Flume experiments on the orientation and transport of models and shell valves

Palaeogeography, Palaeoclimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

F L U M E E X P E R I M E N T S ON T H E O R I E N T A T I O N A N D T R A N S P O R T OF MODELS A N D S H E L L VALVES P. J. BRENCHLEY AND G. NEWALL Jane Herdman Laboratories of Geology, University of Liverpool, Liverpool (Great Britain) (Received January 7, 1969) (Resubmitted July 21, 1969)

SUMMARY Several series of flume experiments were performed in order to investigate the current response and orientation of models and shells over different substrata. Eighteen different models, of simple symmetrical shapes, and eight species of modern shells, six pelecypods and two gastropods, were used on two different sediment bases, medium sand and mud. All movements of objects were recorded photographically and final measurements were made from the photographs. The experiments were begun with model sets in random orientation, all objects being in the concave-up attitude. From the experiments it was concluded that most objects take up a preferred final orientation of longest axes across the current. Only exceptionally, for example, in the gastropod, Turritella, and the very light weight pelecypod, Cultellus, do current parallel orientations result. Some of the models were loaded with lead to produce an eccentric centre of gravity and to simulate the effect of the thickened umbonal regions of shells. These loaded models indicate the tendency for an up-current preferred orientation of centre of gravity ( = load). Some non-loaded models produced unidirectional patterns, reflecting the relative ease of transport in certain positions. By analysing the behaviour of the objects during the experiments it was possible to break down the final orientation patterns into their component parts. This operation demonstrated that the objects are most perfectly oriented during transport and that finally, this pattern can become more or less dispersed due to impedence between objects, irregularities on the substrata and inversion of objects to the convex-up attitude. Experiments on the two substrata indicate that objects can be moved more easily and for greater distances, and are less likely to be inverted on sand than on mud. This is considered to be due, at least in part, to the relative mobilities of the two sediments: sand sized particles are more easily eroded at a given current velocity than are mud sized particles. Observations on the burial of objects indicate that burial begins at lower Palaeogeography, Palaeoclimatol., PalaeoecoL, 7 (1970) 185-220

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velocities on sand, and that, by scour, objects can become buried in bizarre attitudes to the sedimentary interface. It is restated that objects are most stable in the convex-up position. They are soon inverted to this position on mud and because of this it seems difficult to develop drifted assemblages on this substratum. INTRODUCTION

For many years the geological literature has contained records of fossil orientations of one kind or another. In most cases these are mentioned briefly as incidental observations during descriptions of some other stratigraphic, sedimentological or palaeontological subject. Many other instances of oriented assemblages are known to individual workers but for one reason or another go unrecorded. Relatively few orientations are considered in detail, and when the relationship of these orientations to some directional agent (usually water currents) is deduced, the conclusions are very often speculative. The present authors' initial interest was in fossil assemblages which showed marked preferred orientation patterns and it was to provide some data against which these and other orientation patterns could be assessed, that the present work was initiated. Relatively little experimental work on the orientation of shells had been done prior to the initiation of this project and the main papers in this field were those of TRUSHEIM (1931), MENARD and BOUCOT (1951) and SCHWARZACHER (1963), while LEVER (1958) and LEVER et al. (1961, 1964) had performed a variety of experiments on the transport of valves and models in a beach environment. During the course of the work, three further papers have been published dealing in one case with experimental orientations (KEELING and WILLIAMS, 1967), in another with comparative experimental and field orientations (NAGLE, 1967), and in a third dealing with transport problems of artificial shell valves on sandy beaches (LEVER and THIJSSEN, 1968). The conclusions reached in these three papers are, in some instances, at variance and all papers contain some conclusions which differ considerably from those drawn from this present study. Some of these variations in the results are to be expected in differently conducted experiments, but the fact that they exist indicates the need for more experimental work if sufficient data are to be collected to make interpretation of most fossil orientations possible. Natural environments are usually defined by variously interacting physical and biological characteristics, and the behaviour of shells in such environments will be dependent upon any specific permutation of characteristics. Because of this, it cannot be too frequently stressed that any comparison between the results of a single group of experiments on shells and the natural assemblage must be very carefully considered: the very simplified nature of most experimental series will almost certainly make direct comparison invalid. Many aspects of shell morphology and of environment can have an effect on the final orientation taken up by shells. It is the presence of these many aspects Palaeogeography, PalaeoclimatoL, Palaeoecol.. 7 (1970) 185-220

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that makes interpretation of natural oriented assemblages difficult. In order to make the present experiments interpretable, as many variables as possible were eliminated, and only a few characteristics examined: the environment was standardised in that a simple unidirectional current was used over a uniform substratum. The variable nature of shells was avoided by using standard models in which only the shape or the weight distribution varied. Modern pelecypod shells, and two gastropods, were then used for comparison. Two different substrata were used at different times. During the experiments all the models were run on a sand base, and in this paper, this series is used as a basis with which experiments with modern shells and experiments on a m u d base are compared. The following terms will be used throughout this paper in the sense defined below: Objects: all shells and models used in the experiments. Set: all objects of one type. Run: a single experiment with one set of objects. Series: all runs with either shells or models on the same substratum. CONDITIONS OF EXPERIMENT

