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 resided in a nearshore marine atmosphere with standard oxygenation; or it may have dwelled in a more distant, open-sea environment further from the coast, where oxygen would not be accessible beneath the deposited sediment's surface layers.