Lake Margin Deposits Research Articles

Geologic characterization and depositional history of the Uteland Butte Member, Green River Formation, southwestern Uinta Basin, Utah

The 15- to 65-m-thick informal Uteland Butte member of the Eocene Green River Formation represents the first widespread transgression of Lake Uinta in the Uinta Basin, Utah. This study assesses the spatial and temporal variation of Uteland Butte member deposits along a 40-km transect in the southwestern margin of the Uinta Basin using detailed measured sections, organic and inorganic geochemical data, and outcrop gamma ray logs. Fourteen lithofacies are identified, which comprise seven facies associations linked to with lacustrine, palustrine, and deltaic depositional settings. Facies associations are traceable laterally across the study area, where five 4- to 12-m-thick depositional cycles are identified. Each shallowing upwards cycle is defined by a >1.5-m-thick basal package of organic-rich, argillaceous laminated mudstone, and is capped by thick packages of bedded carbonate. In the far western study area (Kyune Creek Canyon), thick deposits of organic-rich mudstone are present and represent the most distal outcrop section; time-equivalent strata in the eastern study area (Minnie Maud Creek Canyon) are relatively organic lean with higher silt and clay content, interpreted to represent proximal lake margin deposits influenced by a nearby delta. The outcrop belt is correlated to more distal cores and well logs across the western Uinta Basin. Similar lithological and petrophysical patterns across the western Uinta Basin are used to subdivide stratigraphy into nine laterally contiguous sub-units based on nomenclature from the oil-producing area of the central basin (from base to top: lower Uteland Butte, D Bench, D Shale, C Bench, C Shale, B Bench, B Shale, A Bench, and A Shale). Siliciclastic clay-rich and carbonaterich intervals are correlated across the region and indicate distinct siliciclastic- and carbonate-dominated lake phases during Uteland Butte member deposition. Climate is interpreted to be the dominant driver of these claycarbonate cycles, in which relatively humid periods resulted in increased fluvially derived siliciclastic sediment into the basin (clay-rich periods), and arid periods resulted in evaporative conditions with decreased fluvial sediment input that favor carbonate accumulation. Climatically driven depositional cycles within the Uteland Butte member reflect, to a smaller degree, the larger scale climatically driven depositional cycles observed at the member- and formation levels of Paleocene and Eocene Uinta Basin stratigraphy. Importantly, the Uteland Butte member clay-carbonate cycles showcase how relatively small-scale climate shifts can impact basin-scale lacustrine deposition.

Paleoenvironment Implication of Red Paleosols in a Late Cretaceous Continental Succession, Songliao Basin, NE China

The limited knowledge of Late Cretaceous terrestrial environments and their response to tectonic events in mid-latitudes can be addressed through continental basin deposits such as paleosols. Paleosols have been discovered in the Late Cretaceous Yaojia Formation in the southern Songlaio Basin and are recognized by evidence of soil structures controlled by pedogenesis. Sedimentary facies research on red paleosols was conducted on the Late Cretaceous Yaojia Formation in the outcrop of the southern Songliao Basin to interpret the depositional environments and tectonic significance of red paleosols during the greenhouse period. Mudflat, lake margin, and shallow lake depositional environments in a semi-arid climate are interpreted from the outcrops based on sedimentary descriptions and interpretation as well as geochemical and micromorphological analyses of paleosols in outcrops. We reconstructed the paleoenvironmental and paleoclimatic conditions through the paleosols in the mudflats and lake margin. The red paleosols in the mudflats and lake margin deposits formed in a stable landscape influenced by the tectonic uplift of the Songliao Basin, which is considered as new important evidence for tectonic uplift influenced by the collision of the Okhotomorsk Block with East Asia. The tectonic uplift process in East Asia is identified from the evolution of the depositional environments and drainage conditions inferred from different types of paleosols. Thus, the paleosols-bearing red bed deposits in outcrops provide an important contribution of the Late Cretaceous terrestrial paleoclimate and the tectonic setting research.

