Regional Variations In Thickness Research Articles

Architecture and Evolution of the Southern Chotts‐Jeffara Basin, Tunisia

AbstractSouthern Tunisia is known to be less deformed and simpler than its neighboring Atlassic domain to the north. This area is complex and basin evolution in the Southern Chotts‐Jeffara (SCJ) basin is debated. In this paper we combined surface and subsurface data with low temperature thermochronology (LTT) to reinvestigate the tectono‐sedimentary evolution of the SCJ basin from Permian to Jurassic. We reconstruct the present‐day architecture of the SCJ basin along two regional sections. In these sections, we focused mainly on regional thickness variations and on internal reflections interpreted from seismic data. We observe three structural elements: (a) A Paleozoic culmination, oriented E‐W, capped by Mid‐Upper Triassic deposits; (b) the Tebaga of Medenine (ToM), a culmination also oriented E‐W but located ∼50 km north of the Paleozoic culmination; and (c) A Triassic culmination in the eastern part of the area, oriented NW‐SE. We note the absence of major normal faults along the sections. The LTT data we present are the first published in this area and allow to reconstruct the timing and magnitude of vertical movements. These data prove: (a) exhumation at ∼230 Ma of the Permian and Lower Triassic units associated with the onset of the ToM removing locally about 900 m of pre‐Cretaceous sediments; and (b) the development of the Triassic culmination ∼180 Ma removing 2000 m of pre‐Cretaceous sediments in the Jebel Rehach. This study demonstrates that vertical movements in the SCJ basin are controlled by long‐wavelength processes developed essentially in shortening regimes.

Preservation of organic carbon in the Cretaceous Hue Shale on the North Slope of Alaska: Insights from pyrite morphology

The Cretaceous Hue Shale is a condensed mudstone section of the Brookian sequence with type II and type III kerogen, and it is one of the major source rocks that fill the world-class conventional oil fields on the North Slope, Alaska. The bottom section of the Hue Shale is referred to as Gamma Ray Zone (GRZ) or Highly Radioactive Zone (HRZ) and believed to be the most organic-rich section. The thermal maturity increases from immature to over oil window towards the southwest. Although, the research on the Hue Shale as an unconventional shale reservoir is still at an early stage, where exploration activities are very limited. The Hue Shale thickens to the east in general. However, regional variations of thickness and more importantly total organic carbon (TOC) have been reported.In this research, a suite of nineteen core samples of the Hue Shale are selected. These cores are collected from six wells covering a wide spectrum of thermal maturity with four wells in the National Petroleum Reserve Alaska and two wells in the state land. A combined geochemical and petrographic methodology is utilized to address the regional redox conditions and the preservation of organic carbon. TOC, Tmax, hydrogen index, and oxygen index are acquired through pyrolysis to characterize the abundance, thermal maturity, the origin, and the evolution of organic matter. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) is used to investigate the mineralogy and diagenesis of the Hue Shale. Pyrite crystal forms, and pyrite framboid size distributions are used as an indicator of the redox conditions during deposition. The results show substantial variance in oxygen levels in the water column. High abundance and small sizes of pyrite framboids, which indicates a depleted oxygen environment, show better preservation potential of organic matter.

Palynostratigraphy and paleoecology of a Lower Cretaceous (upper Barremian–lower Aptian) succession in the Shushan Basin, northwestern Egypt: inferences on intra- and inter-basinal correlations

