bluuebunny/crossref_metadata_2025 huggingface dataset:
Quaternary mud turbidites from the South Shetland Trench (West Antarctica): recognition and implications for turbidite facies modelling
Authors: POREBSKI SZCZEPAN J. , MEISCHNER DIETER , GÖRLICH KRZYSZTOF
Published: June 2006

A piston core from the basinal part (depth of 5188 m) of the South Shetland Trench (West Antarctica) yielded a terrigenous mud section 11 m long, which can be subdivided with great precision into turbidite and hemipelagite layers. Mud turbidites (mean bed thickness = 44 cm) alternate regularly with, and are best distinguishable from, their hemipelagite host (mean bed thickness = 17 cm) by the following features: (i) sharp basal contacts; (ii) terrigenous sand‐free textures (except basal, well‐sorted silt laminae) and the absence of outsized (ice‐rafted) components; (iii) a laminated, little to non‐bioturbated internal structure; (iv) distinct textural and compositional grading; and (v) marked steps on water‐content and sediment‐density logs.

Mud turbidites recovered from the South Shetland Trench differ from an earlier model mud‐turbidite sequence by their: (i) excessive (about six times larger) bed thickness; (ii) complex internal organization, manifested in multiple repetitions (up to four) of the same structural interval(s) in sequential or nonsequential order; (iii) distinctive very fine‐grained cap of highly porous clay, rich in fragments of siliceous biogenics; (iv) widespread zones of penesyndepositional deformation; and (v) evidence of flow reversals. These features are interpreted to record deposition from large, muddy turbidity currents subjected to flow transformations, including soliton‐ and/or seiche‐related reversals, induced by ponding and interactions of the flow with the topographical confinements of the trench.

It is concluded that‘contained’muddy turbidites cannot be adequately modelled using published sequences. Differentiation of single‐model and‘contained’mud turbidites offers obvious advantages in basin analysis and in understanding the plethora of turbidity current‐related depositional mechanisms of deep‐sea mud.
The character of seismo‐turbidites in the S‐1 sapropel, Zakinthos and Strofadhes basins, Greece
Authors: ANASTASAKIS GEORGE C. , PIPER DAVID J. W.
Published: June 2006

The early Holocene S‐1 sapropelic sequence in the northwest Hellenic Trench has been studied in six piston cores from the Zakinthos and Strofadhes basins. The S‐1 sequence, 0.7‐3.5 m thick, consists principally of silt to mud turbidites, with rare, thick, disorganized, sandy turbidites. These lithofacies are described and compared with fine‐grained turbidites from the literature. Petrographical data, including the abundance of organic carbon and planktonic microfossils, indicate that the principal source of sediment to the turbidites was from the continental slope. On the basis of composition and texture, five turbidite units can be correlated between the two basins. These basins are fed by separate but adjacent drainage systems. The apparently synchronous occurrence of turbidites in the two drainage systems suggests that the turbidity currents were seismically triggered. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of travel. Turbidites were deposited more frequently in the S‐1 sapropelic interval than in the over‐ and underlying sediments. Application of slope stability analysis shows that on the 8° slopes above the basins, a 10‐cm‐thick sapropel would have a factor of safety of about 2, and would fail with earthquake accelerations in excess of 0.08 g. The frequency of earthquakes likely to produce such accelerations is similar to the observed frequency of turbidites. The low strength of the sapropelic sediment makes it particularly susceptible to such failure. Similar thin‐skinned slumping may be an important process for the initiation of turbidity currents in other environments where there are steep slopes or high sedimentation rates.
Upper Triassic (Norian) cephalopod limestones of the Hallstatt‐type, Oman
Authors: BLENDINGER WOLFGANG
Published: June 2006

Norian crinoidal/brachiopod limestones and cephalopod limestones of the Hallstatt‐type occur as blocks in a Hettangian(?) calcareous breccia of the Haliw Formation in the Oman Mountains. Crinoidal and brachiopod packstones, up to 12 m thick, prevail in the lower part of the sequence and were deposited on a substrate of Norian forereef breccia. The overlying cephalopod wackestones, up to 4.9 m thick, have a basal white bed followed by red limestones with abundant planar and scalloped, corroded surfaces and local stromatolites. Upward, red, nodular wackestones and, finally, slumped grey wackestones follow.

