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

triassic latemar cycle tops subaerial exposure of platform carbonates under tropical arid climate the triassic latemar platform in the dolomites italy is the site of several ongoing controversies perhaps the most interesting debate focuses on apparent cyclic deposition within the latemar platform whose nature and duration are still open to debate further disagreement concerns the lack of meteoric diagenesis related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence here an evaluation of the nature of latemar cycle tops is presented combining evidence from previous work and new field petrographic and geochemical data cycle tops are ranked according to increasing exposure duration and spatial extent type i surfaces lacking unequivocal evidence of prolonged supratidal conditions type ii dolomite caps formed in warm evaporitic intertidal lagoonal waters followed by exposure of perhaps intermediate duration type iii clastic rich red calcareous horizons with some showing platform wide extent representing prolonged supratidal conditions and type iv discontinuities in tepee belts genetically related to type ii and iii surfaces but likely representing shorter lived exposure stages petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking this is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that where present are here interpreted as marine vadose in origin the scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid triassic climate the low latitude tropical but arid setting of the latemar situated in the western extension of the tethys ocean its isolation from nearby continental areas and overall short term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops high amounts of aeolian clastic material beneath some cycle tops along with high fe and mn elemental abundances argue for intermittent subaerial conditions this study proposes an enhancement of the classical allan and matthews 1982 isotope model for subaerial exposure under strongly arid climates as the subaerial exposure nature of latemar cycle tops and therefore eustasy as the cause for cyclicity have been previously challenged due to the lack of meteoric induced isotopic signatures the outcome of this study is of significance for the ongoing latemar stratigraphic controversy

speculations on the controls on cyclic peritidal carbonates ice house versus greenhouse eustatic controls abstract metre scale peritidal cyclothems which are distinctive components of lower palaeozoic and many mesozoic platform carbonates are not common in carboniferous sequences in their place the platform sequences are characterized by 5 10 m thick largely subtidal limestones with prominent exposure surfaces the former type of platform cyclothems developed during greenhouse phases when in the absence of major continental ice sheets sea level oscillations were caused by small scale obliquity and precession milankovitch rhythms under such conditions platforms rapidly aggraded with the deposition of peritidal cyclothems the carboniferous was an ice house phase when the waxing and waning of gondwanan ice sheets caused by eccentricity rhythm generated climatic changes created high amplitude rapid falls and rises in sea level the resulting carbonates reflect such changes with catch up subtidal dominated sequences and pronounced subaerial exposure surfaces capping the cyclothems

impact of hot fluid advection on hydrocarbon gas production and seepage in mud volcano sediments of thick cenozoic deltas hydrocarbon seeps are ubiquitous at gas prone cenozoic deltas such as the nile deep sea fan ndsf2 where seepage into the bottom water has been observed at several mud volcanoes mvs3 including north alex mv namv4 here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of namv to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments at the centre high upward flux of hot 70 c hydrocarbon rich fluids is indicated by an absence of biomarkers of anaerobic oxidation of methane aom and nearly constant methane ch4 concentration depth profile the presence of lipids of incompatible thermal maturities points to mixing between early mature petroleum and immature organic matter indicating that shallow mud has been mobilized by the influx of deep sourced hydrocarbon rich fluids methane is enriched in the heavier isotopes with values of 13c 46 6 vpdb and d 228 vsmow and is associated with high amounts of heavier homologues c2 suggesting a co genetic origin with the petroleum on the contrary at the periphery a lower but sustained ch4 flux is indicated by deeper sulphate methane transition zones and the presence of 13c depleted biomarkers of aom consistent with predominantly immature organic matter values of 13c ch4 60 vpdb and decreased concentrations of 13c enriched c2 are typical of mixed microbial ch4 and biodegraded thermogenic gas from plio pleistocene reservoirs of the region the maturity of gas condensate migrated from pre miocene sources into miocene reservoirs of the western ndsf is higher than that of the gas vented at the centre of namv supporting the hypothesis that it is rather released from the degradation of oil in neogene reservoirs combined with the finding of hot pore water and petroleum at the centre our results suggest that clay mineral dehydration of neogene sediments which takes place posterior to reservoir filling may contribute to intense gas generation at high sedimentation rate deltas

