Changes In Depositional Environment Research Articles

Structural investigations of Australian coals—I. A characterization of Victorian brown coal lithotypes and their kerogen and humic acid fractions by i.r. spectroscopy

The functional groups of brown coal lithotypes, their humic acids and kerogens were investigated by elemental analysis and quantitative infrared spectroscopy using absorption coefficients. An inverse relationship is reported between the concentrations of kerogen and humic acids for each lithotype and significant variation in the levels of humic acids with lithotype are attributed to changing higher plant input and depositional environments. The structural implications of observed trends in absorption coefficients and their usefulness as a rapid method of functional group analysis has been investigated. Using this infrared technique, kerogen structures as reflected in functional group analyses were found to exhibit a dependance on lithotype, although no significant variation in the functionality of the humic acid structures was observed.

Holocene Facies Succession and Depositional Environments of Semi-Enclosed Windward Lagoon off Great Abaco Island, Bahamas: ABSTRACT

The Abaco windward lagoon, bounded bankward by Great Abaco Island and seaward by the discontinuous Abaco Cays, exhibits up to 6 m or more of lagoonal, non-reef derived sediments, 5 km from the marginal escarpment of northeastern Little Bahama Bank. Six sedimentary facies, distinguished by texture, composition, and molluscan fauna, record the changing depositional environments as water depth and circulation increased during the Holocene transgression. A skeletal grainstone and mixed (skeletal-ooid-aggregate) grainstone are limited to inter-island channel areas. Away from these channel energy windows, a maximum sequence of 4 depositional facies is observed. The bulk of sediments are a normal marine (N.M.) skeletal wackestone or mixed (pellet-skeletal) packstone. The N.M. wa kestone dominates the western half of Abaco lagoon while the packstone occurrence End_Page 949------------------------------ characterizes eastern Abaco lagoon where it overlies the wackestone facies, accounting for 50% of the normal marine section. The N.M. wackestone typically grades downward into a dark gray restricted marine (brackish to hypersaline) skeletal wackestone, ^eqles70 cm thick, and then to a dark brown nonmarine soil zone, <=16 cm thick, above bedrock. Radiocarbon dates indicate flooding of Abaco lagoon at least by 7,446 YBP at - 10 m, followed by the transition from restricted to normal marine conditions as early as 4,716 YBP. Sedimentation rates increase from 16 cm/1,000 years for the restricted marine wackestone, to 58-104 cm/1,000 years and 216 cm/1,000 years for the N.M. wackestone and packstone facies, respectively. The windward lagoon setting illustrates the caution required in pre iction of facies continuity perpendicular to carbonate bank margins. Recognition of an ancient windward lagoon sequence may have important implications regarding sea level history and paleogeographic reconstruction. End_of_Article - Last_Page 950------------

Holocene evolution of a coastal lagoon, Lake of Tunis, Tunisia

ABSTRACTHistorical records demonstrate that the Lake of Tunis, Tunisia, was an open bay that became separated from the Mediterranean by an accreting barrier spit, forming a lagoon. Closure of the lagoon was caused by increased sedimentation as a consequence of Roman deforestation of the Medjerda River drainage basin to the north and subsequent accelerated longshore transport. The separation of the lagoon from the Mediterranean was completed in the early 1500s. At present, the 48 km2 lagoon averages less than 1 m in depth and reaches eutrophic conditions in the late summer. Consistent with the historical records, the lagoon sedimentary column shows three distinct successive environments of deposition: (1) an arid continental environment; (2) an open marine bay; and (3) the present brackish to hypersaline lagoon. These depositional environments are represented by the lower grey layer, which is less than 0‐5 m thick, the middle olive‐grey layer, which varies from 1 to 5 m in thickness, and the upper black layer, which is 1 m thick. All of the strata are predominantly silt plus clay, but usually contain at least 10% sand. The lower grey layer consists of pitted quartz sand, with very few abraded, broken molluscan fragments and benthic foraminifera with thick tests. An arid, subaerial depositional environment of latest Pleistocene time best explains these sediment and fossil assemblages. In the middle olive‐grey layer, coral, coralline algae, open marine graeses and the dominance of foraminifera over ostracods (expressed as a low percentage of ostracods/ostracods plus foraminifera) attest to a depositional environment of an open marine bay. A sharp increase in the percentage of ostracods/ostracods plus foraminifera and organic carbon from the middle olive‐grey layer to the upper black layer signifies a major change in depositional environment. The predominance of ostracods over foraminifera, abundance of gastropods characteristic of eutrophic conditions, high organic carbon content and absence of macro‐fossils characteristic of open marine conditions clearly indicate that the upper black layer was deposited in a brackish to hypersaline, eutrophic lagoon which has persisted to the present. Seismic records indicate a karstic bedrock surface underlying the lagoon. The surface is marked by considerable relief, and shows a linear depression which may represent an early sixteenth century ship canal. Heavy metal analyses of total samples in five cores demonstrate that Mn and Fe vary randomly, and are apparently derived from natural sources only. Cd, Cr, Pb, Cu and Zn are typically highest in the uppermost unit, which reflects levels of human contamination since closure of the lagoon.

