Braided Stream System Research Articles

Large scale dinoturbation in braided stream deposits: evidence from the Cretaceous Tugulu Group of the Hami area, Eastern Xinjiang, China

Large dinosaur tracks were recently reported from locations in the Pterosaur-Yadan National Geological Park situated about 100 km south of Hami in Xinjiang Province, China. The park comprises a substantial area in a much larger arid region comprising and extensive spectrum of Cretaceous, siliciclastic, Tugulu Group, lithofacies representing proximal, basin margin, alluvial fan and braided stream deposits, grading into alluvial plain, deltaic and lacustrine facies near the depocenter. Due to the difficulties of conducting detailed geological surveys in such a vast and inhospitable area, definitive resolution of the litho-, bio- and chrono-stratigraphy is challenging in some areas, and yet to be published in detail. Nevertheless, the occurrence of large dinosaur tracks and dinoturbated units, here interpreted as sauropodan, in association with root casts, dinosaur bone and fossil wood, points to the potential of this frontier area to yield valuable paleontological information, and show that flora and fauna were found in arid braided stream systems away from the lacustrine depocenters where body fossils are more abundant and better known.

Basin evolution during Cretaceous-Oligocene changes in sediment routing in the Eastern Precordillera, Argentina

The response of sedimentary basins to earliest onset of Andean contraction and lithospheric flexure in the southern Central Andes is debated and not well-resolved. The Upper Cretaceous to Oligocene strata of the Cuesta de Huaco anticline in the Argentine Precordillera record sedimentation, regional deformation, and climate patterns prior to the highly studied Oligocene-Miocene foreland basin phase. These deposits have recently been recognized as Cretaceous and Paleogene in age, prompting a re-evaluation of this depocenter as part of the early Andean system, prior to deposition of the aeolian foredeep sediments of the Oligocene Vallecito Formation. This work presents new data from the Argentine Precordillera fold-and-thrust belt at 30°S that sheds light on new reinterpretations of the timing of sedimentation for an important interval in Andean retroarc foreland basin history. We report the first Paleocene detrital radiometric ages from the Cuesta de Huaco ‘red strata’ of the pre-Oligocene Bermejo Basin. Detailed sedimentology and provenance data from the Cenomanian-Turonian Ciénaga del Río Huaco and Danian-Priabonian Puesto La Flecha formations reveal a Cenomanian-Turonian braided stream system that transitioned into a shallow freshwater lacustrine depocenter in Paleocene-Eocene time. During Late Cretaceous time, sediment in the braided river system was derived primarily from northeastern cratonic sources; during the Paleocene-Eocene, uplift and unroofing of the Andean arc and Frontal Cordillera resulted in an influx of western-derived sediment. We therefore suggest a revised timing of sedimentation for the transition to Andean retroarc foreland basin deposition.

Why terrestrial coals cannot be dated using the Re-Os geochronometer: Evidence from the Finnmark Platform, southern Barents Sea and the Fire Clay coal horizon, Central Appalachian Basin

