Stratigraphic Continuity Research Articles

Conodont Biostratigraphy of Lower Ordovician Rocks, Arbuckle Group, Southern Oklahoma: ABSTRACT

The Arbuckle Group of southern Oklahoma displays the only complete exposure of the shallow-water carbonates that characterize the Lower Ordovician of interior North America. Trilobites have been described from some parts of this sequence and sporadic occurrences of other invertebrates are known, but much of the sequence is sparingly fossiliferous. As a consequence, these magnificent exposures have not contributed notably to continuing efforts toward development of a comprehensive biostratigraphic scheme for the Lower Ordovician of the North American platform. Samples collected at 25-ft intervals through the Arbuckle Group along and adjacent to Interstate Highway 35 on the south flank of the Arbuckle anticline near Ardmore, Oklahoma, produced conodonts in abundances ranging from a few tens to over a thousand elements per kilogram and displaying good to excellent preservation with low CAI. These conodonts document a biostratigraphic continuum that provides a standard for correlation of Lower Ordovician rocks in the subsurface of central US and of the many localized and incomplete outcrops of generally equivalent strata in the Ozark and Upper Mississippi Valley regions. The stratigraphic continuity of the collections makes the I-35 section an ideal standard reference section for graphic correlation of Lower Ordovician rocks containing conodonts of the Mid-Continent more » Province. « less

Depositional Wedge Cycles on Evolving Plate Margin, Western and Northwestern Australia

The Barrow province lies at the transition between the western and northwestern rift domains of the Australian continental margin. As these domains evolved during the Mesozoic, five depositional wedge sequences developed in the eastern half of the province. Each sequence possesses distinct stratigraphic and sedimentological characteristics, which differs from the others as a function of proximity, direction, and style of rift-related diastrophism along the evolving margin. The first depositional sequence developed during the Triassic as part of a regional Tethyan-facing passive margin clastic wedge. This event was followed during the Early and Middle Jurassic by a second more provincial sequence in response to early syn-rift fault-related subsidence. By the Callovian, a deep anastomosing rift complex had formed and deposition of the third Late Jurassic cycle was initially very localized, confined by the new graben topography. However, stratigraphic continuity returned during post-breakup subsidence of the northwestern margin as subregional depositional systems began to bury the fragmented borderland. This cycle was terminated at the close of the Jurassic in response to renewed rifting between western Australia and Greater India. Sediment stripped from t e uplifted rift shoulders fed the northerly prograding Barrow delta system to form a fourth depositional sequence during the Late Jurassic-early Neocomian. The final cycle commenced as Greater India uncoupled from Australia during the middle Neocomian. Post-breakup subsidence is reflected by a major regional transgression and the development of a second margin-wide depositional wedge during the later Cretaceous. The close temporal and spatial relationship that exists between these cycles and the evolution of the western and northwestern Australian continental margin emphasizes the importance of tectonic control upon sedimentation.

Concepts and Methods of High-Resolution Event Stratigraphy

Modern geological analyses involving sedimentary rocks require a high­ resolution system of dating, integrating, and correlating diverse strati­ graphic, geochemical, and paleontological data that also takes into account the possibility that short-term phenomena (100 kyr or less) exercise a strong control on sedimentation. Such phenomena may be extraterrestrial, tectonic, volcanic, oceanographic, climatic, sedimentologic, and/or bio­ logic in origin. Some reflect unpredictable perturbations of local to global scale. But most short-term phenomena are predictable and may be either autocyclic (locally regulated, with limited stratigraphic continuity) or allo­ cyclic in nature (regionally to globally regulated, with extensive strati­ graphic continuity) (Beerbower 1964). A stratigraphic system based on short-term phenomena would ideally be chronostratigraphic, involving the identification and regional tracing of time lines (isochronous surfaces or very thin event deposits), and easily integrated with refined, independently derived biostratigraphic and geochronologic systems. An intensive search for such a new and revolutionary system of stratig­ raphy has been prompted in many parts of the Phanerozoic by attain­ ment of plateaus in stratigraphic resolution utilizing the more standard tools of magnetostratigraphy, biostratigraphy, and geochronology: Most magnetic reversals have been identified; biostratigraphic systems can be no more refined than the maximum evolutionary rates of component taxa and their major structures (known for many favored groups); and geo­ chronology is regulated by the analytical confidence limits of radiometric

