Showing papers in "AAPG Bulletin in 1972"

Deep Mantle Convection Plumes and Plate Motions

Abstract: Evidence shows that volcanic island chains and aseismic ridges are formed by plate motion over fixed-mantle "hot-spots" (Iceland, Hawaii, Galapagos, etc.) and new arguments link these hot-spots with the driving mechanism of continental drift. It is assumed that the hot-spots are surface expressions of deep mantle plumes roughly 150 km in diameter, rising 2 m/year, and extending to the lowest part of the mantle. The rising material spreads out in the asthenosphere, producing stresses on the plate bottoms. Order-of-magnitude estimates show these stresses are sufficiently large to influence plate motion significantly. the total upward flow in the plumes is estimated at 500 cu km/year, which would require the entire mantle to overturn once each 2 billion years.

Aquathermal Pressuring--Role of Temperature in Development of Abnormal-Pressure Zones: GEOLOGICAL NOTES

Abstract: Zones of abnormal subsurface pressures, both above and below hydrostatic, have been described in many areas. The very abrupt changes in pressures and salinities, together with the undercompacted nature of the high-pressure zones, indicate that they are effectively isolated from their surroundings. If this isolation occurred at a shallower depth than the present one, the isolated volume would have been subjected to increasing temperatures as it moved downward. The P-T-density diagram for water shows that, for any geothermal gradient greater than about 15°C/km, the pressure in an isolated volume increases with increasing temperature more rapidly than that in the surrounding fluids. This mechanism for producing excess pressures will operate in addition to most of the ot er processes that have been suggested, but the overall influence in any given area will depend on how well the system remains isolated. If a normally pressured system becomes isolated and is then subjected to a decrease in temperature (for example, if erosion removes considerable quantities of overburden) the pressure in the system will fall below the external hydrostatic pressure. This may have happened in some areas which now have abnormally low pressures.

Late-Stage Dehydration in Deeply Buried Pelitic Sediments

Abstract: Late-stage dehydration of shales containing montmorillonite or highly expandable illite-montmorillonite consists of the conversion of montmorillonite to illite layers, thus removing interlayer water. This conversion involves interlayer potassium fixation and probably is a result of ionic substitution within the montmorillonite alumino-silicate layer. Therefore, the rate at which dehydration takes place--and, indeed, whether it takes place at all--should be dependent on the chemical conditions of diagenesis, particularly pore-water composition. Detailed X-ray diffraction studies of cuttings from several Gulf Coast wells have resulted in a quantitative interpretation of late-stage dehydration of shales, caused by loss of interlayer water in montmorillonite. In the wells studied late-stage dehydration begins in the depth range of 6,000-8,000 ft and continues over a stratigraphic interval of 4,000-10,000 ft or more. Higher geothermal gradients result in shorter dehydration intervals. Montmorillonite dehydration rates reach maxima at the very inception of the diagenetic reaction and very near its close, when new illite layers are introduced into the illite-montmorillonite in an ordered fashion. At the close of the diagenetic reaction the illite-montmorillonite still contains about 20 percent hydrated layers which persist to tota depth in all wells studied. The temperature of inception of late-stage dehydration, as well as of its close, appears to be dependent on pressure, but other parameters such as pore-water chemistry may be equally important.

Longshore Drift, Submarine Canyons, and Submarine Fans in Development of Niger Delta

Abstract: The southwesterly prevailing wind of the Gulf of Guinea strikes symmetrically on the nose of the Niger delta, causing divergent longshore drifts which meet opposing drifts near Lagos and Fernando Poo. Submarine canyons channel about 1 million cu m of sand a year from each pair of opposing drifts to feed submarine fans on either side of the delta foot. In times of low sea level axial distributaries of the Niger feed a third, now inactive, submarine fan in front of the delta. At the time of the last low sea level numerous submarine canyons formed on the front of the Niger delta, and their heads cut back into the Benin Formation continental sands. As the sea rose, these canyon heads formed wide estuaries which have since been filled. All Quaternary canyons except three currently scoured by resedimented longshore drift material have been filled. Because the Niger delta has prograded toward the southwest throughout the Tertiary and because the prevailing wind has blown persistently from the southwest, the longshore drift pattern long has been as it is now, and the two corners of the delta have been areas of high concentrations of submarine canyon formation. There may have been a third area of high concentration of submarine canyons between the Cross River and Niger deltas when these were separate. Recognition of the submarine fan environment leads to a new symmetrical five-layer interpretation of the structure of the Niger delta: (5) top continental sand unit (Benin Formation); (4) transitional sand/shale unit (Agbada Formation); (3) marine shale unit (Akata Formation); (2) transitional shale-sand unit (newly distinguished); (1) bottom submarine fan sand unit (newly distinguished). Other deltas feeding into waters of oceanic depths may have a comparable structure.

