architecture of miocene overbank deposits in northern pakistan abstract understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition overbank deposits of the fluvial chinji formation siwalik group can be divided into sequences 1 20 m thick defined by the alternation of stratified sediments and paleosols preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development rapid deposition of overbank sequences is predicted by models that hypothesize 1 episodic vertical aggradation of the entire floodplain 2 deposition controlled by the growth of alluvial ridges 3 incision and filling of local valleys 4 rapid filling of localized low areas or 5 rapid deposition associated with river channel avulsion these models also have important implications for understanding fossil preservation and floodplain paleoecology paleosol bounded sequences in the chinji formation can pinch out laterally over kilometers indicating that episodes of rapid deposition were restricted to local areas on the floodplain there is no evidence that bases of sequences are erosional and they do not appear to record valley incision and filling sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits suggesting they are not related to the growth of alluvial ridges along active channels lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel this infilling of floodplain topography may reflect a continuous process whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels alternatively the filling of local flood plain topography may have been caused by widespread but short lived events associated with river channel avulsion

geomorphic thresholds and complex response of fluvial systems some implications for sequence stratigraphy first generation sequence stratigraphic models have dealt in a very rudimentary fashion with the response of fluvial systems to eustasy a major element of presently accepted models is that rivers incise when sea level falls and aggrade during the ensuing rise well established geomorphic principles state that fluvial systems are complex process response systems that can adjust to internal and external changes in other ways besides incision and aggradation by modifying their stream patterns and channel geometries application of geomorphic principles to sequence stratigraphic models results in the following observations during eustatic lowstands rivers may adjust to lowered base levels and changes in slope by modifying channel patterns therefore not all lowstands produce type 1 sequence boundaries type 1 sequence boundaries characterized by fluvial valley incision are more likely to develop when sea level drops below the shelf slope break resulting in topological relief near the strandline in which headwardly eroding knickpoints form rates of eustatic change although rapid in a geological context are sufficiently low that geomorphic systems can easily maintain their equilibrium during eustatic changes and migrate back and forth across the shelf without major modifications finally und r conditions of relatively static sea level sequences and parasequences of the same scale in time and space as those which form during eustatic change can be deposited as the result of purely intrinsic causes and responses of a fluvial system in general eustasy controls the location of deposition and erosion but the resultant stratal geometry is controlled by sediment supply and processes acting on the sediments as the shoreline moves across the shelf in response to eustasy

