Trough Cross-stratification Research Articles

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

Analysis of Sediments in the North-East Morainic Upland Edge Zone in the Vicinity of Piaski Pomorskie (West Pomerania, NW Poland)

Abstract the aim of this article is the description of lithofacies characteristics of sediments forming the north-east part of the morainic upland edge zone in the vicinity of Piaski Pomorskie (west Pomerania). the paper undertake also an attempt to reconstruct paleogeographical conditions of the formation of moranic uppland edge. The diversification of glaciofluvial and fluvial processes during the phase of stagnation and recession of uppervistulian ice-sheet in the vicinity of Piaski Pomorskie (western Pomerania) led to the formation of a morainic upland edge distinctly marked in morphology The edge zone separates the undulating moraine plateau from a lower situated Dębnica River Valley. This area is mostly formed by fine-grained sands with occasional layers of a coarser fraction of gravel and coarse- and medium-grained sands. The observed type of low-angle cross-stratification and trough cross-stratification indicates the dominance of diversified energy depositional environment with considerable power of water as transporting medium. the research area was formed at the front of the ice-sheet, which receded from the nearby reach of the Pomeranian Phase of the weichselian glaciation.

The wet desert system of the Upper Jurassic of the Parnaiba Basin, in the region between Formosa da Serra Negra and Montes Altos, Maranhão State, Brazil

O Mesozoico foi marcado por mudanças geológicas significativas decorrentes da Orogenia Gonduanide, que possibilitou a implantação de sistemas desérticos concomitantemente com expressivos eventos magmáticos. No Brasil, o registro desses eventos é observado na porção central da Bacia do Parnaíba, representados pelos depósitos siliciclásticos da Formação Corda, de idade jurássica, que sobrepõem derrames basálticos da Formação Mosquito. O estudo de fácies e estratigráfico de afloramentos dessa unidade, realizado na região entre Formosa da Serra Negra e Montes Altos, estado do Maranhão, possibilitou reconstituir o paleoambiente e inferir condições paleoclimáticas para essa região da Bacia Parnaíba durante o Jurássico. Foram identificadas quinze fácies sedimentares agrupadas em três associações de fácies (AF), representativas de um sistema desértico: 1) canal fluvial entrelaçado (AF1), caracterizado por conglomerados e arenitos finos a grossos, com grãos e seixos de rochas vulcânicas, com estratificações plano-paralela e cruzada acanalada; 2) lençóis arenosos (AF2), constituídos por arenitos finos a grossos e pelitos subordinados com laminação plano-paralela, laminação cruzada cavalgante, marcas de adesão e gutter casts; 3) campo de dunas (AF3), arenitos finos a médios, com grãos arredondados e estratificações cruzadas tabular e tangencial. Fluxos fluviais efêmeros (AF1), adjacentes ao campo de dunas (AF3), frequentemente retrabalhavam o topo das planícies vulcânicas e, esporadicamente, invadiam os lençóis arenosos (AF2). Comparado aos ergs do Permo-Triássico, o sistema desértico Corda foi mais úmido e menos extenso, precedendo os sistemas fluviais e costeiros de clima mais ameno do Cretáceo da Bacia do Parnaíba

Integrated litho-ichnofacies and ichnofabric analysis of the lowermost part of the Kunzum La Formation along the Khemangar khad and the Parahio Valley sections, Spiti Region (Zanskar-Spiti-Kinnaur Basin), Northwest Himalaya, India)

The Cambrian sequence, designated as the Kunzam La Formation, is exposed along the cLassical Parahio Valley section and also on left bank of the Khemangar khad in the Parahio Valley (Spiti), the Latter constitutes the lowermost exposed part of the Kunzum La Formation. The lower part of the Parahio Valley section and outcrop near the left bank of the Khemangar khad were studied for integrated litho-ichnofacies and ichnofabric analysis. The study establishes a genetic reLationship of the strata of these two localities. Five lithofacies are recognised in both the examined sections. The lithofacies indicate a wave-dominated shallow marine environment of deposition for these Cambrian rocks. The overall calcuLated ichnofabric indices in 74 m exposed strata at the Khemangar khad locality range from ii1=80.% (represented by 25.8 m), ii2= 17% (represented by 5.4 m) and ii4=2.5 % (represented by 0.8 m). The 42 m rocks examined in the main Parahio Valley section indicate ichnofabric indices ranging from ii1=37% (represented by 17.8 m), ii2=21% (represented by 8.0 m), ii3=40% (represented by 15.2 m) and ii4=2.0% (represented by 1.0 m). The presence of archetypal Cruziana ichnofacies, parallel and wavy Laminations, hummocky cross stratification (HCS) and trough cross stratification (TCS) supports a wave-dominated shallow water environment from upper shoreface to middlelower shoreface setup for the deposition of the lowermost part of the Kunzum La Formation in the Parahio Valley.

