Gentle Dome Research Articles

Tectonic Structures of the Mangazeya Ore Cluster (Verkhoyansk Range, NE Asia)

The Mangazeya ore cluster is located in the central part of the West Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt, in the zone of junction of the Kuranakh anticlinorium and the Sartang synclinorium. High-tonnage Ag deposits and occurrences are concentrated here, which are thought to be associated with the zones of longitudinal strike-slip faults and bedding-plane thrusts. Structural studies were conducted in the dome and the eastern limb of the Endybal anticline, in the areas of the Nizhne-Endybalskoe, Verkhne-Endybalskoe, Bezymyannoe, Vertikalnoe, Sterzhnevoe, and Semenovskoe ore deposits. The Endybal asymmetric anticline 10-15 km wide and 60 km long, with a wide gentle dome is the major tectonic structure of the territory. The conducted structural and statistical analyses of tectonic structures in the study area allowed better understanding of their specific features and trends in development. Morphology of the fold structures indicates manifestation of concentric-type folding produced by bending of strata as a result of bedding-plane slipping. The same NS orientation of the cleavage and folds gives unanimous evidence for their genetic relationship and formation in the conditions of EW horizontal compression. It is established that the structural paragenesis represented by fractures, normal faults, and extension structures in the hinge zone of the concentric Endybal anticline, which determines the general tectonic pattern of the Mangazeya ore cluster and controls localization of ore bodies, was formed at the first deformation stage or in course of bedding-plane faulting and fold-and-thrust deformations rather than as a result of strike-slip motions. It is shown that strike-slip motions of the second deformation stage not only produced new structures but also inherited disturbances of the previous stage. It is found that normal faults in the study area formed synchronously with the fold-and-thrust structures. A similar orientation of the major faults and the strike-slip faults suggests that the regional faults have strike-slip kinematics. The occurrence of different types of tectonic deformation is controlled by rock lithology.

Laccolithic, as opposed to cauldron subsidence, emplacement of the Eastern Mourne pluton, N. Ireland: evidence from anisotropy of magnetic susceptibility

The structural evolution and emplacement of the Eastern Mourne pluton was investigated using anisotropy of magnetic susceptibility (AMS) measurements (carried out on 112 oriented block samples) and structural data from the host rocks. From these new data cauldron subsidence, as the emplacement mechanism, is disputed and evidence for an alternative, laccolithic style model involving inflation is presented. This includes deflection and uplift of host-rock bedding close to contacts and the magnetic fabric pattern, which has a gentle dome geometry, even close to contacts. The magnetic lineations usually plunge down-dip near the external margins but otherwise have a general SSW–NNE trend that diverges northward. This suggests a northward-directed inflow direction. The model for the emplacement of the Eastern Mourne pluton is a laterally fed laccolith, emplaced south to north. The eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical ‘breached’ laccolith.

Diapiric emplacement in the upper crust of a granitic body: the La Bazana granite (SW Spain)

The ascent and emplacement of granites in the upper crust is a major geological phenomenon accomplished by a number of different processes. The active processes determine the final geometry of the bodies and, in some favourable cases, the inverse problem of deducing mechanisms can be undertaken by relying on the geometry of plutons. This is the case of the La Bazana granitic pluton, a small Variscan igneous body that intruded Cambrian rocks of the Ossa-Morena Zone (SW Iberian Massif) in the core of a large late upright antiform. The granite shows no appreciable solid-state deformation, but has a late magmatic foliation whose orientation, derived from field observations, defines a gentle dome. The regional attitude of the main foliation in the country rock (parallel to the axial plane of recumbent folds) is NW–SE, but just around the granite, it accommodates to the dome shape of the pluton. Flattening in the host rock on top of the granite is indicated by boudinaged and folded veins, and appears to be caused by an upward pushing of the magma during its emplacement. The dome-shaped foliation of the granite, geometrically and kinematically congruent with the flattening in the host rock, can be related in the same way to the upward pushing of the magma. The level of final emplacement was deduced from the mineral associations in the thermal aureole to be of 7–10 km in depth. Models of the gravity anomaly related to the granite body show that the granite has a teardrop–pipe shape enlarged at its top. Diapiric ascent of the magma through the lower middle crust is inferred until reaching a high viscous level, where final emplacement accompanied by lateral expansion and vertical flattening took place. This natural example suggests that diapirism may be a viable mechanism for migration and emplacement of magmas, at least up to 7–10 km in depth, and it provides natural evidence for theoretical discussion on the ability of magmatic diapirs to pierce the crust.

