controls and predictability of carbonate facies architecture in a lower jurassic three dimensional barrier shoal complex djebel bou dahar high atlas morocco spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics external controls and degree of predictability of the facies architecture of shallow water carbonate platform tops to quantify the spatial distribution and vertical stacking of lithofacies within an outer platform shoal barrier complex integrated facies analysis and digital field technologies have been applied to a high relief carbonate platform exposed in the djebel bou dahar lower jurassic high atlas morocco the outer platform is characterized by subtidal cross bedded coarse grainstone to rudstone grading into supratidal pisoidal packstone rudstone with tepees that together formed a 350 to 420 m wide shoal barrier belt parallel to the margin this belt acted as a topographic high separating a restricted lagoon from the subtidal open marine region low energy tidal flats developed on the protected flank of the barrier facing the lagoon lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model the outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts in addition the spatial heterogeneity was captured in multiple time slices i e during different phases of cyclic base level fluctuations in general the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top establishing low energy subtidal conditions across the whole platform and when the accommodation space was filled with tidal flat facies heterogeneity increased during deposition of the relief building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation cycles commonly are composed of a thin transgressive tidal flat unit followed by coated grain rudstone bar facies lateral to the bar facies pisoidal grainstone beach deposits accumulated these bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand there the bars either graded into supratidal pisoidal facies with tepees when accommodation space was filled or were capped by subaerial exposure due to a sea level fall modified embedded markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval constant depositional mode analysis of individual sections did not reveal any ordering which may be related to the limited thickness of these sections composite sections however rejected the null hypothesis of randomness the addition of stratigraphically significant information to the markov analysis such as exposure surfaces and lateral dimensions of facies bodies strengthens the verdict of unambiguous preferential ordering through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation 2009 the authors journal compilation 2009 international association of sedimentologists

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

holocene climatic change in the zacapu lake basin michoacan synthesis of results following archaeological investigations of the preclassic occupation of the western part of the zacapu lake basin a series of analyses were made from four cores taken along a transect between the centre and the shore of the former marsh drained at the beginning of this century eighteen samples were radiocarbon dated yielding ages between 8220 60 years bp and 2170 60 years bp uncalibrated here we present a summary of the results of the sedimentological palynological and diatom studies undertaken which indicate changes in the water level in zacapu and changes in local climate taking into account the effects of local volcanic activity

early mid pleistocene environments in the valsequillo basin central mexico a reassessment the valsequillo basin in central mexico has been of interest due to the presence of megafauna and evidence for early human occupation but research has been controversial it has been suggested that extensive and deep lakes characterised the early pleistocene environment but sediment exposure is highly fragmentary and reliable dating has been difficult here we report for the first time early pleistocene palaeoenvironmental reconstructions using stable isotopes diatoms tephra and pollen we studied several stratigraphic sections of mainly non volcanic rocks containing the 1 3 ma xalnene ash as a stratigraphic marker the isotope and other proxy data show that topographically low points in the basin were occupied by spring fed shallow water lakes during the early mid pleistocene with a trend to drier conditions the basin was a dynamic volcaniclastic environment during this period with the production of the toluquilla volcano sequence and other rhyolitic dacitic volcanic ashes interbedded with the lake sediments at the sections studied there is no evidence from the sections for extensive and deep lakes before or after the xalnene ash deposition the presence of lakes in the basin during the early pleistocene would have made it attractive for megafauna

vegetation in western central mexico during the last 50 000 years modern analogs and climate in the zacapu basin the late glacial and the transition towards the holocene marked a period of significant environmental change at a global scale in western central mexico few records span beyond the holocene and little is known about pleistocene climatic and environmental variability here we report on the pollen record of a composite sequence made of three cores that cover the last 50 ka thousands of calibrated radiocarbon years before present in the zacapu basin western central mexico 1970 m asl the conjunction of modern pollen rain from the area with the composite fossil pollen sequence from cores cantabria cantabria 1 and cemca point 1 provided a detailed history of the vegetation of the area recognition of modern environmental patterns through individual modern pollen taxa proved difficult but multivariate analysis separated different vegetation types no modern analog communities relative to the modern vegetation cover of the area were evident especially during periods of drought apparently summer precipitation was significant before 40 ka and after the deglaciation however this moisture source declined from 40 to 10 ka because tradewinds from the gulf of mexico were blocked by the eastern mountains of central mexico as a result of lower temperatures by contrast westerly and northerly winds were enhanced by maximum ice cover from 19 to 17 ka providing abundant winter precipitation and therefore preventing extreme drought in the area copyright 2012 john wiley sons ltd

preservation of forcing signals in shallow water carbonate sediments abstract no consensus has been reached on whether the metre scale cycles that commonly occur in peritidal carbonates are predominately a product of external relative sea level variations allocycles or an intrinsic property of carbonate production generated via the interaction of non linear processes autocycles for any forcing signal such as eustatic sea level change to be detectable in stratigraphy its effects must be preserved here a deterministic three dimensional geological process model is used to explore how such cycles are preserved in the geological record in the presence of autocyclic processes each simulation produced cycle thickness distributions that are statistically indistinguishable from a theoretical poisson process regardless of whether auto or allo cycles dominated spectral analysis of depositional time series constructed from idealised geological sections showed that all detectable signals occurred within the milankovitch forcing frequency bands even when no milankovitch forcing was present thus it is deduced that from any geological section alone external forcing signals are detectable but are not distinguishable from autocyclically produced signals interestingly there is no correlation between the percentage of sediment preserved and the accuracy with which signals are detectable in the preserved sediment in some model realisations even with preservation as low as 40 the correct forcing signal can be detected accurately while conversely sections with preservation as high as 90 can have poor signal preservation the reverse can also be true in other models it is therefore concluded that distinguishing allocyclic and autocyclic forcing in shallow marine or peritidal carbonate successions is likely to be extremely difficult except in cases of extraordinary sedimentary preservation and dating accuracy