Disconformities in Grants Mineral Belt and Their Relation to Uranium Occurrence: ABSTRACT

At least two major, regionally extensive, intraformational disconformities and numerous local ones are present in sedimentary rocks of Triassic, Jurassic, and Cretaceous ages in the Grants mineral belt. These disconformities, some of which have been known for many years, have proven useful in stratigraphic correlation and in differentiating genetically related rock sequences in the mineral belt. In addition, two of these disconformities, one local and the other regional, are significantly associated with the distribution of sandstone-type uranium deposits in the Jurassic Morrison Formation and the overlying Cretaceous Dakota Sandstone. The Westwater Canyon Member and laterally equivalent beds of the Recapture and Brushy Basin Members of the Morrison Formation are separated from underlying Jurassic rocks by an intraformational disconformity which marks a major change in depositional environment from predominantly sabkha-eolian dune to fluvial-lacustrine within the Jurassic sequence. This disconformity is economically significant in that all of the large uranium deposits in rocks of Jurassic age in the mineral belt occur within fluvial sandstone facies of the Morrison above the disconformity. Recognition of this disconformity in outcrop and in the subsurface is important in delineation of exploration target areas and resource assessment in the San Juan basin. Uranium deposits in the basal part of the Dakota Sandstone are associated with the well-known regional, southward-beveling disconformity present at the lower boundary of the Dakota throughout the San Juan basin and adjacent region. In the western part of the mineral belt, impermeable Brushy Basin shales have been truncated at this disconformity and uranium-bearing groundwaters from the Morrison Formation have migrated into basal organic-rich sandstone of the Dakota to produce several small to medium-sized uranium deposits. End_of_Article - Last_Page 689------------

Borehole Gravity Study of Density and Porosity of Selected Frontier, Tensleep, and Madison Reservoirs in Bighorn Basin, Wyoming: ABSTRACT

Borehole gravity surveys in the Gebo, Garland, and Big Polecat oil fields of Wyoming uniquely assess the density and porosity of the reservoir and associated rocks. The borehole gravity method is unique because (1) unlike other well-logging techniques, gravity measurements depend directly on rock bulk density; (2) the large radius (and volume) of investigation ensures that the measurements are unaffected by borehole fluids and by rugosity, casing, cement, or any area close to the borehole that may be modified by flushing or invasion by drilling fluids; and (3) the high precision of the measurements makes this method sensitive to very small variations in formation density (usually 15%) and/or gas-filled sandstone units, principally in the Frontier Formation, were easily detected behind casing in the three oil fields. An abrupt and possibly widespread downward increase in porosity in the upper part of the Frontier Formation may (1) reflect lithologic and mineralogic variations owing to changes in the depositional environment, (2) be related to a previously proposed unconformity, nd (3) have exploration significance. End_of_Article - Last_Page 822------------

Clay Mineralogy of Lewisville Member of Cretaceous Woodbine Formation in Arlington, Tarrant County, Texas, Area: ABSTRACT

