Structure Contour Maps Research Articles

Block rotation and termination of the Hosgri strike-slip fault, California, from three-dimensional map restoration

The Hosgri fault is located immediately offshore of south-central California and is part of the transform boundary between the Pacific and North American plates. This fault terminates to the southeast into east-trending folds and reverse-separation faults of the western Transverse Ranges. Our new structure-contour maps of deformed horizons show a spatial relationship between faulting and folding consistent with right-lateral slip. Restoration of these digital maps quantifies post-Miocene right-lateral slip across the southern Hosgri fault to be 3.5 km. This slip is absorbed by folding, thrust overlap, and clockwise vertical-axis rotation of elongate blocks between strands of the fault. The restored part of a block located to the east has rotated 8° clockwise. Extrapolating this restored rotation to the 50 km block length produces an estimate of 7 km of dextral shear for a total, including the 3.5 km fault slip, of 10.5 km of post-Miocene displacement. Our three-dimensional approach precludes interpretations for reverse slip on the Hosgri fault, and is not consistent with models for more than 80 km of late Cenozoic right-lateral fault slip.

Contrôle morphostructural de l'histoire d'un réseau hydrographique: le site de capture de la Moselle

By comparing the topographic contour map, the Bajocian limestone structural contour map and alluvial terraces in the Toul and Nancy area, it is possible to reconstitute the changes in the drainage pattern and the Moselle and Meuse cuestas (côtes de Moselle et de Meuse) retreat that have taken place since the late Tertiary. The highest reworked alluvial deposits of the palaeo-rivers (‘Haute-Moselle’ and ‘Paléo-Meurthe’), are now situated on the inverted relief top of the Moselle cuesta. They suggest that the course of these rivers drifted and that the relief in the late Tertiary was subdued. The clearing of the back slope surfaces only started at the beginning of the Pleistocene (at 350 m) as a result of a climatic deterioration. Since this time, downcutting erosion and the correlative removal of clays have strongly emphasized cuesta relief although there has been a certain retreat which varies about 6 to 21 km, depending on the basement dip. The main rivers are almost entirely superimposed on the palaeo structural frame-work. On the contrary the tributaries of the left bank originally extended and changed on the Callovo-Oxfordian clays before penetrating into the Bajocian limestones, according to the cuestas retreat during the Plio-Pleistocene. Only small strike rivers were in accordance with structural features; sinking into the underlaying Bajocian limestones these entrenched rivers quickly dried up. On the opposite, the flow of the eastward rivers, independent of the structure, grew upstream to downstream, because these rivers first drained the Callovo-Oxfordian clays, before cutting across the Bajocian limestones groundwater. It is the same nowadays. The piracy of the Moselle can be explained by the evolution of the ‘Paléo-Terrouin’, a major eastward palaeo tributary of the ‘Paléo-Meurthe’, and by the retreat of the Meuse cuesta. The significant retreat of this cuesta in the ‘Toul syncline’, changing from an inverted relief to a structural relief in the Dieulouard structural basin, led to a widening of the Callovo-Oxfordian clay plain and a broadening of the ‘Paléo-Terrouin’ 's hydrographic basin. As a result, the ‘Paléo-Terrouin’ captured the ‘Rivière de la Haie Plaisante’, a small strike tributary of the Upper Moselle, and then eastward, the Upper Moselle. In the future, it will probably also capture the Upper Meuse.

Subsidence of a volcanic basin by flexure and lower crustal flow: The eastern Snake River Plain, Idaho

