Regional Facies Research Articles

Timing of middle Paleozoic (Acadian) metamorphism in the southern Appalachians: K-Ar studies in the Talladega belt, Alabama

The Talladega belt of Alabama and Georgia is located at the southernmost exposed part of the southern Appalachian orogen and is one of the westernmost metamorphic terranes in this part of the orogen. The presence of the Lower Devonian Jemison Chert of the Talladega Group provides a maximum age for the timing of greenschist facies metamorphism and penetrative slaty cleavage formation in this terrane. Six slate collected form upper Precambrian( )-Lower Cambrian, and Silurian( )-Devonian rocks of the Talladega belt have an average K-Ar age of 399 {plus minus} 17 Ma. This age is interpreted to be the approximate time of regional greenschist facies metamorphism. The timing of metamorphism in the Talladega belt is similar to the age of the Acadian orogeny in the Maritime and northern Appalachians. Unlike the accreted nature of the Acadian terranes in the northern Appalachians, the Talladega belt appears to have been part of the early to middle Paleozoic cratonic margin of North America. The results of this study indicate that Acadian dynamothermal events extended into parts of the Laurentian margin in the southern Appalachians. Paleozoic igneous activity in the southern Appalachians was less extensive compared with the voluminous Acadian plutonism of the northern Appalachians;more » however, synkinematic to postkinematic Devonian plutons are present in the central and western parts of the orogen.« less

Stratigraphic and environmental patterns of ironstone deposits

Summary Evaluation of the accumulation of Minette- and Clinton-type ironstones in terms of stratigraphic successions and regional facies patterns provides a sequence of types from condensed ‘iron shot’ muddy carbonates over barrier bars to distal tempestite deposits. A lagoonal origin of iron-oolites is derived and discussed. Biogenetic carbonate production and a biogenic contribution to iron-oolite formation, as well as replacement patterns of carbonate by iron-minerals indicate that (1) ironstone formation is related to environmental changes in the course of transgressive-regressive cycles and (2) formation of iron-ooids occured in environments with short term fluctuations between oxidizing and reducing conditions. Adopting this dynamic viewpoint allows dissolution and precipitation of iron ‘side-by-side’ without contradicting classical models.

Paleo-Oceanographic and Tectonic Settings of Early Miocene Reefs and Associated Carbonates of Offshore Southeast Asia

The late Oligocene to early middle Miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast Asia. A generally rising eustatic sea level during this period allowed the accumulation of thick reefal units. An expansion of the coral-algal regional facies belt north to Japan and south to New Zealand occurred during the early Miocene as the result of a reduced oceanic latitudinal temperature gradient. An episode of enhanced organic carbon burial is reflected in a positive ^dgr13C excursion from 17.5 to 13.5 Ma. Increasing paleo-oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen-minimum zone. Marine oil-prone or anic carbon was deposited in silled anoxic basins both bounded by, and within, island-arc systems. A combination of tectonic and paleo-oceanographic factors, therefore, created conditions that favored contemporaneous deposition of organic carbon-rich source beds and reef reservoirs during the latest early to early middle Miocene. Paleogeographic and facies reconstructions for an 18-m.y. interval indicate that deposition occurred in three tectonic settings: (1) passive continental margins, (2) convergent plate boundaries, and (3) obliquely convergent plate boundaries. In all these settings, the vertical-motion histories of fault-bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups, their scal and morphology, duration, diagenesis, and relationship to siliciclastics.