The flume and current conditions The flume was a long open channel, 12 inches (30.5 cm) wide; during these experiments a 54-inches (134 cm) stretch of the flume floor was covered with a layer of sediment, approximately ½ inch (1.3 cm) thick. The experimental base was limited on the down-stream side by a weir, 1.4 inch (3.6 cm) high, which served both to retain the sediment base and to allow a shallow depth (ca. 1 inch = 2.5 cm) of standing water when the current was switched off. At the up-current side the sediment base was delimited by a low-angle run-up plate, which retained the sediment and also allowed smooth flow of water onto the sediment. The current velocities in the flume were calculated by weighing the volume of water flowing over the weir in a given time: the current velocity was finally calibrated against the height of water over the weir, which could easily be read off from a micrometer screw gauge. The relationship between depth of water and current velocity is given in Fig.lA. The range of current velocities used was from 0-20 inches/sec (0-50 cm/sec). The flow was uniform and turbulent, with Reynold's numbers ranging from 0 to 200,000 depending on current velocity (Fig. 1B). Two types of sediment were used for different series of experiments. A medium sand of mean grain size 0.4 m m and maximum size 1.0 m m and a fine mud/silt of maximum grain size 0.08 m m and mean 0.02 mm. Starting with a smooth sand surface in the flume, a bed roughness developed as the current velocity increased. At velocities of about 20 cm/sec scour around some of the objects commenced and at velocities of about 30 cm/sec there were Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 185-220

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crescentic scour hollows upcurrent of most objects. At velocities greater than 35 cm/sec small-scale asymmetrical ripples with length of about 5 cm and ripple height 0.5-1.0 cm formed. With these higher current velocities the sand surface was highly irregular with deep scour marks, ripples and interference bed forms where objects interfered with the formation of regular ripples. With a mud base scour marks and small scale asymmetrical ripples formed at similar velocities to those on sand, but the scour marks were less deeply eroded. It was mainly the silt fraction which was eroded and put into transport by the currents. The ripples had a length of about 2.5 cm and a height of 0.25 cm or less. The models and shells The test models were constructed to give a wide variety of simple sym-

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metrical shapes. The models can be referred to one of two main groups depending upon their symmetry: (1) Monosymmetrical models. Those with one plane of symmetry, e.g., semicircles. In such models two orientations can be recognized; a lineation and a unidirectional orientation (see, for example, Fig.2h, i; Fig.4b, c). (2) Disymmetrical models. Those with two planes of symmetry at right angles, e.g., rectangles. In such models a single orientation can be recognised--a lineation, measured here as long axes (see, for example, Fig.2b, d, e). The range of shapes used can be seen in Fig.2 and are referred to in the text as an outline shape together with a code number. The models were moulded to give a fairly standard convexity. The models were made from plastone modelling clay, homogenised with lead dust to give a specific gravity of 2.60, near to that of shell calcite. To protect the models while in water and for ease of recognition, the models were sprayed with black paint on the convex side and white on the concave. Finally the shells were numbered. The clay was always 1.55 mm thick, and the mean weight, length, width and height (i.e., convexity) for each of the shapes is given in Table I. A second group of models fewer in number, was made identical to the others except that the models were loaded at a point on the periphery with lead shot (only the loaded equilateral triangle LT has no unloaded counterpart). This caused pronounced eccentricity of the centre of gravity and represented the effect of shell thickening around the umbones. Eight species of modern pelecypod and two gastropods were also used. Of these, two--Glycimeris glycimeris and Venus casina--were abandoned since their mass was too great for the shells to move under the experimental conditions. The shells finally used were: Venerupis rhomboides, Gari tellinella, Ensis sp., Cultellus pellucidus, Nucula turgida, Mytilus edulis, Turritella communis and Capulus ungaricus. Nearly equal numbers of left and right pelecypod valves were used in the experiments but systematic differences in their behaviour were not recorded (cf. LEVER and THIJSSEN, 1968).

Procedure On the sand base, experiments were performed using all the shells and models. On mud a selection of models and shells were chosen to represent the main range of shapes: thus five plain and four loaded models were used and five shells (omitting the two peculiar forms, Capulus and Turritella). Before each run the sediment base was carefully levelled and each object was placed under standing water on the segment in the concave-up position. This starting position was chosen since it is less stable than the convex-up position and, therefore, allows the greatest scope for movement. Also because in natural assemblages some considerable proportion of valves would be expected to fall into the concave-up position after death of the organism and, therefore, begin movement from this position. In each run about ten objects were used. The

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objects were arranged so that their measured axes were in random orientation. This randomness was arranged by eye and it will be apparent from some of the initial orientation patterns on the figures, that the eyes do not always select the most random positions. At the beginning of each run the current was started and the velocity was gradually increased by small amounts: at each increase steady flow was maintained until any movements of the objects had ceased. The run was ended when the orientation pattern of the objects was stable through two or more current increases. All records of movements were made photographically, by a camera fixed permanently over the flume. Photographs were taken at short intervals and as often as was necessary to record movements: at least one photograph was taken for every increase in current velocity. All measurements were made from photographic prints and were plotted on rose diagrams, using 10 ° class intervals. Initial and final orientation patterns were plotted for each experimental run, and other tracings were made to show the sequence of movements during transport: examples of these are given in Fig.10, 11. It is important that concave-up and convex-up objects be distinguished in the final orientation pattern, especially when a unidirectional orientation is being considered. Fig.3 demonstrates how inversion modifies the orientation patterns taken up during transport. Fig.3A shows how a bimodal pattern can be converted to a unimodal one, and Fig.3B the converse, where a unimodal pattern during transport is converted to a bimodal final orientation by inversion of some of the objects.

