metre scale cyclicity in permian ramp carbonates of equatorial pangea venezuelan andes implications for sedimentation under tropical pangea conditions abstract palmarito strata in the mid permian of the venezuelan andes show three different types of metre scale cycle type a cycles are mixed clastic carbonate shallowing upward peritidal cycles the upper boundaries of the cycles are exposure surfaces with calcrete this kind of cycle shows a significant degree of randomness in its thickness pattern and this is attributed to an autocyclic origin through tidal flat progradation and variations in carbonate productivity and clastic input to the depositional area type b cycles consist entirely of shallow subtidal facies with a shallowing upward arrangement and an alternation between heterozoan and photozoan assemblages type c cycles were deposited in a middle to outer ramp setting and are characterized by a heterozoan assemblage and shallowing upward trends that includes spiculitic wackestone packstone passing up into neomorphic bioclastic wackestone or bioclastic crinoidal bryozoan wackestone packstone all cycles are interpreted as 4th 5th order 104 to 105 years duration and some can be linked to milankovitch orbital rhythms notably short eccentricity 100 000 years overall the origin of the palmarito cycles was a complex combination of autocyclic and allocyclic controls however the evidence suggests that cycles in the lower part of the succession were dominated more by autocyclic processes whereas those in the upper part were more the result of allocyclic controls climate driven processes such as migration of the intertropical convergence one and the waxing and waning of polar ice caps had significant effects on deposition of the palmarito succession and the cycles therein controlling the clastic sediment supply nutrient availability and thus carbonate production and sea level change and these factors determined the vertical stacking pattern of the cycles

evolution of terrestrial proto co2 atmosphere coupled with thermal history of the earth abstract the effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto co2 atmosphere on the earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle carbon is assumed to circulate among five reservoirs atmosphere ocean continents seafloor and mantle and the carbon flux of each reservoir is calculated under varying conditions such as an increase in solar luminosity continental growth and a decrease in tectonic activity with time we consider processes such as continental weathering carbonate precipitation in the ocean carbonate accretion to the continents metamorphism of carbonates followed by co2 degassing through arc volcanism carbon regassing into the mantle and co2 degassing from the mantle the degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume the mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle we use a parameterized convection model to calculate the thermal evolution of the mantle from which we estimate the seafloor spreading rate and the melt generation depth in the mantle numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state this is because the response time of the carbon cycle system against the perturbation for surface carbon is short being estimated at about 900 million years under the present conditions thus the present amount of surface carbon would not be affected by the initial amount of surface carbon the seafloor spreading rate would have been almost constant throughout the earth s history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3 5 billion years ago the surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the archean period in any event co2 in the proto atmosphere on the earth is suggested to have decreased with the growth of continents resulting in stabilization of the terrestrial environment against the increase in solar luminosity

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

effect of nutrient availability on marine origination rates throughout the phanerozoic eon the diversity of marine life has varied throughout the past 500 million years statistical analyses suggest that fluctuations in the availability of marine nutrients has been one important regulator of rates of origination during this time

towering sponges in an early cambrian lagerstatte disparity between nonbilaterian and bilaterian epifaunal tierers at the neoproterozoic cambrian transition epifaunal suspension feeding bilaterian animals in the cambrian lived close to the sediment water interface and hence their ecological tiering levels were low 10 cm here we report an early cambrian diandongian or probably tommotian atdabanian lagerstatte from the hetang formation in anhui province south china the hetang biota is characterized by high tiering to 50 cm sponges and small 0 5 cm bilaterians including orthothecid hyoliths and bivalved arthropods nonbilaterian suspension feeders sponges cnidarians and archaeocyathids as high tiering animals and bilaterian suspension feeders as low tiering animals also characterize other neoproterozoic cambrian assemblages such as the ediacaran chengjiang burgess shale and sinsk biotas these data are consistent with medium to high tiering levels in neoproterozoic cambrian epifaunal communities but suggest that nonbilaterians achieved such tiering levels long before bilaterian suspension feeders did so in the early ordovician the disparity between bilaterian and nonbilaterian tierers during the neoproterozoic cambrian transition and the delayed appearance of high tiering bilaterians demand phylogenetic and ecological explanations the cambrian substrate revolution may have triggered a cascade of ecological evolution including the rise of bilaterian animals in high tiering levels during the ordovician radiation of the paleozoic fauna