the genesis of coal from the viewpoint of coal petrology abstract the genesis of the microscopic constituents of coal macerals and of maceral associations microlithotypes representing various coal facies is discussed in the light of recent advances that have been made in the study of coals and modern peats peat microscopy especially promoted the understanding of humification and biological gelification both of which are decisive for the genesis of huminites vitrinites in brown coals and hard coals respectively microbial activity during peatification has been documented by chemists microbiologists and petrologists and recently by electron microscopical observations in coals this activity leads to the incorporation of lipid substances into the humic matter and later influences the chemical physical and technological properties of vitrinites geochemical gelification transforms the huminites of brown coals into the vitrinites of bituminous coals and is probably caused by the onset of bitumen generation in coal in this process the hydrogel brown coal is transformed to the bitumogel bituminous coal coal as a source rock for oil has become an important research subject for coal petrologists and oil exploration the formation of more or less liquid bitumen and its later cracking to gaseous hydrocarbons leads to the coalification jumps of the macerals especially liptinites and vitrinites and to the generation of secondary macerals like exsudatinite bitumen and micrinite dead carbon bituminization in coal has been revealed by the use of fluorescence microscopy which also found the presence of new liptinite macerals bituminite fluorinite and exsudatinite and suggests a higher proportion of algal material in coals than was thought earlier the genesis of various types of inertinite macerals will probably become an increasingly important subject of research especially with regard to rank inertinites that seem to attain their high inertinitic reflectance initially during coalification besides the origin of macerals the origin of various coal facies in terms of maceral associations is considered in relation to the palaeogeographic depositional milieu the peat forming vegetation palaeoclimate water and nutrient supply acidity marine and calcareous influence and fire with examples from euramerican carboniferous coals gondwana coals and tertiary brown coals vegetation water and oxygen supply eh conditions are most important for the generation of coal facies vegetation has been reconstructed with the help of palaeobotany including palynology cuticular analysis and wood anatomy in brown coals and with the study of petrified permineralized peats preserved in coal balls the microscopical investigations of botanically recognizable plant remains isolated from coals or accompanying rocks has been especially informative however the results of palynological and cuticular analyses should be evaluated with caution considering the varying resistance of palynomorphs and cuticles and in the case of palynomorphs their possible extrapaludal origin microscopical studies of modern peats that were derived from different types of vegetation growing in different hydrological environments have been very helpful in interpreting coal facies especially in brown coals the much disputed origin of brown coal lithotypes is considered in detail for hard coals there seems to be agreement that coal layers rich in vitrites and liptinite poor clarite were deposited in wet forest swamps that microlayered liptinite rich clarites are sub aquatic humic muds and that inertinite rich durites represent oxidized peats the origin of liptinite rich durites is controversial either subaquatic muds or oxidized peats the latter perhaps from raised bogs a recent trend in the literature is the assumption that many if not all mineable coal seams have been deposited in raised bogs under ombrogenous oligotrophic conditions although raised bogs with a dense arborescent vegetation are known from borneo it seems not probable that most seams which were deposited in foredeeps or steadily sinking graben zones are of ombrogenous origin although a succession from relatively wet and eutrophic to relatively dry and oligotrophic conditions may be true for occasional seams but as in modern peats striking changes of coal facies are commonly caused by hydrological events e g inundations as a consequences of crevasses or of fires that burned the peat the origin of the inertinite rich gondwana coals is discussed in relation to their specific climatic conditions and a comparison with sub arctic peats is recommended coals influenced by marine and calcareous conditions are characterized by high contents of sulfur and pyrite as well as by hydrogen rich vitrinites with a relatively low reflectance and high fluorescence all due to a strong bacterial activity in peats of low acidity proposals for further research are made in the conclusions

deltaic coals of ferron sandstone member of mancos shale predictive model for cretaceous coal bearing strata of western interior the upper cretaceous ferron sandstone member of the mancos shale the coal bearing unit of the emery coalfield in central utah shows accumulation of clastic sediments in a lobate river dominated deltaic system that existed along the western shoreline of the interior cretaceous seaway during late turonian time five cycles of deltaic sedimentation each containing one major coal bed or coal zone are represented a clear genetic relationship exists between the geometries of the major coal beds of the emery field and the geometries of the delta front sandstone units with which they are associated the thicker part of each coal bed extends from the vicinity of the landward pinch out of the associated delta front sandstone landward to a distance of about 10 km this genetic relation forms the basis of a predictive model that can be used in designing more cost effective exploration programs in coal bearing strata of cretaceous age in the western interior

episodic growth of holocene tidal marshes in the northeastern united states a possible indicator of eustatic sea level fluctuations development of coastal marshes during the holocene submergence of the northeastern united states seems to have been episodic radiocarbon dating of the modern marshes indicates that they were generally established within the past 3 000 yr initiation of marsh growth was probably related to the marked slowing in the submergence rate that took place in the area about 3 000 to 4 000 yr ago on the basis of the few available dates of marsh peats from the inner continental shelf we suggest that previous episodes of marsh growth took place at about 4 700 5 600 6 600 7 700 8 200 and 8 600 radiocarbon yr ago these times of marsh growth seem to coincide with proposed times of negative sea level oscillations cool climate and glacial advances if these oscillations of sea level are climatically controlled then the data suggest an approximately 1 000 yr cycle of climatic change