Emplacement of the Great Whin Dolerite Complex and the Little Whin Sill in relation to the structure of northern England

SUMMARY By definition, sills are concordant, tabular sheet-like bodies of igneous rock with the Great Whin Sill of northern England often quoted as a primary example. Analysis of surface and subsurface records of the Great Whin shows that it is seldom concordant over wide areas and usually rises and falls in the stratigraphical succession in gentle transgressions and abrupt jumps to new levels. The term ‘sill’ is not used here for the Great Whin complex, but is used for the related Little Whin Sill of Weardale, Co. Durham. Studies of the petrology of the dolerite Whin complex have brought out significant differences between the Little Whin and the Great Whin. The Little Whin Sill is olivine-bearing and believed to be composed of an early differentiate of the Whin dolerite magma. The Great Whin, non-olivine-bearing and slightly density graded, is a later differentiate of the Whin magma. Two separate periods of Whin dolerite injection are confirmed by studies of vitrinite reflectance over the Alston Block where two periods of Whin contact metamorphism have been recognized. The two periods of Whin dolerite emplacement form part of the end-Carboniferous earth movements in northern England. They can be shown to have occurred between a period of compression from a W-SW direction and later gentle doming of the Alston Block near the Westphalian-Stephanian boundary, dated about 300-295 Ma.

The morphology and evolution of coronae on Venus

Coronae on Venus are large, circular to ovoidal surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. New data from the Magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail. Typical coronae are dominated by concentric tectonic features and have a raised rim, a central region higher than the surounding plains but in many instances lower than the rim, and, commonly, a peripheral depression or “moat”. Some coronae also show significant amounts of radial tectonic structure, and in most cases this predates the concentric features. In addition, there are other features on Venus, recognized for the first time in Magellan data, that consist of domical rises with intense radial tectonic patterns and little or no concentric structure. All of these features commonly are associated with moderate to large quantities of volcanism. In fact, some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography. We explore a model of corona formation that links these forms into a genetic sequence. The model begins with the ascent of a mantle diapir. Upward mantle flow driven by its ascent forces the lithosphere above the diapir upward, producing a gentle dome with a radiating pattern of extensional fractures. As the diapir impinges on the underside of the lithosphere it flattens and spreads, transforming the uplift to a more flat‐topped shape. In this flattened, near‐surface configuration the diapir can cool rapidly. With the resultant loss of buoyancy the raised plateau can relax to form a central sag, a raised rim, and a depressed moat. Concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae. Volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs. Our analyses to date suggest that this scenario is broadly consistent with many of the coronae on Venus. However, there is enormous diversity in corona morphology, and features are present that require substantial deviations from this simple model. In particular, some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas. Some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the Venusian lithosphere.

The Petroleum Geologic Characteristics of Sichuan Basin, Central China

Sichuan basin is the main gas producer of China. It covers an area of 230,000 km{sup 2}. The evolution of this basin since Meso-Cenozoic was influenced by both trans-Eurasia Tethys tectonism from the west and the circum-Pacific tectonism from the east. So it has dual characteristics, compressional and tensional. The northward-moving Indian Plate resulted in a series of thrust fault zones along the Longmenshan western margin of Sichuan basin. Jurassic oil pools and Triassic, Permian, Carboniferous, and Sinian gas pools are present, where a series of box-like anticlines, comblike anticlines, and gentle slope dome anticlines, and gentle slope dome anticline, carbonate reef buildups are the main trap types. Significant role of fractures and caves of carbonate reservoir formations in Sichuan basin affects the production capacity of gas/oil wells and abundances of gas/oil reserves. Three-dimensional seismic methods are used to predict the unconformities and the paleokarst and fracture zones. Acidizing treatments were used for well completions.

