Low-angle Cross-bedding Research Articles

Development, sand control mechanism and hydrocarbon accumulation of beach-bar sandstone in a saline lake basin: A case from the Neogene of southwestern Qaidam Basin, NW China

Based on the data of field outcrops, drilling cores, casting thin sections, well logging interpretation, oil/gas shows during drilling, and oil/gas testing results, and combined with modern salt-lake sediments in the Qinghai Lake, the Neogene saline lake beach-bars in southwestern Qaidam Basin are studied from the perspective of sedimentary characteristics, development patterns, sand control factors, and hydrocarbon accumulation characteristics. Beach-bar sand bodies are widely developed in the Neogene saline lake basin, and they are lithologically fine sandstone and siltstone, with wavy bedding, low-angle cross bedding, and lenticular-vein bedding. In view of spatial-temporal distribution, the beach-bar sand bodies are stacked in multiple stages vertically, migratory laterally, and extensive and continuous in NW−SE trending pattern in the plane. The stacking area of the Neogene beach-bar sandstone is predicted to be 3 000 km2. The water salinity affects the sedimentation rate and offshore distance of beach-bar sandstone, and the debris input from the source area affects the scale and enrichment of beach-bar sandstone. The ancient landform controls the morphology and stacking style of beach-bar sandstone, and the northwest monsoon driving effect controls the long-axis extension direction of beach-bar sandstone. The beach-bars have a reservoir-forming feature of “one reservoir in one sand body”, with thick beach-bar sand bodies controlling the effective reservoir distribution and oil-source faults controlling the oil/gas migration and accumulation direction. Three favorable exploration target zones in Zhahaquan, Yingdong−eastern Wunan and Huatugou areas are proposed based on the analysis of reservoir-forming elements.

Ediacaran cap dolomite of Shennongjia, northern Yangtze Craton, South China

Ediacaran cap dolomites are strong evidence for the glaciation during the Neoproterozoic. Stratigraphic-sedimentological studies combined with δ13C and δ18O isotope analyses are used for defining the processes of post-glacier environmental changes and sea-level rise caused by glacier melting and the reconstruction of the depositional environments for the Doushantuo cap dolomites in Shennongjia, northern Yangtze Craton (YC). Like many other cap dolomites in the YC and worldwide, those in Shennongjia exhibit strongly negative δ13Ccarb excursions, ranging from −7.3 to −2.5 ‰ (averagely from −5.4 to −4.1 ‰). A laminated clast-free calcareous siltstone commonly occurs between the cap dolomite and the underlying diamictites, indicating a low-energy, shallow-water depositional environment, wherein alkaline conditions developed before the deposition of the cap dolomite. The most prominent features of the cap dolomites in Shennongjia are the very thin thickness and monotonous sedimentary structures. The maximum thickness of the cap dolomites from the six sections in Shennongjia is 1.8 m, and the thinnest cap dolomite is 0.4 m with simple laminations. Sedimentary structures such as low-angle cross-bedding, giant wave ripples, sheet-crack cement, and tepee breccias are rare in Shennongjia except in the area near the Three Gorges. These evidences, therefore, point to the deposition of the Shennongjia cap dolomite in restricted shallow-water tidal environments.The space–time distribution of sedimentary facies in Shennongjia and the southeastern areas to Shennongjia indicate that the Ediacaran cap dolomite was formed on a ramp. Accordingly, we propose a sedimentary facies model for the formation of the cap dolomite, covering the inner and middle ramp facies. This model explains the thickness variations and distributions of the cap dolomites at both the local and regional scale, including their sedimentary structures, compositions, and barite occurrences. The model successfully illustrates why the cap dolomites in the Shennongjia area share a unique set of sedimentary features. Finally, the cap dolomite is interpreted as a diachronous deposit (bottom to top), tracking glacioeustatic flooding and recording ocean-wide changes over the time following deglaciation, and a correlation and cause of negative δ13Ccarb excursions in cap dolomites worldwide, particularly in those in the Yangtze Carton, are further discussed.

Evolution of Jurassic intertrap deposits in the Parnaíba Basin, northern Brazil: The last sediment-lava interaction linked to the CAMP in West Gondwana