Recent national and international demand for additional resources of the fossil fuel resulted in extensive hydrocarbon exploration activities in Egypt to further explore possible petroliferous rock units that were conventionally deemed as barren or having non- commercial reserves such as the Alam El Bueib Formation. This increased the stress to enrich the lithological correlation of this formation across the north Western Desert to better understand its regional lithological characteristics and thus its potential regional reservoir/source rock quality. Palynostratigraphic analysis of Lower Cretaceous successions in the Tut-1X and Amoun-1X wells in the Shushan Basin, north Western Desert enabled the recognition of the upper Alam El Bueib Formation in both wells, in contrast to a previous study, which suggested non-deposition of this formation in the Amoun-1X well. This highlights the important role of the palynology as a key stratigraphic tool, which helped effectively in recognising this problematic monotone clastic formation, where other marine microfossils are lacking. This also shows that recognition of this formation based on lithological criteria only was impractical and misleading. Marker terrestrial and marine palynomorph taxa recovered from the upper Alam El Bueib Formation enabled dating it as late Barremian– early Aptian and were used to identify two palynological biozones that can be traced out regionally across the Shushan, Matruh, and other nearby basins. These biozones are the early Aptian Murospora florida – Afropollis operculatus – Pseudoceratium securigerum Assemblage Zone and the late Barremian Stellatopollis barghoornii – Pseudoceratium anaphrissum– Cyclonephelium vannophorum Assemblage Zone. An undifferentiated interval was recorded just below the upper Barremian strata in the Tut-1X well due to very low palynomorph recovery and absence of marker forms. Composition of terrestrial and marine palynomorph assemblages suggests the upper Alam El Bueib Formation was deposited in fluviodeltaic to proximal inner shelf settings. The Alam El Bueib Formation shows notable regional variations in thickness and considerable lateral and verti- cal changes in lithology due to local tectonics and relative difference in its depositional settings in the Shushan and Matruh basins. Regionally, the Alam El Bueib Formation shows dual reservoir/source rock characteristics, where thick and poorly fossiliferous sandstone units act as reservoir rocks, while moderately fossiliferous finer clastics and carbonates already acted as intraformational source rock intervals in some parts of the Shushan and Matruh basins.

Corticosteroids and Regional Variations in Thickness of the Human Cerebral Cortex across the Lifespan.

Exposures to life stressors accumulate across the lifespan, with possible impact on brain health. Little is known, however, about the mechanisms mediating age-related changes in brain structure. We use a lifespan sample of participants (n = 21 251; 4–97 years) to investigate the relationship between the thickness of cerebral cortex and the expression of the glucocorticoid- and the mineralocorticoid-receptor genes (NR3C1 and NR3C2, respectively), obtained from the Allen Human Brain Atlas. In all participants, cortical thickness correlated negatively with the expression of both NR3C1 and NR3C2 across 34 cortical regions. The magnitude of this correlation varied across the lifespan. From childhood through early adulthood, the profile similarity (between NR3C1/NR3C2 expression and thickness) increased with age. Conversely, both profile similarities decreased with age in late life. These variations do not reflect age-related changes in NR3C1 and NR3C2 expression, as observed in 5 databases of gene expression in the human cerebral cortex (502 donors). Based on the co-expression of NR3C1 (and NR3C2) with genes specific to neural cell types, we determine the potential involvement of microglia, astrocytes, and CA1 pyramidal cells in mediating the relationship between corticosteroid exposure and cortical thickness. Therefore, corticosteroids may influence brain structure to a variable degree throughout life.

Echocardiographic pearl a rare complication of infective endocarditis one of the rarest complication of infective endocarditis being diagnosed by echocardiography

