uncommonly high in comparison to coeval pennsylvanian examples the platform exhibits a self nourishing prograding microbial boundstone dominated slope thus the slope shedding model applies well to valdorria however phase ii recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea level stands this is marked by common slope red stained breccias synchronous to platform top subaerial exposure phases contrarily periods of relative high sea level and rapid subsidence in phase iii registered a greater development of cemented microbial boundstone these observed partly opposing relationships of sea level stands shedding modes and slope architecture provide an improvement of the currently used slope shedding model the overall architecture of the valdorria outcrop compares well with that of other contemporaneous platforms such as sierra del cuera and bolshoi karatau valdorria shares the high relief and flat topped steep slopes cyclothemic patterns and occurrence of karst features with the pricaspian basin platforms tengiz karachaganak and kashagan with minor variations in facies distribution of the internal platform furthermore the continuous seismic scale outcrop of valdorria together with its isolated setting and asymmetrical growth makes it an additional good candidate for potential subsurface analogues of hydrocarbon bearing systems

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

scalloped bank margins beginning of the end for carbonate platforms bathymetric seismic reflection and satellite data have revealed the common existence of a new type of carbonate bank margin scalloped margin throughout the gulf of mexico and the caribbean region these large up to tens of kilometres across irregular convexbankward embayments are interpreted as predominantly erosional features produced by large scale failure of platform margins in tectonically active areas they may be triggered by earthquake shocks whereas in stable regions they may more likely be the result of undercutting by dissolution and or deep water currents scalloped margins are common in the south eastern bahamas this suggests that these relatively small isolated bahama banks may be erosional remnants of once larger platforms the processes that create scalloped margins may result in erosional deep water carbonate basins and potentially the demise and disintegration of carbonate platforms

hydrocarbon geochemistry of the puget sound region ii sedimentary diterpenoid steroid and triterpenoid hydrocarbons cyclic components of the aliphatic hydrocarbon mixtures extracted from puget sound sediment cores include a suite of c19 and c20 diterpenoid hydrocarbons of which fichtelite sandaracopimaradiene and isopimaradiene have been identified although apparently also derived from vascular plants these diterpenoid hydrocarbons have relative abundances distinctly different from the co existing plant wax n alkane suite five c27 c28 and c29 diasteranes and four c29 c30 and c31 17 h 21 h hopanes occur in relatively constant proportion as components of a highly weathered fossil hydrocarbon assemblage these chromatographically resolved cycloalkanes along with the strongly covarying unresolved complex mixture have been introduced to puget sound sediments from adjacent urban centres at increasing levels over the last 100 yr in the absence of any major oil spill naturally occurring triterpenoid hydrocarbons including hop 22 29 ene diploptene are also present a new group of c30 polyenes has been detected which contains compounds apparently structurally related to a co existing bicyclic c25 diene and to c20 and c25 acyclic multibranched hydrocarbons described in a previous paper barrick et al 1980

radiaxial fibrous calcites a new look at an old problem abstract marine radiaxial fibrous and fascicular optic calcites are very common but poorly understood pore filling cements in paleozoic and mesozoic marine neritic and upper bathyal limestones the main diagnostic feature of these cements is their converging or diverging crystal c axis respectively the reasons for this anomalous texture are at present unknown another controversy is due to the relative lack of occurrences in cenozoic strata and their apparent absence in quaternary marine limestones despite these uncertainties marine fibrous cements are thought by some to be amongst the best proxies for the geochemistry of ancient oceans because of the absence of metabolic effects during their precipitation applying electron backscatter diffraction analysis we here for the first time document radiaxial fibrous and subordinate fascicular optic fibrous biominerals from well preserved jurassic and cretaceous low mg calcite belemnite rostra the finding of fibrous biogenic calcites in combination with the recent description of holocene and modern fibrous calcites in stalagmites represents in the view of the authors a significant advance in carbonate research here these findings are placed in their wider processes oriented context and the significance of biogenic and speloan fibrous calcite for their marine counterparts is assessed comparing the physico chemical and organomineralic properties of different precipitation sites indicates that the diagnostic variations in the crystal c axis orientation are not related to a specific nucleation environment or substratum in contrast preliminary crystallographic analyses suggest that kinetic factors during nucleation or subtle gradients in the statistical replacements of ca 2 by mg 2 in the crystal structure may lead to local strain resulting in converging or diverging crystal c axes

