lower permian sediment gravity flow sequence eastern california abstract the lower permian middle wolfcampian zinc hill sequence a 65 to 110 m thick series of beds in the owens valley group in east central california comprises sediment gravity flow deposits consisting of carbonate sediment that originated on and siliciclastic sediment that may have been generally ponded behind a carbonate shelf to the east and northeast thickness patterns and paleocurrent indicators show that the sediment forming this sequence was transported primarily southeastward and deposited in a southeast trending lobe shaped body evidently the sediment was carried from the shelf by sediment gravity flows that travelled westward down the slope and then turned southeastward upon reaching a southeast trending basin at the base of the slope data derived from the study of this basin which paralleled the shelf edge and is thought to have formed parallel to a southeast oriented segment of the early permian continental margin constitute one of the most important arguments favoring a pennsylvanian to early permian age of truncation of the western north american continental margin

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

stratigraphy depositional history and tectonic evolution of paleozoic continental margin rocks in roof pendants of the eastern sierra nevada california during the late devonian early mississippian antler orogeny at a considerable distance to the west later in post early permian time rocks of the morrison block were deformed rocks of the antler belt were emplaced against the morrison block and the facies boundaries and structural belts defined here were offset on northwest trending dextral faults

a kinematic thermal history of the earth s mantle kinematic convection models of the earth s thermal history are computed assuming that subducted slabs along with some entrained material sink all the way to the core mantle boundary the temperatures within the lithosphere and within the mantle away from sinking slabs are computed assuming that the return flow from the core mantle boundary to the surface can be considered as slow vertical advection in the models a drop in asthenospheric temperature occurs near the end of the archean when subducted material first returns to the surface this drop is about 200 c if the subducted material does not mix thermally with the mantle on its way down and if only a thin thermal layer exists at the core mantle boundary we consider the possibility that this temperature drop is an explanation for the transition between archean and proterozoic tectonic styles an implication of the kinematic scheme is that the asthenospheric temperature increases gradually during the archean until the cooler material reaches the surface after the archean the thermal evolution is similar to a well mixed model a difficulty with the convection scheme as well as with any temperature profile in the mantle which is basically adiabatic is that excessively low temperatures in the core are implied if a 100 km thick thermal boundary layer at the core mantle boundary is included then higher core temperatures occur but there is a smaller drop in asthenospheric temperature at the end of the archean

the first 800 million years of earth s history the earth grew initially from hot planetesimals of reduced material and began to differentiate early into a metal rich core a silicate rich mantle and a volcanic surface accretion ended with volatile rich oxidized material which did not interact with the core because of a mantle barrier quenching of magmas at the surface and foundering of cool material was effective in cooling the earth by 1600 j per gram of magma magmas transported radioactive elements and volatiles to the outer 50 km the lower mantle became stranded below the critical adiabat for melting the core may have been fully molten radiogenic and non radiogenic heat sources produced a surface heat flow that decreased from ca 1 3 x 10 14 w at 4 45 ga b p to the present value of 4 x 10 13 w in spite of uncertainties in the production and transport of heat it seems safe to conclude that intense volcanism occurred throughout the pre archaean era before survival of crust from phase equilibrium data for peridotites and calc alkaline rocks it is deduced that a range of ultrabasic to basic magmas was produced from a peridotitic upper mantle and that calc alkaline magmas were produced during foundering of crust an anorthositic crust was not formed from observations of lunar basins it is concluded that a minimum of ca 500 1000 impact basins were formed on earth in ca 100 200 ma before 3 95 ga b p plausible calculations suggest that ca 20 times as many basins were formed in the preceding ca 300 ma impact debris would be cooled by water in wet crustal rocks and oceans it is assumed that after 4 0 ga b p impacts would have only temporary local effects on the crust and emphasis is placed on upper mantle convection as the main driving force for geochemical transport sedimentation and metamorphism were important factors during foundering of crust oceanic9 and continental9 regions developed above up welling and down welling segments of convection cells and these regions resembled greenstone belts and high grade regions of the archaean era as heat flow declined polygonal tectonics was replaced by symmetrical linear tectonics during the archaean era and then by asymmetrical linear tectonics during the proterozoic era