Our patient is a 41 years old male, born and living in Cairo, working as a constructor worker and has 3 children the older of which is 13 years old. He was admitted to the internal medicine department by fever and shortness of breath for about 1 week associated with weakness of his right upper limb for 12 h before his presentation which made him sought medical advice. He was not known to be hypertensive nor diabetic.He was a heavy Cigarette and Shesha smoker for about 17 years .He denied history of any substance abuse. He has no family history of any cardiac disease O/E: The patient appeared pale, toxic, orthopenic, a little confused however he was oriented to time, place and persons.There was mild weakness of his right upper limb with intact sensation. BP: 100/70 bilaterally, HR: 110, regular, of average volume, peripherally felt, Temp: 38.8 °C, RR: 20/min. Bilateral fine basal rales on deep inspiration, Normal abdominal examination. Cardiac examination: The cardiac impulse was hyperdynamic at the 5th intercostal space just outside the mid-clavicular line with no palpable thrill. Auscultation revealed S3 gallop apically with grade III–IV pan systolic murmur radiating to the anterior axillary line. Hgb: 11.2,WBC’s: 13,000, Platelet count: 270,000. Total billirubin: 1.1, BUN: 17, Creat: 1.4, Na: 135, K: 3.9, SGOT: 45, SGPT: 37. Left atrial dissection (LAD) is a rare complication and the literature reveals only a small number of cases. LAD is by Gallego et al. as a gap from the mitral or tricuspid annular area to interatrial septum or left atrial wall, creating a new chamber with or without communications into the true left or right atrium. The most common etiology of LAD is mitral valve surgery. Debridement of much calcified valves annulus, improper suturing of the annulus to the prosthetic cuff, excessive traction on sutures in the posterior annulus, and the hemodynamic influence of the paraprosthetic leak extended the dissection into the left atrial wall, developing a false cavity. Also left atrial thrombectomy can be associated with injury to the left atrial endocardium as a mechanism of primary tear.A rare case of left atrial dissection as a consequence of infectious endocarditis was reported. They present a patient with infectious endocarditis with involvement of mitral and aortic valves; in whom the trans-esophageal echocardiography was able to visualize the left atrial dissection. The LA has a venous component that receives the PVs, a fingerlike atrial appendage, and shares the septum with the right atrium. The major part of the atrium, including the septal component, is relatively smooth-walled whereas the appendage is rough with pectinate muscles. The smoothest parts are the superior and posterior walls that make up the pulmonary venous component, and the vestibule. Seemingly uniform, the walls are composed of one to three or more overlapping layers of differently aligned myocardial fibers, with marked regional variations in thickness. Why the posterior wall of the left atrium:A sagittal section through the left atrium of a cadaver shows the proximity of the esophagus to the posterior wall of the left atrium The wall is particularly thin at the level of the superior pulmonary veins. Clinical presentation may be the appearance of a new systolic murmur, associated with or without symptoms of heart failure and low-output manifestations, hours to days after the operation but there were patients in whom clinical onset occurs years after surgery. Rarely, LAD can be an incidental finding on TEE in an asymptomatic patient. LA dissection typically appears as a hypoechoic space from the mitral/tricuspid origin extending along the interatrial septum or LA wall. M-mode is excellent at distinguishing subtle movement of the intima or the endocardium in relation to the cardiac cycle. Similar to what is seen in aortic dissections, the false cavity is compressed during systole as the LA is being filled. Other entities that should be considered when an LA mass is visualized are: Thrombi most common left atrial myxoma, cysts, coronary aneurysms. Pericardial blood impinging on the LA wall may mimic these findings. Color flow Doppler can be used to examine the endocardium for a tear and point of communication with the chamber. Pulsed wave Doppler can also be used to identify flow across a tear. TEE is the diagnostic modality of choice for LAD. No definitive criteria exist to help guide management of LAD. Prompt surgical repair is usually required because of coexistent significant mitral regurgitation, intra-cardiac shunt, mycotic aneurysm, pseudo aneurysm or fistulous communication. However, in the absence of these findings, surgery may not always be necessary and occasionally successful repair has been performed years after diagnosis.

Controls on the deposition and preservation of the Cretaceous Mowry Shale and Frontier Formation and equivalents, Rocky Mountain region, Colorado, Utah, and Wyoming

Regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin. Preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages. The preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present. This is the case for the Cretaceous Mowry Shale and Frontier Formation and equivalent strata in the Rocky Mountain region of Colorado, Utah, and Wyoming. Biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level, which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography. The most highly subsiding parts of the Rocky Mountain foreland basin, near the fold and thrust belt to the west, typically contain a low number of coarse-grained sandstone channels but limited sandstone reservoirs. However, where subsidence is greater than sediment supply, the foredeep contains stacked deltaic sandstones, coal, and preserved transgressive marine shales in mainly conformable successions. The main exploration play in this area is currently coalbed gas, but the enhanced coal thickness combined with a Mowry marine shale source rock indicates that a low-permeability, basin-centered play may exist somewhere along strike in a deep part of the basin. In the slower subsiding parts of the foreland basin, marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities, providing conditions for the development of stratigraphic and combination traps, especially in areas of repeated reactivation. Areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals. Paleogeographic reconstructions are used to show exploration fairways of the different play types present in the Laramide-modified, Cretaceous foreland basin. Existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons, which results from the partitioning of facies within the different parts of the foreland basin.