Aptian-Late Cenomanian Fluvio-Lacustrine Lithofacies and Palynomorphs from Mamfe Basin, Southwest Cameroon, West Africa

The sedimentary sequences in the Mamfe Basin are generally thought of as continental (fluvio-lacustrine) in origin. But the wide spread occurrence of salt springs and salt accumulations and in places gypsum, often puts to question the exclusive continentality throughout this basin’s history. The sequences studied portray a wide range of complex lithologies and lithofacies relationships and include basal and intra-formational conglomerates, sandstones, shales, carbonaceous, carbonate and halite facies respectively. All these are well exposed at several outcrop sections distributed all over the basin with some measuring up to and above 30 m in height. Sedimentary structures include: parallel aligned imbricated prismatic pebbles, plannar to trough-cross stratifications, fining upward grain size distribution and thin frequently alternating sandstone-shale beds. Palynological data have been obtained for the first time from the shally and carbonate intervals and integrated with five lithofacies associations, permitting the determination of the ages and paleo environmental reconstruction. The palynomorphs are characterized by pteridophytic spores dominated by Cicatricosisporites sp.,Cyathidites sp. and Deltoidspora sp., pollen grains include gymnosperms dominated by Classopollis annulatus and Classopollis todosus and angiosperms represented by Retitricolpites sp. and Retimoncolpites sp., fungal remains include some hyphae and Fusiformisporites sp. Only a few dinoflagellate cysts dominantly Spiniferites were encountered at Nfaitok outcrop, suggesting a proximal sea water influence. A fluvial, lacustrine-deltaic and an anoxic deep bottom lake setting sub-environments were reconstructed with no marine strata encountered. The brines are here suggested to have originated from sea water splay over a barrier and complimented by the chemistry of the drained surrounding country rocks. The palynomorph assemblage suggests an Aptian-Turonian age for the studied sequences and a chronostratigraphic sequence in which the Nfaitok and Satom Bridge units are both Late Cenomanian-Turonian (lateral equivalents of each other) are younger and overlie the Aptian-Early-Cenomanian Okoyong sandstone unit.

Fossilized intestine casts located within closed bivalve shells: implications for palaeoecological and sedimentological studies

Marine organisms inhabiting soft-bottom sediment are particularly susceptible to rapid sedimentation and erosion events. This article presents a novel example of fossilized intestine casts located within closed bivalve shells in relation to rapid sedimentation event from the Pleistocene sediment of the Oga Peninsula, Akita Prefecture, northern Japan. The mollusc shells were loosely packed in well-sorted medium-grained to coarse-grained sandstone associated with low-angle trough cross-stratification. The closed shells of Glycymeris yessoensis were present in the shell concentration. The internal parts of the shells were almost hollow, being partially filled with yellow fine-grained clay minerals (median grain size = 30.63 μm). The characteristic material within the shells clearly differed from the surrounding sediment, which consisted of coarse-grained felsic minerals (median grain size = 449.73 μm). Furthermore, the yellow fine-grained clay minerals within the shells were tube-shaped, and located near the posterior adductor scar. On the basis of anatomical observation of living Glycymeris, we confirmed that part of the intestine and the anus are also placed near the posterior adductor muscle. Therefore, the yellow fine-grained clay minerals within the shells represent the fossil remains of particles ingested by the G. yessoensis individual through suspension-feeding, and the tube-shaped material is interpreted as being fossilized intestine cast. These results suggest that G. yessoensis individuals were buried alive, as rapid sedimentation prevented ejection and destruction of the filling material of intestine. The presence of intestine cast within the mollusc fossils can be used for recognizing rapid sedimentation.

