• Energy Research
• 15. Life on land close
• AAPG Bulletin close

Sequential restorations of a north-south megaregional cross section across the north-central Gulf of Mexico Basin from east-central Louisiana to the abyssal plain define a dynamic, complex history of sedimentation, salt flowage, and salt evacuation. Proprietary composite seismic profiles (590 km), 33 wells, and published depth-to-basement maps were used to constrain the section in depth. Thirteen sequential structural restorations, incorporating both decompaction and isostatic subsidence (thermal and tectonic), were then constructed from the Late Cretaceous to the Holocene. The restorations highlight and constrain a protracted history of deformation that is primarily controlled by gravity and the progradation of Cenozoic sediments over salt. Early stages of the tectonic history of the northern Gulf of Mexico Basin were related to differential thermal subsidence resulting from Early-Middle Jurassic rifting. During the Cenozoic, the evolution of the basin was dominated by the influx of large clastic depocenters, which caused the basinward evacuation of autochthonous Jurassic salt. Salt extrusion from the autochthonous layer was accomplished by inclined salt bodies, which flowed into salt glaciers or sheets near the sea floor. Evacuation of allochthonous salt layers provided significant sediment accommodation, and unusually thick sedimentary sections were deposited, such as the Terrebonne Trough of southern Louisiana (3-7 km of Miocene strata). Salt sheet formation and evacuation occurred progressively basinward through time in response to basinward shifts of major Cenozoic sedimentary depocenters. As a salt sheet neared complete evacuation, the underlying autochthonous salt layer would begin to evacuate, providing additional sediment accommodation that caused autochthonous salt flowage basinward, and the formation of the next allochthonous salt sheet basinward of the depocenter. The area of autochthonous salt progressively decreased through time and currently represents at most 45% of its maximum along this transect. The total area of salt through time was more stable, with variations of only 30% from its maximum. This relationship is a function of lateral salt flow into and out of the plane of section, and possible salt dissolution. The restorations indicate that very little translation or extension (1.46%) occurred at the autochthonous salt level during the evolution of the basin. The majority of translation or extension occurred above allochthonous salt sheets (25%) and was compensated laterally by salt flow. Displacements above allochthonous salt sheets were driven by gravitational instabilities caused by the slope gradient.

The Campos basin is situated in offshore southeastern Brazil. The Lagoa Feia is the basal formation in the stratigraphic sequence of the basin, and was deposited during rifting in an evolving complex of lakes of different sizes and chemical characteristics, overlying and closely associated with rift volcanism. The stratigraphic sequence is dominated by lacustrine limestones and shales (some of them organic-rich), and volcaniclastic conglomerates deposited on alluvial fans. The sequence is capped by marine evaporites. In the Lagoa Feia Formation, complex lithologies make reservoirs and source rocks unsuitable for conventional well-log interpretation. To solve this problem, cores were studied and the observed characteristics related to log responses. The results have been extended through the entire basin for other wells where those facies were not cored. The reservoir facies in the Lagoa Feia Formation are restricted to levels of pure pelecypod shells (''coquinas''). Resistivity, sonic, neutron, density, and gamma-ray logs were used in this work to show how petrophysical properties are derived for the unconventional reservoirs existing in this formation. The same suite of logs was used to develop methods to define geochemical characteristics where source rock data are sparse in the organic-rich lacustrine shales of the Lagoa Feia Formation. Thesemore » shales are the main source rocks for all the oil discovered to date in the Campos basin.« less

End_Page 311------------------------------Upper Ordovician (Cincinnatian) strata in Ohio, Indiana, and Kentucky were deposited as cyclic sequences. Three types of cycles are represented: thin, graded storm cycles, moderately thick megacycles (carbonate to shale sequences), and thick shoaling-upward cycles (shale-rich, grainstone-poor facies that grade upward into shale-poor, grainstone-rich facies). Cincinnatian strata were deposited on a gently sloping, shallow-marine carbonate ramp. Sedimentation was episodic; periods of in-situ carbonate accumulation were frequently interrupted by storm events. Tropical storms affected sedimentation and benthic ecology in seven ways by: (1) eroding sediments; (2) transporting allochthonous clays and silts onto a carbonate ramp; (3) winnowing, transporting, and redistributing carbonate sediments; (4) generating downslope gravity flows; (5) mixing benthic fauna from different communities; (6) periodically interrupting the process of community succession; and (7) creating favorable conditions for the evolution and success of opportunistic species. Because of the excellent preservation of episodic storm events and their influence on sedimentation and paleoecology, the Cincinnatian Series is recognized as an example of an ancient storm-dominated, carbonate ramp. The following characteristics are diagnostic of storm domination in the rock record: (1) abundant storm sequences occur in all facies; (2) storm sequences are variable; (3) inner shelf facies have thin, discontinuous bedding; (4) rudites dominate in inner shelf facies; (5) fine grainstones are concentrated in outer-shelf facies; (6) textural inversions are common; (7) carbonate rock types are widely variable; (8) storm-generated structures occur in all facies; (9) in-situ faunal communities are rare; and (10) most beds contain a mixture of fossil-preservational states.End_of_Article - Last_Page 312------------