The Eastern Snake River Plain (ESRP) is a linear volcanic basin interpreted by many workers to reflect late Cenozoic migration of North America over the Yellowstone hotspot. Thermal subsidence of this volcanic province with respect to Yellowstone has been documented by several workers, but no one has characterized subsidence with respect to the adjacent Basin and Range Province. This paper documents crustal flexure along the northwest edge of the ESRP, uses flexure to model the dimensions of a dense load beneath the basin, and presents evidence in support of density‐driven subsidence and lower crustal flow away from the basin. Crustal flexure adjacent to the ESRP is reflected by the attitudes of Mesozoic fold hinges and Neogene volcanic rocks. Fold hinges formed with a subhorizontal plunge and a trend perpendicular to the ESRP but now show a southward plunge near the ESRP of as much as 20°–25°. We present a contour map of equal fold plunges proximal to the ESRP that shows flexure is roughly parallel to and extends 10–20 km north of the average edge of the ESRP. Flexural profiles indicate the minimum amount of ESRP subsidence, with respect to the Basin and Range; subsidence ranges from 4.5 to 8.5 km. The structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models. These models indicate the flexed crust is very weak (flexural parameter of 4–10 km), interpreted to be a result of the high heat flow of the ESRP. Assuming subsidence was induced by emplacement of a dense crustal layer beneath the ESRP, a midcrustal “sill” identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure. New dimensions include a thickness of 17–25 km and a half width of 40–50 km, which place the edge of the sill beneath the edge of the ESRP. The dimensions of the ESRP sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30+ km and because ESRP seismic, gravity, and heat flow data support lower crustal compensation. Density‐driven lower crustal flow away from the ESRP is proposed to accommodate subsidence and maintain isostatic equilibrium. Timing of subsidence is constrained by ESRP exploratory wells, where 6.6 Ma rhyolites at a depth of 1.5 km indicate most subsidence occurred prior to their emplacement, and by strong spatial correlations between plunge contours and Quaternary volcanic rift zones. Two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at ∼10 Ma, which provided the heat source for the initial rhyolitic volcanism on the ESRP, and continuing, localized loads from dikes and sills associated with Quaternary basalts. Widespread ∼10 Ma magmatism and subsidence conflicts with simple time‐transgressive migration of the Yellowstone hotspot, indicating a need for revision of the hotspot paradigm.

Abstract: Subsurface Stratigraphy and Water Resources of the Dothan Area, Houston County, Alabama

ABSTRACT The city of Dothan is located in northwestern Houston County in the extreme southeastern corner of Alabama. Currently, water is supplied to the Dothan area through 30 wells whose screened intervals vary in depth from 100 to 1,080 feet. During 1997, these wells supplied about 19.4 million gallons of water per day to a population of about 75,000 citizens. Prior to the 1997 completion of geological investigations conducted in support of a wellhead protection program for Dothan's water supplies, the probable source(s) of Dothan's water was poorly known. Guesses based primarily on drillers' logs or a combination of well depth integrated with the regional dip of stratigraphic units were invariably incorrect or involved multiple geological units as possible sources. The most fundamental question--"From which unit is our water being produced?"--could not be answered. In order to define the stratigraphy of the area, over 14,000 feet of samples from 11 wells, including two deep oil and gas test wells, in the Dothan area were microscopically examined and described in detail. These data were then integrated with available electric logs. The resulting stratigraphy was used to define the subsurface geology of over 50 additional area wells lacking sample or log control. This stratigraphy defined the precise locations of screened intervals within individual formational units. The construction of a series of strike and dip cross sections, as well as isopach and structure contour maps, for the approximately 150-square-mile study area, also delineated major and minor aquifers and their associated boundary confining units, documented their areal facies and thickness changes, and permitted the projection and characterization of potential aquifers below current production zones.

Cretaceous Subsurface Geology of the Middle East Region

ABSTRACT A regional structure contour map at Near Top Cretaceous is based on hundreds of well tops from extensive bibliographic references from throughout the Middle East. This structure map shows strong basin asymmetry. Major faults are shown in outcrop and/or suspected at basement or intermediate levels, based in part on gravity and magnetics modeling, published and in ‘open file’ studies. Major uplifts associated with several super-giant oil and gas fields are clearly indicated even at the shallow Cretaceous level (Ghawar Anticline, Qatar Arch, Burgan-Khurais Trend, etc.), even at a very regional scale with a contour interval of 1,000 feet. Isopach maps of Upper, Middle, and Lower Cretaceous are contoured at intervals of 500 feet. Each of these three isopach maps is overprinted in color to show generalized lithofacies trends. Lower and Middle Cretaceous deltaic sandstone fairways on the western shelf provide excellent reservoir rocks for a trend containing many of the world’s very largest oil fields. Somewhat more basinward, predominantly carbonate facies include oil reservoirs in the Upper, Middle, and Lower Cretaceous. Deepest facies lie beneath the Zagros Foothills Belt in coastal Iran and eastern Iraq. This is particularly true for the Upper Cretaceous, where Coniacian to Maestrichtian thick, deep basinal shales, cherts, clastic turbidites, and slumped exotic blocks of ophiolites mark the northeastern border of the Late Cretaceous basin as it approaches the Main Zagros Fault and an assumed subduction zone underthrusting the Iranian Plate or Eurasia.

