**5. Drowning unconformities**

*Drowning unconformities* are "maximum flooding surfaces" (= drowning surfaces *sensu* Posamentier and Allen [17]) specific for carbonate platforms.

In the subsurface, drowning unconformities usually make good seismic reflectors with basinal strata onlapping carbonate slopes and platform tops [72]. On the outcrop or core face, these contacts are characterized by condensed sections (e.g., shell concentrates) and non-deposition surfaces with hardgrounds sometimes impregnated with phosphate and/or glauconite [73, 74]. The surface of a drowned carbonate platform can be table-flat, rimmed by a reef crest (empty-bucket configuration), or outgrown with backstepped carbonate mounds or "pinnacles" [31, 75]. Drowning unconformities are within-trend drowning ("flooding") surfaces in sequence stratigraphy [20]. Fundamental genetic difference from subaerial unconformities renders certain reluctance in accepting them as formal sequence boundaries (e.g., [76]). However, drowning surfaces are more practical in subsurface surveys as they produce vivid onlap pattern on carbonate slopes and sharp impedance contrasts, in difference to feeble expression of subaerial disconformities embedded in carbonate platforms. Also, submarine corrosion can mimic subaerial karst to some degree, complicating its workflow recognition and leading to misinterpretations. These considerations led to proposal to legalize drowning unconformities in sequence stratigraphy as *Type 3 sequence boundaries* [18, 31].

Factors leading to demise and drowning of carbonate platforms are rapid relative sea level rise and/or carbonate production shutoff by eutrophic turbid waters, either loaded with siliciclastics or upwelled from deep ocean [72, 74], but these factors can only smother photozoan or tropical carbonate factories (*T-factories*; [31, 77]). Non-actualistic mud-mound carbonate factory can likely produce thick carbonate buildups in dimmed or aphotic settings and at elevated nutrient levels (*M-factories*; [31, 77]). Drowning unconformities are usually produced in settings of tectonic subsidence, e.g., in extensional rifted basins [78] or foreland basins [79–81]. It remains unclear to what extent high-amplitude eustatic rise of sea level, without aid of other factors, is capable of shutting off carbonate platforms. Another factor is the slowdown in ocean circulation under greenhouse condition of the Earth or even shutdown of thermal ocean circulation under extreme hothouse condition [82], which should lead to lateral expansion and shallowing of the oxygen minimum zone in the ocean (OMZ; [83]). The OMZ under such conditions should develop a thick and permanent euxinic environment in its core and should be able to rapidly shut down benthic carbonate factory across broad expanse of an ocean-facing carbonate shelf even in tectonically quiet setting. Such OMZ expansions are seen as the condition imprinted in severe form in oceanic anoxic events [84]. Possible link between synchronous and widespread demise of carbonate platforms and oceanic anoxic events has been indicated based on the Cretaceous "Selli event" (OAE 1a; [85]). Even during icehouse epochs, anoxic waters similar to those in present-day eastern tropical pacific OMZs were likely able to encroach far into interiors of epeiric seas during interglacial highstands and switch carbonate deposition to black phosphatic shales [86].