Coal horizons in terrestrial sedimentary sections are key to understanding the structural and thermal evolution of sedimentary basins. Precise and accurate age information for coal horizons can define correlations and the temporal evolution of sedimentary successions. While the biostratigraphy for coal horizons yields relative ages, tied to radiometric ages through the continuously refined geological time scale, direct radiometric ages of coal horizons have been derived only from tonsteins, lithified ash beds. The Re-Os geochronometer has been proven to record accurate and precise depositional ages for sedimentary rocks rich in organic matter. However, the reliability of the Re-Os geochronometer has not yet been assessed for coals deposited in strictly terrestrial environments. Here, in order to test the Re-Os geochronometer, we measure the Re, Os concentrations and isotopic composition of terrestrial coals from two distinct environments: the Finnmark Platform, southern Barents Sea and the Central Appalachian Basin (CAB). Samples from the Finnmark platform (Soldogg Formation) also include an adjacent organic-rich shale horizon. The shale and two coal horizons were also analysed for total organic carbon (TOC) and Rock-Eval data. Results show that the Soldogg coals and shale reached the oil generation widow (Tmax = 448 ± 2 °C), with Type III or mixed Type II/III organic matter, supporting the model for deposition in overbank deposits in a fluvial setting. In contrast, the Fire Clay coals in the CAB were deposited in a broad expanse of mires. Average Re (0.5 ± 0.4 ng/g) and Os (23 ± 26 pg/g) concentrations of terrestrial Soldogg and Fire Clay coals are similar to those of terrestrial Maghara coal and a few orders of magnitude lower than the marine-influenced Matewan coal. Elevated Re/Os* ratio (Os* = Common Os; Os corrected for ingrowth of 187Os since deposition of the sample) for the Fire Clay coals suggests highly reducing and/or euxinic conditions prevailing in the water column of the extensive Carboniferous peat-forming mire in which these coals were deposited. In contrast, lower Re/Os* for Soldogg coals highlight less reducing conditions, likely in a short-lived mire in a dynamic braided stream system. Re and Os* concentrations of terrestrial coals are on the order of Re and Os of plant materials and river sediments, suggesting that debris from plant materials and weathered continental crust are significant contributors of Re and Os to terrestrial coals, with some contribution from water column of the coal mire.This study is the first attempt to apply the Re-Os geochronometer on terrestrial coals. In case of the Soldogg coals and shale, the 187Re/188Os vs. 187Os/188Os relationship displays scatter with no discernible linear trend. The scatter is ascribed to heterogeneity in the initial 187Os/188Os ratios (Osi) and lack of post-depositional isotopic equilibration during subsequent diagenesis. For the Fire Clay coals, 187Os/188Os and 187Re/188Os are well correlated; linear regression of the data, however, does not yield a geologically reasonable age. Rather, the linear relationship results from binary mixing between two end-members: The Fire Clay coal and tonstein, suggesting that Re-Os systematics of terrestrial coals depositing very close to tonstein beds can be significantly disturbed. We conclude that the post-depositional equilibration with a large Re-Os reservoir, as with a marine incursion, is necessary to record a meaningful Re-Os radiometric age for the coals.

Evidence of early C4 grasses, habitat heterogeneity, and faunal response during the Miocene Climatic Optimum in the Mojave Region

Major changes to landscapes, climate, and mammalian faunas occurred at the regional scale in western North America during the Miocene Climatic Optimum (MCO) between ~17 and 14Ma, but few studies have looked at how the MCO affected basin-scale environments. Paleoenvironmental reconstructions coupled with mammalian fossil assemblages from the Crowder and Cajon Valley formations in the Mojave Desert, California, contribute insights into local-scale (10s of kilometers) responses to warming during the Hemingfordian and Barstovian North American Land Mammal Ages. By integrating lithological descriptions, phytolith analyses, carbon isotope composition of preserved soil organic matter (δ13CSOM), and elemental geochemistry of paleosols, we provide reconstructions of depositional environments, vegetation, and precipitation through the MCO. Phytolith and δ13CSOM evidence suggest that paleoenvironments were predominately C3 grasslands, with the earliest potential presence of C4 grasses in the region found within the Crowder Formation at ~17Ma. Based on elemental geochemistry of paleosols, mean annual precipitation estimates are 807 (±182) mm yr−1 at 17Ma in the Crowder Formation and 740–800 (±182) mm yr−1 from 16 to 15Ma in the Cajon Valley Formation. Our multiproxy approach indicates that the Crowder and Cajon Valley basins were stable, large-scale braided stream and floodplain systems with intermittent paleosol development and heterogeneity in vegetation and moisture conditions across spatial scales. Overall, we find a signal of local paleoenvironmental stability during the MCO; however, a significant drying trend in the Crowder Formation does correspond with faunal turnover and reduction in the number of taxa present. High species diversity within faunal assemblages, especially among small mammals, and fewer shared species between the Crowder and Cajon Valley assemblages than expected based on bootstrap analysis indicate that peak mammal diversity was accommodated at both local and regional scales during sustained tectonic activity and the MCO in western North America.