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FLUME EXPERIMENTS OF MODELS AND SHELL VALVES

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RESULTS OF EXPERIMENTS

Results o f experiments on a sand base Models. Starting with an initial random orientation of models, each run finished with a final preferred orientation with the longest axis lineated across the current. The influence of the longest axis was always most important, so that any other factor of the model's shape was of less significance in controlling its orientation. For example, in the model rounded triangle, $4 (Fig.2j), the presence of the straight edge and the symmetry of the shape were unimportant in controlling the orientation: the longest axis, perpendicular to the straight edge ,was the factor which influenced the resultant cross-current orientation. All the models, whatever their shape, showed no tendency for current-parallel orientation of their long axes. When monosymmetrical models are considered and their orientation patterns plotted unidirectionally, some show no preference for up- or down-current orientations, e.g., the triangles, T3 (Fig.4g), and the rounded triangles, $4 (Fig. 4d); this endorses the fact that in these cases only the long axis influences the orientation. In other monosymmetrical models, e.g., semicircles, S1 and $2 (Fig.4a, b), and triangles, T2 (Fig.4f), a preferred unidirectional orientation is seen: this orientation is of the longest straight edge (the "hinge lines") down-current while the models remain in the concave-up position. When the models are inverted to the convex-up position, the expected up-current orientation of the "hinge lines" results. The loaded models can also be measured unidirectionally, and the resultant orientation patterns demonstrate the interaction between long axis and centre of gravity. In general, the preferred orientation is with the centre of gravity (i.e., the loaded portion) up-current of the long axis which itself maintains a basically cross-current orientation (see Fig.4h, i, j). The loaded equilateral triangle, L T (Fig.41), which has three equally long axes, demonstrates the influence of the centre of gravity in producing a preferred unidirectional orientation with the centre of gravity up-current. Where the loaded model had a distinct long axis and was weighted at one end of it, e.g., loaded ovals, LO (Fig.4h), and rectangles, L R (Fig.4k), the long axis is still the most important control on the final orientation. The centre of gravity comes to lie on the up-current side of the long axis, but is of secondary importance. (The asymmetry of the final orientations, seen in Fig.4h and k, to the left or right of the current flow, have apparently occurred totally by chance and do not seem to have been current-induced.) It should be made clear that the orientation patterns described above are related to models of a certain weight and size and with a given amount of loading. It would be expected that if the amount of eccentric load was increased, that this could come to have a greater influence on the final orientation; ultimately coming to act like an anchor. Palaeogeography, PalaeoclimatoL, Palaeoecol., 7 (1970) 185-220

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F L U M E EXPERIMENTS OF MODELS A N D S H E L L VALVES

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edge. The groups demonstrate further the importance of the long axis in controlling the cross-current orientation pattern. Also they demonstrate that overall shape influences the position taken up by the model during transport, these models being most successfully transported with the straight edge ("hinge line") down-current. (These models may later be inverted and, therefore, finally come to rest in the convex-up position with the straight edge up-current--see Fig.3.) The centre of gravity does not, in these cases, influence orientation during transport, but it is of utmost importance in producing the final orientation. Models loaded on the straight edge and being transported with this edge down-current are relatively unstable and are most easily inverted to the convex-up position (Fig.4i), while models loaded on the rounded edge and being transported with this edge up-current, are most stable and do not easily invert (Fig.4j)--see p. 196 for further discussion of mechanisms of transport).

Shells. The reaction of shells to currents over a sand base can be compared in general terms with that of models. Detailed absolute comparison is not possible due to the irregular shapes and other variables within the shells. In general, shells take up the same type of final orientation as was seen in the models, i.e., a lineation of longest axes across the current. Venerupis is basically oval in outline and takes up a very pronounced crosscurrent lineation (Fig.5d) and can be compared with the oval model, 03 (Fig.2c). Mytilus responds on a similar way, though the final pattern is more dispersed due to the irregular shape and weight distribution (Fig.5a). This cross-current lineation of Mytilus in these experiments is totally unlike the bimodal, but predominantly current-parallel orientation pattern recorded by KELL1N6 and WILLIAMS (1967). The significance of this difference will be discussed later (p. 204). Nucula is basically triangular in shape and reacts similarly to the triangular models, T2, preferring the long axis across the current (cf. Fig.21 with Fig.5f). This result is interesting in that in Nucula the longer axis is only slightly longer than the other (100~/88%) and yet this difference is apparently amply sufficient to cause a marked lineation. Ensis shows no final preferred orientation (Fig.5h) and this can be attributed to the large size of the shells used: shells frequently collided with each other during transport, so that their final attitudes were irregular and did not bear a direct relationship to current direction (further discussion p. 216). Of the shells used in the present experiments, Cultellus is the only exception to the general cross-current lineation rule (Fig.5i). Here a marked current-parallel orientation is produced. This is due to the remarkably low mass of Cultellus valves, allowing transport at relatively low current velocities. At higher velocities Cultellus behaved differently to any of the other shells or models used, exhibiting a tumbling motion during transport. When coming to rest, the valves appeared to touch the substratum with one end and then rotate into a current-parallel position. Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 185-220

