Statistical methods for analysis and mapping of flysch-type sediments

To present a model for flysch deposition involves the simultaneous representation of many parameters. The most significant of these are arenite-lutite bed thickness ratio, mean and variance of arenite-bed thickness, mean palaeocurrent direction and the primary directional structures yielding them. Field and flume data suggest that arenite-bed thickness decreases exponentially away from source, with a complementary increase in siltstone bed thickness in a turbidite fan. If H_A, H_L. are arenite and lutite bed thicknesses in a couplet (the unit of bedding at a flysch outcrop), then where m and R are constants of the outcrop in the turbidite wedge. Field evidence suggests m= 1, so that R-values allow for a classification of couplets at an outcrop or within a zone. Maximum H_A, H_L. values are indicative of proximal deposition from turbidites, and usually provide transverse directional structures, as does (H_A) max. Arenite bed thickness can be considered as thick beds or thin beds according to whether they exceed a chosen thickness, say t cm. If 15 cm be chosen, the percentage thick arenite-bed thickness H_A1 and thin arenite bed thickness percentage, H_A2 can be represented with H_L. (siltstone bed thickness percentage) over twenty-five couplets, as one point on a triangular diagram. The mean palaeocurrent direction Nφ° E is represented as an arrow through the point, a colour (point or arrow) being used to distinguish the directional structure (flute, ripple, groove, foreset, etc.) producing it. Flysch examples from Ordovician, Silurian and Devonian in Central Victoria are thus interpreted in terms of proximal and distal emplacement. In cases where bedding detail is clear, A + B, C and D + E divisions substitute for H_A1H_A2 and H_L. respectively, for a unit of 1 m thickness of outcrop on the triangular diagram. If thick arenite bed thickness percentage be plotted against total arenite bed thickness percentage, the linear plot of points for outcrop samples results on log-log paper. Limiting lines of the form log H_A1= m log H_A+ log C (m, C constants) demarcate the field. Parallel and equally spaced lines with values C₁, C₂, C₃,…C_n subdivide this into sub-fields. If the determinations for H_A1 in any one sub-field be plotted on a locality map considering only MAXIMUM H_A1 values, and the H_A1H values conveniently contoured, trends emerge which closely agree with the modal palaeocurrent directions in the case of the Llandovery Deep Creek Flysch 25 km NNW of Melbourne, Australia.