Well-preserved Sedimentary Structures Research Articles

The occurrences, characteristics, and implications of chaotic rocks (accretionary rocks) within the Trans-Saharan orogenic belt in Nigeria

The present research paper reveals the occurrences of metachaotic rocks/accretionary sediments (mélange) in Igarra, southwest Nigeria. These metachaotic rocks are associated with schistose, marble, pegmatite, granitoid, and gneissose units. The mineral assemblage of the surrounding rocks shows garnet, biotite, staurolite, quartz, and chlorite assemblage, which suggest that the rocks were metamorphosed to the epidote–amphibolite facies grade. The chaotic rocks were classified into four groups based on major differences in color, grain size distribution, and stratigraphic and structural characteristics of observed blocks and matrices surrounding them. Group I metachaotic rocks are melanocratic in color, and form pockets of coarse lineation. They are highly deformed with mullions, rodding structures, and burrow-like cavities and show quartzite veins. Group II occurs as a leucocratic to melanocratic mixture of meta-siltstone and metapelite with greenish rock fragments trapped within the meta-siltstone layers. This unit has a cover of meta-siltstone layers occurring in pockets as striations forming mullion and rodding structures as well as ripple-like lineation. Group III metachaotic rocks occur as multiple benches of different strata tilted between 70 and 80º, with mild folding and burrow-like cavities at the base and well-preserved sedimentary structures. In Group IV, there are evidences of pre-metamorphic soft sediment deformational structures that are well-preserved in the metachaotic rocks, with discontinuous blocks of different units trapped within mixtures of metapelite and meta-siltstone. In this group, micro-faults and quartzite veins are scattered within the rocks. Meta-siltstone occurs as horizontal and vertical layers and web-like structures showing dendritic patterns. Overall, the field and petrographic characteristics suggest the Igarra metachaotic rocks are tectonic in origin and may have been formed within the trench basin of an outer accretionary complex.

Magnetic fabric and sedimentological constraints on the paleocurrent changes within the early devonian strata from the northwestern Tarim, China

The magnetic fabric technique is widely used to measure the direction of paleocurrent and paleostress due to its speediness, precision, and cheapness. However, correlating unique magnetic fabric patterns in weakly deformed rocks with hydrodynamic or tectonic regimes remains challenging. The Early Devonian sedimentary rocks in the Keziertage Formation have well-preserved sedimentary structures and were deformed by the Cenozoic non-plunging fold. This sequence offers a prime opportunity to explore the relationship between magnetic fabric evolution, paleocurrents, and weakly deformation. Furthermore, the paleocurrent characteristics of the Keziertage Formation reflect the provenance changes during the opening of the South Tianshan Ocean. Here, we report 120 sedimentary structure-based paleocurrents and 382 magnetic fabric data from the Keziertage Formation in the Kalpin area, Xinjiang. We find that: (1) During the weakly deformation stage, the minimum axis of susceptibility's imbrication direction serves as a reliable indicator for detecting paleocurrents at the coarse sandstone layer with well-developed cross-bedding; (2) While the maximum axis of susceptibility is more consistent with the tectonic stress rather than paleocurrents. Consequently, the complexity of magnetic fabric needs to be clearly verified when it is applied in the weakly deformed strata. In the northwest margin of Tarim, both the minimum susceptibility axes of magnetic fabrics and sedimentary structures indicate a shift in the paleocurrent direction, transitioning from southwest to northeast in the mid-to-upper Keziertage Formation. This indicates significant channel changes, potentially linked to a regional change in provenance due to the Early Devonian opening of the South Tianshan Ocean.