The analysis of geopetal fabrics in orientated samples shows that bedding of these facies is, in fact, inclined bedding. Inclinations varied between 15 and 29°. In addition, the restored dip directions demonstrate rotation, indicating deposition on a gliding block.

The preferred orientation of orthoconic cephalopods and imbrication of discoidal ammonoids coincide with the dip direction measured from geopetal fabrics. Such features, generally interpreted as current‐induced, are here interpreted as gravity‐induced. The overall mud‐supported rock fabric thus indicates deposition under very low‐energy conditions.

The common mud‐supported texture of the rocks contrasts with evidence for current activity found in the scalloped surfaces and shell lags, particularly in the crinoidal/brachiopod facies and the lower, stratigraphically condensed, cephalopod limestones. This indicates that deposition of lime mud alternated with periods of elevated current strength. A comparison of the Hallstatt‐type limestones and current‐influenced sediments on the northern slope of the Little Bahama Bank suggests that condensed sequences of the Hallstatt‐type are restricted to relatively shallow depths with strong fluctuation of contour‐following currents undersaturated with respect to aragonite along carbonate shelf margins facing the open ocean. On steep slopes, sediment bypassing may be an additional factor for stratigraphic condensation.
Volcanic and tectonic controls of lacustrine and alluvial sedimentation in the Stephanian coal‐bearing sequence of the Malpàs‐Sort Basin, Catalonian Pyrenees
Authors: BESLY B. M. , COLLINSON J. D.
Published: June 2006

The infill of a small Variscan basin shows a complex succession of lacustrine and alluvial sediments whose pattern of distribution through time appears to have changed in response to contemporaneous volcanism. Following initial deposition of pyroclastic breccias and extrusion of lavas, small lakes were established on the floor of the basin. Their infills were probably locally controlled by relief on top of the lavas but the pattern was disrupted by phases of mass movement from the basin margin which emplaced debris flows and a sedimentary mélange containing large blocks of limestone and led to extensive soft‐sediment deformation. This complex sequence of events culminated in the emplacement of a widespread ash‐flow bed.

An alluvial plain, established during a period of rather uniform subsidence, was rapidly drowned to give an extensive lake whose progradational infill led to re‐establishment of an alluvial plain. The deposition of a widespread airfall tuff coincided with the onset of differential subsidence during which major channel sandstones were stacked in association with poorly drained palaeosols in the east of the area, whilst a mudstone‐dominated sequence characterized by well‐drained palaeosols developed in the west. A return to more uniform subsidence appears to have coincided with the cessation of volcanic activity.
Stable isotope signatures associated with palaeosols, Pennsylvanian Holder Formation, New Mexico
Authors: GOLDSTEIN ROBERT H.
Published: June 2006

The Holder Formation (Pennsylvanian, Virgilian) of southern New Mexico, USA, consists of limestones interbedded with siliciclastics. It was deposited during times of glacio‐eustatic sea‐level change and was exposed subaerially during multiple sea‐level lowstands. Microcomponents and whole‐rock samples of limestones were analysed for δ13C and δ18O values to examine the method of whole‐rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure. Whole‐rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces. Apparently, whole‐rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation. However, the isotopic shifts are present in calcareous, soil‐precipitated microcomponents, such as rhizoliths, laminated crusts, and soil‐precipitated cements. The components are heterogeneous in isotopic composition, but converge on a meteoric calcite line at about δ18O=−5.5‰. These microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon. Without petrographic selection of palaeosol components, the detection of whole‐rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment. Only minor whole‐rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting subaerial exposure was unlikely.
A middle Proterozoic palaeokarst unconformity and associated sedimentary rocks, Elu Basin, northwest Canada
Authors: PELECHATY SHANE M. , JAMES NOEL P. , KERANS CHARLES , GROTZINGER JOHN P.
Published: June 2006

A major palaeokarst erosion surface is developed within the middle Proterozoic Elu Basin, northwestern Canada. This palaeokarst is named the sub‐Kanuyak unconformity and truncates the Parry Bay Formation, a sequence of shallow‐marine dolostones that were deposited within a north‐facing carbonate platform under a semi‐arid climate.

