AKHILESHWAR TIWARI

ASSOCIATE PROFESSOR

E content for B.Sc. Geology (Hons.) Part-III

Sedimentary Petology

Sedimentary Petrology

Erosional Sedimentary Structures

Erosional Sedimentary Structures

The pre-depositional structures are formed before the deposition of the overlying beds. The structure occur on the surface between beds.

The bed forms in cohesive sediments are cut by the erosive effect of turbulent fluid with or without entrained sediment grains.

The most familiar erosional structures are

1. Sole marks –➢Scour marks &

➢ Flute marks,

2. Tool marks.

Sole marks are characteristically theproduct of environment with episodicsedimentation.

Prolonged deposition of muds ispunctuated by sudden influx of coarsersediments in event comprising an early erosivephase and an immediate succeedingdepositional phase.

Most common event generating such bedsare turbidity current.

SOLE MARKS

Nomenclature for sole marks

Scour Marks

Scour marks are distinguished by their

➢Generally smooth shape, and

➢ Streamlined appearance.

On the basis of shape can be grouped as

i. Obstacle scour,

ii. Flutes,

iii. Longitudinal scour.

i. Obstacle scour

ii. Flutes

They are discontinuous elongated hollows.

Often described as hell shaped consisting ofrounded upstream end, which flaresdownstream and merges into bedding plane.

In section they are asymmetrical withdeepest part at the upstream end. Commonlyrange in length from 5- 50 cm, in width from 1-20 cm, and in depth up to 10 cm.

Formed by localised erosion by eitherturbidity current or sand laden current movingover cohesive mud surface

Flutes

iii. Longitudinal scour - occurs as closely spaced parallel ridges and furrows

Tool marks-Produced by objects carried by flow interacting

with bed rather than flow itself.

The are of two types

➢ Continuous-

i. Groove marks

ii. Cheveron marks

➢ Discontinuous-

i. Bounce and Prod marks,

ii. Skip marks

Groove marks- continuous,elongated ridge, occur inisolation or in parallel groups,result from infilling oferosional relief by someobjects dragged by a current.

Cheveron marks – linear zoneof v-shaped crenulation ,consistently close in onedirection, 2-3 cm wide, reliefis generally less than 5cm.

Bounce marks – low approach angle, digs gently

Prod marks – larger impactangle, digs deeply to leave ablunt nose at the downstream end.

Skip marks – series of similarbounce marks arrangedlinearly.

Groove marks

Chevron marks

Bonce Marks and Groove marks

Prod marks and flute casts

Skip marks