The Dalradian rocks of the Shetland Islands, Scotland

Metasedimentary and metavolcanic rocks to the east of the Walls Boundary Fault on Shetland have lithological similarities to those of the Dalradian Supergroup of the Scottish mainland. In particular, the middle part of the succession, termed the Whiteness Group, includes numerous metalimestones and associated pelites in a shallow-marine succession that recalls the upper parts of the Appin Group and the Argyll Group of mainland Scotland. Metavolcanic rocks within the deeper water turbiditic sequence of the succeeding Clift Hills Group might be broadly coeval with those of the Southern Highland Group of Scotland. Beyond that, correlations with the established Dalradian succession are tenuous and are not possible at formation level. A local succession immediately west of the Walls Boundary Fault is of even more-dubious Dalradian affinity.The dominant structure is the regional-scale, downward- and east-facing East Mainland Mega-monocline. This has a vertical western limb, which youngs to the east, and an eastern top limb that dips to the north-west at 20–30°. Strata on the eastern limb are inverted on Mainland, Whalsay and Out Skerries but are right way up on the west side of Unst, having been folded around the tight Valla Field Anticline. The Shetland Ophiolite-complex has been thrust over the inverted limb of the Valla Field Anticline on the east side of Unst. The regional monocline folds earlier small- to medium-scale, tight to isoclinal folds with associated planar and linear structures, which are all assigned to a single ‘Main Deformation’. It also post-dates the regional metamorphism, which ranges from chlorite to garnet grade, with localized development of staurolite-kyanite, gneissose fabrics, and the emplacement of schistose granitic sheets in the Colla Firth Permeation Belt.The GCR sites have been selected mainly to be representative of the East Mainland Succession with its associated structures and metamorphism. Highlights include well-preserved sedimentary structures, high-grade gneisses permeated by granitic material, basaltic pillow lavas and serpentinized ultramafic rocks. Some of the latter contain enigmatic skeletal pseudomorphs after olivine and have been interpreted as former high-magnesium lavas.

The conservation of unconsolidated Pleistocene strata: an experiment at Wood Farm Pit, Bubbenhall, Warwickshire, UK

Abstract Wood Farm Pit, Bubbenhall, Warwickshire, central England, was one of a complex of sand and gravel quarries within unconsolidated Middle Pleistocene fluvial and glaciogenic sediments of the Baginton and Wolston formations. These strata document the development of the pre-Anglian Bytham River and its eventual destruction during the Anglian glaciation. They have yielded a freshwater and terrestrial fossil biota and Lower Palaeolithic handaxes, and demonstrate well-preserved sedimentary structures. These features underline the value of the local Middle Pleistocene sites as scientific and educational resources. The Warwickshire Geological Conservation Group introduced an experimental soft-sediment conservation scheme at Wood Farm Pit during 2005–2007. This involved designing, installing and monitoring a screen to protect a representative geological exposure. The structure proved highly effective in terms of low cost, ease of installation, protection of the face and allowing rapid re-exposure. The experiment was halted by flooding within the quarry, following high rainfall.

“Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: a sequence through neomorphism of aragonite to calcite” by Webb et al., Sedimentology, 56, 1433–1463: Discussion

