Gulf Coast Geosyncline Research Articles

The History, Development, and Regional Significance of Ruston Field

The Ruston gas field lies just north of Ruston, Lincoln Parish, Louisiana, 70 mi east of Shreveport, Louisiana. The field covers a surface area of 64.5 sections or approximately 41,280 ac. The Ruston lies within a trend containing many Lower Cretaceous and Upper Jurassic gas reservoirs on the northern flank of the Gulf Coast geosyncline, along the northern flank of the North Louisiana Salt Dome basin. Within the belt of production, several prolific Cotton Valley strandline plays exist at Ruston along with many different Hosston pays. More than 40 zones produce natural gas and oil from seven formations in the field. Several of the sands are totally structurally controlled, but many have a minor to major stratigraphic trapping component. Ultimate recovery from the field is estimated to be 1,600,000 bbl of oil, 1,730,000 bbl of condensate and 614 bcf of gas. The most prolific reservoir is the strandline Cotton Valley [open quotes]D[close quotes] sand averaging over 12 bcf per well, a total of over 195 bcf of gas, and more than 600,000 bbl of condensate. The best well in the fields is the Arkansas Louisiana Gas No. 2 Dowling in Sec. 30, T19N, R2W, which has produced 22 bcf ofmore » gas and 110,000 bbl of condensate. The upper, middle, and lower Hosston produce at Ruston. The lower Hosston produces from approximately 14 fields in northern Louisiana and from 11 wells in the field. The middle Hosston produces from approximately 28 fields in the basin and has made over 46 bcf of gas from six sands. The upper Hosston is regionally the most extensive in northern Louisiana producing from approximately 41 fields. At Ruston, the Hosston has produced over 15 bcf of gas from seven sands.« less

Hydrocarbon Production in the Gulf Coast Region from Organic-Rich Source Beds of Ancient Intraslope Basins

We have recently presented an empirical model to explain how low oxygen levels in intraslope basins of the northwestern Gulf of Mexico have been a common mechanism for the accumulation of sediments with significantly increased amounts of marine organic carbon. In that model (Model I) progadation of the shelf-slope and regional salt tectonics were invoked to control the occurrence and stratigraphic distribution of source beds throughout the Tertiary of the Gulf of Mexico. In turn, the maturation history of these organic-rich sediments is influenced by the high thermal conductivity of the underlying salt structures. In Model II, the topic of this paper, we use random number theory to suggest that the occurrence of organic-rich black muds in intraslope basins of the northwestern Gulf of Mexico had sufficient capacity to more than account for the known oil and gas reserves in sediments of the Gulf Coast Geosyncline.

Eocene Paleotectonics and Sedimentation in the Rocky Mountain-Colorado Plateau Region: ABSTRACT

End_Page 1331------------------------------The Laramide orogeny (c. 80 to 40 m.y.B.P.), which culminated during early Eocene time, resulted in the development of numerous uplifts and basins in the foreland of the western United States. Uplifts are assignable to three general classes: (1) Cordilleran thrust belt uplifts, (2) basement-cored, fault-bounded uplifts of the classic Laramide Rocky Mountains, and (3) monocline-bounded uplifts of the Colorado Plateau. Basins were also of three types: (1) Green River type--large equidimensional to elliptical basins bounded on three or more sides by uplifts and commonly containing lake deposits, (2) Denver type--asymmetrical, synclinal downwarps with a related uplift along one side, and (3) Echo Park type--narrow, highly elongate basins with through drainage and of strike-slip origin. Gr en River-type basins exhibit quasiconcentric zonation of facies, in contrast to the unidirectional, proximal-to-distal facies tract of Denver-type basins. Facies distribution in Echo Park-type basins is complex and often difficult to reconstruct due to faulting, erosional truncation, and cover. The prevalence of en echelon structures in the deformed zone east of the Colorado Plateau, and evidence for significant crustal shortening north of the plateau, suggest that the major structural features of the Laramide foreland were produced by large-scale, north-northeastward translation of the relatively rigid Colorado Plateau block. The magnitude of this motion, as indicated by dextral offset of lineaments which cross the eastern margin of the plateau and by the amount of crustal shortening in the Wyoming province, may be as great as 65 to 120 km (40 to 75 mi). This translation probably resulted from the interaction of relatively competent Colorado Plateau lithosphere with the underlying, gently dipping Farallon plate, which was being overridden by the western United States in Lar mide time. Evidence for increased strain rates in early Eocene time includes: (1) markedly higher rates of deposition and sand/shale ratios in the Gulf Coast geosyncline (Wilcox Group), (2) formation of several new basins in the southern Rocky Mountains in which Eocene deposits rest unconformably on pre-Cenozoic rocks, and (3) the generally coarser and more arkosic nature of Eocene sediments, as compared to older Laramide deposits, in many areas throughout the foreland. The early Eocene culmination of Laramide tectonism appears to result from two factors. First, the subducted Farallon plate achieved its shallowest dip at about 55 m.y.B.P., resulting in increased viscous coupling with the overriding continental lithosphere. Second, changing spreading-center geometries in the Labrador Sea, Norwegi n Sea, and Arctic Ocean caused the maximum horizontal stress direction to shift to a northeasterly orientation, causing the Colorado Plateau block to increasingly decouple from the craton along north-trending wrench faults in the southern Rocky Mountains. Translation of the Colorado Plateau to the north-northeast during Laramide time resulted in a series of transpressive uplifts and basins along its eastern margin and large-scale crustal shortening in the Wyoming province to the north. End_of_Article - Last_Page 1332------------