Stratigraphic control of the Hills Pond lamproite, Silver City Dome, southeastern Kansas

The Silver City Dome contains a rare occurrence of a lamproite sill complex. Interpretation of seismic reflection data suggests that the character of the Late Cretaceous Hills Pond lamproite is a consequence of local stratigraphic conditions—namely, the dense layering and hydrologic impermeability of Pennsylvanian strata that host the intrusion. The structure developed in at least two phases. First, a gentle dome and marginal ring grabens formed as a result of deep magmatic activity. Later, a smaller and steeper dome and a centrally located down-warp were superimposed upon the gentle dome as a result of sill injection into the Pennsylvanian sequence and collapse of the rock column above the magma source.

Fault Patterns Associated with Domes--An Experimental and Analytical Study

Experimental (clay) and analytical models suggest that regional strain, either extension or compression, significantly affects the fault patterns produced by doming. Our models specifically simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied, regional horizontal strain. The models of circular domes show that without regional strain, normal faults develop on the crests and flanks. On the flanks, the normal faults have radial trends. With regional extension, normal faults on the crests of circular domes trend perpendicularly to the regional extension direction, whereas many normal faults on the flanks trend obliquely to it. Strike-slip faults trending 60° from the regional extension direction fo m near the peripheries. With regional compression, many normal faults on the crests and flanks of circular domes strike parallel with the regional compression direction. Strike-slip faults trending 30° from the regional compression direction also form on the flanks, and reverse faults striking perpendicularly to the regional compression direction develop on the peripheries. Our models show that regional strain affects the fault patterns produced by elliptical doming similarly. The fault patterns of our models resemble fault patterns of domed strata above salt diapirs and above uplifted blocks of basement.

Effect of Regional Strain on Fault Patterns Produced by Doming: Experimental and Analytical Study: ABSTRACT

Experimental (clay) and analytical models suggest that regional strain, either extension or compression, significantly affects fault patterns produced by doming. Our models simulate the shallow deformation produced by gentle doming of a homogeneous material with and without a simultaneously applied, regional horizontal strain. The models show that without regional strain, randomly oriented normal faults develop on the crests, and radial normal faults form on the flanks of circular domes. With regional extension, normal faults on the crests and flanks of circular domes trend perpendicular to the applied extension direction, and strike-slip faults trending 60° from the regional extension direction form on the flanks. With regional compression, normal faults on the cres s and flanks strike parallel to the applied compression direction. Strike-slip faults trending 30° from the regional compression direction also form on the flanks, and reverse faults striking perpendicular to the regional compression direction develop on the peripheries. Our models show that regional strain affects the fault patterns produced by elliptical doming. This study has important implications for hydrocarbon exploration. The models provide guidelines for determining the strike of faults on domes and suggest that strike-slip and reverse faults, as well as normal faults, may form during doming. These faults may influence hydrocarbon migration and entrapment. Strike-slip faults develop on domes formed in the presence of regional extension (for example, many Gulf Coast domes). Strike-slip and reverse faults develop on domes formed in the presence of regional compression (for example, several domes of the Rocky Mountain foreland province). End_of_Article - Last_Page 1008------------

Geological development of Jabal al Akhdar, Libya

The northern Cyrenaic headland situated north of 32 degrees latitude between Benghazi and Tubruq was subject of a geological mapping. This paper draws attention to the results of this geological survey concerning Upper Cretaceous and Tertiary facies development, paleogeography, tectonic and geomorphologic evolution. The essential part of the investigated area corresponds to Jabal al Akhdar (Green Mountains), a gently uparched plateau built of Upper Cretaceous and Tertiary sediments (mostly limestones, subordinate dolomites and marls). These sediments were deposited at the southern margin of the Tethys sea and were moderately folded, mainly during the intra-Senonian (pre-Campanian) and early Ypresian intervals. From Middle Eocene till Middle Miocene, the area was subject to a slight warping followed by oscillating transgressions of a shallow sea. The youngest tectonic movements resulted in a gentle doming of the area associated with downfaulting of certain zones. Some prominent faults revealed rejuvenated activity, partly with movement inversion. Deep faulting probably controlled the paleogeographic and tectonic development of certain zones. The present gross geomorphology of Jabal al Akhdar roughly corresponds to its final uparching dated after the Middle Miocene. Topographic evolution of the northern slope of the mountains has been importantly influenced by marine erosion. Two broad littoral terraces bordered by cliffs were formed successively.

