Mississippian Time Research Articles

Geochronology and geochemistry of Putnam-Nashoba terrane metavolcanic and plutonic rocks, eastern Massachusetts: Constraints on the early Paleozoic evolution of eastern North America

The Nashoba Block, the northern portion of the Putnam-Nashoba terrane, is a fault-bounded fragment of Late Proterozoic to early Paleozoic crust located in eastern Massachusetts. The Marlboro and Nashoba gneisses of the Putnam-Nashoba terrane are composed of a sequence of mafic, intermediate, and felsic volcanic, volcanogenic, and probable plutonic rocks. This sequence was intruded by plutons ranging in composition from granite to gabbro. Major and trace element data for the Marlboro and Nashoba gneisses and some of the granitic plutons are consistent with formation in a calc-alkaline arc setting; some of the later granitic plutons are probably the result of crustal anatexis. The gabbroic plutons are slightly alkaline. U-Pb zircon age determinations for the Marlboro and Nashoba gneisses indicate ages ranging from 584 ± 8 to 425 ± 2 Ma, but most are in the range 473 to 430 Ma. These ages indicate that significant arc volcanism occurred during Late Ordovician and Silurian time in an ocean basin separating Laurentia and Avalonia as Avalonia progressed toward its eventual collision with and accretion to Laurentia. A metamorphic and deformational event, interpreted to record the docking of the Putnam-Nashoba terrane (volcanic arc) with Laurentia as the Boston–Rhode Island terrane (Avalonian fragment) impinged upon it, is constrained by a 425 ± 3 Ma monazite age for the Fish Brook gneiss and the ca. 390 Ma ages of the Straw Hollow diorite and Salem gabbro-diorite. These mildly alkaline mafic plutons, which intrude the Putnam-Nashoba terrane and the Boston–Rhode Island terrane, respectively, are members of a group of roughly contemporaneous intrusions that yield ages from 430 ± 5 to 385 ± 10 Ma. The ages and chemical similarity of these intrusions support the interpretation that the two terranes were proximal to each other by Early Silurian to Late Devonian time. The 360 ± 9 Ma syntectonic, peraluminous Andover Granite and the 349 ± 4 Ma calc-alkaline phase of the Indian Head Hill granite indicates additional igneous activity in Early Mississippian time. Later metamorphism of rocks of the Putnam-Nashoba terrane was presumably due to changing pressure-temperature conditions during oblique overthrusting and subsequent unroofing of the Putnam-Nashoba terrane during tectonic shuffling of the accreted Nashoba and Boston–Rhode Island terranes outboard of the Laurentian margin during the approach and collision of Gondwana. An important event was the migmatization of the Fort Pond and Beaver Brook members of the Nashoba gneisses about 340 Ma.

Production and preservation of organic matter during deposition of the Bakken Formation (Late Devonian and Early Mississippian), Williston Basin

The organic-rich (up to 35% TOC) lower and upper black mudstone members of the Bakken Formation (Late Devonian and Early Mississippian) accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic, palaeoceanographic and palaeoclimatic conditions. During Bakken deposition the semi-isolated Williston Basin was located close to the Equator (5° to 10°N) in a zone of prevailing east to west winds. A pattern of estuarine-like marine circulation between the Williston Basin and open-ocean conditions at the western craton edge of North America is proposed. Surface water discharged from the basin was replaced by nutrient-rich deeper water sourced from the equatorial undercurrent in the Pacific Basin. Undercurrent flow into the Williston Basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition. In response to increased productivity, that led to a high rate of organic sedimentation, and quiescent, deep-water conditions, due primarily to the geographic isolation of the basin and the restriction of estuarine-like circulation to the upper water column, anoxic bottom water developed on the basin floor. As sea-level rose during lower and upper black mudstone deposition, anoxic bottom water expanded across the basin enhancing organic matter preservation. Conditions in the Williston Basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic-rich mud deposition. These results have potentially important implications for the accumulation of similarly organic-rich deposits across the North American interior during Late Devonian and Early Mississippian time.