Thin- and thick-skinned salt tectonics in the Netherlands; a quantitative approach

AbstractThe Zechstein salt in the Dutch part of the North Sea Basin played a key role in the generation of successful petroleum plays. This is not only because of its sealing capacity, but also because the salt occurs in structures that provide lateral and vertical traps. The structural styles of areas with thick salt and those with none- or thin salt are completely different during phases of extensional or compressional tectonics. This indicates that, indirectly, the depositional thickness of the main Zechstein salt is essential in regulating the loci of the Dutch petroleum systems. In this paper we aim at quantifying current ideas on the relationship between 1) depositional salt thicknesses; 2) structural style of the main structural elements identified in the Dutch subsurface; 3) timing of deformation; and 4) thickness of the overburden. By finalisation of TNO's subsurface mapping program (see Kombrink et al., this issue), several data products are available that allow evaluation of these relationships. The depositional thickness of the salt was estimated using iterative smoothing of the present day thickness, the results of which account both for regional thickness variations and volume preservation (99%). Fault-distribution analysis shows that faults are only able to penetrate salt with a depositional thickness of <300 m, a transition that demarcates the division between thin- and thick-skinned salt tectonics. In the southern offshore where the salt is thin or absent, the overburden shows the same fault pattern throughout the stratigraphic sequence. In the northern realm, where salt is thicker than 300 m, the salt layer acted as decollement and sub- and supra salt strain are dissimilar. A strong genetic and temporal relationship exists between periods of regional tectonism, halokinetic intensity and thickness distribution of the Zechstein overburden. This relationship is further proven by burial history analysis across two selected profiles in the northern offshore. The analysis focuses on the vertical distribution of the salt by taking into account the depositional and erosional history of the salt overburden, without a-priori defined periods of salt flow. The results corroborate the notion that platforms and highs experienced less extension during the major phases of Jurassic rifting and further suggest that the absence of a thick Jurassic overburden precludes major salt flow during this tectonic phase. Main salt flow was triggered during the Sub-Hercynian and later phases of compression resulting in salt pillow geometries. In the basinal areas, where the Jurassic succession is thickest, salt diapirs and walls formed that are almost exclusively linked to major subsalt faults. Main salt flow occurred during Late Kimmerian rifting, whereas some minor structuration occurred during Sub-Hercynian inversion.

Neogene sediment thickness and Miocene basin-floor fan systems of the Celebes Sea

In the Celebes Sea Basin the Middle Miocene turbidites were correlated from ODP site 767 throughout the studied area. Differences in their regional thickness variations and distribution indicate two source areas. The Middle Miocene turbidite–fan complexes of the central and southern Celebes Sea Basin are controlled by the paleo-Tarakan delta system, the tectonic events and the basin floor morphology, respectively. The main source area for the time correlative turbidites along the southern Sulu Arc is assumed to be Mindanao.The correlation of the Middle Miocene to Pleistocene sequences exhibit tentative ages for the development of the accretionary wedges along the Cotabato Trench and along the North Sulawesi subduction. A post-Middle Miocene to pre-Pliocene age is inferred for the Cotabato wedge and a Plio-Pleistocene age is assumed for the North Sulawesi wedge.

Regolith Layer Thickness Mapping of the Moon by Radar and Optical Data

Mapping of regolith layer thickness for the lunar nearside is carried out with Arecibo radar data at 70 cm wavelength (T. W. Thompson 1987, Earth Moon Planets 37, 59–70) and distributions of iron and titanium content derived from Earth-based optical data (Yu. Shkuratov et al. 1999, Icarus 137, 222–234). For the mapping a new simple model of radio wave multiple scattering in the regolith layer was used. A comparison of this map with independent estimates of regolith layer thickness for landing sites and other areas of the lunar surface showed a good correlation. It was found that regional variations of thickness are almost the same for maria and highlands, though the average thicknesses are different (5 and 12 m, respectively). A relatively thin regolith layer (4 m) covers a portion of Mare Serenitatis, Mare Tranquillitatis, and Mare Humorum, while the thickest regolith layer occurs in Mare Nectaris (9 m). A thin regolith layer is a characteristic of the cryptomare Schiller-Schickard. Very small regolith thickness corresponds to the crater floors. In this case, the model cannot yield reliable estimates of thickness; it predicts only a characteristic distance between rock inclusions buried in regolith. A thick highland regolith occurs at the southeast portion of the lunar disk and the highland to the north of Mare Imbrium and Sinus Iridium. For the lunar nearside, the regolith thickness generally correlates with the surface age: the greater the age, the thicker the regolith. The results are consistent with a higher rate of regolith growth for times earlier than 3.5 byr ago, when meteorite flux was much higher.

