The Map
Because planimetric maps are the most significant and tangible product of geological/volcanological investigations, we place great emphasis on detailed, accurate, and legible maps that portray as much qualitative and quantitative data as possible. The spatial relationships of observations and data collection points are not only the key to understanding the subsurface structure of an exploration property but also necessary in planning drilling operations—especially for issues concerning topographic accessibility. The three-dimensionality of geologic investigations also dictates the need for maps with associated cross-section interpretations. The exercise of producing maps and cross sections is one way to validate spatial observations—the spacial and temporal laws of superposition, topographic control, and cross-cutting relationships must be satisfied by data and observations before the field geologist can produce a technically accurate map. During the map-editing stage, inconsistencies in observations and the completeness of a field study become obvious.
The stages of mapping are well described in classical texts on geological field methods.
Fig. A.18
Median size (Mdf ), sorting coefficient (sf ), and frequency distribution of pumice (light gray),
crystals (white), and lithic clasts (black) for the Fogo A tephra. (a) These variations show
vertical changes that are documented within a stratigraphic section. (b) These
variations show lateral changes documented within the deposit with
increasing distance from the vent.
(Adapted from Walker and Croasdale, 1972.)
For instance, Compton (1962) discusses the scale and detail, types of data and observations to be included, ways maps address specific themes or problems, note-taking and location protocol, field equipment required, sampling procedures, sample density, and traverse plans.
Generally, mapping is first approached from the reconnaissance level where previous reports and maps are compiled, accessibility is determined, and land ownerships are determined. Many of these issues were covered at the beginning of this appendix. Often maps and orientations provided by previous workers can be compiled into a working reconnaissance map. The mapping process then progresses from observations of type localities for development of the stratigraphic framework to field checking major geological contacts determined by previous work. Most useful reconnaissance traverses are along areas with outcrops, such as streams, roads, ridges, and trails that cross structural and stratigraphic contacts. At the end of the reconnaissance stage, when the area of interest has been placed in a regional context, the size of the area to be studied and the level of detail required can be determined; in addition, specific areas can be selected for detailed mapping.
Fig. A.19
Variation of pyroclast constituents for samples from the scoria cone section shown in Fig. A.12.
The curves depict variations for the size fractions indicated.
(Adapted from Wohletz, 1986.)
Fig. A.20
SEM photographs illustrating four common pyroclast textures. (a) Vesicularity is well
developed for this pyroclast sampled at Surtsey. (b) Grain angularity is prominent in a
hydroclastic sample from Surtsey. (c) Grain rounding indicates transport abrasion in this
poorly vesicular pyroclast from Kilbourne Hole maar in New Mexico. (d) Surface
alteration coats this pyroclast from the Coliseum Diatreme maar in Arizona.
Scale and Graphic Detail
Topographic maps for most areas are available at scales of 1:250,000 to 1:25,000. The scales of satellite and aerial photo images vary, but satellite images are generally more regional. It is often satisfactory to photographically enlarge topographic base maps for more detailed investigations, but this process can make it difficult to judge absolute distances correctly and triangulation methods will be necessary.
For geothermal fields, the scale of the map may be determined by the size of the volcanic field with which it is associated. For example, if a hydrothermal system strongly interacts with regional aquifers, related hydro-thermal manifestations, geochemical survey points, and sampling locations may extend over regions of up to several hundred square kilometers. In such cases, a large-scale geological reconnaissance map helps identify the geological control of the hydrothermal manifestations and their possible relationships with primary hydrothermal prospects. These large-scale maps are valuable in establishing hydrologic recharge and hydrothermal outflow areas, which are a function of the regional hydrologic gradient. As a part of the geological and hydro-
Fig. A.21
Variation plot for pyroclast textures determined by SEM vs scoria cone
stratigraphy (Fig. A.12). The vesicularity in magmatic (cone-forming eruptions)
samples is greater than that in samples from the hydrovolcanic tuff ring, but
the hydrovolcanic phases show greater grain alteration and blocky (angular) textures.
Pyroclast rounding is increased in samples abraded during surge transport.
(Adapted from Wohletz, 1986.)
geochemical survey, evaluations of the regional groundwater budget often play a major role in modeling the productivity of a hydrothermal system.
When exploration has progressed to the drilling/coring stage and production drilling plans are being considered, a detailed plane-table map showing the target area's topographic contours, geological contacts, lithology, and structures is beneficial. This process may require scales in the range of 1:1,000 to 1:10,000, which will make it easier not only to determine the well site, but also to locate geologic details and project depths accurately.
