Lithological Cycles Research Articles

High-frequency cycles in Upper-Miocene ramp-temperate carbonates (Sorbas Basin, SE Spain)

Uppermost-Tortonian temperate carbonates occur at the southern margin of the Sorbas Basin (Almeria, SE Spain). These carbonates, included in the Azagador Member, formed in a gentle, shallow-water ramp. Six facies cycles in ramp deposits comprise alternating bivalve-shell concentrations and coralline algal beds. The basic cycle reflects the landward advance, as relative sea level rose, of coralline algal deposits, which were the facies of the outer ramp, over bivalve biostromes, which grew in the shallower areas of the mid-ramp. Biostromes were mainly built by oysters and locally by Isognomon. In many cases, however, the removal of smaller shells by storms left only thin, discontinuous patches of large bivalves as residual remains of the oyster biostromes. Some original cycles might be missing due to complete removal of bivalve shells from the biostromes. The six cycles recognised, therefore, should be considered as the minimum number of original cycles in the Azagador carbonates. The available age constraints suggest these cycles were forced by orbital precession or some higher-frequency process. Lithological cycles forced by precession are characteristic of the basinal deposits laterally equivalent to the Azagador carbonates.

Trace-element and Sr–Nd isotopic geochemistry of the PGE-bearing Xinjie layered intrusion in SW China

The Xinjie layered intrusion is one of a number of major ultramafic–mafic bodies hosting Fe–Ti–V deposits and Cu–Ni–PGE sulfide in the Pan–Xi area of the Sichuan Province, SW China. The intrusion is a 7.5 km long, 1 to 1.5 km wide, and 1200 m thick ultramafic–mafic sill, which is associated with Permian plume-related Emeishan flood basalts. The intrusion is characterized by three lithological cycles, each representing a sequence from ultramafic to basic/felsic composition. The basal part of the intrusion is composed of three lithological units, namely, the Marginal, Peridotite and Pyroxenite Units. Two major PGE-mineralized layers occur within the Marginal and Pyroxenite Units. The Marginal Unit (MU) is characterized by marked Nb–Ta, Sr and Ti enrichments and obvious Th depletion, while the Peridotite Unit (PeU) shows significant Sr, slight Zr depletion and pronounced positive Ti anomalies. The Pyroxenite Unit (PyU) is depleted in Nb and Zr but significantly enriched in Sr and Ba, with lower ratios of Nb/La and Zr/Sm, compared to those of the underlying two units. The PGE-enriched layer in the Marginal Unit has elevated and highly variable Rb/La and Ba/Th values and the lower part of the Pyroxenite Unit, which includes a PGE-enriched layer, exhibits high and decreasing Rb/La and Ba/Th ratios upwards. Sr–Nd isotopic compositions of samples from the three units (ca. 90 m thick) were systematically analyzed and fall in a range of 0.7056 to 0.7074 for ( 87Sr/ 86Sr) i and of −5.26 to +2.79 for ε Nd(i). Within the Marginal and Peridotite Units, high ( 87Sr/ 86Sr) i and low ε Nd(i) values are demonstrated. By contrast, an enriched Sr-isotopic (( 87Sr/ 86Sr) i∼0.7074) and slightly depleted Nd-isotopic ( ε Nd(i)∼+2.5) signature occurs in the basal Pyroxenite Unit, 8 m below the PGE-enriched layer in this unit. The Marginal Unit is interpreted as the product of contamination of a plume component (e.g., ε Nd(i)∼+5 and ( 87Sr/ 86Sr) i∼0.704) by continental crust. The Peridotite Unit has undergone decreasing degrees of crustal contamination. At the bottom of the Pyroxenite Unit, a new magma input, contaminated by upper crust, is proposed, where its slight Nd–Sr decoupling may be attributed either to the much higher Sr content than the Nd content of the contaminant or the selective contamination of Sr rather than Nd. The PGE mineralization in the Pyroxenite Unit was caused by magma mixing between the new magma and the resident magma, whereas the PGE-enriched layer in the Marginal Unit resulted from wallrock contamination. However, the displacement of the isotopic pulse from the PGE-enriched layer in the Pyroxenite Unit is due to later sulfide saturation being achieved in both end members. Both of the PGE-enriched layers are linked directly to the segregations of the Fe–Ti oxides and sulfides.

