The really cool bit in these images is seeing how the black igneous
dolerite rock on the left cooks the fossils in the limestone on the right.
As you back away from the contact zone the ghost-like images of the
original fossils are still visible in the marble rock. See the steps shown
Chillagoe , Queensland Australiia has very interesting geology.
On what was once the edge of the continental shelf, fossil bearing
limestones are intruded by granite and shot through with dolerite dykes.
Red hot dolerite moved into the limestone along joints (straight cracks)
that formed when as the limestone was being formed it lost .
When something like rock shrinks you get two sets of straight joints .
Igneous rocks also get joints but for another reason. Liquid rock takes up
more space than solid rock so as the rock turns solid it shrinks and
voila! - two sets of joints form
You can also get joints by putting rock under tension that is pulling
These hot igneous rocks cook the limestone and like a good cook, without
melting the limestone transform it into clean beautiful marble.
The marble is mined as decorative stone.
The cooked contacts of the igneous
and sedimentary rocks
form calcsilicate rich mineral skarn
mineral deposits and are mined for copper and gold
Comment #1 Cooking
The red hot black igneous rock dolerite (left) intrudes along a joint into
the white and grey sedimentary fossil bearing limestone (right)
The greenish band on the contact is newly created calcsilicate minerals
which form the basis for skarn type mineral deposits
The process is called "contact
The fossils, still visible below are transformed, without melting,
from limestone to marble
The resultant marble has larger crystals, is denser and has fewer
impurities than the original limestone.
The fossils gradually "ghost" and "fade" into the marble as the crystal
growth obscures and overgrows their edges.
Comment #3 The Full Sequence from right to left...
the unaffectedv fossiliferous limestone
heat from dolerite cooks the fossils transformng them into
gradually fossils are gradually disappearing into coarser marble,
on the contact, green calcsilicates are newly formed
some of the calcium from the limestone contaminates the
contact margin of the dolerite
the dyke of unaffected black igneous dolerite
Comment #4 Contact Metamorphism
In contact metamorphism
the heat and pressure of a nearby intrusion "cooks" the adjacent rocks
transforming it into a metamorphic rock.
Metamorphic rocks tend to be more dense and have coarser crystals than
their origin rocks.
The process of contact metamorphism on the "dirty" fossil bearing
limestones of the Chillagoe area transforms the limestone into clean
massive marble and creates concentrations of copper and gold along
the contact skarns.
Joints (also termed extensional fractures ) are planes of
separation on which no shear displacement has taken place.
The two walls of the resulting opening typically remain in tight
Joints may result from
regional tectonics (i.e. the compressive stresses in front of a
folding (due to curvature of bedding),
or internal stress release during uplift
or cooling (ie shrinkage)
They often form under high fluid pressure (i.e. low effective
stress), perpendicular to the smallest principal stress.
so in sedimentary beds they generally form perpendicular to the
sets regularly spaced of joints are formed but their spacing is
offset (skewed) in sectional view
Joints are ubiquitous features of rock exposures and often form families
of straight to curviplanar fractures typically perpendicular to the layer
boundaries in sedimentary r ocks. A set is a group of joints with similar
orientation and morphology. Several sets usually occur at the same place
with no apparent interaction, giving exposures a blocky or fragmented
appearance. Two or more sets of joints present together in an expos ure
compose a joint system . Joint sets in systems commonly intersect at
constant dihedral angles. They are conjugate for dihedral angles from 30
to 60°, orthogonal when the dihedral angle is nearly 90°
the thinner the bed the more joints,
the stiffer the bed the more joints
from the collection of Earth Science Australia