One hundred clay-sized samples selected from two cores and two measured sections of the Lewisville Member, three samples from the overlying Arlington Member, and five from the underlying Dexter Member of the Cretaceous Woodbine Formation in eastern Tarrant County, Texas, were analyzed. The amounts of each clay mineral present were calculated from X-ray diffractograms of natural, ethylene glycol-saturated, and heated samples. The average clay mineral assemblage was approximately 28 percent illite, 36 percent kaolinite, and 36 percent expandable lattice clays. Floral and faunal assemblages and sedimentary structures demonstrate that the Lewisville Member in the Arlington area is part of a deltaic interdistributary complex association where admixing of fresh and saline water creates a transition zone. Vertical changes in clay-mineral distributions correlate well with the changes in depositional environments from fluvial to swamps and bays and finally to marine by the end of Lewisville deposition. Apparently for the Lewisville Member specifically, and probably for the Woodbine Formation in general, clay-mineral distributions are environmentally significant. The predominant expandable lattice clay in the Lewisville Member is Ca-Mg montmorillonite. In his studies of the Woodbine in east-central Texas, Beall found these exchangeable cations to be most abundant in nearshore sediments with Na + cation content increasing basinward, reflecting the more marine character of the sediments. Kaolinite is the most abundant clay mineral in the five Dexter Member samples and in samples from the lower 5 to 10 ft of the Lewisville Member. Several lenticular channels, extensive sands, and interbedded silts and clays provide evidence that the Dexter Member is part of the meander-belt facies. The basal Lewisville marks the transition between a fluvial environment below and swamp and bay environments above. Randomly interstratified montmorillonite/illite is present throughout the sampled intervals, but no clear relation between this type of clay and depositional environment is apparent, as the semiquantitative method used in this investigation is useful only in determining the relative amounts of the end members of the mixed-layer clay. An increase in kaolinite and expandable-lattice clays with respect to the average composition in the upper Lewisville and in three Arlington Member samples reflects an increase in transport energy and a westward shift in the strandline. End_of_Article - Last_Page 2209------------

Environment of stratiform sulphide deposition; variation in Mn:Fe ratio in host rocks at Heath Steele Mine, New Brunswick, Canada

The varying response of Fe and Mn to changes in pH and Eh suggests that their ratio may be a sensitive indicator of changes in environment of deposition of sedimentary rocks. A volcanic-sedimentary model for the deposition of a massive stratiform sulphide deposit in northern New Brunswick is proposed and the physico-chemical nature of the various environments discussed. The disposition of the Mn:Fe ratios in sedimentary rocks surrounding the sulphide deposit is entirely consistent with the proposed model.

Morphological Evolution of Cambrian Algal Mounds in Response to a Change in Depositional Environment

An evolutionary trend in algal mound morphology within the Morgan Creek Limestone, upper Cambrian of central Texas, is associated with a change in depositional environment. Morphological modification of the mounds is a response of the algal communities to changes in water depth and turbulence. This change occurs through a stratigraphic interval approximately 20 m thick. The lower part of the Morgan Creek Limestone exhibits substantial evidence of having been deposited in a more turbulent environment than the upper part. This is indicated by an upward decrease in abundance of oolite bodies; an increase in micrite and corresponding decrease in spar; a decrease in cross-stratified deposits upward; and, in the middle of the section, the first appearance of a green alga. The lowest mounds, stratigraphically, are discrete club-shaped, simple, highly arched, nonbranching, concentric structures. These mounds, which grew in a high energy environment during deposition of the lower part of the Morgan Creek, have maximum dimensions of 0.5 m in thickness and 0.75 m in diameter. In the middle Morgan Creek mounds are larger, 0.25 to 1 m thick and 0.3 to 1.7 m in diameter, and have a complex digitate internal structure. Near the top of the Morgan Creek Limestone is a horizon of the largest-sized algal mounds, biconvex lenses up to 1.7 m thick and 8 m in diameter. The over-lying strata contain some flat, algal-laminated structures. This evolution in mound form, a decrease in height to width ratio and from simple to complex internal structure, is associated with a decrease in water turbulence and a shift from shallow marine to intertidal to supratidal site of deposition. This environmental response demonstrates that changes of mound morphology can be useful in interpreting depositional environments.

Sandstone: Secular Trends in Lithology in Southwestern Montana

Long-term secular trends in the composition and texture of sandstones in southwestern Montana reflect changing provenance and depositional environment, which in turn reflect changing tectonic patterns in the Cordilleran mobile belt just to the west.

