Downthrown Block Research Articles

Effect of Differential Subsidence in Growth-Faulted Regions on E-Log Patterns and Preservation Potential: ABSTRACT

Detailed electric log correlation, supplemented by cores, in the Eocene Wilcox Group and the Oligocene Frio Formation of the Texas Gulf basin contradicts a commonly held notion that changes in log character across growth faults exclusively reflect changes in environment. An invariable consequence of growth faulting is thickening of a depositional unit in the downthrown block, reflecting a greater subsidence rate. The growth ratio (downthrown or upthrown) varies from just over 1:1 to as much as 10:1. Analysis and mapping of log character indicate that the basic unit of both deposition and physical correlation is regressive coarsening-upward sequences. In Wilcox deltas, prodelta shales pass up into delta-front sandstones while in Frio barrier-bar or standplains, shelf and lower-shoreface deposits pass up into upper-shoreface sandstones. Regressive packages grade downdip in environment from delta plain and bay or lagoon to offshore marine. Growth faults had no significant surface expression and did not separate contrasting environments. A marked change in log character (e.g., from smooth to serrated) across a growth fault in a regressive shoreface sequence appears to indicate that the subsidence rate of the downthrown block exceeded a threshold value, enabling preservation of low-energy muddy layers and possibly episodic waning-flow storm deposits that were largely destroyed by fair-weather wave reworking on the upthrown block. This concept has implications both to regional stratigraphy and reservoir properties. Correlative units can abruptly change log character across growth faults, impeding correlation. Sandstones in the downthrown End_Page 251------------------------------ block may contain shale barriers to vertical fluid flow if the threshold subsidence rate was exceeded. End_of_Article - Last_Page 252------------

Sedimentary Basin Thermal Histories Through 40Ar/39Ar Analysis of Detrital Microcline: ABSTRACT

The 40Ar/39Ar age spectrum technique has the demonstrated ability to resolve gradients of 40Ar* within crystals resulting from geologic heatings. A practical application of this observation is the analysis of detrital microcline from sedimentary beds to assess the source age of the feldspar, the time of basin heating, and the thermal intensity of the heating event. This intensity of the heating event is available through knowledge of the amount of 40Ar* lost from the sample and the temperature-dependent rate of argon transport within microcline. Both of these parameters are obtained as a by-product of the age spectrum experiment. Results from a variety of sedimentary basins are encouraging and demonstrate the quality of information available from this technique. 40Ar* gradients in samples from the Basin block of the southern San Joaquin Valley, California, indicate a heating duration of 500,000-1 m.y., which is consistent with the stratigraphy. In conjunction with the present heat-flow data, these results suggest an equilibrium thermal gradient ~7°C/km higher than that presently observed--a consequence of rapid burial. K-spar separates from deep drill holes in the Albuquerque basin, New Mexico, reveal age spectra characteristic of substantial 40Ar* loss. Thermal calculations based on these data indicate a simple conductive history for samples above a present dep h of about 6 km, although hydrothermal activity is evident in deeper material. Microcline separates have been obtained from a Kimmeridgian Sandstone (Tartan field, North Sea basin) that has been displaced about 1 km by a normal fault. Age spectra from these samples reveal minor 40Ar* losses of 4 and 6% from the samples in the upthrown and downthrown blocks, respectively. These loss profiles correspond to temperature histories that agree well with the formation temperatures and burial histories estimated for these wells. The subtle contrast in argon loss between these 2 samples suggests that this structure is relatively recent. End_of_Article - Last_Page 263------------