Laurentian magmatism of the southern Appalachian Blue Ridge: Post-Iapetan rifting

Tectonically significant postrift metaigneous rocks occur within three genetically distinct cover-rock sequences along a 400 km strike segment of the southeastern margin of Laurentia, in the southern Appalachian western Blue Ridge. These include (1) Neoproterozoic rift facies rocks, (2) lower Paleozoic drift facies rocks, and (3) unconformably overlying postdrift Paleozoic successor basin sequences. This region is divisible into two along-strike, ensialic, continental margin volcanic belts, separated by a probable Neoproterozoic transfer fault. To the northeast, igneous rocks are intrusive into sequences 1 and 2 above, whereas in the overlying successor basins on both sides of the transfer boundary, they occur predominantly as eruptive rocks. These igneous rocks can be separated into several suites that are distinct from the Neoproterozoic rift-related igneous rocks, based on stratigraphic position, geographic location, and composition. Mafic dikes and sills intrusive into the rift facies and mafic metavolcanic rocks of the Hillabee Greenstone represent low-K tholeiitic magmatism associated with derivation from a “depleted” mantle source, whereas most igneous rocks in the northeastern belt exhibit an alkaline basalt affinity. The successor basin sequences associated with the volcanic activity formed above extended and thinned continental crust, near and parallel to the southeastern Laurentian margin. This igneous activity can be constrained broadly between stratigraphic position and age of metamorphism (Middle Ordovician to earliest Mississippian time). Most likely this magmatic activity was associated with a destructive plate boundary during Paleozoic A-type subduction, but was largely decoupled from slab-derived magmatism, being instead more likely associated with backarc or pull-apart basin evolution.

Nature and regional significance of unconformities associated with the Middle Ordovician Hagan K-bentonite complex in the North American midcontinent

Stratal patterns of the Middle Ordovician Hagan K-bentonite complex and associated rocks show that the Black River–Trenton unconformity in the North American midcontinent formed through the complex interplay of eustasy, sediment accumulation rates, siliciclastic influx, bathymetry, seawater chemistry, and perhaps local tectonic uplift. The unconformity is diachronous and is an amalgamated surface that resulted from local late Turinian lowstand exposure followed by regional early Chatfieldian transgressive drowning and sediment starvation. The duration of the unconformity is greatest in southern Wisconsin, northern Illinois, and northern Indiana, where the Deicke and Millbrig K-bentonite Beds converge at the unconformity. On the basis of published isotopic ages for the Deicke and Millbrig beds, it is possible that in these regions erosion and nondeposition spanned a period of as much as 3.2 m.y. Two broad coeval depositional settings are recognized within the North American midcontinent during early Chatfieldian time. (1) An inner shelf, subtidal facies of fossiliferous shale (Spechts Ferry Shale Member and Ion Shale Member of the Decorah Formation) and argillaceous lime mudstone and skeletal wackestone (Guttenberg and Kings Lake Limestone Members) extended from the Canadian shield and Transcontinental arch southeastward through Minnesota, Wisconsin, Iowa, and Missouri. (2) A seaward, relatively deep subtidal, sediment-starved, middle shelf extended eastward from the Mississippi Valley region to the Taconian foreland basins in the central and southern Appalachians and southward through the pericratonic Arkoma and Black Warrior basins. In the inner shelf region, the Black River–Trenton unconformity is a composite of at least two prominent hardground omission surfaces, one at the top of the Castlewood and Carimona Limestone Members and the other at the top of the Guttenberg and Kings Lake Limestone Members, both merging to a single surface in the middle shelf region. The inner and middle shelves redeveloped later in approximately the same regions during Devonian and Mississippian time.