Cyclic Carbonate Sedimentation in the Upper Triassic Dachstein Limestone, Austria: The Role of Patterns of Sediment Supply and Tectonics in a Platform-Reef-Basin System

ABSTRACT Factors that controlled the deposition of (i) peritidal Lofer cycles on a carbonate platform, and (ii) platform margin (reef-slope) deposits have been deduced at localities in the Austrian Alps (the Steinernes Meer and Hochkonig Massif). These locations are part of an entire Late Triassic platform-reef-basin system that is preserved with most original stratigraphic relationships intact. Platform Lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolomitic mudstones to a supratidal weathering horizon (soil). Lofer cycles show random, non-hierarchical stacking patterns, limited lateral continuity, varied progradation directions, complete shoaling (98% of cycles), and very low stratigraphic completeness (only 1-20%). Exponential frequency distributions of cycle thickness suggest random, aperiodic cycle deposition, rather than regular deposition in response to regular eustatic sea-level oscillations. Sediments in the adjacent reef complex record storms and the lateral migration of sand shoals and stromatolite-capped banks, not sea-level fluctuations and intermittent subaerial exposure. On the basis of these observations, in contrast to many previous interpretations, Lofer cycles are interpreted to be mostly autocycles formed within a tidal-flat island system by lateral migration of wide, low, emergent banks separated by shallow subtidal areas. Preservation potential of individual cycles is thought to have been low; reworking was almost certainly very important in this system. Extensional regional tectonics in the Late Triassic exerted a long-term control over the development of the tidal-flat island system on the platform top, and appears to have left a overprint in Lofer cycle successions. Differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations (1200-3000 m) in the Norian/Rhaetian platform carbonates of the Northern Calcareous Alps. Two important deepening events in the Steinernes Meer section almost caused platform drowning, and correlate with a lowermost Rhaetian transgression in the Western Tethys. Many other events may have gone unrecorded on the platform. Within a 716 m thick measured succession of Lofer cycles, intervals of enhanced paleokarst development nd stacked intertidal to supratidal beds are present with 20-75 m vertical spacing. These platform units are interpreted to represent prolonged periods in which greater areas of the platform were occupied by intertidal to supratidal sediments. These units correlate with thick units of debris flows on the reef slope. What may be tectonic highstands (the result of a vertical movement) are recorded as packstone to grainstone deposition on the reef slope. A holistic model driven by aperiodic fault-controlled downdropping, resulting in switching loci of sediment export patterns from a continuously operative subtidal carbonate factory (the reef complex) best explains vertical facies patterns in the platform and reef-slope successions. It is not a static sea-level model, although thi d-, fourth-, and fifth-order eustasy is not required. The model could explain the sporadic occurrence of shallowing-up cycles in the adjacent Kossen Basin. Late Triassic eustatic sea-level fluctuations were ineffective in controlling sedimentation as a result of the processes described above.

An introduction to the crystalline basement of Africa

Abstract The crystalline basement of Africa comprises three major suites of rocks: the granite-gneiss-greenstone association of Archaean cratonic nuclei; strongly deformed metamorphic suites in mobile belts, mainly of Proterozoic age; and anorogenic intrusions which include Phanerozoic intrusive magmatic rocks related to rifting. This heterogeneous basement is extensively concealed beneath a variable thickness of diverse, unmetamorphosed sedimentary and extrusive volcanic rocks and weathering products. The Archaean cratons have similar geological histories, which generally culminate with anorogenic potassic granites emplaced at about 2500 Ma. The history of the Kaapval Craton is prematurely curtailed at about 3050 Ma. The Limpopo Mobile Belt is uniquely Archaean: its development is linked to differential movement between the oldest cratonic elements of southern Africa. Elsewhere in Africa, the oldest Archaean record suggests an absence of thick continental crust: early greenstone belts formed above mantle plumes on mobile, thin crust. The development of the Proterozoic mobile belts is related both to collision of older cratons (Wilson cycle orogenesis) and to ensialic disruption of single cratons. Early extensional orogenic phases produced both active (new oceanic crust development) and failed rifts. The earliest Proterozoic mobile belts cut across Archaean cratonic domains and later belts may be superposed. Older fractures are commonly reactivated by new stress systems. Much of the Phanerozoic anorogenic magmatism is related to Mesozoic and Cenozoic continental fragmentation and rifting, possibly associated with hot spots (White & McKenzie 1989). Notwithstanding the relative antiquity of most of the basement, it is the effects of high-level brittle fracturing and weathering which largely control groundwater storage. The fracturing varies in age from Archaean, within the greenstone belts, to Phanerozoic, for faults related to movement of the African Plate, with ongoing Quaternary faulting in the major tensional rifts such as the East African Rift System. The thickest weathering profiles occur above the oldest erosion surfaces; regional thickness variations are controlled by past and present climatic differences.