In a recent article, Webb et al. (2009) examined the behaviour of the rare earth elements (REEs) in scleractinian coral during diagenesis from original aragonite to secondary calcite and showed that the mobilization of REEs in the coral skeleton was minimal. In their argument against mobilization of the REEs during carbonate diagenesis they used a paper that I co-authored in 2006 (Johannesson et al., 2006). Specifically, Webb et al. (2009) stated: ‘‘...and Johannesson et al. (2006) recently showed that some groundwaters have REY patterns that are similar to those of sea water (sic) and inferred that ancient sea water-like trace element patterns in limestones and dolostones might represent diagenetic patterns rather than the original fluids in which the limestones originally precipitated’’. Here, REY indicates the REEs and yttrium. Later on in the Implications portion of their manuscript, Webb et al. (2009) write: ‘‘Hence, although terrestrial weathering and groundwater processes are capable of producing fluids with REYSN patterns (where SN indicates shale-normalized) similar, in some ways, to those of sea water (Johannesson et al., 2006), it is very unlikely that the extremely low concentrations of REEs in such fluids could adequately contaminate an existing limestone so as to impose a radically different REE pattern than that which it already contained’’. However, despite these implications, we did not discuss the effects of diagenetic fluids on REE concentrations and shale-normalized REE patterns of carbonate rocks, either modern or ancient. In fact, the word ‘diagenesis’ does not even appear in Johannesson et al. (2006), and the word ‘diagenetic’ is used only once in our Introduction when reviewing previous studies that examined preservation of REE patterns in palaeo-sea water proxies (i.e. biogenic skeletal carbonates and phosphates, hydrogenous Fe–Mn minerals, abiotic carbonates, phosphates and silicates). Specifically, we stated that: ‘‘These proxies fail for a number of reasons including post-diagenetic mobilization of REEs, fractionation of REEs during precipitation/ uptake, and/or because they exhibit exceedingly low REE concentrations that are easily contaminated by exogenous sources (Palmer, 1985; Elderfield & Pagett, 1986; Palmer & Elderfield, 1986; Sholkovitz & Shen, 1995; Bau et al., 1996; Reynard et al., 1999; Shields & Stille, 2001)’’. Consequently, it is important to emphasize that, although it is possible to infer from Webb et al. (2009) that we argued that diagenetic fluids with: ‘‘...extremely low concentrations of REEs... could...contaminate an existing limestone so as to impose a radically different REE pattern than that which it already contained’’, this contention was never put forth by Johannesson et al. (2006). Instead, our principal objective was to reiterate the fact that shale-normalized REE fractionation patterns reported for modern sea water samples are not unique to sea water. As reviewed at length in Johannesson et al. (2006), modern sea water samples commonly exhibit heavy rare earth element (HREE) enriched, shale-normalized REE fractionation patterns that additionally possess anomalously low, shale-normalized Ce values (i.e. negative Ce anomalies), anomalously high shale-normalized La and Gd values (positive La and Gd anomalies) and large, superchondritic Y/Ho ratios (e.g. Zhang et al., 1994; Bau et al., 1995; Nozaki et al., 1997; see fig. 1 from Johannesson et al., 2006). Broadly similar shale-normalized REE patterns reported for some ancient chemical sediments have commonly been cited, along with well-preserved sedimentary structures, as evidence of a marine origin for these chemical sediments (Derry & Jacobsen, 1990; Shimizu et al., 1990; Danielson et al., 1992; Bau & Moller, 1993; Kamber & Webb, 2001; Van Kranendonk Sedimentology (2012) 59, 729–732 doi: 10.1111/j.1365-3091.2011.01264.x

Tsunamis versus storm deposits from Thailand

Along the Andaman (west) coast of Thailand, the 2004 tsunami depositional features associated with the 2004 tsunami were used to describe the characteristics of tsunamis in a place far away from the effect of both recent and ancient storms. The current challenge is that a lack of precise sedimentological characteristics have been described that will differentiate tsunami deposits from storm deposits. Here, in sedimentological senses, we reviewed the imprints of the sedimentological characteristics of the 2004 tsunami and older deposits and then compared them with storm deposits, as analyzed from the deposits found along the eastern (Gulf of Thailand; GOT) coast of Thailand. We discuss the hydraulic conditions of the 2004 tsunami and its predecessors, on the Andaman coast, and compare them to storm flows found on the coast of the GOT. Similar to an extensive tsunami inflow deposit, a storm flow overwash has very similar sedimentary structures. Well-preserved sedimentary structures recognized in sand sheets from both tsunami and storms include single and multiple normal gradings, reverse grading, parallel, incline and foreset lamina, rip-up clasts, and mud drapes. All these sedimentary structures verify the similarity of tsunami and storm inflow behavior as both types of high-energy flow start to scour the beach zone. Antidunes are likely to be the only unique internal sedimentary structures observed in the 2004 tsunami deposit. Rip-up clasts are rare within storm deposits compared to tsunami deposits. We found that the deposition during the outflow from both tsunami and storms was rarely preserved, suggesting that it does not persist for very long in the geological record.

The Goli d’Aget Member: Early Permian volcanoclastic and volcanic rocks within the Briançonnais Grand St-Bernard Nappe (Valais, Switzerland)

The well-preserved sedimentary structures of the Goli d’Aget Member (Early Permian) allow to distinguish four successive environments of deposition: 1) An alluvial plain with important channeling and evidence of volcanic activity; 2) A shallow lacustrine environment with a low water level, deposition of massive carbonates and an increasing contribution of volcanic materials; 3) A fluviatile plain with the deposition of volcanic tuffs; 4) A deltaic to lacustrine environment occasionally covered by pyroclastic flows. An ignimbritic rhyolite from the upper part of the Goli d’Aget Member has an age of 267-282 Ma (U-Pb on zircon). Major and trace elements, as well as zircon typology point to a calc-alkaline and volcanic-arc affinity for that high-silica rhyolite. The Goli d’Aget Member may be the best-preserved unit among the Permian units from the Swiss Penninic Western Alps. This series provides an important point of comparison with other local Permian units (Dent de Nendaz and Cleuson Members) and with similar series in the Western Alps including the Melogno porphyroids in the Ligurian Alps and the Rochachille Series near Briancon (France).

