Salt Provinces Research Articles

Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)

The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern (nonvolcanic) segment of the margin was studied by a 490‐km‐long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the wide‐angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles. The continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5.7–6.9 km/s. Crustal thinning down to 3 km occurs in a 180‐km‐wide zone and the sediment cover in this area is up to 15 km thick. Farther seaward, a 150‐km‐wide transition zone is observed with a 5‐km‐thick lower layer (7.2–7.6 km/s) interpreted as partially serpentinized mantle. At the landward end, this layer is overlain by highly altered continental crust (5.4 km/s) extending up to the seaward limit of the Jurassic salt province. Farther seaward, the upper layer is interpreted as exhumed and highly serpentinized mantle (5.1 km/s) separated from the lower layer by subhorizontal reflectivity, which probably represents a serpentinization front. Oceanic crustal thickness is 4 km with layer 2 velocities of 4.6–5.0 km/s. Layer 3 velocities of 6.4–6.55 km/s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile. This reduced magma production might be related to the proximity of the Newfoundland transform margin.

Neoproterozoic allochthonous salt tectonics during the Lufilian orogeny in the Katangan Copperbelt, central Africa

The Neoproterozoic Katangan rocks in the Copperbelt of Shaba (Democratic Republic of the Congo) and Zambia contain the world's largest concentration of stratiform copper-cobalt ores. We propose that during the Lufilian orogeny, the Katangan basin was radically transformed first by extrusion of allochthonous evaporites, then by orogenic shortening. Salt tectonics plausibly explains spectacular breccias underlying 25,000 km2 and containing gigaclasts up to 10 km long. The brecciated Shaban Roan Supergroup—the lowermost Katangan unit—forms regional detachments and diapirs. The former existence of Roan evaporites is indicated by sabkha facies, crystals, and pseudomorphs of gypsum and anhydrite, stratigraphic gaps underlain by collapse breccias, chloride inclusions in ores, hydrothermally propylitized ore hosts, and saline springs. Salt tectonics began during deposition of the Roan Supergroup between 1050 and 950 Ma. In mid-Roan time, small walls and extrusions of evaporite-gigabreccia began to be emplaced. In early Kundelungu time (940–850 Ma), evaporitic diapirs enlarged. Lufilian deformation began between 850 and 650 Ma by laterally squeezing diapirs to form salt welds. Then, a large sheet of commingled Roan evaporite and carbonate-dominated sediments and ores was extruded northward by ∼65 km. Extrusion was fast enough to blanket a uniform preorogenic footwall unit without overriding any synorogenic deposits. Continued shortening then emplaced large thrust sheets, lubricated by the preexisting salt-sediment extrusive sheet. Restoration suggests that the Lufilian foreland in this area shortened from 193 km to 85 km. The depositional northern edge of the evaporite basin controlled the shape of the Outer Lufilian Arc. We infer three original salt provinces. The large-scale salt tectonics inferred here has implications for ore genesis and may yet be recognized in other Precambrian basins.

ABSTRACT: The Scotian Salt Province, an Unexplored Deepwater Salt Basin

ABSTRACT: The Scotian Salt Province, an Unexplored Deepwater Salt Basin

3-D Visualization and Isochore Analysis of Extensional Diapirs Overprinted by Compression

Palinspastic restorations elucidate geologic and hydrocarbon-migration histories in salt provinces; however, salt dissolution and salt flow in and out of the section plane make it difficult to determine the shape of salt bodies before deformation, which hampers accurate restorations. Three-dimensional computer visualization of a physical model and analysis of isochores provide clues to the original shape of allochthonous salt bodies that were emplaced during extension and later compressed.

Isochores and 3-D visualization of rising and falling slat diapirs

Diapir fall, which was predicted by physical models, has been identified in salt provinces, such as the South Atlantic margins, the North Sea, and the Paradox Basin (Colorado–Utah). However the 3-D geometry of falling diapirs and their country rock is still poorly understood. 3-D visualization and isochore patterns from a physical model help elucidate this geometry. The model initially comprised a unit of viscous silicone overlain by a prekinematic sand unit. Sand units representing brittle sediments were deposited episodically during gravity gliding and spreading. Regional extension triggered and eventually widened salt walls, causing them to sag. The 3-D visualization shows that regional hydrocarbon migration, which tends to be seaward during diapir rise and landward during diapir fall, can potentially be orthogonal to local migration along grabens at soft-linked zones of relay ramps. Furthermore, anticlinal culminations may form (1) in horsts that bend along strike and (2) adjoining the fork of Y-shaped salt walls. Sequential isochore maps of the overburden show how patterns of sedimentation, deformation, and underlying salt thickness changed through time. Isochores of prekinematic units record only strain: thinned belts record early extension. In contrast, isochores of synkinematic units record mostly thickness variations due to deposition on actively deforming topography. Isochores above sagging diapirs identify the thickest part of crestal depocenters, where the most rapid sagging occurred in regions of maximum extension near the unbuttressed downdip part of the gravity-spreading system. Additionally, asymmetric isochore patterns may reveal underlying half-grabens or tilted symmetric grabens. In relay systems, overlying isochores may indicate which part of a salt wall rose to compensate for sagging elsewhere in the relay.