archean plate tectonics constraints and inferences the earth has been cooling since archean time the higher temperatures beneath archean ridges resulted in more partial melting which extended down to greater depths than at present the archean oceanic crust was much thicker 20 km than modern crust 5 km this inference is compatible with previous ideas of suspected oceanic crust the thicker oceanic crust in the archean tended to resist subduction similar to modern aseismic ridges but could not prevent it short lived episodes of intra arc spreading followed by cordilleran type compression may have produced archean greenstone belts archean meta tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures

use of intrusive and calc silicate compositional data to distinguish contrasting skarn types in the darwin polymetallic skarn district california usa the polymetallic pb zn ag w cu au skarn and vein ores of the darwin area california represent a metal association not common in skarn deposits detailed and regional geologic mapping combined with modal and chemical analyses of igneous rocks and electron microprobe and assay studies of calc silicate rocks shows that there are three separate mineralizing systems which have been superimposed by structural and intrusive events scheelite bearing skarns are associated with the alkaline darwin stock small cu au skarns are adjacent to sheared thrust emplaced granite granodiorite masses and pb zn ag vein skarns are zoned around small granite porphyry dikes and plugs calc silicate mineral compositions are also indicative of three different mineralization systems w skarns have high fe pyroxenes and moderate fe garnets cu skarns have low fe pyroxenes and high fe garnets and pb zn skarns have high mn pyroxenes and garnets patterns seen in other skarn deposits vein ores contain metals compatible with the subjacent skarns and apparently represent lower temperature remobilization of skarn ores

underplating of oceanic lithosphere in the archean a possible mechanism for the formation of archean komatiites abstract the thermal evolution of a subducting archean oceanic lithosphere suggests that amphibole and phlogopite can transport water to depths of about 100 km water released from the oceanic crust hydrates the overlying depleted peridotitic lithosphere lowering its melting temperature and causing a high degree of partial melting temperatures at the top of the archean asthenosphere only about 150 c higher than the present value are sufficient to produce over 50 melting in the bottom of the overlying lithosphere under wet conditions and generate peridotitic komatiites

evolution of the ordovician through middle pennsylvanian carbonate shelf in east central california carbonate shelf rocks marked by various sedimentary features suggesting intertidal to supratidal deposition or bearing rich benthic faunas and their corresponding slope and basin rock sequences containing planktic fossils and sediment gravity flow deposits have been identified in the inyo mountains argus range region of east central california for all of the paleozoic from ordovician through middle pennsylvanian four major depositional sequences each beginning with a marine transgression and all but the third ending with a progradation were developed representing 1 early ordovician into early silurian time 2 early silurian into earliest middle devonian time 3 middle to late devonian time and 4 early to middle late mississippian time a period of basinal infilling in the late mississippian was followed by a fifth major marine transgression in the early pennsylvanian the carbonate shelf itself evolved from a homoclinal ramp in the ordovician to an accretionary rimmed margin by the early devonian this margin was drowned probably in the middle devonian by the early mississippian a new homoclinal ramp had been developed before the end of the mississippian this ramp became distally steepened and then was transformed into an accretionary rimmed margin the early to middle pennsylvanian marine transgression gave rise to still another homoclinal carbonate ramp subsidence of 2 300 m occurred between early ordovician and middle pennsylvanian time in the area immediately east of the inyo mountains it increased westward to probably 3 500 m at the edge of the owens valley as the continental margin subsided and bent downward 1 between early ordovician and middle late mississippian time as a result of subsidence and relatively low rates of sedimentation the water depth in the western part of the study area increased from close to sea level at the beginning of the ordovician to as much as 3 000 m in the late mississippian because of higher rates of sediment accumulation and lower rates of subsidence in the eastern part of the region shallow water environments were maintained throughout most of this time interval even so the shelf slope break retreated at least 45 km southeastward from middle ordovician to middle late mississippian time and at least 35 km farther southeastward by middle pennsylvanian time

the magma ocean concept and lunar evolution the model of lunar evolution in which the anorthositic plagioclase rich oldest crust of the moon is formed over a period of 300 myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies consideration is given to the classification of lunar rocks the evidence for primordial deep global differentiation constraints on the depth of the molten zone the effects of pressure on mineral stability relationships mainly liquid vs mainly magmifer ocean models and the evidence for multiple ancient differentiation episodes a synthesis of the model of primordial differentiation and its aftereffects is presented and the generalization of the model to the earth and to mars mercury venus and the asteroids is discussed

