Fining-upward Succession Research Articles

Sedimentology, palaeontology and palaeo-environments of the Middle (?) to Upper Permian Emakwezini Formation (Karoo Supergroup, South Africa)

The little-known Emakwezini Formation (Adelaide Subgroup, Beaufort Group, Karoo Supergroup) crops out in a narrow strip just inland of the eastern coast of South Africa in northern KwaZulu-Natal. It is an actively mined coal-bearing succession characterised by fining-upward successions of coarse- to fine-grained sandstones together with mudstones containing a fossil assemblage of animal traces, insects, molluscs, arthropods, fish and plants. Previously, only Glossopteris leaves and Phyllotheca australis had been described from this formation, on the basis of a few very small, scattered and poorly provenanced collections. Recent regional-scale sedimentary facies analysis (based on field relationships, provenance studies, palaeocurrent and subsurface data) together with palaeobotanical studies of a newly discovered, well-preserved and diverse palaeoflora from the Emakwezini Formation, have permitted a more detailed interpretation of the depositional environment. The current investigation also reveals the first evidence of the plant fossils Dictyopteridium flabellatum , Rigbya arberioides , Lidgettonia spp., and Trizygia speciosa in the upper parts of the Emakwezini Formation, strongly supporting a Late Permian age based on correlation with floras from Upper Permian units in the main Karoo Basin. In the lower part of the unit, within the partings of actively mined coal seams, our preliminary investigations also show the presence of a new flora including both the glossopterid fructification Ottokaria sp. (only known from the Lower Permian in South Africa) and the sphenopsid Schizoneura gondwanensis (typical of the Upper Permian in South Africa). With sediments sourced from a north-easterly continental interior, the Emakwezini Formation was deposited rapidly in a fluviolacustrine setting under moist conditions capable of supporting an abundant and diverse biota. The permanently moist environment in which the Emakwezini Formation was deposited contrasts with that of the time-equivalent lower Beaufort Group units in the main Karoo Basin where environmental indicators suggest deltaic, lacustrine and meandering fluvial systems developed in a seasonally dry and more arid setting. Thus, within southern Africa, during the Middle to Late Permian, the environmental conditions, including climate, were varied leading to a mosaic of continental depositional settings.

Late Quaternary palaeoenvironmental evolution of the Adriatic coastal plain and the onset of Po River Delta

Despite abundant geologic literature about the Po coastal plain, stratigraphic architecture beneath modern Po River Delta is virtually unknown. In this paper, combined sedimentological, micropalaeontological and geochemical investigations of three continuous cores (cores 1 to 3), about 40 m thick, enable for the first time the detailed reconstruction of subsurface stratigraphy in the Po River Delta area, along with its late Quaternary palaeoenvironmental evolution. Lowstand deposits, formed during the Last Glacial Maximum, are expressed as a fining-upward succession of fluvial-channel and related overbank deposits, capped by a stiff, overconsolidated horizon. Above this unconformable surface, the lower post-glacial succession (transgressive systems tract or TST) is less than 7 m thick and includes a deepening-upward succession of back-barrier, shoreface and inner-shelf deposits. On the other hand, the overlying highstand systems tract is considerably thicker (up to 27 m) and comprises the bulk of recent delta progradation ( i.e., vertically stacked prodelta, delta front and delta-plain deposits). Stratigraphic correlations within early transgressive, back-barrier deposits show vertically stacked, thin shallowing-upward successions, reflecting an internal subdivision of TST into small-scale parasequences. The homogeneous shallow-marine clay that accumulated at the turnaround from transgressive to highstand conditions can be palaeontologically differentiated into two packages ( Miliolidae-dominated and Ammonia/Cribroelphidium-dominated), reflecting open-marine and river-influenced conditions, respectively. Timing of maximum marine ingression (maximum flooding surface-MFS) is recorded within the open-marine clays at about 5500 calibrated yr BP by minor changes in microfauna. Geochemical characterization of Core 1 testifies that distinct changes in sediment provenance accompanied the palaeoenvironmental evolution of the study area under changing relative sea-level conditions. At lowstand times, the study area was an alluvial plain under the influence of Adige River, as suggested by comparatively high Ba/Al values within pre-Holocene fluvial deposits. Increasing upward values of Cr/Al 2O 3 and Ni/Mg within TST suggest an increasing sediment provenance from the Po River during early stages of transgression, via the longshore drift. The onset of modern-age Po Delta in the study area is precisely identified in Core 1 at 25.10 m depth, and has a distinctive, double signature. In terms of microfaunal assemblages, Po River sediments are clearly marked by the development of a characteristic foraminiferal association dominated by the opportunistic species Nonionella turgida, a taxon typically abundant in the marginal zone of present northern Adriatic Sea mud-belt. Po River deposits are also fingerprinted by a peculiar geochemical composition, corresponding to maximum Cr and Ni concentrations.

