In my view the lines would be:
And from this chasm, with [ceaseless turmoil seething,]
As if this earth in fast thick pants were breathing,
[A mighty fountain momently was forced :]
The chasm definitely is a reference to Chaos, the Greek void state of the Cosmos before creation. Chaos does actually mean chasm in ancient Greek. Chaos in Greek mythology was the confused state of matter and mind. A sort of primordial jumble which contained everything that would and could be. According to Greek mythology it was also “seething with ceaseless turmoil”, meaning that all of its elements were filling up with energy and about to emerge from it into creation (imagine it as the Greek version of the Big Bang).
Then the Earth is and it is “breathing in fast thick pants”, in other words the Earth is palpitating with the turmoil of creation, life and matter and water, and winds gushing and rushing all over the surface of the planet.
And then life emerges and the imagery of a mighty fountain is very sexual, a “fountain was forced” clearly evokes the semen pouring out of the male reproductive organ during sexual intercourse. Mother Earth is being impregnated with Life by Water.
Coleridge is obviously using Kubla Khan’s Xanadu as an allegory for Creation.