1) Find one or more volcanic tuff layers bracketing the shale unit and do U/Pb (SHRIMP or TIMS) on the freshest/youngest zircons; or Ar-Ar on feldspars (preferably sanidine) or mica flakes. The youngest detrital zircon/feldspar/mica ages are a strict maximum age for the sediments (because the sediments can only rework mineral grains that already 'exist', so the sediment is younger than or equal in age to the youngest mineral grains dated).

2) Rb/Sr has been applied to whole-rock shale samples, but the system is easily disturbed (as rubidium is a rather mobile element).

thanks Martin..