stacking pattern of cyclic carbonate platform strata lower cretaceous of southern apennines italy detailed sedimentological analysis of aptian albian shallow water carbonate platform strata from monte faito southern italy reveal composite sea level change the organization of the lithofacies and their related diagenetic overprint have allowed the identification of 50 elementary cycles they often show exposure related diagenetic features superimposed on subtidal deposits suggesting drops of relative sea level elementary cycles are grouped into 16 bundles forming six superbundles the hierarchical organization of the cycles are interpreted to be related to composite eustatic sea level fluctuations driven by the earth s orbital perturbation in the milankovitch frequency band moreover the systematic variation of superbundle thickness and the general evolution of the lithofacies reflects variations of accommodation potential defining longer term environmental changes different lithofacies forming elementary cycles are frequently separated by bedding planes that when viewed in a sequence stratigraphic context are interpreted as transgressive surfaces maximum flooding surfaces or sequence boundaries this allows the recognition of lowstand transgressive and highstand deposits in the cyclic units considering both the bundle superbundle ratio and the sequence stratigraphic interpretation of the superbundles a chronostratigraphic diagram illustrating both gaps at bundle level and their expected location along the section was created it establishes that the section developed over about 2 4 ma

dolomite formation in the shallow seas of the alpine triassic the alpine triassic units of switzerland northern italy and western austria offer an extensive geological archive in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context recent studies clearly demonstrate that large amounts of the alpine triassic dolomites are late diagenetic or hydrothermal nevertheless as part of multiple generations of diagenetic overprint some generations of fine crystalline ca rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments in this review three cases of alpine triassic dolomites are discussed where dolomite rocks may have formed during or soon after sedimentation the sedimentary facies indicate contrasting palaeoenvironmental conditions and hence document three different possible processes of dolomite formation i in the dolomite mountains northern italy dolomite beds of the partly isolated middle triassic anisian ladinian latemar platform are confined to the very top of shallowing upward lagoonal facies cycles ii dolomite beds of the san giorgio basin southern switzerland an intra platform basin that opened during the anisian ladinian transition are associated with organic carbon rich shales which were deposited in a deeper water environment under anoxic conditions iii in the entirely dolomitized platform facies of the dolomia principale hauptdolomit formation a very early generation of fine crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles this platform extended over thousands of square kilometres along the tethys margin during the late triassic carnian and norian and large amounts of carbonate were deposited under hypersaline sabkha like conditions representing three distinct depositional environments these three different triassic systems show features in common with several dolomitization models developed from the study of modern dolomite forming environments for example the sabkha model the evaporative lagoon lake model the organogenic model and the microbial model although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation dolomite forming processes during the triassic were apparently quite different from the modern world in terms of distribution and scale recent developments in stable isotope geochemistry and high resolution geochemical probing offer the possibility to make better reconstructions of triassic palaeoceanographic conditions and suggest a non actualistic approach to better understand dolomite formation during the triassic

platform to shelf edge transect triassic of northern israel north arabian plate abstract a transect of the middle to late triassic from the epicontinental belt to the shelf edge of the north arabian margin is presented based on new data from two boreholes in northern israel two sedimentary systems are encountered along this traverse the inland meged 2 borehole represents the seaward extension of the epicontinental belt with shallow to deep water carbonates salina evaporites and basinal shales while ga ash 2 on the levant margin was positioned on an isolated platform separating the internal shelf basin system from the open tethys displaying a shallower range of facies that is free of evaporites proximal distal facies shifts are bundled into five low order lo t r cycles ranging from anisian pelsonian to norian each composed of numerous high order ho shallowing upwards cycles meged 2 paleoenvironments imply an interior basin trapping sediments of the epicontinental belt with numerous ho facies shifts indicating pulses of subsidence ga ash 2 contains lofer like peritidal cycles and a late anisian early carnian hiatus in place of the second lo cycle the two orders of cycling are controlled by extrabasinal regional or eustatic change and synsedimentary vertical movements the divergent sedimentary patterns of the late anisian late carnian can be attributed to nw se trans tensile movements impacting on earlier neotethyan extensional trends synsedimentary movements represented locally by uplift at ga ash 2 and subsidence at meged 2 controlled the influx of sea water into the epicontinental zone and consequently controlled the alternation of evaporite vs carbonate systems in the latest carnian early norian these facies belts become locked together forming a shallow high energy shelf this change reflects broad uplift and shifting of the locus of active differential movement into the levant basin