EemianWeichselian Pleniglacial fluvial deposits in S Poland (an example of the Vistula River valley in Kraków)

A fragment of the middle terrace in the VistulaRivervalley, nearby the railway station in Kraków, is formed by fluvial channel and overbank deposits of the PrądnikRiver, which bear a record of various environments affected by changing climatic conditions. The sedimentary succession includes two complexes that differ in lithofacies. The older complex comprises fining-upward deposits (channel sand and gravelly sand with medium- and large-scale trough cross-stratification) and, less frequently, sand with planar cross-stratification overlain by silt with intercalations of biogenic deposits of abandoned channels. Vegetation accompanying the deposition of biogenic layers was typical of boreal coniferous forests, dominated by Pinus sylvestris with a small admixture of Larix, Pinus cembra, Picea, Betula, and Populus . Periodically, the landscape passed into open areas overgrown by woody tundra. The complex developed as a result of activity of a meandering river under conditions of a moderately cool climate. The younger complex includes the sand lithofacies with horizontal stratification and low-angle cross-stratification, overlain by alternating sands and silts. The topmost part is represented by sands with large- and medium scale planar cross stratification. Lack of biogenic deposits and considerable amount of frosted quartz grains in alluvial sediments indicate aeolian processes of greater intensity, periglacial conditions and evolution towards a braided or transitional river. Pollen successions, absolute dating and studies of structural and textural features of the sediment suggest that the time of its deposition may be estimated at a range between the close of the Eemian Interglacial and the Weichselian Middle Pleniglacial (OIS 5e–OIS 3).

Sequence stratigraphy of the Upper Cambrian (Furongian; Jiangshanian and Sunwaptan) Tunnel City Group, Upper Mississippi Valley: Transgressing assumptions of cratonic flooding

New data from detailed measured sections permit comprehensive analysis of the sequence framework of the Furongian (Upper Cambrian; Jiangshanian and Sunwaptan stages) Tunnel City Group (Lone Rock Formation and Mazomanie Formation) of Wisconsin and Minnesota. The sequence-stratigraphic architecture of the lower part of the Sunwaptan Stage at the base of the Tunnel City Group, at the contact between the Wonewoc Formation and Lone Rock Formation, records the first part of complex polyphase flooding (Sauk III) of the Laurentian craton, at a scale smaller than most events recorded by global sea-level curves. Flat-pebble conglomerate and glauconite document transgressive ravinement and development of a condensed section when creation of accommodation exceeded its consumption by sedimentation. Thinly-bedded, fossiliferous sandstone represents the most distal setting during earliest highstand. Subsequent deposition of sandstone characterized by hummocky or trough cross-stratification records progradational pulses of shallower, storm- and wave-dominated environments across the craton before final flooding of Sauk III commenced with carbonate deposition during the middle part of the Sunwaptan Stage. Comparison of early Sunwaptan flooding of the inner Laurentian craton to published interpretations from other parts of North America suggests that Sauk III was not a single, long-term accommodation event as previously proposed.

Paleocurrent direction measurements in a volcanic setting by means of anisotropy of magnetic susceptibility: A case study from the Lower Miocene Tepoztlán Formation (Transmexican Volcanic Belt, Central Mexico)

Sources of ancient volcanic rocks are often unknown if they are either eroded and/or covered by younger deposits. This problem, as well as the provenance of reworked volcaniclastic, fluvial and mass-flow deposits, can be partially solved by the application of anisotropy of the magnetic susceptibility (AMS). For massive and poorly sorted volcaniclastic rocks in particular this may be the only way of finding reliable transport directions and therefore allowing for paleogeographic reconstructions. Here, we present a data set of 428 AMS measurements and 249 measurements of sedimentary paleocurrent indicators from the Miocene Tepoztlán Formation at the southern edge of the Transmexican Volcanic Belt (Central Mexico). The highest degree of reliability of AMS measurements is gained for data from lava samples and the lowest from mass flows. Sedimentary structures in sandstones and conglomerates such as trough cross-stratification, asymmetric ripple marks, and the shape of scours and channels could be used to calibrate the results from AMS data and to prove their reliability. AMS data on fluvial deposits point to a drainage system with a W–E flow direction, indicating an outflow of the river system into the ancient Gulf of Mexico.