Sedimentary characteristics of braided stream—braided fan of Shawan Formation in Chunfeng Oilfield, northwest margin of Junggar Basin, China

On the foundation of the geological background, textural maturity analysis, compositional maturity analysis, mudstone color index study, and other provenance research approach were fully made use of during this study to make certain the provenance of Shawan Formation in Chunfeng Oilfield. Provenance analysis of the Shawan Formation in Chunfeng Oilfield, conducted as the first stage of this study, revealed that the main provenance direction changed from northeast to northwest gradually, with a little fluctuation, while Shawan Formation was deposited. And then synthetically using seismic data, log data, and drilling data, according to petroleum geology and reservoir sedimentology, this study analyzed the sedimentary facies indicators, single-well facies, and logging facies in Shawan Formation; clarified the distribution of the sandbodies; and reached new insights on the types of the sedimentary facies of Shawan Formation in Chunfeng Oilfield. The research indicated that braided streams were developed during the deposition of the first member of Shawan formation and in the following two periods where braided fans became dominant little by little. These three microfacies, braided channel, channel bar, and braided plain, make up a typical stacked upward-fining cycle in the braided stream system. And the braided streams flowed from the northwest to the southeast in the research region, while the first member of Shawan Formation was formed. While the second member of Shawan Formation was deposited, the braided streams shrank gradually, and the braided fan coming from the northwest began to grow, with the flow direction moving from the northwest to the northeast. From the upstream to the downstream, the braided fan can be divided into three subfacies, proximal fan, middle fan, and distal fan. And when the third member of Shawan Formation was developed, the area of the braided fan became even larger than ever before, covering over almost the whole Chunfeng Oilfield.

Paleoecological and paleoenvironmental changes during the continental Middle–Late Permian transition at the SE Iberian Ranges, Spain

The Middle and Late Permian are characterized by a pair of mass-extinction events that are recorded in both marine and continental environments. Here, we present the first continental western peri-Tethyan record of an extinction event located in the Middle–Late interval. In the SE Iberian Ranges, Central Spain, the transition between the Lower and Middle subunits of the Middle Permian Alcotas Formation indicates a significant paleoclimatic change from arid and semiarid conditions towards more humid conditions. Coincident with the onset of humid conditions there were changes in the sedimentology, mineralogy, and geochemistry that indicate significant environmental changes including a shift in weathering intensity and a change of fluvial style from braided to meandering systems. Near the top of the Middle Subunit, a local biotic crisis is recorded by palynomorph assemblages. Following this crisis, there is a total absence of coal beds, plant remains, and microflora that defines a barren zone in the uppermost part of the Alcotas Formation which is recorded throughout the basin. The barren zone is accompanied by a shift back to braided stream systems, but not by a return to carbonate-bearing paleosols indicative of arid or semi-arid conditions. This combination of features is consistent with other Middle–Late continental basins related with mass extinctions, so the barren zone is interpreted as the extinction interval. The regional character of the extinction interval and its proximity with the Middle–Late Permian transition could be related with the global mid-Capitanian biotic turnover described in this period of time in other marine basins. However, the common difficulties of dating with precision non-marine rocks make this relationship difficult to probe in the Iberian Basin and in other Middle–Late Permian basins. Further work, including high resolution carbon-isotope analyses and complete studies of the magnetostratigraphy, should be desirable in order to obtain a better age constraint and to produce reliable comparisons with marine sections.

Sedimentary Facies, Architectural Elements and Trace Fossils of Kashkan Formation, Folded Zagros Zone in SW Iran

The Kashkan Formation (Paleocene to Middle Eocene), consists of conglomerate, sandstone, and siltstone in the Zagros Folded zone, southwest Iran. Type of the sedimentary facies, architectural elements and trace fossils show that Kashkan deposits were formed in a low sinuosity braided stream system, with north to south flow direction. The formation displays coarsening-upward succession reflects an overall regressive sequence. The clast–supported conglomerate is the major components of the formation. A thinning of the coarser–grained sediments toward the south, and southwest indicates that the source for the Kashkan formation was to the north and northwest from the study area. Trace fossils of Kashkan Formation are related to Scoyenia ichnofacies, include Arenicolites isp., Diplocraterion isp., Ophiomorpha isp., Skolithos isp., Steinichnus isp., Thalassinoides isp., Escape structure and vertebrate footprints. Sedimentological analysis and type of the trace fossils indicate the south the stream was restricted by a shoreface environment. Kashkan Formation becomes thinner and fine–grained in the southern part of the study area.

