The paradox of drowned reefs and carbonate platforms WOLFGANG SCHLAGER Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, Univer- sity of Miami, Fisher Island Station, Miami, Florida 33139 ABSTRACT Browning Accum. < Rise Shallow-water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate ac- cumulation, and benthonic carbonate pro- duction 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 /um/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 /xm/yr in the Atlantic-Caribbean area, reefs and plat- forms started to recover, built to sea level, and prograded seaward, 1,000 Aim/yr is thus a conservative estimate of the average growth potential of modern reefs and plat- Figure 1. Schematic diagram of initiation, growth, and drowning of carbonate plat- forms. Independently, accumulation rates forms and reefs. Platforms and reefs start to grow where sea floor reaches into the euphotic of prograding platforms in the geologic rec- zone in low-latitude seas devoid of massive terrigenous influx, they continue to grow as ord suggest growth potential in excess of long as sediment accumulation equals or exceeds relative rise of sea level and are drowned several hundred microns per year. when sea-level rise exceeds accumulation. Drowning is complete when the flat platform The growth potential of 1,000 ¡xmlyt ex- tops are submerged below the euphotic zone. In the geological record, complete drowning ceeds any relative rise of sea level caused by is indicated by a cover of pelagic sediments or deep-water hardgrounds on the platform long-term processes in the geologic record. top. Newly formed ocean crust subsides at a maximum of 250 fimlyt, basin subsidence averages 10 to 100 fJ-mlyr, and sea level INTRODUCTION quence of neritic deposits, rapidly passing rises due to increased sea-floor spreading upward into deep-marine sediments. Com- amount to less than 10 /um/yr. Rapid pulses Drowning of carbonate platforms and monly, hardgrounds with crusts of fer- of relative rise of sea level or reduction of reefs is here defined as an event where rela- romanganese oxide, phosphate, or glauco- benthic growth by deterioration of the envi- tive rise of sea level (that is, tectonic plus nite separate neritic and deep-water de- ronment remain the only plausible expla- eustatic movements) outpaces carbonate posits and indicate a period of nondeposi- nations of drowning. accumulation so that the platform or reef tion within the marine environment. Some The geologic record shows examples of becomes submerged below the euphotic platform deposits show the effects of both of these processes. Global mass ex- zone of prolific carbonate production. This meteoric diagenesis, suggesting exposure of tinctions of reefs and platforms occurred in definition excludes termination of carbon- the platform prior to drowning. The causes the middle Cretaceous (eustatic rise due to ate growth by terrigenous influx. "Plat- of platform drowning have never been submarine volcanism or desiccation of a form" is used throughout this report for analyzed in great detail. It was tacitly or small ocean basin?) and the Late Devonian large (several square kilometres) carbonate explicitly assumed that subsidence and eus- (global crisis of ocean environment, ex- bodies with a more or less flat top in the tatic rise of sea level, possibly in conjunc- traterrestrial cause?). Drowning controlled euphotic zone. This includes "platforms" tion with environmental stress, caused the by regional tectonics prevailed in the Juras- and "offshore banks" in the sense of Wilson drowning. Indeed, if we accept the above sic and Early Cretaceous of the Tethyan (1975, p. 21). definition, then each drowned platform in- realm, and the drowning of Mesozoic plat- Drowned platforms are common in the dicates a situation where the combined rate forms in the western North Atlantic seems geologic record and have been reported for of subsidence and eustatic rise of sea level to have been dictated by plate-tectonic drift most epochs of the Phanerozoic (Table 1). was faster than the platform could grow to higher latitudes. Drowned platforms typically exhibit a se- under the particular environmental condi- Geological Society of America Bulletin, Part I, v. 92, p. 197-211, 14 figs., 2 tables, April 1981. 