Alluvial Plain Facies Research Articles

Sedimentology of the latest Permian to Early Triassic in the terrestrial settings of the North China Basin: Low-latitude climate change during a warming-driven crisis

Abstract In terrestrial settings, the Permo-Triassic mass extinction is commonly linked to major changes in sedimentological and climatic conditions that include a switch from meandering to braided fluvial systems and increased aridity. We examined the predominantly terrestrial strata of North China to reveal that, on the contrary, there was little substantial sedimentological change during the Permo-Triassic mass extinction. The crisis level occurs in the upper Sunjiagou Formation, where a range of environments, including low-sinuosity rivers, distally terminating alluvial fans, and mud-dominated coastal plains are recorded. A major sedimentological change occurred slightly higher, within the lowest Triassic, and involved a switch to braided fluvial and shallow lacustrine/fluviodeltaic conditions of the Liujiagou Formation. Later in the Early Triassic, fine-grained, playa lake, and alluvial plain facies became widespread (Heshanggou Formation). Paleosols are present throughout the basin fill and record a transition from Aridisols and Vertisols in the Late Permian to Inceptisols in the Early Triassic followed by gleyed Aridisols, Vertisols, and Inceptisols later in the Early Triassic. Previously reported aeolian facies from Lower Triassic strata were not encountered in this study and were unlikely to have occurred given the prevalence of lacustrine and fluvial conditions at this time. Overall, the North China Basin experienced an increase in seasonal rainfall in the latest Permian and a base-level rise that introduced marine conditions into the southwest of the basin. During the Early Triassic, seasonal or interannual rainfall was frequent, with strong evaporation in a hot climate. Overall, substantial sedimentological changes occurred in the terrestrial Permo-Triassic environments of North China, but despite earlier claims from studies in North China and elsewhere, there was no abrupt transition in fluvial styles around the Permo-Triassic mass extinction.

Lithobiotopes of the Nemegt Gobi Basin1

Three distinct but overlapping dinosaur-dominated faunas characterize the Upper Cretaceous Djadokhta, Baruungoyot, and Nemegt formations of the Nemegt Basin of Mongolia. Documented faunal differences cannot be explained easily by temporal succession, but can be understood in light of the physical processes controlling the life, death, and burial of taxa. The stratigraphy of the Gobi Desert region records tectonically driven geometries, clearly documenting preservational processes that differ from those acting in most other dinosaur-dominated beds worldwide. Small, asymmetric tectonic grabens were filled with Upper Cretaceous dinosaur-bearing deposits showing asymmetric distributions of facies, here termed lithobiotopes. The water-lain fluvial and alluvial plain facies of the Nemegt lithobiotope supported and preserved a fauna dominated by gigantic dinosaurs, but had a preservational bias against smaller animals. The Nemegt passed laterally into the interdune facies of the Baruungoyot lithobiotope, which represented a hostile environment for large species, but preserved smaller animals. This in turn passed laterally into the aeolianite facies of the Djadokhta lithobiotope, which is characterized by the remains of small dinosaurs and a rich fauna of other animals. The Nemegt Gobi Basin can be visualized as an oasis with a central pond supplied with water from ephemeral channels and surrounded by a semi-arid alluvial plain and dune fields.

A Periglacial Palaeoenvionment in the Upper Carboniferous-Lower Permian Tobra Formation of the Salt Range, Pakistan

The Upper Carboniferous–Lower Permian (Upper Pennsylvanian–Asselian) Tobra Formation is exposed in the Salt and Trans Indus ranges of Pakistan. The formation exhibits an alluvial plain (alluvial fan–piedmont alluvial plain) fades association in the Salt Range and Khisor Range. In addition, a stream flow facies association is restricted to the eastern Salt Range. The alluvial plain facies association is comprised of clast-supported massive conglomerate (Gmc), diamictite (Dm) facies, and massive sandstone (Sm) lithofacies whereas the stream flow-dominated alluvial plain facies association includes fine-grained sandstone and siltstone (Fss), fining upwards pebbly sandstone (Sf), and massive mudstone (Fm) lithofacies. The lack of glacial signatures (particularly glacial grooves and striations) in the deposits in the Tobra Formation, which are, in contrast, present in their time-equivalent and palaeogeographically nearby strata of the Arabian peninsula, e.g. the Al Khlata Formation of Oman and Unayzah B member of the Saudi Arabia, suggests a pro- to periglacial, i.e. glaciofluvial depositional setting for the Tobra Formation. The sedimentology of the Tobra Formation attests that the Salt Range, Pakistan, occupied a palaeogeographic position just beyond the maximum glacial extent during Upper Pennsylvanian–Asselian time.