Abstract: Applications of Gravity and Magnetic Data to Oil Exploration in Salt Provinces

Abstract: Applications of Gravity and Magnetic Data to Oil Exploration in Salt Provinces

Abstract: Structural Overview of the Deepwater to Ultra Deepwater Northern Gulf of Mexico Area based on a Recent Regional Seismic Program

This interpretation project is based on an extensive 2-D seismic program (known in the industry as Phase 45) that consists of 60,000 line miles, covering the central and western parts of the deepwater and ultra deepwater northern Gulf of Mexico region. This paper concentrates on the central part of the project area (Garden Banks, Keathley Canyon, Green Canyon and Walker Ridge) covering 38,000 mi 2 and utilizing a 4 by 4 mile recently acquired seismic grid. Four horizons have been interpreted: top and base salt along with the Discoaster surculus (DS - approximate top Pliocene) and Discoaster berggrenii (MDB - approximate top Miocene) horizons. Abundant well data, in the form of electric logs, micropaleontology and velocity data, were available in the northern Green Canyon and Garden Banks regions. All interpretation was performed on a workstation. The mapping of these horizons shows how salt geometry changes moving from north to south. Around the continental shelf break a series of coalesced but discrete allochthonous sheets with significant intersalt basins occur. To the south a virtually continuous salt sheet, with numerous suprasalt basins, develops out to the edge of the present day northern Gulf of Mexico salt province, marked, in part, by the Sigsbee Escarpment. In these distal parts of the salt province, particularly in the Walker Ridge area, there is comparatively little sedimentary cover above the salt except where suprasalt basins have locally depressed salt. These basins have both symmetric and asymmetric geometries. The Mio-Pleistocene section is oilen railed above the allochthonous salt in these basins and seems to override deeper Mio-Pleistocene identified outboard of, and carried beneath, salt.

Influence of Active Subsalt Normal Faults on the Growth and Location of Suprasalt Structures : ABSTRACT

ABSTRACT The location and deformation style of salt and overburden structures that form during subsalt normal faulting vary greatly between and within salt provinces. In some areas, diapirs and overburden faults appear to have formed independently of the location of the underlying basement faults. Elsewhere, diapirs and faults formed systematically above or near the subsalt faults, or the overburden was force-folded above the subsalt fault into a monocline whose shape reflects the offset along the underlying basement blocks. We used dynamically scaled physical experiments and conceptual models to (1) understand how much subsalt faulting controls the formation and localization of structures in the salt and overburden, and (2) evaluate how this control is affected by changing selected parameters, such as salt thickness, strain rate, or sedimentation rate. Both experimental and conceptual models predict that initially thick salt or slow subsalt faulting favors decoupling of the overburden from the subsalt basement. Instead of transmitting stresses and strains upward from basement, through salt, and into the overburden, the salt layer diffuses localized basement slip by laterally flowing from the footwall to the hangingwall. Thus, the salt layer thins above the basement footwall and thickens above the hangingwall. Once the source layer above the footwall has been thinned or depleted, salt can no longer flow fast enough to entirely accommodate subsalt faulting. Hence the fault propagates upward as a broad forced fold whose upper hinge is affected by crestal normal faults, along which further extension is accommodated. Experimental and conceptual models indicate that initially thick salt, slow or moderately fast subsalt faulting, and slow or absent syntectonic sedimentation promote decoupled deformation. Conversely, initially thin salt, very rapid subsalt faulting, rapid syntectonic sedimentation, or all three promote coupled deformation and formation of diapirs or overburden faulting above or near the subsalt fault.

