Halliday & Matthiessen (2014: 434):
A sequence of projection or expansion may be realised by two clauses that are combined structurally to form a clause complex, as in a happened and then b happened or after a happened, b happened. But there are two alternative forms of realisation. On the one hand, the sequence may be realised by two clauses that are not combined structurally but are linked cohesively instead: A happened. Then b happened. Here the grammar provides a ‘clue’ as to the nature of the semantic link; but it does not integrate the two clauses into a grammatical construction. On the other hand, the sequence may be realised by a single clause with a phrase (or adverbial group) serving as a circumstantial element within it: after the time of a, b happened.