Guidelines for lithological, structural and geomorphic interpretation
- 1. The interpretation of aerial photographs for common geological purposes basically involves study of geomorphology, lithology, stratigraphy and structural geology. The aerial photo-interpretation can provide sufficient preliminary data, resulting in broad separation of litho-units. The stereoscopic examination of aerial photographs makes it possible the identification, selection, demarcation and delineation of lithologic units and enables to establish the stratigraphic sequence. In black and white photographs several criteria such as tone, texture, landform, topographic expression, slope, drainage patterns and texture, soil, vegetation, mode of weathering and surface features help in identification of rock types. Major guides for lithological, structural and geomorphologic interpretation can be summarized as follows:- 1. LITHOLOGIC INTERPRETATION: ( Lithologic guides) The term lithologic interpretation refers to the recognition of rock types from photographical data, supported by field experience. Identification of rock types from aerial photograph is of fundamental importance and only the principle rock units can be identified from the photos. The main tool for identification is selective or differential erosion caused by a variable resistance and consolidation of rock units and is expressed by positive topography and development of landforms. Several factors which are commonly used in lithologic photo- interpretation are: i) Differences in Topography- The differential relief of landforms is more significant on aerial photos than in the field. Generally the more hard and tough rocks like granites, sandstones and quartzites etc. give rise to higher topography while the soft and weak rocks like shales, clays and unconsolidated rocks form lower topography. ii) Surface slopes- The surface slope often gives an important clue regarding underlying material but may often be unrelated to lithology. Generally, more resistant rocks have steeper slopes. iii) Drainage- The drainage texture is often related to lithology. iv) The Geometry of Rock Units- The geometric pattern is a significant criterion in the identification of litho-units. It leads the interpreter to distinguish consolidated from unconsolidated sediments and sedimentary from igneous rocks. v) Surface Features- Surface features are rather too small to appear on topographic maps but when present are very useful in lithlogic interpretation from aerial photographs. GL-March-2020 Remote Sensing Q U A R T Z – E C I E S Nikhil V. Sherekar GUIDELINES FOR LITHOLOGICAL, STRUCTURAL AND GEOMORPHIC INTERPRTATION
- 2. vi) Colour and Tone- In black and white photographs, colour of rocks is expressed intone of gray. The photographic tones of the rock bodies relative to that of adjacent rocks are more important. vii)Soils and Vegetation- Soils are often helpful in determining and differentiating the rock types. Similarly, vegetation may be selective and may prefer to grow over particular suitable rocks. 2. STRUCTURAL INTERPRETATION: (Structural Guides) Interpretation of aerial photographs for geological structures has always been one of the strong points of photogeology. In an area of differential erosion in which structures are not too complex, almost a complete picture of the pattern of folds and faults can be obtained. Now a day it has become a routine procedure to make use of aerial photographs either prior or during the field work for many purposes and structural interpretation is one of them as:- 1. The images on the aerial photograph can be related more reliably to geometry of geological structures. 2. Due to vertical exaggeration it has become possible to deduct structures of bed and rocks with confidence. 3. Photographs of sedimentary terrain give greater amount of information regarding structures than those covering igneous and metamorphic. The localization of ore deposits and oil fields by structural control is very well known to most geologists. Systemic structural analysis of the entire tectonic province as an exploration tool has been employed to several photo-geologists. The various major structures’ interpretation is as follows: 1. Slope Asymmetry:- Dip and strike are the basic elements of a bed. The direction of strike and the direction and amount of dip define the attitude of the beds. The tracing of the lithological contacts and the determination of the attitude of beds become the fundamental tasks in photo-interpretation of geological structures. In a series of dipping, alternate hard and soft sedimentary rocks, the soft rocks are eroded away easily and the hard rocks are exposed which results in slope. This slope exhibits the attitude of the dipping beds and such slope is known as dip-slope. These dip-slopes are one of the most important features in aerial photo- interpretation. On aerial photographs the major geological structures are more identifiable than any other features.
- 3. 2. Horizontal and flat lying beds:- These can be easily interpreted and can be easily recognized where different sedimentary rocks exhibit contrasting photographic tones, expressed as regular bands. The slope characteristics are suggestive of horizontal strata. Resistant beds have steep slope, non-resistant will have gentle slope. 3. Dipping Beds:- This is expressed in many ways on aerial photo. Direction of dip is conspicuous where topographic surface coincides with bedding where bedding is expressed by bands of different tones. 4. Folded Structures:- Folds are undulations or wave like features in rocks and constitute significant features in structural geology. Aerial photographs have been found extremely suitable for making interpretation of folded structures. They are commonly expressed as obvious feature of aerial photo if shown in single view. The drainage character may also aid in recognizing folds, also by pattern of beds when folds are plunging. The major streams usually curve around the nose of fold. 5. Faulted Structures:- Faults are fractures in rocks along which relative displacement has taken place. Faults are usually associated with folds. Aerial photos are very useful in recognizing faults. In a single photograph the faults may be traced for tens of Kms. There are a number of geological features and indications on aerial photographs which help on recognition of faults such as fault scraps, fault blocks and abrupt change of dip and strike etc. The abrupt termination of landforms, lithology, geological structure or drainage patterns along straight line are common indication of faulting. 6. Unconformities:- On aerial photographs unconformities can be observed directly when contact between the two sets of beds or two rock types is along tilted plane. 7. Fracture and Joints:- Some of the most prominent features on air photos are the straight or gently curved alignments of topography, vegetation, tonal boundaries, stream courses which mark the rock fracture. Like faults, they are commonly expressed as linear features. Good considerations for identification of joints are relatively undisturbed sedimentary rocks, large number of short lineation etc. 3. GEOMORPHIC INTERPRETATION: (Geomorphic Guides) The use of aerial photographs, satellite imageries has revolutionized not only geomorphological studies but also other branches of geology. Geomorphological studies by aerial photographic techniques are less expensive, more reliable, accurate, efficient and fast. The greatest advantage of this technique is that aerial photographs, unlike maps are the true photographic images of the terrain which can be viewed in three dimensional perspectives.
- 4. Practically all types of mappings and analysis of geomorphic interest like morphometric, morphological, land form, drainage pattern etc. may be done by this technique. Geomorphological interpretation can be categorized in following types. 1) General or Elemental Geomorphology. 2) Direct or Supplemental Geomorphology. 3) Indirect or Complemental Geomorphology. 4) Independent Geomorphology. Geomorphological interpretation depends on the ability of the observer to deduce the significance of land form type from studies of its internal constitution, external form, pattern arrangement and environmental relationship.