Biosedimentary record of postglacial coastal dynamics: high‐resolution sequence stratigraphy from the northern Tuscan coast (Italy)

Integrating analysis of the benthic palaeoecological record with multivariate ordination techniques represents a powerful synergy able to provide an improved characterization of coastal depositional facies in a sequence stratigraphical perspective. Through quantitative analysis of benthic foraminifer, ostracod and mollusc associations from the postglacial succession of Core M3 (Arno coastal plain, Tuscany, Italy), and application of detrended correspondence analysis (DCA) to the mollusc sub‐data set, we offer a refined picture of stratigraphical variations in faunal content from a paralic depositional setting, and reconstruct the palaeoenvironmental gradients that account for such variations. Despite distinct ecological behaviours, and taphonomic and sedimentological constraints, a strong ecological control on meio‐ and macrofaunal biofacies and taxa turnover is documented across the study succession. Amongst all possible mechanisms that may play a role in ‘shaping’ fossil distribution, the ecological signal driven by salinity represents the most prominent factor controlling the composition of fossil associations in the cored succession. Molluscs can even provide outstanding quantitative estimates of palaeosalinity along the sampled core. When plotted stratigraphically, the three fossil sub‐data sets show consistent patterns of vertical evolution that enable prompt identification of the key surfaces for sequence stratigraphical interpretation in otherwise lithologically indistinguishable deposits. The concomitant maximum richness of species with strong marine affinity, paralleled by the highest DCA salinity estimates, allows recognition of the maximum flooding zone, dated to ∼7.7 cal. ka BP, within a homogeneous succession of outer lagoon clays. These clays are sandwiched between early transgressive, swamp to inner lagoon deposits and overlying prograding coastal−alluvial plain facies.

Sedimentological, palynological and geochemical studies of the terrestrial Triassic–Jurassic boundary in northwestern Poland

AbstractThe Kamień Pomorski IG-1 borehole (Pomerania, NW Poland) yields a profile through the Triassic–Jurassic (T–J) transition in continental deposits. An integrated study of the sedimentology, sequence stratigraphy, palynology, biostratigraphy and geochemistry of these deposits has been carried out on the boundary interval, which represents a time of major environmental change. Two lithological units within the transitional section are distinguished: the Lower–Middle Rhaetian Wielichowo Beds of alluvial plain facies, which shows evidence of a semi-arid climate, and the Upper Rhaetian to Lower Hettangian Zagaje Formation, lying above a marked erosional sequence boundary, composed of mudstone-claystone and sandstone deposited in a fluvial-lacustrine environment. Carbon isotope values obtained from palynomaceral separates, and thus reflecting isotopic changes in atmospheric CO2, show significant fluctuations through the Rhaetian; the most conspicuous negative δ13Corgexcursion is correlated with the Rhaetian ‘initial’ excursion and shows two sub-peaks, pointing to short-term carbon-cycle disturbances of lesser magnitude. Above the ‘initial’ negative excursion, there is a positive excursion followed again by more negative values, representing subordinate fluctuation within a positive excursion and is correlated with the T–J boundary. Seventy-two miospore taxa have been determined from the studied T–J transitional section. Two major palynological assemblages have been distinguished: the lower one, typically Rhaetian, named theCingulizonates rhaeticus–Limbosporites lundblandiiassociation, which corresponds to theRhaetipollis–Ricciisporites(=Rhaetipollis–Limbosporites) Zone; and the upper one, typically Hettangian, named theConbaculatisporites mesozoicus– Dictyophyllidites mortoni–Cerebropollenites thiergartiiassociation (with the age-diagnostic pollenC.thiergartii), which corresponds to thePinuspollenites–Trachysporites(= Trachysporites–Heliosporites) Zone. The T–J palynofloral turnover occurred in a humid period and is more conspicuous then palynofloral changes observed in Greenland, the Tethyan domain or other parts of NE Europe. The osmium isotope system is studied herein for the first time from T–J continental deposits and shows marked disturbances similar to those measured in marine deposits and attributed to volcanic fallout. Carbon and osmium isotope correlation and coeval increase in polycyclic aromatic hydrocarbon (PAH) content and darkening of miospores confirm that eruptions of the Central Atlantic Magmatic Province (CAMP) contributed to the perturbances in climate and crisis in the terrestrial biosphere. A series of periodical atmospheric loading by CO2, CH4or alternatively by SO2, sulphate aerosols and toxic compounds is inferred to have caused a series of rapid climatic reversals, directly influencing the ecosystem and causing the Triassic floral crisis. A floral turnover period commenced at the ‘initial’ δ13C excursion, with the onset of CAMP volcanism. Obtained values of initial187Os/186Os between 2.905 and 4.873 and very low iridium content (about 5 ppt) lend no support to a role for an extraterrestrial impact at the T–J boundary event. The position of the ‘initial’ negative carbon isotope excursion about 12 m below the T–J boundary, position of sequence boundaries (emergence surfaces) and other isotope excursions allow reliable correlation with marine profiles, including St Audrie's Bay (UK), Csövár (Hungary) and the GSSP profile at Kuhjoch (Austria).