The last deglaciation (20,000 to 11,000 B. P.) on Andoya, northern Norway

Cores representing a 5.5m long sequence recovered from lake Æråsvatnet have been investigated for lithostratigraphy, micro‐ and macrofossils and radiocarbon chronology. For the first time in Fennoscandia the maximum Weichselian advance has been closely bracketed with radiocarbon datings (19,000–18,500 B.P.). A continuous stratigraphy from 18,500 B.P. and onwards, partly marine and partly lacustrine, discloses the local shoreline displacement, the palaeovegetation, the palaeoclimate and, together with other data, the deglaciation history. Two phases with a prevailing High Arctic climate have occurred: 18,000 to 16,000 B.P. and 13,700 to 12,800 B.P. Important climatic amelioration accelerating the deglacial recession occurred 16,000, 12,800 and 12,000 B.P. The continental ice sheet was situated close to its maximum position until about 16,000 B.P. The following deglaciation was interrupted by (a minor ?) readvance/halt about 15,000 B.P. (the Flesen event), 13,700‐13,000 B.P. (the D‐event), 12,500 B.P. (the Skarpnes event) and 11,000–10,000 B.P. (the TromsØ‐Lyngen event). The deglaciation chronology and pattern can be correlated with the suggested deep‐sea‐stratigraphy‐based stepwise pattern relying on the old age alternative for termination IA.

Temporal Variations in the Deep Ice-Core Chemistry Record from Dye 3, Greenland (Abstract)

Measurements of the chemical constituents in polar-snow deposits translate into chronological records representing a history of atmospheric composition for the periods involved. The 2037 m deep continuous and undisturbed ice core recovered at Dye 3, Greenland between 1979 and 1981 contains a temporal record of sequential snow deposits for the past 9 × 104 a B.P. (Dansgaard and others 1985). The upper 90 m of the deep core were unsuitable for chemistry studies, but stratigraphic continuity with present-day accumulation was obtained by hand-excavating a 5.4 m deep pit and augering two shallow cores to 138 and 113 m depths. The pit and shallow cores represent the last two centuries of snow precipitation.To date, over 6000 individual samples of the pit, shallow and deep ice cores have been measured by ion chromatography for Cl−, NO3−, and SO42− in the field and laboratory (Herron and Langway 1985, Finkel and Langway 1985, Finkel and others 1986), under clean-room conditions. All pit and shallow-core samples were prepared in a continuous sequence of eight samples per year, as identified by other stable and radioactive isotope-dating methods. The deep ice-core samples were selected and prepared from core intervals spaced over the 2037 m profile from time units which showed evidence of abrupt or transitory periods in climate change or volcanic disturbances, as defined by stable isotopes (Dansgaard and others 1985), atmospheric gases (Oeschger and others 1985) and dust (Hammer and others 1985).Approximately 1700 new measurements from the Dye 3 samples are included in this study. Variability in the chemical constituents and their concentration levels is present and meaningful on a short-term and long-term basis. The time units measured represent seasons, years, decades, centuries and longer geological periods. Particular attention is given to two new high- and low-frequency detailed chronological data sets from (1) a continuous 26 m core profile, representing 3000 years, extending from the Holocene/Wisconsin boundary back into the late Wisconsin and (2) measurements made on 106 samples spaced every 2 m over the Wisconsin-age ice from 1786 to 2008 m.

Temporal Variations in the Deep Ice-Core Chemistry Record from Dye 3, Greenland (Abstract)