The sub‐Kanuyak unconformity exhibits up to 90 m of local relief, and also formed under semi‐arid conditions when Parry Bay dolostones were subaerially exposed during a relative sea‐level drop of about 180 m. Caves and various karren developed within the meteoric vadose and phreatic zones. Their geometry, size and orientation were largely controlled by northwest‐ and northeast‐trending antecedent joints, bedding, and lithology. Near‐surface caves later collapsed forming valleys, and intervening towers or walls, and plains. Minor terra rossa formed on top of highs. Karstification was most pronounced in southern parts of Bathurst Inlet but decreased northward, probably reflecting varying lengths of exposure time along a north‐dipping slope.

The Kanuyak Formation is up to 65 m thick, and partially covers the underlying palaeokarst. It consists of six lithofacies: (i) breccia formed during collapse of caves, as reworked collapse breccia and regolith; (ii) conglomerate representing gravel‐dominated braided‐fluvial deposits; (iii) sandstone deposited as braided‐fluvial and storm‐dominated lacustrine deposits; (iv) interbedded sandstone, siltstone and mudstone of sheet flood origin; (v) dolostones formed from dolocretes and quiet‐water lacustrine deposits; and (vi) red‐beds representing intertidal‐marine mudflat deposits. Rivers flowed toward the northwest and northeast within karst valleys and caves; lakes were also situated within valleys; marine mudflat sediments completely cover the palaeokarst to the north.

A regional correlation of the sub‐Kanuyak unconformity with the intra‐Greenhorn Lakes disconformity within the Coppermine homocline suggests that similar styles of karstification occurred over an extensive region. The Elu Basin palaeokarst, however, was developed more landward, and was exposed for a longer period of time than the Coppermine homocline palaeokarst.
Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined field, petrographic and isotopic study
Authors: GAO GUOQIU , LAND LYNTON S.
Published: June 2006

Nodular chert from the middle and upper Arbuckle Group (Early Ordovician) in the Slick Hills, SW Oklahoma, was formed by selective replacement of grainstones, burrow fillings, algal structures, and evaporite nodules. Chert nodules are dominantly microquartz with minor fibrous quartz (both quartzine and chalcedony), megaquartz, and microflamboyant quartz. Lepisphere textures of an opal‐CT precursor are preserved in many (especially in finely‐crystalline) chert nodules. The δ18O values of microquartz chert range from +23.4 to + 28.80/00 (SMOW), significantly lower than those of Cenozoic and Mesozoic microquartz chert formed both in the deep sea and from near‐surface sea water. The δ18O values of chert decrease with increasing quartz crystal size.

Silicification in the Arbuckle Group occurred during early diagenesis, with the timing constrained by the relative temporal relationships among silicification, burial compaction, and early dolomite stabilization. Silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica‐enriched sea water. Chert nucleation appears to have been controlled by the porosity, permeability, and organic matter content of precursor sediments. This conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter. Growth of chert probably occurred by a maturation process from opal‐A(?), to opal‐CT, to quartz, as indicated by the presence of opal‐CT precursor textures in many chert nodules. Although field and petrographic evidence argues for an early marine origin for chert in the Arbuckle Group, the light δ18O values are inconsistent with this origin. Meteoric resetting of the δ18O values of the chert during exposure of the carbonate platform best explains the light δ18O values because: (i) the δ18O values of chert nodules decrease with decreasing δ18O values of host limestones, and (ii) chert nodules from early dolomite, which underwent more extensive meteoric modification than associated limestones, have lighter δ18O values than chert nodules from limestones. Increasing recrystallization of chert nodules by meteoric water resulted in progressive 18O depletion and (quartz) crystal enlargement.
Late‐stage calcites in the Permian Capitan Formation and its equivalents, Delaware Basin margin, west Texas and New Mexico: evidence for replacement of precursor evaporites
Authors: SCHOLLE PETER A. , ULMER DANA S. , MELIM LESLIE A.
Published: June 2006

Comparison of Upper Guadalupian fore‐reef, reef and back‐reef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution‐enlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and fore‐reef sections and only partial remnants of evaporites are retained in back‐reef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions.

Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, late‐stage development of large‐ and small‐scale dissolution breccias.

The restriction of calcite cements to very near‐surface sections, petrographic evidence that the calcites post‐date hydrocarbon emplacement, and the highly variable but generally ‘light’carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stages of hydration/dehydration during conversion to gypsum, dissolution of gypsum and precipitation of sparry calcite. Such evaporite dissolution and replacement processes are probably continuing today in near‐outcrop as well as deeper settings.

This study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements. The extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic carbonate rocks.
Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands
Authors: BURNS STEPHEN J. , SWART PETER K.
Published: June 2006

The sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions.