Probable palaeogeographic influences of the Lower Cretaceous Iberian rifting phase in the Eastern Cameros Basin (Spain) on dinosaur trackway orientations

The Cameros Basin sedimentary infill comprises a large, essentially continental, megasequence ranging from the Tithonian (Upper Jurassic) to the Albian (Lower Cretaceous). It occupies an area of some 5500 km 2, and is home to around 300 dinosaur tracksites. Most of these tracksites are found in either the Huérteles Formation, which is part of the Oncala Group (Berriasian), or the Enciso Group (Lower Aptian), both of which represent early Cretaceous lacustrine episodes. Dinosaur trackways (n = 1170) from both episodes were analysed in order to establish the preferred direction of dinosaur movement, and to determine whether these movements were influenced by the palaeogeographic and palaeoenvironmental conditions of the area. The Huérteles Formation is interpreted as a complex record of alluvial plain systems distally connected with a playa-lake. Its dinosaur tracksites are distributed throughout its alluvial plain facies and trackways show two preferential unidirectional orientations: 1) NW, more or less parallel to the distribution of the facies belt and 2) NNE. The Enciso Group is represented by a wide and shallow lacustrine system connected with marine environments towards the SE (Iberian Basin realm) and in close proximity to marine settings to the NW (Basque–Cantabrian Basin realm), rendering the Cameros Basin as the only continental connection between the Ebro and Iberian Massifs. The fluctuating (but always shallow) water level of the system with frequent desiccations probably allowed dinosaurs to pass through the lake basin. As a consequence of these paleogeographic restrictions, the dinosaur trackways generally show a bidirectional NE–SW orientation. Despite the temporal and geographical differences between the Huérteles Formation and Enciso Group, the ichnocenoses of both are dominated by theropod dinosaur trackways (85% as a mean value). This is probably explained by these dinosaurs being more active than others, a consequence of their searching/hunting behaviour.

Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao basin, northeast China

ABSTRACTThrough tectonostratigraphic analysis of the nonmarine, intracontinental Songliao basin in northeast China, four episodes of deformation are recognized: mantle upwelling, rift, postrift thermal subsidence and structural inversion. The episodes are related to regional geodynamics and plate motions. Each episode is associated with a specific stratigraphic signature. The first period of deformation occurred during the Middle and Late Jurassic when asthenospheric upwelling heated, thinned and stretched the lithosphere. These events may have been caused by the narrowing of the Okhotsk Sea through subduction. This deformation is characterized by doming, extension, widespread volcanism and intrusion, and erosion. Volcanics interfinger with alluvial fan and alluvial plain facies systems tracts. The second rifting episode began in the latest Jurassic and continued into the Early Cretaceous. It resulted in the formation of a large number of isolated, NNE‐trending fault blocks of ‘basin‐and‐range’ style. Rifting may have been caused by the formation and subduction of the Izanagi and Pacific Plates. Coal‐bearing fluvial, floodplain, lacustrine and fan‐delta strata and widespread volcanic rocks filled the fault‐block basins. Volcanic strata hundreds to several thousand meters thick in the Huoshiling and Yingcheng Formations record multiple intrusive events during the rifting stage in the basin. These events were concurrent with episodes of intrusion and volcanic eruption in northeast China. The third phase of regional postrift deformation and subsidence, which began with the Lower Cretaceous Denglouku Formation, was caused by lithospheric cooling and extension, modulated by multiple compressional events. Subsidence in the Songliao basin permitted accumulation of thick postrift deposits, in contrast with other Cretaceous basins in Mongolia and northeast Asia. Three compressional episodes, which episodically interrupted the long‐term cooling subsidence, originated from development of the Okhotsk suture and subduction of the Pacific plate. In the Early Cretaceous, pronounced compression originated from closure of Okhotsk Sea, forming the mountain ranges of Daxinganling, which provided sediment to the northern part of the basin. In the Late Cretaceous, the intensity of compression from the Pacific margin increased through time, causing westward migration of depocentres and uplift in the east until the end of Cretaceous. Postrift strata, typically 3000–4000 m thick with a maximum thickness of 6000 m, extend beyond the rift blocks and onlap the basin margins to form a large uniform basin. Early thermal subsidence strata include alluvial fan, fluvial, floodplain, shallow lacustrine and delta facies tracts, overlain by large deltaic and lacustrine facies. Late postrift environments featured by large lakes in the basin centre rimmed by delta, fluvial and floodplain environments. Reflection seismic profiles show that strong structural inversion, including folding and uplift, began at the end of the Nenjiang Formation and culminated at the end of the Cretaceous. The main compressional stress field changed from NW to NWW at the end of the Cretaceous, which caused changes in the direction of folding. During the rifting stage, several sets of gas‐bearing reservoirs formed in association with coal‐bearing source rocks in fluvial and deltaic strata with reservoir‐quality volcanics and fluvial and fan‐delta sandstone. Thick, widespread mudstone deposited in deep lakes during the postrift stage contains abundant organic matter, and is an important source rock and regional cap rock. Good reservoirs occur in deltaic and fluvial strata. Because of the episodic compressional events that alternately caused subsidence and uplift, reservoirs are sandwiched between source rocks, providing direct fluid communication from source to reservoir.