Application of Oil-Correlation and Source-Rock Data to Exploration in Williston Basin

Abstract: Most Williston basin oils belong to one of three basic types: (1) a lower Paleozoic type believed to have originated in Winnipeg shales and found predominantly in Ordovician and Silurian reservoirs; (2) a Mississippian type expelled from Bakken shales and produced mostly from Madison reservoirs; and (3) a Pennsylvanian type restricted to the Tyler Formation. The three types are isolated vertically by evaporites in basin-margin areas. All three oil types can be correlated to their source facies. Lithofacies maps of these source sequences, when combined with thermal alteration measurements, provide effective oil-source-area definition. The time of expulsion and amount of expelled oil from each source rock can be estimated when its volume, organic content, and thermal history are known. Paleostructure maps and carrier-bed isopach maps indicate the direction and extent of secondary oil migration, both vertically and horizontally from the source facies. Ultimately, the subsurface distribution of each oil type is predicted in order to define high-grade areas in which to concentrate exploration activity.

Algal Mats, Cryptalgal Fabrics, and Structures, Hamelin Pool, Western Australia: ABSTRACT

Abstract: Cryptalgal sediments and structures in Hamelin Pool, Western Australia, are formed by interactions between blue-green algae, which trap and bind sediment particles, and a variety of mechanical and diagenetic processes. The algae form a cohesive mat that tends to cover intertidal, supratidal, and some shallow subtidal surfaces. The mat is differentiated into seven intergradational types as an expression of variations in algal species present, ratio of filamentous to unicellular forms, quantity of mucilaginous matrix, life habits, and quantity and nature of the host sediment. The distribution of mat types is controlled by environmental factors such as elevation of substrate, drainage, depth and nature of interstitial groundwater, and sediment influx. In tidal flats with gen le gradients, there is a broad zonation of mat types, whereas the mat is highly differentiated and has a condensed, patchy development of types on headlands and locations with irregular topography. The sediments trapped and/or bound by the algal-mat communities are imprinted with distinctive fabrics. These fabrics, which can be related to specific mat types, reflect a complex interaction between the algae and processes of sedimentation and diagenesis. Important factors in the development of fabric are surface texture and internal structure of the mat, rate and frequency of sediment influx, and processes such as oxidation, cementation, and lithification. Changes in mat type with changes in environmental conditions (e.g., shoaling and sediment influx) lead to the development of successions of fabrics in the sediment pile. The mat-sediment complex is shaped by physical factors into a variety of structures: (1) extensive flat-lying sheets, (2) ridge and rill structure, (3) rings and crescents, (4) linked ellipsoids and columns, (5) discrete ellipsoidal and circular columns, and (6) calyx and tiered calyx structures. The size range of structures varies from a few centimeters to several meters; confluent and branched structures also are common. The gross morphology of the structures is largely independent of the mat type (or types) involved in the primary trapping and/or binding processes. Major environmental factors involved in shaping of structures are waves, currents, substrate gradient, and long-term sea-level change; minor factors include burial, exhumation, growth of epiphytes, activity of browsing organisms, gas evolution, corrosion, precipitation, desiccation, and variation in sediment type. These factors also influence the external surface texture of structures.

