Halliday & Matthiessen (2004: 413):
The combination of enhancement with parataxis yields what is also a kind of co-ordination but with a circumstantial feature incorporated into it; the most frequently occurring subtypes are those of time and cause. The circumstantial feature is typically expressed
(a) by the conjunctions then, so, for, but, yet,still;
(b) by a conjunction group with and: and then, and thus, and so, and yet; or
(c) by and in combination with a conjunctive (ie a conjunctive expression that is not structural but cohesive) such as at that time, soon afterwards, till then, in that case, in that way.
Note also that some conjunctives, such as meanwhile, otherwise, therefore, however, nevertheless, are extending their use in modern spoken English so as to become paratactic structural conjunctions; in this function they are unaccented (spoken without salience).