Comparing electromagnetic and seismic geophysical methods: Estimating the depth to water in geologically simple and complex arid environments

An integrated electromagnetic (EM) and seismic geophysical study was performed to evaluate non-invasive approaches to estimate depth to shallow groundwater (i.e., < 5 m) in arid environments with elevated soil salinity, where the installation of piezometers would be limited or prohibited. Both methods were tested in two study areas, one serving as a control site with relatively simple hydrogeology and the other serving as the experimental site with complex hydrogeology. The control site is located near the shore of Utah Lake (Palmyra, Utah, USA) where groundwater is shallow and unconfined in relatively homogeneous lacustrine sediments. The experimental site is in Carson Slough, Nevada, USA near the Ash Meadows National Wildlife Refuge in Amargosa Valley. Carson Slough is underlain by valley fill, with variable shallow depths to water beneath an ephemeral braided stream system. The geophysical methods used include frequency domain electromagnetic induction with multiple antenna–receiver spacings. High-resolution P-wave seismic profiles using a short (0.305 m) geophone spacing for common depth-point reflection stacking and first arrival modeling were also acquired. Both methods were deployed over several profiles where shallow piezometer control was present. EM results at both sites show that water surfaces correspond with a drop in conductivity. This is due to elevated concentrations of evaporative salts in the vadose zone immediately above the water table. EM and seismic profiles at the Palmyra site accurately detected the depth to groundwater in monitoring wells, as well as interpolated depths between them. This demonstrates that an integrated approach is ideal for relatively homogeneous aquifers. On the other hand, interpreting the EM and seismic profiles at Carson Slough was challenging due to the laterally and vertically variable soil types, segmented perched water surfaces, and strong salinity variations. The high-resolution images and models provided by the geophysical profiles confirm the simple soil and hydrological structure at the Palmyra site as well as the laterally complex structure at Carson Slough. The integrated approach worked well for determining depth to water in the geologically simple site, but was less effective in the geologically complex site where multiple water tables appear to be present.

The interaction of aeolian and fluvial processes during deposition of the Upper Cretaceous capping sandstone member, Wahweap Formation, Kaiparowits Basin, Utah, U.S.A.

Detailed examination of the Upper Cretaceous capping sandstone member of the Wahweap Formation in the Kaiparowits Basin, Utah reveals the presence of aeolian stratification. Deposition by aeolian processes is recognized and distinguished from subaqueous deposition by the presence of centimeter-scale, inversely graded wind-ripple stratification. The aeolian stratification in the Wahweap Formation is the first occurrence reported from Upper Cretaceous strata on the Colorado Plateau of the western U.S.A. Aeolian stratification occurs as thin wind-reworked caps of fluvial-bar sandstones deposited within low-sinuosity braided streams and as more extensive deposits of small-scale dunes that developed in geographically restricted dune fields. Aeolian reworking of bars took place during low-stage flow and was possibly controlled by intermittent (seasonal) discharge variations. Prolonged aridity led to increased sand supply entering the aeolian system, dunes nucleated and grew rapidly between the braided stream systems. These small-scale dunes rarely developed extensive or tall slip faces as evidenced by the rare preservation of grain flow strata and the dominance of wind-ripple stratification. The dune field was characterized by sinuous-crested dunes, probably barchans, amid and linked to the extensive braided stream systems. Based on modern analogs, the resulting change in sand storage/supply probably is related to a short-term shift to a more semi-arid/arid climate, possibly seasonally. However, a longer-term climate change to semi-arid/arid may be indicated for the dune complex near the boundary with the overlying Kaiparowits Formation.

Landscape and sediment processes in a proglacial valley, the Mittivakkat Glacier area, Southeast Greenland