Reworked pyroclastic beds in the early Miocene of Patagonia: Reaction in response to high sediment supply during explosive volcanic events

Two meter-scale pyroclastic levels are interbedded within the early Miocene succession of the Estancia 25 de Mayo (Patagoniense transgression) and Santa Cruz formations in the foreland Austral (or Magallanes) Basin, Argentina. The Lower Pyroclastic Level (LPL) is a tabular body interbedded within offshore marine deposits, laterally continuous for 30km and varying in thickness from few centimeters to around 4m. Grain-size grades from coarse to extremely fine ash with upward-fining along with a northeastern-fining trends. Structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures. The Upper Pyroclastic Level (UPL) lies within low energy fluvial deposits and is laterally discontinuous, and it is composed by lenticular bodies reaching a maximum of 15m thick and 100m wide, with a concave-up base and a plane top. Grain-size range is similar to the LPL but it coarsens upward. The lower portion of the UPL shows parallel lamination, current ripple lamination and mud drapes with large pumice lapilli and plant debris, whereas the upper portion shows parallel lamination and trough cross-stratification. Both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments. The LPL shows no mixing with epiclastic material whereas the UPL shows an upward increase in epiclastic material, and an upward increment in the scale of cross-bedding.The large thickness in relation to the possible emission center and the content of plant debris of the LPL does not suggest a direct, submarine, ash-fallout origin. The LPL is interpreted as a deposit of hyperpycnal-flows generated at the coastal zone when tephra-laden rivers plunged into the ocean. Large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments. The UPL is entirely composed of tractive deposits, so an ash fallout origin is disregarded. This, together with the lenticular shape and the alluvial plain origin of the encasing sediments, suggests accumulation within fluvial channels. Cycles of upper-flow-regime parallel lamination, current-ripple lamination and mud drapes at the lower portion, suggest short-lived turbulent flows that initially filled semi-abandoned channels. They were followed by sheet floods and channel reactivation, expressed by large-scale cross-bedding. The low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over-saturated with pyroclastic sediments during and after the eruption. The grain-size distribution of the LPL and geochemical data indicate a contemporaneous volcanic source located to the west/southwest in the Andean ranges, where the South Patagonian Batholith is presently located.

Late Cretaceous (Campanian/Maastrichtian) freshwater to restricted marine mollusc fauna from the Loncoche Formation, Neuquén Basin, west-central Argentina

This study is the first detailed account of freshwater to restricted marine molluscs from the Upper Cretaceous sedimentary rocks in the northern sector of the Neuquén Basin. The fossils are from the Campanian–Maastrichtian Loncoche Formation in southern Mendoza, west-central Argentina, which records the initial connection of the Neuquén Basin to the Atlantic Ocean. Six species of bivalves (Diplodon bodenbenderi, Pleiodon? sp., Isognomon? mechanquilensis, Mactridae? indet., Panopea? sp., and Laternula sp.) and three of gastropods (Paleoanculosa macrochilinoides, Paleoanculosa ameghiniana, and a possible cerithioidean) are described. Specimens were collected from fine to coarse sandstones, which may be massive or with planar stratification, planar-cross stratification or trough-cross stratification, and a few from bioclastic limestones and mudstones. Although the sections are from 50 to 300 m thick, the specimens are found only in the lower 120 m. Molluscs represent autochthonous/parautochthonous assemblages composed of mostly non-broken gastropods and articulated bivalve specimens, some of which show signs of postmortem transport; however, they were not removed far from their original habitat. Review of the habitats of living genera supports the inference of dominantly freshwater palaeoenvironments in the lower and middle part of the Loncoche Formation, with restricted marine influence in the southernmost localities studied where there are a few samples that contain specimens belonging to predominantly marine groups (e.g., Laternula, Panopea).