Design And Field Trial Application of Foam In Production Wells to Improve Conformance

Abstract Many of the hydrocarbon vertical miscible floods in the Keg River carbonate reefs of the Rainbow Field are at a mature stage, of reservoir depletion, and are characterized by thin oil sandwiches, high gas recycle and declining oil production. Under these adverse conditions, conventional workover techniques to control gas recycle and improve oil productivity have diminishing success. In considering alternative strategies to reduce gas coning, Husky felt that injection of a gas-blocking foam barrier into a gas cone hadthe potential to be a cost-effective treatment. This paper describes the experimental foam treatments conducted on two production wells, each in a different miscible flood pool. Aspects of the laboratory work, numerical coning simulation, well selection process and foam placement strategy that went into the field treatment design are presented. On one well, the foam-forming surfactant was injected as an aqueous solution to allow in situ foam generation, and also as a surface generated preformed foam. Neither method was successful in improving the performance of this well. On the other well, where only preformed foam was injected, the treatment was successful in reducing gas-oil ratio (GOR) and improving-oil production over a 14 month period. Much of the experience gained from the field treatments will facilitate the efficient planning, design, implementation and evaluation of future foam projects. Introduction Husky Oil Operations Ltd. operates a number of mature hydrocarbon vertical miscible floods in the Keg River carbonate reefs of Rainbow Field, Alberta, Canada. Many of the wells in these floods are already completed at or close to the base of the remaining oil sandwich, and are experiencing increasing solvent/chase gas coning and declining oil productivity as the oil zone thins under continued injection and production. With little or no room to move in the oil pay, conventional workover strategies of cement squeezing and reperforating lower have diminishing application. In evaluating innovative methods to mitigate gas production, foam chemicals were considered. The potential of foam as a blocking or mobility reduction agent to gas flow has been well documented(1–5). Due to potentially high adsorption losses, foam only has a chance of working from the injection side if it is able to reduce mobility in a relatively small channel volume between injection and production wells, or if the channel is isolated from the rest of the reservoir away from the injection well (minimum volume of rock contacted). However, most of the Keg River reefs have relatively good communication throughout, and in most FIGURE 1: Keg River "A" Pool. Keg River structure contour map. Available In Full Paper. cases, tracing has shown the channels to be relatively large. Consequently the focus was diverted away from an expensive, large volume foam treatment at an injector to a much smaller, near wellbore treatment at a producer. Adsorption of foam-forming surfactant onto reservoir rock would not be a big problem if the amount of rock contacted could be kept to a minimum.

Holocene Geologic Framework of Lake Pontchartrain Basin and Lakes of Southeastern Louisiana : ABSTRACT

The Lake Pontchartrain Basin is a 12,170-km2 (4,700-mi2) watershed in southeastern Louisiana, stretching from the State of Mississippi on the north and east to the Mississippi River on the west and south, and to Breton Sound at the Gulf of Mexico. The Pontchartrain Basin is about 200 km along strike and 75 km along dip with modern lakes (Maurepas, Pontchartrain, and Borgne) covering the southern portion of the basin. Lake Pontchartrain and its adjacent lakes form one of the largest estuaries in the United States. Nearly 1.5 million people (one-third of the entire population of Louisiana) live in the 14 parishes within the Lake Pontchartrain Basin. The basin is bounded by incised Pleistocene terraces to north, the Mississippi River delta plain to the south/southwest, and the Pine Island barrier shoreline to the south/southeast. Over the last 150 years, urban growth of New Orleans and the north shore communities and associated exploitation of natural resources have severely altered the environmental quality of the basin. In 1994, the USGS began a multidisciplinary evaluation of the geology, geomorphology, coastal processes, and environmental quality of the Pontchartrain Basin for use by Federal, state and local officials in coastal management and restoration planning. Existing geological information has been integrated with newly acquired high-resolution seismic profiles (>700 line km), 76 vibracores, and more than 1000 samples for geochemical data to develop a geologic history and record of sediment distribution of the basin (Fig. 1). The Pontchartrain Basin has a complex depositional history controlled by sea-level change. Lake Pontchartrain originated as a result of sedimentary processes. During the late Wisconsin lowstand, the region was entrenched by rivers. A buried incised channel of the ancestral Mississippi River, identified from seismic profiles (see Fig. 2 A-A’), underlies the southern margin of Lake Pontchartrain. The incision is three to four km across, was cut to a depth of 40 m, and can be traced from west to east into Lake Pontchartrain (Fig. 3). Sea-level rise during deglaciation truncated the filled paleochannel and the surrounding region of the Pontchartrain Basin. The ravinement surface is a sharp contact with siderite nodules and has been identified from vibracores as the Pleistocene-Holocene contact. The late Pleistocene unit is typically described as a stiff, olive-gray to light grayish-yellow clay that is highly bioturbated. The burrows are fill with oxidized organics or sand and silt. The structure contour map (Fig. 3) of the Pleistocene surface shows the contact is shallow (near the sediment surface, 2 m below sea level) in northeast Lake Pontchartrain and deeper (20 m below sea level) to the southwest. Cross section B-B’ (Fig. 3) illustrates how the Pleistocene contact crops out along the northeast shore and dips to the southwest. Sea-level rise flooded the “Pontchartrain Embayment” and