Thematic Mapping
Geological maps can be very different, depending on the theme of the map—whether it focuses on bedrock lithology, detailed volcanologic or tectonic structure, rock facies determined by chemical or physical properties, or geothermal manifestations. The most useful type of map is
Fig. A.22
SEM of microcrystalline alteration materials coating a vesicular pyroclast from Surtsey.
The mineralogy of these materials can constrain the alteration environment.
geovolcanological and shows aspects of bedrock lithology, volcano structure, and cognate lithologies (suites of rocks all erupted from the same volcanic edifice). A geovolcanological map adds these structural interpretations to more classical geologic base maps. Producing this type of map requires that the field geologists
· recognize related rock types that can be grouped as co-genetic suites related to the evolution of particular vents;
· delineate subunits or facies of rocks that reflect their genesis;
· map geomorphological changes that reflect concealed vent structures; and
· distinguish between regional tectonic structural fabrics and local volcanic ones.
Several map themes we have found particularly useful in specific areas are discussed in more detail below. Creating multiple maps of a geothermal area can be very useful in separating different data sets and observations so they can be judged on their internal consistency; however, multiple maps are also useful as a group when they are over-laid so as to determine areas of greatest data correspondence. (Map overlays will be discussed in the later section on 3-D models.)
Lithological
A geological map that emphasizes only rock data and observations illustrates the greatest number of mapped geological units and structural contacts but places little emphasis on volcanic or tectonic structure. These maps are employed to show rock sample locations and subtle variations in rock properties. However, because the detail can obscure volcanological and structural interpretations, this type of map may not be the most suitable for illustrating key rock and structural elements that control a geothermal system. The geological map of Usu volcano (Fig. 5.34) is typical of a lithological approach that shows variations of rock types according to their age, petrography, and geomorphology.
Structural
A map that emphasizes tectonic and volcanological structure may have some interpretive elements that are based on grouping of lithological units and the delineation of individual volcanic edifices by geomorphological features. Because only major rock units are shown, much greater emphasis is
Fig. A.23
Variation in glass-surface chemistry for samples taken from stratigraphic sections of four
volcanoes: Crater Elegante and Cerro Colorado in Sonora, Mexico, and Panum
Crater and Obsidian Dome from the Inyo-Mono volcanic field of California.
Sample types are designated as fall (F), sandwave surge (SW), massive surge (M),
and planar surge (P). Fall samples are most representative of magmatic compositions
(stippled patterns), whereas the surge samples show hydrovolcanic tephra compositions
that result from the rapid alteration of the magma through its interaction with water.
The vertical line on plots for Crater Elegante and Cerro Colorado separates essentially
magmatic samples (left) and later-erupted hydrovolcanic samples (right).
(Adapted from Wohletz, 1987.)
placed on structural features that affect subsurface conditions and locations of hydrothermal systems. Relative ages and the amount of recent fault movement can be depicted by variable thicknesses of contacts. Figure 5.24 provides a prototype in the detailed structural map for the Coso volcanic field.
Facies
Volcanic facies (lateral and vertical variations in single eruptive units) are manifest as gradation changes in physical or chemical properties. Some examples of mappable facies discussed in previous chapters include
· downslope variations in composite cones, from lavas to lavas intercalated with pyroclastic units to dominantly epiclastic and pyroclastic textures;
· variations in pyroclastic units related to median grain size, sorting, and bedding structures;
· pyroclastic and lava facies of silicic domes;
· contrast between caldera fill and caldera outflow rocks;
· plateau forming, horizontally outcropping rocks of basin fills; and
· dipping and unconformable rock strata of near-vent facies.
Other examples are described by Cas and Wright (1987).
Facies variations shown in plan view can provide information on porosity/permeability relationships that are meaningful when a potential hydrothermal reservoir must be delineated. In other cases, facies variations in some pyroclastic units can point to potential vent areas that have been eroded or concealed under younger units. Wohletz and Sheridan (1979) discussed one example of ways in which pyroclastic surge facies can indicate vent area, and another example, shown in Fig. 2.34, suggests how dry-to-wet pyroclastic rock facies might help constrain the degree of aquifer interaction for a given eruptive unit.
Geothermal Manifestations
Chapter 3 outlined aspects of geothermal manifestations, including thermal spring and fumarole locations, silica sinter and travertine deposits, hydrothermally altered rocks, and phreatic explosion craters and breccias. A map indicating locations of such manifestations is very useful for hydrogeochemical surveys; it not only points to individual sample localities, it also becomes a base map (like that shown in Fig. A.25) for plotting hydrogeochemical samples and subsequent interpretations.
Cross Sections
Construction of cross sections should begin before field work actually commences. The following approach works well to stimulate ideas about the area's geologic history and framework; it also allows the geologist to identify inconsistencies and deficiencies while still in the field.
