As William Gibson famously observed (in his forward to the Vintage Edition), Dhalgren "is not there to be finally understood. I believe its 'riddle' was never meant to be 'solved.'" With that said, I do have my own, unprovable theory on how Bellona is constructed: The entire city is in a closed time loop. (Closed time loops are a favorite of Delany's --he explored one, less successfully, in Empire Star. Compare also Diana Wynne Jones' Tale of Time City.) Events there are cyclical, they happen over and over again --not necessarily identically, and not necessarily with the same cast. Thus, the events that are in the recent past at the beginning of the book are in the near future at the end of the book. As part of this, the city is self-renewing. It burns, but is not consumed. It is looted, but never exhausted.
The narrator/main-character is one of the archetypal inhabitants of the city. He enters at one point of time, and exits at another, but yet in some sense he, or some other version of him, has always been there. Through the diary entries it is suggested that he experiences the complete loop, and that he is present in the city both before his arrival, and after his exit. Due to his amnesia, it's unknowable how many times he has actually lived through the cycle, or if all the versions of him we follow throughout the book are actually the same person.
So what then is the disaster that has created this time anomaly? I think that too can be discerned by a careful reading between the lines. What can create a singularity capable of warping time in on itself? The crashing together of two celestial objects. In this case, the sun and the moon of Bellona are June, a young sheltered white girl, and George, a crude, powerfully built Black man privately and publicly lusted over by half the population of the city. The two of them circle each other at a distance throughout most of the city's cycle, but the consummation of their sexual attraction is the actual disaster that causes the city to detach from the rest of America. Their pairing appears, at a superficial level, to be the exact scenario used by white supremacists to justify the oppression of Black people, the brutal rape of a young innocent white girl by a brutal, powerful, bestial Black man. However, Delany subverts this by suggesting the young girl has cold-bloodedly murdered her own brother to prevent him from revealing her hidden desires, that the Black man is a quiet humanitarian, and that their coming together is a bit of mutually consensual play-acting for their own sexual pleasures --the white supremacist's worst nightmare. It's the contradictory nature of these two scenarios, and the way they go straight to the heart of the American psyche, and its sustained racial dysfunctions, that tears Bellona right out of time and space. (It's a uniquely Delanian conceit that the disaster is sociological and psychological, not physical.) Subsequently a white sniper shoots at Black protestors, riots ensue and buildings are burned, and the city descends into a repeating set of scenarios that feel as familiar and as inescapably current in the year 2021 as they must have back in 1975. Maybe Delany was onto something.