a reappraisal of dolomite abundance and occurrence in the phanerozoic critical assessment of dolomite abundance throughout the phanerozoic suggests that most large scale dolomite has had an origin related to salinity elevated seawaters the distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large scale evaporite basins lagoons existed the development of such depositional systems is controlled largely by eustasy paleogeographic tectonic setting and paleoclimate extensive peritidal sequences are characteristic of mature passive margin carbonate platforms that formed during long periods of earth history generally devoid of major continental glaciation e g late precambrian through early paleozoic and mesozoic sea level fluctuations of relatively low amplitude during such periods allowed extensive cyclic peritidal dominated platforms to develop such platforms typically lack raised rims had depositional surfaces that stayed near sea level for tens of millions of years and show little evidence of major drops in sea level that would have exposed the shelf for long periods this would have allowed repeated replenishment of pore waters by mg rich marine fluids over very extensive supratidal surfaces also since the cycles are very thin the downward moving brines could penetrate several cycles on the inner platform causing massive dolomitization in contrast during major glacial periods e g middle to late mississippian to early permian and late cenozoic higher amplitude ice driven fluctuations in sea level led to extensive meteoric water flushing of shallow platforms but related dolomitization has been of minor significance under conditions of elevated mg ca ratio in hypersaline brines however large scale dolomitization could occur in response to the formation of extensive evaporite basins lagoons suitable climatic and paleogeographic tectonic conditions have allowed such dolomitization at various times during the phanerozoic regardless of specific styles of eustatic sea level change lack of massive dolomitization in holocene carbonates may be because neither extensive peritidal sequences nor large scale evaporite basins lagoons are prevalent in modern carbonate environments

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

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

late pleistocene and holocene paleoenvironments of chalco lake central mexico abstract in order to establish paleoenvironmental conditions during the late quaternary four cores from the basin of mexico central mexico were drilled in chalco lake located in the southeastern part of the basin the upper 8 m of two parallel cores were studied using paleomagnetic loss on ignition pollen and diatom analyses based on 11 14c ages the analyzed record spans the last 19 000 14c yr b p volcanic activity has affected microfossil abundances both directly and indirectly resulting in absence or reduction of pollen and diatom assemblages important volcanic activity took place between 19 000 and 15 000 yr b p when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry cold climate during this interval abrupt environmental changes with increasing moisture occurred from 15 000 until 12 500 yr b p the lake level increased and the pollen indicates wetter conditions the highest lake level is registered from 12 500 to ca 9000 yr b p the end of the pleistocene is characterized by an increase in humidity from 9000 until ca 3000 yr b p chalco lake was a saline marsh and the pollen record indicates warmer conditions after 3000 yr b p the lake level increased and human disturbance dominates the lacustrine record

architecture and depositional sequences of tertiary fault block carbonate platforms an analysis from outcrop miocene gulf of suez and computer modelling abstract this study combines outcrop with seismic data and tectonostratigraphic modelling to characterise the stratigraphy of tertiary fault block carbonate platforms seismic sections from the middle east and southeast asia indicate that carbonate platforms commonly develop on fault blocks in the late syn rift to post rift stage of basin evolution they are characterised by retrogradational or drowning morphologies with deep water clastics evaporites sealing the platforms a new tectono stratigraphic modelling program simulates the development of carbonate platforms on domino style fault blocks and predicts a distinctive tectonic control on the evolving syn rift stratigraphy detailed field work from a three dimensionally exposed miocene syn rift platform from the gulf of suez gebel abu shaar egypt shows the existence of unconformity bound depositional sequences which are predicted by the program and are interpreted to have formed in response to fault block rotation the sequences are characterised by synchronous hangingwall subsidence and footwall uplift and erosion the seismic images the modelling and the outcrop study provide different levels of information on fault block platforms and their characteristic stratigraphy the scale of the modelling is of particular value as it provides a link between the seismic scale imaging and outcrop data

the last glacial maximum in the basin of mexico the diatom record between 34 000 and 15 000 years bp from lake chalco lacustrine sediments from a section of a core extracted from the central part of the former lake chalco were analysed for diatom content radiocarbon dates and tephra correlations bracket the period under study between 34 and 15 ka bp according to the diatom stratigraphy chalco was an alkaline saline lake during the interval 34 31 5 ka bp after 31 5 ka bp it went through a shallower and more concentrated phase until ca 23 5 ka bp at ca 23 5 ka bp a major environmental change took place during which lake chalco was transformed from an saline alkaline pond into a freshwater lake previous archaeological work at tlapaocya indicates that humans entered the basin slightly after this environmental change by ca 22 ka bp at 19 ka bp another reduction in water level occurred with the onset of a period of quick fluctuations between slightly acidic and neutral water these variations were probably associated with intensified volcanic activity that affected the area between 19 and 15 ka bp

holocene climate change and human impact central mexico a record based on maar lake pollen and sediment chemistry abstract this paper presents multiproxy pollen magnetic susceptibility sediment chemistry records from two maar crater lakes in the valle de santiago of guanajuato mexico hoya san nicolas and hoya rincon de parangueo sediment cores from the two sites have basal dates of ca 11 600 and 9600 cal yr bp respectively the san nicolas results show that the lake desiccated several times during the holocene and this resulted in the selective destruction of the less resistant pollen types a comparative analysis of data from both sites indicates that during the earliest holocene ca 11 600 9000 cal yr bp there were short term oscillations of wetter and drier climate during the early holocene ca 9000 5700 cal yr bp it was on average wetter during the middle holocene ca 5700 3800 cal yr bp drier and during the middle to late holocene ca 3800 2200 cal yr bp wetter increasing human disturbance during the late holocene of environment obscures the climatic record agricultural activities in the area began as early as ca 5700 cal yr bp and intensified around 2400 cal yr bp