holocene sedimentation in the tectonically active tekirdag basin western marmara sea turkey abstract the effects of various depositional conditions and active tectonics on the late holocene sedimentation in the tekirdag basin west of the marmara sea were investigated in 11 gravity cores and on 3 seismic profiles representing shelf to slope to deep basin floor environments except for the two coarser grained avg 60 sand and gravel and calcareous avg 25 caco 3 shelf cores sediments in the slope and basin floor cores constitute low calcareous avg 12 caco 3 and terrigenous mud 90 silt and clay the occurrences of relatively higher total organic carbon contents in sediments from the southern slope 1 14 1 38 compared to that from the northern slope 0 37 1 2 indicate significant input from the southern rivers the presence of silty sandy laminations within the mud matrix of the cores and chaotic and mounded reflection configurations on the seismic profiles from the slope and basin floor environments all suggest the effects of gravity mass movements of sediments slides slumps turbidites most likely due to seismotectonic events earthquakes however it was difficult to establish a basin wide correlation between the historical earthquake events in the region and the positions of silty sandy laminations in dated deep sea sediment cores due to complicated other events such as repeated river run off and floods increased sediment load and slope failure the water contents of sediments from the basin floor avg 10 30 are significantly lower than those from the slope sediments avg 32 46 the former can be explained by the influences of synsedimentation with tectonics and compaction between the north and south boundary sub faults of the naf system whereas the latter shows fluidization of slope sediments during sliding and slumping radiocarbon dated base sections from three cores revealed that sediment deposition at these sites commenced about 4700 bp when the late holocene depositional conditions changed from the formation of upper sapropels to the present state the increases of organic carbon contents at or near the base sections of the cores support this contention

subsidence and sedimentation on jurassic passive continental margin southern alps italy the southern alps of italy preserve a tectonically intact array of jurassic facies that record the evolution of a part of the margin of the apulian plate from its ancestral beginnings in a complex of permian and triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs then as the new ligurian oceanic basin began to open farther west and as subsidence gradually slowed accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility carbonate dissolutio levels and the strength of bottom currents estimates of the history of seafloor depths based on a simple subsidence law of the form subsidence k age 12 provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the jurassic in this region the rapid 1 km deepening of the compensation depth for calcite during the late jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling fertile silica rich carbonate dissolving to downwelling less fertile silica poor carbonate preserving

the changing climate framework and depositional dynamics of triassic carbonate platforms from the dolomites abstract the triassic of the dolomites offers a valuable opportunity to investigate the relationships between climate fluctuations and the changing depositional dynamics of carbonate platforms the reconstruction of the large palaeoclimatic modification is based on the synthesis of multiple sedimentological palaeobotanical pedological and geochemical proxies the triassic climate fluctuations were generally fast in nature and were probably associated with a latitudinal shift of the monsoon belts during the triassic the dolomites were placed at an intertropical northern latitude of about 16 18 and were generally dominated by dry climate and elevated temperature at least five pulses toward moister climate are however documented dated to early olenekian middle anisian late ladinian middle carnian and late norian times within this variable climatic scenario the carbonate systems record a global evolutionary trend from late permian early triassic regional shelves rich in loose micrites and bio calcarenites to anisian ladinian synsedimentary cemented steep sided 35 40 high relief platforms and back to carnian low angle ramps moist transgressive episodes were surprisingly prone to the spreading of corals and other colonial reef organisms probably because of the reduced space competition by syndepositional cementation structures the permo triassic boundary extinction dramatically impacted on the calcified organism communities but the carbonate depositional architecture nevertheless stayed almost unaffected through the boundary the early triassic and earliest anisian were commonly associated with hyperhaline sometimes evaporitic conditions intervals of moister climate and efficient river discharge are however documented by earliest induan and early olenekian sediments the latter terrigenous deposits matched a crisis in the carbonate production after a return to arid conditions during the anisian a fluctuation toward moister climate developed associated to a massive continental vegetation cover the anisian tectonic activation of the region fragmented the previous carbonate shelf triggering the development of three generations of fast prograding but globally backstepping isolated platforms recording an increasing role of the synsedimentary cementation a dry climate dominated the late anisian and early ladinian interval at the time a fast subsidence pulse was associated with platform drowning and basinal anoxia only a few aggrading pinnacles were able to survive providing the nuclei for larger prograding platforms during a dry climate interval massive syndepositional cementation generated a major source of carbonate carbonate production stayed active through the ladinian magmatic phase but the onset of volcanism was associated with a large modification in the platform carbonate facies which became dominated by automicrites moist climate phases are well documented again during the late ladinian followed by a return to dry conditions the late ladinian early carnian platforms prograded toward the eastern dolomites depocentre eventually infilling almost the whole of the accommodation space the middle portion of the carnian recorded renewed sharply moist phases associated with the demise of the rimmed platforms and with the return to loose sediment ramps at least four separated humid fluctuations can be identified in this carnian interval alone climate then returned to persistent aridity until the late norian moist phase