Measurements of the chemical constituents in polar-snow deposits translate into chronological records representing a history of atmospheric composition for the periods involved. The 2037 m deep continuous and undisturbed ice core recovered at Dye 3, Greenland between 1979 and 1981 contains a temporal record of sequential snow deposits for the past 9 × 104 a B.P. (Dansgaard and others 1985). The upper 90 m of the deep core were unsuitable for chemistry studies, but stratigraphic continuity with present-day accumulation was obtained by hand-excavating a 5.4 m deep pit and augering two shallow cores to 138 and 113 m depths. The pit and shallow cores represent the last two centuries of snow precipitation. To date, over 6000 individual samples of the pit, shallow and deep ice cores have been measured by ion chromatography for Cl−, NO3 −, and SO4 2− in the field and laboratory (Herron and Langway 1985, Finkel and Langway 1985, Finkel and others 1986), under clean-room conditions. All pit and shallow-core samples were prepared in a continuous sequence of eight samples per year, as identified by other stable and radioactive isotope-dating methods. The deep ice-core samples were selected and prepared from core intervals spaced over the 2037 m profile from time units which showed evidence of abrupt or transitory periods in climate change or volcanic disturbances, as defined by stable isotopes (Dansgaard and others 1985), atmospheric gases (Oeschger and others 1985) and dust (Hammer and others 1985). Approximately 1700 new measurements from the Dye 3 samples are included in this study. Variability in the chemical constituents and their concentration levels is present and meaningful on a short-term and long-term basis. The time units measured represent seasons, years, decades, centuries and longer geological periods. Particular attention is given to two new high- and low-frequency detailed chronological data sets from (1) a continuous 26 m core profile, representing 3000 years, extending from the Holocene/Wisconsin boundary back into the late Wisconsin and (2) measurements made on 106 samples spaced every 2 m over the Wisconsin-age ice from 1786 to 2008 m.

Molecular genetic markers and maturity indices in intermontane lacustrine facies: Kishenehn formation, Montana

Molecular markers have considerable promise as thermal maturation indicators in source rocks having a well-defined depositional environment. This occurs in the Kishenehn Formation (northwest Montana and southeast British Columbia), an Oligocene unit comprised of fluvial and lacustrine shales. Up to 3 km of these non-marine sediments were deposited in a 900 sq.km elongated half graben atop the Precambrian rocks of the Lewis Thrust sheet. Specific outcrops of the Coal Creek Member of the Kishenehn Formation are known to contain excellent potential source rocks, including extensive oil shales. The source rock potential and molecular marker geochemistry of most of the major Kishenehn outcrops have now been investigated, and regional results are presented in this paper. source rock analysis of a regional suite of Kishenehn samples indicates excellent petroleum potential (immature Type I kerogen) throughout the basin. Several classes of distinctive diagenetic molecular markers occur within the Coal Creek Member of the Kishenehn Formation, including diasterenes, spirosterenes, methylspirosterenes, B-ring monoaromatic anthrasteroids, ββ-hopanes and moretanes. Molecular markers indicative of non-marine deposition include dehydroabietane (conifer input) and an onocerane recently identified in leaf fossils of Miocene lacustrine beds in adjacent Idaho. Triterpane ratios are significantly more sensitive than vitrinite reflectance methods in assessing thermal maturation in the Kishenehn, and are successfully used in this study to order outcrop locations according to thermal maturity level. The systematic variation of specific markers with increasing thermal maturation suggests that molecular correlation is feasible, and could provide a method of assessing stratigraphic continuity in the basin.

Stratigraphical and structural relationships of the Dingle Group (Silurian), County Kerry, Ireland

AbstractPrevious views on the relationship between the thick continental deposits of the Dingle Group and the older fossiliferous Silurian rocks of the Dunquin Group are briefly reviewed. Detailed field evidence is presented showing that in the coastal sections of Coosgorrib, Coosglass, and Coosshaun there is faulting between the two groups. The same situation obtains at the northern end of the Great Blasket island. Fortunately inland exposures are sufficient to show that there is stratigraphical continuity from the lower Ludfordian (Ludlow) beds of the Croaghmarhin Formation, at the top of the Dunquin Group, into the purple siltstones of the Dingle Group. The Dingle Group is followed uncomformably by Upper Old Red Sandstone (essentially Upper Devonian).

LATE PLEISTOCENE ZELKOVA EXTINCTION IN CENTRAL ITALY

SUMMARYThe genus Zelkova contains species which are nowadays distributed in an extremely fragmentary, mainly Asiatic, area. The Island of Crete retains geographically the species nearest to Italy. Fossil leaves and pollen discovered in some sites of Central Italy furnish evidence of the distribution of these trees before and during the Last (Eemian) Interglacial and of their persistence during part of the Last (Wurmian) Glaciation. The evidence is seen in two pollen diagrams from boreholes at Castiglione and Fucino which have stratigraphic continuity; at the base of the series Zelkova reaches conspicuous peaks in a phase which can be correlated with the Eemian Interglacial. During the climatic fluctuations of the last glacial Zelkova disappears in steppe periods and spreads again in favourable times up to the time of extinction, which seems to occur in both series somewhat before the latest glacial acme.