Salt Tectonics and Oil Accumulation in Campos Basin, Offshore Brazil: ABSTRACT

Salt tectonics were responsible for the development of optimum conditions for accumulation of large hydrocarbon reserves in Campos basin. The salt movements, triggered by basin tilting and differential loading, initially formed low-relief salt pillows that controlled the porous facies of the shallow-water Macae limestones. The continuous evolution of halo-kinetic processes produced growth faults, rollover crests, and faulted anticlines. Upper Cretaceous turbidite sandstones were channelized in the downthrown blocks of growth faults. Salt-collapse, residual, and relief-inversion features were formed in areas where the salt was sufficiently thick. The oil, generated in the Neocomian shales of Lagoa Feia Formation, migrated to upper formations through normal faults associated with the rift phase, stratigraphic conduits such as porous rocks and regional unconformities, and gaps created by salt flowage in the Aptian evaporite section. Oil migration to the post-salt rocks was favored by active growth faults and by the presence of a regional unconformity developed at the Cretaceous-Tertiary boundary and covered by turbidite sheet sands. The oils underwent progressive alteration along their paths to the post-salt reservoirs. Oil-oil and oil-source rock correlations suggest that the hydrocarbons trapped in many diverse plays, although affected by bacterial degradation and water washing, belong to only one oil family. The oil degradation was probably enhanced by meteoric water invasions during Tertiary low stands of sea level in areas close to the paleocoastline. These phenomena resulted in large amounts of oil becoming heavier and enriched in sulfur, resins, asphaltenes, and cycloparaffins. End_of_Article - Last_Page 255------------

Potential Hydrocarbon Traps Along Growth Faults of Rio Grande Rift, New Mexico: ABSTRACT

Major tectonic boundaries along the Rio Grande rift are mostly listric normal faults that controlled late Cenozoic sedimentation in the rift. Potential hydrocarbon source and reservoir rocks occur in Cretaceous, Jurassic, and Pennsylvanian units beneath presumably barren upper Cenozoic continental sediments. Largest of the grabens and half-grabens comprising the rift is the Albuquerque basin, which was explored during the last decade with several deep tests. Upper Cenozoic strata generally dip gently toward the basin center except near bounding faults where reverse drag has beds on the downthrown block dipping toward the growth faults marking the basin margins. Most bounding faults are covered with surficial sediments, but at a few localities with good, deep exposures, End_Page 317------------------------------ stratigraphically deeper beds have undergone more reverse drag than higher strata. Thus, these reversals of dip adjacent to bounding faults may provide structural traps along the margins of the Albuquerque basin. Recent deep wells were drilled in the central part of the Albuquerque basin on structural highs, probably intragraben horsts. Most of these recent tests were tight holes, but encouraging hydrocarbon shows were encountered. The margins of the Albuquerque basin have not been tested yet, but there may be numerous traps where reverse drag is present. These structures could have trapped hydrocarbons where potential reservoir rocks (mainly sandstones) and source beds (mainly organic-rich marine shales of the Cretaceous and Pennsylvanian, and organic-rich laminated limestone of the Jurassic Todilto Formation) occur. End_of_Article - Last_Page 318------------

Hydropressure Tongues Within Regionally Geopressured Lower Tuscaloosa Sandstone, Tuscaloosa Trend, Louisiana: ABSTRACT

ABSTRACT A regional study of the Tuscaloosa Formation in Louisiana, undertaken to assess geopressured-geothermal potential, revealed lobate, downdip extensions of the hydropressured zone in lower Tuscaloosa massive sandstone facies below the regional top of geopressure. Normal pressure zones within geopressured section were identified by drilling mud weights less than 13 pounds per gallon on electric logs of massive lower Tuscaloosa sandstone, and cross sections demonstrated updip continuity of these zones with the regional hydropressured zone. These hydropressure are permitted by the anomalously high permeabilities reported from the deep Tuscaloosa trend which have been attributed to both primary and secondary porosity. The hydropressure tongues correspond with lobes of thick net sandstone, principally in Pointe Coupee, East Feliciana, East Baton Rouge, and Livingston parishes in the central Tuscaloosa trend. Limited control suggests at least one hydropressure tonge in the Chandeleur Sound area to the east. Dimensions of hydropressure tongues range up to 17 miles (27 kilometers) parallel to strike and 11 miles (17 kilometers) oblique to strike. In many places, tongues are terminated downdip by faults, which, by acting as pressure seals, prevent the tongues from extending to the downdip edge of the massive sandstone in the expanded sections of the downthrown blocks. The areal extent of geopressured Tuscaloosa sandstone is controlled updip by these fault zones, and downdip by pinch-out of the sandstone units basinward. Local hydropressure tongues diminish the geopressured-geothermal potential of the Tuscaloosa trend, but show no discernible relation to gas-productive areas.

