Reef Trend Research Articles

Porosity Loss by Deep-Burial Thermochemical Sulphate Reduction (TSR) In Upper Devonian Leduc Dolostones, Southern Rimbey-Meadowbrook Reef Trend, Alberta: ABSTRACT

Porosity Loss by Deep-Burial Thermochemical Sulphate Reduction (TSR) In Upper Devonian Leduc Dolostones, Southern Rimbey-Meadowbrook Reef Trend, Alberta: ABSTRACT

Subsurface dolomites in Upper Devonian Leduc Formation buildups, central part of Rimbey-Meadowbrook reef trend, Alberta, Canada

Subsurface dolomites in Upper Devonian Leduc Formation buildups, central part of Rimbey-Meadowbrook reef trend, Alberta, Canada

Silurian Pinnacle Reefs of the Canadian Arctic

Pinnacle reefs are commonly an attractive target for oil exploration because they are usually porous carbonate bodies entombed in impervious, deep-water shales that provide both the source and the seal for hydrocarbons. Silurian pinnacle reefs, the first described in the Canadian Arctic Archipelago, are exposed on Ellesmere and Devon Islands. Two main reef trends occur, one of early middle Llandovery to middle Ludlow age and a second of middle Ludlow to Late Silurian or Early Devonian age. Reefs of both phases contain lime mudstone cores: some are stromatactoid-rich and others consist predominantly of microbialite-rich lime mudstone or microbial boundstone. Facies sequences of both reef phases show evidence of upward-shallowing overall, but, in the older reefs, isochronous capping facies are dominated either by coral-mirian or by stromatoporoid boundstone and floatstone. This difference perhaps reflects variation in wave stress and apparent ability of a few corals,thickly encrusted by or associated with microbial boundstone and skeletal algae, to withstand greater wave energy than a stromatoporoid-coral-rich reef community. These reefs constitute one of the bright prospects of hydrocarbon exploration in rocks of the Franklinian succession. 43 refs., 9 figs.

Correction to “Mercury's atmospheric bright spots and potassium variations: A possible cause” by A. L. Sprague

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Porosity Variations in Upper Devonian Leduc Dolomites, Central Rimbey Meadowbrook Reef Trend, Alberta

Porosity Variations in Upper Devonian Leduc Dolomites, Central Rimbey Meadowbrook Reef Trend, Alberta

Depositional Model for the San Andres Formation, Roberts Unit, Wasson Field, Yoakum County, Texas

The Permian San Andres reservoir at Roberts unit produces from approximately 250 ft of anhydritic dolostones. The reservoir interval, which is more than 500 ft below the top of the San Andres Formation, consists of fossiliferous and pelletal/peloidal dolowackestones and dolopackstones. They were deposited in a shallow-marine environment with local shoaling conditions. Toward the top of the reservoir, intertidal and supratidal deposits interfinger with the subtidal units and form the lateral and overlying seals. A sponge-bryozoan bank lithofacies is recognized within the subtidal deposits at Roberts unit. The banks consist of dolomitized mud-rich boundstones dominated by bryozoans, sponges, and crinoids. Interbedded fossiliferous dolowackestones, dolopackstones, and dolograinstones are common. The restricted nature of the San Andres in the western part of Roberts unit (i.e., shoreward of the banks) indicates that the banks baffled wave energy and inhibited current circulation on the platform, resulting in a mud-dominated, restricted lagoonal facies with very low faunal diversity. The sponge-bryozoan banks occur within a narrow belt across the central part of Roberts unit and continue into the adjacent Willard unit. Their distribution has a distinct northeast-soutwest trend that parallels the subjacent Abo shelf margin reef trend, suggesting that the Abo reef trend influenced subsequent bankmore » development.« less

