Depthless and Preliminary

Depthless adjective - Lacking significant physical depth.

Preliminary and depthless are semantically related. in rough topic. In some cases you can use "Preliminary" instead an adjective "Depthless".

Preliminary adjective - Coming before the main part or item usually to introduce or prepare for what follows.

Depthless and preliminary are semantically related. Sometimes you can use "Depthless" instead an adjective "Preliminary".