Origin and Evolution of Saline Formation Waters, Lower Cretaceous Carbonates, South-Central Texas: ABSTRACT

Systematic chemical variations exist in formation waters collected from a dip section through Lower Cretaceous rocks of south-central Texas. These chemical variations can be explained by an interactive water-rock diagenetic model. Cyclic Lower Cretaceous shelf carbonates of the Edwards Group that dip into the Gulf Coast geosyncline act as an aquifer contained by basement beneath, and relatively impermeable Upper Cretaceous clays and chalks above. The hydrodynamic character of this carbonate system is strongly controlled by major fault systems which serve as pathways for the vertical movement of brines into the Lower Cretaceous section. Formation-water movement in this system has strong updip and upfault components. The parent Na-Ca-Cl brine originates deep in the gulf basin, at temperatures between 200 and 250°C, by the reaction: halite + detrital plagiocalse + quartz + water ^rarr albite + brine. Other dissolved components originate by reaction of the fluid with the sedimentary phases, K-feldspar, calcite, dolomite, anhydrite, celestite, barite, and fluorite. Significant quantities of lead, zinc, and iron have been mobilized as well. As the brine moves updip out of the overpressured deep gulf basin, encountering limestones of the Stuart City reef trend (the buried platform margin), small amounts of galena precipitate in late fractures. Updip and up-fault, the brine becomes progressively diluted with downward-moving meteoric water. On encountering significant quantities of dolomite in the backreef facies, the Ca-rich brine causes dedolomitization. Although thermochemical considerations suggest that small amounts of several authigenic phases should precipitate, they have yet to be found, except for minor amounts of calcite spar. As the brine evolves by dilution cooling, no systematic changes in any cation/Cl ratio occur, except for regular updip gain in magnesium as a result of progressive dedolomitization. The high diluted formation water eventually discharges along faults as hot mineral water. End_of_Article - Last_Page 947------------

Sediments of the Mississippi, Alabama, and Florida (MAFLA) Continental Shelf

ABSTRACT The eastern Gulf (MAFLA) continental margin may be conveniently divided into two parts of opposing history and character. West of Cape San Blas lies the eastern limb of the Gulf Coast Geosyncline whose surface expression is a clastic sand body, called the MAFLA Sand Sheet, grading westward into the muds of the Mississippi pro-delta. These sediments have a clay mineral suite dominated by smectite. East of Cape San Bias lies the West Florida Margin, a sequence of carbonate and evaporitic rocks which has been cut off from a major clastic source since Jurassic time. The surface expression of this sequence is the West Florida Sand Sheet, predominantly a patchy veneer of shell hash and foraminiferal, algal, and even oolitic sands which is subjected to periodic reworking by frontal system st rms and hurricanes. Kaolinite dominates its clay mineralogy. Seaward of the carbonate sands lies the West Florida Lime Mud facies, slope sediments composed of planktonic foraminifera and coccoliths. Inshore of the carbonate sands and separated from them by a zone of mixed composition lies a mature quartz fine sand, which also makes up the beaches of Southwest Florida. West Florida shelf quartz sands appear to have been deposited at lower sea level stands and to have been transported back and forth with no net drift in a longshore current system which changes seasonally from north to south. Clay mineralogy in portions of the MAFLA region shows distinct changes in composition over a period of a year in the benthos and over periods as short as a few hours in the water column. These changes reflect contribution from two distinct provenances. Benthie variation probably results from occasional intrusion of smectite laden Mississippi River or Loop Current water into the eastern zone. Water column variation may be the result of seiching of the Gulf or the pulsing movement of kaolinite laden eastern shelf water to the west.