Origin and enrichment of borates in a Late Paleozoic alkaline lake-playa deposit, Junggar Basin, NW China

Reedmergnerite (NaBSi3O8) and searlesite (NaBSi2O6·H2O) are rare borates minerals over the world. In the Late Paleozoic Fengcheng Formation of the NW Junggar Basin, NW China, the two borates, especially reedmergnerite, occur as locally abundant minerals of potential economic interest. Here, petrology, minerology, and boron isotopes of different reedmergnerite occurrences in the Fengcheng Formation were systematically studied to explore its enrichment mechanism in sedimentary rocks. Reedmergnerite in the Fengcheng Formation is a temperature-controlled (crystallizing at 55–180 ℃, mainly at 90–110 ℃) and diagenetic replacive mineral. In the lake center deposits of the Fengcheng Formation, reedmergnerite is confined within sodium carbonate beds or nodules and shows a clear replacement of sodium carbonate minerals. In the lake slope deposits, reedmergnerite is also very abundant and mainly replaces calcite or dolomite-ankerite within sub-horizontal fractures or shortite/eitelite crystal molds, occasionally seen replacing bedded searlesite or growing both replacively and displacively in organic-rich matrix. No significant reedmergnerite is found in the lake margin deposits. Boron isotopes of reedmergnerite-bearing samples (–0.04‰ to + 7.69‰) are higher than those of lacustrine mudstone samples (–14.2 to + 2.79‰) and overlap those of ignimbrite samples (+2.37 to + 3.87‰). Boron for diagenetic reedmergnerite formation was primarily inherited from the original hydrothermal fluids that flowed into lakes and its concentration was influenced by lake water pH and salinity. Potential early boron reservoirs were searlesite, sodium carbonates, organic matter and tuff materials. Fluids responsible for reedmergnerite replacement were weakly acidic. CO2 generated by organic matter degradation-maturation and carbonate dissolution acted as a mineralizer to promote reedmergnerite formation at a lower temperature range than the previously synthetic temperatures (>270 ℃). Enrichment of reedmergnerite in sedimentary rocks requires a primary volcanic-related evaporated lake with high organic productivity, and meanwhile a deep burial environment, at best reaching the depth of oil generation window.

Early Pleistocene archaeological occurrences at the Feiliang site, and the archaeology of human origins in the Nihewan Basin, North China.

The Early Pleistocene archaeological evidence from the fluvio-lacustrine sequence of the Nihewan Basin (North China) offers an excellent opportunity to explore early human evolution and behavior in a temperate setting in East Asia, following the earliest ‘Out of Africa’. Here we present the first comprehensive study of the Feiliang (FL) site, with emphasis on the archaeological sequence, site integrity, and stone artifact assemblages. Magnetostratigraphic dating results show that early humans occupied the site ca. 1.2 Ma. Archaeological deposits were buried rapidly in primary context within shallow lake margin deposits, with only minor post-depositional disturbance from relatively low energy hydraulic forces. The FL lithic assemblage is characterized by a core and flake, Oldowan-like or Mode 1 technology, with a low degree of standardization, expedient knapping techniques, and casually retouched flakes. The bone assemblage suggests that hominin occupation of the FL site was in an open habitat of temperate grassland with areas of steppe and water. The main features of the FL assemblage are discussed in the context of the early Pleistocene archaeology of Nihewan, for which an assessment of current and future research is also presented.