Variable deep structure of a midcontinent fault and fold zone from seismic reflection: La Salle deformation belt, Illinois basin

Deformation within the United States midcontinent is frequently expressed as quasilinear zones of faulting and folding, such as the La Salle deformation belt, a northwest-trending series of folds cutting through the center of the Illinois basin. Seismic reflection profiles over the southern La Salle deformation belt reveal the three-dimensional structural style of deformation in the lower Paleozoic section and uppermost Precambrian(?) basement. Individual profiles and structural contour maps show for the first time that the folds of the La Salle deformation belt are underlain at depth by reverse faults that disrupt and offset intrabasement structure, offset the top of interpreted Precambrian basement, and accommodate folding of overlying Paleozoic strata. The folds do not represent development of initial dips by strata deposited over a preexisting basement high. Rather, the structures resemble subdued “Laramide-style” forced folds, in that Paleozoic stratal reflectors appear to be flexed over a fault-bounded basement uplift with the basement-cover contact folded concordantly with overlying strata. For about 40 km along strike, the dominant faults reverse their dip direction, alternating between east and west. Less well expressed antithetic or back thrusts appear to be associated with the dominant faults and could together describe a positive flower structure. The overall trend of this part of the La Salle deformation belt is disrupted by along-strike discontinuities that separate distinct fold culminations. Observations of dual vergence and along-strike discontinuities suggest an original deformation regime possibly involving limited transpression associated with distant late Paleozoic Appalachian-Ouachita mountain building. Moderate-magnitude earthquakes located west of the western flank of the La Salle deformation belt have reverse and strike-slip mechanisms at upper crustal depths, which might be reactivating deep basement faults such as observed in this study. The La Salle deformation belt is not necessarily typical of other well-known major midcontinent fault and fold zones, such as the Nemaha ridge, over which Paleozoic and younger sediments appear to simply be draped.

Physical models of regional fold superposition: the role of competence contrast

Physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference. Rheologic contrast affects strain partitioning between layer-parallel shortening and buckling and affects the development of structural anisotropies parallel to first-generation folds. Laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins (Type 1 interference). In addition to folding, the layers accommodate shortening by layer-parallel thickening, preferentially in the fold culminations. Laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces (Type 2 interference). The less competent layers thicken in the hinges; the more competent layers maintain their initial thickness. In plan view, the F 1) hingelines are refolded in a lobate—cuspate to box style, whose axial traces form conjugate pairs. The models are dynamically scaled to represent folds with km-scale wavelengths. The evolution of surface structures, map patterns at different depths, and equal-area projections synthesized from structure contour maps were used for analysis, comparable to regional analysis common to field studies. Gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds, thereby enhancing the formation of Type 2 interference.