Archean seismites of the Ventersdorp Supergroup, South Africa

This contribution documents the discovery of seismites in pyroclastic lithologies of the Kameeldoorns Formation of the late Archean (2.8 to 2.65 Ga) Ventersdorp Supergroup, South Africa. A variety of well-preserved sedimentary structures document the seismites, comprising syn-sedimentary faults, convolute bedding, graben-like down-sagging structures, syn-sedimentary breccias and syneresis cracks. Earthquakes associated with syn-sedimentary faulting and intrabasinal volcanic activity that affected the unconsolidated sediments of the Kameeldoorns Formation explain the origin of these structures.

A Formação Salinas na área-tipo, Ne de Minas Gerais: uma proposta de revisão da estratigrafia da Faixa Araçuaí com base em evidências sedimentares, metamórficas e idades U-Pb shrimp

The Salinas Formation has been considered to be a stratigraphic unit of the Macaubas Group. This group comprises the rift to passive margin sequences of the precursor basin of the Neoproterozoic Aracuai Orogen, eastern Brazil. However, new roadcuts show extensive and spectacular outcrops with very well-preserved sedimentary structures, allowing detailed studies in the type-locality of this formation, located in the Salinas town and surroundings, northeast Minas Gerais State. In its type-locality, the Salinas Formation consists of graywacke, pelite and clast-supported conglomerate, metamorphosed in the greenschist facies. The sedimentary lithofacies are grouped into three facies association (shelf, slope and deep-sea), indicating sedimentation from shelf deposits to deep-water turbidites. The shelf sedimentation was influenced by storm-wave during deposition. The slump and deep-sea deposits were generated by gravitational flows and high- to low-concentration turbidity currents. Shelf sandstone, clast-supported conglomerate and proximal to distal turbidites outline a submarine fan system. U-Pb SHRIMP data from detrital zircons of graywacke samples indicate a maximum sedimentation age of ca . 568 Ma. Thus, the Salinas Formation is much younger than the Macaubas Group, and represents late orogenic deposits ( ca . 568- 500 Ma). The distal, passive margin unit of the Macaubas Group is now called Ribeirao da Folha Formation ( ca . 800 Ma). Keywords : Salinas Formation, Macaubas Group, Aracuai Belt, sedimentary facies, depositional environment, U-Pb geochronology

Depositional facies and sedimentary successions of earthquake-induced tsunami deposits in Upper Pleistocene incised valley fills, central Japan

Two ‘anomalous’ deposits with well-preserved sedimentary structures were discovered in a drowned valley and in relict barrier sand body deposits in the Furuya Mud and Kyomatsubara Sand of the Upper Pleistocene incised valley fills. The sedimentary features of both deposits are interpreted as ‘earthquake-induced tsunami deposits’, which were deposited in incised valley fills. The tsunami deposit in the drowned valley (Furuya Mud) is 150 cm thick and shows a contorted structured bed in the lower part and cross-laminated sand in the upper part. The tsunami deposit in barrier sand body (Kyomatsubara Sand) is 270 cm thick and shows structureless sand with mud clasts in the lower part, cross-laminated sand with mud-drapes in the middle part and mud with small wave-ripples in the upper part. Taken together, these sedimentary features suggest a model for the composite succession, which is comprised of three units: a lower earthquake-shock-influenced (anomalous) convoluted part, a middle part dominated by tsunami-generated tractional currents, and an upper suspended-sediment part. The lower part is interpreted as earthquake event deposits (seismites) and the middle and upper parts as tsunami current deposits (tsunamiites).