New dinosaur tracks from the Lower Cretaceous (Valanginian-Hauterivian) Saniri Formation of Yeongdong area, central Korea: Implications for quadrupedal ornithopod locomotion

Newly discovered tracks of a large quadrupedal ornithopod and a theropod with trail trace, and a bird track from the Cretaceous Saniri Formation of Yeongdong area are described. One ornithopod trackway is very unusual in having large (length width ∼15 and >15 cm respectively) clover-leaf-like manus tracks situated anteromedial to the pes tracks with atypical negative, inward rotation of 45°. Ornithopod pes tracks are quadripartite with three separated, elongate-oval, nearly parallel sided, wide digit impressions and a separate suboval heel impression (mean length and width about 41 and 36 cm respectively: l/w ratio 1.13). Manus track morphotypes are a clover-leaf-shaped configuration of three digit impressions, representing digits II–IV, in triangular configuration and registered just in front of (anteromedial to) pes track digits II and III. The pes morphotype is typical of Caririchnium, but the manus morphotype is quite distinct from previously described ichnotaxa, thus justifying a new ichnotaxon: Caririchnium yeondongensis ichnosp. nov., probably representing a facultatively-quadrupedal Iguanodon-like trackmaker.Theropod tracks are composed of three tapered pes digit impressions with interdigital angles between digits II and IV 45°. Length and width about 22.8 cm and 15.5 cm, respectively. Pace, stride, and pace angles are about 51 cm, 101 cm, and 170°, respectively. Theropod tracks are characteristically associated with a nearly continuous tail trace, which is up to 360 cm in length, 4.5 to 6 cm in width, and broad “U” shaped in cross section. Expulsion rims and dragging striae occur intermittently. In addition, dinosaur skin impressions, poorly preserved large sauropod tracks, a bird track, invertebrate and plant fossils are found from the lake margin deposits also containing rain drop impressions and desiccation cracks. Dinosaur tracks of the Yeongdong area represent the oldest (Valanginian-Hauterivian) dinosaur tracks of Korea.

The response of lake margin sedimentary systems to climatically driven lake level fluctuations: Middle Devonian, Orcadian Basin, Scotland

AbstractLake margin sedimentary systems can provide highly sensitive records of sedimentary response to climate change. The Middle Old Red Sandstone of Northern Scotland comprises a thick succession of cyclic lacustrine sediments. Within this succession the deepest lake phase, the Achanarras fish bed, allows bed‐scale correlation over 160 km across the basin. This provides a unique opportunity to examine the character of synchronous lake margin deposits, and their response to climatically driven lake level fluctuations, across a large continental basin. Detailed characterization of two separate lake margin systems was carried out utilizing multiple sections in western Orkney, in the north, and Easter Ross, in the south. Seven facies have been recognized, which include upper and lower shoreface, deep lake, shallow lake, playa, turbidite and fluvial facies. Differences in vertical and lateral facies stacking patterns reflect the response of these systems to climatically driven fluctuations in lake level. Comparison of the northern and southern systems examined highlights the variable response of lake margin systems to the same climatic change and related lake level fluctuations. In the south, a greater fluvial influence is recognized on the development of the lake margin successions, whereas in the northern example, which lay on the downwind margin of the lake, shore zone facies are more commonly developed. The variability recognized can be accounted for by regional variations in sediment supply, coastal physiography, lake size, bathymetry and potential fetch. Lake level stability is also recognized as a major control on the development of lake margin sedimentary systems, as is the linked or unlinked relationship of the catchment and the lake basin climate for which a conceptual model is proposed.

The earliest Late Paleolithic in North China: Site formation processes at Shuidonggou Locality 7