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

the signal and the noise forward modeling of allocyclic and autocyclic processes influencing peritidal carbonate stacking patterns recent carbonate sequence stratigraphic models suggest that the degree of order present in successions of carbonate parasequences is determined by temporal variations in eustatic amplitude and by varying levels of autocyclicity ordered parasequence stacking is suggested to occur during greenhouse periods dominated by low amplitude eustatic oscillations conversely disordered vertical successions are said to occur when amplitudes of eustatic oscillations are high or when autocyclic processes dominate and thus drown the periodic external signal these assertions of causal mechanism are tested here by application of a three dimensional stratigraphic forward model the three dimensional model generates parasequences via a ginsburg type mechanism of sediment transport and shoreline progradation parasequence stacking is controlled by a superimposed longer term eustatic oscillation and parasequences generated are analyzed using runs analysis and durbin watson autocorrelation results from the modeling suggest that autocyclic parasequences generated by shoreline progradation form ordered vertical successions when accommodation changes driven by a low amplitude relative sea level oscillation suggesting that partly autocyclic strata may not be inherently any less ordered than entirely allocyclic strata addition of stochastic processes such as mosaic carbonate production and fluctuations in regional sediment transport direction tend to decrease the level of measurable order present in the autocyclic strata increasing the frequency and amplitude of relative sea level oscillations also tends to decrease the measurable order in stacked autocyclic peritidal parasequence thicknesses these results suggest that controls on parasequence stacking patterns are likely to be more complex and varied than suggested by recent sequence stratigraphic models more outcrop and modeling investigation is required with appropriately circumspect interpretations applied

a proposed integrated multi signature model for peritidal cycles in carbonates abstract a new integrated multi signature model for meter scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record meter scale shallowing upward micro facies cycles are the most common type of peritidal parasequence peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models however the existence of meter scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on micro facies and stacking pattern taken from previous studies nevertheless the shallowing upwards micro facies interpretive template remains widely used to identify these cycles and parasequences we propose that the sedimentary signatures of meter scale peritidal cycles vary systematically during the various phases of an underlying 3rd order relative sea level cycle although during specific phases of a 3rd order relative sea level cycle meter scale peritidal cycles may be characterized by shallowing upward units e g early transgressive systems tract during other periods the stratigraphic signal of cyclicity may be more cryptic and complex causing them to be overlooked e g late transgressive systems tract and highstand systems tract we also propose that there is a half cycle phase shift in periods of deposition and the formation of drowning surfaces relative to 4th to 5th order relative sea level cycles over the course of a 3rd order relative sea level cycle these points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity specifically parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th to 5th order relative sea level cycles at different stages of a 3rd order relative sea level cycle we propose testable new physical stratigraphic signatures for identifying meter scale cycles that have consistent genetic significance each cycle recording a relative sea level cycle characterized by a 4th to 5th order relative sea level rise and fall meter scale peritidal cycles are recorded by an ordered pattern of systematically varying micro facies signatures related to their position within a 3rd order relative sea level cycle the integrated multi signature model of meter scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record as a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd order relative sea level changes a redefinition of parasequence is required and one is presented here

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

co2 forced climate and vegetation instability during late paleozoic deglaciation the late paleozoic deglaciation is the vegetated earth9s only recorded icehouse to greenhouse transition yet the climate dynamics remain enigmatic by using the stable isotopic compositions of soil formed minerals fossil plant matter and shallow water brachiopods we estimated atmospheric partial pressure of carbon dioxide pco2 and tropical marine surface temperatures during this climate transition comparison to southern gondwanan glacial records documents covariance between inferred shifts in pco2 temperature and ice volume consistent with greenhouse gas forcing of climate major restructuring of paleotropical flora in western euramerica occurred in step with climate and pco2 shifts illustrating the biotic impact associated with past co2 forced turnover to a permanent ice free world

shifts in late paleozoic atmospheric circulation over western equatorial pangea insights from pedogenic mineral δ18o compositions the d 18 o values of pedogenic calcites phyllosilicates and iron oxyhydr oxides from fossil soils throughout the southwestern united states show systematic paleolatitudinal and temporal trends that indicate a significant change in soil moisture conditions and atmospheric circulation patterns over southwestern euramerica throughout latest pennsylvanian and early permian time a progressive depletion of as much as 6 in mineral 18 o values with increasing distance northward to 108 of the paleoequator indicates a weakening or disruption of zonal easterly flow over the study area by latest pennsylvanian time furthermore elevated d 18 o values suggest a proximal source of paleoprecipitation over the study area perhaps due to initiation of reversed equatorial flow over tropical southwestern euramerica coupling of the geographic and temporal trends defined by pedogenic mineral d 18 o values with those defined by climatically sensitive paleosol pedotypes in the study area along with overall elevated mineral d 18 o values provides some of the strongest evidence that northern hemisphere monsoonal circulation was well established over pangea by early permian time