On the factors which contribute to thinning of the villous membrane in human placentae at high altitude. I. Thinning and regional variation in thickness of trophoblast

Sections of human term placentae delivered at low and high altitude to indigenous and nonindigenous populations in Bolivia were analysed by stereological methods. Estimates were obtained of the arithmetic and harmonic mean thicknesses of villous membrane and of trophoblast. These values were then used to derive indices of thickness variation. No significant ethnic, sex or interaction effects were found. At high altitude, the villous membrane was significantly more variable in thickness owing to a lower harmonic mean but the same arithmetic mean thickness. Alterations in the arithmetic and harmonic mean thicknesses of the trophoblast contributed to the thinning of the villous membrane. The trophoblast was thinner and more irregular in thickness at high altitude. These findings are discussed in the context of placental diffusing capacity and the possible mechanisms for formation of vasculosyncytial membranes.

Depositional Patterns and Diagenetic History of Trenton-Black River Formations in Michigan Basin: ABSTRACT

The Trenton-Black River formations (Middle Ordovician) exhibit depositional facies and regional thickness variations indicative of normal marine, carbonate platform accumulation adjacent to a deeper water, basinal environment. Thickening of both formations occurs in the southeast part of the Michigan basin, possibly in response to basement movement accompanied by a shift in the basin depocenter. Early mixed-water diagenesis introduced the first generation of replacive dolomite, which varied in minor element composition and abundance depending on proximity to structural highlands southwest and southeast of the Michigan basin. A ferroan cap dolomite is interpreted to be partly representative of this early dolomite stage. Later stage replacive dolomite and dolomite cements are commonly associated with large fracture systems active during the Paleozoic. Although ferroan dolomitization is common throughout the Trenton and Black River formations, it is most extensive within these fracture zones, which also host significant oil and gas deposits in several locations. In addition, several accessory minerals are associated with late-stage dolomite cements, including pyrite, calcite, anhydrite, barite, celestite, sphalerite, and fluorite. The trade occurrence of these minerals in association with late-stage ferroan dolomite cements indicates that the Trenton-Black River formations in southern Michigan have undergone hydrothermal alteration analogous to a typical Mississippianmore » Valley-type deposit. Evidence based on the stratigraphic distribution of this mineralization indicates that the lower Paleozoic and Precambrian basement rocks were important sources for iron, sulfur, and other trace metals deposited in the Trenton-Black River during the late Paleozoic.« less

Structural Influence on Lower and Middle Cretaceous Sedimentation, Northern Great Plains: ABSTRACT

Sedimentation during the Lower and middle Cretaceous in the northern Great Plains (eastern Montana, North Dakota, South Dakota, and northeastern Wyoming) was influenced by recurrent movement of basement fault blocks. Regional thickness variations of four time-stratigraphic intervals are linear and have dominant orientations of northeast and northwest. Regional distribution of porous and permeable sandstone in the Muddy-Newcastle formation and equivalents (Albian), and in the Inyan Kara Group and equivalents (Aptian-Albian) are also linear and have dominant orientations of northeast and northwest. Similar thickness variations and directions of Lower Cretaceous units on the southwest flank of the Black Hills have been described in detail by Weimer and others in 1983. A depositional model, incorporating structural and sea level changes, shows that marine and nonmarine units thicken in structurally low areas and thin over structurally high areas. A tectonic model to account for the linear orientations, illustrates the effects of simple and pure shear stress systems. The stress systems were formed from the recurrent movement of basement faults possibly caused by movement of the North American plate during the Cretaceous. Major surface expressed folds and faults and surface lineaments have dominant orientations of northeast and northwest. These features either overlap or are parallel to linear thickness trends of Cretaceous rocks. Both surface and subsurface features are thought to have resulted from common basement fault block movement. Sedimentation and tectonic models can be used to predict vertical and lateral distribution of marine and nonmarine rocks. When integrated with these models, orientation of fractures and routes of fluid migration can be predicted by paleostructure analyses. End_of_Article - Last_Page 1329------------