Structural evolution of the northern Los Angeles basin, California, based on growth strata

Subsurface structure contour maps and cross sections show that the northern Los Angeles basin is underlain by a south facing monocline complicated by secondary faults and folds. The monocline and associated structures form a structural high, the northern Los Angeles shelf, which marks the northern margin of the Los Angeles central trough. Analysis of “growth strata,” sediments deposited during structural growth, shows that during the Miocene, the predominant structural style was extension. Growth strata show that at approximately the beginning of the Pliocene (5 Ma), contraction reactivated the Miocene normal faults as reverse faults and formed the monocline, its secondary structures, and the Los Angeles central trough. Lensing‐out of earliest Pliocene (Delmontian Stage) growth sandstones onto the flanks of the monocline and secondary structures (the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines) indicates that these contractional structures all began to form at this time. All of the secondary structures became largely inactive prior to the deposition of upper Pico member of early Pleistocene age (1.2 Ma). However, thick accumulations of growth strata of the upper Pico member of the Fernando Formation within the Los Angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive. The growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline (the Los Angeles fault) can be calculated. In the East Beverly Hills area, the dip of the Los Angeles fault is 61° and at Las Cienegas, the dip is 62°. These are maximum values based on the assumption that the growth strata bed lengths record all shortening. The Pliocene‐Pleistocene average fault slip rates for the Los Angeles fault are 1.1 – 1.3 mm/yr in the East Beverly Hills and 1.3–1.5 mm/yr at Las Cienegas. The resulting Pliocene‐Pleistocene horizontal convergence rates are 0.5 – 0.6 mm/yr and 0.6 – 0.7 mm/yr respectively. The Pliocene‐Pleistocene growth strata show increased dip with age, indicating that the monocline grew by progressive limb rotation rather than by kink‐band migration. Therefore, fault‐bend and fault‐propagation fold models based on kink‐band migration are inadmissible solutions to explain the growth of the monocline. We suggest a basement‐involved shear zone to explain the geometry of the northern Los Angeles shelf.

Structure of the crystalline basement in the West Bengal basin, India, as determined from DSS studies

SUMMARY Deep seismic sounding data were acquired in the West Bengal basin, India, along two profiles: (i) Bishnupur-Palashi-Kandi, along a line about 227 km long in the northsouth direction and (ii) Taki-Arambagh, along a line about 120 km long in the east-west direction. Seismic refraction and wide-angle reflection data were recorded by continuous profiling using two 60-channel digital seismic units (DFS-V) with an 80 m geophone group interval and 4 ms sampling rate. These data were interpreted in order to delineate the basement configuration. The 2-D models of the seismic data both indicate a five-layer velocity structure above the Archaean crystalline basement (5.9-6.2 km s-I). A low-velocity layer (4.0 km s-') is inferred immediately above the basement in the shelf region of the basin corresponding to the Gondwana sediments (Upper Carboniferous to Lower Triassic) below the Rajmahal Traps (Upper Jurassic to Lower Cretaceous) of 4.6 to 4.8 km s-' velocity, which is also confirmed from the nearby well data. The results along the Taki-Arambagh profile and the drilling results at the Jaguli (J-1) well are used to investigate whether Gondwana sediments and the Rajmahal Traps exist in the deep part of the Bengal basin. An additional layer of velocity 5.2-5.3 km s- ', delineated in the Palashi-Kandi profile overlying the basement, may correspond to the Singhbhum group of rocks of the Proterozoic. A structural contour map of the basement prepared from the present results indicates a south-easterly dip of the basement in general. The depth of the basement on the stable shelf of the basin gently increases to about 8 km and dips steeply, plunging to a maximum depth of 14 km in the deep basin. No structural high that can be related to the 'Calcutta gravity high' is found in the basement around the Hooghly River.

Attenuation in Railroad Valley, Nye County, eastern Nevada, and its significance for petroleum exploration in the eastern Great Basin: ABSTRACT

Data from over 100 wells suggest that the boundary between the Grant Range and Railroad Valley is a low angle detachment fault at or close to a metamorphic core complex. The White Pine Detachment, mapped previously in the White Pine and Grant Ranges, is penetrated by many wells because it occurs at a high structural level. A structure contour map based on both surface and subsurface data indicates that the detachment dips uniformly into the subsurface and is not displaced significantly by a high angle fault. In the Grant Range subparallel detachments are developed in the more ductile units. Because the White Pine Detachment is not displaced by high angle faults Railroad Valley cannot have formed by steep normal faulting but instead by attenuation along a series of subparallel detachments. Generation and accumulation of hydrocarbons in Railroad Valley and similar basins may be related to the attenuation that created the basins. For example, the extent to which attenuation juxtaposed potential source rocks and hot infrastructure rocks may influence maturity. Consequently, areas normally considered too shallow for petroleum generation should not necessarily be discounted. Furthermore, failure to find petroleum in a favorable structure may not mean that the basin lacks potentialmore » because, as in Railroad Valley, source rocks may be locally absent as a result of attenuation. Reservoir structures formed by attenuation, such as lenticular stretch structures and localized fracture zones in otherwise impermeable formations, may exist.« less