growth dynamics of pennsylvanian carbonate mounds from a mixed terrigenous carbonate ramp in the puebla de lillo area cantabrian mountains northern spain abstract carbonate mounds are widespread in many places around the world and are especially well exposed in the cantabrian mountains northern spain this study focuses on the composition and growth dynamics of two large mound complexes well exposed near the village of puebla de lillo 70 km northeast of leon and evaluates the impact of sea level accommodation and siliciclastic input on mound growth the mound substrate consists of bedded dark skeletal bindstone the core facies exhibits patchy growth forms of alternating massive microbial algal boundstone and skeletal and nonskeletal wackestone reflecting the overall mound internal composition of smaller coalescent bodies microbial activity as inferred from the volumetrically important clotted peloids appears to be the main factor leading to the high accumulation rate in the cores the steeply dipping flanks presumably resulted from early cementation and subsequent lithification and stabilization induced mainly by microbial activity in the intermound strata carbonate beds and small mounds alternate with fine grained sandstone siltstone and shales the intermound carbonates consist of skeletal and intraclastic mudstone to wackestone whereas algal microbial boundstone is a minor component overall facies transition from the mound core towards the flanks or the mound capping beds is gradual however the volume of bioclasts and intraclasts increases towards the flank and capping facies inversely to amount of clotted peloids or micrite the substrate and the cores of the two mound complexes likely formed in the photic zone as inferred from the common calcareous algae carbonate precipitation favored by microbial activity and algal growth along with gain in accommodation space led to mound core accretion the growth of the small mound complexes was most likely stopped by recurrent sea level fall and siliciclastic input the large high relief mound complex actually kept growing until reaching shallow water and high energy conditions unlike many late paleozoic counterparts these mounds were not subaerially exposed during growth the great thickness and the aggradational mound growth are indicative of a high subsidence rate at a later stage however siliciclastic input also buried the large mound complex rapid subsidence sea level fluctuations and siliciclastic input were thus decisive factors controlling carbonate production this study reveals that mound complexes nucleated and grew on preexisting paleohighs maintaining consistent growth in these specific settings paleohighs recurrent sea level fluctuations presumably linked to glacio eustasy and episodic siliciclastic input are the principal factors that controlled the shape size and growth evolution furthermore the internal architecture as a mosaic of juxtaposed small bodies composing the complexes represents an important finding for better understanding of mound growth and thus for better constraining reservoir heterogeneity particularly for upper paleozoic deposits

a pennsylvanian microbial boundstone dominated carbonate shelf in a distal foreland margin picos de europa province nw spain abstract a pennsylvanian bashkirian moscovian high rising microbial boundstone dominated carbonate platform 12 000 km2 developed in the external margin of a variscan foreland basin intensely studied in the northern domain cuera area the stratigraphy and sedimentology of the southern domain the picos de europa province is here revisited distribution of the bashkirian facies valdeteja formation defines a ramp low angle platform with an internal zone central and eastern sectors of the picos de europa province consisting of huge amounts of massive cement rich microbial boundstones surrounded by an external zone comprising lower slope boundstone derived breccias and basinal lime mudstones from the bashkirian moscovian transition to the early kasimovian picos de europa formation the carbonate platform moderately aggraded acquiring a high rising and flat topped geometry and then prograded rapidly into the external zone where platform derived carbonate sediment and clay sourced from the orogen were deposited seismic scale exposures of the western margin of the shelf show angular bedding relationship between basin toe of slope slope and platform top stratal domains with slope inclination up to 32 and a topographic relief of 550 m the platform evolution above described for the picos de europa province significantly differs with respect to the cuera area where platform mostly prograded during the bashkirian and subsequently aggraded during early moscovian the evolutionary differences between these two carbonate domains which are geographically close are interpreted as changes in tectonic subsidence distribution acting as the major factor controlling the large scale platform geometries in the active foreland basin context finally the tectonic overload due to the emplacement of the cuera thrust sheets during the moscovian kasimovian transition changed the platform growth style from progradational to aggradational and consequently the stacking pattern of the platform top cycles in the picos de europa province