Braided rivers within an arid alluvial plain (example from the Lower Triassic, western German Basin): recognition criteria and expression of stratigraphic cycles

AbstractBased on a detailed sedimentological analysis of Lower Triassic continental deposits in the western Germanic sag Basin (i.e. the eastern part of the present‐day Paris Basin: the ‘Conglomérat basal’, ‘Grès vosgien’ and ‘Conglomérat principal’ Formations), three main depositional environments were identified: (i) braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits; (ii) marginal erg (i.e. braided rivers, aeolian dunes and aeolian sand‐sheets); and (iii) playa lake (an ephemeral lake environment with fluvial and aeolian sediments). Most of the time, aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity, such as wind‐worn pebbles (ventifacts), sand‐drift surfaces and aeolian sand‐sheets. In such arid continental environments, stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension. Elementary stratigraphic cycles, i.e. the genetic units, have been identified for the three main continental environments: the fluvial type, fluvial–aeolian type and fluvial/playa lake type. At the time scale of tens to hundreds of thousands of years, these high‐frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland. Lower Triassic deposits from the Germanic Basin are preserved mostly in endoreic basins. The central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the Hercynian (Variscan)–Appalachian Mountains. Consequently, a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition, whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments.

A Cordaixylon axis from well-drained alluvial plain facies in the Lower Pennsylvanian Joggins Formation of Nova Scotia

Plant remains showing preservation of cellular anatomy are rare in the Lower Pennsylvanian Joggins Formation of Nova Scotia. Here I report an anatomically preserved cordaitalean axis that shows endarch maturation and a sympodial vascular architecture. The specimen belongs to the morphogenus Cordaixylon, but in the absence of extraxylary tissue or attached fertile material, it cannot be assigned to a species. Together with a previously reported Mesoxylon axis with mesarch and non-sympodial vasculature, the new discovery demonstrates the existence of both major organizational types of cordaitalean at this locality. Previous reports have identified Cordaixylon as a plant that preferred peat mire environments. In this paper, the morphogenus is recorded from well-drained alluvial plain facies, thus extending knowledge of its ecological range.
 
 Résumé
 
 Les vestiges de plantes présentant une préservation de l’anatomie cellulaire sont rares à l’intérieur de la Formation du Pennsylvanien inférieur de Joggins, en Nouvelle-Écosse. Je fais part dans les présentes d’un axe de cordaitaléen natomiquement préservé qui affiche une maturation circulaire à partir de l’intérieur et une architecture vasculaire sympodiale. Le spécimen fait partie du morphogenre Cordaixylon, mais en l’absence de tissu extraxylaire ou de matière fertile y étant fixée, on ne peut pas le rattacher à une espèce donnée. La nouvelle découverte, conjuguée à un axe de Mesoxylon précédemment signalé qui comportait une vasculature non sympodiale à éléments en spirale internes, révèle l’existence des deux principaux types structuraux de cordaitaléens à cet emplacement. Des rapports antérieurs avaient défini le Cordaixylon en tant que plante préférant les environnements à bourbiers de tourbe. Le présent document fait état de l’observation du morphogenre d’un faciès de plaine alluviale bien drainée, ce qui étend notre connaissance de son aire de distribution écologique.