Three-Dimensional Visualization of Salt Walls and Associated Fault Systems

Abundant three-dimensional (3-D) seismic data from salt provinces have been acquired over the past two decades. These data have been interpreted partly using concepts based on analysis of two-dimensional (2-D) sections through physical and numerical models. Three-dimensional computer visualization of physical models is a recent development that exploits the full potential of model data by displaying even highly irregular geological structures such as convolute salt contacts, strata disrupted by salt tectonics, discontinuous faults, and bedding traces on fault surfaces. We used computer visualization techniques to display and analyze a physical model simulating salt-related structures produced during gravity spreading and gliding. The visualization shows, in realistic 3-D detail, that (1) structures change seaward from tall steep-sided diapirs to squat salt rollers; (2) salt walls change markedly along strike and form branches and relays; (3) subsidence of underlying salt ridges produces irregular turtle-structure closures that tend to have multiple seals; (4) links that laterally connect reservoirs in adjacent rafts are most common in upper stratigraphic ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received November 1, 1995; revised manuscript received March 4, 1996; final acceptance August 12, 1996. 2Bureau of Economic Geology, University of Texas at Austin, Austin, Texas 78713. Additional full-resolution color 3-D images and digital animations related to this paper are available free on the Internet at http://www.utexas.edu/ research/beg/mmedia/AGL96-MM-004. We wrote this paper while testing an unreleased version of the program EarthVision® and working with its creators at Dynamic Graphics Inc. to enhance the technology for visualizing complex brittle and ductile structures. Dynamic Graphics Inc., especially Raphael Mayoraz, provided invaluable technical support. Shing-Tzong Lin provided unpublished cross sections of the physical model that he created under our supervision. Hongxing Ge digitized the model data. The research was funded by the following companies in the Applied Geodynamics Laboratory consortium: Agip, Amoco, Anadarko, ARCO/Vastar, British Petroleum, Chevron, Conoco/Dupont, Elf Aquitaine, Exxon, Louisiana Land and Exploration, Marathon, Mobil, Petrobras, Phillips, Statoil, Texaco, and Total. K. Prewitt and T. Weaver edited the figures. T. Hentz edited the paper. J. Farre, R. Ray, and B. Blake refereed the paper, which is published by permission of the director of the Bureau of Economic Geology, University of Texas at Austin.

PROBABLE CRETACEOUS-TO-RECENT RIFTING IN THE GULF OF MEXICO BASIN An answer to Callovian salt deformation and distribution problems? Part 1

Abundant evidence from the Northern Gulf of Mexico Basin suggests that late Early Cretaceous to Recent sea-floor spreading is the principal factor in the formation of the structural components of this basin. This would classify the Gulf of Mexico Basin as an active tectonic basin as opposed to the more accepted passive basin status. The following two-part paper presents a model for this lithospheric plate adjustment together with the geological evidence to support this movement. Part I discusses the rift model and supporting evidence. Some of this evidence is: a proposed triple junction rift zone radiating from a large dome centered in the DeSoto Canyon area; the division of the Gulf of Mexico Basin Jurassic salt province into three salt basins (the Interior Exterior and Challenger Basins); the linearity and structurally-positive attitude of the Cretaceous Shelf Edge; an excessive Exterior Basin salt mass that thickens towards the Sigsbee escarpment; large areas of non-diapiric salt found within the Exterior Basin; an extensional fault zone found underlying the Abyssal Plain; and the absence of a Late Mesosoic section from the Exterior Basin a section that would be needed to cover the salt until it was overlain by younger Tertiary sediments.

Rheological and tectonic modeling of salt provinces : Weijermars, R; Jackson, M P A; Vendeville, B Tectonophysics V217, N1/2, Jan 1993, P143–174

Rheological and tectonic modeling of salt provinces : Weijermars, R; Jackson, M P A; Vendeville, B Tectonophysics V217, N1/2, Jan 1993, P143–174

Rheological and tectonic modeling of salt provinces

Abstract Technological improvements and a more-rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis. Salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden. The fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth-converted seismic sections. Scale models are, therefore, useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps. Although even unsealed models may help generating ideas, the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes. Previously, published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden. This paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata. Rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other, intrinsic material properties. Scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics, including those with synkinematic sedimentation.