North American Geosynclines--Test of Continental-Drift Theory

North American Geosynclines--Test of Continental-Drift Theory

Evolution of Northern Gulf Coast Deducted from Geophysical Data: ABSTRACT

Seismic refraction data from the western part of the northern Gulf Coast of the United States indicate that the uppermost crust of the Gulf region consists of a thick sequence of sedimentary rocks locally ranging up to 17 km thick. Beneath the sedimentary sequence a layer with velocities ranging from 5.2 to 6.0 km/sec probably consists of high-velocity sedimentary rocks (salt and carbonates) possibly overlying a thin upper crustal layer. The combined thickness of the sedimentary sequence and the 5.2-6.0 km/sec layer is between 15 and 20 km. The deeper crust is 12-20 km thick beneath the interior of the coastal plain and thins seaward. The velocity of this crust (6.45-6.9 km/sec) is comparable with that of oceanic crust. From available data for the north Gulf Coast, we have constructed 3 profiles across the coastal plain to the Sigsbee Deep. From these profiles, we have reconstructed former Gulf coastal margins for 3 epochs of the Mesozoic and Cenozoic, based on the assumption that the region has remained in close isostatic equilibrium. We have postulated the evolution of the Gulf Coast geosyncline, the continental shelf, and the transition from continental crust to oceanic crust at this margin since the Mesozoic. End_of_Article - Last_Page 1841------------

Geology and History of the Gulf of Mexico

The principal aim of this study has been directed toward a comprehensive interpretation of the historical development of the Gulf of Mexico. The initial stage was an analysis and correlation of seismic profiler records obtained over a considerable period, followed by an endeavor to correlate the results with the surface geology of the land areas surrounding the gulf. Indication of simatic oceanic crust beneath the abyssal gulf has led to the assumption that it had been a permanent ocean basin. A concept developed in this study proposes that the simatic crust was formed in late Paleozoic time. Subsequent environmental conditions remained epicontinental—including the environment of Jurassic salt deposition. Proceeding from this viewpoint, the origin of the Gulf of Mexico is proposed to be related o t the extensive regional subsidence of more than 10,000 ft during Cretaceous time, and its isolation came about by the continuous contemporaneous carbonate growth of the Florida and Yucatan platforms. Minimum rates of sediment deposition, compared to the rate of platform growth, led to consistent deepening of the gulf, which, accordingly, must be underlain by a thin Cretaceous section. Hypothetically, the Straits of Florida and the Yucatan Channel originated from erosion at the front of the Laramide tectogene when carbonate growth was halted, following the inundation by seaways. The Gulf of Mexico has been reduced to its present size by the invasion from the north and northwest of the huge Cenozoic mass of deposits—referred to as the Gulf Coast geosyncline. The last major volume of clastic sediments was deposited on the Mississippi cone in early Holocene time. However, the latest deposits from the Mississippi and other rivers were laid down on the continental shelves and a minimum of terrigenous material has been reaching the abyssal gulf by turbidity currents.

Modern and Ancient Hurricane Deposits--Their Geological Significance: ABSTRACT

The shoreline sands along the coastlines of the northern Gulf of Mexico offer excellent examples of the varying processes that have created them and determine their distribution. Sands of such varying origin as eolian sands of the south Texas sand sheet, the islands of the Central Texas Bay-Barrier Island province, the chenier sands of southwestern Louisiana, the channel sands of the active and inactive passes of the Mississippi River delta complex, the reworked sands of the old distributary channels of the Mississippi delta, and the Mississippi-Alabama island chain, are well documented in this almost unique basin of deposition. These sands are made by nearshore processes from other sand-containing sediments and are not deposited as such from their sourc . They, in essence, are all multicycle sands. The normal shoreline and nearshore processes maintain these sand deposits in their present environments. Major storms, however, completely disrupt these processes and cause unusual sand distributions. Many of the storm-caused distributions are repaired by the normal processes shortly after they are formed. Some, however, remain as a permanent distribution and probably are included in the geologic record as such. Many examples of sand bodies in the subsurface Tertiary of the Gulf Coast geosyncline are directly analogous to modern normal and abnormal sand bodies available for study in the northern Gulf of Mexico. For example, the Oligocene Frio barrier in South Texas is associated with probable storm deposits. End_of_Article - Last_Page 1909------------

