STUDY OF FABRIC OF SEDIMENTS DEPOSITED IN PLANE BED PHASE UNDER UNIDIRECTIONAL FLOWS

Abstract

Primary fabric of the ancient sandstones has been used as a paleocurrent indicator by many workers (see reviews Potter and Petti John, 1977; Allen, 1982). The possibility of use of fabric in deducing the depositional processes has been emphasized by Parkash and Middle ton (1970), Parkash (1973), and Allen (1982). However, these applications of fabric need to be supported and corrobo rated by further experimental and analytical investiga tions. Two plane bed phases with sandy sediments i.e. one in the lower flow regime 3nd the other in the upper flow regime have been recognized by the previous workers. The lower-stage plane bed is marked by traction of indi vidual partioies at the interface. However, in the upperstage plane bed phase, a high particle-concentration layer moves close to the bottom. Review of Church and Jones (1982) shows that plane bed with gravelly sediments is altogether different from the sandy sediments. This plane bed is characterized by traction of particles on the bed similar to that taking place on the lower-stage plane bed in sandy sediments. As the lower and upper-stage plane bed phases with sandy sediments are characterized by different depo sitional- processes, fabric patterns of sediments deposit ed in these phases may be distinctive. These patterns need to be investigated and could prove useful in paleohydraulic interpretations. Also, little is known of the role of bed roughness, size and shape of parti cles and sorting of moving sediment and flow strength on the development of fabric in sandy and gravelly sedi ments. It is planned to investigate these problems experimentally under unidirectional flows. Experiments were carried out by moving particles of different sizes over fixed beds under the influence of flow of water to study the orientation of particles. Experiments with sands and sand-gravel mixtures were carried out using 15 sediment mixtures with size varying from 0-25 mm to 31.5 mm and well (uniform size) to poorly sorted. Experiments were conducted both under equilibrium transport and net-aggradation conditions. Water discharge was varied in different runs within the constraints of the stability of plane beds and of the equipment. In the case of net-aggradation condition, plugs were collected using a technique described by Allen (1964b). Samples from top sediment layer were collected in the case of equilibrium transport condition by gently pressing on the bed a slide with resin sprayed on it or grease pasted on it. In sand-gravel mixtures, photographs of the bed at the end of run were taken and studied for particle orienta tion. Some of the plugs collected from beds deposited in the upper and lower-stage plane bed phases were cut perpendicular to the depositional plane and parallel to