lithofacies character and architecture across a pennsylvanian inner platform transect sierra de cuera asturias spain abstract seismic scale outcrops of lower pennsylvanian steep margined carbonate platforms in northern spain sierra de cuera cantabrian mountains provide the opportunity to link depositional facies to platform interior architecture lithofacies character vertical stacking patterns and spatial distribution of depositional facies and shallowing upward cycles were investigated to provide a semiquantitative depositional model for inner platform strata a 70 m thick succession of eight shallowing upward cycles 2 5 15 m thick was studied across a 2 km wide transect cycles consist of algal bioherms and skeletal packstone lithofacies association b deposited in an open marine and sub wave base environment which are locally lateral to crinoid dominated packstone lithofacies association d lithofacies association b developed on a transgressive one meter thick high energy coated grain grainstone lithofacies association a and is overlain by open marine to restricted lagoonal deposits lithofacies association c indicative of decreased paleo water depth cycle boundaries are marine flooding surfaces occurring at the base of lithofacies a and the top of lithofacies c petrographic and outcrop evidence of subaerial exposure is conspicuously lacking cycle and lithofacies thickness varies laterally and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous terminating within tens to hundreds of meters in most of the cycles the lithofacies succession changes laterally because of lateral facies transitions between lithofacies b and d and pinching out of lithofacies a and c meter scale variations in depositional topography are related to the stacked lens shaped bioherms assigned to lithofacies b and appear to control the thickness and lateral continuity of the overlying facies sierra de cuera strata do not resemble other pennsylvanian cycles driven by high frequency high amplitude glacio eustasy which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits except for the absence of peritidal facies and the presence of low relief algal bioherms on the platform top the necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high frequency 100 240 ky moderate amplitude nearly 40 m sea level fluctuations combined with subsidence rates of 70 140 m my tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined moscovian strata the studied inner platform lithofacies character and architecture were controlled by the interplay of high frequency changes in accommodation and the presence of meter scale depositional topography attributed to lateral variations of carbonate accumulation

application of redox sensitive proxies and carbonate clumped isotopes to mesozoic and palaeozoic radiaxial fibrous calcite cements abstract ancient marine radiaxial calcite cements are commonly exploited as archives of marine porewater properties based on the argument that they lack metabolic effects often assigned to biogenic carbonates here we critically test the significance of conventional versus with respect to these fabrics less conventional proxy data from pennsylvanian triassic and cretaceous case examples conventional proxies include cathodoluminescence carbon and oxygen isotope ratios main and trace elemental concentrations less conventionally applied proxies are clumped isotope 47 redox sensitive and rare earth elements sampled across a succession of triassic radiaxial fibrous calcites radiaxial calcites are subdivided in three groups based on their luminescence characteristics non luminescent patchy luminescent and bright luminescent luminescence patterns are in fair agreement with isotope ratios in particular with those of oxygen the data fall into or are close to the range of reconstructed marine seawater values and often plot to the positive end member of the isotopic range these results disagree with the commonly held view that isotope data from luminescent cements reflect a priori non marine values further evidence for this comes from ree concentration patterns and cerium anomalies suggesting normal marine porewater values for all except the very last generation of radiaxial calcites this implies that luminescent radiaxial calcites must not necessarily represent significant diagenetic resetting kinetic effects during precipitation and different activator elements must be considered marine and earliest burial porewater temperatures of 12 26 c are suggested by conventional calcite 18 o thermometry conversely the application of the clumped isotope thermometer to the same radiaxial calcites suggests temperatures of 180 200 c reflecting solid state resetting of fully cemented limestones under a low water rock ratio redox sensitive elements particularly zn cd u and cu are affected by kinetic processes overriding fluid eh manganese concentrations and ce anomaly data point to gradually decreasing marine porewater oxygen levels from outer to inner cement fringes judging from ree patterns and ce anomalies the cement layers in the central portions of the pore filling cement succession witnessed the end of marine precipitation and the onset of shallow marine diagenesis consequently radiaxial calcite precipitation is suggested to continue in the early shallow marine burial domain this study underscores the potential of radiaxial calcite successions as archives of marine porewater to shallow burial diagenetic pathways the combination of conventional and less conventional proxies is a clear strength of this study and documents that abiogenic carbonate archives are often underexplored

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

210pb derived sediment accumulation and mixing rates for the greater puget sound region abstract recent sediment accumulation rates of 46 1200 mg cm2 yr are calculated from excess 210pb activity profiles in 44 sediment cores from 27 sites in the greater puget sound region rates calculated for nine stations in the deeper waters of central puget sound average 480 mg cm2 yr and are greatest between seattle and tacoma near the southern end of this region the standard error of this mean is 113 mg cm 2 yr magnitudes and geographic trends of 210pb derived accumulation rates agree with limited data on sediment inputs and transport paths their accuracy is further supported by several independent approaches including comparison with particle fluxes collected in year long series of sediment trap deployments at two sites accumulation rates are calculated assuming no mixing beneath surface mixed layers of 2 22 cm sediment mixing coefficients in these surface layers average at least 43 cm2 yr four times greater than those reported in open washington coast sediments the difference is attributed to the greater benthic biomass in puget sound sediments particle residence times in surface mixed layers vary from 1 5 to 68 years and introduce corresponding uncertainties in ages of horizons beneath the mixed layers for most of this region these residence times will cause a 20 30 year time lag in achieving an equivalent response of surface sediment chemical concentrations to changes in concentrations in freshly depositing particles for constituents whose stability and phase associations resemble those of 210pb these time lags limit the ability of sediment chemistry monitoring programs to recognize recent changes in composition and fluxes of chemicals to sediments recent fluxes calculated from the product of surface sediment chemical concentration and accumulation rates will be inaccurate for chemicals if their input concentrations have changed in the last 20 30 years