The opening of the Central Atlantic Ocean was accompanied by ascending plume zones that generated large igneous provinces (LIPs), such as the Central Atlantic Magmatic Province (CAMP), with voluminous lava flows and outpourings. One of the best records of the CAMP in the West Gondwana is the Jurassic volcanic-sedimentary succession exposed in the western Parnaíba Basin, northern Brazil. Diagenetic and hydrothermal processes are discussed, focusing on the lava-sediment interaction. The siliciclastic sediments called “intertrap deposits” record a wet desert system characterized by fluvial-aeolian and shallow lake deposits engulfed by lavas of the CAMP at approximately 200–202 Ma. The facies and petrographic studies, based on a drill core and outcrops, led to the interpretation of the palaeoenvironmental and diagenetic evolution of the intertrap deposits. The intertrap deposits are sandwiched by basalts and may be organized in seven sedimentary facies, grouped into two facies associations: 1) ephemeral braided river deposits (FA1), which consist of medium- to coarse-grained pebbly sandstone with poorly sorted angular to sub-rounded grains (basaltic fragments and quartz) and low-angle cross-bedding, even-parallel stratification, and trough cross-stratification, and 2) pond/shallow lake deposits (FA2) composed of tabular beds of sandstone/siltstone rhythmite and laminated mudstone. The main architectural elements of intertrap deposits are 1) channels on volcanic substrate, 2) sand sheets deposited over pre-existent lava topography, 3) tabular beds with laminated rhythmites, and 4) sand-filled fissures/fractures and clastic dykes. The intensification of magmatic processes due to the disruption of Pangaea promoted the formation of extensive volcanic plains, whereas wet desert settings were developed during the quiescent intervals between lava flow episodes. Fluvial channels with subaqueous dunes and sand sheets were incised into the basaltic substrate. The abandoned fluvial plains were affected by flash floods that promoted mechanical infiltration of clays, forming coatings over grains. The heat flow and hydrothermal activity of volcanic rocks increased the devitrification of glassy clasts, releasing silica and precipitating low-temperature authigenic mineral assemblages. The most common pore-filling process was the massive precipitation of early diagenetic-hydrothermal minerals, including smectite, chalcedony, poikilotopic zeolite, megaquartz, and haematite, which partially obliterated the primary porosity and permeability. These exceptional conditions of cementation triggered by basalt eruptions precluded the modification of a primary framework by late stages (post-Jurassic) burial diagenesis. The intertrap sandstones represent the last lava-sediment interaction related to the CAMP recorded in West Gondwana.

Development of large‐scale sand bodies in a fault‐bounded lake basin: Pleistocene-Holocene Poyang Lake, Southern China

In fault-bounded lacustrine basins, the lake basement may be exposed due to tilting of crustal blocks, forming islands of varying size. Such islands are commonly associated with sandy facies that may serve as important reservoirs for oil and gas accumulation. The present study investigated large sand bodies of Pleistocene-Holocene age on Songmenshan Island in the center of Poyang Lake, Jiangxi Province, southern China. The relationship of sedimentation on Songmenshan Island to its formation history was analyzed by means of Google satellite observations and field studies combined with scanning electron microscope, grain size, and 14 C dating analyses. The area of Songmenshan Island (39 km2) represents 8% of the total lake area (500 km2), and its height (81 m) is several times greater than that of the depth of the surrounding lake (13 m). The island has been uplifted since ~ 5 Ma at a rate of ~ 16 m Myr− 1 as a consequence of intrabasinal block faulting. The surficial deposits of the island consist of wave-controlled beach-bars in the lower part (< 36 m elevation) and wind-controlled eolian dunes in the upper part. Sedimentary characteristics were different between beach bars and sand dunes in terms of bedding, sedimentary structures, and grain size and texture. The beach bars are characterized by low-angle cross-bedding, fan-shaped conchoidal, and disciform fractures, and small V-shaped impact craters on quartz grain surfaces. The eolian dunes are characterized by large-scale high-angle cross-bedding, dish-, crescent-, and V-shaped impact craters on quartz grain surfaces. Whereas sand movement in the beach-bar facies was dominantly through traction, saltation was the major process in the eolian dune environment. The depositional history of Songmenshan Island can be divided into 3 stages: (1) deltaic sedimentation from the Ganjiang and Xiushui rivers, which are connected to the Yangtze River via a channel; (2) wave reworking of deltaic sediments in shoreline beach-bar facies after initial uplift; and (3) wind reworking of sands in eolian dune facies following further uplift. Later in its history, Songmenshan Island was reduced through wind and wave erosion into two subequal parts (19.6 km2 and 19.8 km2), producing its present configuration. The large sand bodies comprising this island may eventually be buried in a cocoon of organic-rich lacustrine muds, yielding an exploration target. Songmenshan Island may thus serve as a model for development of sand-rich reservoir facies in fault-bounded lacustrine basins.

RECOGNITION OF FOSSIL NEBKHA DEPOSITS: CLUES FROM NEOICHNOLOGY AND SEDIMENTOLOGY

ABSTRACTThis study includes the first neoichnologic characterization of the burrow systems of Tympanoctomys barrerae (Rodentia: Octodontidae) and also considers sedimentologic features of the modern nebkhas where they occur. Tympanoctomys is a South American solitary and fossorial rodent that has ecomorphofunctional adaptations for living in saline environments and constructs its burrow in nebkhas with halophyte shrubs. The purpose of this work is to identify the ichnologic signatures of T. barrerae burrow systems and to provide combined ichnologic-sedimentologic criteria for identification of Cenozoic nebkha deposits. Tympanoctomys barrerae burrow systems are subhorizontal, typically with ten or more entrances, two or three levels, closed circuits, average complexity of 48, average tortuosity of 3.25, and an average ratio of total chamber volume to total tunnel volume of 0.04. The size of the tunnels averages 85 mm in horizontal diameter and 64 mm in vertical diameter, and cross-section shape ranges from elliptical flattened to plano-convex with incipient bilobed floor. Surface ornamentation is typified by a coexistence of primary (sets of four claw traces forming an arcuate pattern produced during digging) and secondary (numerous arthropod burrows excavated from the burrow lumen) surface ornamentation. Nebkha deposits in upper Cenozoic sequences can be recognized by the combination of ichnologic and sedimentologic features: fossil burrows having the ichnologic features characteristic of T. barrerae burrow systems and presence of rhizoliths of shrubby plants occurring in well-sorted sandy deposits with low-angle crossbedding. These criteria can be potentially applied to fossil sequences dating back to the early Oligocene.