Comparison of Ordovician lithistid sponge communities in the Great Basin

Lithistid sponges are an important component of many Ordovician faunas and form some of the earliest reef communities of the Paleozoic Evolutionary Fauna, yet they have received relatively little attention. Fossiliferous strata with abundant sponges occur at many horizons and localities within the Lower and Middle Ordovician Pogonip Group of the Great Basin. One of the best-known occurrences is near the middle of the Antelope Valley Limestone in central Nevada, where sponge-bearing units occur within the lower Whiterockian Stage. Lithistid sponges and associated species were also examined within the Shingle Limestone of eastern Nevada, where the sponge communities occur as small reef mounds about 55 m below the top of the formation in beds corresponding to the uppermost Ibexian Series. To acquire a more regional view of the environments and faunas present in Nevada during the Ordovician, contemporaneous units in the Pogonip Group were examined elsewhere in Nevada and in the Ibex area of western Utah.The general depositional setting of the Great Basin at this time was that of a carbonate ramp in the upper Ibexian developing into a carbonate platform during the Whiterockian. The sponge mounds in the Shingle Limestone, forming in water 1 to 15 m deep, may have participated in this development by interfering with water circulation along the outer portion of the carbonate ramp. The mounds are several meters in size and are often stacked, with grainstone-filled channels cutting through the mounds in several places. The mounds themselves have muddy cores and are flanked by well-sorted echinoderm grainstones. The reef trend extends for nearly two kilometers before being faulted out, and was probably even more extensive during its development. Besides sponges, the assemblage contains abundant calcareous algae and cyanobacteria, as well as trilobites, brachiopods, echinoderms, and other groups characteristic of the Paleozoic Fauna. These mounds are very similar to those found in the Wahwah and Juab Limestones of the Pogonip Group, suggesting that these latter buildups also formed in very shallow water.The younger sponge communities in the Antelope Valley Limestone at Ikes Canyon probably formed in about 20 to 40 m of water along the uppermost slope of a carbonate platform. The sediments consist of interbedded wackestones, mudstones, and shales with occasional hardgrounds. Some of the sponges have root tufts, indicating that they settled on soft substrate. Besides lithistid and heteractinid sponges, the unit contains a diverse assemblage of metazoans and occasional receptaculitids and other algae.Estimates of relative species abundances were calculated for random samples from the sponge-bearing units. The Shannon Diversity Index for the Antelope Valley Limestone fauna is 3.24 with an equitability value of 0.68, while the Shingle Limestone community has values of 3.44 and 0.74, respectively. Thus, not only were both communities diverse, but the species within each community were comparable in abundance. These values compare favorably with those of modern marine communities, especially when the effects of preservational bias are considered, and suggest that well-integrated ecological communities with substantial amounts of biotic interaction had evolved by the Middle Ordovician.The sponge communities of the Antelope Valley Limestone and Shingle Limestone are thus quite similar, yet only in the latter did the organisms build bioherms. The sucession of communities within these buildups indicate that the greater abundance of cyanobacteria and calcareous algae in the Shingle Limestone probably helped to bind and stabilize the sediment, creating a firm substrate. This was then settled by cylindrical and branching lithistid sponges and other species that not only acted as bafflers, causing mud to accumulate, but produced additional sediment when they died. As the mounds grew, the communities diversified, with cup-shaped lithistids becoming dominant as the bioherms approached sea level. The importance of calcareous algae and cyanobacteria in determining whether mud mounds developed in this depositional setting suggests that factors controlling the distribution of algae, primarily substrate, turbidity, and water depth, also control the distribution of mud mounds. This is probably true not only for these sponge mounds, but for mud mounds throughout the Paleozoic.

Multistage Diagenesis of the Upper Devonian Rimbey-Meadowbrook Reef Trend in Central Alberta, Canada: Possible Implications for Large-Scale Fluid Flow

Multistage Diagenesis of the Upper Devonian Rimbey-Meadowbrook Reef Trend in Central Alberta, Canada: Possible Implications for Large-Scale Fluid Flow

Dolomite-rock textures of Leduc dolomites in the Leduc, Westerose, and Wizard Lake buildups in the Rimbey–Meadowbrook reef trend

Dolomite-rock textures of Leduc dolomites in the Leduc, Westerose, and Wizard Lake buildups in the Rimbey–Meadowbrook reef trend

