ASBOG - Association of State Boards of Geology Field Geology and Mapping Questions and Answers 2 — Questions and Answers
Question 1: When measuring a planar feature's orientation in the field using a Brunton compass, what two measurements are recorded?
- Latitude and longitude
- Strike and dip (Correct answer)
- Azimuth and plunge
- Bearing and inclination
Correct answer: Strike and dip
Strike is the compass bearing of a horizontal line on the plane, and dip is the maximum angle of inclination measured from horizontal perpendicular to strike, together fully describing the plane's orientation.
Strike and dip are the fundamental measurements for describing the orientation of planar geologic features (bedding, faults, foliation, joints) in the field. Strike is recorded as a compass bearing (N30°E, for example) or a quadrant bearing of the horizontal trace of the plane. Dip is the steepest angle the plane makes with the horizontal, measured perpendicular to strike, with a direction indicator (e.g., 35°SE). Together they allow the geologist to reconstruct subsurface geometry and map structural patterns.
Question 2: On a geologic map, what does a hachure pattern (short lines pointing downslope) on a contact indicate?
- A thrust fault with the upper plate moving in the hachured direction (Correct answer)
- A younger rock unit overlying an older one due to normal faulting
- A nonconformity where igneous or metamorphic rock underlies sedimentary rock
- An overturned fold limb
Correct answer: A thrust fault with the upper plate moving in the hachured direction
Hachures on a fault contact indicate a thrust or reverse fault, with the hachures pointing toward the overriding (hanging wall) block, indicating compression and upward movement.
Thrust fault symbols on geologic maps use a line with triangular teeth or hachures on the hanging wall (overriding) side, indicating the direction of relative movement of the upper plate over the lower plate (footwall). Thrust faults dip at low angles (<30°) and result from compressional tectonics, placing older rocks on top of younger rocks. Normal fault map symbols use tick marks on the downthrown side.
Question 3: A geologist measures multiple joint orientations in a rock outcrop and plots them on a stereonet. What does a tight cluster of poles represent?
- A widely dispersed set of randomly oriented joints
- A consistent joint set with nearly uniform orientation (Correct answer)
- A fold axis plunging at a steep angle
- A shear zone with mylonitic fabric
Correct answer: A consistent joint set with nearly uniform orientation
When poles to joints cluster tightly on a stereonet, it indicates a well-defined joint set where multiple joint surfaces have nearly the same strike and dip orientation.
Stereonets (equal-area or equal-angle projections) are used to analyze the orientation of planar and linear structural elements. Poles to planes are points on the stereonet representing the normal to each measured plane. When poles cluster tightly, it indicates a systematic joint set with consistent orientation, suggesting a common stress history. Widely scattered poles indicate random fracturing. The center of a cluster gives the mean orientation of that joint set, used in rock slope stability analysis.
Question 4: What is the purpose of a stratigraphic measured section in field geology?
- To determine the absolute radiometric age of rock units
- To systematically document lithology, thickness, and vertical succession of rock units at a specific location (Correct answer)
- To map the lateral extent of a single rock formation across a region
- To collect geochemical samples for ore deposit evaluation
Correct answer: To systematically document lithology, thickness, and vertical succession of rock units at a specific location
A measured stratigraphic section records the thickness, lithology, sedimentary structures, fossils, and vertical sequence of rock units in detail at one location to establish a reference column for correlation.
Measured stratigraphic sections are fundamental tools in sedimentary geology and stratigraphy. They are constructed by systematically measuring and describing the rock column at a specific outcrop or cliff face, typically using a Jacob staff and clinometer or measuring tape. The section documents bed thicknesses, lithology, grain size, sedimentary structures, fossils, contacts, and any breaks in the record. Sections from multiple locations are correlated to reconstruct basin stratigraphy and paleogeographic history.
Question 5: When mapping in an area with poor outcrop, what remote sensing technique is most useful for identifying linear features that may represent faults or fractures?
- Ground-penetrating radar (GPR)
- Satellite-based synthetic aperture radar (SAR) or multispectral imagery (Correct answer)
- Seismic refraction surveys
- Electrical resistivity tomography
Correct answer: Satellite-based synthetic aperture radar (SAR) or multispectral imagery
Satellite multispectral and SAR imagery reveals lineaments (topographic and drainage alignments) that may correspond to faults, joint zones, or lithologic contacts not visible in the field due to vegetation or soil cover.
Remote sensing techniques, especially multispectral satellite imagery (Landsat, Sentinel, ASTER) and synthetic aperture radar (SAR), are powerful tools for regional geologic mapping in areas with poor outcrop or dense vegetation. They reveal lineaments — straight or curvilinear features in topography, drainage patterns, and vegetation — that often correspond to faults, fracture zones, or lithologic contacts. ASTER thermal infrared data can also help distinguish rock types. These data guide fieldwork by identifying priority areas for ground investigation.
Question 6: In describing sedimentary rocks in the field, a geologist notes 'graded bedding' in a sandstone sequence. What does this indicate about the depositional process?
- Repeated cycles of wind deposition and erosion
- Deposition from a waning turbidity current, with coarser grains settling first (Correct answer)
- Bioturbation mixing sediments of different sizes
- Cross-bedding formed by migration of subaqueous dunes
Correct answer: Deposition from a waning turbidity current, with coarser grains settling first
Graded bedding, where grain size decreases upward from coarse at the base to fine at the top, records settling of sediment from a waning turbidity current as flow velocity decreases.
Normal graded bedding occurs when a sediment-laden turbidity current loses velocity and particles settle at rates proportional to their settling velocity — coarser grains first, finer grains last. Each graded bed represents a single turbidite event. Graded bedding is a key tool for identifying paleoup direction (younging direction) in deformed sequences and for recognizing deep-water turbidite environments. Inverse grading (fine-to-coarse upward) occurs in debris flows and some storm deposits.
ASBOG - Association of State Boards of Geology Field Geology and Mapping Questions and Answers 2
When measuring a planar feature's orientation in the field using a Brunton compass, what two measurements are recorded?