Upper Triassic—Lower Jurassic salt basin southeast of the Grand Banks

A grid of multichannel seismic reflection (MCS) profiles has been used to delineate and map a large, apparently tectonically controlled basin and associated diapiric province beneath the continental slope and rise southeast of the Grand Banks of Newfoundland. The depocenter, for which we propose the name Salar basin, is defined both landward and seaward by pronounced hinges in acoustic basement. It is approximately 400 km long, 35–70 km wide, and covers an area of approximately 20,000 km 2. The southern end of the basin is marked by the intersection of the Southeast Newfoundland Ridge with the South Bank High on the Grand Banks, and the northern end is bounded by the southwestern margin of Flemish Cap. The Salar basin generally parallels the much smaller Carson basin, which lies beneath the Grand Banks immediately to the west, and it merges with this basin north of 45°22′N. Based upon correlation with contiguous deposits sampled by drilling in the Carson basin, we infer piercements filling the Salar basin to be mobilized Late Triassic-Early Jurassic evaporites, probably predominantly halite. A consequent early Mesozoic age for the Salar basin implies a multi-stage rifting history for the Newfoundland Basin, as has been documented for the adjacent Grand Banks. Initial extension, which probably formed the Salar basin's bounding basement hinge zones, occurred in Late Triassic—Early Jurassic time, and was associated with rifting of North America from Africa and extension in western Europe. The presence of early Mesozoic evaporites in a basin southeast of the Grand Banks suggests development of a significant, but shallow and restricted, seaway between the Grand Banks and the Iberian peninsula, probably connected northward with coeval salt provinces of northwestern Europe. A second rift phase related to separation of Iberia and the Grand Banks affected the Grand Banks and Salar basin in Late Jurassic to Early Cretaceous time, after a period of epeirogenic subsidence during the Early and Middle Jurassic. Uplift associated with this extension led to the development of the widespread “U” (Avalon) unconformity across the Grand Banks, the Salar basin and the Newfoundland Basin out to the J-Anomaly and may have induced the first halokinesis in the Salar basin. The second rift phase culminated in the initiation of seafloor spreading in mid-Early Cretaceous (Aptian) time at a location now marked by the J-Anomaly.

Alleghenian reconstruction and subsequent evolution of the Gulf of Mexico, Bahamas, and Proto‐Caribbean

A detailed model for the evolution of the Gulf of Mexico, the Bahamas and the Proto‐Caribbean is built within the framework provided by a detailed initial Alleghenian (western Pangean) reconstruction and an accurate subsequent relative‐motion history between North America and Gondwana (northern Africa and South America). The Alleghenian reconstruction closes all pre‐Jurassic oceans; accounts for Jurassic attenuation of continental crust by restoring that attenuation to original prerift continental thicknesses; incorporates an improved Equatorial Atlantic fit between northern Brazil and the Guinea margin of Africa; quantitatively removes changes in shape of northern South America due to Late Cretaceous and Cenozoic accretion and internal deformation; includes pre‐Mesozoic continental crust presently underlying the western Bahamas and southern Florida; and correlates Late Paleozoic geology of Yucatan with its neighboring continental masses. Extension occurred within the Gulf of Mexico from Late Triassic to earliest Cretaceous time, but seafloor spreading was delayed until the Late Callovian. This divided a single Gulf‐wide salt basin into the Louann and Campeche salt provinces. The Yucatan block progressively rotated about 43 degrees counterclockwise away from the Texas‐Louisiana margin around a pole in northern Florida. The Tamaulipas‐Golden Lane‐Chiapas fault zone of eastern Mexico is interpreted as the remains of an initially intracontinental transform system along which Yucatan migrated. Attenuated continental crust beneath southern Florida and the western Bahamas, termed here the Florida Straits block, migrated approximately 300 km out of the eastern Gulf, approximately along Central Atlantic flow lines. These rotations are consistent with recently suggested magnetic anomaly trends in the Gulf of Mexico (Shepherd et al., 1982; S. Hall, personal communication, 1984). The Proto‐Caribbean formed synchronously by a fan‐like rotation of Yucatan away from Venezuela.

Seismic Modeling: Geologic Predictions and Pitfalls: ABSTRACT

Structural geologists have made tremendous strides in unraveling the architecture of the earth with recent studies of seismic reflection data. End_Page 1836------------------------------ This is true not only for exploration studies, but also for crustal studies. However, the seismic time section is not an undistorted cross section of the earth. Seismic sections across most geologic structures are distorted by sideswipe and/or lateral velocity changes in the subsurface. Invariably, the distortion on the 2-D migrated section hides the features that are most desired. However, through seismic models of similar geologic structures, the interpretational pitfalls caused by sideswipe and velocity are turned into practical prediction tools. Migrated seismic lines across domes and anticlines normally exaggerate the size of the anomalies. Migrated seismic lines across synclines and basins are characterized with false expressions that include grabens, contemporaneous deformation, cross-stratification, high amplitudes, and crossing reflections. Geologic areas that have large lateral velocity contrasts, such as reefs, diapirs, or fault blocks, exhibit false seismic expressions. These include relief faults, basement-controlled tectonics, facies changes, and structures that are located in geologically ambiguous positions. Even the polarity of the seismic reflection is 3-D dependent. Modeling examples show that interpretational pitfalls, such as mapping from migrated sections and interpreting from the basement upward, must be supplemented with pseudo-3-D interpretational techniques. Geologic models and their seismic analyses from salt provinces, reefs, overthrusts, etc, illustrate these pseudo-3-D interpretational tools. End_of_Article - Last_Page 1837------------