Record of palaeoenvironmental changes in a Lower Permian organic-rich lacustrine succession: Integrated sedimentological and geochemical study of the Rudník member, Krkonoše Piedmont Basin, Czech Republic

This study presents an integrated sedimentological and geochemical analysis of the Lower Permian Rudník member — the most extensive lacustrine deposits in the Krkonoše Piedmont Basin. Grey to black and variegated lacustrine mudstones, laminites and carbonates of the Rudník member have a lateral extent of more than 400 km 2. In the studied sections four facies associations were recognised: A) anoxic offshore, B) suboxic to oxic offshore, C) nearshore and mudflat, and D) slope deposits. In the northern part of the E–W elongated basin, anoxic to suboxic organic-rich offshore lacustrine facies dominate and form a succession up to 130 m thick. Fan-delta and turbidite facies occur locally along the faulted northern basin margin. The central part of the basin is occupied by anoxic to oxic offshore facies interfingering with nearshore carbonate and mudflat facies of the low-gradient lacustrine margin. In the central part of the basin, the thickness of the lacustrine deposits of the Rudník member reaches up to 60–70 m. In the southern part of the basin fluvial and alluvial plain facies dominate and alternate with minor lacustrine nearshore facies. The lateral facies distribution indicates that subsidence along the northern basin fault was the main mechanism generating the asymmetric infill geometry in the basin's half-graben setting. The δ 1 8O values of primary and early diagenetic calcite range between − 11.0‰ and + 1.3‰ (V-PDB) and δ 1 3C values between − 5.1‰ and + 3.7‰; most of the data fall within the range of freshwater limestones. Coarser-grained pure microspar laminae show more positive δ 1 3C values in comparison to clayey organic-rich micrite laminae, and are interpreted as a record of bioinduced precipitation during seasonal eutrophication. The obtained δ 1 3C TOC values range from − 29.0‰ to − 24.0‰, the total organic carbon (TOC) content from 0.26% to 23%. Maceral analysis and Rock-Eval pyrolysis indicate that most of the samples contain a mixture of aquatic and terrestrial organic matter, but two minor, distinctive groups of samples with algally-dominated and terrestrially-dominated organic matter composition, respectively, were also found. The study of vertical changes in boron content in the clay fraction of the lacustrine mudstones shows that high lake level stages were periods of lower salinity with the lowest boron contents (from 73 to 268 ppm), and periods of falling lake level were followed by significant increases in salinity with much higher boron values (293–603 ppm). Lake-level fluctuations of the Rudník lacustrine system, which are recorded by shallowing-up units of sedimentary facies within most of the sections throughout the basin, can also be traced within the monotonous black shale dominated sections, where no sedimentological evidence of these lake level changes exists. Good indicators for such changes seem to be the δ 1 8O and δ 1 3C values of primary calcite, δ 1 3C TOC and HI. These lake level fluctuations are interpreted as driven by climatic oscillations in the order of tens of thousands years, which could reflect climatic changes connected with the last glaciation event of Gondwana.

Key geological controls on migration and accumulation for hydrocarbons derived from mature source rocks in Qaidam Basin