Sedimentary facies, processes and environments of the Akakura caldera lake, the South Kurikoma geothermal area, northeast Japan

The Pleistocene Kan-nodai Formation was deposited in the collapse basin formed by felsic volcanism of the Akakura caldera, South Kurikoma geothermal area, northeast Japan. The middle and upper members of the Kan-nodai Formation represent postcaldera lacustrine slope apron and basin plain deposits of the Akakura caldera. The slope apron deposits consist mainly of interbedded sandstone and mudstone and massive tuff breccia deposited from turbidity currents and debris flows that resulted from slope failures of the unstable caldera wall. The beds are laterally continuous and show neither systematic vertical depositional patterns nor distinctive erosional features. Slump beds are common, indicating slope instability. The basin plain deposits overlie the slope apron deposits and are dominated by fine-grained turbidites and debris flow deposits, and suspension fallout deposits, indicating that coarse-sediment supply is not affected. Plant fossils are abundant. The slope apron and basin plain deposits include voluminous graded pumice lapilli tuff. A fining-upward succession from the slope apron to the basin plain deposits reflects retreat of the caldera wall due to slope failures, and rise of lake level with increasing water volume within the caldera basin. The lake deposits record volcanic activities of the felsic pyroclastic eruption in the area east of the lake and the postcaldera andesitic effusive eruption in the southeastern part of the caldera.

Climatic and Tectonic Signature in the Fluvial Infill of a Late Pliocene Valley (Siena Basin, Northern Apennines, Italy)

Abstract Incised valleys entirely filled with fluvial deposits are rarely described in the literature, and even rarer are accounts of incised valleys whose filling is not driven by relative sea-level changes. Valleys of this type have not been widely recognized in the stratigraphic record, because they are likely to occur in areas undergoing intense erosion or to be encased within alluvial deposits with no strong lithologic contrast. In this paper, an upper Pliocene fluvial valley fill, encased within shallow marine middle Pliocene deposits and pre-Neogene bedrock (eastern Siena Basin, Northern Apennines, Italy) and accumulated in response to tectonics and climate changes, is described. The recognition of this fluvial body (up to 40 m thick) in an incised-valley setting is based on its overall geometry, lithofacies characteristics, gravel composition, and fossil content. Upvalley fill is represented by amalgamated gravels emplaced by heavily sediment-laden flows, whereas downvalley fill shows a more organized depositional style and consists of two fining-upward successions. Notable is the presence at the top of the lower succession of decameter-thick floodplain fines, which extend from one valley wall to another and have no fine-grained correlatives in the upvalley fill. Valley incision resulted from a drop in relative sea level, arising from late middle Pliocene regional uplift. Stratigraphic and paleontological data constrain valley filling to the late Pliocene–early Pleistocene time span. Such a filling stemmed from an increase in sediment supply, which resulted from the interplay between uplift of the Chianti Ridge and a climatic change toward humid conditions, as recorded in the coeval strata of the adjacent Valdarno Basin. Beyond the regional significance, an important implication of the case study is that the incision and filling factors need not be genetically related. Moreover, the stratigraphic architecture of the studied fluvial fill is discussed in terms of the relations between sediment supply and sediment storage en route from the source area to the depositional site and their association with tectonic movements. Specifically, the valley-wide floodplain fines are thought to record the response of the fluvial system to a tectonic rejuvenation of catchments, which modified the valley gradient and promoted upstream gravel storage, whereas the overlying gravels manifest the reestablishment of a new equilibrium river profile.

