But Durham's Chris Rushworth hit back with the second new ball, grabbing three wickets to finish with 5 for 40 as the visitors slipped from 182 for 3 to close the first day on 221 for 7.
Having elected to bat in baking heat, Glamorgan ground along all day. losing only one wicket in each of the first two sessions in reaching 147 from 66 overs.
Selman spent 249 balls over his hundred, getting there by inside-edging Rushworth for his 13th four. Three balls later he was lbw for 103. Offspinner Ryan Pringle conceded only 23 runs in 19 overs as Durham maintained their accuracy, with the other two wickets going to academy seamer Matty Potts in his second appearance.
Despite a minute's silence for the London fire victims at 11am, and a drinks break at noon, Durham got through 33 overs in an attritional first session.