Successful integration of remote sensing and ground based exploration techniques in an arid environment : R. F. E. Jones & M. Oehlers, Oil & Gas Journal, 93(10), 1995, pp 47–51

Twenty years ago, remote sensing promised to revolutionize exploration; unfortunately, many of the early promises made were unfulfilled and remote sensing tended to drop out of mainstream exploration. Both these extremes are unrealistic, and projects undertaken by Clyde in Yemen illustrate some of the ways remote sensing can become a successful and cost-effective part of an exploration program. Firstly, the remote sensed data, integrated with a minimum of ground control work, provided maps to use in subsequent fieldwork, a surface geology map, and a digital elevation model with its derived topographic contour maps. Secondly, the remote sensed data enabled the authors to create a structural contour map of a near surface horizon at a very low cost per square kilometer. Thirdly, the remote sensed data became a crucial planning tool for seismic operations to optimize data quality and minimize acquisition cost without having to resort to costly and time-consuming swath shooting or similar high-effort techniques. Finally, the surface geological map derived from the image interpretation enabled them to create geological cross sections along the shot seismic lines in a matter of hours without having a field geologist mapping along the lines. Remote sensing can provide highly cost-effective benefits to anmore » exploration program in an arid region, and many of the applications can also be developed for use in areas with vegetation cover.« less

The development of the Gorleben salt dome (northwest Germany) based on quantitative analysis of peripheral sinks

Abstract The structure of the most important stratigraphic horizons in the Gorleben salt dome area, northwest Germany, has been studied using reflection-seismic data and drilling results. Epeirogenic, tectonic and halokinetic movements in the area can be reconstructed on the basis of structure-contour maps, isopach maps, and palaeostructure-contour maps. For this purpose the increase of sediment volume in the rim synclines was calculated in three dimensions and compared with the results of the ‘normal’ subsidence. The additional subsidence in the rim synclines was used for calculation of salt flow by volume. By budgeting the salt movements, the flow rate of salt into the diapir and the salt loss by erosion and leaching were determined. The movement of the salt from the Late Buntsandstein (Röt) until today is depicted in 13 successive stages in three-dimensional diagrams using computer programs.

Upper mantle reflector structure and origin beneath the Scottish Caledonides

The mantle reflector pattern beneath the Scottish and Irish Caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle. Previous investigations interpreted the reflector as a thrust related to the Caledonian orogeny (Silurian‐Early Devonian) or as a normal fault or shear zone active during post‐Caledonian (e.g., Devonian or Permo‐Triassic) extension. This study uses all available deep seismic reflection data to map mantle reflector structure along the Caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology. The distribution of the mantle reflector shows that it is not spatially related to either Caledonian compressional or subsequent basin extensional structures in a simple way. A structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform‐synform pair beneath the West Orkney basin (north of the Scottish mainland) and the Færoe‐Shetland basin west of Shetland, respectively, separated by an inferred zone of structural disruption. A working hypothesis for this study is that these two surfaces may have once been part of a continuous, more or less planar surface that has been offset and deformed by either (or perhaps a combination) of two processes: (1) differential extension of crust that was at least partially coupled to the uppermost mantle beneath the West Orkney and Færoe‐Shetland basins; (2) a broad, approximately orthogonal (i.e., east‐west trending) zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust. Our results also suggest that the mantle reflector structure may be inherited from Neoproterozoic events associated with rifting of the Laurentia‐Baltica supercontinent.

Automated, Quantitative Assessment of Basin History from a Multiwell Analysis

A set of formation tops from an array of wells can be simultaneously analyzed, providing rapid quantitative insights on basin development. The structural configuration of each formation can be defined in terms of relative proportions of a small number of end-member structural states. The relative proportion of each end member as a function of time is a succinct quantitative form of basin history analysis. The procedure works with mappable, as well as unmappable, data. With mappable data, the end members can be displayed as structure contour maps. The end members can also be decomposed into trends and residuals, providing additional details about the structural history of the basin. An example employs formation tops data from 12 Cretaceous formations and 99 wells in the de p basin of Alberta, Canada.