Seismic Stratigraphy of Veracruz Tongue, Deep Southwestern Gulf of Mexico

The Veracruz Tongue is a region of continental slope and rise sediments bounded bathymetrically by the Mexican Ridges fold belt to the west and the Campeche Knolls salt province to the east. Study of two multichannel seismic lines and single-fold sparker data enables five post-early Miocene seismic stratigraphic sequences to be distinguished in the tongue. Sedimentary processes responsible for deposition of each sequence are inferred on the basis of external geometry, internal reflection configuration, and, where possible, core data. The five sequences are each interpreted to consist of turbidites grading laterally into hemipelagites. These primary deposits may be modified later by downslope creep or sliding and slumping. Because bathymetry is a major control on location of turbidity-current flows, slides, and slumps, the depositional history of the Veracruz Tongue provides indirect evidence of tectonic evolution of the Mexican Ridges fold belt and the Campeche Knolls salt province. Salt domes first formed a barrier to turbidity-current flow from the east at the close of the middle Miocene. The Mexican Ridges fold belt evolved gradually until it formed a complete barrier to turbidity-current flow from the west at the close of the Pliocene. As a result of shift in direction of turbidity-current flow through time, potentially sand-prone Miocene turbidites are overlain by hemipelagic clays and clayey oozes--a situation favorable to hydrocarbon entrapment.

A Three-Dimensional Seismic Study of Challenger Knoll: ABSTRACT

Several intersecting multichannel seismic lines shot by The University of Texas Marine Science Institute in the deep Gulf of Mexico provide a three-dimensional grid in the vicinity of Challenger Knoll. Seismic stratigraphic sequences identified on the lines include the Sigsbee, Cinco de Mayo, Upper and Lower Mexican Ridges, Campeche, and Challenger units. Isotime End_Page 548------------------------------ maps document a detailed growth history of the Challenger diapir. DSDP drill holes 2, 3, and 87 provide age data for the grid. Differences in the style of diapirism between the northern (Louann) salt province and the Sigsbee salt province to the south are due to the influence of the basement. Both the original distribution of evaporites and subsequent diapirism in the vicinity of Challenger Knoll are controlled by basement fault blocks of transitional crust that formed during the initial opening of the Gulf of Mexico. End_of_Article - Last_Page 549------------

Seismic Stratigraphy of Veracruz Tongue, Southwestern Gulf of Mexico: ABSTRACT

The Veracruz tongue is an area of continental slope and rise sediments bounded topographically by the Mexican Ridges foldbelt and the Campeche-Sigsbee salt province. Study of two multichannel lines and single fold sparker data enables five End_Page 900------------------------------ seismic sequences to be distinguished in the tongue. Correlation of reflectors with well data allows ages, ranging from middle Miocene to Holocene, to be assigned to the seismic sequences. Sedimentary processes responsible for deposition of each sequence are determined on the basis of external geometry, internal reflection configuration, and where possible, core data. Primary deposits consist of proximal and distal turbidites, and hemipelagites and pelagites. Sediments may be later modified by creep or slumping and sliding. This seismic stratigraphic study allows the depositional history of the Veracruz tongue to be elucidated and aids in the dating of folding and salt movement at the edges of the tongue. Detailed interpretation of seismic waveforms indicates that gas accumulations are present in the study area. Future seismic stratigraphic studies will allow quantitative evaluation of hydrocarbon potential of the Veracruz tongue and the adjacent foldbelt. End_of_Article - Last_Page 901------------