Some Relationships between Crude Oil Properties and Geology in the Kubiki (Katamachi) Gas and Oil Field, Japan (Part 1)

The purpose of this study is to reveal any relationships between various crude oil properties and geology in the Kubiki (Katamachi) gas and oil field, Japan. Taking the reservoir conditions into account, we found out tendencies that some properties of crude oils vary with the geologic conditions in the field. The Kubiki gas and oil field is one of the largest field in Japan and includes several producing structures (Figure 1). The general stratigraphic column is shown in Figure 3. Many reservoirs are intercalated in the dark grey mudstone of the late Miocene Teradomari Formation. Amoung these structures and reservoirs the Ie reservoir in the Katamachi structure is producing much oil. Thus, our study was carried out mainly on the Ie oils and Ie reservoir there. The Katamachi structure is a gentle dome cut by many minor faults (Figure 2). The Ie reservoir is more or less tuffaceous or shaly sand and remarkably changes in thickness (Figure 4).We sampled crude oils from 73 producing wells (Table 1) and measured specific gravity and absolute viscosity. Furthermore, we conducted fractional distillation and made a gas-chromatographic analysis of lighter cut of the crude oils (lower than 105°C). On the other hand we collected various reservoir data including initial water-oil ratio.Through the above-mentioned work, we have come to the following conclusion.1) The initial boiling point of these crude oils is relatively high and the amount of lighter fraction is comparatively small (Figure 9). As for fraction of 250-275°C the crude oils are generally of naphthenic base.2) Specific gravity of crude oils from a reservoir in a small area vary very much (Figure 5). Taking the initial water-oil ratio (Figure 6) and the thickness of the reservoir into account, it is suggested that wells producing heavier crude oil are generally situated at the places where the sand reservoir is thin and initial water-oil ratio is large and vice versa, almost regardless of the setting of the wells in the Katamachi structure. This tendency is quite remarkable in the block east of d-d' fault. This is probably because in the portion where the reservoir is thin the reservoir is shalier and contains much of water. But this tendency is not clearly recognized in the block west of d-d' fault. Consequently we propose schematic models of the Ie reservoir in the Katamachi structure (Figure 7).3) Absolute viscosity has a strong correlation with specific gravity (Figure 8), however, the above tendency (described in 2)) cannot be ascrived to the variation of viscosity of the crude oils. Relation of viscosity vs. specific gravity of the Katamachi crude is quite different from that of Khafji field, Middle East. This fact can be explained from the distillation histograms (Figure 9).4) Relative amount of lighter hydrocarbons shows a considerable variation (Table 2). However, the Kubiki crude has much of cycloparaffins and branched paraffins, and thermodynamical equilibrium is not found out. These facts suggest that the Kubiki crude is young and less-evolved oil.

Yates Oil Field, Pecos County, Texas: ABSTRACT

The Yates field, discovered in October 1926, is extraordinary in amount of oil produced, size of original oil accumulation, and well productivity. During the 41 years since discovery, it has produced more than 500 million bbl of oil. Material balance calculations, based on past performance of the Yates field reservoir, indicate an oil-in-place of between 3.7 and 4.3 billion bbl. It has been estimated that ultimate recovery from the reservoir will be between 1.5 and 2 billion bbl. The Yates field reservoir, mostly dolomite, may be the largest single oil accumulation ever found in a North American carbonate. It occurs as a gentle dome with structural closure in excess of 350 ft and covers an area of about 21,700 acres. The principal marker in the reservoir is found at depth ranging from approximately 1,000 ft to about 1,900 ft. Nearly a third of the 637 wells potentialed for more than 10,000 bbl of oil per day, and 26 potentialed for volumes ranging from 80,000 to 205,000 bbl per day. Cavernous and high matrix porosity in the reservoir contributes significantly to the remarkable productivity of some wells in the field. The cavernous porosity probably is related to subaerial erosion. Both the Yates field reservoir and its seal are of middle Permian age. The reservoir is largely marine bioclastic dolomite equivalent to the San Andres-Grayburg. It also includes sandstone approximately equivalent to the Queen, between the Grayburg and Seven Rivers anhydrite, the latter forming the reservoir seal. The huge Yates field reservoir oil accumulation probably resulted from favorable location relative to coarse beds and regional migration routes. The trap, a local and regional structural high, adjoins basinal strata on two sides and is situated at the southern tip of the northward-tilted Central Basin platform. End_of_Article - Last_Page 523------------

WOOLNOUGH HILLS AND MADLEY DIAPIRIC STRUCTURES, GIBSON DESERT W.A.

