bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calciturbidite composition related to exposure and flooding of a carbonate platform (Triassic, Eastern Alps)
Authors: REIJMER J. J. G. , TEN KATE W. G. H. Z. , SPRENGER A. , SCHLAGER W. 
Published: June 2006

ABSTRACTThe compositional variation of calciturbidites (Pedata/Pötschen Formation), deposited adjacent to the Dachstein Formation carbonate platform in the Triassic Hallstatt Basin, was analysed using detailed field measurements and point‐counting of thin sections. The 35 m long section is located in the Northern Calcareous Alps of Austria. Six point‐count groups were distinguished separating basinal from platform‐derived input. Summary statistics, cluster and correspondence analysis of the point‐count data reveals a close relationship between the biota present on the Dachstein carbonate platform and the calciturbidite composition. The variations in turbidite composition are attributed to fluctuations in sea level and resulting flooding and exposure of the platform, which alternately created and destroyed shallow‐water habitats on the platform top.
Stratal geometries, facies and sea‐floor erosion in Upper Cretaceous Chalk, Normandy, France
Authors: QUINE MARK , BOSENCE DAN 
Published: June 2006

ABSTRACTUnusual lenticular stratal geometries and facies of the Upper Cretaceous Chalk of coastal Haute Normandie, France, are described and interpreted. Thirteen facies within these chalks are described and illustrated on the basis of field, thin‐section and SEM investigations: nannofossil mudstones, nannofossil hardgrounds, echinoderm wackestones and packstones, echinoderm hardgrounds, bryozoan mudstones and wackestones, bryozoan hardgrounds, bryozoan packstones and wackestones, inoceramid wackestones, inoceramid hardgrounds, sponge hardgrounds, marly chalks, conglomeratic chalks and debris flow chalks.These facies occur within lenticular bedded structures with both concave‐up and concave‐down geometries which have been previously interpreted as megaripples, mud mounds or tectonic structures. Detailed examination of the structures and the associated facies indicates that the concave‐up geometries were formed from submarine erosion, and redeposition in NW‐SE longitudinal channels. The concave‐down geometries developed between adjacent channels. Assessment of the regional and temporal setting indicates that the erosion occurred in the Armorican‐Cornubian straits of the Anglo‐Paris Basin during sea‐level lowstands. Within these straits channelling is preferentially developed on the positive, south‐western block to the Lillebonne‐Fécamp‐Cotentin Fault.
Geochemistry of meteoric calcite cements in some Pleistocene limestones
Authors: SALLER ARTHUR H. , MOORE CLYDE H. 
Published: June 2006

ABSTRACTIn this study, the stable isotope and trace element geochemistries of meteoric cements in Pleistocene limestones from Enewetak Atoll (western Pacific Ocean), Cat Island (Bahamas), and Yucatan were characterized to help interpret similar cements in ancient rocks. Meteoric calcite cements have a narrow range of δ18O values and a broad range of δ13C values in each geographical province. These Pleistocene cements were precipitated from water with stable oxygen isotopic compositions similar to modern rainwater in each location. Enewetak calcite cements have a mean δ18O composition of −6.5%0 (PDB) and δ13C values ranging from −9.6 to +0.4%0 (PDB). Sparry calcite cements from Cat Island have a mean δ18O composition of −4.1%0 and δ13C values ranging from −6.3 to + 1.1%0. Sparry cements from Yucatan have a mean δ18O composition of −5.7%0 and δ13C values of −8.0 to −2.7%0. The mean δ18O values of these Pleistocene meteoric calcite cements vary by 2.4%0 due to climatic variations not related directly to latitude. The δ13C compositions of meteoric cements are distinctly lower than those of the depositional sediments. Variations in δ13C are not simply a function of distance below an exposure surface. Meteoric phreatic cements often have δ13C compositions of less than —4.0%0, which suggests that soil‐derived CO2 and organic material were washed into the water table penecontemporaneous with precipitation of phreatic cements.Concentrations of strontium and magnesium are quite variable within and between the three geographical provinces. Mean strontium concentrations for sparry calcite cements are, for Enewetak Atoll, 620 ppm (σ= 510 ppm); for Cat Island, 1200 ppm (σ= 980 ppm); and for Yucatan, 700 ppm (σ= 390 ppm). Equant cements, intraskeletal cements, and Bahamian cements have higher mean strontium concentrations than other cements. Equant and intraskeletal cements probably precipitated in more closed or stagnant aqueous environments. Bahamian depositional sediments had higher strontium concentrations which probably caused high strontium concentrations in their cements. Magnesium concentrations in Pleistocene meteoric cements are similar in samples from Enewetak Atoll (mean =1.00 mol% MgCO3; σ= 0.60 mol% MgCO3) and Cat Island (mean = 0.84 mol% MgCO3; σ= 0.52mol% MgCO3) but Yucatan samples have higher magnesium concentrations (mean = 2.20 mol% MgCO3: σ= 0.84mol% MgCO3). Higher magnesium concentrations in some Yucatan cements probably reflect precipitation in environments where sea water mixed with fresh water.
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

ABSTRACTA 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

ABSTRACTThe 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

ABSTRACTNorian 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

ABSTRACTThe 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

ABSTRACTThe 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

ABSTRACTA 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

ABSTRACTNodular 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

ABSTRACTComparison 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.