holocene aridification vegetation change sedimentation regime and limits of carbon isotope data as indicated by the alluvial pollen sites of arroyo grande and el molino in north central mexico abstract a multi proxy record of palynological sedimentological archaeological and carbon isotopic data of alluvial sites in the upper nazas basin arroyo grande durango 1855 m amsl and the parras basin el molino coahuila 1245 m amsl is introduced describing environmental changes across a short grassland desert ecotone from the eastern margins of the sierra madre occidental smo these sites reconstruct holocene vegetation history in semi arid north central mexico and integrate fluvial geomorphology and palynology data in particular pollen data links a rise of xeric flora with higher energy deposition and increased erosion in upland basins of the eastern smo according to archaeological finds at examined sites human occupation is linked to mesic or afforested conditions however limitations of carbon isotope data for environmental historical reconstruction are indicated by somewhat stable values at arroyo grande durango especially while pollen sites show a replacement of short grasses and park woodlands by a more xeric flora at higher elevations with a development of chihuahuan desert flora at low elevations el molino coahuila vegetation reconstructions are supported by pollen analog data collected from the cuatro cienegas region in coahuila mexico

orbital cycles differential subsidence and internal factors controlling the high frequency sequence architecture in a sinemurian shallow carbonate platform mallorca island spain abstract the 125 m thick lower to lowermost upper sinemurian peritidal to shallow subtidal platform carbonates in the llevant mountains of mallorca spain have been analysed from four stratigraphic sections to provide new data on the hierarchical stacking pattern of high frequency depositional sequences due to the fact that in shallow water environments the stacking of carbonate facies can be controlled by external allocyclic and internal autocyclic processes deciphering the dominant controls on the high frequency sequence architecture of these platforms is a challenge the studied carbonates encompass a high variety of facies representative of open lagoon internal bars restricted lagoon and tidal flats with local beach sands based on a detailed analysis of vertical facies trends and bounding surfaces large medium and small scale sequences have been identified within the long term transgressive regressive facies cycle defined by the entire succession large scale sequences 10 30 m thick and medium scale sequences 1 10 m thick are generally shallowing upward sequences bounded by sharp facies changes to relatively deeper facies and have been related to sea level variations driven by long 400 kyr and short 100 kyr eccentricity cycles respectively the overprinting of differential subsidence probably related to extensional tectonic and carbonate production and accumulation processes varied throughout time controlling the lateral continuity and preservation potential of the 100 kyr medium scale sequences shallowing upward locally aggradational small scale sequences 0 3 to 5 m thick are very variable in number and thickness and cannot be correlated between sections pointing out that their most important controlling factor was the internal processes hiatuses and erosion related to subaerial exposure at the peritidal caps lateral migration of internal bars local wave and currents patterns depositional and erosional processes related to spring tides and storms in the frame of a complex mosaic of facies within the studied platform

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

field and modelling studies of cambrian carbonate cycles virginia appalachians reply abstract field studies coupled with computer modelling of carbonate cycles in the elbrook conococheague formations middle to upper cambrian in the appalachians have helped define the mechanisms of formation the sequence is up to 1 6 km thick and composed of cyclic peritidal carbonates these formed on an aggraded rimmed shelf on a mature passive continental margin the cycles 1 7 m thick have lower parts of subtidal intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite shale and breccia sedimentation rates for these peritidal carbonate environments exceeded long term subsidence of the platform 0 01 to 0 1 m 1 000 yr thus the shelf stayed near to sea level i e was aggraded milankovitch 20 000 to 100 000 yr low amplitude sea level fluctuations few meters superimposed on 3rd order sea level fluctuation of 1 to 3 m y duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform less than 0 02 m km slopes resulting in deposition of over 400 peritidal cycles in about 26 m y cycles that formed during 3rd order sea level rise are thick and limestone rich whereas those formed during 3rd order fall are thin quartzose and highly dolomitized the modelling suggests that fewer and thinner cycles formed during the long term falls compared to the rises reflecting decreased accommodation cycle frequency decreases into areas of lower subsidence rate open marine cycles typical of the outer shelf and cycles formed during 3rd order sea level rise contain digitate bioherms grainstone and ribbon carbonates in lower parts they reflect high subsidence rate and an open marine setting inner shelf cycles and cycles that formed during 3rd order sea level fall commonly are thrombolitic or have thin subtidal parts grainstone conglomerate or stromatolite and reflec the low subsidence rates interior areas of quaternary carbonate shelves are characterized by incipiently drowned facies and pinnacle reefs and are punctuated by soil caliche horizons and karst surface that reflect 100 m glacio eustatic sea level fluctuations in contrast many ancient shelves like the cambro ordovician shelf of the appalachians were flat topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea level fluctuations such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low amplitude milankovitch sea level fluctuations as opposed to the large scale fluctuations that typified the quaternary

a liassic isolated platform controlled by tectonics south iberian margin southeast spain the middle and upper members of the gavilan formation near velez rubio almeria province southeast spain are interpreted as carbonate platform deposits the studied outcrops form geologically part of the external zones of the betic cordillera south iberian palaeomargin a genetic unit limited by intra sinemurian and intra upper carixian sedimentary breaks is defined the evolution of the parasequence sets in the area and the development of deeper sedimentation in its northern outcrops suggest that this platform was isolated with regard to the south iberian margin three distinct parasequence sets are proposed progradational parasequence set only locally present retrogradational parasequence set middle member of the gavilan formation and aggradational parasequence set basically composed of the upper member of the gavilan formation sedimentary features such as neptunian dykes sharp lithological changes thickness variations the presence of palaeofaults and the subsidence history all lead to the conclusion of a significant regional tectonic influence in the genetic unit evolution normal extensional faulting has been proposed as a major control on sedimentation on this isolated platform this regional factor has completely distorted the effect of the eustatic fluctuations for this period of time

sequence stratigraphy in the middle ordovician carbonate successions yeongheung formation mid east korea paleogeographic implications for a land detached ramp type platform abstract this study reveals the origin of the stratigraphic discrepancy between the yeongweol and taebaek groups and reconstructs paleogeographic model for the taebaeksan basin on the sino korean craton skc the global sea level fall in the earliest middle ordovician would have subaerially extensively exposed the lower ordovician peritidal successions forming the solution collapsed breccia zones on the sauk tippecanoe bounding surface the successive rise in sea level generated the fine grained siliciclastic successions in the taebaek group during early transgression since the carbonate production of the group was hampered by the strong input of siliciclastic sediments from terrestrial sources however such successions are absent in the yeongweol group because the yeongweol platform was distant from the siliciclastic sources and sustained carbonate production due to isolation from terrestrial influences the yeongweol group is interpreted as a land detached isolated ramp type platform as it responded to sea level fluctuations with restricted terrestrial influence whereas the taebaek group can be described as a land attached ramp type platform affected by a large input of siliciclastic sediments during lowstand and early transgression based on the comparison of stratigraphic patterns and configurations of land attached and detached ramp type successions this study concludes that the yeongweol sequence would have been formed on a land detached isolated carbonate ramp type platform located at the skc along the northeastern gondwana margin