Origin and Chronology of Alabama River Terraces: ABSTRACT

Investigation of high fluvial terraces along the Alabama River indicates that the highest terraces (115+ m above the floodplain) may predate the Citronelle Formation. Preliminary radiometric age determinations suggest that all of Quaternary history may be represented by those terraces less than 50 m above the present floodplain. The Citronelle surface in southwestern Alabama shows differential warping caused by subsidence of the Gulf Coast geosyncline and uplift of the adjacent upland. Rates of uplift were roughly 1.2 cm/103 years. The average rate of river entrenchment prior to North American Pleistocene glaciation was about 2.5 cm/103 years. During glacial Pleistocene time, the maximum net rate of river entrenchment may have been as great as 7 cm/103 years. Observation of fluvial features, such as meander scrolls, shows that, in terraces more than 60 m above the floodplain (i.e., more than 1.1 × 106 years old), meander scrolls have been obliterated. Likewise, terrace surfaces are mostly destroyed where more than 1.6 × 106 years old. In deposits more than 3.5 × 106 years old, primary structures have been obliterated and the only evidence of these oldest terrace deposits is chaotic blankets of alluvium. End_of_Article - Last_Page 1697------------

Crustal studies in the Gulf of Mexico

The eastern margins of the Gulf of Mexico are areas of carbonate buildup. They are represented by the south Florida and Yucatan platforms which are similar in respect to topography, sediment type, seismic velocities, depth to equivalent age horizons, and bordering buried reefs. The existence of the Lower Cretaceous reefs is of particular importance as they have been the major factor in controlling the sedimentary history during the Mesozoic and Cenozoic in the eastern Gulf of Mexico. The areal extent of the Lower Cretaceous reef is best known on the Florida Platform but its existence on the edge of the Yucatan Platform suggests the possibility of a Lower Cretaceous reef complex that nearly encircles the Gulf. In this case these buried offshore reefs would be part of a system including the Golden Lane (Faja de Oro) of eastern Mexico and the well known oil producing Lower Cretaceous reef trends of Texas and Louisiana. Reef material recovered from topographic highs in the Straits of Florida suggests that the bordering reefs of the west Florida Platform continue to northern Cuba. These similarities between the Yucatan and Florida platforms allude to a geographical connection between these areas in the geologic past. If this is true, the mode of separation of the platforms suggested by seismic reflection data is erosion (controlled mainly by the Gulf Stream) and faulting. The western margins of the Gulf of Mexico are characterized by thick deposits of terrigenous elastics. The main structural element is in the north; the Gulf Coast geosyncline. Structure within the geosyncline is very complex, structural mobility being caused by the presence of extensive buried Triassic-Jurassic salt. Similar conditions exist in the southwestern Gulf (Bay of Campeche) where salt structures dominate the topography. Although there is no conclusive proof, there is much evidence that indicates the north-south trending ridges of the western Gulf of Mexico are also related to buried Mesozic salt. If these folds off eastern Mexico prove to be salt anticlines at depth, the post-Paleozic development of the western margins of the Gulf of Mexico becomes clear; 1. (1) during Mesozoic, salt was deposited on the western margins of the Gulf of Mexico; 2. (2) the salt was covered by pelagic and continental derived sediments; 3. (3) the salt was folded into a series of linear ridges parallel with the seaward extent of the salt; and 4. (4) as sedimentation over the anticlines continued the salt became more mobile, more complex structures developed and diapirism became the dominant mechanism. Both phases three (3) and four (4) are evident in the western Gulf. The linear features on the central portion of the eastern Mexican slope are examples of the relatively undeformed salt anticlines and the structures of the Bay of Campeche represent an intermediate stage where the ridge outlines can still be detected but secondary growth features are prevalent. The continental slope off Texas and Louisiana represents the final stage of development i.e., there is no preferred alignment of structures and diapirism predominates. The crustal structure of the central basin is quasi-oceanic. Earthquake seismology studies and gravity measurements both indicate that the oceanic type crust extends some distance inland on the northern margin of the gulf coastal plain. The northern extent of the oceanic crust and the great thickness of unconsolidated sediments present across the central basin suggest a long history of filling. This is consistent with a hypothesis which considers the Gulf of Mexico as an old ocean basin which is being flooded by sediments. The effect of the filling has been to decrease the water depth in the basin and to increase the depth to the Mohorovičić discontinuity. In terms of the new global tectonics, this concept of the Gulf representing a remnant of an old ocean with the characteristics mentioned above, it appears that the Gulf of Mexico moved passively with the North American continent.

