O K blueland...
next stage go to the bottom of the surface layer requestor in the Coverage Tab set Coverage to .5 and Fractal Breakup to 2.
Next add the same image map mentioned previously to the box with Fractal breakup 00 in it by clicking the (...) next to it and go Create new shader>Image map shader and then add image as before, This will mask out the colours in the Colour Tab of the main requestor where you can now put an image that you want to see in the sediment/whatever area you've just masked out in the previous step.. ....