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

pennsylvanian sea level cycles nutrient availability and brachiopod paleoecology abstract as with modern organisms the spatial and temporal distribution of fossil communities was controlled by both the physical setting in which the organisms lived and by the organisms physiology and interactions by studying the sedimentological and geochemical context of fossil communities it is possible to assess the relative importance of the physical setting and the organisms physiology comparison of pennsylvanian brachiopod associations with changing sedimentological context water depth facies and nutrient availability indicates that body size is a function of water depth and nutrient availability for most spire bearing athyridids and spiriferids brachiopods but rarely for productid brachiopods spire bearing brachiopods dominate the associations in high nutrient settings and productid brachiopods dominate the associations in low nutrient settings this difference suggests that physiological differences between brachiopod orders such as lophophore filtering efficiency play an important role in controlling their distribution

updip sequence development on a wave and current dominated mixed carbonate siliciclastic continental shelf paleogene north carolina eastern u s a abstract cores quarry exposures and exploratory wells in the paleogene sediments of the albemarle basin north carolina span the transition between the southern carbonate and northern siliciclastic provinces of the eastern u s continental shelf succession this swell wave and current dominated shelf succession differs from the standard carbonate sequence stratigraphic model because available accommodation is limited by the depth of wave abrasion and scour the study area is on the thin updip part of the shelf which includes the relatively positive cape fear arch onslow block and the slowly subsiding 1 5 cm ky albemarle block to the north the paleogene supersequence set has a basal boundary that is a hardground on cretaceous shoreface shallow shelf mollusc facies it is overlain by a thin paleocene sequence of transgressive deeper offshore glauconitic fine sands to deep marine silt shale five regionally mappable vertically stacked eocene sequences 0 30 m thick contain coastal sands shoreface sandy mollusc rudstone offshore bryozoan grainstone packstone and subwave base fine wackestone packstone and marl eocene sequences commonly are bounded by hardgrounds overlain by thin local lowstand shallow marine sands they consist of a thin transgressive shallow marine unit commonly absent overlain by an upward shallowing highstand marine succession on the arch lowstand and transgressive units may be condensed into lags the lower oligocene succession on the arch has a single marl to fine foram sand dominated sequence whereas downdip two to three siliciclastic rich sequences are developed capped by nearshore sandy molluscan facies the upper oligocene section is dominated by possibly three sequences composed of basal thin sands grading up into variably sandy mollusc rudstone sequence development was influenced by differential movement of basement blocks which influenced localization of fines in lows coupled with increasing third order eustatic sea level changes during later paleogene global cooling this was coupled with swell wave and current sweeping of the shelf that effectively decreased available accommodation by 20 30 m and generated the distinctive hardgrounds at sequence boundaries and variable development of lowstand and transgressive system tracts the well developed highstand deposits reflect maximum accommodation allowing deposition of an upward shallowing succession that terminated at the depth of wave abrasion and current scour on the open shelf the sequence stratigraphic development contrasts markedly with that from tropical shelves which tend to be aggraded to sea level

cool water carbonates in a paleoequatorial shallow water environment the paradox of the auernig cyclic sediments upper pennsylvanian carnic alps austria italy and its implications distinct parts of limestones within the upper paleozoic auernig group of the carnic alps austria and italy are characteristic of cool water carbonates the carnic alps were between 5 n and 10 s paleolatitude during the late carboniferous a position confirmed by dasyclad algae and fossil plants the floral association occurrence of coal seams and absence of evaporites indicate a humid tropical environment the entire section lacks abiotic components of typical warm water limestones no ooids and no aggregates occur within the auernig group parts of the limestones show surprisingly a cool water association of high diversity bryozoans brachiopods crinoids red algae sponge spicules and entomozoan ostracodes the genesis of these limestones atypical for a paleoequatorial setting cannot be explained by changes in salinity bathymetry or terrigenous input the water temperature possibly linked with upwelling nutrient supply and paleoceanographic currents is the most convincing cause of this unusual association paleoceanographic changes are interpreted as linked to contemporaneous glaciation deglaciation cycles in gondwana this paper shows that cool water carbonates in shallow water environments are not necessarily nontropical as generally interpreted future studies should consider water temperature oceanic circulation and availability of nutrients supplementary to bathymetry salinity and latitude related climate in interpretation of carbonate components