The mature oil/gas source rocks exist mainly in Jurassic strata in the northwest and in Tertiary strata in the west of Qaidam Basin. Oil and gas generated from the mature hydrocarbon source rocks in the Jurassic and Tertiary mainly accumulate in Tertiary formations in the Qaidam Basin. The main sedimentary facies in Tertiary formations include alluvial plain, lakeshore, shallow lake, semi-deep lake, alluvial fans, fan delta and sublacustrine fan. However, there were no fluvial facies. Most of the alluvial plain facies and lakeshore facies are very narrow and they become wide only where basin-bounding faults are present. Most parts of the basin are filled with shallow lake sediments such as pelitic siltstone and silty sandstone. The deposition systems have determined the hydrocarbon migration and accumulation patterns in Tertiary formations in different parts of the basin. In the inner basin where continuous permeable formations are absent, faults and fractures are the main pathways. Fractured rocks are the main reservoir types. Along the basin margins where continuous permeable formations occur in abundance, faults and fractures are also the main pathways. Therefore, the faults and fractures are very important for the hydrocarbon migration and accumulation. Four migration and accumulation models have been developed based on this study: (1) Migration and accumulation model through thrust faults. This model is mainly for migration and accumulation at the basin margin. The mudrock associated with the major fault is about 10 m thick so that there exist the hanging wall and footwall migration and accumulation zones. Where the major fault extends into source rocks areas, hydrocarbon migrated to and accumulated in the reservoir in the hanging wall. Otherwise, the faults just seal the reservoir in the footwall. (2) Migration and accumulation model through combined syn-depositional reverse faults and strike-slip faults. This model is also mainly for migration and accumulation at basin margins in which the strike-slip faults control different kinds of sedimentary fans and connections with hydrocarbon source areas; they are very suitable for hydrocarbon accumulation. (3) Migration and accumulation model through recoil faults. This model is mainly for migration and accumulation in fractured reservoirs in the inner basin region. (4) Migration and accumulation model through superficial thrust fault and lower strike-slip fault complex. This is mainly for migration and accumulation in the north of Qaidam Basin where the reservoirs are at the footwall of the superficial thrust faults.

Sequence Stratigraphy of Upper Cretaceous Alluvial Plain Facies, Tuscaloosa Formation, Alabama: ABSTRACT

Sequence Stratigraphy of Upper Cretaceous Alluvial Plain Facies, Tuscaloosa Formation, Alabama: ABSTRACT

Middle Tertiary Extension Recorded by Lacustrine Fan-Delta Deposits, Plush Ranch Basin, Western Transverse Ranges, California

ABSTRACT The Plush Ranch Formation (upper Oligocene and lower Miocene) consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the Plush Ranch basin. Distinctive depositional facies, provenance, and sediment transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half-graben origin. The northern basin margin consists of sandstone-dominated alluvial-plain deposits (0.1-1.5 m thick, normally graded, lenticular sandstone beds). Small deltaic sequences 1-2 m thick were formed where these alluvial systems flowed southward into a lake. Lenses of massive, boulder-rich granitic breccia that represent rockslide deposits derived from a nearby northern granitic provenance nterfinger with the alluvial-plain facies. In contrast to the northern margin, the southern basin margin is represented by coarse-grained fan-delta deposits. Matrix- and clast-supported lenticular conglomerate beds 0.2-5 m thick with interbedded trough-cross-bedded pebbly sandstone represent braided-stream and flood-flow and/or noncohesive debris-flow deposits of alluvial fans that drained a highland area to the south. The alluvial-fan deposits interfinger to the north with several types of subaqueous sediment-gravity-flow facies including turbidite sandstone beds and matrix-supported debris-flow conglomerate. Each of the basin-margin depositional systems grades basinward and to the east into lacustrine deposits that include organic-rich dark shale, evaporite, and limestone. The lacustrine deposits represent the central and eastern parts of the Plush Ranch basin, which received little coarse siliciclastic sediment. Basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies. The southern margin of the Plush Ranch basin formed along a north-dipping, normal-slip fault along which dip separation increased toward the southwest; the northern margin developed on the tilted hanging-wall block of this fault. This fault was later reactivated in post-middle Miocene time as the present left-lateral strike-slip Big Pine fault. The Plush Ranch is one of several extensional and transtensional basins that formed in southern California and western Arizona about 25-20 Ma as a response to the change from a convergent to a strikeslip tectonic regime along western North America.

Genetic stratigraphy and depositional systems of the upper Devonian-lower Mississippian Price-Rockwell deltaic complex in the central Appalachians, U.S.A.

