bedrock. Contact -called biomarkers are used. Among them are: 1 .- Color  
 pollen and spores (palynomorphs) that with increasing temperatures ranging from shades of yellow to brown and finally black. 2 .- 
 vitrinite reflectance. This maceral reflectance increases with temperature (increasing the size of aromatic ring structures). 3 .- 
 conodont color alteration (CAI) on a scale similar to that of pollen and spores.  
 SOURCE: http://cig.museo.unlp.edu.ar/docencia/sed/organogenas.pdf