Petroleum Potential of Lower and Middle Paleozoic Rocks in Nebraska Portion of Mid-Continent: ABSTRACT

Central North America during the Paleozoic was characterized by northern (Williston) and southern (Anadarko) depositional regimes separated by a stable Transcontinental arch. Nebraska lies on the southern flank of this arch and contains the northern zero edges of the lower and middle Paleozoic rocks of the southern regime. Most of these rocks are secondary dolomites with zones of excellent intercrystalline porosity. The Reagan-LaMotte Sandstones and the overlying Arbuckle dolomites are overlapped by Middle Ordovician rocks toward the Transcontinental arch. Rocks equivalent to the Simpson consist of a basal sand (St. Peter) and overlying interbedded gray-green shales and dolomitic limestones. An uppermost shale facies is present in the Upper Ordovician (Viola-Maquoketa) eastward and southward across Nebraska. The dolomite facies extends northward into the Williston basin. The Silurian dolomites, originally more widely deposited, are overlapped by Devonian dolomites in southeastern Nebraska. Upper Devonian rocks exhibit a regional facies change from carbonate to green-gray shale to black shale southeastward across the Mid-Continent. Mississippian carbonates overlap the Devonian westward and northward across the Transcontinental arch. Pennsylvanian uplift and erosion were widespread, producing numerous stratigraphic traps. Sands related to the basal Pennsylvanian unconformity produce along the Cambridge arch. Arbuckle, Simpson, Viola, and Hunton production ismore » present in the Forest City basin and along the Central Kansas uplift. Although source rocks are scarce and the maturation is marginal, current theories of long-distance oil migration encourage exploration in the extensive lower and middle Paleozoic reservoirs in this portion of the Mid-Continent.« less

High-resolution seismic-reflection interpretations of some sediment deposits, Antarctic continental margin: Focus on the western Ross Sea

High-resolution seismic-reflection data have been used to a varying degree by geoscientists to interpret the history of marine sediment accumulations around Antarctica. Reconnaissance analysis of 1-, 3.5-, and 12-kHz data collected by the U.S. Geological Survey in the western Ross Sea has led to the identification of eight echo-character facies and six microtopographic facies in the sediment deposits that overlie the Ross Sea unconformity. Three depositional facies regions, each characterized by a particular assemblage of echo-character type and microtopographic facies, have been identified on the continental shelf. These suites of acoustic facies are the result of specific depositional processes that control type and accumulation of sediment in a region. Evidence of glacial processes and products is uncommon in regions 1 and 2, but is abundant in region 3. McMurdo Sound, region 1, is characterized by a monospecific set of acoustic facies. This unique assemblage probably represents turbidity current deposition in the western part of the basin. Most of the seafloor in region 2, from about latitude 77°S to 75°S, is deeper than 600 m below sealevel. The microtopographic facies and echo-character facies observed on the lower slopes and basin floor there reflect the thin deposits of pelagic sediments that have accumulated in the low-energy conditions that are typical of deep-water environments. In shallower water near the boundary with region 3, the signature of the acoustic facies is different from that in deeper water and probably indicates higher energy conditions or, perhaps, ice-related processes. Thick deposits of tills emplaced by lodgement during the most recent advance of the West Antarctic Ice Sheet are common from latitude 75°S to the northern boundary of the study area just south of Coulman Island (region 3). The signature of microtopographic facies in this region reflects the relief of the base of the grounded ice sheet prior to decoupling from the seafloor. Current winnowing and scour of shallow parts of the seafloor inhibits sediment deposition and maintains the irregular, hummocky relief that characterizes much of the region. Seafloor relief of this type in other polar areas could indicate the former presence of grounded ice.

Sedimentological studies of the fluviatile-shallow marine Upper Triassic to Lower Jurassic succession in Jameson Land, East Greenland

A three-year research fellowship programme supported by British Petroleum Development, London, was initiated in the summer of 1987. The main subject of the study is the Upper Triassic to Lower Jurassic succession in Jameson Land, East Greenland. This stratigraphic interval includes the Kap Stewart and the Neill Klinter Formations which have many features in common with some of the largest coeval hydrocarbon reservoir formations known in N.W. Europe (e.g. Statfjord fieid). The core of the project is a lithofacies analysis but ichnology, palynology, source-rock analyses and porosity/permeability analyses will be included where relevant. If possible, corresponding intervals from the Norwegian continental shelf will be included in the project. The aim of the project is: (1) to provide detailed and regional facies models for the two formations. Special stress wiIl be laid on the physical stratigraphic relations in order to ascertain if regional unconformities are present and what order of magnitude they may represent. (2) to establish a reservoir geological model which might help in the understanding of comparable reservoirs in the Norwegian-Greenland region. Particulal' attention will be paid to the geometry ofindividual sand bodies.