Sea-Floor Spreading and Structural Evolution of Southern Red Sea

Abstract: The Red Sea has entered the early stages of continental dispersal, and its structural evolution fundamentally has been the rifting and breaching of continental lithosphere by normal faulting attendant on the process of sea-floor spreading. In Oligocene time, the continental lithosphere of the Red Sea area was bowed into a large regional arch with normal faults across the crest. Subsequently, rifting by normal faults that propagated upward through the brittle part of the lithosphere caused strong subsidence on horst and graben and tilted blocks; this rifting led to an extensive marine incursion, and a thick evaporite sequence was deposited in the restricted, hot, arid, low-latitude setting of the Miocene Red Sea trough. A second rift west of the earlier central or axial Mi cene rift originated in the southern Red Sea area in Pliocene time, and the two features now are evolving concurrently over a distance of at least 400 km. An evaporite section of very shallow marine origin accumulated in the western rift during the Quaternary; it constitutes a modern example of salt accumulating in a narrow, restricted, rifted trough which is forming as a consequence of continental breakup. Although considerable separation has occurred in the Red Sea, the present opposing coastlines were never in contact, because the fragmentation of continental lithosphere was very largely attained by normal faulting. Only if a vertical fault cuts the entire thickness of lithosphere could points on opposite coastlines ever have been contiguous.

Permian Rotliegendes of Northwest Europe Interpreted in Light of Modern Desert Sedimentation Studies

Abstract: The Permian redbeds of northwest Europe, termed the "Rotliegendes," are continental clastic sediments laid down under desert and semidesert conditions. Recent drilling in the North Sea, Netherlands, and north Germany has shown that these beds are in a sedimentary basin up to 2,000 km long and 500 km wide. This basin is in the foreland of the Variscan mountains. (Hercynian orogeny) and is of part-orogenic origin and partly fault bounded. Volcanic rocks are present locally, especially in the east. The Rotliegendes strata attain a maximum thickness of approximately 1,500 m. A central shale and halite facies (Haselgebirge facies), corresponding to a salt lake, changes toward the southern basin margin first to anhydritic clay (Ten Boer member) and then to sandstone and conglom rate of the Slochteren member (wadi deposits and eolian sands). Paleowind directions were essentially from east to west. Deflation of the alluvial fans spreading northward from the Variscan mountains resulted in accumulation of up to 200 m of dune sands in the southern North Sea area. The dune sands form the reservoir rock for important accumulations of gas. At present, the proved and probable reserves of Rotliegendes gas in the North Sea, Netherlands, and western Germany are about 2,500 × 109 cu m (85 × 1012 cu ft). The interpretations of Rotliegendes facies are based on a study of modern deserts and their sediments (wadi sediments, dune sands, and the sediments of inland and coastal sabkhas).

Marginal Fracture Zones as Structural Framework of Continental Margins in South Atlantic Ocean

Abstract: A plate tectonic model of the early opening of the South Atlantic is used to describe the structural framework of the continental margins and adjacent oceanic and coastal areas on both sides of the ocean. It is proposed that major offsets of the continental margins necessarily induce the subsidence of coastal basins where fracture zones intersect the continents. The configuration of the accreting plate-margin boundary gradually developed throughout the opening of the ocean after Late Jurassic time.

Review of Upper Cretaceous-Lower Tertiary Stratigraphy in Central and Southern Egypt

Abstract: The Upper Cretaceous-lower Tertiary sedimentary rocks in central and southern Egypt are characterized by gradual facies differentiation of three main types--Nile Valley, Garra-El Arbain, and Farafra. These facies are present in three different basins within the marginal trough on the northern flank of the African shield. Two main parvafacies are recognized, one in the Maestrichtian and the other in the Paleocene; the Nubia Sandstone in southern Egypt represents a heterochronous magnafacies correlatable with several rock units in central Egypt.

Contemporaneous Dolomitization of Middle Pleistocene Reefs by Meteoric Water, North Jamaica: ABSTRACT

Abstract: Middle Pleistocene reefs of northern Jamaica, and elsewhere in the Caribbean, are distinctly different from the late Pleistocene 120,000 year-old "Sangamon" reefs. They are more extensively developed, better lithified, and commonly dolomitized. In Jamaica, the contact between the two units, the "Sangamon" Falmouth Formation and the middle Pleistocene "Yarmouth"(?) Hope Gate Formation, is erosional. This contact demonstrates that the diagenesis and dolomitization of the Hope Gate took place before Falmouth deposition. The Hope Gate dolomite replaces micrite and fills pores created by the dissolution of aragonite, demonstrating the near contemporaneity of aragonite solution and dolomite precipitation. The dolomites have low Sr (220 ppm) and Na (350 ppm) contents, precluding their precipitation from marine fluids. Dolomitization took place as meteoric waters first infiltered the seaward growing, submarine-cemented reefs. The composition of the fluid from which the dolomite precipitated was controlled by the composition of the influxing meteoric water, but more significantly by the composition of the primary reef carbonate. Continued meteoric flushing, after the available Mg was consumed by dolomitization, resulted in low Na, low Sr, and C12- and O16-enriched calcites. The vadose textures superimposed on the intense phreatic-meteoric diagenesis consists only of insignificant amounts of speleothems, despite more than 300,000 years of subareal exposure. End_of_Article - Last_Page 635------------