spatial and temporal distribution of microbial carbonates skeletal and non skeletal grains in a pennsylvanian carbonate platform valdorria northern spain abstract the valdorria carbonate platform in northern spain features a well preserved and continuous outcrop of a bashkirian platform to basin transect during the asatauian late bashkirian the platform growth changed gradually from a progradational to an aggradational mode recording 10 most likely short eccentricity controlled cyclothems the facies of the platform top and slope have been mapped and the architecture of each cyclothem schematically reconstructed the distribution of microbial carbonates microbially mediated precipitates skeletal grazers burrowers corals filter feeders algae foraminifera osagia like oncoids and thartarella terebella worm tubes and non skeletal grains faecal pellets has been quantified for the transgressive regressive periods of deposition transgressive maximum flooding early and late regressive intervals of each cyclothem this study shows that the distribution of microbial carbonates skeletal and non skeletal grains along a carbonate platform transect is variable through time and mainly governed by a set of measurable interconnected factors regulating the local palaeoenvironments the water depth and the wave energy facies along the platform profile inner outer break slope during periods of sea level fluctuations transgressive to regressive intervals of deposition auloporid corals siliceous sponges phylloid algae and osagia like oncoids are characteristic of a low energy environment situated from estimated palaeo water depths of 25 to 80 metres below sea level mbsl onward that formed in the outer platform during the maximum flooding intervals anthracoporella archaeolithoporella boundstone is characteristic of the moderate energy environments created in the horizontal inner platform during the transgressive intervals microbially mediated precipitates are reliable indicators of the slope and masloviporidium indicator of low energy environments of the upper slope on the platform top ungdarella and donezella are more abundant during the early regressive intervals than during any other interval stacheoids are mainly present in the water depth range of 10 to 60 mbsl tournayellids from 15 to 35 mbsl and archaediscids from 15 to 75 mbsl all in low to moderate energy conditions rugose corals are common in the water depth range of 5 to 35 mbsl either during the transition from the early to the late regressive intervals as solitary forms or forming boundstone or during periods of higher sea level stand as solitary forms in floatstone fenestellids fistuliporids and trilobites occur mainly in the slope but are also common in the platform top during the maximum flooding and or the early regressive intervals archaeolithoporella is a reliable indicator of the platform top deposits but its abundance is closely linked to the presence of adequate substrates a high abundance in the assemblage of gastropods endothyrids palaeotextulariids bradyinids and fusulinids characterizes the platform top shallow water coated grain grainstones formed under high energy currents during the late regressive intervals tetrataxids act as encrusters on microbial precipitates of the slope or on phylloid algae and archaeolithoporella of the platform top but are also observed as nuclei of coated grains brachiopods are more abundant in low to moderate energy and 35 to 55 mbsl environments that prevailed during the transgressive intervals faecal pellets were commonly observed in low energy environments from 50 to 325 mbsl at the platform break and in the slope and the same applies for thartarella terebella worm tubes that are virtually absent in high energy deposits finally crinoids lasiodiscids and tuberitinids are randomly distributed and are not indicative of any specific environment the here well constrained distribution patterns of microbial carbonates skeletal and non skeletal grains in valdorria outcrop hold the potential to serve as a reference for interpretation of depositional environments of other platforms using biota associations and depositional texture particularly in regions where the platform geometry is not preserved or is not fully exposed

component specific petrographic and geochemical characterization of fine grained carbonates along carboniferous and jurassic platform to basin transects abstract fine grained carbonates are present throughout much of the geological record and are widely used as geochemical archives even though their origin and diagenetic pathways remain poorly understood here petrographical and geochemical properties of granulometrically separated component spectra of marine mudstones sampled along two proximal to distal transects carboniferous of spain and jurassic of morocco are documented these settings represent end members in terms of platform geometry steep flanked versus gentle ramp and the aragonite versus calcite sea mode the data from spain reveal a bimodal organization of microcrystalline carbonate isotope values from platform top and slope and toe of slope settings the data from morocco lack a clear spatial and bathymetrical pattern the significance of the complex site specific biological and physico chemical parameters is emphasized mudstones have been separated in granulometric fractions of 8 5 5 3 and