Sedimentary and Tectonic Origin of a Transgressive Surface of Erosion: Viking Formation, Alberta, Canada

The upper Albian Viking Formation in the subsurface of Alberta contains at least four major erosional surfaces, or bounding discontinuities. The highest of these surfaces, Viking Erosion surface 4 (VE4), can be correlated basinwide. There is an easily recognized well log marker in the shales 10-50 m above VE4, known as the Base of Fish Scales (BFS). An isopach map of the BFS-VE4 interval has been constructed using 9987 wells over an area of about 85,800 km 2 . The depositional surface represented by BFS was probably smooth and almost flat during deposition, and the isopach map can be interpreted as showing the topography of the VE4 surface. The surface is stratigraphically deepest in the northeast, and rises southwestward. Its topography is characterized by a broad east-west-trending arch and two linear steps that trend northwest-southeast. The VE4 surface is overlain by a coarse lag a few centimeters to about 5 m thick, with clasts up to 15 cm proximally (SW) and 7 mm distally (NE). Topographic relief on VE4 is at least 40 m. VE4 is interpreted as having formed initially during a major lowering of relative sea level, with modification of the subaerial topography and removal of all evidence of subaerial exposure during the ensuing southwestward marine transgression--thus VE4 is a transgressive surface of erosion. Several well log markers onlap the VE4 surface. They represent coarse to granule sandstones with extremely sharp bases. These tongues of sandstone can be traced up to 20 km basinward from their points of onlap and are interpreted as thin lower shoreface sandstones that formed during minor forced regressions super-imposed on the overall transgression. The initial regression that immediately preceded VE4 implies a fall of relative sea level of at least 40 m, and a northeastward movement of the shoreline of at least 250 km. These two figures imply a basinward gradient of about 0.00016. Structure contour maps of the top of the Mannville Group (Albian) and the top of the Wabamun (Upper Devonian) show smooth unbroken surfaces in the study area, with strikes different from those of the arch and steps. Thus there appear to be no older structural elements that influence Viking trends. The broad east-west-trending arch cannot be identified at Wabamun, Mannville, or older Viking horizons, and had disappeared before the BFS horizon was deposited. It is at least 170 km wide and 6 m high, and formed shortly before the VE4 transgression. Thus there may have been structural control confined to the Viking that influenced changes in relative sea level at the end of Viking deposition.

Mesozoic-Cenozoic sedimentation in the Kane Fracture Zone, western North Atlantic, and uplift history of the Bermuda Rise

Seismic reflection profiles, basement structure-contour maps, and bathymetric data are used to analyze sedimentation patterns over a distance of ∼2,300 km along the Kane Fracture Zone and to interpret uplift history of the Bermuda Rise. Sediments within the fracture valley thin seaward on younger crust and with distance from continental sources. Superposed on this pattern are the effects of local and regional basement structure, variations in surface-water productivity, influence of bottom currents, and local effects of sediment redistribution. Preservation of a thicker and more complete sedimentary record within the Kane fracture valley facilitates identification and eastward correlation of seismic reflections, and we have cross-correlated these eflections to synthesize an along-axis transect of the fracture zone. Stratigraphic correlations of key reflections to Deep Sea Drilling Project drill sites determine the composition and ages of the lithologic sequences. Observed pinch-outs of seismic horizons J 1 , β, and A* on fracture-valley crust extend significantly seaward of previously mapped limits, and the younger pinch-out ages accord better with ages defined by deep-sea drilling. Paleosedimentation patterns in successive stratigraphic intervals document the growth of the continental margin/basin sedimentary prism from Late Jurassic time and the uplift of the Bermuda Rise beginning in middle Eocene time. Middle Eocene cessation of turbidite deposition atop the Rise, as identified by Horizon A t , marks initiation of regional uplift. A seismic discontinuity in turbidite ponds east of the Rise records a probable pulse of uplift during early Oligocene time. Structure of the overlying seismic reflection sequence suggests continued uplift, albeit at decreasing rates, through the late Oligocene and possibly through Miocene time. The anomalous elevation of the Rise subsequently has been maintained, but there is no clear stratigraphic evidence for further growth. Turbidite ponds fill the Kane fracture valley east of the Bermuda Rise and terminate against a prominent structural dam at 55°20′W. These ponds began to fill with turbidites in middle Eocene to early Oligocene time, and the sediments were derived from local sources. Within the past 10-12 m.y., turbidity currents began to enter the ponds through connections with the encroaching Nares Abyssal Plain, and they probably now constitute the dominant sediment source for the fracture valley.