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

Abstract This paper summarizes the results of completed and ongoing research in three areas of the Basin and Range Province of the western United States that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated. Given that these examples were influential in the development of ideas about low-angle normal faults, and particularly in making the case for frictional slip at dips of appreciably less than the 30° lock-up angle for μ ≈ 0.6 (where μ is the coefficient of friction), we advocate a critical re-examination of interpreted detachments elsewhere in the Basin and Range Province and in other extensional and passive margin settings. The Sevier Desert ‘detachment’ of west-central Utah is reinterpreted as a Palaeogene unconformity that has been traced to depth west of the northern Sevier Desert basin along an unrelated seismic reflection (most probably a splay of the Cretaceous-age Pavant thrust). The absence of evidence in well cuttings and cores for either brittle deformation (above) or ductile deformation (below) is inconsistent with the existence of a fault with as much as 40 km of displacement. The Pavant thrust and the structurally higher Canyon Range thrust are erosionally truncated at the western margin of the southern Sevier Desert basin, and are not offset by the ‘detachment’ in the manner assumed by those inferring large extension across the basin. The Mormon Peak detachment of SE Nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction. A particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation, with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks. The Castle Cliff detachment in the nearby Beaver Dam Mountains of SW Utah is similarly regarded as a surficial feature, as originally interpreted, and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin. The middle Miocene Eagle Mountain Formation of eastern California, interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ESE from a location close to the Jurassic-age Hunter Mountain batholith of the Cottonwood Mountains, is reinterpreted as having accumulated in a fluvial–lacustrine rather than alluvial fan–lacustrine setting, with no bearing on either the amount or direction of tectonic transport. The conglomeratic rocks upon which the provenance argument was based are pervasively channelized, with erosional relief of less than 1 m to as much as 15 m, fining-upwards successions at the same scale and abundant trough cross-stratification – all characteristic features of fluvial sedimentation and not of alluvial fans. The interpretation of the Eagle Mountain Formation as having been deposited within a few kilometres of the Hunter Mountain batholith, which depends strongly on assumptions about the dimensions of alluvial fans, is therefore not required. The result is important because the Eagle Mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of California, and for the existence of detachment faults of regional dimensions.

Pleistocene macrotidal tide-dominated estuary–delta succession, along the Dong Nai River, southern Vietnam