Red Bluff is an erosional escarpment located on the western margin of the Pearl River flood plain in northwestern Marion County, Mississippi. The bluff shows approximately 30 m (100 ft) of relief and is composed of alternating units of red-to-yellow sand and sandy gravel. The sand grains are composed primarily of quartz, with small amounts of heavy minerals and feldspar. The gravel is composed of varying percentages of chert, flint, jasper, rip-up clasts, quartz, and tripoli, including a small fraction of silicified Paleozoic fossils. Grain-size analysis of the sediment and investigation of the sedimentary structures suggest a braided-fluvial environment of deposition. Particle sizes in the medium sand to pebble range predominate in all units; very little silt and clay is present. The largest "particles" present are boulder-size rip-up clasts. The most conspicuous sedimentary structures at Red Bluff are graded bedding, low-angle to medium-angle cross-bedding, and well-developed paleochannels. A statistical comparison (discriminant analysis) of the seven most abundant heavy minerals of Red Bluff, with the same suite of heavy minerals found at the type section of the Citronelle Formation (Pliocene-Pleistocene), and outcrops of a known Miocene coarse clastic unit indicates a correlation of Red Bluff to the Citronelle Formation. These heavy minerals are kyanite, staurolite, rutile, tourmaline, zircon, black opaques (primarily ilmenite and magnetite), and white opaques (primarily leucoxene). The suite of heavy minerals present at Red Bluff belongs to the east Gulf province. This metamorphic assemblage of heavy minerals implies the source area of the sediments at Red Bluff to be the southern Appalachians. The silicified pebble-size Devonian-Mississippian fossils were derived most likely from formations flanking the southern Appalachians in northern Alabama. End_of_Article - Last_Page 1472------------

The National Coal Resources Data System (NCRDS) of the U.S. Geological Survey has the capability through spatial data bases and computer software to depict coal geology and calculate coal resources by computer. The Birney 1:100,000 map in southeastern Montana and the Canyon coal bed were selected to demonstrate this capability. The Birney quadrangle includes parts of Big Horn, Rosebud, and Powder River Counties. Other political entities are the Northern Cheyenne Indian Reservation and the Custer National Forest. The Canyon coal bed locally splits into an upper and lower unit, whose thicknesses range from 1/3 to 33 ft (0.1 to 10 m). The desired data subset retrieved from the stratigraphic (USTRAT) data base consists of more than 300 drill-hole and 100 outcrop locations and their respective Canyon coal stratigraphic sections. Required digitized information included: X-Y locations (lat. and long.) for each point, township-range intersections, county, and national forest boundaries from the base map, the Indian reservation boundary, the Canyon bed outcrop, and the 200, 400, and 1,000 ft (61, 122, and 305 m) overburden isolines as drafted by the geologist. GARNET, a NCRDS interactive graphics program, produces isopachs and structure maps, does trend analysis, and allows the user to edit data points, expand areas of interest, and calculate coal-resource areas and tonnages for any defined area. The USGS methodology for calculating and reporting coal resources requires that computations be delimited by criteria of coal thickness, overburden thickness, rank, and distance from points of observation as related to land classification and political subdivisions. GARNET allows interactive graphic combination of digitized and computer-derived lines to produce boundaries of these categories. End_of_Article - Last_Page 605------------

A paleontologic survey of the western outcrops of the terrestrial Baca Formation has yielded the first Oligocene fossils from New Mexico. Three sites of Chadronian (early Oligocene) age are known, and their fauna included the small oreodont Leptomeryx, the cameloid ?Eotylopus, the carnivore Hyaenodon, the entelodont Brachyhyops, and a large-horned titanothere Menodus. All these genera are also found in the Porvenir local fauna, Chadronian of Trans-Pecos Texas. Vertebrate finds indicate far less contemporaneity of the formation's isolated outcrops than previous workers had accepted. The formation had been considered Eocene, partly on the strength of the discovery of a specimen of Protoreodon pumilis, a late Eocene-early Oligocene oreodont, in alluvial-fan deposits north of Datil. The Chadronian sites lie in fluvial deposits north of Quemado, approximately 60 km west. Most Baca vertebrates have been recovered from channel deposits or from float. No site with a concentration of small fossils suitable for screening has yet been discovered. Future goals for Baca Formation paleontologic work include location of fossils in the still unfossiliferous outcrop areas and location of screening sites. End_of_Article - Last_Page 568------------