transport models for crust and mantle evolution the exact solutions for the isotopic compositions and the concentrations of two models for mantle crust evolution are given for arbitrary rates of crustal growth and of backflow to the mantle in model i continents are derived by melt extraction over the history of the earth from undepleted mantle in model ii new additions to the continents are derived from a mantle reservoir which becomes increasingly depleted through time by repeated extraction of melts the key parameters are the chemical fractionation factors for crustal growth and refluxing and the integrated fractional mass removal rates from the crust and the mantle the relationships between the mean age of the mass of the continents and the isotopic parameters are given the models suggest that the pre archean era prior to 3 8 ae was dominated by rapid refluxing of crust to the mantle which during the early archean gave way to a regime where transport from the mantle to the crust substantially dominated over refluxing of crust to the mantle the mean age of the mass of the continents appears to be 1 8 ae even for small amounts 10 of refluxing model ii shows that highly incompatible elements have very short residence times in the mantle mass balance considerations implies that the continents were derived from only a small fraction 30 of the mantle

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

the onset and early evolution of life the tension between co 2 dissolved at relatively high atmospheric pressure in the hadean ocean and h 2 generated as ocean water oxidized ferrous iron during convection in the oceanic crust was resolved by the onset of life we suggest that this chemosynthetic life emerged within hydrothermal mounds produced by alkaline solutions of moderate temperature in the relative safety of the deep ocean fl oor exothermic reaction between hydrothermal h 2 hcoo and ch 3 s with co 2 was catalyzed in inorganic membranes near the mound s surface by mackinawite fes nanocrysts and ready made clusters corresponding to the greigite fe 5 nis 8 structure such clusters were precursors to the active centers e g the c cluster fe 4 nis 5 of a metalloenzyme that today catalyzes acetate synthesis viz the bifunctional dehydrogenase enzyme acs codh the water and some of the acetate h 3 c coo produced in this way were exhaled into the ocean together as fl uid waste glycine h 3 n ch 2 coo and other amino acids as well as tiny quantities of rna generated in the same milieu were trapped within tiny iron sulfi de cavities energy from the acetate reaction augmented by a proton gradient operating through the membrane was spent polymerizing glycine and other amino acids into short peptides upon the phosphorylated mineral surface in turn these peptides sequestered and thereby protected the catalytically and electrochemically active pyrophosphate and iron nickel sulfi de clusters from dissolution or crystallization intervention of rna as a polymerizing agent for amino acids also led to an adventitious though crude process of regulating metabolism a process that was also to provide genetic information to offspring the fl uxes of energy and nutrient available in the hydrothermal mound commensurate with the requirements of life encouraged differentiation of the fi rst microbes into two separate domains at the bifurcation the bacteria were to specialize in acetogenesis and the archaea into methanogenesis representatives of both these domains left the mound by way of the ocean fl oor and crust to colonize the deep biosphere once life had emerged and evolved to the extent of being able to reduce nitrogen for use in peptides and nucleic acids light could have been used directly as an energy source for biosynthesis certain bacteria may have been able to do this where protected from hard uv by a thin coating of chemical sediment produced at a subaerial hot spring operating in an obducted and uplifted portion of the deep biosphere

a paleotropical carbonate dominated archive of carboniferous icehouse dynamics bird spring fm southern great basin usa abstract much of our far field knowledge of glaciation and climate during the late paleozoic ice age is built on decades of study of mixed carbonate siliciclastic cyclothemic successions from paleotropical euramerica far fewer carbonate dominated successions have been studied despite their high sensitivity to changes in accommodation space and environmental conditions here we present a sequence stratigraphic framework for the carbonate dominated bird spring formation southern great basin u s a and infer from it a relative sea level history for the latest mississippian through latest pennsylvanian correlation of four successions across the paleo platform documents changes in the lithofacies composition of meter scale cycles their cycle stacking patterns and cycle bounding surfaces that record substantial variation in relative sea level at several temporal scales during the pennsylvanian ten genetic sequences bounded by intervals of restricted peritidal lithofacies karst terra rossa rooted caliche and or protosols are recognized within sequences 126 parasequences stack into parasequence sets that in turn define lowstand transgressive and highstand systems tracts the stratigraphic cycle stacking patterns and across platform architecture of systems tracts suggest a mid carboniferous lowstand in relative sea level followed by a middle morrowan middle bashkirian highstand as previously suggested bishop et al 2010 subsequent progradation of the platform occurred during a gradual long term fall through the late morrowan and atokan late bashkirian early moscovian upon which repeated higher frequency events were superimposed culminating in widespread exposure across the platform proximal to the atokan desmoinesian boundary long term sea level increased into the mid desmoinesian end moscovian after which progradation late desmoinesian and aggradation missourian virgilian of shallow water and peritidal carbonates occurred across the bird spring platform the progradational to aggradational phase involved an abrupt shift from ramp to rimmed shelf geometry in the missourian kasimovian likely in response to progressively increasing sea level dampened high frequency sea level fluctuations and or to regional faulting along the bird spring platform margin inferred magnitudes of short term 105 yr fluctuations superimposed on the longer term rises and falls range from 20 to 70 m during the early to mid pennsylvanian and are substantially reduced 20 m during the late pennsylvanian the early to middle pennsylvanian sea level history inferred from the carbonate dominated bird spring platform corresponds well with a recently published pennsylvanian onlap offlap history defined from the cyclothemic succession of the donets basin and with independent near field reconstructions of glaciation during the pennsylvanian