Rocks of the Price and Rockwell formations comprise the uppermost Devonian-Lower Mississippian deltaic complex in the central Appalachians. The Price Formation is dominantly marine and refers to all uppermost Devonian-Lower Mississippian rocks above either Hampshire Group red beds or the marine Greenland Gap Formation and below either the Greenbrier Limestone Group or Maccrady Formation red beds in the central Appalachians. The Rockwell Formation is dominantly non-marine and refers to equivalent rocks in south-central Pennsylvania and the Maryland-West Virginia panhandle region. Members of the Price Formation are generally equivalent to discrete genetic events. Price deposition was initiated in latest Devonian time when a regional transgression covered delta and alluvial plain facies of the Catskill deltaic complex with protected-bay and strandline facies of the Oswayo Member in West Virginia, Pennsylvania and western Maryland. Farther south, these transgressive facies overlie the Cloyd Conglomerate Member, a thin regressive-transgressive sedimentary wedge, and occur in the lower part of the Sunbury Shale Member. Subsurface Berea Sandstone deposition corresponded to regional exposure and erosion in the outcrop belt. This unconformable surface was overstepped in earliest Mississippian time by a major transgression that deposited a variably thick but persistent basal transgressive sandstone during shoreface retreat. Sandstones that have been referred to the “Cussewago Sandstone” in northern West Virginia and southwest Pennsylvania are part of this basal sandstone. Eastward, these sandstones thicken, coarsen and interbed with a polymictic diamictite at the base of the Rockwell Formation. Shaly facies of the earliest Mississippian transgression overlie the basal sandstone. The Riddlesburg Shale preserves offshore and open-bay facies in West Virginia that grade eastward to estuarine and restricted, back-bar lagoon and marsh facies of a barred-bay complex in south-central Pennsylvania and the Maryland-West Virginia panhandle. From central West Virginia, Riddlesburg facies grade southward into basinal facies of the Sunbury Shale Member. Syndepositionally active basement features, the West Virginia Dome and “Tri-State Block”, were structurally positive areas in the foreland basin. The “Tri-State Block” caused truncation of Lower Mississippian marine facies in western Maryland and large-scale westward erosional overlap of channel-fill sandstones in the upper Rockwell Formation. The West Virginia Dome was also structurally positive and the basal transgressive sandstone contains phosphatic omission surfaces on the dome crest. Lack of subsidence caused scour and incision of Riddlesburg facies by overlying deltaic facies. Progradation ensued with fluvial-dominant, shallow-water deltaic systems (Price Rockwell Member) in northern West Virginia and marine-dominant deep-water deltaic facies farther south. A 150 m minimum paleodepth estimate in the foreland basin is based on stratigraphic thickness in southern West Virginia. Following a transgression near the Kinderhookian-Osagean boundary. Price marine-deltaic facies form a coarsening-upward sequence to delta plain coals and coastal-alluvial plain red beds. A 35 m estimate for average storm wave base utilized tempestite proximal-distal trends.

Dating of Malaysian fluvial tin placers

The richest tin placers in Malaysia—fluviatile and piedmont fan placers—formed mainly within the “Boulder Beds” (BB fan facies), “Old Alluvium” (OA alluvial plain facies) and “Transitional Unit” (TU) which are regional lithostratigraphic units that can be correlated throughout western Peninsular Malaysia. Palaeomagnetic studies show that the TU was deposited during the early part of the Brunhes Normal Polarity Epoch (0–0.73 Ma) whereas the OA and BB mainly formed during the Matuyama Reversed Epoch (0.73–2.48 Ma). Present environments are mostly unfavorable for tin placer formation. Economic placers are generally covered on land by Young Alluvium (YA) overburden or are submerged offshore. As many of the rich OA/BB near-source placers are now largely worked out, the transported TU fluvial placers are becoming more important economically. The latest Pliocene to Mid Pleistocene period represented the principal phase of economic tin placer formation. Evidence from palaeomagnetism and the stratigraphic context of placers, and the relationship between sea level change, climatic change and placer genesis, suggest that the bulk of OA placers formed during a Lower Pleistocene interglacial period. A Mid Pleistocene age for TU placers is indicated by vertebrate fossils, palaeomagnetism, and their stratigraphic position below Upper Quaternary YA and above OA which is older than 0.73 Ma.

Tectonic Setting and Depositional Environments of Hanna Formation, South-Central Wyoming: ABSTRACT