Gulf of Mexico Basin: Interactions Among Tectonics, Sedimentation, and Hydrocarbon Accumulation: ABSTRACT

Gulf of Mexico Basin: Interactions Among Tectonics, Sedimentation, and Hydrocarbon Accumulation: ABSTRACT

Geologic Occurrence of Oil and Gas in Miocene of Gulf Coast of United States: ABSTRACT

Miocene sands have produced, through 1967, approximately 6.76 billion bbl of oil and 35 Tcf of gas in about 650 fields in coastal and offshore Louisiana and Texas. More than 80% of the production has come from fields in the Louisiana segment, where all 19 giant Miocene oil fields are located. The Miocene sediments of coastal and offshore Louisiana and Texas form a seaward-dipping and thickening wedge of interbedded marginal-marine sandstone and shallow-marine shale with maximum thickness at any locality of about 25,000 ft. A composite section in the Gulf Coast geosyncline is at least 45,000 ft thick. Rapid sedimentation in large deltas, where there were prolific organic production and accumulation and where the organic material was preserved by rapid burial, made possible the many large Miocene petroleum accumulations. These favorable conditions were confined mainly to the Louisiana coastal and adjacent offshore areas that subsided at a faster rate than the coastal interdeltaic regions on the east and west, and confined the Miocene Mississippi River to that part of the northern Gulf basin. Downward movement along faults that bound the deeply buried salt-filled grabens also took place as the major deltas prograded, causing diapiric structures and rollover anticlines to form where organic-rich deltaic mud, silt, and sand were deposited. The tectonic-sedimentation history of the Gulf Coast Miocene clearly demonstrates the close relation between depositional environments and petroleum occurrence. End_of_Article - Last_Page 866------------

Late Mesozoic-Cenozoic Tectonic Aspects of the Atlantic Coastal Margin

Data from the middle and northern Atlantic Coastal Plain of the United States and continental slope show that the dominant filling of the Atlantic margin geosyncline occurred in pre-latest Cretaceous time with only thin additions of Cenozoic strata. The influx of significant amounts of detrital material into the offshore areas had largely ceased by latest Cretaceous time, was essentially absent in the early Cenozoic, but increased considerably in the Miocene, indicating a rejuvenation of the Appalachian source area. Basin migration continued throughout the Cenozoic, and the Miocene and Eocene strata are used to illustrate the shifting of the loci of deposition. The distributions of the strata indicate that local to regional tectonic movements are responsible. Comparison with the Gulf Coast geosyncline shows that, although features such as sedimentary thickness, evaporites, and volcanic materials are generally similar between the two margins, major differences in time of filling and regional tectonic movements exist.

Gravity Anomalies, Basement Rocks, and Crustal Structure, Central and Southeast Texas: ABSTRACT

Regional gravity surveys have been conducted (1) between Bryan-College Station and Austin, Texas, across the margin of the Gulf Coast geosyncline and the buried Ouachita fold belt, and (2) across the northwestern Llano uplift, in the Llano-Mason-San Saba-Brady area. These new gravity data have been incorporated with those previously published in order to construct a regional gravity-anomaly map, contoured at a 5-mgal interval, of a large part of central and southeast Texas. Gravity anomalies in the Llano region can be interpreted readily in terms of exposed major Precambrian rock units. Characteristic anomaly patterns then can be used to interpret basement lithology where concealed around the periphery of the uplift. Gravitational effects of the thick wedge of Cenozoic and Mesozoic sedimentary rocks in the Gulf Coast geosyncline were calculated for several different density contrasts; thicknesses were based on extrapolations of regional well data. Similarly, gravitational effects of Paleozoic clastic rocks in the foreland between the metamorphosed Ouachita facies and the crystalline rocks of the Llano uplift were calculated End_Page 1825------------------------------ for several density contrasts and basin configurations. Combined gravitational effects of the models have been applied as corrections to the Bouguer anomaly map to obtain a gravity-anomaly profile that generally represents effects of changes in crustal thickness or density. The granitic crust is considerably thicker under the Llano uplift than under the Gulf Coast geosyncline. At present, the region is at or near isostatic equilibrium, but near the end of the Paleozoic it was out of equilibrium, with an excess mass at the present site of the Gulf Coast geosyncline. The writers speculate that gulfward migration of depocenters during Mesozoic and Cenozoic has taken place in response to a mechanism for gradual restoration of regional isostatic equilibrium. End_of_Article - Last_Page 1826------------

