Sills form through various emplacement mechanisms. Laterally-fed and centrally-fed models are two common types. Laterally-fed sills are saucer-shaped due to magma ascending to achieve hydrostatic equilibrium and forming conjugate climbing sheets at the level of neutral buoyancy. Centrally-fed sills form when a ring dyke reorients to an inclined sheet, inflating and allowing magma to flow downward and form a saucer base. Stress reorientation at sill tips enables upward climbing to form circular or elliptical saucer shapes depending on the magma feed geometry.

1. Question still not answer today ?
How is sill emplaced and what is the
geometry of feeder ?
How do complex sill form ?
Why sill turns into Saucer-shaped sill ?

3. Emplacement mechanisms-
Main structure of sill is
saucer-shape
Bradely (1965) told that sill
follow the surface of natural
buoyancy level(LNB)
subsequently shaped as a
saucer

4. Two type of
model
Laterally – fed model
(structural observation)
Centrally-fed model
(discrete element
modeling)

5. Sill intrusion concept-

6. LATERALLY–FED MODEL
MODEL (A)
 Mainly sill emplacement is based on the neutral buoyancy (Midland Valley sill of Scotland )
The step of emplacement –
Step 1-flank feeder dyke caused to formation of the horizontal sill, due to stress related reasons overshoot the
LNB and developed horizontal sill.
Step 2- following of the horizontal discontinuity of the inter-bedding .
Step 3-To achieve the hydrostatic equilibrium , magma may ascend to up-dip that lead to formation of the
conjugate climbing sheet of saucer-shaped sill
fig- of emplacement controlled at the level of neutral buoyancy (LNB), Modified from
Francis. Sills are fed laterally from one part of the outer sills .
---By Francis (1982)

7. MODEL (B)
Based on the
geometry of Karoo
basin, south Africa.
Steps of sill
emplacement
Step 1- A ring dyke ascends vertically and at a given
point the dyke re-orientates to form an inclined
sheet, inflating as it does so.
Step 2-This inflation causes the overburden of country rock
to be lifted ,allowing magma to flow downwards into the
central section, forming the base of a saucer-shaped sill.
LNB
fig-theradialdykeintrudeintheoutersillandspreadby
hydrofracturing,formingtheinnersill
--Chevalier and Woodford (1999)

8. CENTRALLY –FED SILL MODEL
MODEL (C)
 assumption- non-viscous fluid , injected into the homogeneous elastic medium (host rock-sedimentary)
Step 1- Sill initiation occur at suitable horizon i.e. over pressured shale, rigidity contrasts or discontinuities
between rock layers.
Step 2- When length of the sill = overburden thickness, at this condition due to reorientation of the stress
field at sill tips, inner sill inflation results.
Step 3- This re-orientation of sill tip lead to brittle fracture being open at angle of ~45˚to horizontal, thus it
enabling the sill to climb upward.
Malthe-Sørenssen et al. (2004)
fig- Model of emplacement along horizontal discontinuity,
modified after Malthe-Sørenssen et al.(2004)

9. The inner sill need
to be gain suitable
diameter for re-
orientation of
stress .
At shallow depth
climbing is
occurred due to
the force folding
and at greater
depth due to
flexure folding.
When fed is a
point source
saucer is circular
and if line then
saucer is of the
elliptical shape.
So this model
expertly explain
the shape(saucer-
shape) and the
doming effect of
the overburden.

10. Fig.-sill emplacement model on the basis of seismic data

12. REFERENCE-
 Emplacement of shallow dikes and sills beneath a small basaltic volcanic
center – The role of pre-existing structure, Greg A. Valentine , Karen E.C.
Krogh, Received 28 January 2006
 Journal of Volcanology and Geothermal Research, Christophe Y. Galerne
Hansen, D.M., Cartwright, J.A., Thomas, D., 2004. 3D seismic analysis of the
geometry of igneous sills and sill junctions relationships
 Phillpotts R. Anthony principles of Igneous and Metamorphic petrology 2nd
edition
 Valentine, A. G. and K. E. C. Krogh, 2006. Emplacement of shallow dikes
and sills beneath a small basaltic volcanic center - The role of pre-existing
structure (Paiute Ridge, southern Nevada, USA). Earth and Planetary Science
Letters, 246, 217-230
 Philpotts Anthony and Jay Ague Principles of Igneous and Metamorphic
Petrology,2nd edition ,Cambridge University Press, 29-Jan-2009