A Double-Electrode Method of Spontaneous Potential Logging

Koerperich, E.A., SPE-AIME, Shell Oil Co. In this method of logging, a recording of the potential difference between two SP electrodes yields a curve that may facilitate and improve sand counts, and that might be amenable to automatic computerized interpretation. Spurious SP electrode noise signals can be substantially reduced or eliminated from the potential-difference curve. Introduction Wellbore SP currents and their applicability in the location of subsurface sand bodies were discovered in 1931 and the usefulness of this measurement has persisted to the present day. Through the steadfast persisted to the present day. Through the steadfast endeavors of many researchers, knowledge and understanding of the SP has slowly evolved from the original mere recognition of its existence to its present-day quantitative use. Some common applications present-day quantitative use. Some common applications of the SP curve include bed correlation, sand counting and formation-water resistivity determination. This report relates to the determination of net sand counts from SP data. Many influences combine to complicate SP interpretation and to restrain its use as a unique, infallible indicator of permeable bed boundaries. The distribution and intensity of SP, currents are largely influenced by properties of the drilling and formation fluids. wellbore diameter and invasion depth, and by various characteristics of the subsurface strata. Quantitative use of the SP curve is severely restricted in salt, carbonate, and fresh water provinces, Shaly sand formations create difficulties with SP interpretation, and this topic has been the subject of many publications. An even greater problem is SP noise, which has long been a source of low-quality and erroneous SP curves, particularly since the advent of offshore drilling operations. A principal source of noise is associated with the surface "ground" potential to which the wellbore spontaneous potentials are referenced. Being located at the surface, the reference potential is vulnerable to voltage disturbances created by surface objects. Thus, all these factors that mold SP character should be accounted for, as adequately outlined in the literature, when the curve is being analyzed quantitatively. Many times the exact influence of some or all of these factors affecting SP response cannot be predicted. For this reason, auxiliary tools, whenever predicted. For this reason, auxiliary tools, whenever available, are used in conjunction with the SP to determine net sand. Short spaced, highly focused resistivity devices or pad-type resistivity devices are commonly used for this purpose. Despite the limitations of the SP curve, there are a number of occasions and areas where it satisfactorily reflects permeable bed boundaries. Many times auxiliary logs are not available to refine the SP sand count, leaving the SP as the primary indicator of bed boundaries. In these cases, the method of recording and presenting SP data as outlined in this report can presenting SP data as outlined in this report can usefully supplement the SP curve. This method of presentation is directly derived from a single conventional presentation is directly derived from a single conventional SP curve or from two conventional SP curves recorded simultaneously at different depths in the wellbore. When the data presentation is derived from a single down-hole electrode the resultant curve (Delta V curve) is subject to all the limitations and required corrections associated with the SP recording, including the presence of noise signals when they exist on the SP. When the, Delta V curve is derived from two downhole electrodes, it is subject to the same limitations except the SP noise. JPT p. 1437

Major Transition Zones of Gulf of Mexico: DeSoto and Campeche Canyons: ABSTRACT

Deep well information throughout the Gulf of Mexico coastal plain has indicated that the Gulf margins can be divided into two distinct provinces, the subsided southeastern section which is underlain by carbonate rocks and the northwestern section that is underlain mainly by terrigenous clastic rocks. In the latter area there is complicated surface and subsurface structure controlled mainly by the influence of upward salt migration. Recent geophysical studies in the offshore areas indicate that DeSoto Canyon is the transition zone between the terrigenous clastic and carbonate provinces in the northern Gulf and that the Campeche Canyon plays a similar role in the southwestern part of the basin. Most salt diapirs in the region lie west of a line connecting the two canyons, b t recent work suggests the presence of some diapiric structures east of the line. The geophysical data from the DeSoto Canyon indicate that erosion has played an important part in its development. Two mechanisms for the formation of the canyon are suggested: (1) the loop current of the eastern Gulf of Mexico and associated circulation in the northeastern Gulf have sufficient velocity along the bottom during specific periods of time to effect a scouring action and/or keep sediments in suspension; and (2) erosion by turbidity flows takes place during periods of low sea-level stands associated with glacial stages. The fact that the DeSoto Canyon extends across parts of two distinct geologic provinces, the northeast Florida platform and the Mississippi cone, adds credence to an hypothesis involving erosional rather than tectonic processes. Although there are insufficient data available to determine the origin of the Campeche Canyon, it is suggested that, unlike the DeSoto Canyon, its topographic expression probably is more the result of adjacent salt tectonics than of erosion. Some workers suggested that an alignment from the DeSoto Canyon to Campeche Canyon may represent a fracture zone across the Gulf basin. The hypothesis that this alignment forms the southeastern boundary of the Gulf of Mexico salt province is contradicted by the presence of diapirs in northwestern Matanzas Province, Cuba, and by the discovery of some possible diapiric structures in the Florida Straits and Yucatan Channel. End_of_Article - Last_Page 1831------------