In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca2+, Mg2+ and Sr2+ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio‐irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition.

Stratigraphic analysis and δ13C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr−1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high‐Mg calcite dissolution and low‐Mg calcite precipitation, from 0 to 35 cm; (2) Ca‐ or Mg‐sulphate dissolution and low‐Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X‐ray diffraction analyses, probably because of the comparatively young age of the island.
A Holocene calcrete from North Yorkshire, England: implications for interpreting palaeoclimates using calcretes
Authors: STRONG G. E. , GILES J. R. A. , WRIGHT V. P.
Published: June 2006

Glacial gravels of Late Devensian Dimlington Stadial age (26 000–13 000 years BP) at West Tanfield, North Yorkshire, England, have been cemented by carbonate‐rich solutions to produce a strongly indurated calcrete horizon. The low‐Mg cements occur as drusy spar, needle fibres, alveolar septal structures, micrite and micropinnacles, indicative of vadose‐zone cementation. Some complex pore partition structures attributed to precipitation along meniscus films also occur. These partitions separate air‐dominated and water‐dominated microenvironments of the vadose zone. The abundance of vadose fabrics shows that the accumulation is not a groundwater calcrete. In addition, much of the carbonate appears to have been precipitated by biological mediation.

Carbon and oxygen isotopic data suggest that the carbonate did not form as a result of freezing, as has been suggested for some ‘arctic’soil carbonates. The pollen history of the area since the Devensian suggests that this calcrete precipitated at low temperatures; this contrasts with widely reported occurrences of calcrete in soils of hot arid or semi‐arid regions, and suggests that palaeo‐calcretes should not be used as absolute palaeoclimatic indicators.

The unusual occurrence, albeit localized, of a thick calcrete under a cool and wet climate probably reflects the well‐drained nature of the gravels, the abundance of CaCO3 as limestone clasts in the gravel and a high degree of biological activity beneath a forest cover, which created a local environment favouring carbonate precipitation.
Storm‐influenced siliciclastic and carbonate ramp deposits, the Lower Ordovician Dumugol Formation, South Korea
Authors: LEE YONG IL , KIM JEONG CHAN
Published: June 2006

The Dumugol Formation (Lower Ordovician) in the southern part of the Baegunsan syncline, South Korea, contains mixed siliciclastic and carbonate ramp deposits. The ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone‐mudstone couplets, bioclastic grainstones to packstones, flat‐pebble conglomerates, a skeletal lag layer and laminated calcisiltites. All tempestites are characterized by an erosive to sharp base, poor grading and a transitional upper boundary. The difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality. For example, laminated calcisiltites have developed on the shallow carbonate ramp, flat‐pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps, and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting. The stratigraphic succession of the Dumugol Formation represents an initial transgression followed by a regression. The vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp, to a basin, shallowing to a deep carbonate ramp, and to a shallow carbonate ramp. Storm effects are mostly well preserved in shallow to deep ramp deposits.
Groundwater dolocretes from the Upper Triassic of the Paris Basin, France: a case study of an arid, continental diagenetic facies
Authors: SPÖTL CHRISTOPH , WRIGHT V. P.
Published: June 2006

Thick dolomite‐cemented horizons (dolocretes) occur within a fluvial sandstone‐mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.

The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar‐filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar‐septal fabrics were observed in the thin, nodular dolomite beds.

The dolomite is near stoichiometric, well ordered and non‐ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water.

A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.

Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non‐marine, continental origin for the groundwaters. The poorly ordered and non‐stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near‐stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.
Episodic sedimentation on an early Silurian, storm‐dominated carbonate ramp, Becscie and Merrimack formations, Anticosti Island, Canada
Authors: SAMI TERRY , DESROCHERS ANDRÉ
Published: June 2006

The 150–160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified: (a) laminated and homogenous mudstone; (b) laminated and homogenous packstone; (c) argillaceous mudstone and packstone; (d) calcareous shale; (e) laminated calcisiltite; (f) medium‐ to fine‐grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LF1) calcareous mudstone‐shale; (LF2) laminated‐homogenous mudstone; (LF3) calcareous grainstone‐shale; (LF4) laminated mudstone‐grainstone; and (LF5) laminated calcisiltite‐grainstone. The sequence reflects deposition on a low‐energy, muddy, carbonate to argillaceous ramp subject to short‐lived, episodic high‐energy storms. These events produced fining‐upwards storm units 5–80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium‐grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low‐energy, fairweather mudstones and calcareous shales.