Facies Architecture Analysis for Paleo-environment Evaluation in “Tom” Oil Field, Eastern Niger Delta, Nigeria

The cored section of reservoir C, well 4 of the drilled five wells that penetrated three reservoirs A, B and C in “TOM” oil field, Eastern Niger Delta was analysed and described on the basis of lithofacies, sedimentary structures and trace fossil records by using core data and wireline log motifs, with the aim of carrying out thorough geological core analysis to interpret the depositional environment of the oil field. The lithofacies are sandstones with interbedded mudstones and siltstones, the dominant sedimentary structures are parallel to ripple cross laminations, hummocky and swaley cross stratifications, sandy hetherolitics, planar to low angle cross bedding with traces of Teichichnus and Ophiomorpha burrows. The gamma-ray log motifs were noted and used to further constrain the character of the sedimentary facies and depositional environment of the field. A tidal incised – fluvial dominated shallow marine (lower, middle, upper shoreface) comprises of tidal channel sands and tidal flat of the coastal shelf depositional setting in the marginal marine mega depositional environment had been inferred for the “TOM” field.&#x0D; Keywords: Shoreface, Reservoir, Lithofacies, Structures

Architecture of intereruptive and syneruptive facies in an Andean Quaternary palaeovalley: the Huarenchenque Formation, western Argentina

The Huarenchenque Formation is a volcano sedimentary unit deposited to the east of the Plio-Quaternary Andean Magmatic Arc. In order to define depositional settings, two lithofacies associations (fluvial and pyroclastic) were defined. The fluvial facies association is composed of polymictic conglomerates with the predominance of basalt- dominated clasts, coarse- medium-grained conglomeratic sandstones and medium- to coarse-grained sandstones. These deposits occur as stacked or single bodies, display both sheet and channelized geometries, and contain a range of internal sedimentary structures, such as planar, low angle stratification and cross-bedding. This facies association is interpreted as the deposit of a multichannel fluvial system characterized by high bed load, steep gradient and non-cohesive bank materials. Facies and architecture of the fluvial deposits are the result of high bank full discharge related to rapid deglaciation of the Andean Last Glacial Maximum. The pyroclastic facies association is characterized by lapilli and ash tuffs deposited from air fall, pyroclastic density current, and density stratified surge mechanisms. In the Huarenchenque Formation the fluvial and the pyroclastic facies associations show a clear physical separation, suggesting that sedimentation occurred in two distinct (intereruptive and syneruptive) phases. During the long-lived intereruptive phases the sedimentary record corresponds mainly to the deposits of the gravelly braided fluvial system, whereas during syneruptive phases the fluvial valley was almost entirely occupied by primary pyroclastic deposits related to high-explosive episodes of the neighbor Andean strato-volcanoes.&#x0D; Although most of the cross-bedded sandstones and conglomerate sandstones are rich in basaltic fragments, some strata are composed almost entirely of pumiceous fragments, while in others there is a marked alternation between “basalt” and “pumiceous” foresets. These attributes reflect the preservation of intrabasinal pyroclastic fragments and allow suggest that: i. explosive volcanic events could be more frequent than reflected by the pyroclastic deposits themselves; ii. syneruptive pyroclastic materials could be eroded (even eliminated) by the fluvial system; iii. contributions of primary pyroclastic material persisted during intereruptive (fluvial-dominated) phases.

Sedimentary diagenesis of rudist shoal and its control on reservoirs: A case study of Cretaceous Mishrif Formation, H Oilfield, Iraq

Abstract Based on the core, cast thin section, whole rock analysis, conventional physical properties and high pressure mercury intrusion test, the sedimentary diagenesis characteristics of rudist shoal in Cretaceous Mishrif Formation of H Oilfield, Iraq and its control on the reservoir were studied. The rudist shoal of the Mishrif Formation develops in the high-stand systems tract and is distributed in the high places of paleogeomorphology on the edge of platform with strong hydrodynamic force. According to the relative sea level changes, lithologic evolution and sedimentary structure characteristics of the rudist shoal, the single rudist shoal is divided into four lithologic sections: A, B, C and D, that is, low-angle cross-bedding pelletoids-rudist packstone, low-angle cross-bedding and parallel bedding arene-rudist grainstone, parallel bedding rudist gravel limestone, and horizontal bedding carbonaceous mudstone. The complete sedimentary sequence of a single rudist shoal is often disrupted. Several rudist shoals superimpose to form thick rudist shoal sediment. The single rudist shoal thickness and lithologic sections assemblage change regularly in vertical direction. The rudist shoal has the characteristics of “strong dissolution, weak cementation and strong compaction”, forming pore-type reservoir with intergranular pores, intergranular dissolved pores, mold pores, and dissolved pores. With mainly coarse pore throats larger than 5 μm, the reservoir is of medium-high porosity and high permeability. There is lithological reverse cycles inside single shoals and between single shoals, with content of mud crystals decreasing from the bottom to the top, dissolution increasing, cementation decreasing in strength, pore throats getting larger, and physical properties turning better. The rudist shoal of MB2-1 at the top of the high-stand systems tract has the largest thickness, moreover, subject to the strongest atmospheric freshwater leaching, this layer has the most significant dissolution and the largest pore throat, so it is the best reservoir of the Mishrif Formation.