Tidal dunes versus tidal bars: The sedimentological and architectural characteristics of compound dunes in a tidal seaway, the lower Baronia Sandstone (Lower Eocene), Ager Basin, Spain

The Lower Eocene Baronia Formation in the Ager Basin is interpreted as a series of stacked compound dunes confined within a tectonically generated embayment or tidal seaway. This differs from the previous interpretation of lower Baronia sand bodies as tidal bars in the front of a delta. The key architectural building block of the succession, the deposit of a single compound dune, forms a 1–3m-thick, upward coarsening succession that begins with highly bioturbated, muddy, very fine to fine grained sandstone that contains an open-marine Cruziana ichnofacies. This is overlain gradationally by ripple-laminated sandstone that is commonly bioturbated and contains mud drapes. The succession is capped by fine- to coarse-grained sandstones that contain both planar and trough cross-strata with unidirectional or bi-directional paleocurrent directions and occasional thin mud drapes on the foresets. The base of a compound dune is gradational where it migrated over muddy sandstone deposited between adjacent dunes, but is sharp and erosional where it migrated over the stoss side of a previous compound dune. The cross strata that formed by simple superimposed dunes dip in the same direction as the inclined master bedding planes within the compound dune, forming a forward-accretion architecture. This configuration is the fundamental reason why these sandbodies are interpreted as compound tidal dunes rather than as tidal bars, which, in contrast, generate lateral-accretion architecture. In the Baronia, fields of compound dunes generated tabular sandbodies 100s to 1000s of meters in extent parallel to the paleocurrent direction and up to 6m thick that alternate vertically with highly bioturbated muddy sandstones (up to 10m thick) that represent the low-energy fringes of the dune fields or periods of high sea level when current speeds decreased. Each cross-stratified sandstone sheet (compound-dune complexes) contains overlapping lenticular “shingles” formed by individual compound dunes, separated by 10–30cm of bioturbated muddy sandstone, which migrated over each other in an offlapping, progradational fashion. Each compound-dune complex (the best reservoir rock) thins as it downlaps, at average rates of 3–4m/km in a dip direction. These reservoir units can be comprised of discrete compartments, each formed by a single compound dune, that extend for 500–1000m in the direction of the current, and are at least 350–600m wide in a flow-transverse direction. Distinguishing between tidal bars and tidal dunes in an ancient tidal succession can be difficult because both can contain similar cross-bedded facies and have overlapping thicknesses; however, the internal architecture and sandbody orientations are different. Tidal bars have their long axis almost parallel both to the tidal current direction and to the strike of the lateral-accretion master surfaces. In inshore areas, they are bounded by channels and fine upward. Large compound tidal dunes, in contrast, have their crest oriented approximately normal to the tidal currents and contain a forward-accretion architecture. Coeval channels are uncommon within large, sub-tidal dune fields. The above distinctions are very important to reservoir description and modeling, because the long axis of the intra-reservoir compartments in the two cases will be 90° apart.

Stratigraphic application of Thalassinoides ichnofabric in delineating sequence stratigraphic surfaces (Mid-Cretaceous), Kopet-Dagh Basin, northeastern Iran

This study integrates ichnological and sedimentological data to refine depositional sequences and interpretations of sea-level dynamics for the shallow marine, Albian–Cenomanian Aitamir Formation in northeastern Iran. Three ichnofabrics are present in a succession of glauconitic mudstone and sandstone. This is a sequence that grades upward from a lower glauconitic sandstone unit with trough cross-stratification, hummocky and ripple cross-lamination into a fining-up unit of mudstone with intercalated sandstone beds. The lower unit contains an ichoassemblage of the Ophiomorpha–Palaeophycus ichnofabric (upper shoreface), whereas the upper unit bears ichnoassemblages of the Thalassinoides ichnofabric (in a distinctive level at the top cycle which demarcates the base of the next cycle) (middle shoreface) and the Chondrites–Planolites ichnofabric (lower shoreface). An upper shoreface–lower shoreface trend from the Ophiomorpha–Palaeophycus ichnofabric to the Chondrites–Planolites ichnofabric represents a deepening-upward sequence. An integrated sedimentological and ichnological approach has allowed the recognition of the internal organization of the sequence and the characterization of significant discontinuity surfaces at sequence scales. Thalassinoides ichnofabric reveals colonization of firmgrounds during prolonged times between erosion and deposition related to transgressive surfaces. Transgressive surfaces (sequence boundaries) are generally well-cemented and marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, unlined, and uncompacted burrows (omission suite) and are associated with rare highly abraded and fragmented shell remains.