Recent research on large sized modern anastomosing river systems (upper Columbia River, British Columbia, Canada, and Magdalena River, Colombia, South America) has recognized six depositional environments: channel, levee, crevasse-splay, lacustrine, marsh, and peat bog or swamp. Average sedimentation rates in both river systems are 5 mm/yr and 3.8 mm/yr, respectively, Such rapid sedimentation rates (vertical accretion) are keeping pace with equivalent rates of basin subsidence. High rates of sedimentation and basin subsidence are most likely to be found at proximal locations in molasse basins during major orogenic pulses. Such conditions were present during the Columbian and Laramide orogenies during the early Cretaceous and Tertiary in the foreland adjacent to the Rocky ountain system. Thus, channel and crevasse-splay shale-encased sandstone reservoirs and coal, common in anastomosed fluvial rock sequences in proximal molasse settings, should be encountered in parts of the Western Interior sedimentary basin. Such deposits probably have been interpreted as deltaic or alluvial plain and should be reexamined to better predict sandstone trends for hydrocarbon exploration. End_of_Article - Last_Page 949------------

Two communities of shelled benthic macrofauna are recognized south of the Mississippi delta by means of cluster analysis. The faunal pattern is correlated closely with water depth, pelecypod feeding type, and substrate texture. Correlation of faunal, lithologic, and environmental characteristics reflects joint sedimentation and biotic production in response to the present hydrologic regime. East of the Mississippi delta, 8 communities of benthic shelled macrofauna are recognized by means of cluster analysis. Distribution patterns of these communities (biofacies) appear to reflect the primary environmental factors controlling the nature of the water mass: distance from the delta front; water depth on the shelf away from the influence of the delta; and subdivision of the shelf by the Chandeleur-Breton islands. Faunal and substrate patterns are poorly correlated; histograms of sediment texture for each of the biofacies are not significantly different from the histogram of sediment texture for the whole area. The poor correlation of fauna with substrate texture is the result of the formation of the substrate distribution pattern during deposition of the St. Bernard delta. Fa nal distribution patterns are determined primarily by the environmental factors controlling present water-mass characteristics and only secondarily by the relict substrate texture pattern. If preserved in the geologic record, the co-occurring fauna and sediments would represent 2 different periods of deposition. End_of_Article - Last_Page 1699------------

A quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads, the lithosphere, and the associated foreland basin. The model treats the lithosphere as a two-dimensional nonuniform elastic plate whose strength is controlled thermally. The thermal and flexural evolution of a margin is followed from initial rifting and passive-margin development, through overthrusting and foreland-basin deposition, to postdeformational erosion. Our models indicate the following. (1) Foreland-basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting. Sensitivity decreases the farther the overthrust load is empl ced onto the craton. (2) Foreland-basin depth is strongly influenced by the size of the overthrust load. Maximum depths in excess of 10 km (6 mi) can occur given overthrust topography similar to the present-day Himalayas. (3) A few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained. Very little of this eroded material is deposited in the foreland basin; most must bypass the basin because it also is subjected to erosion and associated flexural uplift. (4) Thick overthrusts, as interpreted from COCORP profiling, are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere.

Raymond Canyon is located on the west side of the Sublette Range, Lincoln County, Wyoming. The study area is just east of the Idaho border and 10 mi (16 km) southeast of Geneva, Idaho. It contains an ideal view of a thrust fault (Tunp thrust), excellent exposures of vertical strata, small-scale folding, and minor strike-slip faulting formed during development of the Idaho-Wyoming thrust belt. Formations exposed range in age from Late Pennsylvanian to Tertiary (Pliocene) and include: the lower part of the Wells Formation (Pennsylvanian, total thickness 720 ft or 219 m); the upper part of the Wells Formation and the Phosphoria Formation (both Permian, 153-210 ft or 47-64 m); the Dinwoody Formation (185 ft or 56 m); Woodside Shale (540 ft or 165 m); Thaynes Limestone (2,345 ft or 715 m); and Ankareh Formation (930 ft or 283 m), all of Triassic age; the Nugget Sandstone (1,610 ft or 491 m), Twin Creek Limestone, Preuss Sandstone, and Stump Formation, all of Jurassic age; and the Salt Lake Formation and the Sublette conglomerate, both Pliocene postorogenic continental deposits. Generally these formations are thinner than in nearby areas to the west and northwest. Raymond Canyon lies on the upper plate of the Tunp thrust and the lower plate of the Crawford thrust of the Idaho-Wyoming thrust belt. Thus, it lies near the middle of the imbricate stack of shallowly dipping thrust faults that formed in the late Mesozoic. Study of the stratigraphy, structure, petrography, and inferred depositional environments exposed in Raymond Canyon may be helpful to those engaged in energy development in the Idaho-Wyoming thrust belt. End_of_Article - Last_Page 949------------