(a) Describe the principles of geological mapping. Using sketches, describe the different types of symbols used in geological…

(a) Principles of geological mapping and map symbols (20 marks)

Geological mapping is the plotting of rock units, their contacts and structures on a base map by direct field observation. Its principles are as follows.

• A suitable base map (topographic sheet or air photo) is chosen, at a scale to suit the purpose (1:50,000 for regional and 1:10,000 or larger for detailed work), and studied beforehand with photo and satellite interpretation and earlier reports.
• The area is mapped in traverses across the strike, or by tracing contacts, with each observation located by GPS or compass and pacing.
• Mappable units are defined on distinctive lithology, and are recorded with their contacts, attitude (strike and dip), fossils, structures (folds, faults, joints, foliation) and samples.
• The laws of superposition, original horizontality and cross-cutting relations are used to interpret the sequence, and the rule of V's is used to relate outcrop pattern to dip and relief.
• Contacts are followed, correlated and drawn, a legend, cross-section and stratigraphic column are prepared, and the map is checked in the field.

Symbols. The signs fall into categories: bedding attitude (inclined, horizontal, vertical, overturned); contacts (observed, inferred, concealed); faults (normal, thrust, strike-slip); folds (anticline, syncline, plunge); unconformity; and other features (joints, foliation, dykes, fossil localities, samples and bore holes).

(b) Drainage in relation to lithology and structure; antecedent and superimposed drainage (15 marks)

Lithology. Resistant, impermeable rocks such as shale and clay give a high drainage density, while permeable rocks such as sandstone and limestone give a low density; in karst, streams vanish underground. Homogeneous rocks or horizontal strata develop a dendritic pattern.

Structure. Alternating hard and soft folded strata give a trellis pattern, with subsequent streams along the strike of the weak beds. Joints and faults in massive rocks give a rectangular or angular pattern. Domes and volcanic cones give a radial pattern, basins a centripetal pattern, and eroded domes an annular pattern. Faults also produce linear valleys and offset streams.

Antecedent drainage is a river that existed before an uplift and kept its course by cutting down as fast as the land rose, forming a gorge across the rising range. The Indus, Sutlej and Brahmaputra (Dihang gorge) crossing the Himalaya are examples.

Superimposed drainage is a river that was established on an overlying younger cover, and was lowered on to the older, structurally different rocks beneath, cutting across their structure with no adjustment to it. Examples are the Subarnarekha and Chambal in India and the Green River across the Uinta Mountains in the USA.

(c) Aerial photographs and satellite imagery; multispectral data (15 marks)

Aerial photographs. Merits: very high spatial resolution, stereoscopic coverage giving relief, dip and structure, flexible timing and scale, and low cost for small areas. Limitations: a small area per photo (so many are needed), scale variation and relief displacement, a limited spectral range, and costly repeat coverage.

Satellite imagery. Merits: synoptic, repetitive coverage of large or inaccessible areas, multispectral and thermal bands, digital data suited to processing and GIS, and low cost per unit area. Limitations: coarser resolution (except costly high-resolution data), cloud cover, limited stereo, and atmospheric effects.

Multispectral data. Rocks reflect and absorb differently in different bands, so combining bands identifies lithology. Iron oxides absorb in the visible and near-infrared, clays and carbonates absorb in the SWIR (about 2.2 and 2.3 μm), and silicate rocks are separated by thermal-infrared emissivity. False-colour composites, band ratios, principal components and classification map rock units. Structures are recognised from lineaments, offsets, tonal and drainage anomalies, and from fold-shaped outcrop patterns, which are enhanced by filtering and combined with DEMs.