nature and evolution of middle triassic carbonate buildups in the dolomites italy during the middle triassic an evolution from carbonate bank to buildup highly elevated in respect of the adjacent basins is observed in the alps the dolomites in the southern alps italy are here considered as an example during the anisian subtidal banks of the upper serla fm with rare peritidal cycles show only a limited biologic density and diversity with the late anisian contrin fm a gradual development of binding and framework building communities can be observed the sinking of wide areas of the contrin carbonate bank into anoxic conditions plattenkalke fm allowed the birth of isolated buildups their increased carbonate production due to a greater diversity and density of organisms was able to keep up with the rate of subsidence the latemar buildup with additional data for the slope taken from mt coldai civetta and sass da putia is used to illustrate the main character of a lower ladinian buildup the restricted platform sediments display peritidal cycles prevalent in the subtidal portion is dasycladacean packstone grainstone associated with gastropods ammonoids and blue green algae in the supratidal portion desiccation and tepee structures are present scleractinian and sphinctozoan frameworks are rare on the upper part of the slope which is bound predominantly by encrusting organisms mainly blue green algae porifers and tubiphytes the prevailing sediments on the upper part of the slope are bindstones whilst packstones and rudstones are in the lower part of the slope the latter carbonates were cemented early under submarine conditions mostly by isopachous palisade cement polyphasic internal sediments with phreatic vadose influence are present and can in the middle lower part of the slope replace most of the original sediment

carbonate platform lagoon and delta high frequency cycles from the carnian of lombardy southern alps italy abstract the early and middle carnian succession of the bergamasc alps southern alps italy was deposited in three different environments a deltaic depositional system passing seaward to a lagoon or bay and then to a carbonate platform the corresponding lithofacies sandstones and siltstones shales and limestones are organized in high frequency cycles fining upward trends are typical of the subaerial delta plain cycles and shallowing upward trends are widespread in the lagoon and platform cycles the genesis of the lagoonal and carbonate platform cycles is possibly accounted for by sea level changes mainly controlled by small scale eustatic oscillations sea level changes are interpreted as exerting a major control also in the development of the deltaic cycles

salina margin tepees pisoliths and aragonite cements lake macleod western australia their significance in interpreting ancient analogs tepee structures banded aragonite cements and pisoliths are currently forming in lake macleod a carbonate evaporite salina in western australia although lake macleod is separated from the indian ocean by a barrier it lies 3 4 m below sea level which promotes the seepage of seawater through the barrier and its discharge from vents and seepage mounds around the margin of the salina discharging waters have precipitated and diagenetically altered carbonate sediments within these seepage mounds to form tepee structures of lithified protodolomite overlying cavities that are lined with banded aragonite cement and floored by both cement and pisoliths significant variations in 18o 5 1 pdb and 13c 5 5 pdb of the aragonite cements were documented and are thought to record shifts in the isotopic composition of the water brought about by the effects of evaporation influx of meteoric water and oxidation of organic water carbon 14 dating of cements indicates that cementation began about 3400 b p and has proceeded at a rate of about 0 2 to 0 4 mm 100 yr the highest rate occurring during evaporative episodes by analogy with lake macleod and other australian salinas peritidal tepee structures and associated diagenetic carbonates in the permian capitan reef complex may owe their origin to speleanlike diagenesis operative in a marine groundwater discharge zone