Karst-Controlled Reservoir Heterogeneity in Ellenburger Group Carbonates of West Texas

Petroleum production from restricted shelf carbonates of the Lower Ordovician Ellenburger Group is commonly considered to have been a result of a pervasive, relatively homogeneous tectonic fracture system within the reservoir rock. However, regional facies and diagenetic (paleokarst) studies of Ellenburger strata, based on cores and wireline logs, have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit. Ellenburger Group carbonates, which attain a thickness of over 1,700 ft (520 m), record sedimentation on a shallow-water restricted shelf occupying most of west Texas. Logging of over 10,000 ft (3,050 m) of core from 63 wells has allowed recognition of six facies assemblages: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone-grainstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod-intraclast packstone-grainstone. These facies assemblages record initial transgression and subsequent progradation and aggradation. Paleoslope was generally south and east from the Texas arch toward the Ouachita and Marathon orogenic belts. All facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, are characterized y very low intergranular and intercrystalline porosity. Porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand prior to transgression of Simpson Group siliciclastics. During this episode, a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolomites (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor. Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, Block 13, and several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized laterally extensive collapse structures, such as in the Shafter Lake reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model.

Petroleum Geology of Pennsylvanian and Lower Permian Strata, Tucumcari Basin, East-Central New Mexico: ABSTRACT

The Tucumcari basin of east-central New Mexico is a structural depression that existed as a depositional basin from Strawn (Middle Pennsylvanian) until late Wolfcampian (Early Permian) time. Depth to Precambrian ranges from 6500 ft to more than 9000 ft. High-angle faults form the north, east, and west edges of the basin. No major structural discontinuities separate the basin from a shallow shelf to the south. Faults cut Pennsylvanian and Wolfcampain strata but generally do not offset post-Wolfcampian strata. Faults and regional structure control facies and thickness patterns within the Pennsylvanian and Wolfcampian. Coarse arkosic sands in the Pennsylvanian and Wolfcampian are good reservoirs. Those sands were deposited in northern and western parts of the basin and were derived from highlands of Precambrian granitic rocks that formed the northern and western margins of the basin. High-energy limestones are possible reservoirs in the southern part of the basin. Porous Wolfcampian dolostones cover the Frio uplift on the east side of the basin. Pennsylvanian and Wolfcampian marine shales and micritic limestones are source rocks within the Tucumcari basin. Post-Wolfcampian strata are thermally immature. Two presently noncommercial pools of oil and gas, the Latigo Ranch pool and the T-4 Ranch pool, have been discoveredmore » in Strawn sands in the northern part of the Tucumcari basin. Oil generated in the upper Paleozoic has migrated vertically into the Triassic; two oil accumulations, the Santa Rosa tar sands and the Newkirk oil pool, are in Triassic sandstones and have combined reserves of 153 million bbls of oil.« less

Tectonic Influence on Sedimentation Patterns, Upper Ordovician of Eastern North America: ABSTRACT

The upper part of the Champlainian Series and all of the Cincinnatian Series (both parts of the Upper Ordovician Series on the newly published COSUNA charts) can be divided up into seven time slices; the late part of the Blackriverian Age, Rocklandian, Kirkfieldian, Shermanian, Edenian, Maysvillian Ages, and early part of Richmondian Age. Analysis of the rocks, using these time slices in eastern North America, shows five regional facies packages: (1) clean carbonates, (2) mixed carbonates and terrigenous clastics, (3) shale, (4) terrigenous clastics coarser than shale, and (5) terrigenous clastics prograded over carbonates. The latter is considered a facies because of this style of sedimentation is integral to the Upper Ordovician. Regional tectonic events related to plate collision along the eastern margin of North America had a direct influence on the sedimentation pattern of these facies packages. the extensive clean carbonate platform represented by upper Blackriverian rocks was replaced by wide-spread argillaceous carbonates during Rocklandian, Kirkfieldian, and Shermanian time. Also, by Shermanian time, a linear belt of shale deposition bisected the carbonate platform from the southwest to the northeast. South of this trend, carbonate sedimentation continued essentially without interruption in response to tectonic stability until the end of the more » Ordovician Period. To the north, the carbonate platform deepened and was later flooded by shale during Edenian and Maysvillian time. Starting during the Maysvillian and continuing into the Richmondian, upwarping and erosion of the Taconic highlands caused large-scale terrigenous clastic progradation over the northeastern part of the platform. « less