Geology of Subsurface Eocene Cockfield Formation, Southern Allen Parish, Louisiana: ABSTRACT

A regional study of the subsurface Cockfield Formation (Eocene), southern Allen Parish, Louisiana, reveals that it was deposited as a result of the action of longshore currents and deltaic distributary channels within a nearshore marine environment. The juxtaposing of reservoir quality sands and marine shales within this environment created favorable conditions for the formation and entrapment of hydrocarbons. The Cockfield trend, a major oil and gas producing belt through the study region, appears to be composed mainly of fields that have combination stratigraphic-structural style entrapments. Localized sand pinch-outs and permeability barriers on rollover anticlines that developed on the downthrown sides of faults are responsible for most accumulations. Faults evident y controlled sand deposition in some parts of the study area, resulting in thicker deposits of sand on their downthrown blocks. Thickening of section within the Cockfield is present in isolated areas, and is confined to relatively short stratigraphic intervals. End_of_Article - Last_Page 1678------------

Thermal History by Fission-Track Dating, Tejon Oil Field Area, California: ABSTRACT

Data that have been obtained from deep drill holes in several areas where the approximate duration of heating events is known suggest that fission tracks in apatite and zircon are totally annealed when the grains are heated at temperatures of 135 to 105°C and 200 to 175°C, respectively, over periods of 106 to 108 years. Annealing can pose problems for determining the primary ages of samples, but if offers a powerful method for studies of their thermal and tectonic history. This paper explores the application of this method to determining the thermal history of sedimentary basins using detrital grains separated from drill-hole samples. Detrital apatite and zircon have been separated from Eocene to Miocene sandstone recovered from deep drill holes in the southern San Joaquin Valley of California (Tejon oil field area). The Tejon area is divided by the seismically active White Wolf fault. Fission-track data show that apatite in the downthrown block immediately northwest of the fault is totally annealed at a maximum paleotemperature (reconstructed from laumontite geothermometry) of 135 to 140°C, suggesting heating of 106 years' duration. The higher paleotemperature (> 150°C) indicated for total annealing of apatite in the upthrown block shows that these samples could have been at the suggested maximum paleotemperatures for no more than 105 years. The relative short duration of heating over the whole Tejon area is supported by the lack of annealing in zircon, even at paleotemperatures as high as 179°C. These conclusions are consistent with the thermal history suggested by laumontite crystallization viewed with stratigraphic and structural evidence. End_of_Article - Last_Page 906------------

Geologic Setting and Oil and Gas Potential of Eastern United States Continental Margin North of Cape Hatteras: ABSTRACT

Two sedimentary basins, the Georges Bank basin off New England and the Baltimore Canyon Trough off the Middle Atlantic states, have been examined for geologic setting and hydrocarbon potential. Georges Bank basin, a complex-shaped trough, is on a block-faulted basement of igneous, metamorphic, and sedimentary rocks. The deepest part of the basin (deeper than 8 km) and the oldest sediments are restricted to south-central Georges Bank. Toward the northeast and southwest, the sedimentary section thins to less than 2 km over the Yarmouth Arch-LeHave platform and the Long Island platform. The only deep wells in the area are the COST G-1 and G-2; data from these wells will not be released until 60 days after the first oil and gas lease sale. Seismic correlation with the Shell M hawk B-93 well on the Scotian Shelf indicates that most of the sedimentary rocks in the Georges Bank basin are Jurassic and older. Jurassic sandstone and limestone units serve as potential reservoir rocks. Potential hydrocarbon traps may occur on structural highs associated with draping of Jurassic and basal Lower Cretaceous strata over basement blocks. The Baltimore Canyon Trough is an elongated northeast-trending basin that contains at least 14 km of Jurassic and younger marine and nonmarine sedimentary rocks. Lithologic and stratigraphic data from the COST B-2 and B-3 wells indicate that Lower Cretaceous and Jurassic rocks are predominantly nonmarine to shallow-marine sandstone and shale. Analyses of organic carbon and identification of low thermal maturity suggest that gas rather than oil will be produced. Nineteen wildcat wells have been completed; three are significant natural-gas discoveries. The largest discovery is probably associated with a large rollover trap on the downthrown block of a Cretaceous and Jurassic growth fault. Potential hydrocarbon traps in carbonate rocks beneath the present continental slope have not yet b en explored. End_of_Article - Last_Page 746------------