late quaternary volcanic record from lakes of michoacan central mexico this paper describes the initial stages of the development of a tephrochronology for the region of the michoacan guanajuato volcanic field mgvf in central mexico there are two elements to this the geochemical characterisation of volcanic glass and the linkage of tephra deposits to eruptions of known age the mgvf is dominated by cinder cones and shield volcanoes which erupt only once there are only two stratovolcanoes multiple eruptions which are common elsewhere in the trans mexican volcanic belt tephras were sampled from sub aerial sites close to cones of known age and from lake sediment cores from the zirahuen patzcuaro and zacapu basins in the state of michoacan multiple samples were collected to ensure that each tephra was well represented the glass was analysed by electron microprobe and found to be calc alkaline in composition sio2 abundances varied from 52 to 75 full results are available at http www geo ed ac uk tephra the ages of the dated cones ranged from the 20th century ad to ca 17 000 14c years bp tephras from eruptions of el jabali 3840 14c years bp jorullo 1759 1774 and paricutin 1943 1952 have been identified in lake cores these provide a means of correlating between basins and have the potential to provide a basis for understanding the volcanic history of this area and for dating a wider range of sediment sequences

volcanic impact on the southern basin of mexico during the holocene the interstratification of volcanic and glacial deposits in central mexico has been helpful in correlating the time sequence of these events two glacial advances and many eruptions have occurred during the last 10 000 years although conditions have changed drastically since the last eruptions in the 1920s popocatepetl the origin and evolution of several lakes basin of mexico are related to vulcanism and periods of warmer and colder climatic conditions fifty monogenetic volcanoes have erupted near mexico city during the holocene probably during 20 22 different eruptive periods since some are superimposed vents similar to the jorullo vents which erupted 200 years ago in michoacan mexico the youngest of these volcanoes are chichinautzin and xitle 2030 60 years bp whose lavas covered approximately 80 km each the xitle eruption began with ash emission mainly towards the nw and was later dominated by lava flow fed by tubes this lava covered the cuicuilco prehispanic urban center and the agricultural areas near it the lava dammed and caused silting of the magdalena xicotitla and chicuautitla rivers and deflected the drainage pattern in general the eruption of chichinautzin and the other holocene volcanoes have drastically changed the topography and caused fires as can be seen in the carbonized branches at the base of the flows chichinautzin volcano was formed by successive thin lavas which flowed to the south and by thicker flows to the east west and north older holocene volcanoes are responsible for the formation of the zempoala and xochimico lakes popocatepetl is a 5450 m high stratovolcano to the east of mexico city which has been subject to glacial processes its eruptions have been documented since the 14th century the last being in the 1920s most of these eruptions caused ash fall mainly to the east and northeast but lava emissions and flooding have also been reported larger eruptions in the earlier history produced pyroclastic flows and surges lahars plinian pumice falls and two debris avalanches a preliminary map of the type of volcanism which has affected the different areas and its implications for future hazards is presented

geology and radiometric dating of quaternary monogenetic volcanism in the western zacapu lacustrine basin michoacan mexico implications for archeology and future hazard evaluations the zacapu lacustrine basin is located in the north central part of the michoacan guanajuato volcanic field mgvf which constitutes the west central segment of the trans mexican volcanic belt geological mapping of a 395 km2 quadrangle encompassing the western margin of the basin 40ar 39ar and 14c radiometric dating whole rock chemical and petrographic analyses of volcanic products provide information on the stratigraphy erupted volumes age and composition of the volcanoes although volcanism in the mgvf initiated since at least 5 ma ago rocks in the western zacapu lacustrine basin are all younger than 2 1 ma a total of 47 volcanoes were identified and include 19 viscous lava flows 40 vol 17 scoria cones with associated lava flows 36 vol seven lava shields 15 vol three domes 6 vol and one maar 2 vol erupted products are dominantly andesites with 42 km3 86 vol followed by 4 km3 of dacite 8 vol 1 4 km3 of basaltic trachy andesite 3 vol 1 km3 of basaltic andesite 2 vol and 0 14 km3 of rhyolite 0 3 vol eruptive centers are commonly aligned ene wsw following the direction of the regional cuitzeo fault system over time the high frequency of eruptions and consequent accumulation of lavas and pyroclastic materials pushed the lake s shore stepwise toward the southeast eruptions appear to have clustered through time one cluster occurred during the late pleistocene between 27 000 and 21 300 bc when four volcanoes erupted a second cluster formed during the late holocene between 1500 bc and ad 900 when four closely spaced monogenetic vents erupted forming thick viscous a a to blocky flows on the margin of the lacustrine flats for still poorly understood reasons these apparently inhospitable lava flows were attractive to human settlement and eventually became one of the most densely populated heartlands of the pre hispanic tarascan civilization with an average eruption recurrence interval of 900 years during the late holocene the western zacapu lacustrine basin is one of the most active areas in the mgvf and should hence be of focal interest for regional volcanic risk evaluations