Provenance of late Wisconsinan (Woodfordian) till and origin of the Decatur sublobe, east-central Illinois

Stratigraphic continuity of late Wisconsinan (Woodfordian) tills in the Decatur sublobe area with those in the Peoria sublobe area of the Lake Michigan lobe and garnet-to-epidote ratios indicative of a Superior and Southern Province source rather than a Granville Province source are evidence that the Decatur sublobe was part of the Lake Michigan lobe rather than the Saginaw Bay, Huron, or Erie lobes. Garnet-to-epidote ratios indicate that till of the Iroquois Moraine was deposited by an eastern source lobe, probably a coalesced Huron-Erie lobe. The Decatur sublobe formed in a broad pre-Woodfordian lowland between drift-covered bedrock uplands to the northwest in McLean and Livingston Counties and to the southeast in Vermilion and Edgar Counties, Illinois. A coalesced Huron-Erie ice lobe at times deflected flow of the Lake Michigan lobe ice westward. Slightly lobate end moraines in the Decatur sublobe area probably are the result of strong advances or readvances of a broad ice margin; more lobate end moraines probably formed during recession when local topography exerted greater control on the shape of the ice margin. Some of the moraines may have formed as a result of surge-like readvances of a portion of the ice margin.

Metamorphosed melange terrane in the eastern Piedmont of North Carolina

The Falls Lake melange crops out in the eastern Piedmont of North Carolina between the Carolina slate belt and the Raleigh belt. The melange is composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, dispersed without apparent stratigraphic continuity, in a matrix of pelitic schist and biotite-muscovite-plagioclase-quartz gneiss. Textures and structural relationships suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to Early Cambrian volcanic-arc terrane of the Carolina slate belt were thrust upon a probable continental terrane of the Raleigh belt before overprinting by late Paleozoic folding and metamorphism.

Synchronous and time-transgressive Neogene radiolarian datum levels in the Equatorial Indian and Pacific Oceans

Fifty radiolarian events of early Pleistocene and Neogene age were identified in an E-W transect of equatorial DSDP sites, extending from the Gulf of Panama to the western Pacific and eastern Indian Oceans. Our objective was to document the degree of synchroneity or time-transgressiveness of stratigraphically-useful datum levels from this geologic time interval. We restricted our study to low latitudes within which morphological variations of individual taxa are minimal, the total assemblage diversity remains high, and stratigraphic continuity is well-documented by an independent set of criteria. Each of the five sites chosen (503, 573, 289/586, 214) was calibrated to an “absolute” time scale, using a multiple of planktonic foraminiferal, nannofossil, and diatom datum levels which have been independently correlated to the paleomagnetic polarity time scale in piston core material. With these correlations we have assigned “absolute” ages to each radiolarian event, with a precision of 0.1–0.2 m.y. and an accuracy of 0.2–0.4 m.y. On this basis we have classified each of the events as either: (a) synchronous (range of ages <0.4 m.y.); (b) time-transgressive (i.e., range of ages > 1.0 m.y.); and (c) not resolvable (range of ages 0.4–1.0 m.y.). Our results show that, among the synchronous datum levels, a large majority (15 out of 19) are last occurrences. Among those events which are clearly time-transgressive, most are first appearances (10 out of 13). In many instances taxa appear to evolve first in the Indian Ocean, and subsequently in the western and eastern Pacific Ocean. This pattern is particularly unexpected in view of the strong east-to-west zonal flow in equatorial latitudes. Three of the time-transgressive events have been used to define zonal boundaries: the first appearances of Spongaster pentas, Diartus hughesi, and D. petterssoni. Our results suggest that biostratigraphic non-synchroneity may be substantial (i.e., greater than 1 m.y.) within a given latitudinal zone; one would expect this effect to be even more pronounced across oceanographic and climatic gradients. We anticipate that the extent of diachroneity may be comparable for diatom, foraminiferal, and nannofossil datum levels as well. If this proves true, global “time scales” may need to be re-formulated on the basis of a smaller number of demonstrably synchronous events.