What is Franciscan

Abstract: Newly established early Cenozoic fossil localities within the "coastal belt" of Franciscan rocks recall the furor which developed before the last decade over occurrences of Cretaceous fossils within the Franciscan "Formation" of presumably Late Jurassic age. It is timely to consider again the question--what is Franciscan?--not only in regard to what physically constitutes the Franciscan, but also to what it should be called. The Franciscan is herein designated a complex, and is defined as the folded, faulted, and stratally disrupted basement terrane of the California Coast Ranges--including extensions into Oregon and Mexico--which is composed of graywacke, shale, minor conglomerate, radiolarian chert and siliceous shale, minor limestone, volcanic rocks, mafic-ultramafic p utonic rocks, and their zeolite-to-blueschist-facies metamorphic equivalents. Thus, the Franciscan Complex is shown to have both structural and lithologic significance. Other terms used in connection with Franciscan rocks are also defined.

Clay Minerals and Petroleum-Forming Reactions During Burial and Diagenesis

Abstract: Model experiments have been carried out to investigate the effect of montmorillonite as a catalyst in promoting organic reactions of importance in hydrocarbon generation. Montmorillonite catalyst promotes the decarboxylation of fatty acids to form long-chain alkanes. It likewise promotes subsequent cracking of these alkanes to produce shorter chain alkanes with molecular distribution similar to those of petroleum. From kinetic considerations the activation energies for these two model reactions can be estimated at about 36 kcal/mole for decarboxylation and about 46.5 kcal/mole for catalytic cracking of the C21 hydrocarbon. The kinetic models are applied to a natural situation, where the geothermal gradient and rate of subsidence in a sedimentary basin permit consideration of the extent of organic transformation as a function of changing depth (temperature). A geochemical model is proposed which relates, in sequence, alkane production (by decarboxylation), maturation (cracking), and migration (water expulsion during diagenesis). The model postulates depth (temperature) zonation which is qualitatively in agreement with fatty acid and hydrocarbon molecular distributions observed when young and ancient sediments and petroleum are compared.

Distribution of Carbon in Crust of Earth: GEOLOGICAL NOTES

Abstract: The total carbon in crust of the earth is about 9 × 1022 g. Continental and oceanic sediments contain 1.2 × 1022 g of organic carbon and 6.4 × 1022 g of carbonate carbon. There is about three times as much organic carbon in claystones and shales as in carbonate rocks and sandstones. The carbon in the oil and gas of petroleum reservoirs totals 1 × 1018 g or about 0.01 percent of the organic carbon in sedimentary rocks. The hydrogen in the organic matter of sedimentary rocks totals 9 × 1020 g or about 32 times as much as is needed to form all the petroleum in both reservoir and nonreservoir rocks. This inventory shows not only that there is adequate hydrogen to form oil in the sediments, but als that the entire process of the origin, migration, and accumulation of petroleum is extremely inefficient.

Economic Evaluation of Reykjanes Thermal Brine Area, Iceland

Abstract: The Reykjanes thermal brine area is in southwesternmost Iceland on the subaerial continuation of the Reykjanes ridge. This area is unique among thermal areas for its fluid composition. In the reservoir, where temperatures are between 250 and 290°C, the brine has the same salinity as seawater. However, the concentrations of some ions are different and can be explained by relatively simple interaction of seawater with the rock. The content of trace metals is low. In the upflow zone, hot brine has been exchanged many times during the lifetime of the hydrothermal system. Surface activity covers an area of 1 sq km, and a resistivity survey indicates that the system is of similar area above 900 m within the hyaloclastite formation where it is surrounded by cold groundwater of seawater composition. By contrast, the system is more extensive in the underlying basalt formation consisting of lava flows and thick interbeds of hyaloclastites and sediments. At 2,600 m depth P-velocity increases from 4.2 to 6.5 km/sec. At this depth the basalt formation is underlain by a denser formation considered to be similar to the "oceanic layer." Because of its temperature and composition, feasibility studies indicate that the brine could be exploited economically for the production of NaCl, KCl, CaCl2, and possibly other components. Aquifers are abundant in the basalt formation. It therefore is recommended that production wells penetrate to depths of about 2,000 m to withdraw brine within this permeable formation, so as to ensure highest mass flow and minimize risk of cold seawater intrusion.