A diapiric origin is indicated for several domal structures with arcuately linear trends, in the western Gibson Desert, Western Australia. The most prominent of these trends is approximately latitudinal, although a minor trend north of Woodnough Hills, is north to northeasterly.Foundered blocks of Upper Proterozoic, Permian and possibly Jurassic sediments occur within the gypsum-cores of several of the diapirs. The sharp angular unconformities between Upper Proterozoic and Permian and Permian and Jurassic sediments respectively, suggests that diapiric uplift and piercement has continued, at least intermittently, for extensive periods of geologic time. Gentle doming of the late-Tertiary laterite surface evidences the continuation of diapiric uplift into post-Tertiary times.

Structural Features of North Tejon-Wheeler Ridge Area: ABSTRACT

Both the North Tejon and Wheeler Ridge oil fields lie almost directly over the deeply buried portion of the White Wolf fault, but are found to occupy structures quite different in origin. The Wheeler Ridge anticline is one of a series of folds occurring along the southern border of the San Joaquin Valley. These folds and extensive thrust faults are easily recognized as the product of northerly directed compressive stresses. The principal North Tejon structure, located along the western side of Highway 99, has a north-south anticlinal axis striking nearly at right angles to the Wheeler Ridge axis, and does not have the related thrust faulting common to the other anticlinal folds. Development of the North Tejon field showed the presence of numerous high-angle faults and revealed a segmentation of oil reservoirs into blocks containing oil of different gravities. The peculiar structure and distribution of Miocene basalt, capping most of the faulted reservoir rocks, and the gentle doming of formations lying above it indicate that the shape of the North Tejon structure resulted from an upwarping of the probably highly fractured basement rock with little regard to the existing faults. A northeasterly regional tilt, accompanied by depression of gas-condensate-filled, former structural highs to positions below the level of present heavier oil- and water-bearing sandstones, has long been recognized in the segmented reservoirs at North Tejon field. Such a tilt may well have been caused by the large-scale sinking of the portion of the positive block of the White Wolf fault in the area just northeast of the field, which, like the negative block, was exposed to the very rapid End_Page 1091------------------------------ accumulation of Plio-Pleistocene debris derived from adjacent uplifts. End_of_Article - Last_Page 1092------------

Grandison Complex, Lafourche and Jefferson Parishes, Louisiana: ABSTRACT

The Grandison area, centering in southeastern Lafourche Parish, on the Mississippi River delta in South Louisiana, is an interdomal basin surrounded by peripheral salt domes and anticlines. Hinge line flexures and normal faults help to make correlative zones 2,000 feet lower in the center of the basin than on the flanking anticlines. The main producing sands of the basin either grade into shale or are truncated onto the flexures and against the faults. One gas sand produces over a distance of more than 12 miles along its pinch-out on the flanks of four major anticlines. Isopach maps were valuable in predicting where the pinchout would occur. The foraminiferal assemblages in the marine zones reflect ecological changes that occur across the faults and flexures and on the flanks of the anticlines and domes. The most extensive producing sand contains gas wherever the ecological conditions under which it was deposited are constant; but where these conditions change across the closely controlled Coffee Bay fault and onto the anticlines, no accumulation occurs. Although the central part of the Grandison complex is a structural basin at depth, above 7,000 feet it is a broad, very gentle dome that makes a prominent photogeologic anomaly that has aided exploration. End_of_Article - Last_Page 2518------------