birdseyes fenestrae shrinkage pores and loferites a reevaluation abstract birdseyes birdseye limestone fenestrae fenestral fabric shrinkage pores and loferites are considered similar if not synonymous when occurring in lime mudstone or syndepositional dolomite especially in association with mudcracks and stromatolites compaction experiments indicate however that in the absence of early cementation these rugs can be completely obliterated whereas mudcracks and stromatolites remain relatively unchanged lumping of burrows and root tubes under the general terms birdseyes or fenestrae such as for example tubular fenestrae burrow fenestrae or root tube fenestrae is discouraged they should be called burrows or root tubes because the birdseyes fenestrae and shrinkage pores are so intimately associated with tidal flats in the minds of most geologists submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the bahama banks has resulted in rugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and assoc ated sedimentary structures

diagenesis in limestone dolostone successions after 1 million years of rapid sea level fluctuations a case study from grand cayman british west indies abstract meteoric diagenesis in young marine carbonate sediments has commonly been linked to fluctuations in quaternary glacio eustatic sea levels the extent to which these sea level changes are recorded in these carbonate successions however remains questionable this is amply demonstrated by the diagenetic record found in the limestones and dolostones of the cayman formation miocene on the cayman islands on the eastern part of grand cayman dolomitization that ceased by 1 million years ago created an architecture whereby the limestones in the central part of the island were surrounded by dolostones in coastal areas of the island since then the upper 90 m of the cayman formation has been repeatedly cycled through many different marine and meteoric diagenetic zones as large rapid eustatic oscillations in sea level affected the island the records of these diagenetic cycles in the dolostones and limestones are however different and impossible to match to the cyclic changes in sea level in the peripheral dolostones post dolomitization diagenetic features are sparse in contrast the limestones in the interior of the island exhibit a wider variety of meteoric diagenetic features including extensive dissolution and calcite cementation the dolostones have low porosity

role of autochthonous versus detrital micrite in depositional geometries of middle triassic carbonate platform systems middle triassic platforms formazione di contrin upper anisian in the italian dolomites southern alps record large changes in carbonate production and depositional geometry the changes include variation in the abundance of detrital micrite i e allochthonous calcareous mud relative to the autochthonous micrite which formed in situ and was syndepositionally lithified carbonate factory dynamics controlled the geometry of the clinostratified slope units and were probably associated with changes in oxygenation early anisian low relief platforms were followed by late anisian high relief buildups associated with basinal dysoxic anoxic environments the geometric evolution of the platform slopes records a progressive increase in the dip angle of clinostratifications matched by a deepening evolution of the basinal environments at the same time the slope sediments record a gradual change from loose detrital micrite characterizing the lower portion to autochthonous micrite dominating the upper part and recording massive syndepositional lithification the development of the autochthonous micrite was associated with the preservation of significant amounts of organic matter this sharp increase in automicrite formation was probably induced by a rapid change from oxic to suboxic conditions in the carbonate slope and margin environments while anoxic conditions developed in the adjacent organic rich basin moena formation the low oxygen level promoted the preservation of organic matter and the activity of sulfate reducing bacteria which in turn induced in situ deposition of autochthonous micrite through biomediated processes this pervasive early cementation and lithification induced the development of steep platform slopes the conceptual model distilled from this middle triassic case can support the interpretation of analogous buildups where the skeletal framework is subordinate to the micrite component

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

cyclostratigraphy and sequence boundaries of inner platform mixed carbonate siliciclastic successions barremian aptian zonguldak nw turkey abstract barremian aptian shallow marine mixed carbonate siliciclastic successions of the zonguldak area nw turkey have been studied in detail along three measured stratigraphic sections field and laboratory analysis of microfacies and sedimentary structures have demonstrated that sections are composed of meter scale shallowing upward cycles cyclicity of shallow water mixed carbonate siliciclastic successions are generally observed in the form of alternating sandstone siltstone or sandy limestone and limestone a prominent level of charophyte packstone within the successions of zonguldak and kozlu is interpreted as a sequence boundary in the cengellidere section cyclicity is generally represented by an alternation of sandstone conglomerate and bioclastic limestone the first level of conglomerate within the section is interpreted as a sequence boundary smaller scale cyclic arrangements are also recorded within meter scale cycles 4th 5th and 6th order cycles are superimposed and create a hierarchical stacking however 6th order cycles present a relatively random distribution and termed as episodic thickness variation of cycles along the section and frequency of sand and or silt rich cycle types are used to interpret systems tracts within the sequences

