1.Create a procedural terrain. I doubled the displacement in the terrain editor, Heightfield shader 01 Tab
2.In top view make a Paint shader at the center of the terrain with fall off on the brush (see the .tgd) Play around with the fall off till you get a beach area if you need one, I didn't go that far. I'd advise playing around with the Paint shader's fall off on a flat terrain at first to see what it does re: slope etc. with displacement. Remember to use negative numbers if you want the displacement downwards.
3.Use the Paint shader as an inverted mask in the Mask by shader slot.
4.In the Shaders tab I added a Displacement shader and once again used the Paint shader this time for negative displacement to give more depth in the lagoon created.
5.I then applied the desert_cliffs .tgc that comes with the presets as a quick texture for display, mess around here to get the texturing you are after.
Hope this helps some.