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Apart from the lineations described above some of the shells exhibit a preferred unidirectional final orientation, which as with the models, was controlled by long axis, centre of gravity and sometimes overall shape. Nucula has preference for the umbones up-current of the long axis, when the shells are in the concave-up (transportable) position (Fig.5g). Venerupis and Gari reflect the long axis and centre of gravity controls, having a marked preferred orientation of umbones (i.e., centres of gravity) up-current (Fig.5k). When the final orientation of Mytilus is plotted by measuring the direction at right angles to the straight edge (Fig.5b), a unidirectional orientation is apparent. This indicates that Mytilus is more easily inverted when the straight edge is down-current, comparable to the situation in the semicircular models (see, e.g., Fig.4b). If the direction to which the umbones point is plotted this shows that the umbones are preferentially oriented a little up-current of the transverse position (Fig.5c). This pattern is reminiscent of that of oval models, loaded at one end (Fig.4h), but the analogy may not be an accurate one, as the umbones of Mytilus are not greatly weighted. These two different unidirectional plots of Mytilus yield different interpretations, which at first seem paradoxical although derived from the same final assemblage of shells. This exemplifies the difficulties in the analysis of natural shell assemblages and nicely indicates why models, as well as shells, were used in these experiments. It is also interesting that Mytilus is, to date, the most commonly used shell and yet its one-sided shape makes it perhaps the most unsuitable shell for empirical experiment. The two gastropod species show unidirectional orientations: Capulus, a peculiarly shaped open, twisted cone, has a dispersed final pattern, with apices up-current (Fig.5j; influence of long axis is apparent in causing the dispersion). Turritella shows quite the most remarkable preferred orientation of apices upstream (Fig.5k). This is a quite different situation to that described by KELLIN6 and WILLIAMS(1967) in the gastropod Nucella: the discrepancies will be discussed later (p. 213).

Results from experiments on a mud base Models. The data from the models run on mud were grouped. Those models having a pronounced long axis and being disymmetrical are plotted on the same diagram: the lineation of these models is very pronounced across the current (Fig.6f). In the triangles, T2 (Fig.6g), the same cross-current lineation is seen but with the addition of a secondary current-parallel group. Here, the transported models come to lie across the current and invert to the convex-up position, while those models which remain unmoved are mainly in a current-parallel orientation in the concaveup attitude giving a second maximum. The loaded ovals, LO (Fig.6i), and the loaded equilateral triangles, LT (Fig.6j), show poorly developed lineations and their orientations are more readily interpreted when unidirectional plots are examined. Palaeogeography, PalaeoclimatoL,Palaeoecol., 7 (1970) 185-220

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Fig.6. Initial and final orientation patterns of models on mud, measured as unidirectional orientations (a-e) and lineations (f-j). Palaeogeography,

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Unidirectional plots indicate the up-current situation of the centres of gravity (Fig.6a-e), but it is important to note that the orientations arise from different causes. In both the loaded ovals, LO (Fig.6e), and equilateral triangles, LT (Fig.6d), the orientation results from transport in an oblique position with the load up-current. The asymmetry of the up-current modes in these models is fortuitous, reflecting the asymmetry of the initial orientation: it is the presence of these modes, visible in the unidirectional plots, which produces the oblique appearance of the lineations (Fig.6i, j). The semicircles loaded at the rounded edge differ in their orientation from those on sand: the resultant diagram (Fig.6c) shows the same up-current distribution of the centre of gravity, but it should be remembered that in this case on mud, the models were transported with the rounded edge downcurrent and most of the models are inverted to the convex-up position (see Fig.3). Semicircles loaded at the straight edge react in the same manner as do those on sand (cf. Fig.6b with Fig.4i). Thus, in general, the final orientation patterns on mud are very similar to those on sand: only occasionally are there differences. However, it is noticeable that on mud the preferred orientation patterns tend to be more strongly developed than on sand: on a mud base models are transported only a relatively short distance before being inverted and consequently there is less interference. On sand interference between models in an assemblage is common, giving a more dispersed orientation pattern. This will be fully discussed on p. 215.

Shells. Venerupis and Gari take up a very pronounced cross-current lineation (Fig.7a), reacting like the disymmetrical models (Fig.6f). Ensis too takes up a cross-current lineation (Fig.7g), with a secondary mode developing at right angles to the main mode, in a current-parallel direction. This contrasts to the situation on sand where there was no obvious preferred orientation. Nucula takes up a preferred lineation of long axes across the current (Fig.7e), while Cultellus becomes aligned in a current parallel direction (Fig.7h), a similar reaction to that on sand. Mytilus lineates across current, though the pattern is dispersed being slightly bimodal (Fig.7c). Mytilus and Venerupis and Gari also take up preferred unidirectional orientations of umbones up-current during transport in the concave-up attitude but m a n y valves invert to give the orientation seen in Fig.7b and Fig.7d. Nucula, however, plots randomly when considered unidirectionally (Fig.7f), a result unlike that on the sand base. The major difference between the final orientations of objects on sand and on mud is that in the latter case more objects become inverted to the convex-up position. This should be borne in mind when comparing the final orientations of assemblages on sand with those on mud.

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TRANSPORT PATTERNS AND FACTORS INFLUENCING THE FINAL ORIENTATION

In the following paragraphs the behaviour of models and shells during transport will be discussed. Analysis of the transport patterns can explain some of the differences in the final orientations obtained in this present work and in the experiments of previous workers. When examining the final orientation patterns, the relationship between current directions and preferred orientations is, in most cases, readily seen. When considered in detail, however, the disposition of individual objects can be outside the modal peaks of the orientation. It is often impossible to explain these minor anomalies.

