The paradox of drowned reefs and carbonate platforms

Abstract

Shallow-water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation, and benthonic carbonate production ceases. Drowned platforms are common in the geologic record, but they present a paradox if one considers rates of processes involved. During the early Holocene, when sea level rose at rates of 6,000 to 10,000 µm/yr (= mm/1,000 yr), most reefs and platforms were outpaced by the rising sea. During the late Holocene with sea level rising 500 to 3,000 µm/yr in the Atlantic-Caribbean area, reefs and platforms started to recover, built to sea level, and prograded seaward, 1,000 µm/yr is thus a conservative estimate of the average growth potential of modern reefs and platforms. Independently, accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year. The growth potential of 1,000 µm/yr exceeds any relative rise of sea level caused by long-term processes in the geologic record. Newly formed ocean crust subsides at a maximum of 250 µm/yr, basin subsidence averages 10 to 100 µm/yr, and sea level rises due to increased sea-floor spreading amount to less than 10 µm/yr. Rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning. The geologic record shows examples of both of these processes. Global mass extinctions of reefs and platforms occurred in the middle Cretaceous (eustatic rise due to submarine volcanism or desiccation of a small ocean basin?) and the Late Devonian (global crisis of ocean environment, extraterrestrial cause?). Drowning controlled by regional tectonics prevailed in the Jurassic and Early Cretaceous of the Tethyan realm, and the drowning of Mesozoic platforms in the western North Atlantic seems to have been dictated by plate-tectonic drift to higher latitudes.