This paper focuses on the facies and sequence stratigraphic interpretation of a tide-dominated incised-valley system that is rare among geological records of the world for its completeness. The subject is the Middle–Upper Pleistocene Ba Mieu and Thu Duc formations exposed along the Dong Nai River, southern Vietnam. Both the Ba Mieu and the Thu Duc formations are interpreted as a transgressive tide-dominated estuary succession overlain by a highstand tide-dominated delta succession. The tide-dominated estuary succession is generalized as follows, in ascending order: fluvial facies in landward portions, tidal flat, upward-coarsening and -thickening succession of marine sands with a tidal ravinement surface (TRS; created by tidal scouring during transgression) at the base, and subtidal lag in seaward portions. This tide-dominated estuary is distinguished from wave-dominated and wave- and tide-dominated estuaries by (1) the lack of a central basin (lagoon), tidal delta, or bayhead delta facies; (2) the widespread presence of marine sands; (3) the presence of a subtidal lag; and (4) the presence of an upper-flow-regime sand flat facies. The tide-dominated delta succession is generalized as follows, in ascending order: upward-convex unit of marine sand and onlapping tidal flat deposits, a thick upward-fining succession of tidal flat–salt marsh deposits, and coarse fluvial deposits. This delta is distinguished from an estuary as follows. (1) It shows thick, regional, upward-fining successions indicating shallowing, whereas upward-coarsening successions indicating deepening are recognized in an estuary. A maximum flooding surface (MxFS) is recognized in the coarsest part of the succession, in marine sand and tidal flat deposits. (2) Fluvial processes dominate the sediment supply, in contrast to an estuary, in which a thick marine sand body fed from marine sources is dominant. (3) Ebb tidal currents are dominant, whereas flood tidal currents are dominant in an estuary. Key features of the incised-valley system are (1) amplification of the incised-valley geometry by tidal ravinement erosion; (2) development of tidal channels filled with mud clasts in the estuary; (3) a thick marine sand body in the estuary; and (4) upward-convex unit of marine sand and a thick tidal flat–fluvial succession in the delta. The Ba Mieu Formation is recognized as depositional sequence (SQ) 1 between sequence boundary (SB) 1 below and SB2 above. SQ1 consists of transgressive systems tract (TST) 1 and highstand systems tract (HST) 1. SQ1 was formed by a transgression–regression cycle during marine isotope stage (MIS) 7. SB2 was formed by fluvial incision during the MIS 6 lowstand and is amalgamated with the TRS of MIS 5. The Thu Duc Formation is recognized as SQ2 between SB2 below and SB3 above, and it consists of TST2 and HST2. SQ2 was formed by a major transgression–regression cycle on which a lower-rank cyclicity was superposed during MIS 5. TST2 thins seaward as a result of tidal ravinement erosion. The landward extent of marine environment in MIS 5 is inferred to have been of greater scale than that in MIS 7 from the distribution of the facies and the TRS.

An unusual fluvial to tidal transition in the mesoarchean Moodies Group, South Africa: A response to high tidal range and active tectonics

The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels. The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.

Sedimentology and stratigraphy of a thick, areally extensive fluvial-marine transition, Missisauga Formation, offshore Nova Scotia, and its correlation with shelf margin and slope strata

Abstract A 100 m thick, more than 20 km wide upward-fining succession at the top of the Lower Cretaceous Upper Member of the Missisauga Formation was investigated in the Panuke Field, offshore Nova Scotia, using core, well logs and 3-D seismic data. The succession consists of 50 m of dune cross-stratified sandstone overlain by 50 m of tide-influenced heterolithic strata, which in turn is overlain by 150 m of mudstone of the Naskapi Member of the Logan Canyon Formation. The succession is interpreted to be a fluvial–marine transition formed during a long-term (3rd order) relative sea-level rise. The two main Panuke reservoirs are thin (<5 m) sheet-like sandstones at the Missisauga–Naskapi contact interpreted to be remnants of a wave-formed barrier system. In seismic data, the Upper Member of the Missisauga Formation correlates basinward with progradational shelf-margin reflections, suggesting that the sheet-like fluvial sandstone at the base of the upward-fining succession transferred sediment, and possibly sand, to the continental slope.

How do we identify big rivers? And how big is big?

