Construction of Triangle facies maps. Numerous facies maps can be designed for
three-component systems by using the method of multiple contouring of ...
Notes on the construction of facies maps reproduced verbatim from Krumbein, W. C. and Schloss, L. L., 1963, Stratigraphy and Sedimentation, Freeman, pp. 460-464
Frontispiece
Construction of Triangle facies maps. Numerous facies maps can be designed for three-component systems by using the method of multiple contouring of percentage lines or ratio lines. A common type is the triangle ratio map, in which two sets of ratio contours are used to delineate areas having certain compositional features in common. The limiting ratio values can be selected to block out clusters of points on the 100-percent triangle, or for preparation of “rockinventory maps," a generalized set of limits may be chosen. By using the same limiting values in a set of maps, ready visual comparison of patterns or colors that represent certain conventional facies groups can be made. Preparation of multiple-ratio facies maps is more complicated than is the preparation of individual isopacb, isolith, or percentage maps. The following remarks on map construction facilitate interpretation of the examples to follow. The use of two ratios to
prepare a facies map is illustrated in Figure 12-10. Component A includes carbonates (limestone and dolomite) and evaporites (anhydrite, gypsum, and salt). Component B represents sandstone and conglomerate, and C represents siltstone and shale. The ratio (B + C)/A thus expresses the relationship between detrital (clastic) sediments and the nondetrital (nonclastic)sediments. This is the clastic ratio. The ratio B/C contrasts coarse and Pine elastics and is the sand-shale ratio.
The maps in Figure 12-10 show the several stages in constructing a triangle facies map. The isopach base commonly used in such maps is shown in the upper left. The clastic ratio and sand-shale ratio values are entered at each control point on separate base maps. Contour lines of equal ratio values are then drawn through the fields of numbers, commonly on a geometric contour interval, as shown. Ratio maps, like isopach maps, are contoured as continuous functions. Thus, the three upper control points on the clastic ratio map of Figure 12-10 have the values 0.7, 7.0, and 44. The elastic ratio lines 1, 2, and 4 must be drawn between the first and second points, and lines 8, 16, and 32 must pass between the second and third points. The clastic ratio and sand-shale ratio maps are directly useful for inter- pretation of some features of the stratigraphic unit. However, for preparing a “rock inventory map" that shows interrelations among all three components, it is conventional to superimpose one map on the other, as shown in Figure 12-10. In this map, the triangle is divided into
segments at sand-shale ratio lines 8, 1, and 1/8 and at elastic ratio lines 8, 1, and 1/4. This selection serves two purposes. First, the lines divide the map into selected “pattern areas” that show where the stratigraphic unit is mainly sandstone, shale, or nonclasties or where a given lithologic component constitutes more than 50 percent of the unit. Further simplification is achieved by eliminating the sand-shale ratio line of 1 where the clastic ratio falls below 1/4 (that is, where the strata are more than 80 percent carbonate or evaporite), and by using the sand-shale ratio lines 8 and 1/8 only in those map areas where the clastic ratio exceeds 8 (that is where approximately 90 percent or more of the section is sandstone or shale). For many conventional facies maps, the limits used in Figure 12-10 have been adopted, and the legend triangle has become known as the "standard' triangle. Table 12-4 summarizes the composition of each of the nine lithologic groups associated with the standard triangle. Use of the standard triangle facilitates comparison of rock inventory maps of different stratigraphic units. The frontispiece is an example of a conventional facies map illustrating the effective use of color on maps of this type. The limiting ratio values are those indicated in Table 12-4, except for the sand and shale end members, which are here defined by the clastic-ratio line 16 and the sand-shale ratio lines 16 and 1/16. It has become standard practice to use colors that approximate those of the frontispiece; if the shale and shale-sand groups were to appear within the map area, these would be colored bright green and yellowish-green, respectively. In many facies maps prepared as described, the clastic ratio and sand-shale ratio lines may cross and recross one another, as they do in the area TABLE 12-4. LIMITS AND CHARACTERISTICS OF LITHOLOGIC GROUPS GROUP NAME
CLASTIC RATIO LIMIT
SAND-SHALE LIMIT
GENERAL FEATURES
Sandstone
>8
>8
More than 79%sandstone
Sand-shale
>8
8-1
More sand than shale; less than 11% limestone
Shale--sand
>8
1-1/8
More shale than sand; less than II% limestone
Shale
>8
1
More sand than shale; 11 to 50% limestone
Shale-lime
1-8
1
50 to 80% limestone; more sand than shale
Lime--shale
1/4-1