Sedimentary structures made by shore ice in muddy tidal-flat deposits, St. Lawrence estuary, Québec

Well-preserved sedimentary structures formed by shore ice are exposed in emergent muddy tidal-flat deposits along the shoreline of the St. Lawrence estuary. The sedimentary features are exposed in high-marsh micro-cliffs 1 to 2 m high. Features from two localities where they are the most abundant (Sainte-Anne-de-Beaupre, north shore, and Montmagny, south shore) are reported here. Two categories are distinguished: first, bowl-shaped circular and elongated structures 30 to 100 cm across, which are former scoured depressions subsequently filled by stratified mud, fine sand and organic debris; and second, deformations (folds) a few decimeters to a few meters in size produced by ice-block pressure. Pebble- to boulder-size dropstones are associated with these structures. Most features were observed in mud-flat and low-marsh units (facies) vertically exposed at these localities. The age of these tidalite deposits ranges from about 200 to 600 years. These ice-made structures and deformations, well-preserved in a sequence of Recent emerged tidal sediments, suggest that similar features may be preserved in older rock formations. Consequently, they can be considered as a promising tool for interpreting the former climatic conditions in which tidalites and other shallow marine and lacustrine formations may have been deposited.

Unreworked Archaean basement and Proterozoic supracrustal rocks from northeastern Disko Bugt, West Greenland: implications for the nature of Proterozoic mobile belts in Greenland

Whole-rock Rb–Sr and Pb–Pb and zircon U–Pb isotope data yield an age of approximately 2800 Ma for the Atâ granite from northeastern Disko Bugt, West Greenland. Field observations and isotope data suggest that the surrounding gneisses were formed by deformation and recrystallization of granitoid rocks similar to the Atâ granite some 100 Ma after the emplacement of the granite. Rb–Sr whole-rock data on siltstones at low metamorphic grade give an age of 1760 ± 185 Ma, which is interpreted as the time of closure of the isotope systems after metamorphism. The initial 87Sr/86Sr ratio demonstrates that the sediments were probably deposited during the early Proterozoic.Field observations and isotope data show that Proterozoic (Hudsonian s.l.) deformation and metamorphism were weak in the investigated area. The Archaean basement is well preserved, and the metasediments have well-preserved sedimentary structures. The area lies between the Proterozoic Nagssugtoqidian and Rinkian mobile belts of West Greenland, which are thus separated by an area of Archaean rocks little affected by mid-Proterozoic tectono-thermal events. Thus the two belts form separate tectonic units and do not constitute a single contiguous vast area of Archaean rocks reworked by Proterozoic deforma-tion and metamorphism. It is suggested that the Proterozoic mobile belts of West Greenland are orogenic zones of restricted width (a few hundreds of kilometres) that may be interpreted in terms of modern plate tectonic processes.

Petrology and Diagenesis of Early Miocene Deep-Sea Fan Deposits near Point Arena, California: ABSTRACT

Early Miocene deep-sea fan deposits of the Skooner Gulch and Gallaway Formations near Point Arena, California, were examined to determine which factors influence or control diagenesis in fine-grained turbidite deposits. Well-preserved sedimentary structures (including complete Bouma Ta-e beds) are nearly continuously exposed in a vertical sequence along the coast. This sequence is interpreted to represent outer fan depositional lobes, lobe fringe, and basin plain deposits. Four representative stratigraphic sections were sampled and analyzed using the scanning electron microscope, energy dispersive X-ray, X-ray diffraction, nuclear magnetic resonance, electron microprobe, and thin section analysis. Rock types examined include medium to very fine-grained arkoses, lithic arkoses, mudstones, siltstones, and hemipelagites. Diagenesis of a turbidite sequence is complex and does not involve uniform changes in detrital and authigenic minerals. Factors affecting diagenesis include variations in the original depositional environment, sediment composition, texture, stratification, frequency of interbedded shale beds, bioturbation, degree of compaction, and original porosity and permeability. Diagenetic changes recognized were: (1) alteration and resorption of unstable detrital grains, such as feldspar, rock fragments and detrital clay; (2) formation of authigenic quartz, potassium feldspar, and clay cements which reduced porosity and permeability; (3) formation of iron oxide and pyrite; and (4) incorporation of detrital and authigenic minerals in late-stage carbonate cement. All of the observed diagenetic cha ges can occur at relatively low temperatures and shallow to moderate burial depths. End_of_Article - Last_Page 1007------------

