What happens when fossils get cooked?

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What happens when fossils get cooked?

brachiopod shells in limestone unaffected limestone with brachoipod shell fossils

Let the rocks tell their story
Comments on the in the cooking process...
      see also Metamorphic Rocks...
      see also Skarn Mineral Deposits...
Joint Basics - a very general overview





The Story



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 below

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 apart forces.

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

 Comments on the in the cooking process:


 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 metamorphism"
dolerite dyke in contact with limestone

Comment#2 Transformation 
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.

recrystallised fossils still visible as they transform to marble
 

Comment #3 The Full Sequence from right to left...
                                                                    
right to left limestone transformed to marble with calcsilicates on contact with doleritelimestone with fossils


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.
contact metamorphism with limestone

a Chillagoe marble mine
Chillagoe marble quarry


Joint Basics - a very general overview...



joint basics Two 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°

joints perpendicular to the sedimentary bed
source: https://lifeinplanelight.wordpress.com/tag/metamorphism/
             http://www.files.ethz.ch/structuralgeology/JPB/files/English/4joints.pdf
             from the collection of Earth Science Australia