97/02243 Physical modeling of coal-fired power plant stack flows

This chapter is concerned with the southern portion of the craton, the southern craton margin, and its geological relationship with the Ouachita Orogeny. The southern continental margin was shaped by extensional and transform faulting, during the breakup of Rodinia, and includes the Oklahoma Basin, a basin formed by transtensional faulting. Carbonate reef systems flourished along the entire continental margin from Texas to Newfoundland during the Early Paleozoic. Shallow-water, carbonate, and siliciclastic sedimentation continued in the Oklahoma Basin from Middle Ordovician through Earliest Mississippian time. The ocean that lay south of the North American Plate through much of the Paleozoic was closed by collision with Gondwana, starting in the Mid-Mississippian. Northwesterly directed contractional stress generated a regionally distinctive style of transpressive fault and block-uplift deformation across the Southwestern United States. Mild warping of the Texas–Oklahoma area during this period accentuated existing differentiation of the continent into a series of basins (including the Delaware and Midland basins) and intervening uplifts (Diablo Platform, Central Basin Platform, Eastern Shelf, Llano Uplift, Ozark Uplift). That portion of this basin and uplift system located within Texas now constitutes the classic Permian Basin. The carbonate platform and basin configuration of the Permian Basin is similar in topography and scale to that of the Cretaceous to modern Bahamas Platform. The basin is probably best known for the distinctive “clinoform” stratigraphy of the Capitan Reef, one of the first places where the lateral facies transition from back reef, through reef crest, foreef slope to basin floor was mapped and described in detail, based on superb outcrops in the Guadalupe Mountains.Sediments of the Absaroka Sequence (Uppermost Mississippian to Lower Jurassic) are widely distributed across the continental interior. For much of the Carboniferous to Permian, the Earth was under the influence of repeated, high frequency glacioeustatic sea-level changes. These caused transgressions and regressions that shifted shorelines and depositional systems back and forth hundreds to thousands of kilometers across low-relief continental interiors. The classic Pennsylvanian cyclothems of the southern US Midcontinent are the most characteristic and well-known products of this process.Crustal loading of the continental margin by colliding Gondwana terranes commencing in the Mid-Pennsylvanian generated typical foreland-type basins which now constitute the Ouachita system, and including such foreland basins as the Forth Worth, Arkoma, and Black Warrior basins. In the Ouachita Mountains of Oklahoma and Arkansas some 3,500 m of structurally disturbed Late Cambrian to Mississippian pre-orogenic strata have been mapped. They consist mainly of shales and sandstones, the latter representing a variety of sediment-gravity flow mechanisms.

A Southern Antler Foredeep Submarine Fan: The Mississippian Eleana Formation, Nevada Test Site

ABSTRACT Strata of the Eleana Formation in southern Nevada record the history of the Late Devonian through late Mississippian Antler foreland basin in southern Nevada. The Antler foredeep was created in latest Devonian or early Mississippian time, but did not receive significant sediments in southern Nevada until early-middle Mississippian (late Osagean) time. During late Osagean and Meramecian time, sediments deposited in the foredeep were dominantly mature, recycled siliciclastics derived from the uplifted Antler allochthon and foreland basin to the north in central Nevada. At the same time, a heterolithic sediment source, including mafic volcanic rocks and eroded, older limestones, supplied sediment intermittently and sparsely from the west-northwest. In late Mississippian (Chesterian) time siliciclastic sedimentation waned and deposition was dominated by fine-grained, carbonate- and siliciclastic-debris turbidity currents derived from an organically productive shelf or slope, probably to the north and northeast. The Eleana Formation on the Nevada Test Site is preserved in at least two pre-Tertiary thrust sheets that are separated by a fault with potentially large horizontal displacement. Although palinspastic reconstruction is not yet possible, understanding of this thrust juxtaposition allows a preliminary reconstruction of the Eleana basin paleogeography. The stratigraphically lower part of this Mississippian submarine fan comprised mainly channel and interchannel deposits. More proximal fan deposits are found in the upper thrust plate of an east-vergent thrust system, indicating that the axis of the fan was to the west of the present exposures of the Eleana Formation. The Antler foreland basin remained a basin in southern Nevada long after the Antler foredeep had filled in central Nevada. It may have been a peripheral deep marine basin that occupied either a reentrant in the old passive margin or a reentrant in the encroaching allochthon. Foredeep sediments were recycled down the basin axis from central Nevada south into the Eleana basin. Overlying allodapic limestone turbidites in the upper Eleana Formation were deposited as a transgressive deposit and are coeval with a known eustatic sea-level rise in Chesterian time.