Geografisk Tidsskrift, Danish Journal of Geography 108(1):97–110, 2008 During the Little Ice Age (LIA), the present proglacial Mittivakkat Valley, stretching 1.5 km ENE-WSW from the terminus of the Mittivakkat Glacier tongue to a delta terminating in the Sermilik Fjord, Southeast Greenland, was transgressed by the glacier as indicated by a terminal moraine near the valley mouth. The first recordings of the glacier terminus in 1933 show a frontal retreat of about 300 m since the Little Ice Age (LIA). Since then, the glacier has retreated another 1200 m leaving a valley train characterized by steep slopes flanking a 150–300 m wide flat-bottomed, sediment-floored trough. The larger part of the valley is floored by fluvial sediments and shows a typical valley sandur stream, the Mittivakkat stream, with one or two main water-channels, which branches out in a braided stream system with numerous intervening channel bars. In the outer part of the valley train, the Mittivakkat stream is erosive and divides around remnants of ground moraine, end-moraines, and outwash with abandoned meltwater channels. In this area some bank erosion is seen, but rejuvenation of older stream channels takes place during spring time when snowbridges cover the recent outlet and force the meltwater to follow the older channels at a higher level. Most recently, the seaward side of the outermost end-moraine ridge has been eroded by the sea, probably because the delta area is degrading. This could be a result of a decreased delivery of glaciofluvial sediments, due to the sink effect of the proglacial valley, but also because of less sea ice during summer periods and a more recent shift in wind directions during open water periods towards south and west. Geoelectric measurements (Schlumberger) along a number of traverses show sediment thicknesses in the valley trough of 6–23 m, which suggest that 2.2 x 106 m3 of sediment has been deposited in the valley. The estimated volume of glaciofluvial sediments deposited in the valley and delta is 9.9 x 106 m3. Recent sediment transport from the glacier basin of 18.4 km2 through the Mittivakkat Valley to the Sermilik Fjord at the mouth of the valley has been monitored. The annual average is around 7,000 m−3. The investigations confirm that a slight net deposition takes place in the upper part of the valley, showing that the proglacial valley will trap more of the sediment released by glacial erosion if the Mittivakkat Glacier continues to retreat. Thus, the investigation demonstrates the close interaction between the different morphological zones in a proglacial sediment transport system.

Modeling of Paleohydrologic Change during Deglaciation

This paper aims to test whether estimates of paleodischarge made for proglacial terrace sequences support the model of largescale change in hydrologic regime (as inferred from channel pattern, form and sedimentology changes), during deglaciation. Modeling paleodischarges of former braided stream systems on each terrace surface is shown to be at a tentative stage, however, because of numerous error sources and assumptions that remain to be tested. These are discussed in relation to field measurements of channel width, depth, gradient and sedimentology; to theoretical modeling of flow depth, resistance coefficients, velocity, Froude number and discharge; and to geomorphic interpretation of the flow events being modeled. The different discharge models adopted have provided approximate estimates of former peak flows and patterns of change in peak flow magnitudes during deglaciation. Longterm deglaciation appears to have been associated with largescale decreases in peak flows of between 10 and 30 times. On shorter timescales, peak flows produced by short-term meltwater and by non-melt processes, such as volcanically induced floods, mask any longer term discharge trends in the catchment.

Pseudo-gamma ray volume from extended elastic impedance inversion for gas exploration

The Unayzah Formation has been an exploration target in Saudi Arabia for more than a decade, with oil and gas production from the unit now well established. However, exploration in the Unayzah in central Saudi Arabia remains technically challenging. This is due in large part to its complex interplay of continental clastic depositional environments that include braided stream, wadi fill, playa-lake, interfluve, alluvial fan, and eolian dune systems.

The conglomerate of Churn Creek: Late Cretaceous basin evolution along the Insular–Intermontane superterrane boundary, southern British Columbia

Upper Cretaceous coarse clastic rocks exposed in the canyon of Churn Creek, south-central British Columbia, record active basin tectonism and coeval volcanism adjacent to the boundary between the Intermontane and Insular superterranes. Mid to late Albian (~104 Ma U–Pb), calc-alkaline andesite and basaltic andesite flows, with minor conglomerate and reworked epiclastic deposits and tuffs correlative with the Spences Bridge Group of the Intermontane superterrane are exposed in the canyon. In depositional contact above the volcanic rocks is the conglomerate of Churn Creek, which contains a thick (&gt;1 km) sequence of complexly intertonguing conglomerate and sandstone that is divided into two members composed of four lithofacies. The lower member was deposited unconformably on the underlying Albian volcanic unit and contains late Albian–Cenomanian chert-pebble (&gt;50% chert) conglomerate and interbedded chert- and volcanic-lithic sandstone. It is interpreted to have been deposited in a braided stream system flowing from southeast to northwest. The source for the chert was most likely the Bridge River terrane, a Mississippian to Jurassic ocean floor assemblage located to the southwest of Churn Creek, south of the Yalakom fault. Gradationally overlying the lower member throughout much of the basin is a mixed chert, plutonic, and volcaniclastic lithofacies of the upper member. Plutonic debris was provided to the mixed and plutonic lithofacies of the upper member by the Little Basin pluton, which was uplifted along the northeast-directed Little Basin thrust fault on the southwest margin of the basin. The upper member also contains a volcanic-rich lithofacies composed of chaotic volcanic conglomerate and local lithic tuff derived from a coeval proximal volcanic source. The conglomerate of Churn Creek records active northeast-vergent compressional tectonism and development of piggyback basins along the boundary between the Insular and Intermontane superterranes during Albian–Santonian time. The conglomerate of Churn Creek has been correlated to the Silverquick – Powell Creek succession of the Methow terrane, based on age, stratigraphic, lithologic, structural, geochemical, and paleomagnetic similarities, and may, therefore, represent an overlap assemblage linking the superterranes in the Late Cretaceous.