growth and demise of late carboniferous carbonate platforms in the eastern cantabrian zone asturias northwestern spain abstract geometric relationships of seismic scale stratal patterns observed on aerial photographs of late carboniferous carbonate platforms in the cuera region in asturias northwestern spain allow the identification of two discrete stages of platform development with different styles of deposition the first phase represented by the bashkirian valdeteja formation is characterized by progradation of more than 10 km with steep clinoforms of up to 35 and slope heights up to 650 m during the second phase corresponding to the moscovian picos de europa formation the platform aggraded mainly vertically nearly 850 m both phases of platform growth were terminated by major relative sea level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks calibration with outcrops suggests a subdivision into the following lithofacies directly related to the depositional profile 1 platform interior parallel well to thick bedded skeletal pack to grainstones 2 margin algal and sponge dominated bioherms with little internal structure 3 slope clinothems of medium to thick bedded margin derived breccia beds 4 toe of slope alternating grain and packstones with spiculites and 5 basin marly shales shales sandstones and calcareous mudstones relative sea level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases sea level highstands triggered the nucleation and growth of the platforms whereas major sea level lowstands in combination with terrigenous infill of the basins terminated the platform growth in addition it is proposed that the termination of late carboniferous carbonate platforms is caused by the combined effect of a sea level fall subaerial exposure and erosion and finally a rapid transgression that drowned and polluted the platform with shales and mudstones local variations in the infilling of the basins controlled the variations in the style of progradation the subsidence rates for the vertically aggrading phase picos de europa formation were approximately twice that for the progradational phase valdeteja formation and may explain the aggradational versus progradational character

lithofacies and depositional processes on a high steep margined carboniferous bashkirian moscovian carbonate platform slope sierra del cuera nw spain the depositional slope of the sierra del cuera a carboniferous bashkirian moscovian high steep margined carbonate platform provides excellent cross sections of lithofacies zonations and associated stratal geometries the steep 30 40 and nearly planar upper slope is dominated by massive sheet like layers of microbial cement rich boundstone which alternate with intercalations of red stained bryozoan cementstone with crinoids and brachiopods the slightly gentler 20 26 and concave upward lower slope is characterized by clast supported resedimented deposits the upper slope extended from platform break to ca 300 m water depth whereas lower slope sediments were deposited in water depths up to 600 700 m at which level the slope beds flatten to a few degrees toe of slope and interfinger with spiculitic and argillaceous basinal sediments between 250 and 450 m water depth boundstones and breccias alternated in a transitional zone the lower slope sediments include clast supported breccias with radiaxial fibrous cement in interparticle space mud to clast supported breccias with red stained carbonate mud matrix and packstone to grainstone and rudstone beds most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived a pervasive submarine cementation occurs along the upper two thirds of the flank and this stabilized the slope slope deposition is interpreted as follows during active boundstone accretion microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe of slope rock falls and avalanches were generated whenever the shear strength of the substrate of loose or partly lithified sediment was exceeded upper slope boundstone accretion and shedding independent of the depth of light penetration controlled most of the depositional processes on the slope cement dominated intervals are considered to be related to early highstand and or flooding phases relative sea level fluctuations and or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is as yet unclear

three dimensional sedimentary framework of the carbonate ramp slope of central west florida a sequential seismic stratigraphic perspective the sequential post early cretaceous stratigraphic evolution of the carbonate ramp slope of central west florida has been determined via the integration of 1 500 km of seismic reflection profiles with drillcores piston cores and rock dredge hauls our goal has been to gain insight into and increase our knowledge and understanding of carbonate ramps in general as well as to develop a model for potential application to the rock record our results which focus on the neogene indicate that the three dimensional sedimentary framework of the west florida ramp slope has developed in response to the interplay of a number of processes including 1 regional tectonic subsidence 2 long term sea level change 3 short term sea level oscillations 4 large scale gravity collapse 5 a major paleoceanographic event 6 fluctuating intensity of oceanic circulation 7 lateral migration of ocean currents and 8 open ocean upwellang and pelagic carbonate production during the early cretaceous seismic sequence vi the continental margin of west florida was a large shallow water carbonate evaporate platform that had persisted since late stages of continental rifting in the jurassic during the mid cretaceous seismic sequence v terrigenous sediment nutrient influx terminated platform growth and deposited a layer of neritic marl for the next 60 m y during a long term eustatic highstand in the late cretaceous paleogene seismic sequence iv the present west florida slope was an open ocean deep water marginal plateau that accumulated pelagic carbonates following a eustatic lowstand in the late oligocene 30 m y ago the carbonate ramp slope of central west florida began to take shape as a low relief ramp margin built seaward into the early and middle miocene seismic sequences iii and ii in the latest early miocene large scale gravity collapse generated a slope basin that was rapidly filled by prograding clinoforms in the middle miocene 12 15 m y ago intensification of oceanic circulation truncated antecedent clinoforms blocked off shelf sediment transport and stimulated pelagic carbonate production this oceanographic event resulted in the succession of seaward prograding clinoforms by a slope front fill pelagic depositional system that has survived numerous sea level oscillations into the present seismic sequence i