Sedimentary environment and palaeogeography of the ?Palaeocene-Middle Eocene Kashkan Formation, Zagros fold-thrust belt, SW Iran

Sedimentary environment and palaeogeography of the ?Palaeocene-Middle Eocene Kashkan Formation, Zagros fold-thrust belt, SW IranThe Kashkan Formation (?Palaeocene to Middle Eocene) in the Zagros fold-thrust belt, SW Iran, which is intercalated between to marine limestone formations, consists of conglomerates, sandstones and siltstones. The sedimentology and the palaeogeography of the Kashkan Formation had not received any attention thus far, but have now been studied in seven sections, situated in the province of Lorestan. The sediments form twelve lithofacies, three ichnosubfacies and seven architectural elements, which are described, depicted and analysed. The analysis leads to the conclusion that most sediments accumulated in a low-sinuosity, low-gradient braided-river system (characterised by mainly unidirectional palaeocurrent directions, and by sheetfloods), that occasionally showed meandering characteristics (represented by overbank deposits and large bars). The deposits of this system closely resemble those of the South Saskatchewan River in Canada, which is considered as the classical example of a sand-bed braided river. The river flowed roughly from North to South, as deduced by palaeocurrent indicators such as imbrication and large-to medium-scale trough cross-stratification (direction measured in the trough axes). This current direction is supported by the southward to south-westward thinning of the formation and by the diminishing average grain size in the same direction. The trace fossils in the Kashban Formation fit all in theSkoyeniaichnofacies, which has here three ichnosubfacies which allow a more detailed palaeoenvironmental reconstruction, indicating that the braided stream passed into the low-energy shoreface zone of a tidally-influenced sea.

Estratigrafia, arquitetura deposicional e faciologia da formação Missão Velha (Neojurássico-Eocretáceo) na área-tipo, bacia do Araripe, nordeste do Brasil: exemplo de sedimentação de estágio de início de rifte a clímax de rifte

The aim of this study was to carry out a stratigraphic revision of the Missão Velha Formation (Araripe Basin, northeastern Brazil) based on detailed facies analysis, architectural elements, depositional systems and palaeoflow data. The main facies are: 1. coarse-grained conglomeratic sandstones and locally pebbly conglomerates, with abundant silicified fossil trunks, several large-to medium-scale trough cross-stratifications and predominantly lenticular geometry; 2. medium-to coarse-grained lenticular sandstones with granules, abundant silicified fossil wood and trunks, and large-to medium-scale trough cross-stratifications, cut-and-fill features and mud drapes on the foreset cross-strata; 3. conglomerates and poorly sorted medium-grained sandstones with sparse pebbles and horizontal stratification; 4. fine-to very fine-laminated silty sandstone interlayered with 5. decimetric pelitic layers with parallel stratification and climbing-ripple cross-lamination. Ten architectural elements were identified: CH: Channels, GB: Gravel bars and bedforms, SG: sediment gravity flows, SB: Sand bars and bedforms, SB(p): sand bedform with planar cross-stratification, OFch: Overbank flow of channel (levee, crevasse and channel splay), DA: Downstream-accretion macroforms, LS: Laminated sand-sheet, LA: Lateral-accretion macroforms and FF: Floodplain fines. Not all of these features were observed at each outcrop. These elements, which are defined by their geometry and bounding surfaces, form the basis for interpreting depositional environments. The Missão Velha Formation is interpreted as: a. high energy braided fluvial systems with fining-upward cycles, channelized features, truncated cross-strata (diastema), lenticular geometry, channeled sandstones, sparse pebbles and b. meandering river systems due to the presence of flood plain deposits, crevasse splays and point-bars deposits. Aeolian deposits can also occur.