Alaska--an Explorationist's Jambalaya: ABSTRACT

Seaward growth of Alaska since Jurassic time has resulted from terrane accretion and deposition of thick clastic sequences in successor basins. Post-accretionary strike-slip faulting and rifting have fragmented both newly accreted terranes and older continental rocks. Accretion and dispersion have resulted in a jambalaya of geologic units that may be viewed in the context of terrane analysis. This offers a spectrum of opportunities and problems for exploration. Large continental fragments (e.g., North Slope, Nixon Fork, and Tatonduk terranes) consist predominately of Paleozoic rocks with relatively continuous stratigraphy and the greatest potential for regional source and reservoir trends. Other exploration targets may include continental rocks beneath oceanic terranes (e.g., Kagvik, Angayucham, Innoko, and Tozitna terranes), which occur as extensive, relatively thin thrust sheets. An island-arc terrane, the Peninsular terrane, has acted as a source both of hydrocarbons from its organic-rich oceanic sediments, and of reservoir-quality deposits shed into the Cook Inlet successor basin. Post-accretionary rifting and strike-slip dispersion of the growing continental framework of Alaska have resulted in formation of a series of basins filled wit clastic sediments, including the Bering Sea and Interior basins, which are current targets for hydrocarbon exploration. End_of_Article - Last_Page 244------------

La stratigraphie des loess et formations fluviatiles d'Achenheim (Alsace) : nouvelles données bioclimatiques et corrélations avec les séquences pléistocènes de la France du Nord-Ouest

New data (malacology and palynology) of the detailed study of Achenheim (Alsace, Bas-Rhin) confirm the presence of a continuous stratigraphy which is the record of the cyclic climatic evolution during the Upper and the Late Middle Pleistocene. Four loess cycles are separated by partly interglacial pedocomplexes. The vosgian terrasse Formation displaced by an important fault system corresponds to a prior glacial cycle which age is discussed. Taking fauna data into account correlations are proposed between Achenheim and the typical sites of the stratigraphy of loess (Saint-Pierre-les-Elbeuf Formation) and fluviatile terraces of North- Western France.

The Vema Channel: Physiography, structure, and sediment—Current interactions

The east flank of the Vema Channel in the southwestern Atlantic is an opportune location for interpreting the Pleistocene/Holocene record of deep thermohaline circulation. The channel and the adjacent sediment-covered Rio Grande Rise intersect each of the principal sub-thermocline water masses of the western Atlantic. We analyzed a suite of 25 giant gravity cores in transects extending between 1400 and 4150 m depth; this interval includes the upper and lower branches of Circumpolar Water, North Atlantic Deep Water, and Antarctic Bottom Water. The late Pleistocene/Holocene sediments along these transects are pelagic carbonates with excellent stratigraphic continuity; shallower and deeper regions are relatively unsuitable due to poor core recovery or widespread erosional unconformities. Extensive hydrographic and direct current observations near the location of our prime coring transect at 30°S indicate a notable cross-channel asymmetry in the abyssal hydrography and velocity structure which is reflected in the underlying lithofacies distribution. The near-bottom flow field shows marked temporal and spatial variability on a local scale, even in the channel axis where one would anticipate relatively steady, uniform flow. Much of this variability appears to reflect the effects of transient mesoscale eddies rather than regional variations in the integrated north ward transport of AABW. In spite of temporal variations in the flow field, the NADW/AABW transition zone appears to have remained remarkably stable over time scales of 10 5–10 6 yrs, on geological evidence discussed elsewhere in this volume (Jones, 1984; Jones and Johnson, 1984). This observation is somewhat surprising, because in the modern oceans the density contrast between NADW and AABW is relatively small. If the transition zone between the two water masses has indeed remained relatively stationary, then the density-determining water properties ( T and S) may also have remained relatively constant, with only the rates of production changing. If this interpretation proves correct upon further studies, it implies that pale-oceanographic interpretations of deep thermohaline flow should in some instances utilize geological indices of non-conservative water properties (e.g. CO 2, δ 13 C for monitoring paleocirculation changes.