Structural divergence and transpression in the Teslin tectonic zone, southern Yukon Territory

The structural and tectonic evolution of the Teslin tectonic zone, a complex belt of ductilely deformed rocks forming the southern extension of the Yukon‐Tanana terrane in the northern Canadian Cordillera, is elucidated with new field data and by the reinterpretation of existing data. The zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the Nisutlin and Anvil assemblages characterized by S and L‐S tectonite fabrics. Primary contact relationships and ages show that most rocks in the zone were contiguous by Mississippian time. Mapping identified a number of strain domains which preserve S fabrics dipping northeasterly or southwesterly, variable mineral lineation orientations, and a variety of shear directions including easterly and westerly vergent thrust shear, down‐to‐the‐east and down‐to‐the‐west normal shear, and dextral strike‐slip shear. Regional constraints and microstructures suggest that latest ductile deformation was Late Triassic to Early Jurassic in age. Structural characteristics are explained most effectively by convergent‐dominated transpression in combination with tectonic wedging and associated back thrusting above a low‐angle west dipping detachment. Oblique eastward and upward movement of the rocks over the detachment, which may have coincided with the top of thinned North American continental crust, produced easterly vergent shear, and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear. The transpressive deformation postdates formation of the Nisutlin assemblage by Mississippian time and subduction of the Slide Mountain Ocean beneath the Nisutlin assemblage in Permian to Early Triassic time. The western margin of the Teslin tectonic zone was truncated by Cretaceous strike‐slip faults and translated northward. Rocks of the peraluminous orthogneiss assemblage of the Yukon‐Tanana terrane in central Yukon acted as the tectonic wedge and were overthrust by rocks of the Nisutlin and Anvil assemblages in Early Jurassic time before unroofing by local extension in Cretaceous time.

Mississippian assembly of the Nisutlin assemblage: evidence from primary contact relationships and Mississippian magmatism in the Teslin tectonic zone, part of the Yukon–Tanana terrane of south-central Yukon

Metamorphosed and ductilely deformed sedimentary, plutonic, and volcanic rocks of the Nisutlin and Anvil assemblages make up the Yukon–Tanana terrane in the Teslin tectonic zone study area. The Nisutlin assemblage consists of siliceous schist–quartzite and graphitic phyllite that share a primary depositional contact, and Early Mississippian tonalite to quartz diorite that intrudes the siliceous schist–quartzite and possibly the graphitic phyllite. The Anvil assemblage includes metagabbro and mafic schist–greenstone that share an intrusive contact relationship. Tonalite to quartz diorite of the Nisutlin assemblage is characterized by minor zircon inheritance with an average Proterozoic age, εNd(350 Ma) values of −2.5 to −6.2, and Nd model ages of 1.50–1.79 Ga. These data suggest that the magmatic bodies have inherited a component of continentally derived material. Primary contact relationships and age data indicate that the Nisutlin assemblage had formed by Mississippian time, and regional correlations show that this assemblage makes up a large part of the Yukon–Tanana terrane of southern Yukon. Assembly of the Nisutlin assemblage by Mississippian time indicates that it did not form as a late Paleozoic and early Mesozoic subduction melange, and it suggests that its tectonic fabrics did not result from the progressive growth of a Permo-Triassic subduction complex. We suggest that the Nisutlin assemblage was part of a crustal block that lay outboard of North America in Mississippian time, and that it lay in the hanging-wall plate of a Permo-Triassic subduction zone as a relatively coherent assemblage, rather than forming within the zone as a subduction complex.