peat accumulating depositional systems of sarawak east malaysia many coal deposits originate in deltaic estuarine and coastal plain settings and a knowledge of interrelationships between the tectonic and depositional elements active at the time of sediment deposition is necessary to formulate basin scale models the prograding coastal depositional systems of sarawak all contain domed peat accumulating environments in which low ash low sulfur peats are being deposited in areas of active clastic siliciclastic sedimentation these depositional systems are as large as 11 400 km2 and individual peat deposits within systems are in excess of 20 m thick and 1000 km2 in area the geographic positions and drainage basin areas of each depositional system are controlled by fault and fold systems although prograding into the same receiving basin individual system geomorphology is variable and ranges from a wave dominated microtidal delta to a wave dominated meso to macro tidal delta coastal plain system to a tide dominated macrotidal estuarine embayment along a 450 km stretch of coastline system variation is a function of sediment supply shelf and embayment geometry wave climate and tidal range these factors which control depositional system geomorphology also control the resulting long axis orientation of the thick domed peat deposits the surface vegetation and internal characteristics of most domed peat deposits however are similar internal characteristics consist of basal high ash high sulfur degraded peats overlain by low ash low sulfur well preserved peats in vertical profile these systems demonstrate variable responses to late pleistocene holocene sea level rise and in these instances the variation is most attributable to local differences in siliciclastic sediment supply which is a function of the drainage basin area

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

glacial eustatic base level climatic model for late middle to late pennsylvanian coal bed formation in the appalachian basin abstract late middle late pennsylvanian middle allegheny and lower conemaugh marine units in the appalachian basin represent the distal edges of glacial eustatic marine incursions of the midcontinent sea onto the detrital apron of the appalachian highlands because these marine units typically overlie coal beds the main environment of formation of this type of coal bed appears to have been that of a coastal swamp migrating ahead of transgression as base level rose the water table continually rose and the swamp was further nourished and surrounding vegetation kept thick enough to inhibit detrital influx by rainfall from the expanding nearby source of moisture even in parts of the appalachian cyclic succession with no marine units upper allegheny monongahela dunkard the coal beds rep esent the wettest part of the climate cycle of cecil 1990 sandwiched between detrital units representing seasonal wet dry climates when sediment was most readily mobilized and antipodal to nonmarine limestones representing the driest climates throughout this time the appalachian basin was in the center of the tropical zone of a nearly accreted megacontinent quite distant 4000 km from permanent oceanic sources of moisture climate modeling without considering inland seas or orographic effects on winds shows that the tropical zone of such a large land mass would have suffered from a severe deficit in rainfall and soil moisture although further modeling shows that the appalachian orogenic highlands would have drawn in moisture to ncrease rainfall in their vicinity they also may have impeded the normal westward flow of moisture laden easterly winds the western source for enough moisture to form coal beds was most likely the north american midcontinent sea during major highstand this sea would have covered about 2 million km2 and provided both high base level and abundant nearby moisture for enough continual rainfall to keep the coastal peat swamps widespread fresh and relatively detritus free in the appalachian basin in contrast during major lowstand it would have covered only about 200 000 km2 with its shoreline about 1500 km away and it apparently provided less consistent moisture to the appalachian basin then because most nonmarine limestones and paleosols formed there during the e times contain evidence of drier climates therefore upper allegheny monongahela and dunkard coal beds most probably formed when the midcontinent sea was approaching highstand and stood just west of the presently preserved outcrop thus they likely represent the coastal lowland equivalent of a marine unit at highstand farther west this constrained time of peat formation and preservation in a variety of coastal environments prior to and during marine highstand is responsible for the discrete and widespread nature of the coal bed interval in a typical depositional cycle or cyclothem a pattern that stands in contrast to the complicated relations of overlying and underlying detrital units which reflect the segregation of sand and mud into a complex mosaic of local deltaic and ter estrial subenvironments that developed when sea level rainfall and the water table were lower

organics versus clastics conditions necessary for peat coal development abstract the absence or lack of detrital influx into ancient peat forming swamps is critical to the formation of low ash coal modern and ancient coal forming swamps of continental basins show a separation of peat and clastic sediments which is partially fault controlled in the african rift valleys as well as in the stephanian intermontane coal basins of france thick peat free from clastic input may be the result of tectonic activity in the paralic basins of morocco westphalian b and nigeria late cretaceous coal occur landward of the shoreline turnaround and are related to a relative high stand of the sea which curtailed detrital influx into the basins thus peat formation occurred during an early transgressive phase