Early Cretaceous Sedimentation and Tectonics in Southeastern Arizona: ABSTRACT

Sedimentary strata of Aptian to Albian age are widespread in southeastern Arizona, southwestern New Mexico, and northern Mexico. These rocks comprise a transgressive/regressive marine/nonmarine basinal sequence, locally over 3,000 m thick, of sandstone, shale, limestone, and conglomerate mapped as the Bisbee Group. In general, the nonmarine lower third of the sequence is composed of coarse clastic alluvial fan and basin-fill deposits of local origin and rests unconformably on rocks of Jurassic to Precambrian age. These syntectonic sediments were deposited during regional southwest-northeast extension where local fault-block uplifts bounded by northwest-trending normal faults created a basin-and-range paleogeography with isolated clastic-filled basins and mountain ranges r mmed by alluvial fans. The marine and marginal-marine shales and limestones of the middle third are restricted to the southeastern part of the region and represent the northwestern end of a shallow marine sea (Bisbee Sea) which advanced northwestward across northeast Mexico from the Jurassic rift basins of the Gulf of Mexico. Shallow-water platform carbonates were deposited over a wide area, with coral-algal-rudist patch reefs localized over paleo-structural highs. The upper part of the Bisbee Group is the regressive facies of this sea and is composed primarily of nonmarine deltaic (fan delta), lacustrine and fluvial sandstones, and siltstones. This facies reconstruction suggests that differential vertical displacements along northwest-trending normal faults controlled the regional variations in thickness, lithology, and grain size in the Bisbee Group. End_of_Article - Last_Page 549------------

Solution thinning of the M bed host rock limestone in the Tri-State District, Missouri, Kansas, Oklahoma

Regional thickness variations of the M bed limestone in the Tri-State zinc-lead district were investigated by determining the elevations of the top and bottom of that important ore-bearing horizon in 447 selected churn drill hole logs. This bed is the principal host rock for the ore deposits in the Picher field, and it is an important ore-producing horizon in many of the other mining fields of the Tri-State district of southwestern Missouri, southeastern Kansas and northeastern Oklahoma.The M bed was deposited with a greater original thickness, about 100 to 110 feet, in the vicinity of the Oronogo-Webb City-Duenweg field, than it was to the west near the Picher field where it was deposited in thicknesses of 70 to 80 feet. Post-Keokuk erosion has caused local removal of the top of the M bed in the western portion of the Picher field, and westward from that field the bed is progressively thinned by post-Keokuk erosion.In the Picher field, the principal ore-producing field of the Tri-State district, the M bed is markedly thinned and the isopachous bands exhibit a zonal pattern centered upon the ore deposits. On the scale of this regional study, more than one-half of the principal mined area in the Picher field exhibits M bed thicknesses of less than 20 feet; nearly all of the mined area occurs where M bed thicknesses are less than 30 feet. Outside the mining field there are progressively thicker M bed zones up to a maximum of 70 to 80 feet. The Galena and Melrose mining fields exhibit similar, but less well-defined, zonal patterns of M bed thickness.The thinning of the M bed associated with the ore-producing fields has originated principally by solution of the host rock limestone. The writers believe that the solution of the limestone was associated with the post-Keokuk erosion surface, and that solution of the M bed continued during some of the subsequent periods of erosion, but that some solution of the M bed host rock limestone was accomplished by the ore fluids themselves.

Effect of Regional Variations of Crude Oil and Reservoir Characteristics on In Situ Combustion and Miscible-Phase Recovery of Oil in Western Canada