The Paleocene Hanna Formation was deposited during tectonic events that led ultimately to the development of the Hanna and Carbon structural basins of south-central Wyoming. Early Paleocene uplift prior to deposition of the Hanna Formation resulted in a regional unconformity as observed in the Carbon basin, on the east side of the Hanna basin, and in the area south of these two basins. Subsequent movement of the thrust-fault system on the north side of the present Hanna basin resulted in southward filling of this asymmetrical foreland basin by clastic wedges, which make up facies associations. The Hanna Formation deposited in the Carbon basin has the same facies associations that occur in the southern part of the Hanna basin. Facies associations in the Hanna Formation of the Hanna basin suggest deposition in alluvial-fan and alluvial-plain environments. The alluvial-fan facies can be subdivided into proximal, medial, and distal subfacies. The proximal and upper-medial subfacies, which have been eroded away on the north side of the Hanna basin, probably consisted of medium to coarse gravels and muds deposited by debris flow and sheet floods. The lower-medial subfacies consists of tongues of conglomerate interbedded with mudstones and was probably deposited by sheet floods. This subfacies grades laterally southward into the distal conglomeratic sandstones and gray mudstones that were deposited by braided streams and sheet floods. In the Hanna basin, these distal-fan subfacies grade southward into an alluvial plain facies. In the area of the Carbon basin, all the fan facies are isolated in the Medicine Bow Mountains and associated mountain front, and only the alluvial-plain facies occur in the basin. The alluvial-plain facies in the Hanna Formation can be divided into two subfacies on the basis of thickness and the occurrence of coal and carbonaceous shale. One subfacies consists of overbank and backwater-deposited gray shale and claystone and splay sandstone and siltstone interbedded with backswamp deposits of thin carbonaceous shales and thin coals. These deposits lie laterally from sandstone-filled channel systems. In the Hanna basin this subfacies is thickest near the base of the Hanna Formation. The other subfacies is similar in lithology but contains more backswamp deposits of carbonaceous shale and coal beds. Stacked channel sandstones are also more common in this subfacies. This subfacies is thickest in the central part of the Hanna basin, and makes up most of the alluvial plain strata in the Carbon basin. Analysis of the alluvial-plain sequences in Hanna basin is hampered by poor outcrops and a lack of subsurface correlation. Complete analysis of the coal-bearing subfacies has been possible in the Carbon basin where a stratigraphic framework has been established. From this stratigraphic framework, a sequential strata model has been constructed. Analysis of several coal-bed-bounded sequences in the Carbon basin confirms the sandstone domination of the sequences as shown by the sequential strata model. Also, mapped southeasterly trends of the sandstone bodies indicate the fluvial channel systems of the Hanna basin were continuous into the Carbon basin. End_of_Article - Last_Page 1340------------

Braided fluvial sedimentation in the lower paleozoic cape basin, South Africa

Lower Paleozoic braided stream deposits from the Piekenier Formation in the Cape Province, South Africa, provide information on lateral and vertical facies variability in an alluvial plain complex influenced by a moderate to high runoff. Four braided stream facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures. A lower facies, dominated by upward-fining conglomerate to sandstone and mudstone channel fill sequences, is interpreted as a middle to lower alluvial plain deposit with significant suspended load sedimentation in areas of moderate to low gradients. These deposits are succeeded by longitudinal conglomerate bars which are attributed to middle to upper alluvial plain sedimentation with steeper gradients. This facies is in turn overlain by braid bar complexes of large-scale transverse to linguoid dunes consisting of coarse-grained pebbly sandstones with conglomerate lenses. These bar complexes are compared with environments of the Recent Platte River. They represent a middle to lower alluvial plain facies with moderate gradients and no significant suspended load sedimentation or vegetation to stabilize channels. These bar complexes interfinger basinward with plane bedded medium to coarse-grained sandstones interpreted as sheet flood deposits over the distal portions of an alluvial plain with low gradients and lacking fine-grained detritus or vegetation.

Mauch Chunk Alluvial Plain and Mud-Flat Sediments in Northern West Virginia: ABSTRACT

Regional subsurface analysis of red beds of the Mauch Chunk Group (Upper Mississippian) in northern West Virginia, using oil and gas well logs and cuttings, suggests that deposition was in alluvial-plain environments grading basinward into mud flats. Thickest net sandstone in the Mauch Chunk Group is in Barbour, Tucker, and Preston Counties in a belt 10 to 15 mi (16 to 20 km) wide striking north-northeast parallel with and just west of major fold axes of the Appalachian Plateau. This belt comprises numerous dip-oriented (west to northwest), vertically stacked, anastomosing subbelts and dendroids. Belt position reflects a gradient change from what were higher elevations on the east onto more level and tectonically stable areas of northwestern West Virginia. Lithofacies interpreted for the area include (1) channel fill-levee deposits as gray-green sandstone, siltstone, or non-red shale, and (2) flood-basin (overbank-levee) sediments as red and green shale. Streams carried a large suspended load and very fine to fine-grained sand. To the northwest, sandstone percentage decreases, and alluvial-plain facies interfinger with mud flats. Distal mud facies include laterally persistent limestone beds, and tidal-channel units with massive sandstone fill. The overall genetic aspect of Mauch Chunk stratigraphy is a general regressive facies shift to the northwest. The boundary between Mauch Chunk red beds and coarse clastic alluvial sediments of the overlying Pottsville Group reflects changes in gradient, supply, and source area relief. End_of_Article - Last_Page 1586------------