Paleozoic transcurrent conjugate shear zones in the central Appalachian piedmont of southeastern Pennsylvania

Well-documented tectonic events in the central Appalachians of Pennsylvania are: (1) the early Paleozoic Taconian orogeny that occurred during convergence of Laurentian and the Chopawamsic-Wilmington complex magmatic arc over an east-dipping subduction zone, and resulted in intense metamorphism and deformation in the Piedmont, and (2) the late Paleozoic Alleghanian orogeny that resulted in the thrust and fold belt in the Valley and Ridge province and dextral shear zones in the Piedmont. Unlike the Paleozoic tectonic history for the northern and southern Appalachians, the north-central part of the orogen in Pennsylvania lacks evidence for Acadian deformation and metamorphism. The relative chronological order of deformation and metamorphic events in the eastern Piedmont of Pennsylvania, combined with published geochronology suggests the previously undocumented Acadian deformation possibly exists as a transcurrent conjugate shear zone pair. The Rosemont shear zone is a dextral transcurrent shear zone that is the boundary between the type-section Wissahickon Formation of the Philadelphia structural block (to the southeast) and the West Chester and Avondale Grenvillian basement massifs (to the northwest). The Crum Creek shear zone is the sinistral antithetic structure to the Rosemont zone, and developed internal to the Philadelphia block. Geometric and metamorphic history similarities, opposing offsets, angular relationships, and relative timing of local deformation events supports a conjugate model for these shear zones. East-west oriented bulk shortening and north-south oriented bulk elongation directions are inferred from the conjugate geometry. The Rosemont-Crum Creek system crosscuts and deforms regional Taconian structures and metamorphic zones. In turn, the Rosemont zone is truncated by the Alleghanian Pleasant Grove-Huntingdon Valley shear zone. The available geochronology brackets the movement on the Rosemont-Crum Creek system from Devonian to early Mississippian time. This timing correlates with the Acadian metamorphism in the central Appalachians, suggesting that the Acadian orogeny was manifest as a strike-slip shear system in the Piedmont of eastern Pennsylvania. The lack of regional thrusting and subsequent crustal thickening associated with transcurrent deformation could explain the lack of regionally extensive Acadian metamorphism in the Pennsylvania Piedmont.

Ages of Detrital White Mica from Devonian-Pennsylvanian Strata of the North Central Appalachian Basin: Dominance of the Acadian Orogen as Provenance

Detrital micas (mostly white mica) from Devonian-Middle Pennsylvanian strata of the north central Appalachian Basin at 12 sites were dated by conventional K/Ar. Three small pilot samples were less precisely dated than the rest. The youngest stratigraphic sample of Pennsylvanian depositional age (Alleghenian) produced 414/Ma muscovite and 361 Ma biotite. Excluding this sample, a narrower 35 m.y. range of Early to Middle Devonian detrital dates (from 406-371 Ma) were obtained. The precise dates from the proximal and distal Catskill wedge represent an even narrower 19 m.y. span (406-387 Ma), which is entirely Early Devonian and only approximately 30 m.y. older than the Late Devonian-Early Mississippian time of deposition. All these detrital dates from pre-Alleghenian samples correspond to reported primary ages of crystallization, or cooling therefrom, of regional metamorphism and plutonism during the Acadian orogeny, believed to be the principal Paleozoic orogeny in New England and eastern Canada. The dominance of the Acadian Orogen as a source region is evident. Even though these are conventional K/Ar dates of multi-grain separates, the mean provenance ages from so many individual samples are so close to the age of deposition to preclude almost any Precambrian contribution from either the nearby Canadian Shield or the uplifted cores of the orogen. They imply Acadian sources dominated considerably over Taconic ones. Furthermore, our results for the Mauch Chunk (377 Ma), Pottsville (371 Ma), and our youngest (Alleghenian) sample suggest that the Acadian Orogen persisted as the major provenance for post-Catskill clastic pulses that prograded into the central Appalachian Basin at least up until Middle Pennsylvanian time.