early dolomitization and recrystallization in shallow marine carbonates mississippian alida beds williston basin canada evidence from petrography and isotope geochemistry abstract the mississippian lower carboniferous alida beds of the northern williston basin canada are carbonate rocks that contain a number of significant hydrocarbon reservoirs these shallow marine carbonate platform deposits consist of abundant very finely to finely crystalline dolomite that occurs mainly in mud dominated sediments dolomite crystals are 4 to 30 m in length subhedral to anhedral in shape and typically replace the matrix but also occur as thin lamellae within ooid cortices textural relations indicate that the dolomite predates all other diagenetic phases including coarse calcite cement anhydrite and bitumen dolomite 18o values 9 3 to 4 5 pdb are depleted relative to that expected for precipitation from osagean late tournaisian seawater 0 5 to 1 5 pdb while 13c values 2 3 to 3 4 pdb are within the range of osagean seawater values 1 8 to 4 8 pdb dolomite 87sr 86sr values 0 70816 to 0 70921 are somewhat elevated with respect to osagean seawater 0 70768 to 0 70817 a trend of increasing average dolomite crystal size with decreasing 18o is observed in matrix replacive dolomite based on petrographic relations 13c compositions and the pervasive occurrence of finely crystalline dolomite initial dolomitization is interpreted to have taken place in the presence of slightly evaporated seawater the absence of depositional evaporites in dolomitized intervals suggests that dolomitization was driven by reflux of slightly evaporated seawater that had not reached gypsum saturation depleted 18o values elevated 87sr 86sr ratios and correlation of dolomite crystal sizes and 18o values suggest that the early formed dolomite was variably recrystallized during burial

earthquakes recorded stratigraphically on carbonate platforms the longest records of earthquake activity1 may be preserved near the edges of carbonate platforms where reefs have bordered active faults because the carbonate sediments build up almost to low tide level because their character and lithification are very sensitive to deviations from that datum and because they commonly lithify soon after deposition2 strata can precisely record changes in lithospheric flexure3 which in turn record the buildup and release of elastic strain energy during stick slip faulting the approximate stratigraphy is predictable for a given history of faulting comparison of predictions with observations leads to an explanation for some of the hemicyclic sedimentary sequences common4 6 on carbonate platforms these records are important for characterizing the seismicity of atlantic type continental margins in general after rifting faults evidently remain active7 but major earthquakes are so rare that their recurrence times cannot be established from direct observations

low and high frequency sea level changes control peritidal carbonate cycles facies and dolomitization in the rock of gibraltar early jurassic iberian peninsula the rock of gibraltar comprises two tectonically separated limbs of an isolated klippe of liassic gibraltar limestone formation both limbs have similar c 400 m thick sequences of inner carbonate platform facies arranged in high frequency metre scale shallowing upward peritidal cycles with emergent caliche caps four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures lithologies facies and biota when compared with other liassic cycles from fault bound platforms of the western mediterranean region all are found to be of similar scale facies and cycle type likely common origins are through milankovitch band allocyclicity or autocyclic tidal flat progradation superimposed on regional subsidence within the gibraltar limestone high frequency cycles are superimposed on a low frequency third order cyclicity that is revealed through the use of fischer plots to control the occurrrence of facies biota high frequency cycle types and dolomitization falling sea level and lowstand phases with reduced accommodation space are typified by restricted inner platform facies and cycles and by early reflux dolomitization transgressive and highstand phases with more accommodation space are characterized by the absence of early dolomites the incoming of inner platform microfossils i e foraminifera and calcareous algae and by less restricted marine facies i e oncoids shelly rudstones packstones and grainstones fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas

paleosols bones phytoliths and δ13c signatures of humus and teeth in the alluvial sequence of axamilpa puebla inferences for landscape evolution and megafauna paleoecology during mis 3 2 in southern mexico abstract alluvial paleosol sedimentary sequences containing animal fossils and paleobotanical and paleopedological archives have a potential for reconstructing quaternary paleoenvironments in southern mexico and for understanding the paleoecology of pleistocene megafauna in the section of axamilpa river four pedostratigraphic units were recognized the lower gleyic and vertic pedological units are at the end of marine isotopic stage mis 3 while the upper humic and most probably calcic units correspond to mis 2 according to the radiocarbon dates of humus the micromorphological analysis indicates an incipient development and syn sedimentary character of paleopedogenesis in the periodically water saturated environment indicated by redoximorphic features hydrogenic carbonates and low values of magnetic susceptibility in most strata the 13 c values of the paleosol humus range from 22 6 to 24 7 indicating a c3 vegetation whereas in the modern soil it is higher 15 2 pointing to an abundance of the c4 and cam plants the 13 c signature of the vegetation inferred from the teeth enamel of the fossil fauna appeared to be less negative than that of humus from corresponding paleosols the alimentary preferences of the paleofauna which fed on grasses with high proportion of c4 plants explain this difference the phytolith analysis shows dominance in conifer forms followed by c4 and c3 grasses the content of conifer phytoliths is progressively reduced towards the top of the sequence we interpret the late pleistocene landscape of this area as a heterogeneous forest grassland landscape with the forest shrinking through time and the grass areas increasing swampy floodplain meadows inferred from paleopedological results could provide a habitat for grassy vegetation both isotope and phytolith records demonstrate a contrasting difference between pleistocene and recent vegetation dominated by xerophytic shrubs and succulents this vegetation change could play important role in the extinction of pleistocene megafauna

optimal dynamic rock fluid physics template validated by petroelastic reservoir modeling separation of fluid pore pressure and saturation using inverted time lapse seismic attributes is a mandatory task for field development multiple pairs of inversion derived attributes can be used in a crossplot domain we performed a sensitivity analysis to determine an optimal crossplot and the validity of the separation is tested with a comprehensive petroelastic reservoir model we simulated a poorly consolidated shaly sandstone reservoir based on a prograding near shore depositional environment a model of effective porosity is first simulated by gaussian geostatistics well known theoretical and experimental petrophysical correlations were then efficiently combined to consistently simulate reservoir properties next the reservoir model was subjected to numerical simulation of multiphase fluid flow to predict the spatial distributions of fluid saturation and pressure a geologically consistent rock physics model was then used to simulate the inverted seismic attributes finally we conducted a sensi