Palynological evidence for the astronomical origin of lignite–detritus sequence in the Middle Pleistocene Marathousa Member, Megalopolis, SW Greece

The Marathousa Member, Middle Pleistocene strata in the fluvio-lacustrine Megalopolis basin, southwest Greece, displays distinct but complicated lithological cycles comprising first-order alternation of lignites and detrital muds and second-order alternation expressed by frequent intercalation of organic layers. Palynological evidence indicates that the lithological cycles are driven by the Earth’s orbital forcing. All the lignite seams yield temperate oak forest whereas the detrital beds provide semi-arid steppe mainly of Artemisia. This means that the first-order lithological cycle represents the glacial/interglacial cycle (i.e., the 100-kyr eccentricity cycle), providing a timescale of at least 350 kyr to the Marathousa Member. Pollen also detects smaller-scale climate fluctuations in many of the subordinate organic layers, with the total number of fluctuations being five in a complete lignite–detritus couplet. This means that the second-order lithological cycle reflects the 21-kyr insolation cycle. A tentative phase relation between the lithological cycles and the astronomical cycles is shown based on palynostratigraphy and electron spin resonance dating. Lacustrine environments with increased water tables are implied for the glacial periods sedimentologically, in contrast to local swamp vegetation for the interglacial periods. The subordinate organic layers were formed under intermediate environments (climate, water depth, etc.) between full glacials and interglacials.

Cyclostratigraphic analysis of pelagic carbonates at Monte dei Corvi (Ancona, Italy) and astronomical correlation of the Serravallian-Tortonian boundary

Cyclostratigraphic analysis of pelagic carbonates at Monte dei Corvi (Ancona, Italy) and astronomical correlation of the Serravallian-Tortonian boundary Spectral analysis of high-resolution calcium carbonate data from a 2-m.y.-long section from the upper Miocene pelagic limestone-marl sequence at Monte dei Corvi, Italy, indicates orbital forcing of depositional cycles at frequencies corresponding to eccentricity, obliquity, and precessional cycles. Identification of these Milankovitch cycles, dated by 4 0 Ar/ 3 9 Ar ages from two biotite-rich volcanic-ash layers within the section, allows us to correlate this section with the astronomical time scale by matching low-frequency variations in calcium carbonate content with the orbital eccentricity. This correlation yields precise numerical ages for any stratigraphic level within the analyzed section, including the two volcanic-ash layers and the Serravallian-Tortonian boundary, dated here as 11.32 Ma. The eccentricity-based correlation also allows us to tune the data, resulting in enhanced power in the obliquity and precessional frequencies, supporting the initial hypothesis of orbital control of these lithologic cycles.

Orbital forcing in Pliocene–Pleistocene Mediterranean lacustrine deposits: dominant expression of eccentricity versus precession

Milankovitch forcing of climate is expressed in the sedimentary record as lithological cycles that can have one or more of four typical periods related to precession (21kyr), obliquity (41kyr) and eccentricity (100 and 400kyr). In several Mediterranean continental successions, striking differences in the expression of particularly precession and eccentricity appear. We present the results of an additional lower Pliocene lacustrine succession from Lupoaia (southern Romania), and compare the cyclic expression in this lignite–siliciclastic basin in detail with the time-equivalent lignite–carbonate basin of Ptolemais (northern Greece), the middle Pleistocene lignite–siliciclastic basin of Megalopolis (southern Greece) and the Miocene carbonate–clay basin of Orera (north eastern Spain). It appears that carbonate basins dominantly express precession, while siliciclastic basins dominantly express eccentricity in their lithological cycles. This can be explained by a more linear response to insolation forcing in carbonate basins than in siliciclastic basins. Alternatively, it can be explained by the low amplitude of 100-kyr eccentricity at the times the carbonate sections were deposited. This reduced 100-kyr amplitude was caused by minima in the 2.35Myr eccentricity cycle, that co-occurred with the deposition in both carbonate basins. Finally, the expression of 100-kyr eccentricity appears to be independent of glacial cyclicity.