the himalayas a modern analogue for archaean crustal evolution abstract present crustal evolution models fail to account for the generation of the large volumes of continental crust in the required time intervals if the oceanic crust was appreciably thicker in the archaean as geothermal models would indicate then oceanic crustal collision on the scale of the present day himalayan continental collision zone may have been a frequent occurrence in the archaean resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts leaving a depleted stabilised basic residuum

a speculative reconstruction of the middle paleozoic continental margin of southwestern north america geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern north america with respect to the middle paleozoic continental margin the southwestern margin of the middle paleozoic continent based on these data is quite irregular in its present configuration but most of the irregularities can be accounted for by strike slip displacement of continental fragments during several post middle paleozoic tectonic events we conclude that large scale late paleozoic sinistral displacements and mesozoic and cenozoic dextral displacements have been the major events affecting the configuration of this margin our preferred model includes the following elements 1 500 km of left lateral displacement of a large part of the margin during a late paleozoic truncation 2 slivering of the margin by right lateral faulting during the mesozoic amounting to about 275 km of total displacement and 3 further deformation of the margin during the cenozoic primarily by right lateral faulting oroclinal bending and crustal extension

interpretation of the last chance thrust death valley region california as an early permian decollement in a previously undeformed shale basin abstract the last chance thrust discontinuously exposed over an area of at least 2500 km2 near the south end of the cordilleran foreland thrust belt in the death valley region of east central california is controversial because of its poorly constrained age and its uncertain original geometry and extent we interpret this thrust to be early permian in age to extend throughout a sedimentary basin in which deep water mississippian shale overlain by pennsylvanian and earliest permian limestone turbidites accumulated to represent about 30 km of eastward displacement and to be related to convergence on a northeast trending segment of the early permian continental margin last chance deformation occurred between the times of the antler and sonoma orogenies of late devonian early mississippian and late permian ages respectively and followed early to middle pennsylvanian truncation of the continental margin by transform faulting in the western part of the mississippian shale basin in east central california the originally recognized exposures of the last chance thrust show neoproterozoic and early paleozoic strata above lower plate mississippian shale farther east faults subparallel to bedding above below and within the mississippian shale are interpreted to mark the thrust zone and to represent a regional decollement at the eastern margin of the basin upper plate thrust slices of deep water late paleozoic strata are interpreted to have piled up against the margin of the mississippian carbonate shelf to form a large antiformal stack above the lee flat thrust which we regard as the easternmost exposure of the last chance thrust thrust loading depressed the western part of the shelf creating a new sedimentary basin in which about 3 5 km of younger early permian deep water strata were deposited against the antiformal stack later probably in the late permian other thrusts including the inyo crest thrust which was subsequently overlapped by early to middle triassic marine strata cut across the last chance thrust we interpret the last chance thrust as similar in many ways to appalachian type decollements in which the zone of thrusting is localized along a shale interval the last chance thrust however has been dismembered during later geologic events so that its original geometry has been obscured our model may have unrecognized analogs in other structurally complex shale basins in which the initial deformation was along a major shale unit