Changes in geochemical conditions within the Silesian basin (Polish Flysch Carpathians) at the eocene-oligocene boundary

A section through the Upper Eocene Globigerina marls and succeeding Oligocene sub-Menilite Beds, exposed near Roznow, Nowy Sacz district (Poland), displays marked vertical variations in both petrographic properties and chemical composition, which reflect important changes in depositional environment at the Eocene-Oligocene boundary. The Globigerina marls consist of frequently bioturbated, marly and argillaceous shales, passing upwards into marls, and are characterized by the presence of calcareous algae, plant fragments, agglutinating foraminifers and numerous globigerinids. An upward increase in calcium-carbonate content is accompanied by a change from a predominantly agglutinating to a largely calcareous foraminiferal fauna. The proportion of siliceous phytoplankton among the microfossils is low. The occurrence of only small amounts of sulphide and bituminous material suggests an oxidizing environment. No significant enrichment in the rare metals was observed. The sub-Menilite Beds are bituminous, marly and argillaceous shales with interbedded chert and sandstone layers and include acidic tuff layers in the upper part. There is no extensive biogenic disturbance of internal sedimentary structures, and diatoms are the predominant microfossils. The development of reducing conditions during sedimentation is indicated by a vertical increase in sulphide content and a relatively high proportion of organic material. The calcium-carbonate content of the sediments decreases upwards. Vanadium, molybdenum, copper and lead are locally concentrated. At the outset of the Oligocene, the open, well aerated basin, in which the Globigerina marls had been deposited, was affected by vertical movements, resulting in poor circulation of basin waters, reactivation of a source area for sands, transported in turbidity currents, and initiation of volcanic activity. These conditions in turn led to the development of a prolific flora of siliceous phytoplankton and a reducing environment, favourable for the concentration of rare metals.

Morphologic Evolution of Cambrian Algal Mounds, Texas, with Changing Depositional Environment: ABSTRACT

Algal mounds within the Morgan Creek Limestone (Upper Cambrian) of central Texas, exhibit an overall change in morphology with height in the section. This evolutionary trend is associated with a change in depositional environment. The modification in algal-mound morphology is believed to be a response of the algal communities to change in the level of water turbulence and water depth. The earliest forms are discrete club-shaped mounds exhibiting a relatively simple, highly arched, non-branching, concentric structure. They are up to 1.5 ft thick and 2 ft in diameter. They are succeeded by larger mounds, 0.75-3 ft thick and 1-5 ft in diameter, with a complex, digitate internal structure. Near the top of the Morgan Creek Limestone are the largest algal mounds, biconvex lenses up to 5 ft thick and 25 ft in diameter. The overlying strata contain some flat, algal-laminated structures. This evolution in mound form, a decrease in height-to-width ratio and from simple to complex internal structure, is associated with a decrease in water turbulence and a shift from shallow marine to intertidal to supratidal site of deposition. This environmental response demonstrates that changes of mound morphology can be useful in interpreting depositional environments. End_of_Article - Last_Page 608------------

Geologic Outline and Oil Fields of Sergipe Basin, Brazil: ABSTRACT

In the Sergipe basin of northeastern Brazil a Lower Cretaceous unconformity marks a change in general depositional environment and tectonic style. Below this unconformity, Carboniferous to Lower Cretaceous beds are nonmarine, whereas the overlying Lower Cretaceous to Tertiary beds are dominantly marine. A period of intense normal faulting which preceded the unconformity resulted in an uplift which exposed Precambrian rocks in an area north of Aracaju, while in adjacent grabens thick wedges of syntectonic conglomerates were deposited locally over older sediments. Irregularities on this unconformable surface were filled by the Carmopolis Member of the Muribeca Formation, a conglomerate and coarse sandstone; the more extensive overlying Ibura Member evaporites of the same fo mation also covered the areas where basement was still exposed. Late Cretaceous tectonism is characterized by small-scale faulting; the Riachuelo-Siririzinho and Vassouras-Carmopolis oil trends resulted from a combination of northwestward subsidence of basin-margin grabens and a regional southeastward tilting that started somewhat later. Oil production in this basin from the Carmopolis, Siririzinho and Riachuelo fields comes mostly from the Carmopolis Member. Some oil is also produced from Lower Cretaceous reservoirs in contact with the unconformity. Depth range of all reservoirs is 400-800 m. Favorable conditions for oil accumulation are the result of adequate structural evolution during Late Cretaceous time, presence of evaporites and probable oil source organic shales at the top of the reservoirs, and younger unconformities not reaching down to the trap. Locally, lateral permeability barriers or reservoir pinchouts complement the structural trap closure. The oil is of mixed base ranging from 24 to 30.5° API. Cumulative production to December 31, 1969, with Siririzinho and Riachuelo fields still being developed, was 23.65 × 106 bbl of oil. This production comes from 205 completed wells drilled in a grid of 400 m. End_of_Article - Last_Page 353------------

