education geology and geophysics (docs39)
TRANSCRIPT
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Interpretation of Interpretation of Landforms, Lithology & Landforms, Lithology &
Structure of Structure of Sedimentary RocksSedimentary Rocks
S.S. NayakS.S. Nayak
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Lithological Interpretation
Bedding Tone Texture Drainage characteristics Drainage density Landforms Vegetation
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Intermediate rocks
Limestone, Dolomite,
Rock salt, etc
Carbonates + Evaporites
Coarse Clastics Fine Clastics
Conglomerate, Sandstone,
Quartz-arenite, etc
Clay, Shale, etc
Classification of Sedimentary RocksClassification of Sedimentary Rocks
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LIMESTONE
SANDSTONE SHALE
Water soluble
Internal drainage,
Sink holes
Widely spaced External drainage
Close spaced
External drainage,
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Horizontally Bedded Sandstone• Topograhy:
• Bold, massive, flat topped hills of equal elevation
• Nearly vertical hill sides
• Drainage• Coarse textured, joint controlled modified dendritic• Often rectangular
• Erosion• Few gullies, V-shaped if present in residual soil
• Photo tone• Light due to light colour of the rock• In humid areas dense tree causes dark tone
• Vegetation• Commonly forested in humid while sparse in arid
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Horizontally Bedded Sandstone
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Horizontally Bedded Shale• Topograhy:
• Minutely dissected with steep stream/gully in arid climate
• Gently to moderately sloping softly rounded valleys & hills
• Drainage•Fine textured, dendritic patten with gently curving streams in arid climate•Medium to fine in humid climate
• Erosion• Gullies have gently rounded cross section in humid• But sharp angular in arid terrain
• Photo tone• Generally dark tone
• Vegetation• Barren in arid except for desert vegetation• Intensely cultivated in humid areas.
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Horizontally Bedded Shale
Arid
Humid
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Horizontally Bedded Limestone• Topograhy:
• Karst topography in humid areas
• Forms table rocks in arid climate
• Drainage• Internal drainage in humid climate• Angular drainage with medium to fine density in arid terrain
• Erosion• Chemical weathering in humid climate• Erode very little and forms cap or table rocks in arid terrain
• Photo tone• Varies from light to dark depending on cover
• Vegetation• Barren in arid except for desert vegetation• Dense vegetation resulting from clay soil in humid areas.
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Limestone
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Limestone
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Sink holes in Limestone
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Interbedded Sedimentary Rocks – Flat Lying
• Topograhy:
• Terraced hillsides with hilltops at same elevation
• Slopes more gradual in humid while sharp in arid terrain
• Drainage
• Dendritic medium to coarse in humid
• Dendritic medium to fine in arid
•Photo tone
• Varies depending on rock type on cover
• Vegetation – varies depending on climate and lithology
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Thick Interbedded
Flat-Lying Sedimentary Rocks
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Thin Interbedded
Flat-Lying Sedimentary Rocks
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PLATEAU – MESA - BUTTE
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Escarpment
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Interbedded Sedimentary Rocks – Flat Lying
Thickly bedded
Thinly bedded
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Thinly bedded flat-lying interbedded sedimentary rocks
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Interbedded Sedimentary Rocks – Tilted
• Topograhy:
• Series of parallel ridges with alternating valleys
• Slopes and ridges are smooth in humid while sharp in arid terrain
• Drainage
• Trellis and dendritic
• Coarse in humid terrain and fine in arid terrain
• Photo tone - Varies depending on rock type on cover
• Vegetation - Varies with Climate and the lithology
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Interbedded Interbedded Sedimentary Sedimentary Rocks – TiltedRocks – Tilted
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Landforms When the resistant strata are inclined
the ridges eroded from the strata are asymmetrical in cross section
The slope which has the same direction as the dip is usually a consequent or resequent or dip slope
The slope opposed to the dip is obsequent slope
Where dip is less than 30o the obsequent slope is steeper than resequent and the landform is described as Cuesta
With dip becoming steeper, the ridge becomes symmetrical and are called hog’s back
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Tilted Interbedded sequence of sand stone and marl giving rise to a series of parallel ridge and valley topography
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Tilted Interbedded Sedimentary RocksTilted Interbedded Sedimentary Rocks
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Hogback
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Flatirons – Steeply dipping beds – indicate direction of dip
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Interbedded Tilted Sequence Sandstone and Shale
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Criteria for Relating Topography & Structure Homoclinal ridges with infacing escarpments mark anticline
Homoclinal ridges or plateau like areas with outfacing
escarpments mark syncline
Homoclinal ridges of a plunging anticline will converge in the
direction of plunge
Homoclinal ridges of plunging syncline converge in the direction
opposite to plunge
Alternating homoclinal ridges and valleys develop where there is
a series of alternating strong and weak beds on the flanks of
anticline & synclines
The noses of anticlines are smooth and rounded
The noses of synclines are abrupt, narrow & scarp like
Anticlinal mountains are typically of rounded summit and smooth
Synclinal mountains may vary from elongate narrow ridges to
broad plateau like expanses
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Criteria for Relating Topography & Structure
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Adjustment of topography to fold structure
Differential degradation of strong and weak beds cause the topography to show the position and outlines of folds
Plunging anticlines and synclines give rise to a zigzag pattern in the ridge
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Anticlinal Fold
Antiformal closure
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A syncline
Note: 1. Dipping beds, escarpments facing outside, Flatirons, Radial (centripetal) drainage
2. A Hogback (H) in the background with displacements along several faults
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Recognition of faults
Straight line segments of stream courses Alignment of lakes and streams Alignment of vegetation Conspicuous changes in photographic tone,
drainage, or texture across a linear feature Alignment of topography, including saddles or straight
scarps Horizontal or vertical offsets of beds or recognisable
rock units Presence of fault scarps and faultline scarps Low angle faults are more difficult to interpret –
dicordance of structures within rock unit with strongly curved or irregular traces
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Recognition of faults
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Recognition of faults
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Recognition of faults
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Recognition of faults
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Features Diagnostic of fault scarp
Presence of small scarps, sag ponds, or basins and small wedge shaped hills in discordant relationship with each other
It a scarp faces upstream and intersects a transverse valley to form a lake at its base
Small alluvial fans at the base of a scarp indicates sinking of a fault block
Displacement of older topographic surface
Displacement of Recent or Pleistocene deposits
Warped terraces – steep slope or reversal
Scarp across alluvial deposits
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Unconformities
1. Angular unconformity: An unconformity where the attitudes of
the rocks on opposite sides are not parallal
2. Disconformity: The attitudes of the rocks on opposite sides are
parallel – essentially covers a large area
3. Local unconformity: Similar to disconformity but of local extent
4. Non conformity: Where the older rock is of plutonic or
metamorphic origin
A surface of erosion or non-deposition
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Angular Unconformity
The older folded sequence abuts against the younger horizontally disposed sequence
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Angular Unconformity
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Non-Conformity
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