Initial movement As can be seen from Table I the current velocities at which movement commences varies from one object to another. This variability is presumably due to a combination of factors, including individual total mass of the objects, convexity and shape. Any one of these characters separately has a random or only poorly correlated relationship to the current velocity of the initial movement; e.g., from Table I it can be seen that the mass of the different objects does not correlate closely to the velocity at which movement commences, but that in general the very light objects do tend to move at lower velocities than the rest. The initial movement of individual objects within an assemblage is influenced by the attitude of the objects (convex-up; concave-up) the initial angle of the object to the current and slight variations in roughness of the sediment substratum. The importance of the initial attitude of objects with a convexity is demonstrated by the relative ease with which objects were transported in the concave-up position, and their stability once they had been inverted to the convex-up position. Similar experiments to the present ones were conducted by KEELING and WILLIAMS (1967) who placed Mytilus in an initial convex-up attitude, which contrasts to the concave-up initial attitude of all objects in the present experiments. This difference of initial attitude explains some of the differences in the results of the two groups of experiments. Mytilus valves placed in the convex-up attitude (KEELING and WILLIAMS, 1967) re-oriented in the current but were rarely transported, so that the affects of transport on the final orientation patterns could not be investigated in the experiments of Kelling and Williams. With the sets of models used in our experiments there was found to be little correlation between initial orientation of particular objects in any one set and the current velocity requited to initiate movement (competent velocity; see Fig.8). In a general sense, however, it can be seen that models starting in near currentparallel orientations require the greatest velocities to initiate movement, while models in all other positions are equally easy to move. As an extension of this conclusion it can be seen from Fig.9a that models which do not move at all during Palaeogeography, PalaeoclimatoL, Palaeoecol.,7 (1970) 185-220

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Fig.9. Analysis o f final orientations in terms o f the c o m p o n e n t parts considered as lineations, a. Models showing n o movement, b. Models which rotated hut were not transported. c. Models which were transported but h a d transport stopped by impedence, d. Orientation during transport, e. Models which were deposited without impedence. P a l a e o g e o g r a p h y , Palaeoclimatol., Palaeoecol., 7 (1970) 185-220

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P. J. BRENCHLEY AND G. NEWALL

the experiments tend to be those which are aligned in a near current-parallel direction: this can also be seen in rectangular models 1 and 2 in Fig.10C. The component of the original sample which shows no movement has considerable effect on the total final orientation pattern of the assemblage, since the preferred orientation of long axes is at right angles to the trend of those models which become current oriented during transport. Allied to those models showing no movement, are those which rotate in response to the current passage, but do not get transported. This is the smallest component group. The models rotate from their initial position into one which is more nearly cross-current (Fig.9b). This rotation can be seen, for example, in the semicircular model 2 in Fig.10A and shell 5 in Fig.11C. The poor correlation found between competent velocity and the initial angle of models to the current contrasts to the good correlation found by SCRWA~ZACHER (1963) in his experiments with crinoid stems. The difference in results is likely to have arisen from the contrasting behaviour of crinoid stems, which move by rolling, and convex plates, which rest with only a small part of the convex surface on the substratum and move by saltation (see below). The first movement of a convex object was commonly a rotation of its long axis into a cross-current position: this initial rotation, preceeding transport, diminished the importance of initial orientation in determining the velocity at which transport commenced.

Transport Except in a few exceptional cases, which will be discussed below, all the models and shells used in the experiments were transported in a concave-up position with long axes across the current: this orientation is very pronounced (Fig.9d). The cross-current lineation can be seen in Fig.10A, especially model 1. During transport, the elongate objects move by saltation coupled with an oscillatory motion about a mean cross-current position of the substratum. It seems, from observation, that movement of the shells or models depends not only upon water currents getting under protruding surfaces and creating lift, but also upon the production of a local moving carpet of sediment grains beneath and to the sides of the object, as water is accelerated past it. Sediment grains beneath the objects are in motion while transport continues, and scour marks develop at the lateral extremities of the objects and are continually formed as the model or shell is transported: this results in parallel, shallow scour grooves being developed. A comparison of transport patterns developed on mud with those developed on sand, indicates that the same style of movement takes place on both substrata, but on mud the same object requires a higher velocity to initiate movement (see Table I). Further, it is found that objects can be transported over much greater distances on sand than on mud. It is suggested that the differences in the competent velocities and the extent of transport of objects on sand and mud is largely controlled by the more pronounced development of a moving traction carpet on the Palaeogeography, PalaeoclimatoL,Palaeoecol., 7 (1970) 185-220

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Width of flume is one foot ( 30"5cm ) Fig.10. Series diagrams, drawn from photographs, showing the positions taken up by certain models on sand and mud substrata, from the beginning to the end of the runs.

Palaeogeographv, Palaeoclimatol., Palaeoecol.. 7 (1970) 185-220

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1,. J. BRENCHLEY AND G. NEWALL

Venerupis on sand

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Width of flume is one foot ( 30'5cm ) Fig.ll. Series diagrams, drawn from photographs, showing the positions taken up by certain shells on sand and mud substrata, from the beginning to the end of the runs.