Piper Oil Field: ABSTRACT

The Piper field lies in UK block 15/17 near the eastern part of the Moray Firth Basin, 125 mi northeast of Aberdeen, Scotland. The field was discovered in December 1972. Five appraisal wells and one exploratory well were drilled in 1973. A steel platform with 36 well slots and space for two drilling rigs was centrally located over the field in 474 ft of water in June 1975, and readied for production drilling by October 1976. Production is from the Upper Jurassic Piper Sandstone, a high-energy, marine sandstone with gross thickness, 177 to 453 ft (56 to 138 m); net sand, 132 to 378 ft (40 to 115 m); net oil sand, 132 to 296 ft (40 to 90 m); average net oil sand, 160 ft (49 m); average porosity 26%; permeabilities 200 to 1,200 md in lower energy bioturbated sands, 2,000 to 0,000 md in higher energy sands. The reservoir is sealed by Kimmeridge shale over most of the field and by Upper Cretaceous marlstones along some fault scarps. The main area of the field, a gently folded fault block dipping 5° northeast and a downthrown northwest-southeast fault block, covers 6,300 acres (2,550 ha.), has a common oil-water contact at 8,512 ft (2,594 m) subsea, and a gross reservoir column 1,300 ft (396 m), which is 7,200 ft (2,195 m) subsea to the oil-water contact. A small accumulation on a parallel fault block on the southwest has a separate oil-water contact at 9,199 ft (2,804 m) subsea. The P1 production well spudded October 10, 1976, established commercial production December 7, 1976 at more than 30,000 B/D, restricted by 5½-in. tubing. The P7 well complet d in April 1977 produced more than 50,000 BOPD, restricted by 7-in. tubing. Twenty-four wells have been drilled, four as water injectors to support an active water drive. Production is 300,000 BOPD and recoverable reserves are estimated to be 700 million bbl. Extensive use of seismic data and excellent cooperation by partners in the consortium and the United Kingdom regulatory bodies allowed maximum use of production and new well data to improve subsequent development. End_of_Article - Last_Page 489------------

Sedimentation and Trapping Mechanism in Upper Miocene Stevens and Older Turbidite Fans of Southeastern San Joaquin Valley, California

Most of the stratigraphic oil production in the southern San Joaquin Valley comes from the sandstones of the upper Miocene turbidites. Updip, time-equivalent rocks of the shallow-marine environment are second in productivity. Third in importance are the deep-marine, fractured, siliceous shales deposited beyond distal margins of the fan complex. Many underexplored areas remain in the San Joaquin Valley despite many years of field development and wildcat drilling. The Miocene of the Bakersfield arch is representative of the classic arcuate type of turbidite deposits characteristic of modern submarine fans. Further paleogradient of the fans has been accentuated and not reversed through geologic time; stratigraphic oil fields are numerous. As in most California deep-marine troughs, the turbidite facies most easily reached by the drill, and in which most of the oil reserves have been developed, are in the mobile zone of basin-margin wedging. Two additional trapping aspects related primarily to depositional stratigraphy within the midfan facies are (1) intrabasin contemporaneous faults (analogous to Gulf Coast growth faults) resulting in expanded section and reverse drag on the downthrown block, and (2) compaction anticlines formed by post-sediment ry accommodation over submarine channel sands resulting in structural inversion with depth. South of the Bakersfield arch is an earlier (pre-late Mohnian) turbidite basin, virtually unexplored and referred to informally as the Maricopa subbasin, where drilling depths to potential Miocene pays range from 12,000 to 20,000 ft (3,600 to 6,000 m). Basic stratigraphic relations recognized in the Bakersfield arch may be applied in the subbasin to develop new hydrocarbon reserves.

Holocene faulting in the western Grand Canyon, Arizona

The Toroweap and Hurricane faults and a subsidiary fault in the western Grand Canyon exhibit evidence of Holocene movement. This evidence includes scarps in alluvium and sediments ponded against a fault on the downthrown block. These displacements are the latest in a well-exposed record of recurrent movements along the major faults in the region.

