Recognition Of Sedimentary Structures Research Articles

Sedimentological and lithostratigraphic study of the Foum El Kous Senonian, Central Hight Atlas of Morocco

The study area belongs to the southern edge of the central High Atlas. It is limited to the North by the Sub-atlasic accident and to the South by the extension of the Eastern Anti Atlas formations. To determine the depositional environments of the Senonian series, a sedimentological analysis of the facies was carried out based on field observations by the recognition of sedimentary structures and textures. An exoscopic analysis of the quartz grains was also carried out by a systematic prospecting of the grain relief and of all the characters that will appear on the surface of the grain after the formation of the mother rock at the origin place, during transport, immobilization phases and deposition. It seems very interesting to prospect for the clayey side in these Upper Cretaceous reddish formations because the evolution of the clayey and non-clayey assemblages in this central basin of the High Atlas will depend mainly on the combination of tectonic, eustatic, climatic and other influences. The Senonian of Foum El Kous which has a less important thickness corresponds to continental deposits that act the role of a natural barrier that separates on both sides the two sub-basins: Sidi Ali Oubork in the East and Ait youl in the West.

Geological Note: Hummocky cross‐stratification from the Boxvale Sandstone Member in the northern Surat Basin, Queensland

The Boxvale Sandstone Member (BSM) in the upper part of the Early Jurassic Evergreen Formation, in the northern Surat Basin, consists of up to 90 m of interbedded mudrocks and sandstones, with minor coal and ironstone. The unit was interpreted by Exon (1976) as representing a marine transgression over a coastal plain environment. The upper part of the BSM consists of a 10-40 m thick coarsening-upward sequence and contains several very well-sorted quartzose sandstone beds with trace fossils and low-angle cross-bedding. Mollan et al (1972) suggested that the physical and biogenic sedimentary structures, good primary porosity and concentrations of heavy minerals within these sandstones pointed towards deposition in a lacustrine shoreline setting. Exon (1976) suggested that the presence of acritarchs was more consistent with a marine origin, and Gerrits (1985) considered these beds to represent sediment accumulation in marine shoreface and offshore environments. All evidence gathered to date is ambiguous in that it does not allow discrimination between possible marine or lacustrine environments of deposition. Recent fieldwork has resulted in the recognition of sedimentary structures, including hummocky cross-stratification, which have a bearing on this problem. Hummocky crossstratification has not previously been identified in the Evergreen Formation.

Sedimentation and tectonics in the Scottish Dalradian

Synopsis Sedimentological research on the Scottish Dalradian has progressed from the recognition of sedimentary structures in the 1930s, via the identification of sedimentary facies from the 1950s onwards, to the integration, in the 1970s, of sedimentological data with that from studies of stratigraphy, tectonics and volcanism. This has now led to an understanding of the pre-orogenic evolution of the Dalradian terrane in terms of progressive lithospheric stretching associated with the break-up of the Proterozoic Supercontinent. The Appin and Argyll Groups were deposited on the NW side of a late Precambrian marine gulf which developed over a complex zone of crustal thinning between the Laurentian and Baltic parts of the Super-continent. As extension accelerated, subsidence rates increased and the Dalradian area of the gulf evolved from a relatively shallow shelf into a series of turbidite basins. Thinning of the lithosphere gave rise, in Argyll Group times, to locally intense igneous activity. Subsequently, complete continental rupture along the gulf axis led to the birth of the Iapetus Ocean. By Southern Highland Group times the Dalradian terrane had become part of the new, thermally-subsiding, Laurentian continental margin. One can envisage the geometry and facies variations of many horizons within the Dalradian in terms of a pattern of numerous fault blocks defined by listric normal faults, dipping SE towards the site of continental rupture, and NW–SE trending transfer faults, which divided the gulf and subsequent margin into a series of compartments. It was movements on these faults that largely controlled Dalradian stratigraphic evolution. For example, pulses of rapid stretching, and consequent fault activity, produced basin-deepening sequences which mark the base of the Easdale and Crinan Subgroups.

Gravelly flood deposits of Irvine Creek, Ontario, Canada

ABSTRACTIn May 1974 a powerful flood flushed the Grand River basin, Ontario. The effects on the bedload were drastic in a narrow (30 m) and deep (40 m) rock walled and floored gorge near Elora, Ontario. Along Irvine Creek, the tributary occupying the gorge, the gravel cover was reworked in several types of bars, predominantly transverse and point bars. The bars formed very rapidly in response to essentially steady, non‐uniform flow that developed during a brief period of high flood.Superimposed on major bars are several minor sedimentary features such as coarse transverse ribs, chute channels and bars, longitudinal ribs, imbrication clusters, backsets with well developed imbrication, that were formed under very high stream discharge. Structures like imbrication clusters, transverse ribs and small riffle bars require a ‘live bed’ situation to form, and they develop when stones come to a stop either because they cluster during transport, or because keystone effects occur along shallow channels. In Irvine Creek, very few sedimentary features were formed during waning and low flood stages: only some shadow deposits and a few Ostler lenses. The few fines that were available were lost downstream or filled in lower parts of gravel beds.This study confirms that in streams that experience strong seasonal fluctuations in discharge, bedforms that develop during high floods have a high probability of preservation. In gravelly deposits, foreset structures and plane beds are most commonly preserved, although they may be difficult to recognize in old deposits, which may appear massive, particularly if the gravel has been infilled with finer pebbles and sand. In the case of Irvine Creek, all deposits are organized, and lateral and vertical variations in textures, particularly imbrication and packing, are very useful in the recognition of sedimentary structures.

Discriminant Analysis Versus Factor Analysis of Grain-Size Data from Different Environments and Sedimentary Structures: ABSTRACT

Grain-size data analyzed by multivariate computer techniques allows (1) discrimination of environment of deposition and (2) recognition of sedimentary structures. Multiple discriminant analysis where used in conjunction with variables derived from ¼-phi-sieve grain-size analysis was found to be the most useful multivariate technique. Grain-size samples from 3 environments (Arkansas River, Great Sand Dunes National Monument, Colorado, and Gulf Coast beaches) were used as environments in discriminant analysis. With the intention of classifying 49 samples from Bijou Creek, Colorado, they were listed in the discriminant analysis program as unknowns. Thirty-six of 39 unknowns were correctly classified by the program as river sediments. Q/mode factor analysis correctly classified 32 of the Bijou Creek samples with known river samples. Variations in grain-size distributions within a given environment were studied in an Arkansas River sand wave. Foreset beds, climbing ripples, and horizontal laminations were designated as sedimentary structures. By use of multiple discriminant analysis 20 of 21 samples from a second sand wave were classified correctly. Using Q-mode factor analysis all but 3 samples were classified correctly. By analyzing separately each of the 2 or 3 populations appearing as straight lines on cumulative plots on normal probability paper, greater discrimination between environments was obtained than by using standard grain-size parameters calculated by assuming each sample represented a single population. End_of_Article - Last_Page 873------------