Shuidonggou (SDG) has long been recognized as the type site complex for the Chinese Late Paleolithic, as defined by the presence of blade technology and by its chronology. Recent field and laboratory research conducted in the past decade has revealed the presence of Paleolithic ornaments and two different technological components of an industry equivalent to the early Upper Paleolithic – Levallois-like blades and simple core and flakes which appear at SDG localities. Although clearly important for Paleolithic research in China and broader Old World prehistoric studies, relatively little is known about the SDG site formation processes and the specifics of site context. Here, we explore the various formation processes and sedimentary context associated with the stone artifact assemblage from one key site – Shuidonggou Locality 7 (SDG7).Contrary to earlier suggestions that the SDG7 lithic accumulations are the result of secondary deposition, our study indicates that SDG7 has been preserved in near-primary context. Features that were previously argued to reflect fluvial disturbance are in fact localized collapse features caused by saturated sediment load in loess deposits in a lakeshore context. Multiple lines of evidence support the argument that the site preserves the original foraging behavior of the SDG hunter–gatherers. This evidence consists of the sedimentary matrix and the distribution patterns of archaeological materials (particularly the artifact assemblage composition, debitage size distribution, artifact conditions, orientation analysis, inclination, and spatial patterning). The SDG7 archaeological deposits were buried rapidly in shallow lake margin deposits of fine sands, silts, and clays that were minimally disturbed and subjected only to relatively low energy hydraulic forces. This indicates that the SDG7 occurrences are suitable for early hominin behavioral studies in North China at the start of the Late Paleolithic.

Time, Space and Structure on the Korea Cretaceous Dinosaur Coast: Cretaceous Stratigraphy, Geochronology, and Paleoenvironments

The stratigraphy, geological ages, and paleoenvironments of the Korea Cretaceous Dinosaur Coast (KCDC) are reviewed and synthesized in order to understand the occurrence and diversity of the vertebrate fossils and track remains in time and space. Various absolute age measurements obtained during the last decade yield new age data to correlate with the dinosaur fossil-bearing Cretaceous deposits in Korea. The radiometric age of the lower (Sindong Group) and middle (Hayang Group) parts of the Gyeongsang Basin (the largest basin of the KCDC) located in the eastern part of the KCDC gives dates ranging from Aptian to Campanian, which is younger than the previous biostratigraphic age estimates, but the results may be influenced by metamorphism. Likewise, radiometric dates from the Cretaceous sequence in the western part of the KCDC give numerical values suggesting a correlation with the Upper Cretaceous Yucheon Group of the Gyeongsang Basin. Dinosaur evidence (e.g., tracks, eggs, bones) along the KCDC is variable in relation to stratigraphy and paleoenvironment. Dinosaur tracks are preserved mostly in stratigraphically higher Cretaceous lake margin deposits, whereas dinosaur bones occur mostly in older Cretaceous floodplain deposits. Most dinosaur eggs are found in Upper Cretaceous deposits, and they occur in floodplain deposits of alluvial fan and meandering river settings. Thus, we conclude that dinosaurs inhabited volcanically influenced, alluvial fan, fluvial plain, and lake margins on the Korean Peninsula throughout the Cretaceous under a semi-arid climate with strong seasonality. The rarer occurrence of dinosaur tracks in older Cretaceous deposits in some basins might be attributed to the limited exposures of lake-margin deposits. It is inferred that the presence of large lakes as water sources, rich vegetation of gymnosperm trees as food, and semi-arid paleoclimatic conditions formed a general landscape and environmental setting favorable for dinosaurs and their preservation in the Cretaceous deposits of South Korea.

Use of fine-scale stratigraphy and chemostratigraphy to evaluate conditions of deposition and preservation of a Triassic Lagerstätte, south-central Virginia

The rich, fossiliferous Triassic sediments exposed in the Virginia Solite Quarry include a 34-mm-thick “insect layer” that is notable for detailed preservation of soft-bodied invertebrate and vertebrate remains. We describe this unique Konservat-Lagerstatte and use sedimentologic and geochemical analyses to interpret the environmental conditions necessary to preserve such delicate fossils. This work is among the first attempts to apply detailed geochemical/stratigraphic analysis to the study of Lagerstatten and we report on a 332-mm-thick section that includes the insect layer and the rocks immediately below and above it. Our analysis successfully constrains various aspects of the depositional and diagenetic history of the Lagerstatte and permits a detailed analysis of changing conditions prior to, during, and after deposition. Geochemical and sedimentologic analyses of the insect layer and surrounding lithologies reveal a change from siliciclastic-dominated layers (Unit 1) to dolomite-siliciclastic laminites above (Unit 2 and the insect layer), separated by a boundary dolostone layer that is traceable for over 200 m. We interpret this sedimentary shift as the initial stages in the transgression of a shallow, saline, alkaline rift-basin lake over lake margin deposits. The absence of bioturbation by plants and benthic organisms, as well as a lack of predation on the insects, is not explained by significant water depth, but is instead more reasonably considered a result of the chemistry of the water at the lake margin, affected by groundwater seeps, which provided F-, Mg-, and Ca-rich fluids. Although the initial conditions of preservation are remarkable, it is equally impressive that the fossils survived extensive diagenesis, e.g. dissolution of quartz and coarsening of dolomite.