Lithologic variations in Otago Schist, Mt Aspiring area, northwest Otago, New Zealand

Abstract Psammitic schist, 2 types of pelitic schist (grey and porphyroblastic), and 4 types of greenschist (metavolcanic rock — light, spotted, foliated, and epidote rich) are recognisable on outcrop scale in textural zone 4 of the Otago Schist, northwest Otago, New Zealand. Thin horizons of metachert, marble, and ultramafic rock are commonly associated with greenschist. Broad units with 1 predominant rock type (greenschist, psammite, grey pelite, or porphyroblastic pelite) are mappable on a regional scale. These units also contain most or all of the above rock types and have poorly defined boundaries. The degree of original Stratigraphic continuity within and between these broad units is unknown. The studied area can be subdivided into 2 lithologic associations, the “Aspiring association” and the eastern “Wanaka association”, separated by a north-trending, poorly defined but lithologically gradational boundary. The Aspiring association is made up predominantly of pelitic rock types with considerable qua...

The Loch Quoich Line—a limit of early Palaeozoic crustal reworking in the Moine of the Northern Highlands of Scotland

The Loch Quoich Line, a major structural line at least 60 km in length, within the Moine of the Northern Highlands of Scotland, has hitherto been variously interpreted as a major tectonic dislocation. the root zone of a nappe, and an unconformity. The work reported here was carried out at the type locality of the line, and was directed at resolving these ambiguities. The lithostratigraphic succession established by sedimentary structures shows that the rocks of the Glenfinnan Division are older than, but are stratigraphically continuous into, those of the Loch Eil Division. Rocks of the Loch Eil Division crop out extensively to the W of the Loch Quoich Line, and are not restricted in outcrop to the area E of the line, as was previously thought. The early (D 1 –D 2 ) structures were initially sub-horizontal. Steeply inclined, NNE-trending D 3 structures are weakly developed E of the axial trace of a major fold, the Beinn Beag Synform. In this area the metasediments remain essentially in their post-D 2 , gently inclined attitude. W of the axial trace of the synform, the earlier folds and fabrics were severely reworked into steep attitudes during D 3 , producing NNE-trending zones of 'platy' rocks, which mark regions of severe strain on D 3 fold limbs. Earlier (D 1 –D 2 ) structures are clearly preserved in D 3 hinge zones, which mark zones of low D 3 strain. It is concluded that the Loch Quoich Line coincides with the axial trace of the Beinn Beag Synform, and that it is the eastern limit of severe tectonic reworking (probably Caledonian) over-printing early (probably Precambrian) structures. Recognition of structural and stratigraphic continuity between the Glenfinnan and Loch Eil divisions, implies that the Loch Quoich Line cannot be correlated across the Great Glen Fault with the putative unconformity separating the Grampian (cover) and Central Highland (basement) divisions, in the Moine rocks of the Central Highlands.

Origin, sedimentary features, and significance of low-angle eolian "sand sheet" deposits, Great Sand Dunes National Monument and vicinity, Colorado

Low-angle eolian deposits of a sand sheet constitute an areally extensive (710 km 2 ), transitional facies between high-angle eolian dunes and non-eolian deposits near Great Sand Dunes National Monument, Colorado. The bulk of the low-angle eolian deposits of the sand sheet lie in a 15-km-wide belt between the main dune mass at Great Sand Dunes and the Rio Grande River 45 km to the southeast, or upwind. These low-angle eolian deposits have a number of distinctive sedimentary features which are useful in interpreting ancient rocks, including: (1) coarse-grained, high-index ripples (commonly horizontal); (2) gently dipping, curved or irregular surfaces of erosion (several meters in length) within the deposits; (3) abundant bioturbation traces, formed mainly by insects and plant roots; (4) patches or zones (extending up to 1 m below the ground surface) of bioturbated sand resulting from destruction of laminae by grass root growth; (5) convex upward laminations; (6) small scale cut-and-fill structures, perhaps due to scour around plant roots and subsequent infill; (7) gently dipping, poorly laminated layers resulting from adjacent grainfall deposition; (8) sets of normal- and inverse-graded laminae 1- to 4-mm-thick; (9) discontinuous thin 1- to 4-mm-thick layers of coarse sand intercalated with fine sand; (10) intercalation of non-eolian materials, including silty pond sediment and gravelly stream deposits; (11) occasional intercalation of high-angle eolian deposits. Low-angle eolian deposits of the sand sheet originate mainly by gentle deceleration of wind, in the lee of small topographic features, which produces a different style of deposition compared to the more extreme flow separation at the brink of a dune slipface. Low to moderate velocities, typical of those observed during this study, generally remove fine sand from exposed areas, and deposit the sand in nearby topographically lower or sheltered places, resulting in more texturally homogeneous sets of laminae (type a) overlying more texturally heterogeneous deposits (type b). Recognition of low-angle eolian deposits may assist in identification of margins of ancient dune fields. From an economic point of view such information may be of value in predicting either (a) stratigraphic continuity of high-angle eolian reservoir rock, or (b) proximity of high-angle eolian rock.

