摘要:

SUMMARYA wide range of subaqueous sedimentary structures is classified according to the behaviour of sediments at the time of formation of the structures and according to the nature of the motions to which these sediments are subject at that time. Assessments of sediment behaviour and nature of motions are given for each group of structures. The principles and elemental bases of the classification are discussed and its advantages are briefly stated.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02112.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

摘要:

SUMMARYAn overgrowth from an echinoderm fragment is a single crystal of calcite in lattice continuity with the calcite crystal of the host. The development history of echinoderm overgrowths during cementation has been elucidated by the identification of iron‐bearing and iron‐free calcites within the overgrowths by employing staining techniques.The iron‐bearing parts of the overgrowths described do not stain uniformly for iron. Instead they exhibit a succession of light and dark stained sub‐zones, which represent the shapes of the overgrowths during their development.The pattern of zoning around basal sections of crinoid ossicles is comparatively simple. The innermost zones follow the outlines of the ossicles. Later zones adopt first quasi‐hexagonal outlines with curved sides, and then true hexagonal zones of prismatic habit. Such basal sections of ossicles give well‐centred conoscope images and are therefore, oriented perpendicular to the c‐axis directions of their calcites.Vertical sections through overgrowths show a more complex zoning: above and below the articulating surfaces of ossicles the zones are wide, and have a marked zig‐zag outline, with the axes of the spires parallel to the vertical axes of the ossicles and parallel to the c‐axes of the crystals. Along the sides of the ossicles the zones are narrow and straight. The multiple terminations in the c‐axis direction reduce in number away from the surface of the ossicle and, if the original pore‐spaces are sufficiently large, ultimately yield a single scalenohedron‐type termination. The multiple terminations have their origins on the articulating surfaces of the ossicles. In life these surfaces were perforated by the entrances to the numerous minor canals which ramify the ossicles. The multiplicity of terminations resulted from more rapid crystal growth from the surface of the ossicle between canals (c‐axis direction) than from the surfaces of the canals themselves (direction per

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02113.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

摘要:

SUMMARYRed beds are present in the dominantly drab Bima Sandstone at two main localities in northeastern Nigeria. Their lithology and petrography are described and chemical analyses quoted. It is deduced that the climate at the source area was conducive to the production of red soils. Alternating humid and dry periods are believed to have provided the conditions favourable for the accelerated erosion of a provenance of igneous and metamorphic rocks of marked relief. For the most part the pigment appears to have been finely‐divided material that was sorted prior to deposition; local environmental conditions determined whether the pigment subsequently remained red or was reduced. The association of oxidising and reducing conditions with shallow‐water sedimentary structures may best be explained by the postulation of seasonal flooding of an alluvial plain in a savanna‐type enviro

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02114.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

摘要:

SUMMARYExamination of the criteria employed by ALLEN (1963) in his classification of cross‐stratified units suggests (a) that an additional criterion, the three‐dimensional shape of cross‐strata, should be used in describing and recognizing varieties of cross‐stratification, (b) that the character of the lower bounding surface of sets should be expressed in descriptive rather than genetic terms, and (c) that in describing the relation between cross‐strata and lower bounding surfaces, an additional term,asymptotic, should be used.The recent discussion of the morphological features of cross‐stratification by ALLEN (1963) represents a significant advance in our appreciation of the variability of this type of sedimentary structure. The valuable classification of McKEE and WEIR (1953) generated a concern for precise description of cross‐stratification which has, in 10 years, made the classification inadequate. Allen's proposed classification, based as it is on observable morphological criteria and utilising a nongenetic terminology, should promote further rapid progress.Allen has discussed the problems that arise in naming varieties of cross‐stratification. He very properly avoids terms with genetic implications and his use of Greek letters results in commendably brief names. Almost inevitably, his chosen nomenclature is not readily applicable in the field due to the difficulty of remembering which letter is associated with which assemblage of morphological features. This difficulty will increase as the number of defined varieties increases. It seems probable that workers will prefer for the most part to give names to varieties of cross‐stratification only in the laboratory where reference to literature can be made. The basis for recognition of varieties will be detailed field notes and photographs of morphological features. Nor is this undesirable, for it necessitates the making of detailed observations in the field which will lead to the recognition of new varieties. An easily remembered nomenclature too often leads to the suppression of significant differences in the interests of “pigeon‐holing” the structure in question.It is therefore important thatallsignificant morphological features should be noted. Consideration of the differences between certain varieties

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02115.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

摘要:

SUMMARYThe revision of terms under criterion 3 as proposed by CROOK (1965) is timely and should be accepted. Useful new terms are proposed to describe the three‐dimensional shape of cross‐strata, but there is as yet insufficient evidence for using cross‐stratal shape as a criterion for classification in a scheme in which morphological characters are selected for their known genetical significance. The proposed revision of criterion 5 is difficult to accept because it is not in accord with known cross‐stratification

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02116.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02117.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02118.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02119.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1965.tb02120.x

出版商:Blackwell Publishing Ltd    

年代:1965

数据来源: WILEY