Herpetofaunal and vegetational characterization of a thermally-impacted stream at the beginning of restoration

Pen Branch, a third order stream on the Savannah River Site (SRS), located near Aiken, SC, USA, received thermal effluents from the cooling system of a nuclear production reactor from 1954 to 1988. The thermal effluent and increased flow destroyed vegetation in the stream corridor (i.e. impacted portion of the floodplain), and subsequent erosion created a braided stream system with a greatly expanded delta. Restoration of the area began with planting of bottomland hardwood species in 1993. Occurrence of amphibians and reptiles was monitored by daily sampling from 1 January 1995 through 30 September 1996 to characterize the course of the restoration. Vegetation was sampled in the summer of 1996 to characterize the habitats in the unimpacted riparian zone and the impacted stream corridor. A total of 12 580 individuals representing 72 species of herpetofauna were captured. There were no significant differences in relative abundance or diversity of herpetofauna in unplanted versus planted zones within the impacted corridor 3 years after planting. Likewise, there were no significant differences in abundance or diversity of herpetofauna in the upper and lower corridor areas, which differed in site preparation before planting, or in riparian zones of different widths. However, species diversity of amphibians and reptiles in the unimpacted riparian zone was significantly higher than on vegetated islands located between stream braids within the impacted floodplain corridor. There were also significantly more species and individuals within the riparian zone than in the corridor, and the species assemblage within the riparian zone differed from that of the corridor. Woody vegetation within the unimpacted riparian zone was significantly higher in basal area than on islands within the corridor.

The depositional environment and mineralogical and chemical compositions of high ash brown coal resting on early Tertiary saprock (Schirnding Coal Basin, SE Germany)

The late Tertiary high ash brown coal ( R r=0.26%) of the Schirnding Seam Zone is mainly composed of huminite. The seam is underlain by a thick saprock (mica saprock⇒kaolinite-mica saprock) some tens of meters thick which gradually developed from phyllitic basement rocks and a thin Footwall Series of coarse-grained clastic sediments which, however, occur only in a few places. The roof rocks ` Hanging Wall Series' abruptly appear on top of the seam and consist of massive gray mudstones and arenites, intercalated with some tuffites. These Tertiary series were studied in open-pit mines operated for coal and clay and in bore holes. Chemical investigation involved the analyses of TOC, TSC, Ni, Co, Pb, Zn, Ga, Be, V, Cr, U, Th, P, Zr, REE and organic compounds of extracted organic matter (=OM). Early Tertiary chemical weathering was responsible for the monotonous kaolinite-mica-goethite mineral assemblage in the saprock and the footwall series. In the coal-bearing basin a more variegated mineral association including, pyrite, kaolinite, illite, smectite, illite-smectite mixed layers and siderite was brought about by volcanic processes, intrastratal solution set free in course of coalification and early diagenesis and by meteoric waters infiltrating the coal-bearing series from uplifted parts of the basin, which underwent strong oxidation. The environment of deposition is interpreted in terms of peatswamps that were fed by small streams and crevasse splays of a meandering drainage system grading towards the basin margin into a braided stream system on an alluvial fan plain. Strongly fluctuating water levels were responsible for the pronounced cyclicity to be encountered across the coal-bearing series. The processes of element redeposition and element migration in the Schirnding Basin are operative across the interface coal-saprolite (detrital influx of heavy minerals and intrastratal solution, hydrocarbon infiltration and redistribution of organic matter, reworking of duricrusts), in the saprolite proper (weathering leachats) and across the redox boundaries between the Schirnding Seam Zone and the Hanging Wall Series (input of physils and diagenetic alteration) as well as its strongly oxidized stratigraphic equivalents at the basin edge (volcanic influx, intrastratal solutions). Chemical and mineralogical variations across the coal basin which are directly controlled by the variation in the environment of deposition, weathering and groundwater movements may be directly correlated to changes in the saprock due to the immediate contact between coal and bedrock.

