Abstract The concept of chaotic or “diagenetic” seismic facies is introduced as a tool for understanding factors controlling the evolution of Miocene Zhujiang Formation carbonate platforms in the Pearl River Mouth Basin, South China Sea. Seismic data from this basin contain a continuum of chaotic seismic facies, which are present only within the carbonate interval and range in appearance from simple concave-shaped reflectors to highly irregular, high-amplitude chaotic reflectors. Diagenetic seismic facies are significant because they are overprinted on depositional (primary) seismic facies. The common association between chaotic seismic facies and other features, such as truncated zones and hummocky carbonate surfaces, supports a model involving karstification of Zhujiang Formation carbonates. Two contrasting stages of carbonate platform development are recognized from seismic stratigraphic relationships within the Zhujiang Formation interval: (1) the “lower” platform, a broad and areally extensive low-relief ramp, and (2) the “upper” platform, a narrow and high-relief feature. A regional episode of subaerial diagenesis separating the two stages of platform development is inferred from the distribution of chaotic seismic facies and from the presence on seismic of erosional carbonate surfaces. The magnitude of sea level fluctuation is thought to have been on the order of a few 100 m and was likely eustatically controlled. Numerous backstepping events in the upper Zhujiang carbonate suggest this stage of platform development was characterized by continually rising sea level. Smaller-scale exposure events, not resolvable on seismic, are recognized in cores of upper Zhujiang carbonates in the Liuhua 11–1 field and are seemingly at odds with the drowning model. These exposure events are interpreted to be sudden brief excursions in an otherwise continuously drowning sequence.