Deposition progressed from a carbonate mud‐dominated ramp in the Becscie Formation to an argillaceous mud‐dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south‐east into the Iapetus Ocean and shallowed towards a SW—NE‐orientated shoreline to the north‐west. Vertical tempestite proximality trends and lithofacies changes identify third‐order eustatic sea‐level changes. After an initial deepening at the base of the formation, a shallowing‐deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods.
Flow characteristics of Metresa: an Oligocene seismoturbidite in the Dukla Unit, Polish Carpathians
Authors: SLACZKA A. , WALTON E. K.
Published: June 2006

Metresa is a thick, distinctive turburdite within the Menilite Shales in the Carpathians of south‐east Poland. It was deposited by a SE‐flowing turbidity current derived from the Silesian cordillera and can be traced for 55 km downcurrent; it has a width of at least 15 km. The current contained roughly 3 km3 of sediment and conforms to the definition of a seismoturbidite. Grain‐size analyses of samples from the nine localities where Metresa is exposed indicate average velocities decreasing from 2 m s−1 to less than 1 m s−1 over the 55 km. Massive divisions are poorly developed; instead, the turbidite, although graded, tends to be laminated throughout. Flat lamination predominates with intercalated cross‐lamination and erosion structures at various levels. Small‐scale cross‐lamination, presumably associated with ripples, occurs at some upper levels. An exceptional feature is a large‐scale wave‐like structure somewhat resembling hummocky cross‐stratification. The structures are not consistent with the inferred velocities. It is suggested that the stability fields relating structures to stream power (herein called the ‘Allen fields’) are displaced in respect to deposition from waning turbidity currents.
How round is round? A new approach to the topic ‘roundness’by Fourier grain shape analysis
Authors: DIEPENBROEK MICHAEL , BARTHOLOMÄ ALEXANDER , IBBEKEN HILLERT
Published: June 2006

Numerous methods for roundness measurement have been developed. None, however, has been generally accepted, because of conceptual and practical deficiencies. Modern image processing and Fourier grain shape analysis have eliminated the practical shortcomings, but the conceptual ones remained. Single, higher harmonics of the Fourier series, for example, fail to serve as reliable equivalents for roundness evaluation. The concept outlined in this paper recognizes three criteria for the evaluation of roundness. (1) All curvatures, convex as well as concave or plane elements, must be considered. (2) The positions of morphological elements are significant because salient parts of a particle are more easily abraded than protected ones. Consequently, the curvatures have to be weighted by their relative position on the particle. (3) Positions and curvatures of morphological elements have to be compared with the particle's ultimate abraded shape, which is assumed to be an ellipsoid. The ellipsoid reflects the aspect of form or sphericity. The distinction between sphericity and roundness is retained because there is no evidence that sphericity changes significantly during transport. The measurement is based on the outline of a particle's maximum projection plane, which is transformed to a Fourier series. Roundness data are derived from the complete amplitude spectrum. The aspect of sphericity is eliminated by subtracting the amplitude spectrum of the best approximating ellipse from the spectrum of the empirical shape. The residual amplitudes are normalized and summed. In a final step the resulting values are rescaled. This guarantees reasonable boundaries and a normal distribution of roundness values. The procedure is automated and its efficiency permits the calculation of large samples. Tests on fluvial and coastal gravel populations demonstrate that the method is sensitive to abrasional wear during all stages of roundness.
Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River
Authors: GUILLEN JORGE , PALANQUES ALBERT
Published: June 2006

The lowest part of the Ebro River is a microtidal salt‐wedge estuary. Penetration of the salt‐wedge is largely controlled by the fluvial discharge and the morphology of the river bed, although sea level variations caused by tides and atmospheric conditions can also play significant roles. The concentration and distribution of suspended particulate matter in this part of the river and the fluvial sediment discharge are strongly influenced by the dynamics of the salt‐wedge. Damming of the river has caused sediment to be trapped in reservoirs and has regulated the fluvial discharge. Intrusion of the salt wedge has thus also been regulated. At present, sediment discharge is between 1 and 1·5 × 105 tons per year, which is less than 1% of the sediment that the Ebro River discharged into the sea before construction of the dams. This extreme reduction in sediment supply has allowed marine erosional processes to dominate in the delta.
Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of southern Utah
Authors: KIRSCHBAUM M. A. , McCABE P. J.
Published: June 2006

Alluvial strata of the Cretaceous Dakota Formation of southern Utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the Sevier orogenic belt. These strata contain valley fill deposits, anastomosed channel systems and widespread coals. The coals constitute a relatively minor part of the Dakota Formation in terms of sediment volume, but may represent a substantial amount of the time represented by the formation. The coals are separated by clastic units up to 20 m thick.