Integrated analyses constraining the provenance of sandstones, a case study: the Section Peak Formation (Beacon Supergroup, Antarctica)

Sandstones of the Section Peak Formation (late Triassic?–Middle Jurassic), Northern Victoria Land, Antarctica, record a change of palaeotectonic regime, from a stable continental basement setting to the beginning of rifting indicated by widespread basaltic volcanism during the Middle Jurassic. The provenance study of sandstones was performed by integrating conventional microscope study of detrital mode, a varietal study of quartz grains, the detection of mineral inclusions in detrital quartz, and electron microprobe analyses of detrital garnets. This provides the basis for discussing the validity and limits of each of these approaches, highlighting the potential source of pitfalls in using a single provenance approach, especially when clastics related to rapidly changing geotectonic regimes are studied. The resulting palaeogeographic picture is of a braided stream system draining a low relief, crystalline basement area, composed mainly of granitoid and high-grade metamorphic rocks, intermittently interacting with coeval basaltic lava flows that were partially reworked by fluvial currents.

Middle Jurassic non-marine siliciclastic facies in southern central Saudi Arabia

Middle Jurassic deposits of Saudi Arabia show distinct lateral facies variation, where the carbonates in the north are replaced by siliciclastic rocks in the south. In southern central Saudi Arabia, the Middle Jurassic deposits are composed exclusively of siliciclastic rocks, mainly sandstone with minor shale and siltstone, and rare conglomerate. Five stratigraphical sections were measured and studied along the southern Tuwaiq escarpments, where these rocks are well-exposed. A fluvial depositional environment is proposed here for the Middle Jurassic rocks in southern central Saudi Arabia based on sedimentological and petrological evidence. Eight different lithofacies were recognized in these deposits and each is described and interpreted. Cyclic sedimentation is prominent in all the sections studied; the most frequent transition is from medium-coarse pebbly sand to fine sands/silts/muds indicating a transition from sand flat or channel bar deposition, in which the upper flow regime dominates, to overbank deposition, where the lower flow regime prevails. Sandy bedform and downstream-accretion macroforms are the most common fluvial architectural elements. A braided stream system is the most appropriate model for these rocks, but the geomorphology of the streams was not constant in either space or time. However, the streams were braided most of the time, especially in the vicinity of the upstream source area in the southern highlands, and they changed their morphology gradually northward to become more sinuous, but never reached the stage where they become true meandering rivers. The change in stream morphology affected the distribution of fluviual facies and resulted in the presence of a proximal fluvial facies dominating in the south and a distal facies prevailing in the north. The streams flowed from south to north with little or no contribution from east-flowing tributaries. At this time, the Central Arabian Arch was a negative, submerged structural feature subsiding under the accumulating shelf carbonate sediments, whereas the Hadramaut and Mukalla Arches were positive features controlling the sedimentation pattern in southern central Arabia. Although the Middle Jurassic rocks, both in the north and in the south, form a slightly diachronous, contiguous sedimentary body, they differ from each other significantly. Therefore, it is recommended here that the Middle Jurassic siliciclastic facies in southern central Saudi Arabia be separated from the Dhruma Formation and be recognized as a new stratigraphical unit with formational rank.

The Black Reef Quartzite Formation in the western Transvaal: sedimentological and economic aspects, and significance for basin evolution