Seismic stratigraphy of Discovery Bay, Jamaica

High resolution seismic reflection profiling within Discovery Bay, Jamaica has revealed that this 54 m deep, subcircular, reef-protected coastal basin has over 100 m of relief. Partially buried reefal structures at a water depth of −40 m indicate a past sea-level stillstand. In addition, side scan sonar reveals that the slopes leading down into the basin from these upper reefs are nearly acoustically featureless indicating little downslope movement by reefal debris and no slope failure. The basin center is dominated by a 15 m thick upper sequence of mollusk-rich carbonate mud which has accumulated at a rate of 1600 mm 1000 yrs for the past 9000 yrs based upon 14C dates. The total 50 m of basinal infilling consists of two primary sequences both approximately of the same thickness. Without very long cores (> 15 m) we can only speculate on basin infilling history. The deepest subsurface portion of the basin lies on line with a fault-controlled, dry valley and a pronounced change in the seaward reef trend. The north coast of Jamaica consists of a terraced, limestone coastal plain. We propose that Discovery Bay initially formed by dissolution from calcite-undersaturated groundwaters focused along this fault. A reefal barrier rim developed and sealed off this depression from the open Caribbean Sea sometime during the Pleistocene. A similar coastal depression, Rio Bueno, lies just to the west of Discovery Bay, but remained open probably due to continued freshwater influx which retarded reef growth during lowstands throughout the late Neogene and Quaternary.

Wizard Lake: Reservoir Quality As A Key To Successful Miscible Displacement

Abstract The Wizard Lake D-3A pool, discovered in 1951, has been under a vertical gravity stable first contact miscible flood since 1969. In 1983, a tertiary extension to the miscible scheme was implemented. Ultimate oil reserves of 57.8 × 105 m3 or 86% of the original-oil-in-place (OOIP) are predicted for the pool of which 44.2 × 106 m3 is from primary recovery and 13.6 × 105 m3 is the result of miscible displacement. To date (August 1990), 51.9 × 106 m3 of oil or 77% OOIP has been produced. Introduction The Wizard Lake reservoir is one of a chain of Upper Devonian age Leduc reefs located in the Western Canadian Sedimentary Basin in Alberta (Fig. I). The Leduc reefs are aligned approximately north-south to form the Rimbey-Meadowbrook reef trend (Fig. 2). The Leduc reefs in this trend rest on a regionally extensive series of shallow water, platformal carbonate sediments (the Cooking Lake Formation). These form an active aquifer and the reservoirs in the Rimbey-Meadowbrook chain are hydraulically linked as a result. The Wizard Lake reservoir consists of the Leduc reef proper and the uppermost portion of the underlying Cooking Lake Formation. The entire reservoir has been dolomitized and as a result the pool in general has excellent reservoir quality. The Wizard Lake reef is encased in shales and variably argillaceous carbonates of the Ireton and Duvernay formations (Fig. 3), most of which are believed to represent terrigenous sedimentary basin in fill contemporaneous with Leduc reef growth. At the time of its discovery in 1951, the Wizard Lake reservoir was filled with oil to a depth of 1229.6 m subsea. The maximum thickness of the oil column in the pool was 196.5 m, and the pool contained an estimated 67 106 m3 of original-oil-in-place. Cumulative production through August 1990 is 51.9 × 106 m3. Historical Summary of Pool Exploration From 1951 to 1969, the Wizard Lake pool was produced under primary recovery which consisted of vertical water drive from the Cooking Lake aquifer influx with gravity segregation and solution gas drive. By 1954, a gas cap had formed. In 1969, a gravity stable vertical miscible flood was initiated with injection of a 75% HCPV solvent followed by push gas. The pool was repressured by gas and water injection to 14,480 kPa (gauge). The oil-water contact moved up to 1207 mss. This performance is reviewed in Reference 3. In 1983, a tertiary extension of the secondary miscible flood was initiated. Current miscible displacement is now occurring in the lower parts of the reservoir. To fully understand reservoir performance in this area, a geological model must be constructed. Geological Model of Reservoir Setting The geological model presented below is the result of a series of studies initiated in 1986. This work was greatly facilitated by new porosity log data —acquired since 1980 as wells in the reef were redrilled or twinned —and by data provided by the five flank wells drilled since 1987. Additional geological information was obtained from core data from the neighboring, analogous Bonnie Glen Leduc reef.