Felsic maar-diatreme volcanoes: a review

Felsic maar-diatreme volcanoes host major ore deposits but have been largely ignored in the volcanology literature, especially for the diatreme portion of the system. Here, we use two Mexican tuff rings as analogs for the maar ejecta ring, new observations from one diatreme, and the economic geology literature on four other mineralized felsic maar-diatremes to produce an integrated picture of this type of volcano. The ejecta rings are up to 50 m+ thick and extend laterally up to ∼1.5 km from the crater edge. In two Mexican examples, the lower part of the ejecta ring is dominated by pyroclastic surge deposits with abundant lithic clasts (up to 80% at Hoya de Estrada). These deposits display low-angle cross-bedding, dune bedforms, undulating beds, channels, bomb sags, and accretionary lapilli and are interpreted as phreatomagmatic. Rhyolitic juvenile clasts at Tepexitl have only 0–25% vesicles in this portion of the ring. The upper parts of the ejecta ring sequences in the Mexican examples have a different character: lithic clasts can be less abundant, the grain size is typically coarser, and the juvenile clasts can be different in character (with some more vesicular fragments). Fragmentation was probably shallower at this stage. The post-eruptive maar crater infill is known at Wau and consists of reworked pyroclastic deposits as well as lacustrine and other sediments. Underneath are bedded upper diatreme deposits, interpreted as pyroclastic surge and fall deposits. The upper diatreme and post-eruptive crater deposits have dips larger than 30° at Wau, with approximately centroclinal attitudes. At still lower structural levels, the diatreme pyroclastic infill is largely unbedded; Montana Tunnels and Kelian are good examples of this. At Cerro de Pasco, the pyroclastic infill seems bedded despite about 500 m of post-eruptive erosion relative to the pre-eruptive surface. The contact between the country rocks and the diatreme is sometimes characterized by country rock breccias (Kelian, Mt. Rawdon). Pyroclastic rocks in the diatreme are typically poorly sorted, and ash-rich. They contain a heterolithic mix of juvenile clasts and lithic clasts from various stratigraphic levels. Megablocks derived from the ejecta ring or the country rocks are often found in the diatremes. Evidence for multiple explosions is in the form of steep crosscutting pyroclastic bodies within some diatremes and fragments of pyroclastic rocks within other pyroclastic facies. Pyroclastic rocks are cut by coherent felsic dikes and plugs which may have been feeders to lava domes at the surface. Allowing for the difference in magma composition, felsic maar-diatreme volcanoes have many similarities with their ultramafic to mafic equivalents. Differences include a common association with felsic domes, inside the crater or just outside (Wau), although the domes within the crater may be destroyed during the eruption (Hoya de Estrada, Tepexitl); the dikes and plugs feeding and invading felsic diatremes seem larger; the processes of phreatomagmatic explosions involving felsic magmas may be different.

Upper flow regime sheets, lenses and scour fills: Extending the range of architectural elements for fluvial sediment bodies

Fluvial strata dominated internally by sedimentary structures of interpreted upper flow regime origin are moderately common in the rock record, yet their abundance is not appreciated and many examples may go unnoticed. A spectrum of sedimentary structures is recognised, all of which occur over a wide range of scale: 1. cross-bedding with humpback, sigmoidal and ultimately low-angle cross-sectional foreset geometries (interpreted as recording the transition from dune to upper plane bed bedform stability field), 2. planar/flat lamination with parting lineation, characteristic of the upper plane bed phase, 3. flat and low-angle lamination with minor convex-upward elements, characteristic of the transition from upper plane bed to antidune stability fields, 4. convex-upward bedforms, down- and up-palaeocurrent-dipping, low-angle cross-bedding and symmetrical drapes, interpreted as the product of antidunes, and 5. backsets terminating updip against an upstream-dipping erosion surface, interpreted as recording chute and pool conditions. In some fluvial successions, the entirety or substantial portions of channel sandstone bodies may be made up of such structures. These Upper Flow Regime Sheets, Lenses and Scour Fills (UFR) are defined herein as an extension of Miall's [Miall, A.D., 1985. Architectural-element analysis: a new method of facies analysis applied to fluvial deposits. Earth Sci. Rev. 22: 261–308.] Laminated Sand Sheets architectural element. Given the conditions that favour preservation of upper flow regime structures (rapid changes in flow strength), it is suggested that the presence of UFR elements in ancient fluvial successions may indicate sediment accumulation under the influence of a strongly seasonal palaeoclimate that involves a pronounced seasonal peak in precipitation and runoff.