Element mobility during zeolitic and argillic alteration of volcanic ash in a closed-basin lacustrine environment: Case study Olduvai Gorge, Tanzania

Pliocene–Pleistocene Bed I of Olduvai Gorge, Tanzania provides an ideal test for models of element mobility during closed-basin saline–alkaline tephra alteration. Samples of a single phonolitic lapilli-rich layer of marker Tuff IF were collected across a transect of paleo-lake Olduvai, including authigenic K-feldspar and phillipsite bearing samples from the central saline–alkaline lake basin, less altered phillipsite and chabazite-bearing samples from the intermittently dry lacustrine and lake margin deposits, clay-dominated samples from the distal lake margin, and less altered or fresh samples from a freshwater wetlands deposit. In addition to authigenic mineral assemblage, bulk composition (as measured for lapilli separates) also varies across the basin. Of the major elements, Si and Al varied the least across the environments sampled. Ca, Na, and K were variable but did not display significant leaching as would occur under more dilute and neutral conditions. K in particular varied in concentration depending on environment, showing depletion in the lake margin argillic samples and concentration in the phillipsite and K-feldspar dominated zeolitic samples of the central basin. Mg followed the opposite pattern, with higher concentrations in the argillic samples. The overall pattern of element enrichment and depletion helps reconstruct the Olduvai Basin diagenetic history. Ordinarily immobile elements Ti and Zr showed significant variation, especially in the most altered samples. Zr concentrations were up to 50% lower in the zeolitized samples, while they were conserved or slightly enriched in the least altered and argillic samples. Of the trace elements analyzed, only Ta and Nb were consistently immobile, showing variations of under 10%. These observations suggest that Zr and Ti should not be treated as immobile elements if exposure to saline–alkaline fluids is suspected, as they can become more mobile under specific high-pH conditions.

Palaeoenvironmental significance of the clay mineral composition of Olduvai basin deposits, northern Tanzania

Quaternary deposits in the southeastern part of the Olduvai basin, northern Tanzania, consist of lake margin deposits, followed by a series of fluvial sediments. The clay mineral fraction of the lake margin deposits (Bed I and lower part of Bed II) is composed of smectite and subordinate illite. All smectite is largely dioctahedral and shows indications for a limited degree of irregular interstratification by illite. In the overlying fluvial deposits (Beds II–IV), illite is the most abundant clay mineral. Smectite only occurs in lower parts of the fluvial deposits (up to the middle of Bed III), where it generally shows a high degree of irregular interstratification. Differences in clay mineral composition between the lake margin deposits and the fluvial deposits record differences in sediment source area and degree of alteration. Dioctahedral smectite in the lake margin deposits and the oldest fluvial deposits is derived from a region with volcanic material extending to the east and south of the basin, which also supplies a certain amount of illite. Illite in the fluvial deposits of Bed IV originates from an area with a metamorphic bedrock to the west and north. Alteration of detrital clay minerals resulted in Mg-enrichment of dioctahedral smectite in part of the lake margin deposits and partial illitization of smectite in the older fluvial deposits. Formation of clay minerals during diagenesis or soil development is not documented by analysis of the total clay fraction.