Orientation and imbrication of clasts in trough cross-stratification in fluvial sediments

Orientation and imbrication of clasts in trough cross-stratification in fluvial sediments

Proximal–distal ichnofabric changes in a siliciclastic shelf, Early Pliocene, Guadalquivir Basin, southwest Spain

The lower Pliocene deposits, cropping out along the Atlantic coast of Cadiz, in the Conil–Cabo Roche area, formed on a shelf gently deepening towards the W–NW. Up to five types of facies can be recognized, distributed in belts from proximal to distal parts of the shelf: 1) Coarse-grained sandstone with trough cross-stratification facies on the most proximal parts of the shelf (inner-shelf deposits). 2) Medium-grained sandstone with low-angle planar cross-stratification facies deposited in a more distal position due to storm currents transporting sediment offshore. This facies intercalates with the previous trough cross-bedded sandstone. 3) Massive fine-grained sandstone and siltstone facies laterally and vertically related with the previous facies type and representing sediments formed in the middle shelf close to the storm wave-base. 4) Tabular sandstone bed facies corresponding to storm-bed deposits intercalated within the two previous facies. 5) Greenish–bluish shale facies accumulated on the outer shelf below storm wave-base. Three ichnofabrics associated with these facies are recognized: 1) The two cross-bedded facies are dominated by a Macaronichnus ichnofabric, characterized by Macaronichnus and Bichordites traces that represent colonization of a shifting sandy substrate by polychaetes ( Macaronichnus tracemakers) and by sea urchins ( Bichordites producers). 2) A Rosselia ichnofabric, consisting of vertical traces of Rosselia cross-cutting Macaronichnus, characterizes the interval of massive fine-grained sandstone and silt facies and interbedded tabular sandstones. This ichnofabric formed under high sedimentation rates with Rosselia traces exhibiting stacked patterns that indicate vertical readjustment following deposition. 3) Massive fine-grained sandstone and silt facies are distinguished by a Cylindrichnus ichnofabric. This fabric was produced by the activity of a moderately diverse community dominated by sessile, burrow-dwelling, probably superficial-detritus-feeding polychaetes ( Rosselia and Cylindrichnus tracemakers) and vagile, deposit-feeding polychaetes ( Planolites producers). In addition, infaunal deposit/detritus-feeders producing spreiten structures ( Teichichnus) are found in distal areas. Pervasive indistinct mottling of the sediment of this facies was probably due to the activity of very shallow burrowers in the uppermost sediment tiers. This proximal–distal replacement of ichnofabrics reflects the change of benthic communities across the shelf. This distribution does not fit the traditional Seilacherian ichnofacies model for soft-ground shallow-marine settings, in which proximal, vertical-dwelling burrows of the Skolithos ichnofacies dominate high-energy environments. In the Pliocene of Cadiz, shallow-tier burrows of deposit/detritus-feeders characterize the high-energy facies instead, which fit better with the Cruziana ichnofacies. This deviation can be related with the particular palaeoenvironmental conditions. Thus, vagile, opportunistic deposit-feeders colonized the dune foreset when the dunes were stable, and the progradation of the structure favored the preservation of the traces.

Sedimentology, sequence stratigraphy and syn-rift model of younger part of Washtawa Formation and early part of Kanthkot Formation, Wagad, Kachchh basin, Gujarat

The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture and sedimentation processes of these deposits have been studied applying sequence stratigraphic context. Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that deposition was controlled by one single transgressive - regressive cycle. The transgressive deposit is characterized by fining and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium - to coarse-grained calcareous sandstone - mudrocks alternations (2) Association B: fine-grained calcareous sandstone - mudrocks alternations. The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise. Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of: (1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone - siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface, foreshore to backshore and dune environment respectively. Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits (HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift stage.

Ichnological signatures of shallow freshwater lakes in the glacial Itararé Group (Mafra Formation, Upper Carboniferous–Lower Permian of Paraná Basin, S Brazil)