completeness of the geological record the completeness of a sedimentary section of known timespan may be assessed qualitatively by comparing its thickness with the average accumulation for that timespan average accumulations may be estimated from sediment volume and continental area data quantitative completeness estimation methods based on data compiled from the geological literature have been proposed but we argue that the literature data are significantly biased and cannot support such methods interestingly however a comparison of the literature data and accumulation averages computed from sediment volume data suggests that the thickest known sections may be extremely complete

the geology botany and chemistry of selected peat forming environments from temperate and tropical latitudes abstract peat has been studied in several geologic settings 1 glaciated terrain in cold temperate maine and minnesota u s a 2 an island in the atlantic ocean off the coast of maine where sea level is rising 3 the warm temperate u s atlantic and gulf coastal plains where sea level has changed often and 4 the tropical coast of sarawak malaysia and the tropical delta of the batang hari river sumatra indonesia most of these deposits are domed ombrotrophic or partly ombrotrophic bogs in which peat accumulation continued above the surface of the surrounding soil however the bogs of the u s atlantic and gulf coastal plains are comparatively not as domed and many have almost level surfaces in some bogs aquatic or semi aquatic plant materials accumulated replaced water in the depressions and formed a surface on which marsh or swamp vegetation could subsequently live die and accumulate in others the plant materials accumulated initially on level silt or sand surfaces supporting marshes or swamps as the peat dome formed plants growing on it changed from luxuriant ones near the base of the dome where nutrients were brought into the bog by surface and ground water to stunted ones at the top of the dome where the raised bogs are fed by nutrient poor precipitation the physical and chemical changes that take place in the sequence of environments from the pond stage of deposit development through the grassy marsh stage through the forested swamp stage and finally through the heath dome stage can be measured in terms of acidity and ash volatile matter carbon hydrogen nitrogen sulfur and oxygen contents as well as in the kind and distribution of trace elements the organic and inorganic contents of the deposits relate to geomorphology and geomorphology relates to their settings as models of coal formation some domed peat deposits may help in solving problems of distribution and character of ancient coal beds but clearly not all peat deposits are precursors of coal most holocene peat deposits are subject to destruction by erosion fire and decomposition through microbial and chemical oxidation before burial the best environments for coal precursors have biomass accumulation a continuously rising water table within the mass and minimum influx of clay and silt until preservation by burial the most suitable settings for future economic coal deposits are domed bogs that accumulate thick widespread peat having low ash and low sulfur contents the ombrotrophic peat deposits of tropical sarawak and sumatra are thick and extensive contain low ash and low sulfur peat and have high heating values they are considered to be the best tropical coal analogs because of their extent and chances of preservation the base of the peat is below adjacent river levels and chemical and structural conditions are favorable for accumulation

formation of the yangtze shoal in response to the post glacial transgression of the paleo yangtze changjiang estuary china abstract the yangtze shoal lies east of the yangtze changjiang river estuary china at water depths of 25 55 m and is one of the largest subaqueous sheet like sand banks in the world previous studies have not reached a consensus regarding its origin and timing of formation mainly due to the scarcity of coring and high resolution seismic profiling in the shoal we recovered a 70 20 m long sediment core cre 1402 through the sand shoal and acquired 1586 km of high resolution shallow seismic profiles across the estuary and shoal we conducted systematic analyses of the core including its sedimentary characteristics down core changes in benthic foraminiferal assemblages and 14c and optically stimulated luminescence osl dating we also analyzed the seismic stratigraphy and integrated these results with data from other previously well studied cores onshore and offshore of the yangtze river mouth our results indicate that the stratigraphic architecture in the paleo yangtze estuary i e the area from the present day delta plain to the subaqueous delta and yangtze shoal was controlled mainly by changes in sea level during the post glacial transgression the post 14 ka sedimentary successions in the paleo estuary rest unconformably on fluvial or coastal deposits that formed mainly before mis 1 and are separated by three distinctive bounding surfaces corresponding to mwp 1a mwp 1b or the rapid sea level rise at ca 11 5 ka shortly after the younger dryas cold period and the 8 2 ka event respectively and the maximum marine flooding surface mfs the depositional unit of the yangtze shoal ranges in thickness from 6 to 32 m the shoal exhibits large scale sw dipping generally 0 5 2 foresets with local cross cutting of foresets which are composed mainly of very fine to medium grained sands the shoal is underlain by muddy intertidal to subtidal flat deposits that formed during the younger dryas with an erosional surface corresponding to mwp 1b or the rapid sea level rise at ca 11 5 ka between these and the shoal deposits the yangtze shoal came into being mainly between ca 11 5 and 7 0 ka in the form of tidal sand ridges actively moving southwestwards under intense and reciprocal tidal currents on a tidal shelf the tidal sand ridges have evolved into sheet like tidal sand banks with relatively low relief as a result of tidal remobilization under rotary tidal currents since ca 7 0 ka when the mfs formed therefore the yangtze shoal can be considered to comprise quasi active tidal sand banks