Submarine Debris Flow Deposits in an Extensional Setting--Upper Devonian of Western Morocco

ABSTRACT A series of uppermost Devonian-Tournaisian submarine debris flows in the northwestern Meseta of Morocco near Rabat were triggered by abrupt, robust faulting, accumulating in downthrown blocks and adjacent to their upthrown sources. These flows possess features such as a rigid plug and snout predicted by the rheological models proposed by Johnson (1965, 1970). They are adjacent to and probably generated turbidity deposits. The occurrence of the debris flow deposits at the close of the Devonian marks the initiation of the only major Paleozoic orogenesis in Morocco: the Hercynian Orogeny.

Experimental folding of rocks under confining pressure: Part III. Faulted drape folds in multilithologic layered specimens

Drape folds and reverse faults are produced experimentally at confining pressures to 2.0 kb and shortening rates of 10 −3 to 10 −6 sec −1 by displacing a block of brittle sandstone (2 by 3 by 12.6 cm) along a lubricated saw cut into one to five initially intact layers (0.2 to 1.0 cm thick and as much as 12.6 cm long) of limestone, sandstone, and rock salt. The saw cut is inclined at from 30° to 90° to the layer boundary. The deformation is characterized from studies of fault geometry, displacements and sequence, bedding-plane slip, layer-thickness changes, and the development of fault gouge, fold hinges, microfractures, calcite twin lamellae, and dimensional orientations of grains (in the rock salt). Stress trajectories are inferred from faults, microfractures, and calcite twin lamellae, and strains are calculated from layer-thickness changes and from calcite twin lamellae. Reverse faults curving concave downward propagate upward from the saw cut in the forcing block. With increasing displacement along the precut faults, the faults and associated gouge zones in the layer steepen and become progressively younger toward the upthrown block as displacement increases. The faults are preceded by swarms of extension microfractures that form throughout the deformation and that are the best clues to the stress trajectories. The downthrown layers are thickened by uniform flow and by repetition caused by the faulting. They are displaced away from the faults by bedding-plane slip. Trajectories of the greatest principal compressive stress (σ 1 ) are inclined at low angles to the layer boundaries near the faults and become perpendicular to these boundaries away from the fault. The maximum deformation of the downthrown block occurs when the saw cut is inclined at about 65° to the layering. The upthrown layers are all extended parallel to the layering and perpendicular to the fold axes, as indicated by extension fractures, thinned layers, and calcite twin lamellae and the development of graben zones and low-angle normal faults that are conjugate to the reverse faults. The layers are translated by bedding-plane slip away from the fault zone. Trajectories of σ 1 are inclined from 45° to 90° to the layering. The fabric data are internally consistent, and inferred stresses are in good agreement with those calculated from an elastic solution of the experimental boundary conditions. Principal strains calculated from calcite twin lamellae are within an average of 0.01 of those calculated from layer-thickness changes and permit clear resolution of individual events in domains of superposed deformations.

Recent activity of the Wasatch fault, Northwestern Utah U.S.A.

The Wasatch fault trends north—south for a total length of 370 km, between Gunnison, Utah and Malad City, Idaho. The fault is in the eastern Basin-and-Range Province, adjacent to the Middle Rocky Mountains and the Colorado Plateau. The most recent fault displacements indicate normal slip, with the western block faulted down relative to the Wasatch Mountains. The Wasatch fault is within the Intermountain Seismic Belt of the western U.S.A.; however, it has been associated with only minor seismicity within historic time (140 yrs). There is no conclusive evidence of significant historic surface faulting along the Wasatch, although there is a concentrated 50–100-m-wide zone of damaged streets, curbs, houses, and buildings where the fault trends through Salt Lake City. This damage may be interpreted as evidence of tectonic creep; however, other possible interpretations include fault-controlled damage related to slope instability along the steep scarp or fault-controlled damage related to groundwater. Precise geodetic leveling across the Salt Lake City segments suggests that tectonic creep may be occurring, although the time span (7 yrs) has not been long enough to establish a consistent trend. Geomorphic evidence and tree-ring dating suggest that the most recent events of appreciable surface faulting are no older than a few hundred years. Fresh scarps in geologically young glacial and post-glacial deposits are 13–20 m high. Surface manifestations of active faulting along the Wasatch include multiple-fault scarplets in geologically young materials, graben on the downthrown block, backward tilting of the downthrown block near the fault, triangular facets along the fault scarp, lines of springs, and slickensided fault surfaces. In addition to the geologic evidence of active faulting, there are numerous landslides and large debris flows that have been faulted subsequent to their deposition, as linear scarps and other geomorphic features indicative of surface faulting cross them. Our current geologic research on the activity of the Wasatch fault is directed toward determining the recent history of surface displacements and the relationship of the fault with large-magnitude earthquakes, which is based upon the faulted sequence of Pleistocene Lake Bonneville beds and glacial deposits along the Wasatch Front.