Style and pattern of salt diapirs due to thin-skinned gravitational gliding, Campos and Santos basins, offshore Brazil

Portions of seismic lines and a structure-contour map illustrate the patterns and shapes of salt diapirs and related structures in the Campos and Santos areas, off the Atlantic coast of Brazil. We interpret the structures both kinematically and mechanically, drawing on our experience with similar salt structures worldwide, with the results of recent physical modelling and with geometric restorations in section and in plan. Salt diapirs and related structures have a variety of structural styles, distributed in domains and provinces. Near the coast, there is an upper domain, 100–200 km wide, with a suite of structures that we attribute to horizontal downdip extension: these are salt rollers, in the footwalls of listric normal growth faults; salt walls of triangular cross section, beneath intersecting conjugate normal faults; turtle anticlines; and salt welds. Downslope extension started in the Albian and has continued to the present day. In the Campos area, from simple line balancing, the accumulated downslope displacement is about 100 km. Seawards of this, there is a lower domain, 100–400 km wide, with a different suite of structures, that we attribute to downdip contraction: these are growth folds of various wavelengths, in sedimentary sequences of various thicknesses; asymmetric salt walls, emplaced above reverse faults; deep basins, wedged down between conjugate reverse faults; and salt tongues above thrusts. For Campos, we estimate the total downslope contraction, accumulated since the Albian, to be about 100 km. From the balance between extension and contraction, we infer that the thin-skinned salt tectonics are gravitationally driven and independent of any basement tectonics. The structure-contour map on the top of the salt shows that structural style is variable also along strike. Seismic sections along regional contours indicate differing amounts of strain. On this basis, we distinguish five provinces, separated by NW-SE-trending lines. For the Northern Campos province, we infer radially convergent gliding; for the Campos or Cabo Frio provinces, radially divergent gliding; and for Northern Santos, divergent gliding at a large scale. The pattern in Southern Santos is complicated by right-lateral wrenching against the southern edge of the salt.

Sedimentary basins and crustal thickening

We consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust. Well-known examples are foreland basins; others are ramp basins and buckle basins. We have reproduced various styles of compressional basins in experiments, properly scaled for gravity. A multilayered model lithosphere, with brittle and ductile layers, floats on a model asthenosphere. A computer-driven piston provides shortening and thickening, synchronous with erosion and sedimentation. After a first stage of lithospheric buckling, thrust faults appear, mainly at inflection points. Slip on an isolated reverse fault is accompanied by flexure. Footwall flexure results in a foreland basin and becomes accentuated by sedimentation. Hangingwall flexure is less marked, but may become accentuated by erosion. Motion on a fault leads to hangingwall collapse at the surface. Either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault. Slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin. Simultaneous slip produces a symmetric ramp basin, whereas alternating slip results in a butterfly-shaped basin, with superposed foredeeps. Some well-developed ramp basins become pushed down, until bounding faults meet at the surface and the basin disappears from view. At this stage, the basin depth is equivalent to 15 km or more. Slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline, between two distinct foredeeps. In Central Asia and in Western Europe, Cenozoic crustal thickening is due to continental collision. For Central Asia (Western China, Kyrgyzstan, Uzbekistan, Tajikistan), we have compiled a regional structure-contour map on the base of the Tertiary, as well as 4 regional sections. Foreland basins and ramp basins are numerous and associated with Cenozoic thrusts. Large basins (Tarim, Junggar, Fergana, Tajik) occur around and between mountain ranges, but smaller basins (Issyk-Kul, Naryn) occur within them. In Western Europe, the Alps and Pyrenees are surrounded by foreland basins, ramp basins or intermediate styles. In the Andes and its foreland, Neogene thrusts and compressional basins are due to subduction of oceanic lithosphere. In Colombia, they account for much of the Cordillera Oriental; in NW Argentina, for the Altiplano; in West-Central Argentina, for the Sierras Pampeanas. Compressional basins are also common in other areas of older crustal thickening.