197 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/4/197/3434314/i0016-7606-92-4-197.pdf by guest on 01 October 2021 198 W. SCHLAGER TABLE 1. SOME DROWNED PLATFORMS IN THE GEOLOGIC RECORD Time Platform Description Source CAMBRIAN Lower/Middle Cambrian Spain (Cantabrian Mountains) Red, nodular limestone (Griotte) Van Der Meer Mohr (1967), with hardground (glauconitic) Tucker (1974, 1973) over algal limestone and dolomite ORDOV1CIAN Middle Ordovician Newfoundland Black shale and deep-water lime- Hubert and others (1977) stone with slumps (middle Table Head) over peritidal and shallow-marine carbonate (lower Table Head and St. George Fm.) DEVONIAN Middle and Late Devonian Central Europe Red cephalopod limestone over Krebs (1968, 1972), Tucker (1974), (Givetian and Frasnian) (Rheno-Hercynian fold belt) shallow-water carbonates (incl. Eder and Franke (1979) reefs) Central Europe (Carnic Alps) Red cephalopod limestone over Bändel (1974) shallow-water carbonates (incl. reefs) France Red cephalopod limestone over Tucker (1974) shallow-water carbonates (incl. reefs) TRIASSIC Anisian Central and Eastern Europe Cherty deeper-water limestone Schlager and Schoellnberger (1974), (Alps, Dinarids, Greece) with radiolaria, cephalopods Bechstaedt and others (1978) over algal limestone, dolomite, some reefs Rhaetian Central and Eastern Europe Red cephalopod limestone with Garrison and Fischer (1969), (Alps, Dinarids, Greece) hardgrounds over reefs and Schlager and Schoellnberger (1974) shallow-water limestone JURASSIC Lias Central and Eastern Mostly red cephalopod limestone Bernoulli and Jenkyns (1974) Europe, Mediterranean ("Ammonitico Rosso") over Wendt (1963), Jurgan (1969) shallow-water limestone and dolomite Dogger Atlantic off Newfoundland Abenaki Fm.; deeper-water shale Eliuk (1978) (glauconitic) over shallow- water limestone, dolomite Malm Eastern Europe Red cephalopod limestone Bernoulli and Jenkyns (1974) (Greece, Yugoslavia) ("Ammonitico Rosso") over shallow-water limestone, dolomite tions. This straightforward conclusion carbonates and reefs are at least one order reefs and platform sediments produced an turns into a paradox in view of the enor- of magnitude higher than accumulation unusually detailed stratigraphy of Holocene mous growth potential that reefs and plat- rates of ancient carbonate platforms. We carbonates (Land, 1974; Adey and others, forms displayed during the Holocene trans- must conclude that platforms, at least 1977; Halley and others, 1977; Maclntyre gression. modern ones, could grow much faster than and others, 1977; Davies and Marshall, The 19th-century pioneers were im- they had to in order to keep pace with 1979; review in Adey, 1978). Field obser- pressed with what might best be called "reef long-term subsidence and eustatic sea-level vations and geophysical modeling, on the power," the astounding capability of reefs movements. If all factors involved had been other hand, have greatly improved our to grow upward and outward and to repair correctly evaluated in this argument, there knowledge of sea-level history of the past damage by wind and tides. The realization should be no drowning of platforms at all. 15,000 yr (see Walcott, 1972; Bloom, (after World War II) of the rapidity of the The present report attempts to analyze the 1977, for review of sea-level curves; Clark Holocene rise of sea level made reefs and problem and points out options to reconcile and others, 1978, for geophysical sum- platforms all the more impressive, and these seemingly contradictory observations. mary). death by drowning became ever more The past decade has provided us with a The extremely rapid rate of Holocene difficult to imagine. Finally, an observation wealth of data on past sea-level fluctua- sea-level rise along with our detailed most explicitly stated by Wilson (1975, p. tions, on subsidence, and most significantly, knowledge of processes and products make 15) made the paradox complete. The on the Holocene transgression. Drilling, the Holocene transgression almost a con- growth rates of Holocene shallow-water coring, and radiocarbon dating of modern trolled experiment of platform drowning on Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/4/197/3434314/i0016-7606-92-4-197.pdf by guest on 01 October 2021 PARADOX OF DROWNED REEFS AND CARBONATE PLATFORMS 199 TABLE 1. (Continued) Time Platform Description Source CRETACEOUS Hauterivian/Valanginian Atlantic off New foundland Abenaki Fm.; deeper-water Eliuk (1978) siliceous and argillaceous lime- stone over oolitic, shallow- water carbonates Valanginian Atlantic off Morocco DSDP Site 416: circumstantial Schlager (1980) evidence (turbidites) indicates upward change from neritic limestone to pelagics on con- tinental shelf Barremian Atlantic (Blake Plateau) DSDP Sites 390, 392; pelagic ooze Benson, Sheridan, and others (1979) over shallow-water limestone Late Aptian Atlantic (Bay