quantitative characterization of gas hydrate bearing sediment using elastic electrical rock physics models abstract successful evaluation of reservoir resources in gas hydrate bearing sediments depends on the accurate estimation of reservoir parameters such as hydrate saturation porosity and clay content elastic and electrical measurements are affected by several rock and fluid properties and their joint interpretation can provide complementary information and improve the accuracy of reservoir characterization results in this work we propose a new rock physics inversion workflow based on 3d rock physics template of elastic and electrical attributes for the direct estimation of reservoir parameters we first conduct a quantitative analysis of the available well log data and core data to understand the morphology of gas hydrate within the marine sediment and choose the appropriate elastic rock physics model then we construct the 3d rock physics template by discretizing the 3d space of reservoir parameters and implementing the suitable elastic and electrical models to calculate the poisson s ratio p wave impedance and electrical resistivity for each combination of values of rock and fluid properties finally we superimpose the measurements on the templates and use a grid searching method to find the nearest gridded node to the data point and minimize the objective function of the mismatch between data and model predictions to investigate the feasibility of our proposed inversion scheme we test a rock physics model based on effective medium theory and a modified archie s equation and invert for reservoir parameters of gas hydrate bearing sediments at well 1250f and 1247b in hydrate ridge the good match between inversion results and core data suggests that the proposed inversion scheme is applicable to real data and allows combining elastic and electrical data for reservoir parameter estimation of gas hydrate bearing sediments

late holocene vegetation change in the sierra madre oriental of central mexico abstract past vegetation and climate changes reconstructed from a sediment core from laguna atezca molango mexico provide new insights into the environmental and cultural histories of the sierra madre oriental during the last 1700 yr pollen microscopic charcoal sediment chemistry loss on ignition and magnetic susceptibility indicate that three phases of human occupation deforestation and erosion ca a d 280 890 ca a d 1030 1420 and ca a d 1680 present alternate with two phases of abandonment ca a d 890 1030 and ca a d 1420 1680 forest composition of the two abandonment phases differed with cloud forest taxa liquidambar ostrya carpinus ulmus etc dominating the pollen record during the first phase and quercus and pinus pollen characterizing phase two these differences may reflect a climate change in which the second phase was drier than the first alternatively the increase in pinus and quercus may have been caused by a human induced decline in soil fertility the laguna atezca record also differs from several other mesoamerican paleoenvironmental records in that it shows no evidence of drought at the end of the classic period ca a d 900

buried palaeosols of the nevado de toluca an alternative record of late quaternary environmental change in central mexico buried palaeosols of central mexico not previously analysed from a palaeopedological standpoint have helped to develop a reliable regional model of late quaternary climatic change this paper focuses upon morphological and micromorphological properties particle size distribution and extractable fe al and si of seven palaeosols named pt1 pt7 in the vicinity of the nevado de toluca volcano the characteristics of andic cambisol pt1 similar to those of modern soils in semi arid environments indicate a drier climate in the first half of the holocene humic andosols pt2 pt4 are analogous to modern volcanic soils of humid forest environments they show evidence that a moist palaeoclimate prevailed during marine oxygen isotope stages ois 2 and 3 luvisols pt5 and pt6 which are assumed to have formed at the end of marine ois 5 to marine ois 4 also indicate humid conditions we attribute the differences between andosols pt2 pt4 and luvisols pt5 pt7 to variations in the duration in the pedogenesis rather than to changes in palaeoclimate micromorphological features of andosol to luvisol transition confirm that these soils form an evolutionary sequence being for the most part consistent with lacustrine records the palaeosol properties do not reflect the episodes of drier climate during the last glacial maximum however as shown by diatom and palynological data from lake sediments mesoclimate variations and or incompleteness of soil memory reflecting mostly periods of humid pedogenesis probably provide the reason for this disagreement copyright 2001 john wiley sons ltd

misuse of fischer plots as sea level curves fischer plots are graphic representations of cyclic carbonate deposits showing cumulative departure from average cycle thickness plotted against cycle number and corrected for assumed subsidence during each cycle observed cycle thicknesses in excess of subsidence have been interpreted to represent depositional accommodation space formed by eustatic sea level rise however implicit in this interpretation is the assumption that preserved cycle thickness is a proxy for accommodation to test this assumption a survey of carbonate sediment accumulation patterns i e cycle thicknesses developing during a single transgressive event the holocene postglacial sea level rise was conducted on a shallow carbonate platform great bahama bank where accommodation was created primarily by sea level rise over the platform margin i e subsidence was minimal this survey demonstrates that holocene sediment cycle thickness and accommodation are uncorrelated r 2 0 03 consequently fischer plots constructed by using holocene cycle thicknesses are poor representations of the holocene transgression in extreme examples holocene fischer plots would be interpreted to show relative sea level fall during the holocene on great bahama bank because holocene subsidence currently exceeds sediment thickness in addition a simple sensitivity test shows that eustatic sea level interpretations based on interbasinal correlation of fischer plots are equivocal the gross form of fischer plots appears so overly robust as to be insensitive to broad variations in stratigraphic completeness cycle duration or subsidence because cycle thickness apparently is uncorrelated with accommodation and the gross form of fischer plots is relatively invariant it seems prudent to reevaluate the practice of interpreting fischer plots as sea level curves per se in the analysis of ancient carbonate cycles

stress and fluid sensitivity in two north sea oil fields comparing rock physics models with seismic observations during 4d seismic reservoir characterization it is important to have reliable rock physics models for both static e g mineralogy porosity cement volume and dynamic e g saturation pressure temperature reservoir parameters without a good understanding of reservoir geology and associated static rock physics properties it is impossible to interpret time variant changes in pore pressure and saturation andersen et al 2009 the dry rock properties of the reservoir can be obtained from well log data combined with geological information about mineral composition and rock texture and gassmann theory to estimate the effect of pore fluid changes normally core measurements are undertaken to quantify stress sensitivity but these are often affected by induced fractures caused by the coring acquisition that will enhance the stress sensitivity of the rock holt et al 2005 duffaut and landro 2007 showed how calibrated hertz mindlin contact theory could be applied to estimate stress sensitivity

interactive interpretation of 4d prestack inversion data using rock physics templates dual classification and real time visualization obtaining quantitative information from 4d seismic data about changes in the saturation and pressure in a reservoir due to production is very important for improved hydrocarbon recovery and management of the field however in heterogeneous reservoirs where the dynamic behavior varies spatially below the seismic resolution conventional 4d interpretation methods fail to track quantitative changes in individual lithologies the lack of information about the particular lithology to which a given 4d effect is connected can be detrimental to a general understanding of the reservoir and the forecast