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

application of sequence and genetic stratigraphic concepts to carboniferous coal bearing strata an example from the black warrior basin usa carboniferous strata provide an excellent example on which to test the application of genetic stratigraphic and sequence stratigraphic concepts both approaches are employed in the evaluation of the coal bearing strata of the black warrior basin south eastern usa bounding hiatal surfaces have been recognized in the succession of rock that includes the mary lee coal zone within the framework of genetic stratigraphy one genetic sequence has been identified comprised of offlap progradational and aggradational facies and onlap aggradational and transgressive facies components seven parasequence sets have been delimited according to the concepts of sequence stratigraphy these have been ascribed to transgressive highstand and shelf margin systems tracts the identification of components of these contrasting frameworks provide the basis for evaluating other carboniferous strata

the influence of half graben syn depositional tilting on thickness variation and seam splitting in the brunner coal measures new zealand abstract the brunner coal measures buller coalfield new zealand comprise a fluviatile sequence of eocene age that is generally 30 100 m thick the sequence rests on basement and is dominated by coarse to granular very coarse sandstone the measures contain one mineable coal seam the buller seam member that is laterally extensive and up to 20 m thick it is characterized by simple splitting with a thin upper split a thick lower split and a split separation of up to 30 50 m locally the buller seam member exhibits splitting to form numerous thin seams areas of shale out comprise thin linear belts between areas of thick coal with a low ash yield the coincidence of faults and maximum thickness of seam isopachs indicates that the thickness and splitting characteristics of the buller seam member were largely controlled by syn depositional faulting and tilting of small half graben blocks thickness variation in basal sequences is primarily due to syn depositional faulting syn depositional tilting of fault blocks also influenced the migration of river channels and hence the stacking and distribution of sandstone and locally thick mudstone units overlying coal

late pleistocene barrier island sequence along the southern delmarva peninsula implications for middle wisconsin sea levels evidence for a middle wisconsin sea level high at or above modern limits along the east coast of the us has long been controversial most reports have been dismissed as poorly dated or lacking unequivocal documentation of a marine transgression the authors describe here a sup 14 c dated middle wisconsin transgressive sequence with an extant subaerial barrier facies along the southern delmarva peninsula this sequence indicates that sea levels were near their present position between ca 23 and 34 ka it may correlate with other inferred similar age littoral deposits of the area and it suggests that the evidence for glacioeustatic fluctuations during the wisconsin glaciation should be reconsidered

peat accumulation in coastal plain mires a model for coals of the fruitland formation upper cretaceous of southern colorado usa abstract in the northwestern part of the san juan basin colorado thick high volatile b bituminous coal deposits in the upper cretaceous fruitland formation are associated with nearshore marine sandstones of the pictured cliffs sandstone detailed work along the outcrop and examination of drill cores revealed two coal bearing zones in the lower 60 m of the fruitland formation each zone is up to 13 m thick and consists of interbedded bright and dull coal average ash values of 17 and 34 on a moisture free basis respectively thin fine grained clastic partings and abundant altered volcanic ash partings isopachs of the interval between the top of the pictured cliffs sandstone and a marker bed huerfanito bentonite bed in the underlying lewis shale show linear zones where the interval abruptly thickens these zones which trend northwest to southeast represent areas where the shoreline paused during an overall migration to the northeast isopach maps of coal in the lower part of the fruitland formation and subsurface correlation of shoreface sandstones with coal zones show that the thickest accumulation of coal is 20 25 km landward of these coeval shorelines the fruitland coals may be compared to the high ash peats of the dismal swamp in the southeastern u s a which form in pocosin mires about 20 km inland from the atlantic coast clastic deposirion resulting from coastal processes precludes the formation of peat in low lying mires adjacent to the shoreline the high ash yield numerous partings and the relationship with the coeval shoreline suggest that the coals in the lower part of the fruitland formation accumulated in mires that were transitional from low lying to raised