Homeomorphy in the Asteroidea (Echinodermata); a new Late Cretaceous genus and species from Colorado

Codellaster keepersaenew genus and species, from the Upper Cretaceous Codell Sandstone Member of the Carlile Shale of Colorado (U.S.A.), is assigned to the asteroid (Echinodermata) family Goniasteridae. Although clearly a goniasterid, the flattened body form and details of morphology ofC. keepersaeare remarkably similar to corresponding features of the modern astropectinidAstropecten regalisand also of the luidiidLuidia(Platasterias)latiradiata.The discovery facies ofC. keepersaeincludes low-angle crossbeds and asymmetrical ripple marks that are suggestive of a very shallow marine environment. Both the modern species occur in shallow, turbulent settings, and homeomorphy beween ancient and modern asteroids suggests similar selective pressures and evolutionary responses. Limited evidence suggests goniasterids might have been more common in shallower waters during the Cretaceous than they are today, and absence of modernCodellaster-like goniasterids indicates that the family abandoned these habitats, although the fundamental morphological response to such settings remained viable for asteroid organization.

HOMEOMORPHY IN THE ASTEROIDEA (ECHINODERMATA); A NEW LATE CRETACEOUS GENUS AND SPECIES FROM COLORADO

Codellaster keepersae new genus and species, from the Upper Cretaceous Codell Sandstone Member of the Carlile Shale of Colorado (U.S.A.), is assigned to the asteroid (Echinodermata) family Goniasteridae. Although clearly a goniasterid, the flattened body form and details of morphology of C. keepersae are remarkably similar to corresponding features of the modern astropectinid Astropecten regalis and also of the luidiid Luidia (Platasterias) latiradiata. The discovery facies of C. keepersae includes low-angle crossbeds and asymmetrical ripple marks that are suggestive of a very shallow marine environment. Both the modern species occur in shallow, turbulent settings, and homeomorphy beween ancient and modern asteroids suggests similar selective pressures and evolutionary responses. Limited evidence suggests goniasterids might have been more common in shallower waters during the Cretaceous than they are today, and absence of modern Codellaster-like goniasterids indicates that the family abandoned these habitats, although the fun- damental morphological response to such settings remained viable for asteroid organization.

Shallow-water, eruption-fed, mafic pyroclastic deposits along a Paleoproterozoic coastline: Kangerluluk volcano-sedimentary sequence, southeast Greenland

The 200–300 m thick, volcano-sedimentary sequence at Kangerluluk is part of the psammite zone, one of four major zones, which constitute the 1.8 Ga Ketilidian orogen in south Greenland. Three lithofacies are emphasized in the study: (1) the conglomerate-sandstone; (2) the volcanic; and (3) the pyroclastic lithofacies. The 2–40 m thick conglomerate-sandstone lithofacies represents a subaerial to subaqueous fan-delta deposit. Matrix- and clast-supported conglomerates are interpreted as debris flow and longitudinal gravel bar deposits. Erosive-based conglomerate channel fills attest to stream incision. Trough crossbedded sandstone, interpreted as lunate megaripples, planar-bedded sandstone indicative of upper flow regime bar-top sands, and small-scale trough crossbeds reflecting ripples follow up-section, form collectively with the conglomerate, 0.40–2.50 m thick fining-upward sequences. The sandstone-dominated unit, up-section from the conglomerates and composed of planar and low-angle crossbeds, minor ripples and graded beds as well as mudstone is indicative of a lower shoreface deposit below normal wave base. The clastic sedimentary rocks are suggestive of a fan-delta setting. The 100–200 m thick volcanic lithofacies, composed of pillowed and pillow brecciated lava flows, is consistent with shallow-water deposition. Interstratification of lava flows with both conglomerate-sandstone and pyroclastic lithofacies, intrusion of dykes into volcaniclastic rocks, and peperite formation accentuate contemporaneity between volcanism and sedimentation and is a common feature of island arcs. The 1–50 m thick, pyroclastic lithofacies with sharp depositional contacts to the overlying volcanic and underlying conglomerate-sandstone lithofacies, was emplaced in a subaqueous setting. The lithofacies is divided into a planar- to crossbedded tuff-lapilli tuff and a bedded lapilli tuff breccia, whereby both deposits are inferred to result from shallow-water surtseyan-type eruptions. The 5–15 m thick, bedded lapilli tuff breccia with abundant bomb sag structures and graded beds is considered a result of subaqueous eruptions strong enough to form an insulating steam cupola characterized by ballistically emplaced bombs that rapidly collapsed allowing for transport via mass flow processes. The deposits are considered proximal to the vent. The 2–50 cm thick, planar- to crossbedded tuff-lapilli tuff featuring abundant euhedral and broken crystals of feldspar (≤2cm) and minor pyroxene (≤1 cm), are massive, graded, crossbedded and stratified. The planar but laterally discontinuous beds, characterized by abundant low-angle scours, are interpreted as low- to high-concentration sediment gravity flows produced directly from subaqueous tephra jets that collapsed due to massive water ingestion. Local breccia-size pyroclasts disrupting beds are interpreted as bomb sags. The mafic, eruption-fed, Surtseyan-type deposits, postulated to be a subaqueous counterpart of cold, subaerial base surges, originate from subaqueous tuff cones formed along a rugged volcanic-dominated shoreline featuring high-energy fan-deltas.