Three-dimensional controls on subsidence of a foreland basin associated with a thrust-belt recess: Black Warrior basin, Alabama and Mississippi

Tectonic transport from different directions and convergence of separate thrust belts at structural recesses impart three-dimensional variations to the pattern of foreland-basin subsidence. Assuming approximately uniform flexural rigidity of the lithosphere, subsidence is directly controlled by the geometry and density of the separate thrust loads, and flexural deformations caused by the loads interfere within the recess. For example, the Black Warrior foreland basin, which is located in Mississippi and Alabama, within a recess in the Appalachian-Ouachita orogen, formed in response to two separate thrust loads: (1) the Ouachita accretionary prism along the southwest side of the basin, and (2) thrust-imbricated passive-margin rocks in the Appalachian thrust belt along the southeast side of the basin. The subsidence history differs along two profiles, each perpendicular to the Appalachian and Ouachita thrust fronts. Subsidence of the foreland basin began in middle Mississippian (Meramecian) time in response to northeast-directed translation of the Ouachita accretionary prism. Subsequently, northwest-directed Appalachian thrusting began in middle Early Pennsylvanian (Morrowan) time. Subsidence curves from the profile perpendicular to the Ouachita prism show a clear progression of increasing subsidence rates and amounts toward the thrust front, whereas curves from the profile perpendicular to the Appalachian belt indicate out-of-plane effects from the Ouachita thrust front and a later increase in subsidence toward the Appalachian thrust front. The major implication of this work is that the three-dimensional geometry of a foreland basin within a thrust-belt recess includes diachronous interfering elements.

The Chukchi Sea Planning Area, Alaska: Structural Development, Basin Analysis and Hydrocarbon Potential

Seismic stratigraphic and seismic fault evidence both show that the Central Chukchi basin is a north-trending, fault-controlled basin, which developed at the beginning of early Mississippian time. About 20 km thickness of sediments have been deposited in the basin. Using a simple flexural model a series of 3-dimensional basement motion patterns with time was produced. From these characteristic patterns two stages of basement evolution can be recognized: the first is basement subsidence controlled by regional extensional fault systems during the period from early Mississippian to later Jurassic: the second stage is basement evolution as two nearly separate blocks from the beginning of Early Cretaceous. The west block development is related to reactivation of the old fault system, while the subsidence of the east block is caused by tectonism and associated sediment loading due to progressive thrusting northward of the Brooks Range. Quantitative basin modeling techniques show that many uncertainties occur when applying mathematical and computer models to simulate a sedimentary basin, especially in a frontier area where only limited data are available. In this study, quantitative basin modeling techniques applied to the Chukchi Sea Planning Area, Alaska, demonstrate how uncertainty can be handled. The Chukchi Sea Planning Area encompasses about 127,000 square kilometers of the northwestern Alaska continental shelf and lies offshore of the Natural Petroleum Reserve in Alaska. Only seismic data and a few remote onshore wells are available in the Planning Area. Compared with the North Slope of Alaska, the Planning Area has had a significantly different structural history. In addition, most of the strata in the Area are currently thermally mature to overmature and in the stage of generating dry gas. The prospective areas for oil are the margins of the central Chukchi Basin, such as the Chukchi platform, Herald Arch and northeast part of the basin.

Contractional nature of Devonian-Mississippian Antler tectonism along the North American continental margin

Regional tectonic analysis suggests that the entire Cordilleran continental margin, from Nevada to the Yukon, underwent contractional orogeny involving emplacement of deep basinal, submarine-fan, and mafic volcanic strata over autochthonous continental margin strata during Late Devonian to Mississippian time (Antler orogeny). Displaced parautochthonous strata are present from Nevada to east-central British Columbia, and perhaps within the Yukon-Tanana terrane of Yukon and eastern Alaska. Manifestations of the orogenic event in the northern Cordillera are preserved primarily by normal faults and related coarse clastic strata of the Selwyn basin. We propose that these features formed due to flexural extension during foreland deformation, rather than mid-Paleozoic rifting or transtension.