palynological and magnetic susceptibility records of lake chalco central mexico abstract pollen and magnetic susceptibility data from a 11 27 m long core drilled in the central part of lake chalco 99 00 w 19 15 n located at the southeastern part of the basin of mexico are interpreted in terms of paleoenvironmental changes the record of this subtropical highland lake dated by radiocarbon methods spans approximately 24 000 yr b p and documents changes in climate vegetation lake level and volcanic activity seven palynozones were established by using the method of cluster analysis for correlation purposes zones 7 6 and 5 between 20 600 and 18 300 yr b p indicate reduced forests with extensive grasslands relatively high input of sediments by water discharge of surface streams and high lake level the evidences obtained in zone 4 between 18 300 and 17 500 yr b p suggest a dry and warm climate with xerophytic vegetation lower lake level and erosion rates zone 3 between 17 500 and 10 000 yr b p shows a trend to increasing moisture and cooler temperatures volcanic activity during this period is intense and strongly influences the natural environments at around 12 000 yr b p a clear expansion of forests and fulfillment of the lake recorded at the beginning of the holocene an oak forest expansion and a lowering of lake levels are initiated and by the mid holocene zone 2 subhumid temperate conditions are detected although lake level continues to be low the human impact is recorded in the last 4000 yr

spatial variability of environment change in the teotihuacan valley during the late quaternary paleopedological inferences the teotihuacan valley mexico has been the object of considerable interest because of its importance in the context of human history in america although archeological research has been extensively conducted to understand causes of the decline of teotihuacan culture no sufficient scientific evidence has been provided this paper is focused on paleopedological evidence especially properties that constitute soil memory in order to reconstruct the spatial distribution of environment variability during the late pleistocene and holocene three profiles that include modern soils and buried paleosols were studied in different geomorphic positions uplands cerro gordo site cg lower talus slope transition to valley bottom maseca ma and the mouth of the valley at the ancient border of the former texcoco lake this last site is near tepexpan te where some of the oldest human remains in mexico were recovered the oldest paleosol is at cg at 3000 m a s l it is dated at 22 000 yr bp and exhibits a truncated profile with a very well developed bt horizon dominated by kaolinite with phytoliths of c3 plants all of its features are related to a humid probably warm environment it was classified as a luvisol this paleosol is buried by another luvisol dated at 13 000 yr bp with morphological characteristics similar to the previous soil although clay mineralogy consists mostly of halloysite and a higher percentage of c4 plant phytoliths is apparent in ma the oldest paleosol was not recognized it is probable that the two paleosols found in cg merge into one at ma which shows strong redoximorphic features but high percentages of c4 plant phytoliths and halloysitic clays at the te site paleosols are less developed fluvisols modern soil cover also varies in relation to its geomorphic position in cg it is a phaeozem while in ma and te present day soils have carbonate accumulation we interpret prevailing conditions during the last glacial maximum in the teotihuacan valley to be humid with a tendency towards drying at the end of pleistocene dry conditions prevail in the valley today

on the labeling length and objective basis of fischer plots abstract fischer plots of the accumulation of shallow marine carbonate cycles track the cumulative departure from mean cycle thickness through a vertical sequence of contiguous upward shallowing units this is a simple objective treatment of residuals in a time series and is applicable to other stratigraphic phenomena to reflect the noninterpretive construction of fischer plots the vertical axis should be labeled cumulative departure from mean cycle thickness and the horizontal axis cycle number the typical fischer plot is an erratic bridge that begins and ends at the same elevation but has the form of an irregular train of asymmetric waves which rise steeply and fall gradually the asymmetry of the individual waves arises because thinner than average cycles typically outnumber thick r cycles two to one the overall form of the wave train is sensitive to mean cycle thickness but can guide correlation and may have considerable interpretive potential provided that enough cycles are included to ensure a robust estimate of mean thickness runs tests demonstrate that fischer bridges for peritidal carbonates are unlikely to be the result of random stacking processes since this anticipated result does not emerge unequivocally for fischer plots of less than 50 cycles however we recommend against plotting such short sections

epigenetic dolomitization and the origin of xenotopic dolomite texture abstract xenotopic texture which is commonly observed in pre cenozoic rocks is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and usually undulatory extinction xenotopic dolomite texture is similar in appearance to neomorphic limestone textures idiotopic dolomite texture euhedral to subhedral crystals with straight intercrystalline boundaries contrasts with xenotopic texture and is commonly observed in both cenozoic and more ancient dolomites texture may be controlled by the temperature at which crystals grow crystal growth theory predicts that at low temperature a smooth crystal surface is energetically favored and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites this results in faceted crystals and euhedral to subhedral crystal mosaics above a critical roughening temperature crt a rough surface is energetically favored surface nucleation does not require dislocations and atoms are randomly added to the crystal surface resulting in nonfaceted growth and an anhedral crystal mosaic it is hypothesized that a critical roughening temperature exists for dolomite above 50 c xenotopic dolomites are produced by dolomitization of limestone and or neomorphic recrystallization of dolomite at elevated temperature above the crt after burial idiotopic dolomites are produced below the crt by near surface processes and are occasionally produced above crt if growth inhibiting effects of the solution or impurities stabilize crystal faces calcite has a crt below 25 c and therefore develops anhedral crystal mosaics characteristic of neomorphic texture both at near surface and at elevated temperatures synthetic xenotopic dolomite was produced in the laboratory at 250 c and 300 c by dolomitization of aragonite and calcite skeletal fragments and by recrystallization neomorphism of nonstoichiometric cenozoic dolomites xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic hueco dolomite permian texas near the marble canyon intrusion at temperatures between 350 c and 600 c hydrothermal dolomitization of periclase calcite marble near the intrusion also resulted in a xenotopic texture xenotopic dolomite in the galena group ordovician wisconsin was produced by neomorphism of a preexisting dolomite during the emplacement of lead zinc sulfides at temperatures between 80 c and 227 c in the trenton limestone ordovician michig n xenotopic dolomite replaced limestone during the migration of hot 50 c fluids along fracture systems xenotopic dolomite was not observed in cenozoic dolomites that were subjected to temperatures near surface values only