Mixing zone dolomitization of Devonian carbonates, Guangxi, South China

Summary Regional facies relationships indicate that dolomitization around the Lower-Middle Devonian boundary in Guangxi region is largely restricted to platform margin facies. In the Liujin area dolostones are represented by the Tangding and succeeding Yingtang Formations. The latter comprises alternations of laminated fine-grained dolostones (type A dolomite) and massive, more coarsely-crystalline dolostones (type B dolomite). Type B is richer in marine fossils and other allochems, and increases in importance upwards. The Formation is bounded above by a disconformity marked by a haematitic marl, interpreted as a subaerial erosion surface, and is succeeded by limestones lacking dolomite. Petrographic observations indicate that the two dolomite types represent the dolomitization of originally different sedimentary facies by a single dolomitization process. The dolomite is dominantly replacive, but also partly cementing. Variably negative δ 13 C and δ 18 O values (interpreted as original) and covariation of these parameters with Sr and Na points to dolomitization by mixed meteoric-marine fluids. Stratigraphic variation in geochemical data is inconsistent with a single episode of subaerial exposure and therefore points to repeated emergence. Dominantly replacive dolostones can form by the mixing-zone mechanism given sufficiently stable conditions.

Similarity of Grayburg/San Andres Dolomite Reservoirs--Eastern Edge of Central Basin Platform, Permian Basin: ABSTRACT

Porous dolomites of the Grayburg/San Andres Formations (Permian, Guadalupian) form major hydrocarbon reservoirs along the eastern edge of the Central Basin platform of the Permian basin, Texas. A comparison of the lithologic descriptions and depositional models for these reservoirs shows that although substantial local variation exists, the overall generalized vertical sequence and, particularly, the occurrence of porous shoal grainstones are strikingly similar. Data presented in this pilot study comes from the Gulf Oil 223-R J. T. McElroy Cons. well in McElroy field, the Gulf Oil 35 E.N. Snodgrass well in Waddell field, and the Gulf Oil 1359-56 C.A. Goldsmith et al well in Goldsmith field. Grayburg/San Andres reservoirs in all three fields occur mainly in the middle part of a shallowing-upward carbonate-shelf sequence that is 250-300 ft thick. The sequence consists of: (1) basal open-shelf, bioturbated dolowackestones/packstones; (2) middle shallow shelf fusulinid dolowackestones overlain by nonskeletal shoal dolopackstones/grainstones; and (3) upper variable intertidal/supratidal deposits that grade into the Queen Formation. The depositional sequence formed in a regional facies tract of shallow-water shelf and shoal environments with related tidal-flat and evaporitic (sabkha) sediments. The facies developed during easterly progradation across a deeper water open shelf. Porosity was formed by near-surface dolomitizationmore » and minor dissolution. Early and late evaporite plugging reduced porosity, so reservoir zones average 9% intercrystalline and moldic porosity with 10-md permeability.« less

Topographical and geological maps of Hall Land, North Greenland. Description of a computer- supported photogrammetrical research programme for production of new maps, and the Lower Palaeozoic and surficial geology