A sedimentological study of the early Proterozoic Black Reef Quartzite Formation in the south-western parts of the Transvaal province of South Africa was undertaken with the primary aim of examining the sedimentological controls of gold mineralization in the Black Reef placer, which occurs at the base of this Formation. A second aim of the study was to investigate the early history of the basin in which the Transvaal Sequence of South Africa was deposited. The thin, siliciclastic Black Reef Quartzite Formation, which is informally subdivided into a lower Conglomerate Unit and an upper Quartzite Unit, is underlain by Archaean rocks belonging to the basement complex and the Witwatersrand and Ventersdorp Supergroups, and is overlain by a thick succession of carbonate rocks of the Malmani Subgroup. The pre-Transvaal palaeosurface is characterised by elongated northeast to southwest trending grabens and partly-eroded horst blocks. The Black Reef Quartzite Formation, which has a maximum thickness of about 30 m in the study area, typically comprises a succession of interbedded arenites and mudstones, with a sporadically-developed basal Conglomerate Unit. Thickness trends are similar to the dominant structural trend of the pre-Transvaal palaeosurface. At localities where the Conglomerate Unit is absent, the Formation invariably overlies quartzites of the Witwatersrand Supergroup directly. The palaeocurrent distribution of the Conglomerate Unit is unimodal, with modes towards the southwest in the southern parts of the study area and towards the north in the northern regions. Most of the palaeocurrent distributions of the Quartzite Unit are unimodal, too, but bimodal distributions were found at three localities. Pebble size of the Black Reef placer is largest in the northeastern parts of the study area, but no orderly lateral size variation was found. Pebble roundness, too, varies greatly and apparently randomly. The composition of the pebble assemblage is not constant, but no systematic lateral change could be detected. A petrographic study of the arenites of the Formation reveals a remarkable textural and mineralogical maturity, especially for the upper beds. It is concluded that the pre-Transvaal palaeosurface had a palaeorelief of up to 30 m and that the topography of the palaeo-landscape was the dominant factor controlling early sedimentation in the basin. The palaeo-grabens probably constituted the valleys of shallow braided stream systems that drained south-westwards and northwards from a palaeo-drainage divide in the northern parts of the study area. Sediment, including detrital gold, was derived from erosion of Witwatersrand rocks and fed to the graben valleys via several alluvial fans. During a subsequent transgression, the fluvial systems became drowned and transgressive estuarine conditions ensued. During the final stages of siliciclastic sedimentation, the upper quartzite beds of the Formation were probably reworked by shallow marine processes before carbonate precipitation commenced. The cause of the marine transgression is not known beyond doubt. It is suggested, however, that lithospheric rifting, which initiated the extrusion of the underlying Ventersdorp lavas, resumed during early Transvaal times, resulting in complete severing of the continental crust and the creation of a linear sea.

The use of trend surfaces in palaeoenvironmental reconstructions

Trend surfaces can be produced by a number of mathematical techniques and have been used widely to depict the regional variation of various geological parameters. In this paper, it is demonstrated that trend surfaces can be constructed by a simple graphical method and may be applied successfully to palaeoenvironmental reconstructions using elevation contour of tilted or folded contacts. Two case studies are discussed to illustrate different aspects of the technique. In the first example, the upper contact of a shallow marine sandstone outcropping over an area of roughly 4000 km 2 in the southwestern part of the Sydney Basin (Australia), is restored by means of a first order trend surface. This reveals the palaeotopography as a sinuous shoal trending towards the northnortheast. The second example, from the Beaufort Group of the Karoo Basin (South Africa), restores the basal contact of a fluvial sandstone by means of a second order (quadratic) trend surface. The restored surface portrays the individual thalwegs and islands of a braided stream system over an area of about 4 km 2.

The Characteristics of Neogene Sediments and Structures in the Siberuang Area (Central Sumatra, Indonesia) Based on Gravity Data: ABSTRACT

Neogene sediments in the Siberuang area consist of the Sihapas, Telisa, and Petani formations. The Sihapas Formation is a friable, white to brown sandstone with intercalated claystones, siltstones, and some coal layers. It was deposited in channels or bars of a braided-stream system during early Miocene N4-N8 times. The Telisa Formation, which unconformably overlies the Sihapas Formation, is a calcareous claystone with intercalated glauconitic sands and limestone lenses deposited in a middle to outer neritic environment during middle Miocene N8-N10 times. The Petani Formation is a claystone containing clasts of terrestrial material with some coal lenses and tuffs in the upper part of the section. It is a regressive sequence deposited in an outer neritic environment. These Neogene sediments lie unconformably above the Paleogene Pematang Formation, which is mainly composed of brown conglomerate and conglomeratic sandstones. The Pematang Formation lies unconformably above pre-Tertiary rocks. A Bouguer gravity map processed using a density value of 1.9 gm/cc reveals anomalies with wavelengths of 10 and 15 km. Moving average filtering has been applied to make a regional-residual separation. Map interpretation and modeling along a profile run northeast across the structural trend of the region and indicate a graben-type structure with Neogene sedimentmore » thickness increasing from 600 m in the northeast to 2800 m in the southwest. Greatest sediment thickness is attained in the center of the graben, which is delineated by northeast-dipping faults southwest of the area and southwest-dipping faults northeast of the area.« less