Role of Contemporaneous Faulting During Basinal Subsidence

Faulting contemporaneous with subsidence and deposition is known to characterize various depositional basins. For example, movement along major faults occurred during subsidence of the small but active Los Angeles, Hanna, and Ardmore basins and of large basins such as the Ouachita and Gulf Coast geosynclines. The data from these and other basins indicate that faulting is an important, if not the primary, mechanism of basinal subsidence. During subsidence of the basins studied, faulting proceeded independently of the depositional framework. Major fault movement occurred in the Los Angeles basin during the Mio-Pliocene as turbidites were deposited, in the Hanna basin during the Paleocene-Eocene as alluvial deposits formed, and in the Ardmore basin during the Pennsylvanian as shallow-marine and paralic sediments were deposited. During late Paleozoic time, contemporaneous faulting occurred in the Ouachita system as turbidites and, later, shallow-marine beds were deposited along its length. Subsidence of the Gulf Coast geosyncline is thought to have accompanied the same type of faulting which originates in the basement. There paralic and alluvial sediments overlie offshore or deep-water clay in the overall regressive se uence which formed during the Cenozoic. Another type of faulting, involving only the sedimentary basinal fill, occurred during subsidence of the Gulf Coast geosyncline. Salt and some thick shale units have been deformed by uniform flow which, in turn, apparently caused failure by faulting in the overlying paralic sediments. These sedimentary faults, which do not involve the basement directly, are thought to result from conditions analogous to those which give rise to the Recent mudlumps of the Mississippi delta or the slide-flows experienced in foundation engineering.

Lower Cretaceous Geology, Northwestern Karnes County, Texas

Northwestern Karnes County, Texas, is underlain by about 5,000 ft of Lower Cretaceous sediments. Most of this sequence consists of two dolomitic limestone bodies which were deposited in the backreef provinces of two barrier-reef complexes. Each carbonate body underlies a blanket-type deposit of impermeable shale and shaly limestone; hydrocarbon accumulations are in the carbonates below the upper impermeable sequence but have not been located below the lower. The sediments were deposited on the nearly level San Marcos platform and in the more rapidly subsiding Karnes trough which crosses the platform to connect the East Texas and Rio Grande basins. Post-Early Cretaceous regional tilting on the southeast added to depositionally produced dips on the northwestern side of the trough and subtracted from them on the southeastern side. Hydrocarbons probably accumulated in the structural closure formed along the trough's southeastern hingeline before the directions of depositional dips on the southeastern side were reversed. Up-to-the-coast faulting occurred to trap part of these hydrocarbons before further regional tilting emptied the closures. Isopachous maps of key-bed-bounded units reveal the position of the critical southeastern hingeline as ell as the overall configurations of the Karnes trough and surrounding San Marcos platform. Histograms and scatter diagrams of various production, structural, and stratigraphic quantities verify and dramatize the relations established by the conventional isopachous and structural contour maps. Normal faults of the Sample fault system follow the trend of the Karnes trough through the study area. Two down-to-the-coast normal faults with a total vertical displacement of about 1,400 ft exist on the northwestern side of the trough and at least 12 up-to-the-coast normal faults with a total vertical displacement of about 1,400 ft occur on the other side. The total horizontal displacement of all faults is about 2,000 ft. The nearly equal vertical displacements, the opposing fault pattern, and the geometric configuration of the strata closely fit a gravitational slide-block model. The down-to-the-coast faults are thought to flatten at depth to represent the major slip planes along which the slide block moved downdip toward the axis of the Gulf Coast geosyncline. A potential v id 2,000 ft wide was generated at the head of the slide and up-to-the-coast faulting occurred to fill the potential void. The potential void concept can be extended to provide a philosophical basis for a quantitative approach to many extensional tectonics problems, especially those connected with normal faults. The three basic tectonic mechanisms which create potential voids and the resulting normal faults are punch or sag type stresses, extensional flexing, and lateral separation. The spatial characteristics and historical development of the faults associated with the several types of potential voids are distinctive and predictable.