Burial Dolomitization of the Upper Devonian Rimbey-Meadowbrook Reef Trend, Central Alberta, Canada

Subsurface Leduc buildups (Woodbend Group, Frasnian) of the Rimbey-Meadowbrook Reef trend in Central Alberta, Canada, extend along a linear trend for a distance of 320 km. Most buildups are located on top of the western margin of the underlying Cooking Lake platform (Frasnian). The Leduc buildups are dolomitized only where the margin of the Cooking Lake platform is dolomitized, suggesting that dolomitization of both the platform and the buildups is related. The extent of dolomitization in the Cooking Lake platform ranges from scarce to complete, but the overlying buildups are completely dolomitized. Petrographic data suggest selective replacement of limemud matrix and allochems. The most abundant dolomite type is coarse-crystalline planar mosaic dolomite. It forms dense, nonporous mosaics of subhedral to anhedral crystals, or porous zones characterized by intercrystalline porosity. Replaced allochems are characterized by nonplanar dolomite. Both dolomite types show homogeneous luminescence and exhibit similar isotope values ({delta}{sup 13}C: 2.2 to 3.5{per thousand}, {delta}{sup 18}O: -6.7 to -5.0{per thousand}). Only minor amounts of nonplanar dolomite cements (saddle dolomite) are present, lining pore-spaces and fractures. The observed stratigraphic and spatial distribution of the massive replacement dolomite is compatible with subsurface dolomitization. The Cooking Lake platform margin and the overlying Leduc buildupsmore » were probably dolomitized by the same fluid-flow system. Most of the dolomitizing fluids presumably were derived from downdip areas, utilizing the Cooking Lake platform margin and/or a fracture-fault system as a conduit. Ongoing geochemical analyses will help to constrain the origin of the dolomitizing fluids, their composition, and fluid-flow parameters.« less

Structure and Tectonics of the Sierra Madre Oriental Fold-Thrust Belt Near Monterrey, Northeastern Mexico

The Monterrey salient was examined using Landsat TM and MSS images to determine factors controlling the development, distribution, and exposure of Laramide structures. Lateral Mesozoic facies changes influence structural styles and distribution. Exposure of deep foreland structures north of Monterrey is partly related to the location of the Cupido reef trend. Structure along the front of the salient changes abruptly from tectonic wedging to normal overthrusting where deltaic clastics of the Difunta Group grade into incompetent Mendez shale. Salt thickness is an important factor controlling structural development. Areas without evaporites are usually persistent basement highs characterized by less severe deformation. Areas with thin evaporites have complex structural styles, depending on stratigraphy, depth of exposure, and distance from the Sierra Madre thrust front. Thick salt, apparently in a Jurassic rift beneath the salient, facilitated the northward transport of thrust sheets. Late Cretaceous salt movement influenced stratigraphy in La Popa basin where limestone lenses developed in the clastic Difunta Group. Basement topography is the major factor controlling development, style, distribution of structures, and areal distribution of salt and lateral facies changes. The authors propose a tectonic model that explains the large scale structural styles in the region. The Coahuila basement-high block actedmore » as a buttress during Laramide shortening, limiting northward progression of deformation west of Saltillo. East, in the Monterrey salient, the effect of deeper basement and thick salt permitted thrusts to transport material much farther north, resulting in development of a north-south zone of distributed left-lateral shear in the region of Saltillo.« less

Tectonic Position, Hydrocarbon Exploration, and Future Potential in Bulgaria

The oil- and gas-prospective areas in northern and southeastern Bulgaria and the adjacent regions on the Black Sea shelf are accepted as a part of the Carpatho-Balkan oil- and gas-bearing province. The peculiarities of the tectonic structures are conditioned by some specific, mostly collision processes during the Paleozoic, Mesozoic, and Tertiary. They are deciphered now with the help of the depth seismic explorations, and magnetotelluric, thermal, gravimetric, and magnetic data, in combination with geological mapping of south Bulgaria. The usual type of seismic data and well control as well exist in north Bulgaria and the Black Sea shelf zone; parts of this information has been published. Now an attempt is made for general overview of the three regions built by some parts of the Moesian platform; ForeBalkan, Balkan, and Srednogorie region; and the western part of the Black Sea megadepression, as well. Prospects of the Mesozoic and Tertiary sediments in north Bulgaria have been comparatively well studied, but questions remain for southeastern Bulgaria and the Black Sea shelf zone, coming from unresolved problems concerning the geological structure. Future regional exploration is expected to discover new deposits in the reef trends of Paleozoic, Triassic, and Late Jurassic-Valanginian age, and connected withmore » usual local Tertiary and Mesozoic structures and vast overthrust structures of the Balkan and southward, in both the offshore and onshore.« less

Geophysicists

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Legislative action for Earth science

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Geophysicists

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

AGU advocacy for science

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Geophysicists

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Geophysicists

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.