Topographical and geological map sheets covering the northern part of Hall Land (81-82°N) are presented – an area of about 3000 km2. The maps are the products of a research programme in which newly developed photogrammetric techniques have been used in the interpretation and compilation of the topography and the geology (both solid and surficial). The topographical map has been constructed with a minimum of geodetic ground control. The topographic contours have been calculated from a digital elevation model using computer programmes, and automatically plotted out. The geological map has been hand-drawn from 74 manuscript sheets compiled from aerial photograph models on second-order analog stereo-plotting instruments with computer facilities. The maps, the photogrammetric programme and the solid and surficial geology are described in seven chapters. The first two provide an introductory background that explains the motivation for the research, summarises the history of cartographic, geodetic and geologic work and provides a status of research at the start of the programme. The third chapter discusses the various aspects of the photogrammetric programme, instrumentation and the on-line computer facilities utilised, and is followed by a chapter dealing with compilation method, map presentation and assessment of cartographic accuracy compared to previous maps and modern geodetic ground data. The next chapter describes the topography and geomorphology and relates the three main physiographic provinces to the solid and surficial geology. The penultimate chapter outlines the stratigraphy and structure of the Upper Ordovician-Silurian (Llandovery-Pridoli) section through the E-W trending Franklinian basin. In Ordovician-earliest Silurian time, the map area was part of the carbonate platform; in the Llandovery a major shift southwards of the deep-water basin occurred. The Silurian succession displays a regional facies change from platform carbonates in the south, through a major reef belt on the shelf and upper slope to, in the north, clastic turbidites of the lower slope and trough. Facies transitions and interdigitation of shelf-slope-trough lithologies are complex. The northern part of the map exposes the autochthonous margin of the mid-Palaeozoic North Greenland fold belt characterised by E-W folds. The regional structure is an asymmetric synclinorium; a decollement zone probably occurs in the shale sequence that overlies the Lower Silurian carbonate platform. The final chapter describes eight groups of Quaternary deposits and features: moraine, fluviatile-glaciofluvial, marine, lacustrine, colluvial, solifluction, aeolian and periglacial. Hall Land was formerly entirely ice covered, and deposits of several ice advances are preserved; six major marginal moraine systems are defined. Marine deposits are prominent and terrace levels and raised shorelines are well preserved; the Holocene marine limit is at least 125 m above present sea level. Major events are placed within a Pleistocene-Holocene chronostratigraphic framework. Comments on place names are given in an appendix.

Palaeomagnetism and continental collision in the Alpine Belt and the formation of late-tectonic extensional basins

Summary New palaeomagnetic data are consistent with the hypothesis that Adria moved as a promontory of the African Plate during the Mesozoic. This hypothesis is supported by regional facies analysis and by geophysical data from the eastern Mediterranean basins which indicate a continental lithosphere. However, the new data cast doubt upon the concept of a rigid, autochthonous African promontory remaining coherent during Alpine deformation. It is apparent that small-scale relative rotations have occurred during the Tertiary in areas which were previously considered authochtonous. Late-tectonic extensional basins, such as the Tyrrhenian Sea, are superimposed on the continent-continent collision suture and their growth is contemporaneous with thrusting in surrounding arcs. The very rapid evolution of the collision suture into a region of extension is due to a rapid lithospheric thinning process, such as catastrophic delamination of part of the mantle lithosphere. Such delamination would be most likely to occur very soon after a collision involving fast lithospheric thickening.

Hierarchy of facies of pyroclastic flow deposits generated by Laacher See–type eruptions

The upper Quaternary pyroclastic flow deposits of Laacher See volcano show compositional and structural facies variations on four different scales: (1) eruptive units of pyroclastic flows, composed of many flow units; (2) depositional cycles of as many as five flow units; flow units containing (3) regional intraflow-unit facies; and (4) local intraflow-unit subfacies. These facies can be explained by successively overlapping processes beginning in the magma column and ending with final deposition. The pyroclastic flow deposits thus reflect major aspects of the eruptive history of Laacher See volcano: (a) drastic changes in eruptive mechanism due to increasing access of water to the magma chamber and (b) change in chemical composition and crystal and gas content as evacuation of a compositionally zoned magma column progressed. The four scales of facies result from four successive sets of processes: (1) differentiation in the magma column and external factors governing the mechanism of eruption; (2) temporal variations of factors inducing eruption column collapse; (3) physical conditions in the eruption column and the way in which its collapse proceeds; and (4) interplay of flow-inherent and morphology-induced transport mechanics.