Late Jurassic Contemporaneous Faults in North Louisiana and South Arkansas

Southernmost Arkansas and northernmost Louisiana are dominated by two regional positive features, the Sabine and Monroe uplifts. Between them (and extending at least partly across the latter) is a system of normal faults along which movement took place during deposition of Late Jurassic clastics. North of the study area is the South Arkansas graben system, part of the great peripheral fault system which essentially marks the updip (north) limit of thick salt and other Jurassic and Lower Cretaceous sediments of the Gulf coastal province. Still farther north is a series of grabens or half-grabens in which were deposited redbeds and other detritus of probable Triassic age which were intruded by diabase. Far south of the study area are the systems of Tertiary contemporaneous aults of the Texas and Louisiana Gulf coast. Jurassic contemporaneous faults generally parallel regional structural and depositional strike and are slightly younger basinward (south). Most are downthrown toward the basin, but locally, and regionally in one place, compensating down-to-north faults are present. Beds on downthrown sides are greatly thickened, correlation of individual units is usually impossible, and throws increase with depth. Because the downthrown blocks are tilted, sediments are thickest adjacent to the fault. Fault planes are believed to be curved, and although they are of high angle (60-70°) near the top, they may flatten with depth. As a result of this flattening, together with flowage of underlying Jurassic salt away from downthrown blocks, faults are not known to extend below the salt. They do not cut beds younger than Jurassic and die out upward in a conformable section. Studies of better known (peripheral-graben and Tertiary) fault systems of the Gulf coastal province are plentiful and offer a variety of explanations regarding causes of contemporaneous faults. These are briefly reviewed in the text and include basement faulting, flexures, differential compaction, gravity creep, and salt flowage. It is concluded that gravity creep of the Late Jurassic sedimentary blanket down the gently dipping slope between the coastal shelf on the north and the basin on the south initiated faulting in the study area. Important facies changes across the area of contemporaneous faults suggest that active differential subsidence facilitated movement, as did the presence of an underlying plastic medium (salt). Sediment loading on downthrown blocks caused salt flowage aw y from them; movement was maintained and increased by this process, but ceased when the supply of salt was exhausted.

Grabens on Anticlines in Gulf Coastal Plain, and Thinning of Sedimentary Section in Downthrown Fault Block: GEOLOGIC NOTES

Grabens are present on many anticlines in the Gulf coastal plain of the southern United States. They are the results of stress fields caused by gravitational forces. It apparently is geometrically impossible for some types of grabens to develop unless there is plastic deformation of sediments within them. Postdepositional thinning of sediments in the downthrown fault block, as observed in model studies and in field examples, seems to allow grabens to develop without creating volume problems. Thinning in the downthrown block is in sharp contrast to the thickening in the downthrown block commonly observed in association with Gulf Coast faults.

Some discriminatory characteristics of earthquakes near the Kariba, Kremasta, and Koyna artificial lakes

abstractThe behavior of earthquakes near the artificial lakes at Kariba, Kremasta, and Koyna, where earthquakes of magnitude exceeding 6 have occurred, is examined. Foreshock-aftershock patterns of these earthquake sequences correspond with Mogi's type II model, whereas the normal earthquakes of these regions belong to type I. Three similar relations could be fitted in the time distribution of aftershocks of the main earthquakes. Quite contrary to normal earthquakes, foreshock b values are found to be comparable with the aftershock b values in the frequency-magnitude relations. Focal mechanisms of the largest earthquakes of these sequences have been determined and compared. Dip-slip components of the motion are such that the lakes are situated on the downthrown blocks. These regions are characterized by a volcanic past and the presence of rocks such as limestones and red boles which are easily affected by water. These findings are useful in distinguishing the reservoir-associated earthquakes from normal earthquakes and suggest that the artificial lakes are responsible for changing the mechanical properties of the strata and releasing the accumulated strains.