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FLUME EXPERIMENTS OF MODELS AND SHELL VALVES

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former substratum. It has been shown (HJULSTROM, 1935; INMAN, 1949) that the current velocity required to put a grain of 0.4 mm size into transport is 20 cm/sec, while that for a grain of 0.02 mm size is greater: 40 cm/sec. Thus with the two sediments used in these present experiments, the sand grains will become mobile at lower current velocities than mud grains. The effect of the carpet is to reduce friction between the object and the substratum promoting object movement and prolonging the transport period. The loaded models generally showed long-axis control during transport, retaining a cross-current orientation. However, where the model is initially in a near current-parallel position, the tendency is for rotation so that the centre of gravity became directed into the current: transport in this position ensued. The transport of elongate models, loaded at one end, was mainly controlled by long axes, but the effect of the weight was to cause transport in an oblique attitude, the weight being directed up-current. One exception to the rule of cross-current transport is to be found in Cultellus, which behaves differently to the other objects used in that it tumbles downstream in a very irregular manner. This difference in the behaviour of Cultellus is apparently a result of its exceptionally low mass, so that it is caught in turbulent eddies in the current, resulting in the current-parallel orientation described on p. 199. Turritella, also exceptional, takes up a perfect current-parallel orientation which is retained during transport. The current acting upon the conical shape causes the shell to slew round until the apex is directed upstream. Also air trapped within the spiral near to the aperture kept this end buoyant, the apex acting as a frictional anchor on the substratum during transport. The apex up-current orientation of Turritella is similar to that found by NAGLE 0967), using shells of the same genus, but contrasts with the apex down-current orientation which KEELING (1967) found in their experiments with Nucella lapillus. The current orientation of Turritella appears to result from its behaving as a conical roller during transport and from the asymmetrical weight and buoyancy distribution along its length. This orientation of Turritella in the present experiments is most exceptional whereas in the experiments conducted by NAGLE (1967), this parallel orientation was considered to be the "diagnostic current orientation". Capulus reacts peculiarly since the effect of the asymmetrical cone is to produce an orientation of long axis across the current, but for this pattern to be biased towards having the apex of the cone up-current. The result is of transport in an oblique attitude. The effect of increasing current velocity on numbers of objects in transport at any time, is depicted for a representative series of models in Fig.12. On sand, as the current velocity increased so more and more models move into transport, until a maximum is reached at about 25-35 cm/sec (Fig.12B). At velocities greater than about 35 cm/sec, the number of models in transport decreases to zero in many

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P. J. BRENCHLEY AND G. NEWALL

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cases as the models become deposited. On a mud substratum, a similar pattern of models in transport can be seen, though at higher velocities the numbers fall off more rapidly than on sand (Fig.12A). Deposition Deposition here refers to the time at which shells or models ceased movement: cessation of movement can be brought about in various ways. Inversion from concave-up to convex-up position. As has been pointed out by many workers concave-convex objects are hydrodynamically unstable in the concave-up position, but are stable in the convex-up position. This observation is not only supported by the present experiments but is reinforced. In the concave-up position objects could undergo transport, as described above, but if they became inverted to the convex-up position then transport almost invariably ceased, even at high current velocities. Only at velocities high enough to vigorously erode the sediment were a few objects shifted, and under these circumstances they responded irregularly, tumbling downstream in a manner reminiscent of Cultellus. This latter was the only form used which could easily become transported from either attitude at moderate current velocities: this is explained by the extremely low mass and the relatively fiat section of the valves. The inversion of objects from the concave-up to the convex-up attitude is caused by some form of impedence to movement, even a very small obstacle can cause the moving object to invert. Impact of a transported model against a stationary one, or irregularities of the substratum, e.g., embryonic ripples, can cause instant inversion. It is of interest that many more objects invert on mud than on sand, given the same environmental conditions (see Table I, and compare Fig.10B with Fig.10C, and Fig.llB with Fig.llC). It is difficult to correlate this difference in inversion ratio with impedence either by other shells or by substratum irregularities. It seems more likely that it is related to the ease of mobility of the two substrata; mud grains being less easily transported remain in front of the objects as an immobile, "sticky" bed, against which moving objects can become held by friction. Held in the unstable concave-up position, the objects can easily be inverted by water currents under the elevated leading edges. On sand, the mobility of the grain carpet is such that objects can be transported until some impedence causes them to become inverted. The result of this difference in ease of inversion is that objects on sand can be transported for considerable distances (Fig. 10A, 11A, B), whereas those on mud undergo only short transport before movement ceases (Fig.10C, 11C). Also those shells which become inverted without impedence, take up a very well-marked preferred orientation in the convex-up position, especially if the object turns over about a straight side. If the object inverts about a curved side, e.g., semicircles or ovals, then the perfect cross-current orientation taken up during transport may be disturbed due to the model wobbling as it turns over. Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 185-220

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Shells and models may cease transport apparently without impedence; these take up a most pronounced cross-current lineation (Fig.9e), a reflection of their position during transport (Fig.9d).