“Big rivers” are the trunk rivers that carry the water and sediment load from major orogens, or that drain large areas of a continent. Identifying such rivers in the ancient record is a challenge. Some guidance may be provided by tectonic setting and sedimentological evidence, including the scale of architectural elements, and clues from provenance studies, but such data are not infallible guides to river magnitude. The scale of depositional elements is the most obvious clue to channel size, but evidence is typically sparse and inadequate, and may be misleading. For example, thick fining-upward successions may be tectonic cyclothems. Two examples of the analysis of large ancient river systems are discussed here in order to highlight problems of methodology and interpretation. The Hawkesbury Sandstone (Triassic) of the Sydney Basin, Australia, is commonly cited as the deposit of a large river, on the basis of abundant very large-scale crossbedding. An examination of very large outcrops of this unit, including a coastal cliff section 6 km long near Sydney, showed that even with 100% exposure there are ambiguities in the determination of channel scale. It was concluded in this case that the channel dimensions of the Hawkesbury rivers were about half the size of the modern Brahmaputra River. The tectonic setting of a major ancient fluvial system is commonly not a useful clue to river scale. The Hawkesbury Sandstone is a system draining transversely from a cratonic source into a foreland basin, whereas most large rivers in foreland basins flow axially and are derived mainly from the orogenic uplifts (e.g., the large tidally influenced rivers of the Athabasca Oil Sands, Alberta). Epeirogenic tilting of a continent by the dynamic topography process may generate drainages in unexpected directions. For example, analyses of detrital zircons in Upper Paleozoic–Mesozoic nonmarine successions in the SW United States suggests significant derivation from the Appalachian orogen. Many large rivers in SE Asia flow along structural trends generated during the Himalayan orogeny (e.g., Brahmaputra/Jamuna, Irrawaddy, Mekong, Red), but evidence for large channel systems is not always present where it might be expected. For example, in the Gulf of Thailand, up to 8 km of Cenozoic nonmarine sediments have been recorded in fault-bounded basins, but seismic time-slice images of the Pliocene–Holocene section indicate rivers of modest dimensions. The existing major trunk river, the Chao Phraya, which empties into the gulf at Bangkok, may be but a remnant of the Mekong, which was tectonically diverted to its present course at some point during the late Cenozoic.

男鹿半島から見いだされた２０Ｍａ以前の海成堆積物

This paper describes occurrence of marine sediments and associated trace fossils at Shiose-no-Misaki (Cape), Oga Peninsula, NE Japan. The marine sediments constitute an upward-fining succession from sandstone to mudstone, and contain many trace fossils characteristic of the Skolithos ichnofacies. They are sorted well, and the sandstone bed at the middle of the upward-fining succession shows wavy stratification indicative of a high wave-energy. Though the stratigraphic position remains ambiguous, the Shiose-no-Misaki sediments are intruded by a dolerite of 20 Ma and most likely correlated with the late Eocene to Oligocene Monzen Formation. They represent the Oligocene to early Miocene marine sediments that occur along the eastern margin of the Japan Sea from southern Sakhalin Island to Kyushu Island, and likely support the presence of an incipient back-arc rift basin produced precedent to the early to middle Miocene rapid spreading of the Japan Sea.

Miocene sedimentary evolution of the NE–SW-trending Selendi and Gördes Basins, W Turkey: implications for extensional processes

Western Turkey is one of the classic regions for the study of extensional processes. In a recent interpretation, the regional metamorphic basement, the Menderes Massif, was exhumed related to large-scale extensional processes, possibly during latest Oligocene time. Regional-scale NE–SW-trending basins, including the Gördes and Selendi basins, were created, associated with this exhumation. The NE–SW-trending basins are seen as regional-scale “corrugations” (10–15 km across) in the surface of an extensional detachment orientated parallel to the direction of extension. The detached upper plate is represented by erosional remnants of unmetamorphosed Tethyan ophiolitic melange. We describe and interpret the Gördes and Selendi basins separately and then show that their sedimentary facies and evolution are very similar. Basal coarse alluvial fan deposits accumulated after extensional removal of the upper plate. These basal sediments, rarely exposed, were rotated by up to 50°, probably as a result of local extension or transtension. Later during the early Miocene, the Gördes and Selendi basins were infilled by a generally fining-upward succession derived from the south, based on palaeocurrent evidence. Coarse alluvial fan deposition was followed by the deposition of finer-grained alluvial deposits from braided streams flowing northwards over a topographically subdued flood plain. Later, deposition was within shallow ephemeral lakes after a possible change to a moister climate. Calc–alkaline magmatism was initiated with the fall-out of fine-grained silicic ash, followed by coarser-grained tuff derived from local magmatic centres. During early Pliocene time, the basins were dissected by E–W extensional faults, associated with the establishment of the modern pattern of westward drainage to the Aegean Sea. We use the assembled sedimentary evidence to test alternative tectonic models for Miocene basin formation in western Turkey. Our preferred tectonic model emphasises the importance of deposition in regional basins created by exhumation of metamorphic basement that was previously assembled during early Tertiary closure of the Tethys ocean.