Engineering Properties of Some Sea-Floor Deposits

Measurement of mass physical properties such as wet density, shear strength, water content, and grain size have been made on a large number of sediment cores collected from the North Atlantic and North Pacific Ocean basins. Some degree of correlation is found between these properties and sediment type, currents, and local topography. Areas of the deep-sea floor consisting of red clay commonly display lower shear strengths and bulk densities than those areas blanketed by calcareous oozes. In the vicinity of the continental margins, the mass physical properties vary considerably as the sediment supply and depositional environment change. Sediments from carbonate environments or areas of relatively high relief normally have higher shear strengths. This investigation has lead to a delineation or various portions of the sea floor on a basis of the mass physical properties and gives an indication of the range of these parameters for submarine sediments.

A preliminary study of sandstone sedimentation in the Lower Carboniferous of the Tweed Basin

Synopsis The sedimentology of the sandstones is used, together with the available stratigraphic information, to demonstrate the changes in depositional environment which take place in the Tweed Basin during Lower Carboniferous times. The characters of the sandstones of the Cementstone Group indicate deposition in a dominantly fluvial environment, the Fell Sandstone is interpreted as a thick widespread shallow marine deposit and the sandstones of the Scremerston Coal Group are mainly of fluvial origin. An attempt is made to fit these observations into a regional palaeogeographic pattern.

A hypothesis for the change of ocean levels depending on the albedo of the polar ice caps : Bloch M.R., 1965. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1 (2): 127–142.

Upper Miocene and Lower Pliocene shallow lacustrine deposits from the Florina–Ptolemais–Servia Basin in northwest Greece show a distinct m-scale sedimentary cyclicity of alternating marls and lignites or clays, which were shown to be primarily related to precession-induced climate variations ([van Vugt, N., Steenbrink, J., Langereis, C.G., Hilgen, F.J., Meulenkamp, J.E., 1998. Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record. Earth Planet. Sci. Lett. 164 (3–4) 535–551; Steenbrink, J., Van Vugt, N., Hilgen, F.J., Wijbrans, J.R., Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine succession of Ptolemais (NW Greece): Discrepancy between 40Ar/39Ar and astronomical ages. Palaeogeogr. Palaeoclimatol. Palaeoecol. 152, 283–303; Steenbrink, J., Van Vugt, N., Kloosterboer-van Hoeve, M.L., Hilgen, F.J., 2000. Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece). Earth Planet. Sci. Lett. 181 (3–4) 161–173]). Three additional sections and drill cores from the same basin are studied using a combination of cyclostratigraphy, magnetostratigraphy and 40Ar/39Ar dating. Together with the earlier studied sections, they provide a continuous succession that documents the depositional history of the intramontane Neogene basin fill. Sedimentation in the basin started at around 8 Ma ago with deposition of coarse-grained terrigenous-clastics in an alluvial environment. From ∼ 7.1 to ∼ 6.1 Ma ago, diatomaceous marls were deposited in a lacustrine environment, frequently interrupted by deposition of clays and local xylite-type lignite seams. Hereafter, deposition of alternating coarse- and fine-grained terrigenous-clastic alluvial sediments took place, followed by a gradual shift towards lacustrine sedimentation at around 5.5 Ma. Deposition of alternating fine-grained clastics (silts and clays) and biochemical sediments (carbonates, marls and thin lignites), with episodic periods of sub-aerial exposure continued until ∼ 5.23 Ma. From then onwards up to ∼ 3.9 Ma, regular alternating lignites and marls were formed in a shallow lacustrine environment. Precession-induced fluctuations of regional climate primarily defined the m-scale variations in lithology as observed in all members of succession, whereas the 100- and 400-kyr eccentricity cycle had a clear impact on larger-scale lithological alternations in parts of the succession as well. A marked similarity in the ages of fundamental changes in depositional environments between the Florina–Ptolemais–Servia Basin and the marine basins in the Mediterranean is observed, suggesting (but not necessarily implying) that they have a common, possibly tectonic origin.