Traces of evaporites in Upper Cretaceous lacustrine deposits of Korea: Origin and paleoenvironmental implications

Diverse types of halite and sulfate evaporite traces occur in the Upper Cretaceous lacustrine deposits of the Jindong Formation and the Jangdong Tuff in Korea, in which dinosaur tracks are common. The halite traces usually occur as casts or moulds, and they include traces of primary halite hoppers, primary and intrasedimentary skeletal halite, and intrasedimentary tiny halite. The sulfate traces occur as intrasedimentary and displacive casts filled with sediments and sparry calcite, and the casts occur as diverse modes from single crystal casts through nodular aggregates to massive aggregates. Some aggregates of the sulfate casts are aligned in crack pattern, and selective occurrence of the laths in ripple troughs are observed. Soluble sulfate evaporite minerals such as gypsum, glauberite, or mirabilite are probable for the precursors of these traces. These halite and sulfate traces are interpreted to have been formed in a saline lake and mudflats. It is interpreted that meteoric water was responsible for the evaporite precipitation in the Jindong Lake and Jangdong Lake. Recycled brine produced by the repeated dissolution of evaporites might contribute to the evaporite precipitation in the paleo-Jindong Lake and the paleo-Jangdong Lake. The development of saline lake deposits in the Jindong Formation and the Jangdong Tuff suggests that the southern part of the Korean Peninsula remained an inland continental area during the Cretaceous resulting in semi-arid paleoclimatic condition due to an orographic effect by the location of the Korean Peninsula on continental margin in mid-latitude with topographic barriers during the Cretaceous. The occurrence of dinosaur tracks in the lake margin deposits of saline lake with frequent inflow of dilute meteoric water suggests that Upper Cretaceous dinosaurs inhabiting lakes on the Korean Peninsula might have drunken brackish water.

The Nature of Zeolite Occurrences in Deposits of the Olduvai Basin, Northern Tanzania

AbstractThe Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

Facies Model of a Semiarid Freshwater Wetland, Olduvai Gorge, Tanzania

The sedimentology and stratigraphy of a freshwater wet- land in early Pleistocene (; 1.75 Ma) volcaniclastic deposits, Olduvai Gorge, Tanzania, are characterized using texture, mineralogy, micro- fossils, and stratigraphic description to create a depositional facies model. The freshwater wetland was located on the margin of a semi- arid, closed basin containing a playa lake. The lake-margin deposits are dominated by two distinct lithologies: waxy claystones and earthy sediments. Waxy sediments are olive green, dense claystones that con- tain authigenic minerals such as trona, and represent a fluctuating saline, alkaline lake (Hay 1976). Earthy sediments are beige, friable siltstones that contain siliceous microfossils, bone fragments, and minor pebbles; they represent the freshwater wetland. Microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt-tolerant species. Associated lithofacies are: diatomite, carbonate, and sandstone. Interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland, based on ; 50 excavations, identified seven distinct sub- environments: spring deposits, perennial marsh, pool, ephemeral wet- land, fringing wetland, wetland channel, and ephemeral stream. The stratigraphic sequence, estimated to represent an ; 40-50 kyr long interval, indicates that persistent (; 10 3 yr) wet periods prevailed dur- ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands, and equally long intervening dry periods caused the lake to contract, allowing the flourishing of wetlands and accretion of earthy sediment. The lithofacies associations in the Olduvai paleo- wetland provides an important first step in the development of a de- positional environmental model for freshwater wetlands that can be tested elsewhere, particularly in arid and semiarid settings.