the petrogenesis and setting of archaean metavolcanics from karnataka state south india abstract major and trace element data for ultrabasic to silicic metavolcanics from five archaean volcanic sedimentary belts in south india are summarized their total alteration to greenschist and amphibolite facies assemblages poses problems for petrogenetic interpretation of these data however in the absence of unmetamorphosed lavas some interpretation is essential the data suggest that volcanism in each belt was petrogenetically similar the presence of both light rare earth element enriched and depleted lavas in individual volcanic sequences together with variations in zr y ratios is interpreted as a consequence of fractionation within ultramatic mantle diapirs induced by upward vapour migration other chemical variations probably resulted from fractional crystallization of mafic phases from parental high mgo high cr magmas the influence of plagioclase fractionation only being noticeable in late andesites and dacites the consistent depletion of all basaltic samples in the high field strength elements mb and ta relative to the low field strength element th and to la further emphasises the importance of a vapour phase in their petrogenesis in this regard they resemble modern orogenic basalts the favoured tectonic setting is that of ensialic multiple back arc basins above a thinner lithosphere than in modern back arc environments which rode over a shallow angled subduction zone

permian and triassic rocks of the mojave desert and their implications for timing and mechanisms of continental truncation upper paleozoic and lower mesozoic rocks in the mojave desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern united states this change was apparently accompanied by truncation and reorientation of the continental margin in the mojave desert and western basin and range province from late proterozoic to mississippian time the western margin of north america had a northeast southwest trend through nevada and eastern california truncation of the margin began in pennsylvanian time when a northwest trending left lateral strike slip fault zone formed across the antler orogenic belt the cordilleran miogeocline and the more cratonal character mojave desert rocks of the antler belt were apparently displaced southward from east central california to their present position in the northwestern mojave desert sedimentary rocks deposited during this interval record an active continental borderland characterized by alternately subsided and uplifted marine basins this transcurrent faulting continued to late permian time magmatic activity and deformation started in the mojave desert in late permian time and affected both miogeoclinal cratonal and displaced antler rocks magmatic activity ceased and marine sedimentation resumed in early triassic time lower triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal cratonal rocks and possibly on displaced antler rocks facies trends in these rocks are northwest southeast indicating the new margin trend truncation and reorientation of the margin were completed therefore by early triassic time or possibly as early as late permian time upper permian magmatism and deformation in the mojave desert are older than and distinct from the early triassic sonoma orogeny of nevada lower triassic rocks in the mojave desert are post tectonic with respect to deformation in that region whereas correlative lower triassic rocks in nevada are pretectonic to syntectonic with respect to the sonoma orogeny this difference reflects the variation in deformation and tectonic setting along a newly developed cordilleran margin the southern part of the cordillera was apparently a subduction margin as early as late permian or middle triassic time while the central nevada segment was the site of collision tectonics

extent and bathymetry of north american platform seas in the early silurian the north american craton is 25 covered by lower silurian llandovery rocks based strictly upon the geographic area of surviving marine strata preserved in outcrop and the subsurface this figure does not take into account extensive post silurian tectonics and erosion isolated structural basins in which these strata are found today were not necessarily defined as corresponding topographic basins in the silurian analysis of interbasin structural arches and outliers provides important clues as to the actual extent of early silurian seas information on the age of tectonic origin for 15 prominent arches is reviewed the placement and significance of 11 key outliers is also discussed evidence of this kind indicates early silurian platform seas were among the most extensive of the paleozoic probably more than 65 of the north american craton was then flooded nothing on so vast a scale serves as a recent guide to the past the pliocene holocene bahamian banks do provide however worthy models for the role of bathymetry and water circulation as related to the changeover from laterally monotonous blanket deposits to a more complex mosaic of facies the original bathymetry of early silurian environments may be determined correlated and mapped on the basis of widespread carbonate cycles linked to sea level fluctuations the range of fully integrated facies patterns includes 1 coral stromatoporoid stromatolite paleosol cycles williston basin 2 pentamerid brachiopod coral stromatoporoid stromatolite cycles michigan basin and 3 stricklandiid brachiopod pentamerid brachiopod coral stromatoporoid cycles east iowa basin with shallow bathymetries of 0 30 m the williston class tends to represent mosaic facies while with bathymetries of 0 60 m and 10 90 m the deeper water michigan and iowa classes typically involve blanket deposits bathymetry of other basin deposits generally fits one of these three classes maximum and minimum stands in sea level during late llandovery time are represented by bathymetric maps