Experiments (cont’d)
The report of the experiment entitled Experiments in Stratification of Heterogeneous Sand Mixtures was published in (Julien et al., 1993).
This experimental study examines possible stratification of heterogeneous sand mixtures under continuous (non-periodic and non-interrupted) sedimentation. Three primary aspects of stratification are considered: lamination, graded beds, and joints.
(1) Experiments on segregation of eleven heterogeneous mixtures of sand-sized quartz, limestone and coal demonstrate that through lateral motion, fine particles fall between interstices of the rolling coarse particles. Coarse particles gradually roll on top of fine particles and microscale sorting is obtained. Microscale segregation similar to lamination is observed on plane surfaces, as well as under continuous settling in columns filled with either air or water.
Fig. 3. Results of experiments.
(A) Schematic formation of graded beds.
(B) Time sequence of deposit formation for t 1 < t 2 < t 3.
(2) The formation of graded beds is examined in a laboratory flume under steady flow and a continuous supply of heterogeneous particles. Under steady uniform flow and plane bed with sediment motion, coarse particles of the mixture roll on a laminated bed of mostly fine particles. In non-uniform flow, the velocity decrease caused by tail-gate induces the formation of a stratum of coarse particles propagating in the downstream direction. On top of this cross-stratified bed, fine particles settle through the moving bed layer of rolling coarse particles and form an almost horizontally laminated topset stratum of finer particles. A thick stratum of coarse particles thus progresses downstream between two strata of laminated fine particles, continuously pro-grading upward and downstream
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(3) Laboratory experiments on the desiccation of natural sands also show preferential fracturing (or joints) of crusty deposits at the interface between strata of coarse and fine particles.
Rather than successive sedimentary layers, these experiments demonstrate that stratification under a continuous supply of heterogeneous sandy mixtures results from segregation for lamination, non-uniform flow for graded beds (Fig. 4) Superposed strata are not, therefore, necessarily identical to successive sedimentary layers.
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Fig. 4. : Typical cross-sectional view of deposit
Fig. 5 : Horizontal fracturing of the Bijon Creek sand
Our flume experiments demonstrated that Stenon’s assumption (strata are ancient successive sediments) and his principle of superposition can only apply in the absence of a current (transport velocity nil). Moreover, the experiments reported in my second paper to the Academy of Sciences and experiments conducted by P. Julien and presented by video Fundamental Experiments on Stratificationat several sedimentological congresses clearly show that up to the limit of the angle of repose (30o to 40o for the sands), the lamination of the deposit is parallel to the slope (Fig. 6) [video 11 ]. In this case the principle of horizontxality does not apply. It should not, therefore, be concluded that the dip of the strata necessarily implies tectonic movements subsequent to the horizontal deposit of the strata.
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Fig. 6 : Lamination parallel to a slope of 15o