Palaeoenvironments of Early Miocene Kisingiri volcano Proconsul sites: evidence from carbon isotopes, palaeosols and hydromagmatic deposits

Early Miocene ( c. 18 Ma) hominoid sites (Proconsul) from Rusinga Island, Lake Victoria occur in palaeosols and volcaniclastic strata deposited in a semi-arid, seasonal climate on the flanks of the active, low-relief edifice of the Kisingiri volcano. Palaeosols interbedded with sequences of primary pyroclastic deposits have stable carbon isotope values indicative of C3 vegetation from semi-arid environments. Isotopic values of palaeosol organic matter and pedogenic carbonate from the Rusinga Group have average δ 13 C of – 23.8 ± 0.8 and – 7.7 ± l.l‰, respectively, considerably heavier than average C3 vegetation. These isotopic values were most likely caused by reduced photosynthetic fractionation under water-stressed conditions. Prolonged dry seasons and semi-arid precipitation levels for Rusinga Group fossil soils are also supported by the profile depth of calcareous horizons and vertic clay structures. The principal fossil-bearing units of the Rusinga Group (Kiahera and Hiwegi formations) are dominated by sandy strata which are interbedded with palaeosols and have features indicative of hydromagmatic pyroclastic deposition. Pyroclastic surge features include very low-angle cross-bedding, low amplitude and long-wavelength dune bedforms, moderate to poorly sorted layers of tuffaceous and pebbly sandstones, and common, isolated cobbles and boulder clasts of Precambrian basement rocks with associated impact sags. These features are interpreted as the deposits of pyroclastic surge dune bedforms and ballistically implaced volcanic ejecta, both produced by powerful hydromagmatic explosions from the Kisingiri volcano. According to this interpretation, the initial stages of this carbonatite-nephelinite volcano had repeated episodes of hydromagmatic eruptions which built a large (15–20 km radius), low-relief tuff ring or maar volcano. The palaeosol sequences interbedded with the primary pyroclastic deposits represent periods of volcanic quiescence lasting hundreds to thousands of years. Thus, Proconsul and associated fauna and flora inhabited a frequently disturbed landscape that experienced repeated catastrophic volcanic deposition in an overall semi-arid seasonal climate.

Preservation of pre-vegetational mixed fluvio–aeolian deposits in a humid climatic setting: an example from the Middle Proterozoic Eriksfjord Formation, Southwest Greenland

Sedimentological studies of a 30 m thick coastal cliff section within the Middle Proterozoic Eriksfjord Formation in western South Greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams (Type I), semi-perennial streams (Type II), and ephemeral flash floods (Type III). Perennial river sand sheets are characterised by co-sets of medium-scale trough cross-beds, interbedded with isolated medium- and large-scale, high-angle, tabular cross-beds. Indications of desiccation or subaerial exposure are absent. Semi-perennial fluvial sand sheets consist predominantly of low-angle cross-beds, interbedded with isolated sets of high-angle tabular cross-beds with common reactivation surfaces. Horizontal lamination and climbing ripple lamination form subordinate structures. Associated with the sand sheets are adhesion structures and 0.05–0.4 m thick sets of wind ripple-lamination indicating periods of subaerial exposure and aeolian reworking. High-energy ephemeral flash flood sand sheets consist almost exclusively of planar-parallel lamination and climbing ripple lamination with some isolated sets of low-angle cross-bedding. Scouring and internal truncation surfaces are common. The three types of sand sheets are considered to reflect deposition under changing climatic conditions, varying from humid to arid or semi-arid. Aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation (Type II), while sand sheets showing features typical of arid and or semi-arid flow conditions (Type III) contain no preserved aeolian deposits. This selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime. The deposits show that during pre-vegetational times the preservation of aeolian deposits, under certain conditions, may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi-arid or arid circumstances. The occurrence of aeolian deposits within a Precambrian succession of fluvial deposits therefore, need not be an indication of the most arid environmental conditions.