Paleozoic-Mesozoic Sedimentary Basin of Mekele Outlier, Northern Ethiopia

Abstract: Until now, only a schematic sedimentary interpretation for the Horn of Africa has been accepted. The history of the sedimentary section around Mekele--previously called Adigrat Sandstone, Antalo Limestone, and Upper Sandstone--was not so simple, and it already had begun either in the Ordovician or in the Carboniferous. Three basins of deposition were formed successively. In the first one, the lower Enticho Sandstone, which is exposed only east and north of Mekele, was deposited. The second basin was channel-like, trending north-south and filled by Edaga Arbi glacial deposits and the glacial facies of the Enticho Sandstone. This basin probably was coincident with a system of troughs trending from north to south in the Horn of Africa, eastern Africa, and southern Arabia. Th basin of deposition for the Adigrat Sandstone, Antalo Limestone, Agula Shale, and Amba Aradam Formation was the third to be formed; its history coincides with the regional sedimentary history of the Horn of Africa, from Triassic to Cretaceous. Sedimentation during this period was controlled by the widespread transgression covering the continent from the east and south. It involved two major structural highs, as well as two main basins, one of which, in central west Ethiopia, contains the Mekele outlier as a subbasin. These partly new aspects of the sedimentary history of the area may contribute to new ideas for oil exploration.

Liquid Movement Through Kaolinite under Hydraulic, Electric, and Osmotic Gradients

Abstract: Hydraulic, osmotic, and electro-osmotic conductivities of sodium kaolinite were measured as a function of compaction pressure ranging from 1 atm to approximately 600 atm. The data were used to estimate the relative importance of osmosis and electro-osmosis, compared with hydraulic flow, through confining beds at different depths of burial. The results indicate that Darcy's law is not a sufficient basis for predicting either the magnitudes or the directions of liquid movement through deep confining beds.

Environment of Wyoming Tertiary Uranium Deposits

Abstract: Four major uranium districts in Tertiary rocks of central Wyoming are in fluvial sandstones derived from the granitic rock of the ancestral Sweet-water arch and deposited in adjacent intermontane basins. Sediment transported southward into the Great Divide basin was deposited on an apron of alluvial fans. Sedimentation in the Gas Hills area of the Wind River basin was on an alluvial fan in which ridges of older rock disrupted the normal development of the fan. Sediment in west Shirley basin was deposited on an alluvial fan, but in east Shirley basin and in the Powder River basin sedimentation was channel and flood-basin deposits of a meandering stream. The sandstones are subarkosic to arkosic, medium grained to conglomeratic, angular and poorly sorted. Sandstone intertong e with green or carbonaceous shales. Sedimentation was in a warm, humid climate with abundant vegetation. Decay of the organic material created reducing conditions in the sediment which caused partial carbonization of some of the plant debris, formation of pyrite, and precipitation of uranium minerals. Following burial, uplift-induced changes in the hydrodynamic system caused an invasion of the reduced sediment by oxygenated water far below the static water table. This caused destruction of carbonaceous material, oxidation of pyrite, and accumulation of uranium and other susceptible metals in a wave or front just ahead of the invading oxidizing environment. The invading oxidation was a dynamic, expanding process which moved through the permeable zones of the fluvial sequence until its dimensions measured miles in areal extent and hundreds of feet in thickness. This geochemical cell had a sharply defined boundary produced by biochemically controlled changes in physical and chemical conditions. Oxygenated waters, aided by Thiobacillus ferrooxidans, oxidized pyrite to produce sulfuric cid and ferric sulfate, a strong oxidizer, which leached uranium and other susceptible elements. In the reducing part of the cell, anaerobic bacteria, including the sulfate reducer Desulfovibrio, consumed the organic material in the sediments and the sulfates from the oxidizing area, to produce hydrogen, hydrogen sulfide, and a mildly alkaline, strongly reducing environment which precipitated pyrite, uranium, and other metals on the front. Migration of the cell was controlled by the permeability of the sandstone and by availability of carbon and pyrite. The cell advanced faster in the more permeable zones and was retarded in zones of reduced permeability and areas of greater pyrite and carbon content. The position of the mineral front was a function of the initial sedimentary pattern. Se imentation, alteration, and mineralization in the Gas Hills and Shirley basin districts illustrate these conditions and processes.