Dinosaur track-bearing deposits in the Cretaceous Jindong Formation, Korea: occurrence, palaeoenvironments and preservation

Dinosaur track deposits of the Upper Cretaceous Jindong Formation, Korea, were examined from a sedimentological viewpoint to interpret their palaeoenvironments and conditions of preservation. The general depositional environment of the Jindong Formation was lake-margin to shallow lacustrine. Lake-margin deposits comprise most of it but the stable isotope values of the carbonates suggest an open lake. There are two types of dinosaur track beds. One is a discrete trackway deposit and the other is a dinoturbated deposit. All of the dinosaur trackways in the formation are preserved in interlaminated fine-grained sandstone to siltstone-mudstone deposits, which were deposited on a dry mudflat at the lake margin by sheetfloods and then underwent calcareous pedogenesis. The dinoturbated deposit was originally calcareous silty mud, reflecting deposition in a shallow lake of low energy. It was also modified later by calcareous pedogenesis. The trampling traces of dinosaurs in the Jindong Formation are recognized both on bedding surfaces and in sections. In places dinosaur tracks occur as ‘overtracks’ which are preserved in layers above the true tracks. The Jindong dinosaur track deposits are usually associated with pedogenic calcretes, which indicates that the climate at the time was seasonal and arid. Consequently, it is interpreted that the extensive and frequent preservation of dinosaur tracks in the Jindong Formation is the result of repeated deposition by sheetfloods on a mudflat associated with a perennial lake, which was utilized by dinosaurs as a persistent water source during drought, and subsequent development of calcareous pedogenesis in an arid climate.

Palustrine sedimentation in an episodically subsiding basin: the Miocene of the northern Teruel Graben (Spain)

Lacustrine deposits form most of the sedimentary record accumulated throughout the Miocene in the northern part of the Teruel Graben, a landlocked basin situated in NE Spain. Three main stratigraphic units (Units I–III) spanning Late Vallesian to Late Turolian in age, are distinguished in the area. The two former units comprise mainly carbonate lacustrine deposits that were deposited in a palustrine or a more permanent but shallow lacustrine setting, the latter being related mainly to periods of lake expansion under more humid and cooler climatic conditions. In contrast, deposition of Unit III, characterised by gypsiferous lacustrine sediments in central parts of the basin, reflects the influence of source rocks of evaporitic nature (Upper Triassic formations) that were emplaced by diapiric uplift in the eastern margin during the Middle Turolian. Carbonate lake sediments belonging to Units I and II consist of four main lithofacies associations: (a) distal–alluvial lake margin deposits; (b) lacustrine carbonates and pedogenically modified lacustrine carbonates; (c) carbonate fill channels; and (d) organic-rich marlstone and carbonate (marshes). Lithofacies association (a) shows typically a sequential arrangement formed, from bottom to top, of red mudstone, carbonate palaeosols and biomicrite beds (sequence type 1a). These sequences are interpreted as a result of slow but continuous aggradation of the floodplain areas and subsequent rise of the water table, probably reflecting equilibrium between subsidence and sedimentation. Lithofacies association (b) consisting of biomicrite beds passing upward into palaeosols (sequence type 1b) is thought to be caused by marked pulses of tectonic subsidence that created the accommodation space required for fast development of a carbonate lake. Carbonate fill channels, showing erosive lower surfaces dig out on previous channels, marsh deposits and/or lacustrine carbonates, exhibit well-developed prismatic structure at their bed tops. The sequential arrangement suggests similar tectonic constrains than the envisaged for lithofacies 1b. The comparison of depositional styles of the lake systems with the palaeoclimatic evolutionary pattern of the basin throughout the Miocene indicates that the combined effect of tectonism, climate and source rock played a major role in the stratigraphic framework of the basin during this period.