facies analysis and depositional environments of permian carbonates of the venezuelan andes palaeogeographic implications for northern gondwana abstract upper palaeozoic strata in south america were deposited over cratonic areas and in intracratonic basins e g solimoes amazonas parana however in many areas such as venezuela and colombia they are rarely exposed through dense vegetation and the nature of the terrain so that the strata are poorly documented however these strata do have significant implications for the palaeogeography of the time this article reports on a study of the permian carbonates of the palmarito formation exposed throughout the venezuelan andes and discusses the succession of palaeoenvironments and the palaeogeography resulting from detailed field and lab work deposition took place on an extensive ramp which probably dipped gently northwards towards the open ocean much of the palmarito succession shows a general transgressive trend with a few thinner more regressive intervals likely a response to glaciation deglaciation cycles towards the end of the permian in the venezuelan andes region a major sea level fall led to extensive uplift and erosion resulting in a major disconformity between the upper guadalupian carbonates and overlying jurassic red beds the early palmarito tidal flat facies are noteworthy for their sub metre scale mud to sand cycles with incipient palaeosoil fabrics overlying mid outer ramp carbonate facies are characterised by thin storm beds and bioclastic lenses arranged into metre scale coarsening upward cycles succeeding shallow marine inner mid ramp bioclastic grain wacke stones are dominated by an open marine fauna of fusulinids calcareous algae crinoids bryozoans and brachiopods outer ramp facies are wacke mud stones with sponge spicules radiolarians and brachiopod fragments there are similarities between the permian deposits of venezuela and those recorded from guatemala and mexico to the north and from colombia and peru bolivia to the west and southwest where carbonate systems also dominate these facies were all part of a tropical foreland basin setting the mucuchachi basin which contrasts with the other late palaeozoic intracratonic basins in south america evidence provided here demonstrates that a seaway existed between laurentia and gondwana from the early permian through to the late early permian

stratigraphic imprint of the late palaeozoic ice age in eastern australia a record of alternating glacial and nonglacial climate regime stratigraphic and sedimentological data from new south wales and queensland eastern australia indicate that the late palaeozoic ice age comprised at least eight discrete glacial intervals each 1 8 ma in duration here termed glaciations separated by nonglacial intervals of comparable duration these events spanned an interval from mid carboniferous c 327 ma to the early late permian c 260 ma and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late early permian followed by an opposite trend towards the final demise of glaciation in the late permian the alternating glacial nonglacial motif suggests that the late palaeozoic ice age was considerably more dynamic than previously thought these patterns are remarkably consistent with recent interpretations of palaeofloral change eustatic sea level fluctuations and co 2 climate glaciation relationships for this interval of time the detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere notably in far field ice distal northern hemisphere successions

cyclic carbonate sedimentation in the upper triassic dachstein limestone austria the role of patterns of sediment supply and tectonics in a platform reef basin system abstract factors that controlled the deposition of i peritidal lofer cycles on a carbonate platform and ii platform margin reef slope deposits have been deduced at localities in the austrian alps the steinernes meer and hochkonig massif these locations are part of an entire late triassic platform reef basin system that is preserved with most original stratigraphic relationships intact platform lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolomitic mudstones to a supratidal weathering horizon soil lofer cycles show random non hierarchical stacking patterns limited lateral continuity varied progradation directions complete shoaling 98 of cycles and very low stratigraphic completeness only 1 20 exponential frequency distributions of cycle thickness suggest random aperiodic cycle deposition rather than regular deposition in response to regular eustatic sea level oscillations sediments in the adjacent reef complex record storms and the lateral migration of sand shoals and stromatolite capped banks not sea level fluctuations and intermittent subaerial exposure on the basis of these observations in contrast to many previous interpretations lofer cycles are interpreted to be mostly autocycles formed within a tidal flat island system by lateral migration of wide low emergent banks separated by shallow subtidal areas preservation potential of individual cycles is thought to have been low reworking was almost certainly very important in this system extensional regional tectonics in the late triassic exerted a long term control over the development of the tidal flat island system on the platform top and appears to have left a tectonic overprint in lofer cycle successions differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations 1200 3000 m in the norian rhaetian platform carbonates of the northern calcareous alps two important tectonic deepening events in the steinernes meer section almost caused platform drowning and correlate with a lowermost rhaetian transgression in the western tethys many other tectonic events may have gone unrecorded on the platform within a 716 m thick measured succession of lofer cycles intervals of enhanced paleokarst development nd stacked intertidal to supratidal beds are present with 20 75 m vertical spacing these platform units are interpreted to represent prolonged periods in which greater areas of the platform were occupied by intertidal to supratidal sediments these units correlate with thick units of debris flows on the reef slope what may be tectonic highstands the result of a vertical tectonic movement are recorded as packstone to grainstone deposition on the reef slope a holistic model driven by aperiodic fault controlled downdropping resulting in switching loci of sediment export patterns from a continuously operative subtidal carbonate factory the reef complex best explains vertical facies patterns in the platform and reef slope successions it is not a static sea level model although thi d fourth and fifth order eustasy is not required the model could explain the sporadic occurrence of shallowing up cycles in the adjacent kossen basin late triassic eustatic sea level fluctuations were ineffective in controlling sedimentation as a result of the processes described above

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin