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 ?

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

Two type of model

Laterally – fed model (structural observation)

Centrally-fed model (discrete element

modeling)

Sill intrusion concept-

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)

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- the radial dyke intrude in the outer sill and spread by

hydro fracturing, forming the inner sill

-- Chevalier and Woodford (1999)

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)

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.

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

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