Trace fossil analysis of lacustrine facies and basins

Abstract Two ichnofacies are typical of lacustrine depositional systems. The Scoyenia ichnofacies characterizes transitional terrestrial/nonmarine aquatic substrates, periodically inundated or desiccated, and therefore is commonly present in lake margin facies. The Mermia ichnofacies is associated with well oxygenated, permanent subaqueous, fine-grained substrates of hydrologically open, perennial lakes. Bathymetric zonations within the Mermia ichnofacies are complicated by the wide variability of lacustrine systems. Detected proximal–distal trends are useful within particular lake basins, but commonly difficult to extrapolate to other lakes. Other potential ichnofacies include the typically marine Skolithos ichnofacies for high-energy zones of lakes and substrate-controlled, still unnamed ichnofacies, associated to lake margin deposits. Trace fossils are useful for sedimentologic analysis of event beds. Lacustrine turbidites are characterized by low-diversity suites, reflecting colonization by opportunistic organisms after the turbidite event. Underflow current beds record animal activity contemporaneous with nearly continuous sedimentation. Ichnologic studies may also help to distinguish between marine and lacustrine turbidites. Deep-marine turbidites host the Nereites ichnofacies that consists of high diversity of ornate grazing traces and graphoglyptids, recording highly specialized feeding strategies developed to solve the problem of the scarcity of food in the deep sea. Deep lacustrine environments contain the Mermia ichnofacies, which is dominated by unspecialized grazing and feeding traces probably related to the abundance and accessibility of food in lacustrine systems. The lower diversity of lacustrine ichnofaunas in comparison with deep-sea assemblages more likely reflects lower species diversity as a consequence of less stable conditions. Increase of depth and extent of bioturbation through geologic time produced a clear signature in the ichnofabric record of lacustrine facies. Paleozoic lacustrine ichnofaunas are typically dominated by surface trails with little associated bioturbation. During the Mesozoic, bioturbation depth was higher in lake margin facies than in fully lacustrine deposits. While significant degrees of bioturbation were attained in lake margin facies during the Triassic, major biogenic disruption of primary bedding in subaqueous lacustrine deposits did not occur until the Cretaceous.

Geomorphological significance of late quaternary deposits of the Lake George Area, N.S.W.

Summary A dated stratigraphy of surficial deposits is described for the Lake George area in the Southern Highlands of New South Wales. It includes lacustrine‐littoral, alluvial and aeolian sediments. The oldest stratigraphic reference is the highest strandline of the much enlarged late Pleistocene Lake George, at 30–37 m above the present lake bottom and with a dated age range of 21 000–27 000 y B.P. Five subsequent periods of shoreline aggradation have already been established. Four stratigraphic units were identified in aeolian deposits on the eastern side of Lake George. They indicated periodic instability to wind of lake margin deposits during the interval 23 000 to 2000 y B.P. Dated stratigraphies of alluvial fans and valley fill deposits in the Lake George Basin, and in the adjacent Grove Creek Basin, also reflected periodic instability of hillslope terrain during the last 30 000 years. Evidence available in the area suggested that accelerated fan building and valley aggradation were contemporaneous...

Lithofacies and environments of Bed I, Olduvai Gorge, Tanzania

Bed I in Olduvai Gorge spans the interval from about 1.7 to 2.0 m.y.a., and all evidences of hominid activity at Olduvai are within the latter half of this period. Bed I was deposited in a closed basin approximately 25 km in average diameter, and it can be subdivided into five lithologically different but partly time-equivalent rock assemblages, or lithologic facies, each of which was deposited in the same geographic environment or closely related series of environments. These lithofacies comprise lake deposits, lake-margin deposits, alluvial-fan deposits, alluvial-plain deposits, and lava flows. Alluvial-plain deposits form the lowermost part of Bed I, and they presumably interfinger northeastward with lake deposits in an area not now exposed. Discharge of the lavas diplaced the lake westward to a position it occupied through the latter half of the deposition of Bed I. The lake fluctuated greatly in salinity, level, and extent, and at times of low level it was highly saline and generally ranged between 7 and 10 km in average diameter. At times of high level it was as much as 15 km in diameter and was relatively fresh, at least along its southeastern margin. The lake-margin deposits accumulated on the zone of relatively flat terrain that was flooded at times of high level. Lake-margin deposits interfinger eastward with deposits of an alluvial fan of pyroclastic materials produced in an explosive phase of the volcano Olmoti. The climate at the time of Bed I was relatively dry although somewhat wetter than the present climate in the same region today.