It may have been a nearshore marine environment with 'normal' levels of oxygenation; or it may have been a deeper water environment further from the shore, on the open continental shelf; in this setting oxygen would not be available below the surface layers of the deposited sediment.
It may have existed in a shallow coastal marine environment with typical oxygenation; or it may have resided in a more distant, open-sea location further from the shore, where oxygen would not be available beneath the surface layers of deposited sediment.