Cycles in Perilacustrine Facies of Late Mesozoic Rift Basins, Southeastern Mongolia

Detailed study of 365 meters of core recovered from southeastern Mongolia reveals fluvial, marginal lacustrine, and profundal lacustrine depositional environments in an Early Cretaceous rift basin. Multiple examples of cyclic sedimentation are superposed on these large-scale facies transitions. In fluvial facies, more than 20 upward-fining successions (2–10 m thick) are present. These contain cross-stratified sandstone units, with erosional bases, that fine upward to mudstone with root marks and burrows. They are interpreted as channel-fill successions in a fluvial depositional environment, whereas finergrained, less erosional intervals with abundant soft-sediment deformation and climbing ripples may represent a transition to fluvial-deltaic environments at the lake margin. Coarse fluvial-deltaic facies form the most likely hydrocarbon reservoirs, as indicated by frequency of oil staining in the core. Physical transport processes dominate well-oxygenated fluvial and marginal lacustrine facies, whereas cycles in more distal lacustrine facies mainly reflect biogeochemical processes. Profundal lacustrine strata include kerogen-rich laminated micrite, distal turbidite beds, and dolomitic breccia reminiscent of lacustrine facies in the Green River Formation of the western United States. Evidence for anoxia at the lake floor includes framboidal (biogenic) pyrite, high TOC content, mainly light amorphous organic constituents, and paucity of trace fossils relative to the rest of the core. Approximately 25 dolomite layers are interbedded with laminated micrite and calcareous siltstone units in poorly defined meter-scale cycles. These beds typically have brecciated or clotted textures, and suggest evaporative concentration of magnesium ions at the lake floor. The profundal lacustrine sequence displays a distinctive high-resistivity, low spontaneous-potential log character that is observed in well logs from several fields in the Zuunbayan subbasin. Correlative units can be traced at least 60 km to the northeast and southwest of the Zuunbayan field, indicating that this is a widespread lacustrine depositional unit. These lacustrine strata include the most likely petroleum source facies found to date, and are likely contemporaneous with other large lake systems in China and Mongolia.

Sequence Stratigraphy of the Desert System: A Case Study of the Lower Cretaceous in the Kuqa Basin in Xinjiang, Northwestern China

Abstract The Lower Cretaceous strata in the Kuqa Basin in Xinjiang are marked by a set of arid red beds. Several types of sedimentary facies can be identified in this set of arid red beds: mudstones of the plaza and intracontinental sebkha, aeolian sandstones, sandy conglomerates of the intermittent river, conglomerates of the pluvial fan, etc. These types of sedimentary facies constitute a typical desert system. Therefore, the Cretaceous strata in the Kuqa Basin provide a favorable condition for studies of sequence stratigraphic divisions of the desert system. With the rise and fall of the base level of the sedimentary basin, cyclicity is clearly revealed in stratigraphic records, which helps the identification of the third‐order sequences. Based on the cyclicity in stratigraphic records, 5 third‐order sequences can be found in the strata of the Early Cretaceous in the Kuqa Basin. These sequences comprise a second‐order tectonic sequence. The primary feature of these third‐order sequences is of an upward‐fining sedimentary succession formed by a succession of “coarse sediments of the alluvial system‐fine sediments of the lake system”. The result of this study shows that aeolian sandstones are the best reservoirs of natural gas in the Cretaceous strata in the Kuqa Basin, and that the Kela‐2 gas field is the first large gas field dominated by aeolian sandstone reservoirs in China.

Eustatic Control on Alluvial Sequence Stratigraphy: A Possible Example from the Cretaceous-Tertiary Transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.

Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.