Of the more than 20 billion barrels (exclusive of crude oil in the McMurray formation) of original in-place crude oil discovered in Western Canada to date, it is expected that only about 4.75 billion barrels will be produced by primary methods and water flooding. Secondary recovery methods such as in situ combustion and miscible-phase flooding could result in the production of, approximately, an additional 8 billion barrels (exclusive of crude oil in the McMurray formation), and reviews of the pertinent factors concerning the technological application of both methods are presented. The comparatively simple regional variations in structure and thickness of the sediments in the Western Canada sedimentary basin are noted, as well as the recent demonstration of parallel regio al variations in the physical and chemical attributes of the formation fluids. Accordingly, each of fifteen time-rock units is considered from the point of view of its areal distribution, the regional variations in thickness, stratigraphy, lithology, facies changes, and depth and characteristics of the petroliferous zones, and the regional variations in A.P.I. gravities of the crude oils. The regional applicability of the two secondary methods is then considered. In general, it appeared that thermal methods were most suited to crude oils in the subcrop and Sweetgrass Arch regions of the Upper Devonian, Mississippian, and lowermost Cretaceous sediments. For Jurassic crude oils, thermal methods appeared suitable throughout Western Canada. A reverse combustion method was considered most applicable to the crude oil in the McMurray formation. Conventional dispersed gas drives and enriched gas drives appeared to be suitable in most other regions for the other time-rock units, with high-pressure improved gas drives limited to those petroliferous zones deeper than about 5,000 feet. The miscible-slug technique appeared ideally applicable only to the Cardium formation crude oils.

Differential Compaction of Pennsylvanian Sediments in Relation to Sand-Shale Ratios, Jefferson County, Illinois

ABSTRACT In Jefferson County, in the Illinois coal basin, southern Illinois, the Pennsylvanian column between the Shoal Creek and Stonefort Limestones, an interval of about 900 feet, was divided into seven intervals separated by widespread coals or limestones readily recognized on electric logs, namely the Shoal Creek Limestone, coal No. 8, West Franklin Limestone, coal No. 7, coal No. 5, coal No. 4, coal No. 2, and the Stonefort Limestone listed in descending order. These intervals were determined from 578 electric logs in the county, an average of about one per square mile. Sand-shale ratios were determined for each interval and it was found than a channel sandstone of one age occurs in each interval except between the Shoal Creek Limestone and coal No. 8. Two ages of channels occur in one interval. Sandstone percentage maps were made of each interval. Thicknesses of each interval were compared in the channel areas and nearby, outside the channel, to avoid regional variations in thickness. Assuming that the coals and limestones were laid down nearly parallel to each other, variations in interval result from greater compaction of the shales than of the sandstones. Results show the following percentages of differential compaction for the six intervals with channels listed in descending order: 21, 34, 29, 35, 42 and 54 percent.

Madison Group (Mississippian) of Big Horn and Wind River Basins, Wyoming: ABSTRACT

Studies of both surface and subsurface sections of the group in the Big Horn and Wind River basins, Wyoming, indicate that persistent units can be recognized by means of field data and insoluble residue analysis. From top to base the major units are: (1) beds of unknown age herein termed Upper Madison, (2) the Mission Canyon formation and (3) the Lodgepole formation. Upper Madison strata occur sporadically over the area; previous work indicates that these beds may be of late Mississippian age. The Mission Canyon formation is subdivided into an upper and a lower member; this formation is relatively uniform in thickness. Regional variations in thickness of the group are confined for the most part to the Lodgepole formation. This formation thins to the south and east and probably is not present in the southern Wind River Basin. End_of_Article - Last_Page 1107------------

A Study of Some Upper Cretaceous Sedimentation and Diastrophism in Montana

The presence of a considerable variation in the thickness of the interval from the base of the Eagle sandstone of Montana time to the top of the Kootenai or Cloverly of Lower Cretaceous age has long been recognized over east-central Montana. As far as is known, however, no systematic study has been published showing the regional variation in thickness or the possible causes of such a phenomenon. This article is therefore an attempt to show by discussion and graphical representation the regional variation of the interval previously described, an interval formerly supposed to include only the Colorado formation of Upper Cretaceous age, but now known to embrace, over the southern part of the state at least, sediments of Montana time. There is also shown overloading of sedime ts over certain parts of the area and the resultant effects on diastrophism and sedimentation, from which additional light is shed on the problem of correlation of the sediments grading from Colorado to Montana time. In order to show some effects of the difference in sedimentation, it has been thought advisable to outline roughly the probable conditions immediately prior to Dakota time, as well as the continued deposition into Montana time, as criteria upon which to draw generalized conclusions. It is suggested that more detailed study of isopach lines may throw more light on proper subsurface mapping, especially of relatively large areas, as well as being an aid in the study of the deposition of sediments.