Sedimentary history of the island Læsø Denmark

Coastal sections on the island Lres!l in the northern Kattegat, Denmark, expose many well-preserved sedimentary structures. These structures have been studied in order to gain information about the postglacial sedimentary history of the island. Furthermore, numerous beach ridges make it possible to reconstruct the size and shape of the island at all stages of its development. During early postglacial transgressions and alternating regressions the island was represented by a shoal. During the transgres- sions the shoal was inhabited by the heart urchin Echinocardium cordatum, whereas it was inhabited by the Iugworm Arenicola marina during regressions. Heart urchin-burrowed layers are found in three horizons with lugworm-burrowed beds below and above the upper heart urchin-burrowed horizon. The burrows of the two animals are found in different sedimentary facies, thus also suggesting different hydrographic conditions by means of physical sedimentary structures. In places the lugworm-burrowed horizons are characterized by irregular erosion smfaces. Similar structures are found today at the pseudo-tidal flats south of Lres!!, where they are formed by partial erosion of algal mats. The alternation of different sedimentary structures including trace fossils and irregular erosion surfaces is thus believed to reflect postglacial transgressions and regressions.

Possible Non-Turbidite Origin of Deep-Sea Sands in Cretaceous Flysch (Bavarian Alps, Germany) and Recent San Diego Trough (California): ABSTRACT

The Bavarian-Austrian flysch between the Rhine River and Vienna consists of Cretaceous clastic rocks, about 1,500 m. thick, deposited in an east-west-trending trough more than 500 km. long, 20-40 km. wide, and more than 200 m. deep. The directions of sediment supply, as inferred by sedimentary structures, grain-size variations, and heavy- and light-mineral distributions, were remarkably constant and parallel with the trough axis during long time intervals. Filling took place from both ends of the trough. The source area was at the west (Switzerland) during the Early Cretaceous, at the east (Bohemian Massif) during Turonian time, and at the west again during the Late Cretaceous. Within this flysch sequence the quartz-graywacke of the 200-m.-thick Gault Formation of Early Cretaceous (Albian) age was deposited as a continuous blanket. Single beds can be traced continuously for as much as 115 km. toward the western source area. Correlation was accomplished by comparison of the distinctive sequence, thickness, and petrography of individual beds (fingerprint-type identification). Grain-size, thickness, and content of unstable minerals in correlated sandstone beds increase slightly toward the source. Observations in the Bavarian flysch not easily explained by the hypothesis of avalanche-type, turbidity currents include the following. 1. Sedimentary structures indicating breaks in sedimentation within graded beds or fluctuations in current velocity and type of material (concentrations of heavy-mineral layers in the upper part of graded beds; repeated grading within millimeter-thick laminae). 2. The consistency of bed thicknesses and current directions, as well as heavy- and light-mineral associations of individual beds through a distance of 115 km. The enormous amount of sand contained in one single bed (2-10 km.3) is more easily explained as grain-by-grain deposition from constant bottom currents with perennially fluctuating velocities. Study of a large number of oriented, undisturbed box cores from the central San Diego trough and the La Jolla submarine fan shows that most of the sand layers have well-preserved sedimentary structures very similar to those observed in the Bavarian flysch. However, because of the absence of distinctive compositional differences in the mineral associations, individual sand layers could not yet be correlated through End_Page 632------------------------------ longer distances within the San Diego trough. Parallel and current-ripple cross-lamination, as well as imbricated mud pebbles, reworked from the underlying old clay, are more abundant than distinct grading. Study of grain fabrics (imbrication) by the use of magnetic susceptibility anisotropy and evaluation of cross-lamination foresets generally show downslope direction of sediment transport, confined to the canyon-fan valley system. Current measurements and observations from deep submersible vehicles indicate that bottom currents capable of transporting medium to coarse sand have a pulsating (tide-related?) flow (maximum velocity, 10-25 cm./sec.) both up and down La Jolla Canyon. These data suggest the probably most of the sand in water depths up to 1,100 m. was transported, or at lea t reworked, by ordinary tractive bottom currents (or diluted steady suspension currents), rather than by occasional spasmodic turbidity currents. End_of_Article - Last_Page 633------------