The new BDP-98 600-m drill core from Lake Baikal: a key late Cenozoic sedimentary section in continental Asia

The new 600-m drill core BDP-98 from the Academician Ridge of Lake Baikal recovered a continuous sedimentary record of the past 10 Ma. The entire section is represented by lacustrine sediments, which gradually change from distal deltaic facies at the bottom of the section to fine undisturbed hemipelagic sediments of the upper 300-m interval. The entire 10-Ma lacustrine section contains abundant diatoms, thus allowing extension of Plio-Pleistocene diatom and biogenic silica records into the Miocene. Above the Matuyama/Gauss paleomagnetic reversal boundary, the BDP-98 record contains clearly delineated glacial/interglacial lithologic cycles. Below this boundary the diatom signal is quite different: average diatom contents are higher and variations are of lower amplitude. Although most likely paleoclimatic in origin, these variation presumably reflect past changes in the moisture regime of southeast Siberia under conditions of warm subtropical climate during the Miocene and Early–Middle Pliocene. The continuous BDP-98 drill core, which covers the hiati present in the composite continental sections of the Baikal region, is a key section for reconstructing the Neogene–Quaternary climatic evolution of continental Asia. The BDP-98 section also places several important time constraints on the rifting history of Lake Baikal by providing reliable correlation of lithological and physical properties of the drill core sediments with calculated positions of the acoustic reflection boundaries interpreted from multichannel seismic studies. The lithologic composition indicates that, on the stable block of Academician Ridge where the BDP-96 and BDP-98 drill sites are located, acoustic reflection boundaries are not associated with major erosional events, but instead result from changes in sediment density and composition. Several lithologic indices further suggest that significant changes have occurred in the physics and chemistry of Lake Baikal waters, affecting the carbonate equilibrium and oxygen regime of Baikal.

Biogenic Silica Record of the Lake Baikal Response to Climatic Forcing during the Brunhes

This work presents a detailed, orbitally tuned biogenic silica record of continental paleoclimate change during the Brunhes chron. The Brunhes/Matuyama boundary lies within the warm isotopic stage 19 in Baikal, and the boundaries between eight lithological cycles correspond to terminations in the marine oxygen isotope record. The high amplitude and resolution of climatically driven changes in BioSi content in Lake Baikal sediments permits tuning of almost every precessional cycle during the Brunhes and reveals the structure of interglacial stages. For example, the last three interglacial stages (MIS 5, 7, and 9) clearly consist of five substages (a, b, c, d, e) corresponding to precessional insolation peaks. Abrupt and intense regional glaciations in Siberia during substages 5d and 7d were driven by extreme insolation minima. During substage 9d cooling was more gradual in response to more moderate forcing. The impact of strong glaciation is also observed in the middle of stage 15, where full glacial conditions appear to have lasted for over 30,000 yr during substages 15d, 15c, and 15b. Marine oxygen isotopic stage 11 appears to be the warmest period during the Brunhes in the Lake Baikal record, with at least three substages.A new hypothesis is presented regarding the response of the Lake Baikal BioSi record to insolation forcing. Based on the mechanism controlling modern diatom blooms, biogenic silica production is hypothesized to be dependent on changes in the heat balance of the lake and consequently on changes in the thermal structure of the water column. This mechanism is also sensitive to short-term sub-Milankovich cooling events, such as the mid-Eemian cooling, the Montaigu event during substage 5c, and a cooling which appears to be analogous to the Montaigu event during substage 9c. The continuity of the Lake Baikal paleoclimate record, its sensitivity to orbital forcing, and its high resolution make it an excellent candidate for a new “paleoclimatic stratotype” section for continental Asia.

Orbital forcing of palynofacies in the Jurassic of France and the United Kingdom

Palynofacies analysis of marine Jurassic sedimentary successions in France and the United Kingdom demonstrates the pervasiveness of regular cycles in qualitative and quantitative aspects of the microscopic particulate organic content of the rocks, especially that part which is ignored during routine palynology. The cycles are attributed to precession, obliquity, and eccentricity forcing. Most of the palynofacies cycles are independent of lithology, whether it be cyclic, irregular, or uniform. Terrestrially derived organic particles record the effects of orbital cyclicity on land, which may be controlled by orbital cycles different from those controlling marine sediment deposition. The majority of studies on orbital cyclicity in the sedimentary record concentrate on cyclicity in lithology. This study demonstrates that sections lacking regular lithological cycles may still record evidence of orbital forcing of both the depositional environment and sediment source and transport.

Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record

Continental deposits from the early Pliocene lacustrine Ptolemais basin in NW Greece display rhythmical alternations of lignite and marl beds. Three parallel sections from this area are studied using magnetostratigraphy and cyclostratigraphy. The presence of the greater part of the Gilbert Chron enables the recognition of astronomical periodicities in the succession. Especially the precessional influence is evident, as it determines the lithological cycles. The continental Ptolemais composite section is correlated to the most recent astronomical time scale — and thus to the marine reference section: the Rossello composite from Sicily [C.G. Langereis, F.J. Hilgen, The Rossello composite: a Mediterranean and global reference section for the Early to early Late Pliocene, Earth Planet. Sci. Lett. 104 (1991) 211–225] — on a bed-to-bed scale. It is concluded that lignite corresponds to an insolation minimum (beige layer in the Rossello composite), and marl to an insolation maximum (grey layer in the Rossello composite). This implies a precipitation increase during insolation maxima in early Pliocene continental Greece.

Formation and diagenesis of bedding cycles in uppermost Cretaceous chalks of the Dan Field, Danish North Sea

Metre‐scale lithologic cycles, visible in core and on logs from Maastrichtian chalks of the Dan Field, were examined to determine their mechanisms of deposition and relation to hydrocarbon production. The lower parts of cycles consist of porous, cream‐coloured, largely non‐stylolitic, commonly laminated chalk with limited bioturbation (mainly escape burrows). Cycles are capped by thinner intervals of white to grey, hard, stylolitic chalk with concentrations of bioclastic material, intense burrowing and few preserved primary sedimentary structures. The cycle caps contain nearly twice as much Mg as compared to the more porous parts of cycles and also have slightly larger δ18O values (−4·1‰ for the caps; −4·4‰ for porous zones). There is a significant reduction of average cycle thickness, as well as total thickness of the Maastrichtian chalk section, from SW to NE across the Dan Field. The cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the NE side of the field. These data indicate that episodic winnowing removed fine‐grained constituents from highstanding northeastern areas. Porous cycle bases were deposited at relatively high rates that precluded complete bioturbation; preserved laminae, coupled with escape burrows, reflect episodic sediment influx in areas that flank the seafloor highs. Cycle tops apparently accumulated more slowly (throughout the region, but especially on seafloor highs), perhaps because of reduced productivity of planktic organisms. Slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures, and also led to incipient high‐magnesium calcite seafloor cementation (sufficient to yield firmer sediment and enhanced burrow preservation, but not to form true hardgrounds). Thus, the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis.Rates of chalk deposition, as inferred from physical and geochemical evidence, appear to be a significant control on reservoir characteristics in North Sea chalks. The highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized. Topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics, and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties.

Comparative evolution of material flux through two DSDP successions from Central Atlantic and Gulf of Mexico. Preliminary results of a new approach to sedimentary dynamics in pelagic environments

A method for gaining knowledge of sedimentary fluxes is applied to Early Cretaceous sequences containing high-frequency sedimentary cycles. The cycles are used to determine the relative fluctuations of (1) the sedimentation rate and (2) carbonate and silicate terrigenous fluxes expressed as linear values proportional to the real fluxes. Visual and automatic image analysis of black-and-white core photographs have been used to calculate the masses of different materials deposited in equal time intervals and to determine the distribution of the material within lithological units of cores according to a scale of grey intensities. Characters of these distributions allow definition of a flux variability index, assumed to be negatively correlated to ocean-continent distance related itself to sea level. At DSDP Sites 534 (western Atlantic off Florida) and 535 (Gulf of Mexico), the Valanginian-Hauterivian interval has been chosen for determining the relative variations of carbonate and silicate terrigenous linear fluxes, their variability and their distribution within lithological cycles. These data provide information about the relative importance of carbonate sources (lands, platforms and plankton) and their evolution, and of other forcings upon planktonic productivity such as supply of nutrients from the continents and upwelling. By comparing the data in the two sites, it is possible to detect common (climate and eustasy) and local factors (tectonics and upwelling) which have controlled the material fluxes. Compared with Site 534, Site 535 was supplied by more numerous, active and close carbonate sources and less active but closer terrigenous sources. At the two sites, carbonate flux was controlled by common factors, sea level being the most prominent one. Terrigenous flux was submitted to more local controls. This new method for investigating pelagic deposits should be tested on other deep-sea sedimentary sequences.

