How to create a geological model from a complex geology map with intrusives and sediments

Hello I am trying to build a geological model from a geology map. I have the meshes created for intrusives (two intrusions ages Cretaceous and Paleogene), volcanic flows (Neogene), Jurassic limestones and alluvial quaternary. My problem is to create the contact surfaces for the intrusives. I google for tutorials in this matter but no luck. If someone has some hints on this or a tutorial, I will appreciate it very much.