Prograde metamorphism of metapelites, garnet-biotite thermometry and prograde changes of biotite chemistry in high-grade rocks of West Uusimaa, southwest Finland

The metamorphic grade in west Uusimaa, SW Finland, increases from low- to intermediate-amphibolite facies in the west (Island of Kemiö) up to granulite facies in the middle and eastern parts. Mineral assemblages in metapelitic rocks indicate metamorphic conditions of 700–825°C, 3–5 kbar for the high-amphibolite and granulite facies region, which is in agreement with independent estimates. These metamorphic conditions are used to evaluate garnet-biotite thermometry. Zoning in garnet, Fe enrichment and Mg depletion towards rims bordered by biotite or cordierite, and a distinct Fe depletion/Mg enrichment of biotites in contact with garnet are caused by FeMg exchange of these minerals during retrogression (550°C). The general prograde metamorphic trend from medium- to high-grade in West Uusimaa is reflected by a systematic change in the composition of biotites. Thermometry and biotite chemistry are used to indicate that the granulite facies metamorphism in West Uusimaa is probably caused by a thermal event.

Precambrian gneisses and intrusive anorthosite of Smithson Bjerge, Thule district, North-West Greenland

Smithson Bjerge is part of the Precambrian crystalline complex of North-West Greenland. The area comprises the Qaqujârssuaq anorthosite, the Heilprin Gletscher complex, paragneisses, multiphase orthogneisses and diverse minor granitic and basic intrusions. The Heilprin Gletscher complex comprises intimately associated granite, ferrogabbro and ferrodiorite. The Heilprin Gletscher complex and the Qaqujârssuaq anorthosite were intruded into the paragneisses and the multiphase orthogneisses, and may form an anorthosite-granite-ferrodiorite association. All lithological groups apart from the minor intrusions have been affected by large-scale overtumed folding and regional medium-pressure granulite facies metamorphism.

Carbonate diagenetic facies in the Upper Pennsylvanian Dennis Formation in Iowa, Missouri, and Kansas

ABSTRACT The carbonate members of the Dennis Formation can be divided into five regional diagenetic facies that are defined by differing preservation of originally unstable carbonate grains. Such grains contain no relict internal structure in the uppermost facies (A); they are better preserved in middle facies (B and C); and they are well preserved in the lowest facies (D). A fifth facies (E) contains no originally unstable grains and is characterized by a microcrystalline dolomite fabric. The five facies exhibit differing patterns of compaction and dolomitic void filling. The textures and interpreted diagenetic environments of the diagenetic facies were caused by progressive interaction of initially unsaturated meteoric water with the marine sediments during a general regression. Transfer of CaCO3 in that interaction and the resulting distribution of cements governed later patterns of pressure solution and void filling by ferroan dolomite. Paleohydrologic controls caused irregularities in the distribution of diagenetic facies. Paleotopographic and depositional trends derived from diagenetic evidence agree with previous interpretations based on depositional evidence.

Thermal contraction and flexure of intracratonal basins: a three-dimensional study of the Michigan basin

Cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract. Accumulation of sediments in the resulting depression forms a sedimentary basin. Studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometres suggests the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere. Two- and three-dimensional models for flexure of the lithosphere owing to thermal contraction are applied to the Michigan basin. 2-D flexural models are compatible with previously estimated rheological parameters. For an elastic lithosphere, an effective flexural rigidity of about 5 × 1029 dyne cm can explain observed gravity anomalies and structural contours across the Michigan basin. A viscoelastic lithosphere requires an effective flexural rigidity of about 1032 dyne cm for a viscosity of 1025 poise. For a 3-D elastic or viscoelastic lithosphere, effective flexural rigidity must be approximately an order of magnitude smaller: 2 × 1028 and 1031 dyne cm, respectively. Larger values leave an excessive remaining load at the centre of the basin, which is not indicated by observed gravity anomalies. The decrease in effective flexural rigidity is attributed to greater difficulty in equi-dimensional bending of a 3-D plate. The magnitude and spatial distribution of the computed thermal contraction load are consistent with the presumed mechanism. As the thermal anomaly of the initial heating event was present mainly at great depths in the lithosphere, the lack of geological evidence for an initial heating event prior to subsidence is not a fatal objection to the thermal contraction mechanism of subsidence. A driving load, presumably from subsurface processes including crustal stretching and emplacement of dense rocks into the crust, is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled. Without this driving load, the thermal expansion would produce uplift, but the surface would subside only back to sea-level after the lithosphere cooled. Theoretical gravity results indicate that the driving load is centred at a depth of approximately 15 km. Deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply. Spatial variations in sediment load, caused by regional facies changes, produce migration of the centre of maximum deposition. Water and basement depths are determined for sequential time intervals during basin development. The predicted depositional environments are consistent with lithofacies maps of the Middle Devonian.