Geology of Deadman Butte Area, Natrona County, Wyoming

The rocks along the southeast flank of the Bighorn Mountains range from Precambrian to Quaternary, but include no Ordovician, Silurian, or Devonian. Two Jurassic formations characteristic of central Wyoming, the Nugget sandstone and Gypsum Spring formation, are lacking. Tertiary deposits are present along the southern margin and at places higher in the mountains. The top of the Middle Cambrian Flathead sandstone makes an upward migration in time toward the northeast of 22 feet per mile. The Upper Cambrian Du Noir member of the Gallatin formation tongues out eastward within the area. The presence of remnants of the basal Phosphoria Nowood member implies early Permian deposition and erosion prior to deposition of younger Phosphoria strata. Eastward-extending dolomite and limestone Phosphoria tongues alternate with westward-extending red shale tongues. A restricted use of the name Crow Mountain sandstone member is applied to the Triassic Chugwater interval between the Alcova and the Popo Agie members. A high-angle reverse fault in the southern part of the area is probably related to the southward thrusting of the Bighorn Mountains during the earliest Eocene. Intense faulting in Mesozoic rocks at the southern margin of the mountains suggests proximity to a main thrust zone covered by younger Tertiary deposits. Northwest-trending folds and faults probably developed later than the thrust faulting. Five oil tests, including two within the area, are dry holes. Four of these tests had oil staining in the Phosphoria; one had staining in the Tensleep. A gentle dome in the southeastern part of the area has not yet been tested.

Genesis of Oligocene Sandstone Reservoir, Seeligson Field, Jim Wells and Kleberg Counties, Texas

The character, distribution, and probable genesis of a typical lenticular oil-bearing sand body of the Gulf Coast Tertiary, namely, Zone 19B, an Oligocene sand of the Seeligson field, Texas, were determined for the purpose of developing and defining criteria by which such sand bodies can be readily identified and delimited in the subsurface. The stratigraphic framework was specified by cross sections, fence diagrams, isopach maps, and facies maps, prepared from electric logs and core descriptions, and the sediments were characterized by size, shape, thin-section, X-ray, differential thermal, and chemical analyses. Principal results are: (1) the isopachal trend of the sand body is nearly normal to the regional depositional strike; (2) the sand grades from medium grain at the base to fine at the top and can be divided into three zones on maximum grain size; (3) a typical specimen is a mixture of (a) well sorted sand, the framework size fraction, (b) interstitial clay, the tail of the size-distribution curve, and (c) interstitial calcite cement; (4) the mineral composition is similar to that of a Recent river sand, but dissimilar to that of beaches, of the same geological province. The sand body is probably an Oligocene alluvial-plain deposit as indicated by its similarity to such Recent deposits in external form, vertical textural zonation, colors of associated shales, mineralogy, presence of mud fragments, and absence of indigenous fauna. The often stated opinion that oil-field trends of the Gulf Coast, such as the Frio-Vicksburg, are shore-line trends has led many geologists to infer that the reservoir sands of these oil fields are strand-line deposits. The conclusions of the present study emphasize the risk of such deductions. The trends may be indirectly related to shore lines by reason of marine source beds, but many, or possibly most, of the reservoir sands may be of continental origin. The trapping in this zone arises from updip normal faulting and gentle doming of a lenticular alluvial sand body which is oriented in the direction of regional dip. The greatest value of these findings lies in the extrapolation and extension of this knowledge of local genesis to other parts of the section. In such a fashion a regional depositional and tectonic history can be outlined, centers of deposition for various geologic ages can be delimited, and areas favorable for the presence of permeable beds and oil accumulation can be approached.