Geology and Petroleum Prospects in Eastern Red Sea

Abstract: The Red Sea region was covered by marine embayments from time to time in the Jurassic, Cretaceous, and Eocene, but the present tectonic pattern began to evolve in the Oligocene and Miocene sediments were deposited, including the highly organic lower Miocene Globigerina group and a thick sequence of middle Miocene salt and anhydrite. In late Pliocene and Pleistocene times there was renewed block faulting accompanied by igneous intrusion. The Red Sea depression is characterized by rectilinear faults bordering blocks which were rising or sinking concurrent with Neogene deposition. Oil seeps are known in widely separated localities in the Red Sea depression. Seeps in the Farasan and Dahlac Islands led to drilling of several wells on these islands. Exploration in recent years ed to the discovery of the Barqan gas-condensate field in the northern Red Sea and a well that had a gas blowout off the Eritrean coast. Many of the wells drilled in the Red Sea region have encountered high pressures and high temperatures. The chief source and reservoir rocks will be found in the lower Miocene Globigerina group, with the overlying evaporites sealing the accumulations. The prospects for further discoveries in the Red Sea depression are considered good.

Mainstream Mantle Convection: A Geologic Analysis of Plate Motion

Abstract: Within the earth's upper mantle, an eastward-flowing mainstream is postulated from the character of asymmetric fracture zones and the difference in crustal response to westward versus eastward underthrusting along active subduction zones. Coupled to convection cells beneath the oceanic ridges, the mainstream forms a pattern of migrating zones of primary and secondary mantle upwelling which, in a kinematic model, indicates (1) that a form of asymmetric sea-floor spreading has created the small ocean basins behind island arcs associated with westward-underthrust subduction zones; (2) that the depth to the base of convection varies systematically eastward from ridge to ridge, being shallowest beneath the East Pacific Rise and deepest beneath the Indian Ocean ridge; (3) that econdary zones of mantle upwelling are located along the leading edges of eastward-migrating convection systems and underlie such features as the East African rift system; and (4) that plates descend into the mantle along subduction zones in response to the removal of material from below. In the model, the base of the mainstream forms an unbroken spheroid with its poles of rotation near the earth's geographic poles. The maximum velocity of the mainstream is therefore latitude-dependent. This postulated configuration of the mainstream suggests that directional flow within the mantle may be the result of rotation of the base of the upper mantle at an angular velocity which is slightly higher than that of the lithosphere. To produce a 40-cm/year mainstream velocity, the rotational velocity of the lithosphere must be slowed relative to the lower mantle by one part in 10 billion. This slowage would increase the length of day by only 0.5 millisecond per century, or about a quarter of the amount of increase in length of day attributable to tidal forces.

Depositional Environments and Geologic History of Golden Lane and Poza Rica Trend, Mexico, an Alternative View

Abstract: Middle Cretaceous cores from the prolific oil fields of the Golden Lane and the Poza Rica trend in eastern Mexico were studied to determine the environment of deposition of the reservoir and associated rocks, to consider the significance of sedimentary facies for interpreting the geologic history of the Tampico embayment, and to compare the middle Cretaceous carbonate rocks and history of this area with others of the same age along the Texas Gulf Coast. The Golden Lane fields produce from the El Abra Limestone, which was deposited in a shallow-water shelf or lagoon with scattered rudist patch reefs. The structurally lower Poza Rica trend fields contain rocks of the Tamaulipas and Tamabra Limestones. The Tamaulipas Limestone was deposited principally under open-marine, basinal conditions. The Tamabra Limestone is composed of shallow-water coral-rudist reefs, debris derived from the reefs and deposited in shoal-water nearby, and forereef talus mixed with basinal muds. Production in the Poza Rica trend is mainly from the reef debris. No coral-rudist reef was recognized in the small amount of available core examined from the Golden Lane, and present data do not support the prevalent view that the Golden Lane is a barrier reef, or reef-fr nged atoll, or that the Tamabra Limestone represents deep-water deposits transported 8-16 km (5-8 mi) from the supposed Golden Lane barrier reef. The carbonate rocks of the Golden Lane and Poza Rica trend and of the "Deep Edwards" trend in south Texas are approximately the same age and, broadly speaking, were deposited under similar environmental conditions on a shallow shelf and at the shelf edge adjacent to a basin. However, the Golden Lane and Poza Rica trend are only about 60 km (37 mi) from the Sierra Madre Oriental, a major early Tertiary orogenic belt, whereas the "Deep Edwards" trend is hundreds of miles from the same belt. Movements associated with the early Tertiary orogeny caused exposure and subaerial leaching, producing remarkable porosity in the Golden Lane. Thus, although depositional environments of the middle Cretaceous in south Texas parallel to those of eastern Mexico, the subsequent geologic histories of the two regions are markedly different.