a 6 000 year record of environmental change from the eastern pacific margin of central mexico abstract the transition from the mid to late holocene in mesoamerica saw increasing complexity in spatial patterns of change records from the western part of the region are sparse with lacustrine sequences affected by long term anthropogenic disturbance or lacking chronological resolution here we present a continuous palaeoecological and geochemical record from laguna de juanacatlan a remote lake in the mountains of the western tmvb diatom assemblages xrf scanning data and bulk organic geochemistry from a well dated 7 25 m laminated sequence were combined with summary pollen data from a 9 m partially laminated core to provide a continuous record of catchment and lake ecosystem changes over the last c 6000 years relatively humid conditions prevailed prior to c 5 1 cal ka which supported dense oak pine forest cover around a deep stratified lake a trend towards drier conditions began c 5 1 cal ka intensifying after 4 0 cal ka consistent with weakening of the north american monsoon between 3 0 and 1 2 cal ka lower lake levels and variable catchment run off are consistent with increasing enso influence observed in the late holocene in the neotropics from 1 2 to 0 9 cal ka a marked change to catchment stability and more intense stratification reflected drier conditions and or reduced rainfall variability and possibly warmer temperatures after 0 9 cal ka conditions were wetter with an increase in catchment disturbance associated with the combined effects of climate and human activity in recent decades the lake ecosystem has changed markedly possibly in response to recent climate change as well as local catchment dynamics

palaeolimnological records of climate change in mexico frustrating past promising future central and northern mexico contain numerous closed basin lakes that may preserve valuable palaeoclimatic records in their sediments research has however focused on the basins of the trans mexican volcanic belt tmvb in particular the basin of mexico results from the zacapu basin michoacan are used to illustrate both the potential benefits and problems of palaeoclimatic studies in such basins three diatom sequences provide a discontinuous record spanning 30 000 years the presence of a deep lake before 28 000 bp is indicated there are no sediments covering the pleistocene holocene transition during the holocene only minor changes in lake level are indicated with drying about 5000 bp and most importantly about 1100 years bp difficulties in reconstructing past climates in this basin and in many others in central mexico include discontinuous sedimentation obtaining a reliable chronology the possible impacts of tectonism and the effects of long term human disturbance in spite of these problems the records from zacapu and other basins in the region provide a basis for preliminary palaeoclimatic interpretation there is a clear need for long records for high resolution studies to investigate the detailed nature of climate and for more work outside the tmvb

some problems in the late quaternary pollen records of central mexico basins of mexico and zacapu most pollen analyses of late quaternary sediments have been carried out in closed basins of the trans mexican neovolcanic belt in central mexico climatic changes have been inferred from these pollen data however some problems arise from reinvestigations the dominance of a few pollen types in general pinus quercus and alnus is almost constant among these records in spite of the importance of vulcanism and tectonism in the evolution of this region no effects of these factors are evident from the pollen records some of the problems are illustrated in the pollen records from lake texcoco and lake chalco in the basin of mexico and lake zacapu in michoacan

platform to basin correlation by high resolution sequence stratigraphy and cyclostratigraphy berriasian switzerland and france based on detailed analyses of facies evolution and stacking pattern of berriasian carbonate dominated sections in the swiss jura mountains the swiss ultrahelvetic and the french vocontian trough a high resolution platform to basin correlation is proposed biostratigraphical tie points are furnished by ammonites dinoflagellates calpionellids and ostracod charophyte assemblages the hierarchical stacking of small scale depositional sequences reflects milankovitch cyclicity sequences corresponding to the 20 100 and locally 400 ka orbital cycles can be identified elementary 20 ka sequences on the platform generally consist of one bed of shallow subtidal to intertidal high or low energy carbonate facies whereas on the slope and in the basin they are commonly developed as limestone marl couplets these elementary sequences group into small scale composite sequences reflecting the first orbital eccentricity cycle 100 ka which in turn build up large scale 3rd order composite sequences one 3rd order sequence has been analysed in detail according to the cyclostratigraphic interpretation it took 2 myr to form which is in accordance with the duration of the corresponding ammonite subzones sequence stratigraphic and cyclostratigraphic platform to basin correlation shows that for about 900 ka the platform was exposed or only partly flooded whereas on the slope and in the basin lowstand deposits with channel fills and slumps accumulated with rising sea level accommodation space gradually increased on the platform and a thickening upward sequential pattern with transgressive facies developed while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds this situation lasted about 700 ka the following 300 ka were characterized by sediment starvation and increased bioturbation on the platform and by more marly transgressive sediments on the slope and in the basin the maximum flooding phase is more or less isochronous on the platform and in the basin although the surface with the best developed maximum flooding features may be displaced by one or two small scale composite sequences because of superimposed high frequency sea level fluctuations and or local variations in substrate morphology and sediment distribution third order highstand conditions prevailed for only about 100 ka the combination of cyclostratigraphy and high resolution sequence stratigraphy constrained by good biostratigraphy is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another

a high elevation holocene pollen record from iztaccihuatl volcano central mexico pollen data from 3860 m in the agua el marrano valley on the northwest side of iztaccihuatl volcano southeastern part of the basin of mexico 198n combined with data on the glacial chronology and tephrochronology document holocene climatic and vegetation changes near the present timberline glacial advances occurred between 12 000 and 10 500 cal yr bp milpulco 1 and between 8300 and 7300 cal yr bp milpulco 2 a 450 cm core was drilled in sediments accumulated in a milpulco 1 moraine depression the sequence covers the last 11 000 cal yr bp according to three 14 c dates and the presence of a distinctive pumice deposit dated elsewhere at 5700 cal yr bp the pollen record of agua el marrano shows barren conditions after glacier retreat followed by a gradual colonization by elements of the alpine grasslands around 10 900 cal yr bp this plant community remained until c 7200 cal yr bp between c 7200 cal yr bp and 6500 cal yr bp coniferous forest grew close to or at the site but alpine grasslands were re established between 6500 cal yr bp and c 5000 cal yr bp apparently in connection with mid holocene dry conditions recorded in central mexico the upper timberline attained its modern altitude 4000 m in the last 3000 years during this period no significant changes are observed in the pollen spectra comparison with other palaeoecological records from central mexico suggests that the timberline fluctuated in response to both temperature and precipitation changes during the holocene