Iceland and mid-oceanic ridges

Magnetic anomalies over Iceland, measured by Serson et al. (1968), are similar in shape and amplitude to those found over mid-oceanic ridges in general and over Reykjanes Ridge in particular. However, the geology of Iceland does not favour the simple model of sea floor spreading as formulated by Vine and Matthews. The Brunhes period volcanism can neither in place nor in time be related to an opening process of the Central Graben, which actually is a downthrown block and not an opening rift. Furthermore, the structure of Iceland is not symmetric with respect to the Central Graben. The geology of the Central Graben of Iceland does support a model proposed by Thorleifur Einarsson in 1967. In this model elongate ridges of pillow lavas are thought to have piled up on top of parallel volcanic fissures. The actual spreading is negligible. The fissures have been opening at random over a width of about 120 km, and no definite time scale can be set up for the associated magnetic anomalies. This conflict between Icelandic geology and the current views on sea floor spreading, can be evaded by supposing that the mere circumstance that Iceland is an island obscures a spreading process underneath. One might also postulate that Iceland nevertheless should stand as an example of a mid-oceanic ridge which implies that our ideas on sea floor spreading should be thoroughly revised.

Salt Ridge-Salt Dome Origins in Gulf Coast Area: ABSTRACT

A working hypothesis of salt-ridge, salt-dome formation in the Louisiana Tertiary was formulated in 1957 to explain the observed phenomena along the Caillou Island, Timbalier Bay, Bay Marchand, West Delta Block 30, Venice, and Delta Duck trend: (1) regional down-to-the-north faults of very large stratigraphic throw on the southerly sectors of the domes; (2) a salt, deep-water facies shale ridge connecting Caillou Island, Timbalier Bay, and Bay Marchand at depth; (3) shallow occurrence of older deep-water shales beneath a condensed section on the upthrown sides of down-to-the-north faults; and (4) thick normal facies sections on the north flanks of the domes. Application of results of seismic, bathymetric, and core data from the Gulf of Mexico continental slope gathered and interpreted by Lamont-Texas A&M and Shell groups shows that these phenomena are the results of wavelike salt ridge formation basinward from a prograding shelf edge due to differential loading. Such ridges are present on the continental slope and the major down-to-the-north fault planes are postulated to coincide with the northern faces. The salt ridge grows asymmetrically in cross section, lifting the thinly covered upthrown block and allowing a thicker ponding of sediments on the landward downthrown block. Deep-water shales covering the early ridge roots join the diapiric action of the salt and are confined to the upthrown block until the salt breaks upward through the northward-dipping fault plane. As the salt is further buried by the prograding shelf sediments, it may leave the linear control of the fault and form individual domes with the commonly recognized pattern of shallow salt dome faulting and oil and gas accumulation. End_of_Article - Last_Page 356------------

GRAVITY ANOMALIES OF A FAULT CUTTING A SERIES OF BEDS

The gravity anomaly due to a single horizontal semi‐infinite block terminated by a vertical or dipping fault has been discussed by several authors previously. Geldart, Gill, and Sharma (1966) gave a new and simple expression for calculating the gravity anomaly due to a block cut by an inclined fault at an arbitrary angle and used this expression to obtain the gravity anomaly due to a fault cutting a single bed. In their derivation the effects of both the upthrown and the downthrown blocks were taken into consideration. It is, however, only in the unusual situation that faults cut a single bed of uniform density. More often, faults cut a series of beds of different densities and thicknesses. If the densities and the thicknesses of the various beds differ greatly, an interpretation based upon replacement of the series of beds by a single bed of uniform density may be highly erroneous. Starting from the single block expression given by Geldart et al, an expression can be derived giving the gravity anomaly due to a fault cutting a series of beds having different densities and thicknesses.