Structures In Turbidites Research Articles

Convolute laminations and load structures in turbidites as indicators of flow reflections and decelerations against bounding slopes. Examples from the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France)

This work discusses the significance of particular types of soft-sediment deformations very common within turbidite deposits, namely convolute laminations and load structures.Detailed facies analyses of the foredeep turbidites in the Marnoso-arenacea Formation (northern Italy) and Annot Sandstones (south eastern France) show that these deformational structures tend to increase near morphological obstacles, concomitantly with contained-reflected beds. The lateral and vertical distribution of convolute laminae and load structures, as well as their geometry, has a well-defined depositional logic related to flow decelerations and reflections against bounding slopes. This evidence suggests an interaction between fine-grained sediment and the presence of morphologic relief, and impulsive and cyclic-wave loadings, which are produced by flow impacts or reflected bores and internal waves related to impinging bipartite turbidity currents.

Geoheritage values in the Cat Ba islands, Vietnam

Geological wonders have been generally known as natural wonderful products. Resulted from geological processes, geological wonders are diverse in size that have geoheritage values that should be protected from damaging of substance, form and natural development. In a large scale, geological wonders can be geoheritage areas, containing several geodiversity elements that are geologically important or in a smaller scale, they can be geosites of heritage values (or geoheritage sites). In the delimitation of areas, having geoheritage values and the establishment of geoparks, the first thing is to recognise them as geosites and geoheritage areas that indicate great geological values. Besides the Ha Long bay, the world natural heritage with its outstanding aesthetic and geological values, the Cat Ba islands are typical and grandeur karst landscapes formed in tropical condition. Based on the geodiversity elements with their own geoheritage values on aesthetics, uniqueness and grandeur in the Cat Ba islands, the authors have recognised three geoheritage areas: the south cape of the Cat Ba embayment, Tung Gau (shelter), and the Lan Ha bay. Sites where Brachiopods, Crinoids and Tetracorals are exposed on the way through the island are considered as palaeontological geosites. The folds of limestone layers in the northern part of Cat Co 3 beach, with typical turbidite structures in carbonate formations are considered as a lithological geosite. The Devonian-Carboniferous boundary near the Cat Co 3 beach is regarded as a stratigraphical geosite while Que Kem and Turtle islands, etc. are considered as geomorphological geosites.

Sedimentary structures: Sorby and the last decade

Abstract Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.

Sedimentary structures: Sorby and the last decade

Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.

Sedimentary structures tentatively attributed to seismic events

Fingerprints of earthquakes in the sedimentary record are as yet difficult to identify. This is particularly true for failure phenomena (dewatering, slumping, fault-graded and turbidite structures) that could have been triggered also by other kinds of energy input. Other promising candidates are strange biostratinomic features (convex-down stacking of shells; current-orientation in wave-ripple accumulations), as well as certain preservational features found in geosynclinal pelagic limestones. But in every case, verification of the seismic origin is still needed.

Exotic Blocks of Forereef Slope, Cretaceous Valles-San Luis Potosi Platform (Mexico): ABSTRACT

The outcrops of the folded Sierra Madre Oriental in the Xilitla, San Luis Potosi area, allow the study of contemporaneous, shallow-water, rudistid-reef, forereef, and basinal carbonate sediments. The and Mendez Formations represent the forereef and basinal sediments and also the host rocks of the platform-derived exotic blocks. Parts of these formations are composed largely of lithoclastic fragments of rudistid reefs, or shallow-water, fossiliferous carbonates mixed with basinal, fossiliferous micrite containing planktonic foraminifers, radiolarians, calcispheres, and a few ammonites. These sediments show such sedimentary structures as graded bedding, and thin horizontal, and convolute laminations; many of these features are arranged in combinations analogous to Bouma's sequence of turbidite structures. The exotic blocks are formed mainly of rudistid biomicrite (biolithite?); their stratigraphic thicknesses range from 10 to 95 m. Stratigraphic correlations between the reef and forereef or basinal sediments were made by using fossils to determine the probable displacement of the exotic blocks; by palinspastic reconstructions of Barremian, Albian, and Maestrichtian times, approximate displacement of 5.5, 3.5, and 1 km, respectively, were inferred. Although the Valles-San Luis Potosi platform is the counterpart of the subsurface Cretaceous Golden Lane platform, it does not have comparable large displacements of sediments which were derived from the reefs of the latter. These sediments were transported 10-15 km or more from the Golden Lane and are now contained in the Tamabra Formation. Perhaps this large displacement has not yet been discovered in the Valles-San Luis Potosi platform, or perhaps the patterns of sedimentation differed, because the tectonic settings of the platforms were different. End_of_Article - Last_Page 772------------

Relationship of flute-cast morphology to internal sedimentary structures in turbidites; a discussion

For reference to paper under discussion, see this Bibliography Vol. 35, No. 4, 17 E71-11075

Relationship of Flute Cast Morphology to Internal Sedimentary Structures in Turbidites