Conodont paleoecology of the Lower Ordovician St. George Group, Port au Port Peninsula, western Newfoundland

The St. George Group in western Newfoundland is nearly 600 m thick and is subdivided into the Watts Bight, Boat Harbour, Catoche, and Aguathuna Formations. The group is a nearly complete Lower Ordovician succession representing about 15–20 m.y. The depositional environment of the St. George Group is represented by three main lithofacies (supratidal, peritidal, and subtidal), expressed as two first-order cycles and five second-order cycles.In the study of St. George Group conodonts, over 70 multielement species are represented in 45,000 conodont specimens from 432 samples of 10 sections. Computer cluster analysis of most of those samples on the basis of similarity of conodont species was used to determine conodont community structure. Both the principal coordinate analysis (PCA) partitioning method and the agglomerative method (UPGMA with Bray-Curtis) were chosen for the statistical analysis. The four lithological formations and five secondary lithological cycles in the St. George Group emerge as distinct entities when the samples are clustered together on the basis of conodont data for analysis of first-order and second-order variations in community composition.Two deeper water, five shallow-water, and five intermediate communities through this time interval are recognized. The majority of conodonts from these communities are of Midcontinent Realm affinities, but some species represent strong North Atlantic Realm influxes during the transgressive periods.

Well Logging Results from the Newark Rift Basin Coring Project

Wireline logs were acquired at seven sites in the Newark Rift basin using dipmeter, gamma ray, resistivity, velocity, porosity, density, magnetic susceptibility, temperature, and acoustic televiewer tools. The logs indicate that the formations are clay rich and dip on average 9°N-NW. Densities are relatively constant (2.60-2.80 glee) and compressional velocities vary from 4.2-5.5 km/s. Thin uranium-rich layers and basalt flows are clearly delineated. The boreholes are mostly in-gauge, but deviated, and borehole temperature gradients vary between 15 to 26°C/km. These data are potentially useful as indicators of fluid flow and regional stress, lithologic cycles, and for core orientation in the Newark Rift basin.

Pliocene-Early Pleistocene astronomically forced planktonic Foraminifera abundance fluctuations and chronology of Mediterranean calcareous plankton bio-events

The absolute age estimates for calcareous plankton events proposed by the writer in a recent paper are discussed according to the new proposed paleomagnetic time scales. A re-interpretation of the sequence of lithological cycles recognized in the composite makes their number perfectly comparable with the number of planktonic foraminifera abundance fluctuations in the same stratigraphic interval. An absolute age of 5.33 MA is estimated for the base of the Pliocene, as defined in the Capo Rossello section. Periodicity of the relative abundance fluctuations of the planktonic foraminiferal assemblages during the Early Pleistocene stratigraphic interval is estimated at about 41 kyr, well comparable with the periodicity of the obliquiry astronomical cycles. By correlation of the sequence of biostratigraphic events recorded in the Early Pleistocene interval in the Mediterranean sections with the sequence of abundance fluctuations, their absolute ages have been obtained, as well as the absolute age of chronostratigraphic boundaries. The base of the Pleistocene is at 1.83 MA and the top of the Early Pleistocene, coincident with the top of the Sicilian, is at 0.89 MA. A re-evaluation of the Oxygen isotopic Stages of the upper Singa section is proposed, on the base of paleomagnetic and calcareous plankton biostratigraphic data reported for this section. Correlation with the North Atlantic Site 607 proved that rhe G. oceanica s.l. FO, the C. macintyrei LO and rhe Gephyrocapsa sp.3 FO are slightly diachronous between the Mediterranean and the mid-North Atlantic high latitudes.