Upper Paleozoic Sabaneta-Palmarito Sequence of Merida Andes, Venezuela

An upper Paleozoic sedimentation cycle is represented by the rocks of the Sabaneta and Palmarito Formations of the Merida Andes, southeast of Lake Maracaibo. The Sabaneta is a terrestrial deposit, whereas the overlying Palmarito is marine. Both formations were deposited on older, mainly Paleozoic strata in the Mucuchachi basin, south and east of Lake Maracaibo. This basin, which occupied approximately the present area of the Venezuelan Andes, is believed to have existed since Middle Ordovician time. Continued uplift and progressive denudation of the basin rims produced the sediments that comprise the two formations. Marked changes of depositional environments, from subaerial to fluvio-marine and eventually holomarine, occurred. The upper Paleozoic sedimentation cycle is associated closely with the structural and sedimentational history of the Mucuchachi basin during the earlier, preceding two-thirds of the time when the basin existed. Intermittent uptilt of the basin rim, balanced by depression of the trough area, is believed to have been the basic mechanism controlling sedimentation. The recurrent active tilting movements created a hinge zone, a structural feature of primary importance during the two distinct sedimentation cycles which represent the 200-million-year period of the basin's existence. During earlier Paleozoic time, the hinge zone separated an area of trough deposition, where monotonous, fine, terrigenous clastic material accumulated, from an area where a coarse shelf facies was deposited. The hinge zone also may separate metamorphic from non-metamorphic sequences. During a period of tilting that occurred before the upper Paleozoic sedimentation cycle began, the hinge zone is thought to have been a structural axis along which all Devonian and Carboniferous strata of the shelf facies were removed. It probably is significant that later, during Sabaneta time, the inner-basin side of this structural axis was the locus of deposition of the thickest Sabaneta sections. Structural characteristics of the Sabaneta-Palmarito sedimentary rocks indicate that compressional stresses acted from southeast to northwest, tending to narrow the basin. The trough deposits of the older Paleozoic were deformed by stresses with a similar orientation, but the resulting structural style is different. The differences in structural style between the two depositional cycles are attributed to the superior thickness, greater homogeneity, and probably more plastic behavior of the older Paleozoic rocks, and to differences in the depths at which the deformations took place. Thus, the slates of the older Paleozoic have been thrown into wide, gently folded synclines seemingly separated by narrow, steeply overthrust anticlines. In contrast, because of marked variations in thickne s and facies, the upper Paleozoic cover has deformed asymmetrically and, in the course of deformation, internal detachment planes developed, particularly within the thinner frontal edges of the cover, causing local imbrication.

Biota of a Late Glacial Rocky Mountain Pond

The sediments of a late glacial sag pond in Huerfano Park, south-central Colorado, have yielded a varied biota consisting of vertebrates, terrestrial and fresh-water mollusks, sponges, and pollen. Wood from the sediments has a radiocarbon age of 9600 ± 200 years. The vertebrate fauna contains the tooth of a prairie dog and the remains of frog, salamander, snake, and fish—all characteristic of present-day montane lakes and ponds and their margins in Colorado. The mollusks are all living species, and the fauna is dominated by gastropods. With the exception of one form (Vitrina alaskana Dall), whose known range does not extend as far south as Huerfano Park, the fauna is not significantly different from the mollusks living in the area today. The sediments also contain an abundant and unusually diversified sponge fauna in which as many as six species may be represented. A pollen spectrum from the sediments is composed of arboreal pollen characteristic of the present-day Transition Zone flora which, in Colorado, grows on the upper edge of the Great Plains and all of the foothills belt. Vertical variation in the sediment types and the enclosed biota makes it possible to recreate the changing depositional environment and local climate throughout the life of the pond. The basal sediments, interbedded marl and clastic materials, are interpreted as indicating deposition during alternating wet and dry periods. A bed of nearly pure calcium carbonate marl, which reaches a thickness of 2 feet, forms the middle stratigraphic unit. It is highly fossiliferous and is thought to have been deposited during a warm dry period with relatively mild winters. The uppermost stratigraphic unit is a gray carbonaceous clay which filled the basin as vegetation encroached upon the lake and the lake itself was reduced to a marsh. Because of the radiocarbon age and the reconstruction of the climate based upon faunal and sedimentological changes, the lower two stratigraphic units are thought to correlate with the Lubbock Subpluvial, and the uppermost unit, the gray carbonaceous clay, with the early part of the Yellow House Interval as described in the Llano Estacado, New Mexico.