Fluvial architecture and sequence stratigraphy of the Eocene to Oligocene Iwaki Formation, northeast Japan: channel-fills related to the sea-level change

The Iwaki Formation, the lowermost part of the Eocene to Oligocene Shiramizu Group, northeast Japan, consists of conglomeratic braided drainage sediments. The formation displays fining-upward succession that reflects an overall transgressive sequence with two other overlying formations of the group. Although the clast-supported conglomerates are by far the major components of the formation, they also intercalate with many concave-up channel sediments filled-up with sandy deposits. The lower part of the formation mainly consists of multistorey, large-scale channel sandstones with minor amount of small-scale single-storey channel sandstones, which are related to low rates of base-level rise. In contrast, the middle to upper part of the formation comprises single-storey strata with relatively small-scale channels, which were probably associated with more rapid rates of base-level rise. The field evidence shows that the change of scale and internal structures of channel sandstones within the conglomerates most likely reflect the rate of changes of sea-level rise.

Seafloor precipitates and C-isotope stratigraphy from the Neoproterozoic Scout Mountain Member of the Pocatello Formation, southeast Idaho: implications for Neoproterozoic earth system behavior

Abstract Macroscopic seafloor-precipitated crystal fans are rare in post-Paleoproterozoic strata, but they are more common in the unusual carbonates that cap certain Neoproterozoic glacial strata. The uppermost carbonate and marble unit of the Scout Mountain Member of the Neoproterozoic Pocatello Formation (younger than 667±5 Ma) contains seafloor fans, records negative δ 13 C values, and lies within a transgressive sequence, all features consistent with such post-glacial carbonate units. It does not, however, rest directly upon a known glaciogenic deposit or sequence boundary. Instead, it occurs within a cyclic upward-fining succession that overlies an erosional contact on the pink dolostone post-glacial carbonate atop the late Sturtian Scout Mountain Member diamictites. It is possible that the carbonate unit in question is a cap carbonate and thus requires recognition of an additional glaciation recorded in the Pocatello Formation. Alternatively, it may imply that means independent of glaciation, such as overturn of an anoxic ocean, foster the deposition of cap-like carbonates independent of post-glacial processes. Thus, Neoproterozoic carbonate units with unusual lithofacies and negative carbon isotope values need not indicate the aftermath of low-latitude glaciation. Additionally, the radiometrically calibrated δ 13 C record of the Pocatello Formation demonstrates that the interval from 700 to 580 Ma has more variation than previous compilations would suggest.

The First Late Cretaceous Footprints from the Nemegt Locality in the Gobi of Mongolia

In September 2001, large ornithopod footprints were found in the Nemegt Formation at the locality known as Nemegt in the Gobi of Mongolia. Additional hadrosaur ichnites, plus footprints of sauropods and theropods, have been recovered since then. The Nemegt Formation is known for the number and diversity of dinosaur skeletons found there, but footprints have never before been reported. Footprints were noted in three horizons within the formation, and occur at the top of upward-fining successions of floodplain sandstones and mudstones of a meandering fluvial paleoenvironment. Most of the footprints are preserved as natural casts that show good preservation of detail. Skin impressions are found on some, and many have slide marks. The vast majority of the footprints can be identified as having been made by Saurolophus, but two footprints each of Tarbosaurus and Opisthocoelicaudia were also recovered. Three hadrosaur footprints were found in the quarry of a Tarbosaurus skeleton. It appears that after the Tarbosaurus had died and been partially buried, its skeleton was trampled by a hadrosaur. The overwhelming domination of hadrosaurs at the footprint levels suggests there are preservational biases acting on the fossilization of Nemegt skeletons to produce abnormally high predator/prey ratios.

Glacially Driven Cycles in Accumulation Space and Sequence Stratigraphy of a Stream-Dominated Alluvial Fan, San Joaquin Valley, California, U.S.A.