calciturbidite composition related to exposure and flooding of a carbonate platform triassic eastern alps the compositional variation of calciturbidites pedata potschen formation deposited adjacent to the dachstein formation carbonate platform in the triassic hallstatt basin was analysed using detailed field measurements and point counting of thin sections the 35 m long section is located in the northern calcareous alps of austria six point count groups were distinguished separating basinal from platform derived input summary statistics cluster and correspondence analysis of the point count data reveals a close relationship between the biota present on the dachstein carbonate platform and the calciturbidite composition the variations in turbidite composition are attributed to fluctuations in sea level and resulting flooding and exposure of the platform which alternately created and destroyed shallow water habitats on the platform top

marine red staining of a pennsylvanian carbonate slope environmental and oceanographic significance abstract red stained platform facies are a common feature of many carbonate settings throughout the geological record although the mechanisms involved in red staining of subaerially exposed or argillaceous peri platform limestones are reasonably well understood the environmental and oceanographic significance of red carbonates often remains uncertain here sedimentological sequence stratigraphic geochemical paleontological and quantitative bathymetric data from pennsylvanian red intervals across a well exposed carbonate platform top and slope are documented and interpreted in a process oriented context on the upper slope 80 350 meters below the shelf break red intervals alternate with gray mainly microbial algal boundstones on the lower slope 350 600 meters below the shelf break redeposited red stained mud builds matrix supported breccia tongues interbedded with predominantly redeposited clast supported carbonate debris the presence of large volumes of fibrous calcite biocementstones as well as firmgrounds point to low sedimentation rates or omission in terms of sequence stratigraphy red intervals occur within maximum flooding intervals and reflect near platform drowning elevated 18o values 2 3 and an essentially cool water heterotrophic biotic association in red intervals on platform top and slope suggest deposition during sea level highs associated with colder water masses and high nutrient levels the most likely drivers are an elevated thermocline and upwelling the red staining is the result of iron oxidation which occurred during early diagenesis likely by iron bacteria these red intervals provide an important bathymetric benchmark against which other mainly paleozoic red facies can be tested and calibrated

azaarenes in puget sound sediments the first quantitative measurements of azaarenes in marine sediments are reported for puget sound in northwestern washington state u s a two to four ring azaarenes in concentrations as low as 50 ppb relative to organic carbon are measured in 210pb dated sediments using glass capillary gas chromatography combined with nitrogen selective detection azaarene distributions are geographically and temporally variable azaarene distributions in upper sediment layers suggest that total azaarene concentrations are related to proximity to urban areas and may be anthropogenically derived subsurface maxima between 6 and 17 cm in several cores resemble similar maxima in pah and organo sulfur compounds trace quantities of azaarenes present in sediments deposited prior to urbanization of the puget sound region suggest low level natural azaarene sources one core containing particulate coal produced a concentrated and complex azaarene mixture suggesting either introduction of mined coal or erosion of regional coal deposits analyses of possible azaarene sources indicate that although azaarene compositions vary between sources coal azaarenes can be distinguished from petroleum azaarenes by their ratios of 2 ring to 3 ring azaarenes and may identify predominant azaarene inputs in environmental samples gross differences in azaarene composition exist between european and american air particulates and sediments suggesting differences in azaarene sources

sedimentation on a lower jurassic carbonate platform flank geometry sediment fabric and related depositional structures djebel bou dahar high atlas morocco abstract along the southern margin of the high atlas morocco carbonate platforms of lower jurassic age developed in a transtensional basin the south atlas trough complete exposure and preservation of the platform margin flank and basin of the djebel bou dahar allowed the subdivision of the slope into an upper mid and lower slope based on geometry sediment fabric and depositional structures at ca 50 m below the platform margin coral algal patch reefs and ooid sand shoals grade into the depositional slope the straight upper slope showing discontinuous bedding at angles up to 29 extends down to ca 250 m and consists mainly of poorly sorted mixtures of sand and gravel almost devoid of mud avalanching rockfall and shearing of the coarse gravelly sediment are the dominant processes readjusting the upper slope the middle and lower part of the slope profile extending down to ca 500 m below the margin is separated from the upper slope by a distinct slope break slope angles range from 19 near the slope break down to 4 8 where the slope merges with the basin plain the irregularly bedded mid slope is characterized by scouring and deposition of muddy sands and non cohesive gravelly debris flows sedimentation on the dominantly muddy concave upwards lower slope can be separated into evenly bedded shales muddy sands and mud free gravel sand mixtures deposited as debris sheets the results of this study emphasize the relationship between sediment fabric and the slope geometry of a carbonate platform flank the overall geometry internal heterogeneity and depositional structures are suggested to be strongly related to the original character of the sediment supplied to the djebel bou dahar platform flank