quantifying stratigraphic completeness a probabilistic approach using paleomagnetic data although sedimentary hiati of variable magnitude are abundant in most stratigraphic sections quantification of the degree of completeness for individual sections has proven difficult a new method of estimating stratigraphic completeness is developed for sections that preserve a detrital remanent magnetic reversal history because the timing of geomagnetic reversals is independent of sedimentation events the distribution of reversals in a stratigraphic section can be used to evaluate completeness the most direct method in which sections are sampled at the top and base of each stratigraphic unit evaluates the frequency of reversals within units versus at unit contacts the more complete a section the greater the probability that reversals occurred during depositional events and are recorded as within unit reversals the less complete a section the greater the probability that reversals occurred during depositional hiati and are recorded as unit contact reversals monte carlo simulations suggest that

high temperature clockwise p t paths and melting in the development of regional migmatites an example from southern brittany france the southern brittany migmatite belt sbmb which evolved through the metamorphic peak between c 400 ma and c 370 ma ago consists of a heterogeneous suite of high grade gneisses and anatectic migmatites both metatexites and diatexites rare garnet cordierite gneiss layers record evidence of an early prograde p t path in these rocks growth zoned garnet cores and a sequence of included mineral assemblages in garnet from core to rim of qtz ilm ky pl ky st rt bt and pl sil st rt bt constrain a prograde evolution during which the reactions ilm ky qtz aim rt ms chl st bt qtz v and st qtz grt sil v were crossed parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types in all rock types garnet has reverse zoned rims and garnet replacement by cordierite and or biotite and plagioclase suggests the following reactions have occurred grt sil qtz crd hc ilm bt sil qtz crd hc ilm kfs v and na ca k ti grt bt pl qtz microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained clockwise p t path for the sbmb the high temperature part of the path has a steep dt dp slope characteristic of near isothermal decompression it is proposed that the p t path followed by the sbmb is the result of the inversion by overthrusting of a back arc basin and that such a tectonic setting may be applicable to other high temperature migmatite terranes the near isothermal decompression is at least partly driven by the upward diapiric movement of the diatexite anatectic granite core of the sbmb

geomorphic and tectonic process rates effects of measured time interval analysis of the original numerical components of diverse geomorphic and tectonic process rates indicates that process rate is not independent of measured time interval this implies that direct comparison of geologic process rates measured over variable time intervals may not be valid and that conclusions based on those comparisons regarding changes in rates through geologic time may be misleading in some cases calculation of statistically derived scaling functions is one approach that can be used to account for this measurement interval bias and to improve comparisons of geologic process rate data when this approach is used a 10 4 or 10 5 yr variation in the measured time interval accounts for approximately one order of magnitude change in the measured process rate the magnitude of the correction can substantially affect conclusions of studies that compare uncorrected process rates calculated from variable time intervals

generic types of stratigraphic cycles controlled by eustasy thought experiments imply that stratigraphic cycles controlled directly by eustasy include keep up catch up catch down foreshortened or truncated and give up variants different patterns of thick ness and facies being governed by the interplay of sedimentation rate and accommodation space most examples of these types are in addition base cutout cycles wherever lowstands of sea level dropped below the top of the underlying cycle overall thicknesses of multiple cycles at any locale are controlled ultimately by net subsidence rate accordingly even keep up and catch up cycles can faithfully record full accommodation only where tectonic subsidence during cycle formation paced eustatic fluctuation paradoxically however only condensed cycles that fortuitously compensate for syncycle tectonic subsidence can accurately record the eustatic amplitude as adjusted for water loading