Trophic Group Analysis of Upper Cretaceous (Maestrichtian) Bivalve Assemblages from South Dakota

Abstract: The distribution and composition of Holocene benthic assemblages of suspension and deposit-feeding trophic types are primarily controlled by (1) food availability, (2) sedimentation rate, (3) bottom stability and turbidity, (4) salinity, and (5) dissolved oxygen. Diverse and abundant fossil bivalve faunas from the Upper Cretaceous (Maestrichtian) marine deposits of South Dakota are used to evaluate the usefulness of trophic group analysis in paleoenvironmental reconstruction. The recessional history of the Late Cretaceous sea in northwest-central South Dakota is recorded in an offlap sequence in the upper Pierre Shale and Fox Hills Formation representing offshore-shelf, bar-influenced-shelf, offshore-bar, and deltaic environments. Well-preserved molluscan assemblages from most of this sequence permit trophic comparison with assemblages from analogous Holocene environments. Trophic groups reconstructed from the bivalve assemblages closely reflect the hydrographic and sedimentary conditions reconstructed for these paleoenvironments from independent lithologic and stratigraphic evidence. Recognition of fossil trophic groups can provide a useful tool in environmental reconstruction and analysis of fossil community structure.

Influence of Salina Salt on Structure in New York-Pennsylvania Part of Appalachian Plateau

Abstract: The Appalachian Plateau fold belt is coincident with the boundaries of the Salina Salt, terminating on the northeast at the northern extension of the Lackawanna syncline in south-central New York and northeastern Pennsylvania, and on the southwest at the Burning Springs anticline in West Virginia. Present information shows that in the New York-northern Pennsylvania area, Appalachian Plateau folds are decollements on the Salina Salt. The folds above the thickened salt bear no relation to folds or faults which may or may not be present beneath the salt. The decollement may have originated either by gravity sliding from the west or by thrusting from the east. There is evidence to support each mechanism. Deformation caused by thrusting from the east is the preferred interpretation.

Submerged Reefs of Eastern Caribbean

Abstract: Submerged early Holocene or late Pleistocene reefs up to 90 km long and with bottom relief commonly about 20 m were established in relation to preexisting lower sea levels on outer edges of terraces at 30-80-m depths off most islands in the eastern Caribbean Sea. Submerged reefs are far more impressive physiographic features than their modern counterparts and are in depths greater (below about 15-20 m) than those commonly associated with present reef-framework construction by hermatypic corals. Data from echo-sounder profiles, rock dredging, bottom photographs, and in situ observations indicate that off the Virgin Islands, St. Martin, St. Barthelemy, Montserrat, Guadeloupe, Martinique, St. Lucia, and the Grenadine Islands submerged reefs are dead and covered by only a few scattered living corals. Hermatypic corals below 15 m on these reefs cannot cope with skeletal destruction by boring organisms and cannot compete for substrate with other encrusting or attached organisms. Off the west coast of Barbados, however, reef-framework construction is still occurring below about 15 m. The age relations of these reefs are not known, but probably they are no older than late Pleistocene and started to grow no later than 8,000 years ago. Although the eastern Caribbean area is characterized by inner-shelf fringe reefs, the common occurrence of shelf-edge submerged reefs indicates that, during the latter stages of the Holocene transgression, reefs were generally adjacent to deep waters, as are modern Pacific barrier and atoll reefs.