seismic models of a carbonate foreslope to basin transition picco di vallandro dolomite alps northern italy detailed geologic cross sections augmented by laboratory calibration of lithology and acoustic impedance have been used to produce synthetic reflection seismic sections of a carbonate foreslope to basin transition two areas from the picco di vallandro region of the dolomite alps were modeled a progradational section and a retrogradational backstepping section the resulting models show how these complex areas of strata interfingering might be displayed on conventional reflection seismic lines in the area of progradation rapid stratigraphic thinning below seismic detectibility coupled with abrupt impedance changes produces a reflection discontinuity between steeply dipping reflections of the foreslope and gently dipping parallel reflections of the basin section this apparent downlap surface marks the toe of slope for successive clinoforms but does not correspond to a discrete stratigraphic surface in the backstepping example similar stratigraphic thinning and impedance changes create an apparent onlap surface wavelet interference causes complications in both examples these models indicate how stratigraphic complexity can be simplified by the seismic reflection process and suggest that caution should be exercised when using seismic data to construct general models in areas of complex depositional geometries and rapidly changing facies

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

glacioeustatic cyclicity of a pennsylvanian carbonate platform in a foreland basin setting an example from the bachende formation of the cantabrian zone nw spain abstract by analysis of outcrop data sequence stratigraphy was used to interpret the stratal architecture and evolution of a moscovian pennsylvanian carbonate platform bachende formation located in the variscan foreland basin of the cantabrian zone nw spain the bachende formation represents a delta top carbonate platform developed in highly subsiding average subsidence rate of 254 m my distal shelf areas of the basin it reached a thickness of 650 m and covered an area of 170 km 2 after conservative palinspastic restorations the platform interior succession is mostly composed of low relief mound shaped micritic boundstones alternating with bioclastic mudstones wackestones peloidal foraminiferal packstones grainstones calcareous algal bafflestones and skeletal and ooidal grainstones landwards to the w carbonate platform beds alternate and interfinger with deltaic sandstones and shallow marine shales the platform interior consists of three sequences s1 s2 and s3 ranging from 180 to 230 m in thickness and from 1 3 to 0 5 my in age these sequences are in turn subdivided into several meter scale 10 6 m average thickness high frequency 110 to 15 ky subtidal transgressive regressive cycles these cycles are bounded by subaerial exposure or marine flooding surfaces and are inferred to be laterally continuous across the platform top they vary from mixed carbonate siliciclastic to carbonate dominated cycles cyclicity on the bachende platform was strongly influenced by glacioeustatic sea level variations and changes in subsidence and sedimentation rates subaerial exposure surfaces capping high frequency cycles are better developed in the sequence 1 late kashirian early podolskian when subsidence was relatively low and glacioeustatic sea level fluctuations could have exposed subtidal deposits during base level falls on the basis of the conservative estimated duration of each cycle and spectral analysis of cycle thickness the precessional 20 ky and obliquity 40 ky cycles were detected in all the sequences the short eccentricity cycle 100 ky in the sequence 1 and the long eccentricity cycle 400 ky in the sequences 2 and 3 podolskian nevertheless precessional and obliquity frequencies seem to dominate during sequences 2 and 3 which were deposited under significant higher subsidence and sedimentation rates additionally spectral peaks at 60 80 ky are randomly present across the whole succession and they cannot be explained as a linear response of the climate system to the milankovitch forcing

stratigraphic tectonic relations in spain s cantabrian mountains fan delta meets carbonate shelf abstract stratigraphic patterns indicate that the hercynian orogenic belt of northwestern spain underwent an important reorganization in the latest carboniferous period the passive margin of the cantabrian foreland basin initially far removed from southwestern mountains was the early site of a major carbonate platform this distant margin however experienced a late phase of deformation directed from the north limestones of the passive platform then became the thrust faulted source area for three successive fan deltas which prograded across the remaining carbonate shelf the fan deltas are comprised of conglomerates and coarse sandstones of several areally restricted upper carboniferous formations their distribution identifies a local subbasin in the picos de europa province and paleocurrent data confirm a new northern source of sediment these coarse grained deposits are interpreted to have formed in direct response to tectonic uplift along a nearby thrust fault system the stratigraphic succession spans a 7 ma interval from latest moscovian to late kasimovian time and defines three separate tectonic episodes late myachkovskian 304 ma early chamovnicheskian 300 ma and early dorogomilovskian 298 ma rather than advancing basinward though the fan deltas shifted laterally during this period they become younger in an eastward direction that is parallel to the structural grain the underlying subregional unconformity likewise related to thrust faulting also becomes younger to the east the thrust fault system apparently propagated laterally by means of three consecutive tectonic episodes which led in turn to the sideward displacement of sediment source area and fan delta depocenter

