Interpretation of Interpretation of Landforms, Lithology & Landforms, Lithology &

Structure of Structure of Sedimentary RocksSedimentary Rocks

S.S. NayakS.S. Nayak

Lithological Interpretation

Bedding Tone Texture Drainage characteristics Drainage density Landforms Vegetation

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

LIMESTONE

SANDSTONE SHALE

Water soluble

Internal drainage,

Sink holes

Widely spaced External drainage

Close spaced

External drainage,

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

Horizontally Bedded Sandstone

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.

Horizontally Bedded Shale

Arid

Humid

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.

Limestone

Limestone

Sink holes in Limestone

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

Thick Interbedded

Flat-Lying Sedimentary Rocks

Thin Interbedded

Flat-Lying Sedimentary Rocks

PLATEAU – MESA - BUTTE

Escarpment

Interbedded Sedimentary Rocks – Flat Lying

Thickly bedded

Thinly bedded

Thinly bedded flat-lying interbedded sedimentary rocks

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

Interbedded Interbedded Sedimentary Sedimentary Rocks – TiltedRocks – Tilted

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

Tilted Interbedded sequence of sand stone and marl giving rise to a series of parallel ridge and valley topography

Tilted Interbedded Sedimentary RocksTilted Interbedded Sedimentary Rocks

Hogback

Flatirons – Steeply dipping beds – indicate direction of dip

Interbedded Tilted Sequence Sandstone and Shale

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

Criteria for Relating Topography & Structure

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

Anticlinal Fold

Antiformal closure

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

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

Recognition of faults

Recognition of faults

Recognition of faults

Recognition of faults

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

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

Angular Unconformity

The older folded sequence abuts against the younger horizontally disposed sequence

Angular Unconformity

Non-Conformity