The stratigraphically lowest clastic unit of the Dakota Formation lies above an unconformity cut into Jurassic rocks. Incised valleys associated with the unconformity are up to 12 m deep. Two discrete episodes of valley fill sedimentation are recognized, including a lower sandstone unit with conglomerate layers, and an upper, discontinuous, coal‐bearing unit. After the valleys filled, the area became one of low relief where extensive mires formed. Peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west.

The clastic units consist of sandstone, mudstone or heterolithic ribbon bodies, stacked tabular sandstones, and laminated mudstones, and contain minor coal beds less than 0·35 m thick. Ribbon bodies are 1–9 m thick and 15–160 m wide, have pronounced basal scours, and are filled with both lateral and vertical accretion deposits. An anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions, the variation in the structure and grain size of channel fills, and the presence of abundant tabular sandstones interpreted as crevasse splays. Although some sandstone bodies have well developed lateral accretion surfaces, the overall ribbon geometry indicates that mature meandering streams were not well developed. This is in contrast to modern anastomosed systems, which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain. Rather than being a transitional channel pattern related to river avulsion, the anastomosed channels of the Dakota Formation may have formed part of a large inland delta that episodically invaded widespread mires. The mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts.
Relations between dune generations in the Gran Desierto of Mexico
Authors: LANCASTER N.
Published: June 2006

Multiple generations or genetic units of dunes characterized by different morphologies, sand grain size and sorting, and degree of post‐depositional alteration of sediments occur in the Gran Desierto sand sea of Mexico. Dune generations are separated by super bounding surfaces. Episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes. Based on examples from three parts of the sand sea, relations between different generations of dunes suggest that there are two end‐member modes of sand sea accumulation: (1) stacking or superposition of dune generations, and (2) accumulating mosaics of dunes of different morphological units. The mode of accumulation is determined by sand supply and accumulation rate: stacking occurs where sand supply is high and/or the sand sea area is restricted by topography; accumulation of mosaics is the preferred mode of accumulation where sand supply is low.
Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands
Authors: HEIN JAMES R. , GRAY SARAH C. , RICHMOND BRUCE M. , WHITE L. D.
Published: June 2006

Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void‐filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ18O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO3 and broad and weak ordering X‐ray reflections at 2·41 and 2·54 A. The fine‐scale replacement of skeletal grains and freshwater void‐filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ18O=+2·6‰0 PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater‐seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ13C=2·7‰) and magnesium were derived from seawater.

Low‐temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section.

The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.
Sedimentology of 40 000 year Milankovitch‐controlled cyclothems from central Hawke's Bay, New Zealand
Authors: HAYWICK DOUGLAS W. , CARTER ROBERT M. , HENDERSON ROBERT A.
Published: June 2006