Depositional setting and sequence stratigraphic implications of the Upper Sinemurian (Lower Jurassic) sandstone interval, North Celtic Sea/St George’s Channel Basins, offshore Ireland

Abstract The Upper Sinemurian sandstone is a primary reservoir target in the North Celtic Sea/St George’s Channel Basins. Based on core examination, analysis of gamma ray logs, and interpretation of possible sandbody geometry from well log correlations and seismic character, a marine shelf depositional setting has been interpreted for the Upper Sinemurian sandstone on the northern margin of the North Celtic Sea/St George’s Channel Basins. Burrows observed in the cored interval are typical for a marine shelf depositional setting. Wavy, low-angle cross-bedding, reactivation surfaces and truncated cross-bedding suggest deposition by storm-driven, surging geostrophic currents in this setting. Examination of gamma ray logs through the Upper Sinemurian sandstone indicates that it coarsens and cleans upwards and can be divided into at least three units based on changes in curve patterns. Similar log patterns and sedimentary structure sequences in core and outcrop have been observed in elongate single and coalescing shelf ridges in the Ordovician of the eastern Williston Basin in Canada and the USA, in the Cretaceous of Wyoming and Colorado, and the Middle and Upper Jurassic of the North Sea. As will be discussed, the shelf ridge interpretation for some of these examples is controversial, with some workers suggesting an alternative incised shoreface interpretation. These examples range in thickness from 10 to 130m (33 to 425 ft). The 60+m (200+ ft) thickness of the Upper Sinemurian sandstone is well within this range. Such thicknesses suggest that ridge development (sand supply) kept up with basin subsidence. Thus, based on core data, gamma ray log curve examination, and comparisons to other sub-surface and surface examples, the Upper Sinemurian sandstone is interpreted as a series of complex elongate single and coalescing shelf sand ridges. Interpretation of recently acquired seismic data and sequence stratigraphic analysis of the Lower and Middle Jurassic section indicates that the Upper Sinemurian sandstone is part of a series of stacked sequences. These sequences result from increases in sediment accommodation space due to episodes of basin subsidence and their corresponding relative sea-level rises. Six sequences or cycles can be recognized, with each consisting of a basal onlapping, open marine phase which shoals upwards to shallow marine limestone or sandstone. Onlap occurs at and above a marine flooding surface (or its basinward sediment starvation equivalent) and proceeds towards the northern flexural margin of the basin. The next marine flooding surface is cut into or is in conformable contact with the top of the underlying sequence. The upper part of the sequence has a progradational to slightly aggradational seismic character, which results in an overall parallel seismic signature. The best potential Lower/Middle Jurassic clastic reservoir development in the North Celtic Sea/St George’s Channel Basins, such as the Upper Sinemurian sandstone, corresponds to the last phase of progradation at the top of a sequence. Six such sequences, with their upper boundaries at the tops of the Lower Sinemurian sandstone, Upper Sinemurian sandstone, Pliensbachian limestone, Toarcian marker, Bajocian sandstone/limestone and Bathonian limestone, respectively, can be delineated in the Lower and Middle Jurassic section in the North Celtic Sea/St George’s Channel Basins. The Lower and Middle Jurassic was largely a time of thermal subsidence in the North Celtic Sea and St George’s Channel Basins. Early thermal subsidence was interrupted by events of post-rift footwall readjustment along the northern margin of both basins. This resulted in the input of clastic sediments into both basins and deposition of Lower and Upper Sinemurian sandstones in rapidly coarsening-upward cycles and sequences. In contrast, Pliensbachian to Bathonian sequences represent deposition in a period dominated by thermal subsidence and thus coarsen upwards more gradually. With the exception of basin margin uplift and basin centre subsidence influenced by intraplate stresses, extrabasinal effects, such as eustasy, are not thought to be a factor in Upper Sinemurian sandstone deposition and Lower and Middle Jurassic sequence development in the North Celtic Sea and St. George’s Channel Basins.

Evidence for high-energy events and shallow-water deposition in the Chattanooga Shale, Devonian, central Tennessee, USA

The Upper Devonian Chattanooga Shale of central Tennessee, a classical black shale, was deposited in an epicontinental setting, west of the Appalachian foredeep. Its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water. Interbeds of bioturbated greenish-gray shale, indicating oxygenated bottom waters, are commonly ascribed to pycnocline fluctuations. However, laminated fine sand and silt and hummocky cross-stratification (HCS) at the base of some of these beds indicates interaction of storm waves with the seabed, and suggests that greenish-gray shale beds are post-storm mud drapes. Other interesting features are inclined-undulose erosion surfaces that are conformably overlain by shale beds, sets of inclined shale beds that suggest low-angle cross-bedding, and clearly and uniformly developed alignment of clay particles (magnetic fabric studies). These observations show that the seabed was at times subject to prolonged erosion by bottom currents (erosion surfaces), agitation and reworking by storm waves (HCS and greenish-gray shale beds), and sediment transport by long-lived bottom currents (particle alignment). The epicontinental sea setting and the presence of HCS and other storm-produced features suggest a relatively shallow water depth (possibly only a few tens of meters). Together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation.