origin and significance of isotope shifts in pennsylvanian carbonates asturias nw spain abstract the primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water an inadequate understanding of this variability as well as different diagenetic environments severely limit the interpretation of the stable isotope record of shoalwater carbonates in order to investigate this primary and diagenetic variability along a bashkirian moscovian platform to basin transect 13c and 18o analyses have been performed on more than 1000 matrix micrite carbonate cement and brachiopod shell samples in isotope analysis these different carbonate materials tend to complement each other inasmuch as they have different advantages and shortcomings the resulting data reveal spatial trends in 13c and 18o signatures from platform top lower values to basin higher values in the case of 13c from pristine brachiopods this trend can be explained by the long residence time aging of platform top water masses in the case of brachiopod 18o this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break micrite and marine cement isotopic values from the platform interior were reset lowered during pervasive early meteoric diagenesis in contrast micrite and marine cement isotopic values from the outer platform slope and basin show higher values close to the assumed pennsylvanian seawater isotopic composition this implies that isotopic data from shoalwater carbonates including pristine brachiopod shells might not necessarily reflect paleoceanographic trends of the open ocean water masses because of changes in coastal water mass isotope signature and interaction with early meteoric fluids

upper devonian buildup margin and slope development in the southern canadian rocky mountains the geometries of upper devonian mid frasnian carbonate buildup margins of the peechee member southesk formation in the southern canadian rocky mountains reflect the influence of rising sea level punctuated by stillstands most peechee buildup margins in the cordillera retrograded whereas some aggraded or prograded in response to these sea level changes and variations in the relative rates of adjacent basin filling sedimentation peechee margin accumulation and adjacent basin deposition is best explained by a reciprocal model of sedimentation thick tongues tens of meters of stromatoporoid rudstones and bioclastic grainstones aggraded vertically or retreated during periods of sea level rise as sea level decelerated carbonate production was restricted and outbuilding occurred basinal onlapping sedimentation of the perdrix formation muds increased during the ensuing stillstand of sea level subsequently a renewed episode of relative sea level rise caused build up margin retrogradation this pattern of sedimentation driven by episodic fluctuations of relative sea level occurred repeatedly during peechee time a broad scale shallowing upward trend culminated in a progressive increase up section in the abundance of submarine erosion surfaces in the eastern main ranges and paleocaliche horizons in the front ranges southwesterly paleocurrents are inferred to have influenced peechee buildup margin geometries and adjacent basin sedimentation retrograded windward margins formed in response to active currents and breaking waves along the northeastern edge of the fairholme and southesk cairn reef complexes leeward buildup margins protected from the southwest directed currents exhibit prograding geometries these characteristics are exemplified by the ancient wall complex where the windward buildup margins to the northwest and southeast are vertically aggrading whereas the coeval peechee buildup margin to the southwest prograded 2 km onlapping dysaerobic waters during upper cairn and peechee time inhibited the accumulation of forereef skeletal organisms and served as an additional control on buildup margin geometry

three dimensional modelling of an isolated carbonate buildup triassic dolomites italy the middle triassic carbonate buildups of the dolomites show facies similarities with mud mounds but display apparent architectural elements of flat topped carbonate platforms in order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture a three dimensional modelling study of the middle triassic monte cernera buildup has been carried out the cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup separated by an interval with apparent platform geometry and a retrogradational platform interior which is difficult to explain with a flat top platform model for this purpose a number of three dimensional models were constructed using the three dimensional modelling programme petreltm key geological horizons were constructed based on outcrop measurements intermediate horizons were calculated in the modelling program and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs the models were refined stepwise until a best fit with the actual bedding architecture was achieved the best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements the intermediate platform stage about 1 5 km across had probably retained a mounded top with a relief of up to 50 m which is difficult to distinguish from an absolutely flat top but necessary to explain the retrogradational platform interior the present study shows that monte cernera was dominated by mounded geometries at all stages of platform development the mounded geometry plus facies data suggest that the platform is a deep water accumulation below the zone of intense wave energy but within the photic zone the cernera represents a tropical buildup type which did not have the capacity to grow into continuously wave swept environments because of the small size and the absence of a wave resistant energy barrier such buildup types are probably common after major crises in earth history when reef organisms were virtually absent