It is normal for every Marriott I have been to both in the UK and overseas to block off more on your holding card especially so if you wish to charge incidentals to your room. Usually its around £50-100 on top of the stay rate depending on how long my stay is. Once the final bill is through, only the actual total is charged on my card.

Just think of it like filling up with petrol - your card is "charged" more than what you fill up with usually, but it is just the actual final total that comes off your account.

If it is still showing as a pending charge, it might be worth giving your bank/credit card provider a call.