Paleomagnetic calibration of Milankovitch cyclicity in Lower Cretaceous sediments

Bedded pelagic limestones of early to mid Cretaceous age may record cyclic climate variability driven by changes in the Earth's orbit (“Milankovitch cycles”). If properly calibrated, the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy. The lower Cretaceous Maiolica Formation of northern and central Italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper M-series marine anomalies. The magnetic reversals divide measured sections into 0.5–2 Myr increments in which sedimentation rate (assuming continous deposition) can be measured to 15–25% accuracy. Paleomagnetically estimated sedimentation rates over a number of polarity zones establish, with one exception, a narrow range in periods of the carbonate cycles. Carbonate couplets have an estimated mean period of 23.5 kyr, and prominent modulations in bedding thickness (“bundling”) have a mean period of 117 kyr (Kent and Gradstein 1985 time scale). The close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing.Comparison of measured sections from different locations shows that bed-to-bed correlations are possible regionally, cosistent with the Milankovitch hypothesis. Episodes of Barremian “black shale” deposition occur in the troughs of the short eccentricity cycle, as roughly one half of the carbonate-marl couplet, or about 10 kyr duration. It is possible to estimate both the timing (relative to the top of reversed chron M-0) and duration of a period of unusual organic carbon-rich deposition (“Selli Horizon”) in the early part of the Cretaceous Long Normal Magnetic Polarity Interval by tracing the sedimentary cycles up from the Barremian strata.

Stable isotopic signatures of tertiary lake carbonates, Eastern Ebro Basin, Spain

The Tertiary Ebro Basin, Spain, was a region of internal drainage fed by rivers emanating from the fringing Pyrenees, Catalanides and Iberian chains. The stable isotopic composition of bulk carbonates deposited in diverse late Eocene to Oligocene lakes within the eastern part of this endorheic region are investigated. Carbon and oxygen isotopes fall into several distinct hydrographic provinces which are partly palaeogeographical and partly biostratigraphical. The isotopic signatures of the lake carbonates primarily reflect the nature of the river input (duration, seasonality, catchment geology) and autochthonous processes within the lake (biological productivity, evaporation) but diagenetic changes may further complicate interpretations. Seasonal changes in precipitation rates are picked out by light-dark variations in single lithological cycles.

Lithofacies Variations in Green River Formation, Wyoming: ABSTRACT

The Green River Formation of Wyoming is a thick lens of sedimentary rocks which was deposited in a large playa-like complex. These sediments accumulated in three major depositional environments: (1) marginal (siliciclastic sandstone and siltstone); (2) mudflat (marlstone); and (3) lacustrine (oil shale, trona, fossiliferous limestone). In general, the basinward transition of lithofacies is from coarse-grained, cross-bedded, channel sandstones at the basin margins to algal and oolitic limestones to clastic and algal dolomitic marlstones and finally to oil shale and trona at the center of the basin. The lateral intertonguing of these lithofacies and the repetition of large- and small-scale lithologic cycles provide a basis for the reconstruction of the physical, chemical, a d biologic evolution of Eocene Lake Gosiute. Thin, but extensive, algal marlstone units reflect the basinward migration of Lake Gosiute's shoreline during periods of increased aridity. Widespread fossiliferous limestones suggest transgressive shoreline conditions. The occurrence of thin, extensive tuff beds establishes time-stratigraphic relations among the lithofacies. The correct interpretation of lithofacies changes in a closed basin can be an important tool for locating deposits of rich oil shale, trona, and perhaps oil- and gas-bearing channel sandstone reservoirs similar to those now being developed in the Uinta basin. End_of_Article - Last_Page 919------------

Tectonic Control of Sedimentation and the Interpretation of Sediment Alternation in the Tertiary of Prince Charles Foreland, Spitsbergen

