I'm not going to argue about the definition of "holocaust", or the time-scales. However, the Native Americans were subjected to a deliberate, de-facto "clearance", which was arguably on a larger scale and more effective than that suffered by the Jews.
And no, of course that's not taking...
Was it a holocaust? See Portlock Seagull's post, #20.
Were there many millions of deaths over many decades? I don't think anyone is denying that. Of course we don't know exact numbers.
Were there many causes? Yes. Outright killing, starvation on reservations (who put them there?), disease (gosh...
Seriously? There was pretty much official US government policy of genocide towards the indigenous "Indians", as the settlers swept west according to "manifest destiny". Ancestral lands were taken, natives were put on reservations without the means to live, etc, etc.