models for generation of carbonate cycles computer modeling provides a quantitative approach to a better understanding of actual carbonate cyclic sequences to model carbonate cycles the authors can use water depth dependent sedimentation rate for each facies an initial lag time linear subsidence tidal range and period and amplitude of sea level oscillation about a horizontal datum tidal flat capped cycles up to a few meters thick result from low amplitude sea level oscillation of a few meters and short lag times nonerosive caps reflect sea level lowering being balanced by subsidence and basinward migration of the shoreline not exceeding tidal flat progradation rate when higher amplitude sea level oscillations occur the tidal flats are abandoned on the inner shelf during sea level fall because seaward movement of the strandline outpaces progradation rate of flats increased amplitude also results in sea level falling faster than flats can subside so that disconformities with thick vadose profiles develop high amplitude 100 m or more oscillations result in incipient drowning of platforms and juxtaposition of deep water facies against shallow water facies within cycles sea level falls before the platform can build to the sea level highstand and the shoreline migrates much more rapidly than tidal flats can prograde thus cycles are disconformity bounded and lack tidal flat caps 10 references

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations

global milankovitch cycles recorded in rock magnetism of the shallow marine lower cretaceous cupido formation northeastern mexico rock magnetic cyclostratigraphy was measured in the barremian aptian cupido cupidito formation northeastern mexico the goal was to develop an objective evaluation of palaeo environmental variability recorded in the formation that is independent of facies analysis and interpretation anhysteretic remanent magnetization arm was used to estimate magnetic mineral concentration variations for the upper 143 m of the formation which is characterized by metre scale carbonate cycles representative of innerand middle shelf marine environments isothermal remanent magnetization acquisition experiments and scanning electron microscope sem examination indicate that micron sized detrital magnetite from eolian dust carries the arm signal at the sampled sections from garcia and chico canyons 25 km apart arm records a synchronous 30 35 m oscillation with maxima coinciding with fourth order sequence boundaries superimposed with prominent high frequency variability calibrating the 30 35 m oscillation to a 405 kyr period long eccentricity cycle focuses the high frequencies into short eccentricity obliquity and precession index bands the precession band signal modulates with an eccentricity signature the arm signal is correlated between sections but decoupled from the interpreted fifthorder depositional cycles arm amplitudes diminish up section with facies suggesting deepening conditions that diluted magnetite concentration this probably signals a warming increasingly humid climate changing global circulation and or greater dispersal of magnetite grains earth s ancient shallow marine carbonate platforms are important archives of past global change their in situ biological genesis makes them sensitive indicators of physical conditions in shallow marine environments including temperature circulation and sea level wilson 1975 the accommodation space afforded by water depth is thought to govern the amount and type of sediment that accumulates at a given location water depth in turn is controlled by a combination of tectonic subsidence near shore deposition and global sea level which vary significantly through time schlager 2005 the commonly observed stacking of metrescale shallowing upward cycles in carbonate platform deposits has led to proposals of allocyclic v autocyclic forcing mechanisms the most widely cited allocyclic hypothesis is astronomically controlled eustasy fischer 1964 hardie et al 1986 autocyclic mechanisms involve infilling and oceanward progradation of sedimentation without eustatic change ginsburg 1971 to this day the allocycle v autocycle debate remains unresolved in understanding the origin of 10 10 year scale lithologic cyclicity in shallow marine carbonates schwarzacher 2000 burgess 2006 schlager 2010 full shallowing to sea level may not develop in response to every sea level oscillation subtidal missed beats resulting in amalgamated cycles conversely prolonged episodes of subaerial exposure may occur in inner platform settings that are only briefly or never flooded with every sea level oscillation leading to condensed or peritidal missed beats goldhammer et al 1990 ultimately facies analysis of shallow marine sections is limited in teasing apart the interplay of eustasy owing to climate change and subsidence arising from local tectonics this is in large part owing to the relatively subjective interpretation of lithofacies that is required from the stratigrapher therefore we need to develop facies independent from jovane l herrero bervera e hinnov l a housen b a eds 2013 magnetic methods and the timing of geological processes geological society london special publications 373 325 340 first published online april 25 2013 http dx doi org 10 1144 sp373 20 the geological society of london 2013 publishing disclaimer www geolsoc org uk pub ethics by guest on july 5 2013 http sp lyellcollection org downloaded from