Subsurface Stratigraphy of Coastal Georgia and South Carolina: ABSTRACT

Sediments ranging in age from Pleistocene to Early Cretaceous (?) overlie metamorphosed basement rocks in most of coastal Georgia, but in southeastern Charlton County there are Paleozoic strata between the Lower Cretaceous (?) and the basement complex. The thickness of sediments ranges from 2,674 feet in Screven County, in the north, to 4,700 feet in Glynn County at the coast; the section thins somewhat toward Florida. The Quaternary unit includes the surficial clastics of Recent to Pleistocene age. The Tertiary units include the predominantly clastic section of Miocene age; the Oligocene sandstones and their limestone equivalent, the Ocala Limestone; the Lisbon and Tallahatta clastics and their Avon Park-Lake City Limestone equivalents, the Wilcox clastics and their Oldsmar Limestone equivalent, all of Eocene age; and the Clayton and Tamesi Formations and their Cedar Keys Limestone equivalent of Paleocene age. The Upper Cretaceous units include the Lawson Limestone and the equivalent clastics of Navarro age; beds of Taylor and Austin age, both in clastic and limestone facies; and the clastic Tuscaloosa Formation. The oldest unit consists of clastics of Early Cretaceous(?) age; these sediments overl e metamorphosed basement rocks in the north and Paleozoics in the south. Two sections, one coastal and the other inland, show lithologic and faunal relationships which reflect changing depositional environments. Regionally, facies change from clastics in the north to limestones in the south with corresponding changes in microfossil suites. Tentative correlations of the clastic sections with their limestone equivalents are based on lithologic and fossil evidence. There are no local geologic structures, but the sedimentary rocks of the area show the collective effects of the Carolina arch, the Southeast Georgia embayment, and the Ocala uplift. End_of_Article - Last_Page 344------------

Stratigraphy of Lodgepole Formation, Virden Area, Manitoba: ABSTRACT

The increasing importance of Virden-Roselea and North Virden oil production has made it desirous that a workable subdivision of Lodgepole stratigraphy be undertaken for this area and for adjoining regions wherever correlation is satisfactory. The present paper proposes a member breakdown of the Lodgepole of the Virden area, and describes these units in some detail from type wells of the vicinity. Due to lithologic variations resultant from changes in depositional environment within the preserved portion of the Lodgepole of the eastern Williston basin, the proposed stratigraphic units are applicable in general only to the eastern portion of the Manitoba and northern North Dakota sections. Recognizable correlation becomes increasingly difficult and unreliable rather abruptly westward of a northerly-trending narrow zone of demarcation. Correlation east of this zone, however, is generally good and can be referred readily to the Virden type section. Despite the limited areal extent of application, the economic importance of this region warrants systematic stratigraphic subdivision. End_of_Article - Last_Page 422------------

Origin of Hydrocarbons of Uinta Basin, Utah

The Eocene sediments of the Uinta basin, Utah, consist of fluvial and deltaic red and green shales and sands around the margins of the basin, and bituminous siliceous dolomites in the center. The latter, called Green River oil shales, were deposited in a salt lake, probably chemically stratified, which occupied this area during Eocene time. Asphalts and oils ooze from outcrops of the Eocene and are found in joints, veins, fissures, crystal cavities, and in pores of sandstones. There are four chemically different types of these free hydrocarbons. Each type is found closely associated with one particular stratigraphic unit, so that the source rock of each type can be identified by field geology. In addition to the free hydrocarbons, there are other hydrocarbons that can b extracted by organic solvents from the sediments of the various stratigraphic units. Chemical data such as infrared spectra, elemental analyses, and various physical properties show that the extract from each unit is identical with the free hydrocarbons from the same unit, but differs from the extracts and free hydrocarbons of other units. These chemical data, together with the supporting geological data, are believed to constitute an identification of the source rocks of the Uinta basin hydrocarbons. It also was found that the changes in the chemical character of the hydrocarbons are correlative with lithologic changes and therefore appear to be due to changes in depositional environment. There is no evidence that the differences in the hydrocarbons are a result of metamorphism, catalytic cracking, or depth of burial.