ABSTRACT It is hypothesized that if depositional velocities for Bouma A beds are greater than those for B beds, and B beds greater than C beds, then the morphologies of flute casts associated with A, B and C beds might be different. Four width parameters, two length parameters, maximum depth of erosion, and grain size were therefore measured on 1511 flutes representing 264 beds, mainly in the Ordovician Cloridorme Formation of Gaspe, Quebec. Plan view flute morphology can be characterized by nose shape; a spectrum of shapes from pointed to bulbous was demonstrated. In vertical cross-section, the maximum depth, and distance of this point from the nose can he combined into the parameter B ( = Lmd/Dmax). Study of 77, 56 and 58 flutes on one B bed and two C beds, respectively, showed wide ranges in flute morphology, and no systematic differences between the B and C beds Q-mode factor analysis of one flute from each of 200 beds showed a spectrum of shapes from deep bulbous flutes (associated with A beds) to shallower pointed flutes (associated with B and C beds). Grain size on A beds (-3 to +4 phi) was greater than on B (-2 to +4 phi) and C (0 to +5 phi) beds, and flutes on A beds have a greater absolute width than those on B and C beds. Study of associated sole marks showed that 1, flute tend to occur with other flutes on A beds, but occur with tool and organic marks on B and C beds; 2, organic marks are not associated with flutes on A beds; and 3, longitudinal ridges occur with flutes on A beds but not on B and C beds. In the absence of organic marks on A beds, formation of wide, rounded-nosed flutes appear to be accompanied by bulk erosion of the substrate. Because the flutes were filled by deposition from suspension without bed traction of grains, tool marks tend to be absent. There is a strong contrast between A, and B and C beds, because on B and C beds, flutes were cut without bulk substrate removal, and were filled by traction load with simultaneous formation of tool marks.

Relationship of Flute-Cast Morphology to Internal Sedimentary Structures in Turbidites: REPLY

Relationship of Flute-Cast Morphology to Internal Sedimentary Structures in Turbidites: REPLY

The sequence of sedimentary structures in turbidites, with special reference to dunes

Synopsis The relationship between bed form, grain-size of load, and flow power known from equilibrium-state, open-channel experiments, is revised for turbidity currents, whose power is time-dependent with respect to a point in their path. The revised relationship suggests that cross-bedding, indicative of dune beds, would occur in turbidite sandstones only if the parent turbidity currents had been furnished with sufficiently coarse-grained sediment. Otherwise, the turbidite sandstones are expected to show parallel laminations directly succeeded upward by cross-lamination. The available field evidence is consistent with these conclusions, and some previous difficulties regarding dune structures in turbidites appear now to be resolved.

Stratigraphy and Sedimentology of the Woods Hollow Formation (Middle Ordovician), Trans-Pecos, Texas

The Woods Hollow Formation is a shale lithosome 400 to 500 feet thick that is part of the Ordovician sequence (2000 feet thick) in the Marathon geosyncline, Trans-Pecos, Texas. Estimated percentages of rock types in the formation are 65 percent shale, 16 percent quartzarenite, 15 percent calcarenite and 2 percent each of mudstone and conglomerate (including boulder beds). The olive-gray illite clay-shale was intensely burrowed by soft-bodied organisms; the only skeletal remains are graptolites and sponge spicules. Shale was deposited in a marine basin below surf base, probably in water several hundred feet deep. Quartzarenite and calcarenite beds 1 to 6 inches thick are intercalated through the shale and are characterized by bedding structures (grading; laminations overlain by convolute laminations and current-ripple laminations) that indicate they were deposited by waning currents of initially high velocity; these layers are probably turbidites. About 20 percent of the beds have atypical turbidite structures and possibly were transported or reworked by other marine currents. Siliciclastic and calcarenite debris (ostracods, pelmatozoans, oolites and pellets) were probably derived from Llanoria and a flanking carbonate platform to the east. Boulder-bearing shale and mudstone lenses contain clasts as much as 9 feet long. The coarse debris includes a variety of shallow-water and intertidal carbonate rock types and scarce sandstone, felsite porphyry, and schist. The boulder beds formed as plastic mass flows after the shelf to the west was uplifted and eroded to provide clasts that slid into the basins (Wilson, 1954a). Several beds near the top of the formation contain dolomite of inferred deep-water origin.

So-called turbidite structures

The term 9turbidite structures9 recently used by Sen (1967) is better avoided, because only a combination of features together with the absence of others can be diagnostic. The distinctions in meaning between the terms 9turbidites9 and 9fluviatile deposits9 should not be confused. The explanation of convolute lamination by exaggeration of ripple mark is supported by experimental results.

The sequence of internal structures in turbidites

Synopsis An attempt is made to show how both conditions of transportation and conditions of deposition have to be considered in the interpretation of sequences of internal structures in beds of greywacke and flysch sandstone. Interpretation of graded and laminated divisions involves not only velocity distance (or time) curves but also rates of deceleration.

Turbidite structure from Barakar rocks of Ramgarh coal-field, Bahar, India

Flute casts, groove casts, and convolute laminations have been observed in the nongraded continental Barakar sediments which are interpreted as river-laid deposits. Formation of these structures is generally attributed to the turbidity currents and are regarded as turbidites as the structures are frequently reported from flysch sediments. This study suggests that the turbidite structures should not be restricted to any particular environment since they may occur in any environment provided favorable conditions are present.