the current direction and utilized for the analysis of imbrication of particles. Orientation of particles was analysed by using statistical technique described by Curray (1956) and others. Visual examination of histograms of particle orientation distributions and values of different statis tical parameters have been used to identify eight orien tation distributions (or patterns). These are distribu tions with preferred orientation parallel (P) and trans verse (T) to the current direction, orientation pattern with current-parallel primary mode and current-transverse secondary mode (Pst) or vice versa (Tsp), bimodal distri bution with equally strong modes lying parallel and trans verse to the current directions (Bpt), bimodal distribu tion with 90 apart modes lying symmetrically around the current direction (Bobq), unimodal orientation pattern with preferred orientation at 35° to 55° on either side of the current direction (Obq) and random orientation pattern (R). Each sample consisting of a number of sub-samples collected along the centre line of the flume was assigned one of these patterns. Experiments with fixed beds and in lower-stage plane bed phase were carried out in a small til table, recirculatory flume A 4.5 m in length, 0*15 m wide and 3 with a maximum discharge of 0-002 m /s. The bottom of the flume was coated with sands of mean sizes varying from 0.42 mm to 0«06 mm. Sediment fractions moving over the Vll fixed bed under the influence of flowing water had a range of 0.36 mm to 4.75 mm. Major process of transport of particles in the experiments carried out with fixed beds was rolling with their long axes transverse to the current direction. However, the final orientation was determined by reorien tation which was mainly grain size dependent. The coarser particles (> 2.00 mm) usually preserved current-normal orientation taken during transport, but almost all the finer particles (< 0.71 mm) got reoriented to currentparallel position as they came to rest. Particles in the size range 1.00 mm to 1.40 mm were reoriented to different degrees and the degree of reorientstion was affected by velocity of flow, bed roughness, and shape of particles. Analysis of particle orientation in the lower plane bed phase from uniform sand or sediment fractions in the range 0.25 mm to 1.00 mm indicated that orienta tion distribution was invariably a current-parallel one. However, in the case of a poorly sorted sediment with 45 A of coarse particles in the range 1.00 mm to 1.40 mm, coarse fraction (> 0.71 mm) showed bimodal Bpt pattern and the finer fraction (< 0.71 mm) showed current-parallel patterns. Both upcurrent and downcurrent imbrication values, less than 14°, were noted. Major mode of trans portation in this case also was rolling and reorientation of particles depending upon the grain size of a sediment fraction, and sorting of sediment controlled the final orientation. Vlll In the lower-stage plane bed phase, sands developed current lineations (Type 1) with texturslly segregated, current-parallel elongated patches at shear velocity Reynolds number (R^» = U* d/" , where U* is shear velo city, d is average grain size and V is kinematic vis cosity) in the range 11.0 to 13.32. At higher shear velo city Reynolds numbers (> 14.0) fine current-parallel linea tions (Type 2 current lineation) developed. In this phase, texturally distinct, alternating horizontal-laminations developed in the sands. Experiments with medium to coarse sands in the upper-stage plane bed phase were conducted in large recirculatory type flume B, with a length of 30.0 m, width 0.20 mand a maximum discharge of 0-007 m3/s. Analysis of particle orientation data,collected from the various runs indicated that current-parallel orientation distribu tion pattern with r8re development of a secondary currenttransverse mode was the most common pattern in these sedi ments. Imbrication of particles was found to occur only in the upcurrent direction and imbrication values lie usually between 11.2° and 18-9°. In this phase, a thin layer (about 5-0 mm) with a high concentration of parti cles moving close to the bottom was observed. No current lineations on the depositional surface, as reported by Allen (1964a), were observed in the present study in the upper-stage plane bed phase. ix Samples for particle orientation studies were also collected from experiments carried out by Misri (1981) on five mixtures of coarse sand snd gravel with particles in size range from 0.5 mm to 31.5 mrn. These experiments have been conducted in a large non-re circulatory flume, with a length of 16 m, width of 0.75 m and a maximum discharge of 0.1 m /s. Particle orientation studies of samples indicated that in moderately sorted sediments, orientation pattern observed was bimodal with about 90° apart modes symmetrically disposed around the current direction. In poorly sorted mixtures, a particular size fraction showed current-normal or Bpt pattern at low shear stresses and current-parallel mode became prominent with increase in water discharge. Development of prominent current-parallel mode took place for finer size fractions. Also, more elongated particles had a tendency to attain strong current-parallel mode, whereas prominent currentnormal mode came into existence in the case of less elongated particles. On the basis of experiments it is found that with sand-gravel mixtures, particle movement is mainly by roll ing at low shear stresses. At higher shear stresses, sand particles move by sliding and larger particles usually roll with minor sliding. Reorientation of particles depending upon shear stress and particle size plays important role in determining the final orientation of Particles. Present investigation shows that fabric data can be used to distinguish between horizontal-laminated sand beds deposited in the upper and lower-stage plane bed phases. Rolling with long axes normal to the current direction "seems to be major mode of transportation in experiments with fixed bed, sandy sediments with lowerstage plane bed phase and in gravelly sediments with a plane bed. At higher shear stresses, in the case of gravelly sediments sliding of sand particles is observed.Deg ree of reorientation depending upon particle size, sort ing of sediment and velocity/she9r stress along with mode of transport controls the final orientation of particles in such a case. Particle shape is effective in determin ing final fabric in certain cases, where both rolling and reorientation are involved. Nature of substratum normally does not control the fabric of sediment except in the size range (1.0 mm to 1.40 mm) of moving particles. Horizontallaminated beds with current lineations, hitherto thought to have been deposited in the upper-stage plane bed phase, have been found in the lower-stage plane bed phase.