Cyclothemic sedimentary rocks of the Plio‐Pleistocene Petane Group outcrop extensively in the Tangoio block of central Hawke's Bay, New Zealand. They are products of inner to mid‐shelf sedimentation and were deposited during glacio‐eustatic sea level fluctuations along the western margin of a shallow, pericontinental seaway located in a forearc setting. The succession consists of five laterally continuous cyclothems, each containing a fine grained interval of silt and a coarse grained interval of siliciclastic sand ± gravel or limestone. Five sedimentary facies assemblages comprising 20 separate facies have been recognized. Coarse grained intervals of cyclothems were deposited mostly during relative sea level lowstands and contain up to four facies assemblages: (1) a non‐marine assemblage (with three component facies, representing braided river and overbank environments); (2) an estuarine assemblage (with three component facies, representing tidal flat and mud‐dominated estuarine environments); (3) a siliciclastic shoreline assemblage (with six component facies, representing greywacke pebble beach, shoreface and inner shelf environments); and (4) a carbonate shelf assemblage (with four component facies, representing tide‐dominated, inshore and shallow marine environments). Fine grained intervals of cyclothems were deposited during sea level highstands when the Tangoio area was generally experiencing mid‐shelf sedimentation. This produced an offshore assemblage consisting of four component facies. The distribution of facies assemblages during relative sea level lowstands was dependent upon proximity to the shoreline, the type and rate of sediment supply to the basin, and shelf hydrodynamics. Carbonate shelf facies dominate coarse grained intervals in Cyclothems 3–5, but siliciclastic shoreline and non‐marine facies dominate in Cyclothems 1 and 2. The abrupt change from siliciclastic to carbonate sedimentation during relative sea level lowstand deposition is thought to have been induced by rapidly falling interglacial to glacial sea level accentuated by regional tectonic shoaling. This caused most of the terrigenous sediment supply to bypass the Tangoio area. Consequently, carbonate sediment accumulated in inshore and shallow marine settings. Facies assemblages rarely show lateral interdigitation, but are vertically stratified over the entire Tangoio block. Facies successions in each cyclothem preserve a record of relative sea level change during deposition of the Petane Group and are consistent with a Plio‐Pleistocene sea level change in eastern New Zealand of c. 75–150 m, i.e. approximately the magnitude suggested for Late Quaternary glacio‐eustatic sea level changes.
Pillow‐beds: a new type of seismites? An example from an Oligocene turbidite fan complex, Alicante, Spain
Authors: ROEP T. B. , EVERTS A. J.
Published: June 2006

A new type of soft sediment deformed beds, termed ‘pillow‐beds’, originally a packet of calcarenitic turbidites, show sand‐in‐sand loading with plane lower and upper surfaces. The pillow‐beds have some resemblance to ball and pillow structures and they are considered to have formed as a result of dewatering. The pillow‐beds are Oligocene in age and are part of turbidite lobes in a submarine fan system, which was fed through a canyon along the southern border of the ancient Iberian continent. Three hypotheses may explain their occurrence: overloading, sliding or seismic shock. A seismic origin is preferrred on the basis of the flat, undeformed lower surface of the pillow‐beds, excluding dewatering of—and loading into—the previously deposited beds. Furthermore, the pillow‐bed structures are related to grain size and may show repetitions, thus excluding sliding. The scale and nature of the pillow‐beds suggest they could be the result of an earthquake with a magnitude of 6–7 on the Richter scale.
Changes in velocity profiles at roughness transitions in coarse grained channels
Authors: ROBERT ANDRE , ROY ANDRE G. , DE SERRES BERNARD
Published: June 2006

Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles.
Void‐filling deposits in karst terrains of isolated oceanic islands: a case study from Tertiary carbonates of the Cayman Islands
Authors: JONES BRIAN
Published: June 2006

Caves, fossil mouldic cavities, sinkholes and solution‐widened joints are common in the Cayman and Pedro Castle members of the Bluff Formation (Oligocene‐Miocene) on Grand Cayman and Cayman Brac because they have been subjected to repeated periods of karst development over the last 30 million years. Many voids contain a diverse array of sediments and/or precipitates derived from marine or terrestrial environs, mineral aerosols, and groundwater. Exogenic sediment was transported to the cavities by oceanic storm waves, transgressive seas, runoff following tropical rain storms and/or in groundwater.

At least three periods of deposition were responsible for the occlusion of voids in the Cayman and Pedro Castle members. Voids in the Cayman Member were initially filled or partly filled during the Late Oligocene and Early Miocene. This was terminated with the deposition of the Pedro Castle Member in the Middle Miocene. Subsequent exposure led to further karst development and void‐filling sedimentation in both the Cayman and Pedro Castle members. Speleothems are notably absent. The void‐filling deposits formed during these two periods, which were predominantly marine in origin, were pervasively dolomitized along with the host rock 2–5 million years ago. The third period of void‐filling deposition, after dolomitization of the Bluff Formation, produced limestone, various types of breccia, terra rossa, speleothemic calcite and terrestrial oncoids. Most of these deposits formed since the Sangamon highstand 125 000 years ago. Voids in the present day karst are commonly filled or partly filled with unconsolidated sediments.

Study of the Bluff Formation of Grand Cayman and Cayman Brac shows that karst terrains on isolated oceanic islands are characterized by complex successions of void‐filling deposits that include speleothems and a variety of sediment types. The heterogenetic nature of these void‐filling deposits is related to changes in sea level and climatic conditions through time.