Impedence. Already some of the effects of impedence have been mentioned above. Impeded objects need not invert to the convex-up position, but may remain wedged concave-up. Commonly impedence is not by total contact of one object against another, but by partial interference. For example, an elongate model in passing stationary ones during transport, may become impeded at one end, causing slewing of the model and disorientation of the sample (Fig.10B shows the effect in rectangles 3, 5, 6 and 10). Clearly, there is more chance of mutual inference between objects where the population density is high. These impeded forms can have a marked effect upon the final orientation pattern in that they produce an orientation less perfect (Fig.9c) than that produced when the objects are freely responding to the current, during transport (Fig.9d). Burial Only slight burial of objects took place on the mud base, further reflecting the relative immobility of this sediment under these experimental conditions. On sand, however, partial or sometimes total burial of the shells and models occurred. Burial commenced with the accumulation on grains of the concave-up objects; the grains entering from over the down-current edge as eddies develop in the lee of the objects. Burial begins at different times, both in objects of different shape and in different individuals of the same set. The factors predetermining which objects begin to be buried or which become transported, must be complex: the precise attitude of the object on the substratum may be of considerable importance in producing hydrodynamic micro-environments in which grains can be winnowed. Grains are removed from in front of, and beside, the objects and are then carried into the concavities. In some eases burial began at current velocities as low as 15 cm/sec. As current velocities increase, shells or models which had begun to accumulate sediment may become transported while retaining the sediment on the concave side. Other objects may have become sufficiently buried for transport to be impossible, and increased current velocity further develops the winnowing process, so that marked hollows develop in the substratum laterally to, but particularly in front of, the objects. This causes the objects to slide into the hollow and to take up an attitude at a high angle to the sedimentary interface: shells or models which get into these positions are very seldom put into transport. At higher velocities, as ripple bed-forms develop, impeded or unmoved objects can be overridden and so buried by migrating ripples: continued migration will, clearly, re-expose the object as the ripple crest passes. However, once a model or shell has been overridden it seldom becomes transported later since the passage of the ripple deposits sediment within the concave surface, effectively anchoring Palaeogeography, Palaeoclimatol.,PalaeoecoL, 7 (1970) 185-220

FLUME EXPERIMENTS OF MODELS AND SHELL VALVES

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the object. This stabilisation of the assemblage at higher velocities is a reflected form of the curves on Fig. 12. Burial of objects in the convex-up attitude can take place equally effectively, though whereas burial of concave-up objects is responsible for stopping their transport (or preventing transport starting), burial of convex-up objects usually takes place after movement has totally stopped. CONCLUSIONS AND IMPLICATIONS

Under the conditions used in the present experiments, the following general conclusions can be made: (1) Objects take up the most pronounced orientations during transport: long axes control orientation, and become lineated across the current. (2) The cross-current orientation is preserved in the final orientation pattern. (3) Eccentric centre of gravity, induced by loading of models, can modify the basic cross-current pattern; the load being preferentially up-current of the long axis. (4) Unidirectional preferred orientations are sometimes evident in monosymmetrical objects, being governed by relative ease of transport in a certain position. (5) Shells can be generally compared with models, giving cross-current lineations of long axes; exceptions include the light-weight shell, Cultellus. Shells with thickened umbones can be compared with loaded models, umbones directed preferentially upstream during transport (e.g., Venerupis on sand). (6) Final orientation patterns can be divided into components depending on the transport history of individual objects. The components are: (a) impeded; (b) inverted; (c) rotated but not transported; (d) transported and deposited without apparent impedence; and (e) unmoved. Group (d) gives the most perfect orientation related to current direction; group (e) gives the least perfect orientation in fact these are preferentially current-parallel. (7) Transport can be terminated by: (a) impedence by other objects; (b) irregular substratum; (c) inversion; (d) burial; and (e) for no apparent reason. Objects can be deposited, and therefore, removed from the system, before the highest current velocities are reached. (8) There is only a very poor correlation between the initial angle of any object to the current and the current velocity required to move that object. (9) Objects begin transport at lower velocities on sand than on mud. (10) Objects can be transported over greater distances on sand than on mud. (11) The main reason for (10) is that more objects become inverted to the convex-up position on mud than on sand. (12) Inversion can be caused by some impedence, which trips the objects as they are moving. Loaded models indicate that objects transported with

Palaeogeography, Palaeoclimatol.,Palaeoecol., 7 (1970) 185-220

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P. J. BRENCHLEY AND G. NEWALL

the load downstream are more unstable and therefore more liable to inversion. (13) In the final analysis, concave-up and convex-up objects should be distinguished and treated separately to give the most accurate picture. (14) Object in convex-up position are extremely stable, only at velocities at which the substratum is highly mobile do they occasionally become transported. (15) Burial of objects can begin at low velocities on sand. Scour in front of objects leads to objects being buried in positions across the bedding. (16) Under the present experimental conditions, no current parallel orientation of long axes develops, except in case of the shell Cultellus with very low mass, and Turritella. Already points of difference between our results and those of previous workers have been noted and commented upon. Some discrepancies between the various groups of results are of a minor nature, though some indicate radical differences in the main conclusions. NAGLE (1967) concludes that a current parallel final orientation pattern is diagnostic of current oriented assemblages (though the monosymmetrical object, Spirifer, takes up a cross-current lineation). KELLING and WILLIAMS (1967) developed bimodal final orientations, current-parallel and cross-current, in Mytilus, as did SCHWAgZACHER(1963) in experiments with crinoid stems and rods. In our experiments a cross-current lineation of long axes was the general final orientation. From these results it seems premature to make any sweeping conclusions as to the expected final preferred orientation of natural assemblages, modern or fossil. What these series of experiments indicate is that under different experimental conditions and with differently shaped objects a variety of orientation patterns can result: KELLING and WILLIAMS (1967), for example, used a very special series of conditions, where Mytilus shells were initially in a convex-up attitude and with long axes or 45 ° to the direction of current flow; SCHWARZACHER(1963) used objects with totally different dynamic properties than those used in all the other workers' experiments; NAGLE (1967) used a variety of asymmetrical objects in rather ill-defined conditions; our objects and conditions have been given earlier in this paper. From this diverse group of experiments it would be ambitious to make a general case for expected orientation patterns in natural assemblages. To specify our main conclusion: under conditions of the lower flow regime and with mono- and disymmetrical objects on sand and mud substrata, cross current preferred orientation patterns are to be expected in transported assemblages. In the geological record, and in modern oceans, currents in the upper flow regime are rare apart from those in fluviatile and beach environments (the effect of turbidity currents cannot be assessed against the experiments). Thus it is concluded that the cross-current lineation seen in our experiments is the most likely one to be found in fossil assemblages. If fossil unidirectional orientations occur in transported assemblages, then a position with the centre of gravity into the current is perhaps the most likely.

