17.3: Planar Lamination in Fine Sediments

Planar lamination an develop wherever there is fallout of suspended sediment onto a planar sediment surface in the presence of currents that are too weak to transport the newly arriving sediment over the bed. This is what was called fallout without traction in Chapter 14. Planar lamination of this kind is at once the easiest to understand and philosophically the most natural. Whenever there is a planar bed surface with particles raining down from above, from a fluid that is not moving fast enough to transport the particles once they land, planar lamination is formed, provided only that the nature of the sediment that is raining out fluctuates in one or more ways with time. Such planar lamination is usually found in fine sediments—muds and silts—for the obvious reason that it is not easy to imagine scenarios in which sediment with substantial fall velocities can be carried for long distances by near-bed currents too weak to transport that sediment as bed load.

The floor of the deep ocean is exposed to bottom currents that in most (but not all) places and at most (but not all) times are well below the threshold for sediment entrainment. Slow settling of fine sediment onto the sea floor under those conditions leads to planar lamination if the nature of the settling sediment varies in some way with time—and also provided that bioturbation does not disrupt the lamination. I suspect that most marine sedimentologists would assert that by far the most planar lamination in the sedimentary record was formed in just such a way!

Most shales, if you inspect them closely enough, have planar lamination. The study of such planar lamination goes far beyond the scope of these notes, because it involves consideration of the physicochemical interactions among fine clay particles, as well as the microbiological setting.

One way of producing fallout-without-traction planar lamination in coarser sediments, like fine to medium sands, is to appeal to strong and sediment-laden hypopycnal flows in the ocean (or in saline lakes), from which sediment rains down into quieter water below. Such hypopycnal flows develop wherever the river flow that disgorges into the water body has bulk density lower than that of the water body. Provided that the concentration of suspended sediment is not so high as to make the bulk density of the river flow even grater than that of the saline water body—in which case a bottom-hugging density underflow develops—the river flow spreads out across the surface of the water body, from which it is largely uncoupled dynamically because of the tendency for the strong vertical density gradient to damp turbulent mixing.

References

• Allen, J.R.L., 1964, Primary current lineation in the Lower Old Red Sandstone (Devonian), Anglo-Welsh Basin: Sedimentology, v. 3, p. 89-108.
• Allen, J.R.L., 1984, Parallel lamination developed from upper-stage plane beds: a model based on the larger coherent structures of the turbulent boundary layer: Sedimentary Geology, v. 39, p. 227-242.
• Best, J., and Bridge, J., 1992, The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae: Sedimentology, v. 39, p. 737-752.
• Bridge, J., and Best, J., 1997, Preservation of planar laminae due to migration of low-relief bed waves over aggrading upper-stage plane beds: comparison of experimental data and theory: Sedimentology, v. 44, p. 253-262.
• Bridge, J.S, and Best, J.L., 1988, Flow, sediment and bedform dynamics over the transition from dunes to upper-stage plane beds: implications for the formation of planar laminae: Sedimentology, v. 35, p. 753-763.
• McBride, Shepherd, R.G., and Crawley, R.A., 1973, Origin of parallel, near- horizontal laminae by migration of bed forms sin a small flume: Journal of Sedimentary Petrology, v. 45, p. 132-139.
• Paola, C., Wiele, S.M., and Reinhart, M.A., 1989, Upper-regime parallel lamination as the result of turbulent sediment transport and low-amplitude bed forms: Sedimentology, v. 36, p. 47-59.
• Smith, N.D., 1971, Pseudo-planar stratification produced by very low amplitude sand waves: Journal of Sedimentary Petrology, v. 41, p. 69-73.