Geological Note: Hummocky cross‐stratification from the Boxvale Sandstone Member in the northern Surat Basin, Queensland

The Boxvale Sandstone Member (BSM) in the upper part of the Early Jurassic Evergreen Formation, in the northern Surat Basin, consists of up to 90 m of interbedded mudrocks and sandstones, with minor coal and ironstone. The unit was interpreted by Exon (1976) as representing a marine transgression over a coastal plain environment. The upper part of the BSM consists of a 10-40 m thick coarsening-upward sequence and contains several very well-sorted quartzose sandstone beds with trace fossils and low-angle cross-bedding. Mollan et al (1972) suggested that the physical and biogenic sedimentary structures, good primary porosity and concentrations of heavy minerals within these sandstones pointed towards deposition in a lacustrine shoreline setting. Exon (1976) suggested that the presence of acritarchs was more consistent with a marine origin, and Gerrits (1985) considered these beds to represent sediment accumulation in marine shoreface and offshore environments. All evidence gathered to date is ambiguous in that it does not allow discrimination between possible marine or lacustrine environments of deposition. Recent fieldwork has resulted in the recognition of sedimentary structures, including hummocky cross-stratification, which have a bearing on this problem. Hummocky crossstratification has not previously been identified in the Evergreen Formation.

Subaqueous Evaporites of Buckner Member, Haynesville Formation, Northeastern Mobile County, Alabama: ABSTRACT

ABSTRACT The lower part of the Buckner Anhydrite Member of the Haynesville Formation (Upper Jurassic) was deposited as shallowing upward cycles of subaqueous to subaerial deposits on the north flank of the Wiggins Arch in northeastern Mobile Country, Alabama. The unit studied conformably overlies the Smackover Formation and is generally evaporite dominated. The Buckner Anhydrite averages about 35 m (115 ft) in thickness and has been buried to depths of 5.5 km (> 18,000 ft). In spite of this deep burial, it has suffered little deformation since alteration of gypsum to anhydrite. Subaqueous evaporites in the Buckner are dominated by bedded massive, bedded mosaic, bedded nodular, bedded nodular mosaic, and mosaic structural types. Buckner cycles begin with the selenite facies in which bedded nodular mosaic and bedded mosaic structural styles often display a preferential vertical alignment of anhydrite nodular pseudomorphs after gypsum. Selenite crystals were precipitated on the sediment surface possibly below wave base but within the photic zone. Gypsarenite, which vertically succeeds selenite, is a laminated deposit in which ripples and low-angle cross-beds indicate clastic deposition above wave base. The upper part of this facies may have been subaerially exposed. Stromatolite domes formed in relatively quiet water areas. Subaqueous evaporites of this unit commonly grade upward into a generally poorly laminated anhydrite often containing burrow structures, detrital silt, and possible root structures. This may have been a gypsite facies which formed in the zone of soil moisture. Often this facies is located stratigraphically just below probable storm deposits and tidal flat-sabkha deposits which accumulated under subaerial conditions. These deposits are located at the top of the cycle. Subaerial conditions are indicated by desiccation cracks, an influx of oxidized red silt and clay, and erosional scours. There probably was not a significant change in water depth at the transition from the Smackover Formation to the Buckner Anhydrite. Subaqueous Buckner evaporites were deposited conformably over subtidal deposits at the top of the Smackover. The formation of microcrystalline dolomite at the top of the Smackover probably occurred below the sediment/water interface. Relative thickness of cycles, facies correlations, and the frequency and duration of subaerial exposure indicate the relative paleotopograpic position of various locates in the area. Buckner cycles were controlled by the rate of evaporite aggradation versus sea level rise and/or rate of subsidence.

Cambrian Pisolites as Paleoenvironment and Paleotectonic Stress Indicators, Rattlesnake Mountain, Wyoming: ABSTRACT

Pisolitic-rich carbonates occur within the uppermost 0.5 m of the Meagher Limestone member of the lower Gros Ventre formation in exposures near Cody, Wyoming. The Meagher Limestone is overlain by 51 m, and underlain by 63 m of dark gray Gros Ventre shale. Pisolites range in size from 2.0 to 18 mm in diameter and occur in lime grainstones associated with trilobite fragments, peloids, glauconite, fine-grained subangular quartz, and minor oolites. Girvanella grainstones 15-20 cm thick directly underlie the pisolite strata and have contributed to some of the carbonate material within pisolite nuclei. Dolomite and ankerite may occur within pisolitic rocks as finely crystalline irregular patches. Pisoliths commonly show an oblate ellipsoid shape, with maximum flattening perpendicular to bedding. Long-axis to short-axis ratios of these grains in fracture planes perpendicular to bedding average between 2.5 to 3.5, with the long axis parallel or subparallel to bedding. Grains observed in bedding planes have ratios averaging between 1.5 to 2.0. A paleostress state has produced a strain ellipsoid with long-axis ratios ranging from 1.7 to over 3.0. There appears to be little or no tectonic strain on the bedding plane, so the strain can be described as uniaxial, with maximum compression perpendicularmore » to bedding. The majority of carbonate rocks in the Meagher Limestone were deposited in a normal marine subtidal setting, while ooid and pisolitic grain types are suggestive of subtidal-peritidal conditions. Because of the strain deformed pisoliths, a subaqueous versus subaerial environment of pisolite genesis is difficult to assess. A siliciclastic sandstone, 0.6 m thick with low-angle tabular crossbedding, is present immediately beneath the Meagher Limestone. The sandstone is composed of 94% fine to medium sand-size subangular quartz grains and is associated with glauconite, minor biotite, zircon, and ilmenite.« less