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FLUME EXPERIMENTS OF MODELS AND SHELL VALVES

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The present experiments do give some indication as to the way in which fossil assemblages are best treated. A unidirectional feature is best chosen, if possible to combine in one measurement the longest axis and some other element of the shell (e.g., a line at right angles to the longest axis, will indicate its own orientation and will imply the lineation of the longest axis). The feature measured should yield the most significant data for the given situation, e.g., two apparently different patterns resulted from measuring the same assemblage of Mytilus in two ways (Fig.4c, 5b). Convex-up and concave-up shells should be separated so that any differences in their orientations, probably attributable to behaviour during transport, becomes apparent. Ideally our plots suggest that any assemblage of objects is best divided into components, dependant upon their transport history, e.g., impeded objects give a more dispersed pattern than non-impeded ones. In fossils the total breakdown, possible in experiments, is impossible, though certain factors could be taken into account: in the example above, population density will have an important effect; it would, therefore, be anticipated that populations of low density will give the more perfect orientation patterns since interference between objects is less likely. Symmetry of the shell, weight distribution, current velocity (or wave motion), nature of the substratum, population density, the presence of residual nontransported elements, the mode of transport and the amount of inversion, all are associated in bringing about the final orientation. Some implications of this work on the interpretation of fossil assemblages can be stated. The experiments indicate that shells can be buried by scour in attitudes across the bedding, a position common in fossil assemblages. However, while this process readily takes place on sand, it did not happen on mud, though it is possible that at higher current velocities or over long periods of time, some burial may occur. In the terms of our experiments objects can be transported easily on sand for considerable distances, whereas on mud movement of a few inches was the usual case. This may imply that the terms drifted and non-drifted assemblages are of little value in referring to fossil assemblages in fine grained rocks, since, although the shells have been transported, most, if not all the components of the original non-drifted (life) assemblage will be present. Finally, these experiments indicate orientation patterns in dead transported objects. It is worth bearing in mind that life-orientations are known, and that these are usually not caused mechanically by a current. Except in Turritella, we found few really strongly marked unidirectional current orientations, and it is possible that orientations of this sort found in the rocks could represent part or all of a life-oriented assemblage. Separation of articulated from disarticulated shells when measuring orientations may give some clue to the presence of a life-orientation.

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P. J. BRENCHLEY AND G. NEWALL

ACKNOWLEDGEMENTS The authors wish to a c k n o w l e d g e the help given by the D e p a r t m e n t o f Civil Engineering, L i v e r p o o l University, where the experiments were p e r f o r m e d : especial thanks are due to R. B. W h i t t i n g t o n o f that d e p a r t m e n t for his co n t i n u ed cooperation. REFERENCES

HJULSTROM, F., 1935. Studies of the morphological activity of rivers as illustrated by the River Fyns. Bull. Geol. Inst. Univ. Uppsala, 25: 221-527. INMAN, D. L., 1949. Sorting of sediments in the light of fluid mechanics. J. Sediment. Petrol., 19: 51-70. KEELING, G. and WILLIAMS,P. F., 1967. Flume studies of the reorientation of pebbles and shells. J. Geol., 75: 243-367. LEVER, H., 1958. Quantitative beach studies, 1. The "left-right" phenomenon: sorting of lamellibranch valves on sandy beaches. Basteria, 22: 21-51. LEVER, J. and THUS,SEN, R., 1968. Sorting phenomena during the transport of shell valves on sandy beaches studied with the use of artificial valves. Syrup. Zool. Soc. London, 22: 259271. LEVER,J., KESSLER,A., VAN OVERBEEKE,A. P. and THIJSSEN,R., 1961. Quantitative beach research, 2. The "Hole effect". Neth. J. Sea Res., 1: 339-358. LEVER, J., VAN DEN BOSCH, M., COOK, H., VAN DIJK, T., THISDENS,m. J. n. and THIJSSEN,R., 1964. Quantitative beach research, 3. An experiment with artificial valves of Donax vittatus. Neth. J. Sea Res., 2: 458-492. MENARD, H. W. and BOUCOT,A. J., 1951. Experiments on the movement of shells by water. Am. J. Sci., 249: 131-151. NAGLE, J. S., 1967. Wave and current orientation of shells: J. Sediment. Petrol., 37: 1124-1138. SCHWARZACHER, W., 1963. Orientations of crinoids by current action. J. Sediment. Petrol., 33: 580-586. TRUSHEIM,F., 1931. Versuche iiber Transport und Ablergerund von Mollusken. Senckenbergiana Lethaea, 13: 124-139.

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