The western border of the Indian plate: implications for Himalayan geology

The study of regions situated beyond the western margin of the present-day Indian plate (Afghanistan principally) point to the following facts: 1. (1) During the Late Precambrian—Early Paleozoic, stratigraphical continuity existed between western and central Iran, Central Afghanistan, Salt Range and western Pakistan. 2. (2) During the Paleozoic a similar epicontinental cover existed in central Afghanistan, Kashmir and Tibet, with Gondwana tillites and associated cold fauna, such as in India (Umaria); however, a so-called Hercynian zone exists also in northern Iran—Hindu Kush and northern Pamir: it exibits a Middle Paleozoic unconformity (Upper DevonianCarboniferous) on metamorphic Early Paleozoic. 3. (3) The end of the Paleozoic, is marked by: a fracturation of the basement of the Hercynian zone, with powerful volcanic eruptions at the northern part of Hindu Kush, Kashmir (Panjal trap) and also Nepal (Nar valley) the formation of a geosynclinal zone at the southern part of Hercynian zone (Turkman, Penjaw). 4. (4) During the Jurassic: the geosynclinal evolution of the Turkman—Penjaw furrow accelerated, with the accumulation of flysch, radiolarites, ophiolites, olistolites and incipient HP metamorphism. A general subduction took place followed by a Neocimmerian orogenic phase with overthrusting of the central Afghanistan ranges on the scar of the geosynclinal furrows. 5. (5) During the Cretaceous: the geosynclinal evolution ended: Lower Cretaceous lies unconformably on the folded Jurassic flysch. In eastern Afghanistan and northern Pakistan, during the Middle (?) or Upper Cretaceous, a new geosynclinal zone was created. 6. (6) During the Cenozoic, central Afghanistan was emerged; northwards, sedimentary basins were created along the Herat fault, with volcanic and magmatic activity. A southeastern geosynclinal furrow evolved with accumulation of flysch, ophiolites and finally molasse deposits (Katawas—Soleimans). Its western border began overthrusting, but this movement changed into a left lateral fault i.e., the presentday Chaman Arghandeh fault. Conclusion: Two major phases of dislocation took place during the geological history of Gondwana: the first one began during the Permian and ended in the Jurassic; the second one began during the Cretaceous and is still active. The important Eocimmerian orogenic phenomena, existing in the Central Afghanistan and northern Pakistan, took place at the edge of a Gondwana continental fragment, which was larger than the presentday Indian plate. Coeval phenomena may exist in the Himalayan region and perhaps in one of the ophiolitic sutures of Tibet.

The age and extent of the Red Sea oceanic crust

THE evolution of the Red Sea depression and the extent within it of oceanic crust have been widely discussed1–5. It is generally accepted that the axial trough is floored by oceanic lithosphere formed at a constructive margin over the last 3–4 Myr by spreading at a rate of about 1.0 cm yr−1 per flank2,6. It is also clear from stratigraphic continuity and the trend of isofacial lines that the opening of the Red Sea started less than 45 Myr ago after a long period of tectonic and magmatic quiesence7,8. Two areas of contention exist. First, how much of the Red Sea on either side of the axial trough is underlain by oceanic crust, and second, when was this oceanic crust formed?