Sedimentology of Gachsaran Formation (Lower Fars Series), Southwest Iran

Abstract: The Gachsaran Formation, formerly known as the "Lower Fars Series," is exposed extensively in the foothills of the Zagros Mountains in southwest Iran, and forms the caprock at many important oil fields. During recent visits to this area, we were impressed by the almost universal nodular habit of the gypsum-anhydrite rocks, and by the conspicuous cyclic nature of the sediments. This paper presents two field sections which we suggest as typical of the marginal and the central-basin facies. The former was measured near the northern margin of the basin, where the beds at the top of each cycle are red marl, siltstone, or even sandstone, clearly of continental aspect; the latter was measured near the center of the depositional basin, where the evaporitic part of the cycle is dominant. In both sections, the base of each cycle is a marine carbonate rock of demonstrably shallow-water association, which is overlain by nodular gypsum-anhydrite rock, and at the top by dominantly red or variegated marl. The entire sequence is remarkably similar to the shallow-water, marine, supratidal sabkha, and terrestrial environments of the Trucial Oman Coast first described by Shea man in 1963. The Gachsaran Formation records a succession of minor cycles of sedimentation between the fully marine development of the Asmari limestone and the continental redbeds of the Agha Jari Formation.

Proposed Stratigraphic Nomenclature for Kenai Group, Cook Inlet Basin, Alaska

Abstract: The Cook Inlet basin in south-central Alaska is approximately 200 mi long and 70 mi wide. It contains a thick section of predominantly nonmarine Tertiary sedimentary rocks with major reserves of oil and gas. Early geologists working in the area, hampered by scarcity of good outcrops, named these coal-bearing strata the Kenai Group or Series and assigned a probable thickness of 4,000-5,000 ft. Later workers renamed it the Kenai Formation. Oil and gas exploration, beginning in 1957, supplied previously unavailable subsurface control that greatly expanded geologic knowledge of the area. Subsurface data have made it evident that the Kenai is at least 26,000 ft thick in the central part of the basin and that it is divisible into five major mappable lithologic units, each deserving formational rank. These units are distinguishable by lithology and mechanical-log characteristics. Therefore, the Kenai Formation is elevated to group status and renamed the Kenai Group. As herein redefined, the Kenai Group consists of five new formations, in ascending order, West Foreland Formation, Hemlock Conglomerate, Tyonek Formation, Beluga Formation, and Sterling Formation.

Vegetation and Geochemical Prospecting for Petroleum

Abstract: The value of geochemical prospecting for petroleum has been challenged by Smith and Ellis, who reported that soil hydrocarbons, saturated as well as unsaturated, result from vegetation in the form of grass and roots. These investigators claimed that 1 g of this type of vegetation is sufficient to produce the same quantity of saturated hydrocarbons found in 200 g of soil, and imply that it is the source of hydrocarbon anomalies. Unfortunately, no quantitative analytical data were supplied by them and, therefore, the claim is not substantiated. Investigations by the present writer show that less than one part per billion by weight of saturated hydrocarbons, in the range from ethane through pentane, is contributed by 1 g of grass or roots to 200 g of soil. This concentration is much lower than that normally found in the soil of background or barren areas and, therefore, is of no importance in geochemical exploration. Actually, few significant anomalies contain ethane through pentane values lower than 25 parts per billion by weight. Small amounts of saturated hydrocarbons may be generated by heating grass or roots in a partial vacuum, either alone or in the presence of phosphoric acid, but in the presence of concentrated nitric acid the production of saturated hydrocarbons is so inhibited that less than one part per billion of ethane through pentane is contributed to 100 g of soil by as much as 5 g of this vegetation. Therefore, the presence of any of this group of saturated hydrocarbons in grass or roots, in the natural state, is doubtful. As additional evidence to condemn geochemical exploration, Smith and Ellis cited their unsuccessful attempts to find anomalous hydrocarbon values over oil fields. These failures are explained easily by the fact that their soil gas extraction technique is incapable of removing the major part of the saturated hydrocarbons that are adsorbed on the soil, making it difficult to discern an anomaly. Moreover, the near-surface anomalies, originally produced by the fields they sampled, may have become weakened or even have disappeared because of changes in reservoir conditions that take place as petroleum is being produced.