Subsurface structure in the vicinity of an intraplate earthquake swarm, central Arkansas

Over 40,000 events have been recorded in the Arkansas earthquake swarm since its inception in 1982. The earthquakes occur at depths between 3 and 6 km and cluster in a volume of about 25 km 3 beneath the easternmost Arkoma basin, near the town of Enola, Arkansas. A study of proprietary reflection seismic lines reveals that the earthquakes cluster within a graben formed in Mississippian time. This graben is part of a system of steeply dipping normal faults that trends ENE across the region. The regional strikes of the basement faults are not favorably oriented for activation under the regional stress regime. In the swarm area, however, these faults bend to form a 2.5-km long segment trending WNW. The small WNW striking segments are well oriented for left-lateral strike slip and focal mechanisms are consistent with this sense of slip. Additionally, a subset of the most accurately located earthquakes do appear to lie along a WNW trend. The length of the WNW trending fault segments is sufficient to have generated the largest of the swarm events. The reflection data reveal a loss of coherent reflectors within the swarm hypocentral volume. Reflectors above the graben have been uplifted about 30 m in post-Atokan (Pennsylvanian) time. Third order leveling surveys show 14.3 cm of uplift between 1961 and 1986 at a benchmark over the graben relative to a benchmark outside of the graben.

Magnetic Polarity Stratigraphy of the Permian and Carboniferous: The Extension of the Magnetic Reversal Record into the Paleozoic

Magnetic polarity stratigraphy has revolutionized stratigraphic studies in Jurassic to Pleistocene sediments. These studies have been greatly facilitated by the reversal record that is recorded in rocks of the ocean floor. For times prior to the Jurassic, the reversal history of the magnetic field must be determined and eventually related through the type section concept. The magnetic reversal history of the late Paleozoic is dominated by the Permo-Carboniferous reversed superchron (PCRS), which extends from the late Permian to the Carboniferous (Westphalian). Recent studies by the author and his students in Middle Carboniferous sediments of eastern Canada, Pennsylvania, Arizona, and Nevada reveal that the magnetic field has reversed frequently in late Mississippian and early Pennsylvanian times (Meramecian through late Morrowan). The polarity of the magnetic field over this interval is approximately 50% normal and 50% reversed. The frequency of reversal appears to be about one reversal per m.y. The possibility, therefore, exists that this pattern may be used for continental and intercontinental correlation. Attempts are currently underway to correlate this magnetic stratigraphy to fossiliferous marine sections. The base of the PCRS is probably of Atokan age.

Unusually large subsidence and sea-level events during middle Paleozoic time: New evidence supporting mantle convection models, for supercontinent assembly

New evidence from basin modeling documents a nearly synchronous episode of rapid subsidence between Middle Devonian and earliest Mississippian time in almost all preexisting margins and interior basins of North America. The subsidence events appear to have been accompanied by a large sea-level rise relative to the stable cratonic platform. Herein we suggest that the timing and magnitude of basin subsidence and sea-level change are compatible with predictions by recent mantle-convection models for the assembly of supercontinents. If so, the sea-level and subsidence events were consequences of the onset of accretion of Pangea above a major zone of mantle downwelling.

Fertility, Mortality, and the Mean Age at Death: A Model of a Population under Stress

