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The Development of Clast Fabric in Mudflows
John F. Lindsay
Computer simulation of the motion of particles in a viscous fluid in laminar flow shows that strong long-axis fabrics may develop in mudflows in a short period of time. The rate at which the fabric develops in any one situation depends on the velocity gradient. The viscosity of the mudflow one of the main factors controlling the velocity gradient, is increased considerably by the clasts contained in the matrix Fabric development is cyclical and begins as a vague girdle dipping upstream. At the same time a single mode develops, centered on the girdle, and gradually increases in intensity until a maximum is reached when the plunge of the mode is horizontal. The fabric from this point degenerates in reverse order, first to a weak girdle dipping downstream, and then to a fabric similar to the initial fabric when the particles have completed a half cycle of motion. The process then begins again. The strength of the fabric depends on the instant at which the mudflow is arrested. Even though most mudflows travel in a turbulent manner, the fabric is developed in the final stages as the velocity of the flow decreases, and it passes to a laminar phase before coming to rest. Limited experiments and simulations show that a C-axis fabric should also develop with a single vertical mode and girdle transverse to the flow direction.
Settling of the clasts under gravity after the flow has come to rest probably leads to a strengthening of the C-axis fabric, as clasts would tend to reorient themselves with their plane of maximum projection (the A, B plane) normal to the force of gravity.
The Permian Pagoda Formation in Antarctica consists largely of tillite but contains a small number of beds deposited from mudflows. Four A-axis fabrics were measured from the mudflow units and were found to be similar to long-axis fabrics produced by the simulations. All four have dipping girdles, and two display a central mode. When compared with till fabrics from the same section the mudflow fabrics can be distinguished because of the girdles. Had the mudflows been arrested at a stage of maximum fabric development, it would be impossible to distinguish them from most till fabrics. In this situation the problem may be clarified by measuring several fabrics at different levels in the flow. Because the velocity gradient decreases upward the strength of the mudflow fabrics should be dif erent at different levels. An attempt to exploit this method in the field met with very limited success, probably because the change in velocity gradient upward through the mudflow was too small.
C-axis fabrics developed by mudflows have vertical modes while their counterparts in till fabrics plunge steeply in a downstream direction.
It is concluded that clast fabric could be a valuable tool in determining the origin of diamictites, but more field data are needed.
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