Episode 18 Written in stone

Last time we saw a road cut near Santa Fe where worn-down metamorphic and plutonic rocks formed a surface upon which layers of marine limestone and shale were deposited. 

Let's carry this observation forward into the next set of topics I like to discuss, the construction of the stratigraphic record. Stratigraphic literally means "written in strata". The stratified rocks are among our planet's best record-keepers, so this term is well chosen.

Imagine that this box represents a block of crystalline rock, worn flat by ages of erosion, with the top surface of the box just above sea level:

Now imagine sea level rising slightly, with respect to the surface of the land. The water will spread over and gradually cover the block. This phenomenon is called a transgression of the sea.

If the water subsequently retreats, the effect is called a regression of the sea. Notice that as relative sea level rises and falls the shoreline shifts back and forth across the block.    ​

Depositional environments shift accordingly.

During a transgression, what was once land slips under the water. Marine environments overspread non-marine environments. Sediments will write a record of this change of venue: what was once a beach, for example, becomes an offshore sandbar, soon buried in marine mud.

During a regression, the sea drains away.  What was once water has now become land. 

If this is because of delta of mud and sand has built seaward, marine environments will be buried under non-marine environments. Sediments will faithfully record this change.

If instead, the land is simply brought to light by retreating water, weathering and erosion will resume their work, and the work of record-making will be interrupted.

Assuming that there is a supply of sediment, and assuming that space is being made to store those sediments, it is this recurrent shifting of depositional environments that writes the stratigraphic record. 
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