To separate coordinate clauses the following rules on the use of stops are observed.
§ 7. Coordinate clauses joined asyndetically are always separated by a stop.
The most usual stop is the semicolon.
Arthur looked at his watch; it was nine o’clock. (Voynich)
The policeman took no notice of them; his feet were planted apart on the strip
of crimson carpet stretched across the pavement; his face, under the helmet,
wore the same stolid, watching look as theirs. (Galsworthy)
A colon or a dash may be used when the second coordinate clause serves to explain the first. They serve to express the relations which a conjunction would express.
Breakfast over, Aunt Polly had family worship: it began with a prayer built
from the ground up of solid courses of scriptural quotations. (Twain)
Ellsworth advised a triangular piano — the square shapes were so
inexpressibly wearisome to the initiated. (Dreiser)
A comma is used to separate coordinate clauses when the connection between them is very close.
A fly settled on his hair, his breathing sounded heavy in the drowsy silence,
his upper lip under the white moustache puffed in and out. (Galsworthy)