Geological History and Oil and Gas Potential of Central Gulf Coast: ABSTRACT

The area described includes the coastal plain and continental shelf between Texas and peninsular Florida, and includes the Mississippi embayment. The stratigraphic section includes sediments of all ages from Triassic through Holocene. The maximum composite thickness probably exceeds 80,000 ft, but only about 50,000 ft of Mesozoic-Cenozoic sediments is present at any locality in the deepest part of the Gulf Coast geosyncline. Oil and gas currently are produced in numerous fields in this area from both silicate clastics and carbonate rocks of Jurassic, Cretaceous, Tertiary, and Quaternary ages. The sedimentation history clearly indicates that the potential for future discoveries is great. The structure and stratigraphy of this richly petroliferous basin are described and the possibility of discovering more oil and gas than has been found is pointed out. Thickness, lithology, and depositional environments of each major division of the Mesozoic and Cenozoic are shown on maps and sections. End_of_Article - Last_Page 1826------------

The Gulf Coast Onshore Offshore Experiment: Some preliminary results

The purpose of the Gulf Coast Onshore Offshore Experiment, which was carried out in cooperation with the United States Coast and Geodetic Survey, was to determine crustal structure of a section across the Gulf Coast Geosyncline where the sediments are near maximum thickness. There is a longstanding difficulty in conceiving of a mechanism by which the deep sedimentary basins are formed. There appear to be two possibilities: (1) deposition of sediments on a continental crust which was downbuckled to accommodate them by some unknown means; or (2) that wherever the sediments were formed initially, they were finally deposited in deep water. In the latter case one would expect that they would be underlain by a thin oceanic crust.

Evidence for Deep Salt Deposits in Western Florida Panhandle

Salt intrusions derived from the Louann Salt of Jurassic (?) age are abundant in the Gulf coastal plain and are found from Texas to Alabama. Evidence suggesting that the Louann Salt may extend into the western Florida Panhandle includes the following. 1. The Mississippi Interior Salt Dome basin strikes directly toward the western Florida Panhandle: the easternmost proved salt dome in this basin is only 32 miles away. 2. Small round topographic highs occur on the continental shelf off the Florida coast; some of these may be related to salt domes. 3. Seismic-reflection measurements reveal the presence of two anticlinal structures, one very broad, the other dome-like, on the continental shelf south of the western Florida Panhandle; there is a strong possibility (not yet proved) that these are related to salt domes or ridges. The broad structure is cut by graben faults. 4. A recently discovered domal structure broken by a northwest-trending graben is centered under the town of Jay in Santa Rosa County. The faults are similar to those that occur over known salt domes in Texas and Louisiana. 5. The faults making up the Pollard graben, which extends from Alabama into northern Santa Rosa County, Florida, are of a type that could have been caused by repeated flowage of salt resulting from sedimentary loading during subsidence of the Gulf Coast geosyncline. These faults are contemporaneous; that is, the faulting took place during deposition of the beds that are now displaced. The faults are characterized by: (a) numerous small offsets that occurred at least from Late Cretaceous through middle Oligocene time; (b) increasing throw with depth; and (c) thicker downthrown beds compared with the correlative upthrown beds. Currie (1956) has demonstrated that grabens composed of contemporaneous faults may be produced by upward-moving salt plugs. 6. Gravity minima which may be caused by salt domes are present in westernmost Florida and southwestern Alabama. The three deepest oil test holes in Escambia and Santa Rosa Counties, Florida, were drilled to depths of about 12,500 feet and reached total depth in the Hosston Formation of Early Cretaceous age. An oil well in the Citronelle field, Alabama (about 38 miles northwest of Florida) reached the Louann(?) Salt at about 19,000 feet. Thus, if the salt is present in the western Florida Panhandle, it probably is at least 6,000 feet deeper than the greatest depth so far reached by drilling.