Regional Facies Distribution and Tectonic Evolution of Appalachian and Ouachita Thrust Belts: ABSTRACT

A series of 12 tectonic and lithofacies maps representing critical periods of the evolution of the Appalachian-Ouachita orogen were compiled from published sources and interpretation of seismic and subsurface data. The distribution of sediments supports the concept of multiple deformation that resulted from the collision and accretion of small plates or irregular margins of larger plates with North America. During the Eocambrian and Early Cambrian, a series of rifts developed within the craton subparallel to the continental margin of the Iapetus Ocean. Ouachita sediments were deposited in this rift zone along the southern margin of the craton. However, the rift zone did not persist in the Appalachians, and the sediments in that belt were deposited along the continental margin. In the Middle Ordovician, the extensional regime continued in the Ouachita belt while compression associated with plate collision began in the Appalachians. The northwest-southeast-trending boundary between these areas persisted throughout the evolution of the orogens. The sedimentary records indicate that the initial compressional deformation in the Ouachita belt began during the Late Mississippian, and the final phase of deformation in the Appalachians was initiated slightly later, during the Early Pennsylvanian. Structural features associated with thrusting basin sediments over foreland areas were controlled to a great extent by the presence or absence of buttresses. The Ouachita Mountains area provides the best illustration of contrasting structural styles along the thrust belt. Elsewhere along the thrust belts the evidence is either covered by younger sediments or altered by a complex tectonic history. End_of_Article - Last_Page 507------------

Alteration of Magnetite and Ilmenite in Upper Jurassic Morrison Formation, San Juan Basin, New Mexico: Relationship to Facies and Primary Uranium Mineralization: ABSTRACT

Petrographic studies of outcrop and drill core samples in the Westwater Canyon Member of the Morrison Formation along the southern and western margins of the San Juan basin reveal a close spatial relationship among altered (iron-leached) detrital magnetite and ilmenite (FeTi oxides), depositional facies in the overlying Brushy Basin Member, and distribution of primary uranium deposits. Iron leaching of FeTi oxides resulted from passage of solutions containing soluble organic material; concentrations of this organic material are the sites of the primary uranium orebodies. Along the southern and western parts of the basin, FeTi oxides typically have been leached in the upper Westwater Member, but are unaltered in the lower Westwater; however, locally, leaching occurred thro ghout the Westwater. This zone of leaching systematically thins northward to zero, where unleached FeTi oxides occur throughout the Westwater. Regional patterns of alteration of FeTi oxides correspond to regional facies distribution in the overlying Brushy Basin Member. Extensive FeTi oxide leaching characterizes the Westwater beneath the smectite-rich mud-flat facies of the Brushy Basin, whereas negligible leaching characterizes the Westwater beneath the zeolite-rich playa facies of the Brushy Basin. This correspondence between facies and alteration patterns suggests that solutions responsible for solubilization of organic material, which in turn leached FeTi oxides in the Westwater, originated from the mud-flat facies of the Brushy Basin. Organic material that precipitated from these solutions concentrated uranium to form primary uranium orebodies; therefore, distribution of the Brushy Basin mud-flat facies may define, and restrict, distribution of primary orebodies in the Westwater. End_of_Article - Last_Page 937------------