A model of a stable native population under long-term stress from diseases incurred in the wake of the expanding Western civilization is presented. The model is based on the reconstructed demography of an Eskimo-speaking population of southwestern Alaska around 1880. Statistics of the Alaskan Russian Church were used to reconstruct the mortality (16.3 deaths annually over 10 years) and fertility of a small population (319 persons) living at the Naknek River. Life expectancy was calculated at 20 years and between ages 5 and 45 there was a noticeable upswing of deaths. A large percentage of mortality was caused by recently introduced respiratory diseases (including influenza and pneumonia) and by tuberculosis which affected mainly adults. Accordingly life expectancy was corrected to 31 years after removal of the adult group. This population had been decreasing at a rate of 1.5% a year for some decades although typhus malaria smallpox and measles were absent. Respiratory diseases were probably contracted via contact with the Russian trading network starting in the late 18th century and the Naknek people lost 2/3 of their population within a century. A pattern of mortality involving respiratory diseases as a result of expanding contracts also probably affected a prehistoric west-central Illinois population dating from the Middle Woodland to Mississippian times although on a smaller scale than in the case of the Naknek people. A decrease in mean age of deaths over age 5 and an increase in mean age of deaths at age 1-9 reduced life expectancy for the Illinois indigenous people so the it resembled the fate of the Naknek population

Biogeography of Lower Carboniferous crinoids

Abstract The stratigraphic are ranges of all known crinoid genera of Mississippian and Lower Carboniferous age in western Europe and North America compared. Camerate crinoid distribution was published in 1987 (Lane and Sevastopulo). Inadunate and flexible crinoid ranges are presented here. A total of 197 genera are involved, 157 in North America and 93 in Europe, with 53 joint occurrences. Conspicuous endemism occurs at both the familial and generic levels in both Europe and North America. In early Mississippian time camerate crinoid genera are especially endemic in North America and many become extinct. In later Mississippian time many poteriocrinoid and flexible crinoid genera and some families are confined to either North America or Europe. Both environmental and physical barriers contributed to isolation and endemism. Genera tend to originate first, and become extinct first, at about equal rates in North America and in Europe. European genera tend to be somewhat longer ranging than North American genera, perhaps due to differences in breadth of definition on the two continents. Primitive groups like disparid and cyathocrinoid inadunates range up from the Late Devonian. Advanced poteriocrinoid inadunates commonly range up into younger Pennsylvanian-Upper Carboniferous rocks. New families originated in both North America and western Europe with about equal frequency. New Japanese occurrences of Pennsylvanian inadunates and the age relations of Permian Timor crinoids are discussed briefly.

Deposition Analysis Based on Electric-Log Character of Cypress Sandstone (Chesterian) in Illinois: ABSTRACT

Four major deposystems were mapped in the Cypress Sandstone using SP (spontaneous potential) and resistivity log character in a statewide cross-section grid. Illinois was a shallow, gently sloping embayment subject to periodic influx of sediment and changes in sea level during Late Mississippian (Chesterian) time. Cypress log character varies predictably from the eastern margin of Illinois to the western erosional limit. Deposystem interpretations based on log character have identified (1) a deltaic zone characterized by thin sandstones interbedded with shale and siltstone in easternmost central Illinois, (2) an offshore marine zone characterized by sand bodies 50-150 ft thick, displaying blocky SP in a 20-mi wide band in east-central Illinois, (3) an offshore marine zone characterized by sandstone bodies 15-40 ft thick in central Illinois, and (4) an offshore marine zone characterized by predominantly shale log character in west-central Illinois. Presence of the shale coincident with the DuQuoin monocline suggests that Cypress deposition was influenced by structural expression of the monocline.

Social networks of disease spread in the lower Illinois valley: a simulation approach.

This study illustrates the use of disease modeling and simulation techniques to the study of the spread of disease within and between social networks. A Reed-Frost type model of disease spread is used to construct a simulation of the spread of tuberculosis within three prehistoric populations of the Lower Illinois River Valley during Middle Woodland, Late Woodland, and Mississippian times. A high and low population size was modeled for each time period. Late Woodland model 2 (low population estimate) is the only model that experienced pathogen extinction with host survival. The rest of the models experienced rapid and severe host population decline. The results of the simulation suggest that a social network size of between 180 and 440 persons is required under the conditions of this model for host-pathogen coexistence (i.e., endemicity) to occur. The severe population decline experienced by these populations suggests that tuberculosis as modeled here could not have existed in these populations. Future refinements of modeling and simulation techniques can provide additional insights into how disease spreads among social contacts.