Urban Geologic Mapping - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 177

Formation contacts

Formations

4 3 4

4 4

3

2

1

FIGURE 5.9
Composite block map developed from Figures 5.7 and 5.8.

individual strata present at a given region; and Figure 5.9 is a composite map of the under-
lying geology shown in Figure 5.7 and the individual strata depicted in Figure 5.8.
Combining geologic information from many individual sites using facies analysis is the
next step in constructing a three-dimensional geologic map for a region.

5.2.5 Facies Analysis

Facies are the characteristics of a sedimentary rock mass that reflects its depositional envi-
ronment. Facies analysis or cross-dating is a technique used to help understand sedimen-
tary environments and the processes leading to the origin of their sediments (Anderton,
1985). Facies analysis can be performed when two or more stratigraphic columns are devel-
oped for different locations within a given area. Facies analysis is conducted to

• Evaluate the completeness of sedimentary deposits at any one location
• Identify any gaps in the depositional record
• Reconstruct the geologic history of a region
• Identify or confirm the historical occurrence of geological processes within a
given region
• Reconstruct a succession of events
• Create a vertical succession of depositional record

The type of facies analysis most often employed to reconstruct the geologic history of
a particular region is termed relative dating. This technique uses the four principles of
stratigraphy introduced and described in Chapter 2 and repeated here for reference: (1)
principle of original horizontality, (2) principle of superposition, (3) principle of cross-
cutting relations, and (4) principle of faunal and floral succession. Through the use of
these four principles, facies analysis can evaluate and establish the chronology of depo-
sitional events within the area being mapped. For example, during the course of review-
ing historical literature and conducting fieldwork, it usually becomes apparent that a
few locations contain a nearly complete and undisturbed vertical geologic section. These
accessible columns essentially have preserved the depositional history so it can be studied