Over 6000 feet of clastic sediments, probably of early Tertiary age, have been deposited in an actively developing graben. The sequence consists of beds mostly less than a foot thick, ranging in texture from conglomerate to shale, but dominantly of low rank graywacke. While much of the rapid vertical variation in lithology is erratic, some cyclic patterns are present. The sediments were derived from an area of low-grade metamorphic rocks only a short distance to the west and were deposited at the edge of a mountainous land which was actively rising by warping and perhaps by early movements on the faults that now separate the horst of Prince Charles Foreland from the graben of the Foreland Sound. Slump structures indicate considerable downward sliding of the sediments toward the center of the graben. Lithologic cycles in the sediments have been analyzed quantitatively in an attempt to assess their significance. Major cycles are attributed to variation in the balance between the rate of upward movement of the source area and the rate at which erosion reduced the resulting topographic relief. Minor cycles (mainly conglomerate-sandstone alternations and sandstone-shale alternations) are superimposed on the major cycles. Chiefly from an analysis of the ratios of thicknesses of beds in these cycles, it is argued that they represent short-term meteorologic variations. For each kind of rock, the thicknesses of beds have a log normal frequency distribution, and the geometric mean thicknesses increase with coarseness: shales 0.3 foot, siltstones 0.6 foot, sandstones 1.0 foot, conglomerates 1.4 feet. The sandstone thickness distribution, however, is a composite of two populations. The sandstones that are in cyclic alternation with conglomerates have a geometric mean thickness greater than that of the conglomerates; the sandstones that are in cyclic alternation with shales have a geometric mean thickness less than that of the shales. These data are compared with information on bedding thickness from other areas, and the interpretation of such data is discussed. The tectonic setting is taphrogeosynclinal; the rocks are dominantly low rank graywackes. The association of various kinds of sedimentary rocks with this tectonic setting is discussed. If general uplift occurs at the same time as the differential movement that forms the taphrogeosyncline, non-marine deposits will probably develop. Arkoses will be abundant if the source area consists of high-grade metamorphic and granitic rocks. Arkoses and non-marine deposits have developed in the past in a number of situations where block-faulting accompanied the uplift and deep erosion of the central part of an orogenic belt after an episode of orogenesis. In Prince Charles Foreland, however, the horst rose and the graben sank about a mean horizon of roughly constant elevation, and the “accidental” location on the western flank of the much older Caledonian fold belt of Spitsbergen resulted in a source area of low-grade metamorphic rocks and a sedimentary pile dominated by low rank graywackes.

The stratification of the Great Scar Limestone in the Settle district of Yorkshire

AbstractThe Great Scar Limestone in the Settle district and near the Stainmore syncline has been studied in eight representative sections. The sequence of D1 age in this area is divided by pronounced master bedding planes into nine lithological cycles, which can be traced from section to section.Fossils are concentrated near the master bedding planes and indications of increased current activity are recorded. The regularity with which the master bedding planes occur suggests a climatic rather than tectonic control of the cycles.

Stratigraphy of Madison Group of Montana and Wyoming: ABSTRACT

Lithologic sequences characterized by distinct evaporite cycles are recognized in the Madison group of Montana and Wyoming. Throughout the basinal area of Montana and North Dakota, the End_Page 421------------------------------ cycles are represented by vertical variations from oolitic-fragmental and normal marine limestone (or dolomitized equivalents) to evaporitic rocks including anhydrite, locally halite, and associated cryptocrystalline or dense carbonates. Within the Wyoming shelf, the cycles exhibit only partial development in which the evaporitic interval is thinly represented by primary cryptocrystalline or dense dolomite. Dolomitized limestones (fine to coarse crystalline dolomites) generally form the marine intervals in the shelf areas. By recognition of these lithologic cycles, the Lodgepole, Mission Canyon, and Charles formations may be correlated to their respective eroded edges in Wyoming, South Dakota, and north-central Montana. Mississippian sedimentation commenced earliest in the Williston basin and central Montana trough with the deposition of black shales (Bakken formation) followed by dense limestones and shales (lower Lodgepole). Preceding and accompanying the initial marine transgression on the Wyoming shelf, minor terrestrial to near-shore marine clastic deposition took place in the Black Hills area (Englewood formation). Widespread marine deposition formed normal marine and fragmental-oolitic limestones which were subject to dolomitization in the shelf areas (upper Lodgepole, lower Pahasapa, and Guernsey formations). Marine limestone and dolomite sedimentation, interrupted by two episodes of evaporite precipitation, resulted in the development of the Mission Canyon formation and the superjacent evaporites of the basal Charles, and their equivalents in the Pahasapa formation of the Black Hills area. Restoration of normal marine conditions and development of a complex evaporite cycle followed (Charles formation, excluding basal evaporites and uppermost few beds of the Pahasapa). Big Snowy clastics were deposited in Chester time in the basinal areas of Montana. In areas to the south, pre-Amsden, pre-Minnelusa, or pre-Pennsylvanian weathering and erosion affected the Mississippian beds. In north-central Montana, pre-Middle Jurassic erosion strongly truncated the upper Madison strata. Oil occurrence in the Madison group is related to the two main provinces of deposition. In general, the oil from the basinal or limestone province is of relatively high gravity, whereas that from the dolomite or shelf province is of much lower gravity. End_of_Article - Last_Page 422------------