ABSTRACT High-resolution sequence stratigraphy provides a framework to interpret unconformity-bounded depositional sequences in the stream-dominated Kings River alluvial fan, located near Fresno, California. Depositional units in the fan are analogous to systems tracts described from marine deposits. Fan sequences reflect changes in accumulation space (Blum and Tornqvist 2000) associated with Pleistocene glacial cycles in the Sierra Nevada and preservation space created by tectonic subsidence in the San Joaquin basin. Adjustments in accumulation space are driven by changes in the ratio of sediment supply to discharge during glacial advances and retreats. At the end of glacial periods and the beginning of interglacial periods, declines in the ratio of sediment supply to discharge led to fan incision, a basinward shift in the fan intersection point, and loss of accumulation space. In mid- and upper-fan settings, incised valleys and laterally extensive, moderately mature paleosols formed, marking the unconformable base of the depositional sequence. Throughout the interglacial period, relatively low accumulation space existed and deposition was confined to the distal areas of the fan. Rapid aggradation and, thus, accumulation space increase, in response to increased sediment supply during the next glacial event initially filled the incised valley with a fining-upward succession of relatively coarse-grained channel and overbank deposits that contain rare, immature paleosols. Upon filling of the incised valley, the intersection point stabilized near the fan apex. This led to unconfined, open-fan deposition, indicating that widespread accumulation space was available across most of the fan surface. These high-accumulation-space units consist of fluvial deposits from multiple, large glacial outwash channels that radiated outward from the proximally located intersection point. Sequence boundaries and units associated with accumulation-space cycles can be used to understand and predict facies distributions and stratigraphic packaging within glacially influenced fans similar to the Kings River alluvial fan.

Sedimentary facies and Holocene progradation rates of the Changjiang (Yangtze) delta, China

The Changjiang (Yangtze) River, one of the largest rivers in the world, has formed a broad tide-dominated delta at its mouth during the Holocene sea-level highstand. Three boreholes (CM97, JS98, and HQ98) were obtained from the Changjiang delta plain in 1997–1998 to clarify the characteristics of tide-dominated delta sediments and architecture. Based on sediment composition and texture, and faunal content, core sediments were divided into six depositional units. In ascending order, they were interpreted as tidal sand ridge, prodelta, delta-front, subtidal to lower intertidal flat, upper intertidal flat, and surface soil deposits. The deltaic sequence from the prodelta deposits to the delta front deposits showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost part of the delta front deposits to the surface soil. Thinly interlaminated to thinly interbedded sand and mud (sand–mud couplets), and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of Changjiang deltaic facies. Sediment facies and their succession combined with AMS 14C dating revealed that isochron lines cross unit boundaries clearly, and delta progradation has occurred since about 6000 to 7000 years BP, when the rising sea level neared or reached its present position. The average progradation rate of the delta front was approximately 50 km/kyear over the last 5000 years. The progradation rate, however, increased abruptly ca. 2000 years BP, going from 38 to 80 km/kyear. The possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and/or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid-Holocene.

Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression

A large estuary was formed by marine inundation of the paleo-Changjiang (Yangtze) incised valley during the transgression after the Last Glacial Maximum. This paper presents the sedimentary facies and architecture of the estuary fill, based on the analysis of three sediment cores (CM97, JS98, and HQ98) obtained from the present Changjiang delta plain.Estuary fill deposits showing an upward-fining succession were grouped into five depositional facies: tidal river, distributary channel, muddy intertidal to subtidal flats, transgressive lag, and estuary front, based on sedimentary textures, lithology, and physical sedimentary structures. Sand–mud couplets are common in these deposits, indicating that tides played a significant role in producing these sedimentary structures and that the estuary type was tide-dominated. Some of the successive sand–mud couplets probably recorded neap-spring cycles as well as semidiurnal tidal cycles.The nature of the estuary was very different from other representative tide-dominated estuaries in sediment facies, its distribution, and sediment source for estuarine fill. Unlike the other estuaries that receive sediments mainly from the sea, the paleo-Changjiang estuarine fill deposits were supplied largely from the river. This difference would also have a great influence on the sedimentological and morphological component in the estuary. The sediment distribution of the estuary showed fining-seaward and estuary-mouth sand bodies fed by marine-source sand were absent. The architecture model of tide-dominated estuaries should be divided into two types by the degree of fluvial sediment supply. The paleo-Changjiang estuary shows a good example for an estuary of large rivers.