The tectonics of the Tintagel area, North Cornwall

Summary The paper describes the structure of the coastal region of North Cornwall in the neighbourhood of Boscastle, Tintagel and Trebarwith Strand. The area has been remapped, and the tectonics originally outlined by Dewey (1909) have been reinterpreted. The revision has shown that the epidiorite at Tintagel is more extensive than hitherto believed. Massive epidiorite grades into chlorite-schist which is often very similar in appearance to the sheared volcanic rocks of the area; but the schist derived from the intrusive rock can be identified by its local thermal metamorphism of the phyllites which lie above it. The important tectonic episode in the area was that of thrusting, in which the upper beds moved from the south-south-east towards the north-north-west. The direction and sense of the translation is indicated by various minor structures, e.g. slickenside striations, elongations of lava pillows, amygdules etc., in a (the movement direction); and by drag-folds, zig-zag folds, boudinage structure, tension fractures etc. in b (normal to a ). Three main thrust-zones are recognized, and they separate the rocks into three tectonic units or thrust-slices which lie on an autochtonous foundation. The higher thrust-slices lie to the west of the lower ones because they have been preserved from erosion by the local westerly dip aided by normal faulting which downthrows to the west-north-west. The more important of these normal faults pass through Tintagel Haven and form, inter alia , the great fault complex seen in the cliffs of the Island. Similar faulting occurs at Trebarwith Strand, Lill Cove and Hole Beach. In each case the downthrow is such that the flat thrust-plane there is dropped to the west. The general direction of strike in the area is controlled by the gentle doming due to the Davidstow Anticline. This hemidome pitches north-westerly, and it developed later than the thrusting episode. The evidence shows that there is no connexion between the thrust movements which carried the beds forward at least 2½ miles (or more) and the warping of the anticline. The tectonics at Tintagel are thus no longer to be considered the result of thrusting from the west-north-west, but are rather the result of a major movement from the south-south-east, and so become an integral part of the Hercynian tectonic pattern of Cornwall.

Structure of South Louisiana Deep-Seated Domes

This is primarily a study of the subsurface, geologic structure of oil fields in Louisiana within a coastal belt about 75 miles wide. For this report, 1,000 electrical logs of wells were studied; 123 were selected for reproduction in the cross sections. Most of the structures are the result of the rise of plastic salt columns from deep within the sedimentary layers of the earth's crust. The present depth of the top of the salt column determines the structural characteristics of the resulting domes. This study is devoted particularly to the deep-seated domes in which salt has not been reached. Two common types of structures are found in this group. The most common structural type consists of a region of domed sediments crossed by a series of normal faults creating a graben in the center of the domed layers. The graben of deep-seated domes is bounded by major faults. All observed grabens contain several faults, both major and minor, resulting in a complex mass of faulted blocks. Most of the major faults converge downward toward a zone believed to lie just above the head of the salt column. The dome-with-graben type of structure appears to be a fundamental type to which all of the others are related. A less common type may be described as a dome offset by one or more major faults which have similar directions of strike and dip. In many such cases, it is believed that when additional wells are drilled, other faults will be found to complete a central graben. No reverse faults were found. All of the structures contain normal faults. These faults may be broadly divided into major and minor types. Major faults characteristically increase in throw downward. The throws of major faults range from 100 to more than 900 feet. Observed dips of major faults range from 45° to 65°. The throw of these faults on any structure gives a rough approximation of the amount of structural relief. Minor faults take only a minor part in the segmentation of the domes. Two distinctive types are recognized. Radial minor faults extend radially from the center of doming. Complementary minor faults are distinctive in that the throw decreases downward. Such faults create small grabens against major faults. Complementary faults relieve tension in the outer layers of the dome. Other minor faults are less readily classified. They may be parallel and close to major faults or they may be branches or small divisions of major faults. In many, the throw is so small that they can not easily be traced. The following theory is proposed to explain the development of the grabens. First, upward movement of a salt plug or column causes the deeply buried layers to be gently domed. Continued salt movement causes rupture in the form of a major, normal fault extending diagonally upward through the dome from the head of the salt plug. Additional salt movement causes additional faults of the same kind but which in a general way are directed alternately opposite to, and parallel with, the first. The result is a complexly faulted End_Page 1249------------------------------ graben, with the later faults at higher levels within the graben. Continued salt movement results eventually in a shallow salt dome. The present distribution of oil and gas on deep-seated domes is the result of two sets of conditions. The present general location of the oil and gas is related directly to the center of the original gentle dome created by the earlier movements of the salt. Into this simple domal trap, petroleum migrated from the entire deformed area of the dome. The specific location of each individual trap for each sand is now dependent on the segmentation of the dome by faulting. In general, the petroleum will remain in its original geographic location, but with many complications added by the barriers developed by faulting and the further tilting of faulted blocks. Original maps and sections were prepared for the following fields: Barataria, Bateman Lake, Bayou Sale, Erath, Gibson, Gillis-English Bayou, Grand Bay, Grand Lake, Horseshoe Bayou, Lafitte, Lake Long, Lake Mongoulois, North Crowley, Roanoke, St. Gabriel, West Gueydan.