Topographic Profile
After previous topographic maps, aerial photographs, and any other available data have been examined, it is possible to establish lines for cross sections through critical parts of the field area. If topographic maps are available, the geologist prepares topographic profiles for the cross sections at the same scale used for the working copy of the map. After the profiles are completed, several copies are made with indelible ink on sturdy paper (cross-section paper is good) or plastic mylar (Fig. A.26). No vertical exaggeration is used, especially when sketching in lithologic units and structural features; cross sections with vertical exaggeration are often deceptive and can lead to later problems in siting wells.
Preliminary Interpretation
At this point, the geologist has examined all the older data, aerial photographs, and
Fig. A.24
Variation of major-element chemistry for surfaces of pyroclast samples taken from the scoria
cone described in Fig. A.12. Strong variations are evident in major-element abundances
for the hydrovolcanic, magmatic, and lava samples.
(Adapted from Wohletz, 1986.)
topographic maps and has some preliminary ideas about the structure of the area from geomorphological clues. Folds, faults, and major lithologic breaks are sketched in pencil on the cross-sections (Fig. A.26). Studying these preliminary cross sections can help in planning field traverses.
In the Field
Each evening in the field, appropriate changes are made to the working cross sections, based on the day's lithologic descriptions as well as observations of faults, attitudes, areas of alteration, etc. Sometimes it is necessary to erase an earlier interpretation or add new lines. This messy working cross section evolves, along with the geologist's ideas about the field area (Fig. A.26); the daily review exercise is stimulating and sharpens perceptions for the next day's observations.
By the end of the field season, the geologist has a fairly sophisticated set of cross-sections that are consistent with the geologic map, working hypotheses, and the logical framework within which samples were collected.
Additional Information from Drill Holes
The ultimate test of the three-dimensional view of the field area is a comparison of the map, cross-sections, and data gained through drilling. A proposed stratigraphy, based on field work, is created for the drillers; this exercise helps them prepare a drilling plan and cost estimates. In return, the drilling provides the geologist with hard data about subsurface geological features. As drilling proceeds, numerous changes to the cross-sections may be necessary. On the other hand, well-founded cross sections may be useful for interpreting core or cuttings that are difficult to classify.
The ideal exploration well is a corehole that has been sampled to its full depth. Wireline coring is a proven technology and eliminates guessing about the rock types and the degree of alteration or fracturing. Procedures for the curation and description of core samples are outlined in Appendix F. If no cores are to be collected, careful evaluation of drill cuttings can be useful; however, it is important to take into consideration the limitations of this method when cuttings from different strata are mixed during their rise to the surface and there is a time lag involved. Onsite petrographic identification of cuttings is aided by hand lens and a binocular microscope, but ideally a geologist should set up a simple thin-section preparation system with basic equipment: quick-setting glue (super-glue ) or epoxy to fix the cuttings on a glass slide, a hot plate to set the glue, and a grinder or abrasives to grind down the cuttings mounted on the slide to the appropriate thickness. Because this method takes only a few minutes per thin section, it allows the geologist to keep up with the drilling operations.
When calibrated against core or cuttings, geophysical logs provide critical information on lithologies, temperatures, and permeability (see Appendix F). Integrating these data—perhaps with the aid of a professional well-log analyst—is time-consuming but well worth the effort if the geologist is to understand the third dimension within the geothermal field.
"Final" Versions of Cross-Sections
At this stage, the field geologist has confidence in the cross sections. If the cross sections are to be used in a publications or report, it is desirable to use a technical illustrator for the final drafting. However created, the final maps and cross sections should be drawn on a plastic base or good quality paper that will exhibit minimal expansion and contraction with changes in humidity. One way to avoid many problems is to prepare the cross sections and map with a computer that has computer-assisted design programs or a geographic information system
Fig. A.25
Example of a spring map from a site near Azacualpa, Honduras. Descriptions
and measurements of hot springs include details of local landmarks such as streams,
large boulders, and canyon walls.
(Adapted from Eppler et al., 1987.)
program. Distinct patterns or conventional symbols should be used for lithologic units. Horizontal and vertical scales must be included; it is impossible to use either cross sections or maps accurately without clearly labeled scales.
The final cross sections should be laid across the map parallel to the profile lines (Fig.A.26) and several questions should be asked: do the interpretations still appear to be reasonable? Is the scale correct? Do key points on the map (for example, faults) correlate with those same features on the cross section?
The process of creating these maps is lengthy and involves many stages. The last, extremely necessary step is to proof the completed map: checking the data and spelling of place names as well as myriad other details that have been assimilated during the mapping process.