The Upper Carboniferous–Lower Permian glacigenic deposits of the Itararé Group (Paraná Basin) are widely known and cover an extensive area in the southern Brazil. These deposits consist of different arrangements of massive shales, diamictites, rhythmites, and fine- to medium-grained sandstones with trough cross-stratification. Five genetically distinct thickening-, locally coarsening-upward successions of regular rhythmites can be distinguished in the Mafra/Rio Negro region (Santa Catarina and Paraná states, southern Brazil), representing the landward portion of an incised valley filled during deglaciation. A low diversity invertebrate ichnofauna composed of a freshwater, shallow burrow-dominated suite, and trackway-dominated suite occurs in these rhythmites. The freshwater shallow burrow-dominated suite is composed of Cochlichnus anguineus, Cruziana cf. problematica, Gordia arcuata, Gordia marina, Hormosiroidea meandrica, Rusophycus cf. carbonarius, and Treptichnus pollardi, as well as Undichnia consulca and intermittent rusophiciform traces. It represents a Mermia ichnocoenosis and occurs in the rhythmites of the upper sets, commonly overlapped by D. gouldi and D. biformis, in palimpsest preservation. D. gouldi and D. biformis compose the trackway-dominated suite, which occur in all ichnofossiliferous paired mudstone–siltstone of the basal sets of the rhythmite succession, and in almost all of the upper sets. It represents an atypical Scoyenia ichnocoenosis and is generally preserved over microbially induced wrinkle structures. The rhythmic deposition suggests distal underwater gravity-flow deposits, filling shallow lakes or ponds in depressed areas, the siltstones deposited by melting currents and the mudstones by sediment fall-out, after the flux ceased. The abundant wrinkle structures suggest quiet waters. The overlying deposits bearing a brackish-water ichnofauna and marine fossils suggest a coastal setting, connected with the sea. Although atypical, the occurrence of a Scoyenia ichnocoenosis in almost all ichnofossiliferous rhythmite couplets, preserved over wrinkle structures or overlapping Mermia ichnocoenosis, suggests that these shallow lakes were episodically dried up, at least partially. The present data allow interpreting the regular ichnofossiliferous rhythmites of the Mafra Formation as deposited in shallow lakes or ponds developed in outwash plains at the landward side of a fjord embayment during deglaciation.

Re-Evaluation of the Middle Miocene Eagle Mountain Formation and Its Significance as a Piercing Point for the Interpretation of Extreme Extension Across the Death Valley Region, California, U.S.A.

Abstract The Death Valley area of eastern California and southern Nevada has been highly influential in the development of ideas about extreme crustal extension. One of the tightest constraints on Death Valley extension is apparently provided by clasts in inferred alluvial-fan deposits of the Eagle Mountain Formation (~ 15–11 Ma) and their source in the Hunter Mountain batholith, now located > 100 km from some of the deposits. Because alluvial fans are usually less than 10–20 km in radius, the remaining separation has been interpreted as tectonic. New research reported here suggests that the Eagle Mountain Formation at its type location was deposited in a fluvial–lacustrine setting, and provides no constraint on either the magnitude or the direction of tectonic transport and/or crustal extension. Confidence in palinspastic reconstruction thus depends on resolving ambiguities in the correlation of pre-extensional markers or on the recognition of demonstrably proximal facies tectonically distributed across the region. The succession at Eagle Mountain comprises (1) diffusely stratified monolithologic carbonate breccia and sandstone (~ 140 m) onlapping Cambrian carbonate rocks at an unconformity with ~ 110–140 m of relief (fluvial or fluvially influenced); (2) ~ 10 m of tabular-bedded siltstone, diamictite, and sandstone (lacustrine); (3) cross-stratified and channelized sandstone and polymict conglomerate bearing Hunter Mountain clasts, with minor siltstone and carbonate (~ 110 m; mostly fluvial); and (4) tabular-bedded sandstone, siltstone, and minor carbonate (~ 140 m; mostly lacustrine). Eight prominent stratigraphic discontinuities mapped within the third interval are characterized by up to 15 m of local erosional relief, and by abrupt upward coarsening from siltstone or carbonate to conglomerate or sandstone. A fluvial interpretation for the same critical part of the succession is based upon the existence of the mapped surfaces; the ubiquitous development of channels, trough cross-stratification, and upward fining trends (particularly between the mapped surfaces); and the abundance of well rounded clasts in conglomerate. Paleocurrents are generally directed between southward and eastward, although with considerable dispersion, and they shift from approximately southward or southeastward in the mostly fluvial deposits to approximately eastward in the upper lacustrine interval. An unusual feature of the Eagle Mountain Formation at Eagle Mountain is the presence of five crosscutting conglomerate bodies, interpreted as vertically infilled fissures of tectonic origin. Numerous normal, reversed normal, and oblique-slip faults with up to 34 m of stratigraphic separation are thought to postdate sedimentation and tilting of the Eagle Mountain Formation after ~ 11 Ma.