تحميل كتاب علم الطبقات Stratigraphy: A Modern Synthesisp pdf
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The Stratigraphic-Sedimentologic Data Base
A successful basin analysis requires the collection and integration of several, perhaps many, different kinds of data. Direct observation of the rocks may or may not be fundamental to the study. In the case of a surface geological project, they will be preeminent, though perhaps supplemented by geochemical and geophysical information, plus laboratory analysis of collected samples. For subsurface petroleum studies, actual rock material available for examination may be very limited, consisting of well cuttings from rotary drilling, plus a few short cores. Petrophysical well logs and regional seismic lines may provide at least as important a part of the total data base. Investigations for stratabound ores and minerals typically employ networks of diamond drill holes from which a continuous core normally is available. This provides a wealth of material for analysis, although certain types of observation, such as analysis of sedimentary structures, may be difficult or impossible in such small-diameter cores.
Describing Surface Stratigraphic Sections
Vertical stratigraphic sections, whether measured at the surface or derived from subsurface records, constitute the single most important data set that the basin analyst should assemble. Lithostratigraphic or sequence-stratigraphic classification and correlation, and many sedimentological interpretations, depend on the documentation of vertical relationships within and between lithological units.
Methods of Measuring and Recording the Data
- Vertical Stratigraphic Sections
The simplest way to record the details of a surface outcrop is by measuring and describing a vertical stratigraphic section. Ideally, the location of the section should be chosen to include important stratigraphic features, such as formation contacts, but, in practice, the location is commonly determined by accessibility, e.g., the presence of bars or beaches allowing us to walk along a river cut, or a negotiable gully cutting through a cliff section. Only those geologists who use their profession as an excuse to practice their favorite sport of mountaineering will be able to apply sound geological principles to the choice of section. The rest of us take what we can reach.
In reconnaissance work, rapid measurement and description techniques are acceptable. For example, a hand-held altimeter (aneroid barometer) may be used in conjunction with dip measurements to reconstruct stratigraphic thicknesses using simple trigonometry. Another method that is commonly described in field handbooks is the pace-and-compass technique, suitable for estimating thicknesses across relatively level ground, given accurate stratigraphic dip. The same distances may be measured from maps or air photographs. Long experience with these methods has shown that they are not very reliable; errors of up to 50 % can be expected.
- The Construction of Lateral Profiles
Some stratigraphic units are essentially tabular at the scale of the outcrop, and can be quickly and accurately documented using vertical profiles, in the way outlined in the previous section. However, some types of sedimentary assemblage contain complex facies changes, which may be at a small enough scale to observe in individual outcrops, especially in large outcrops. For example, a reef core, with its reef-front talus slope and back-reef lagoonal deposits, or a large fluvial or submarine-fan channel, with its fill of complex bar deposits, may be spectacularly displayed in a road cut or mountainside.
The measurement of a few vertical sections across such an outcrop is a quite inadequate way to document the wealth of facies detail that may be available. Petroleum companies have developed a considerable interest in such large outcrops because of their use as potential analogs of subsurface reservoir units. Internal heterogeneities of these reservoir analogs can be studied and their porosity and permeability characteristics studied, for example by the use of minipermeameters.
In order to document the details in a large outcrop it may be necessary to construct a lateral profile, a long section that encompasses the full vertical stratigraphic height of the outcrop, and also extends along strike as far as possible, to illustrate the facies changes. This may be constructed by careful surveying, but a much quicker method is to make use of photographic mosaics of the outcrop.
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