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

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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

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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 of Biscay) DSDP Site 401 hardgrounds and Montadert, Roberts, and others (1979) pelagics over shallow-water calcarenites Albian/Cenomanian Pacific (Geisha and Carbonate ooze over shallow- Heezen and others (1973), Mid-Pacific seamounts) water limestone (rudists) Ladd and others (1974) Gulf of Mexico DSDP 95 Worzel, Bryant, and others (1973) (Campeche scarp) Pelagic chalk over margin of Yucatan shallow-water platform Florida coccolith chalk over shallow- Bryant and others (1969), water limestone and dolomite Meyerhoff and Hatten (1974, p. 440) Texas Dark, deeper-water limestone Wilson (1975, p. 338) (Georgetown) over rudistid, Young (1977) shallow-water carbonates Bebout and Loucks (1974) Albian/Senonian Northwest Atlantic DSDP Site 384 rudist limestones Tucholke, Vogt, and others (1979) overlain by nanno-foram chalk Cenomanian/Campanian Atlantic (Blake Plateau) Inferred from seismics, dredging Benson, Sheridan, and others (1979) and nearby DSDP sites: pelagics Shipley and others (1978) over shallow-water limestone (reefs?) Santonian Israel Shallow-water biomicrite overlain Schneidermann (1970) by nanno-foram limestone and chalk TERTIARY Eocene or older Pacific (Ita Matai seamount) DSDP Site 202 Hesse (1973) pelagic limestone over oolite Heezen and others (1973) Eocene Pacific (Koko seamount) DSDP 308 and dredge hauls: ooze Davies and others (1972) over reef and oolite sdst. Larson, Moberly, and others (1975) Pacific DSDP Site 433 Jackson, Koizumi, and others (1978) Miocene Suiko seamount Carb. ooze over reefal calcarenite Greene and others (1981) Miocene Atlantic off Florida Pourtales and Miami Terrace: Gomberg (1975) phosphoritic hardgrounds and Mullins and Neumann (1979a) carb. ooze over terrace of shallow-water limestone.

a global scale. My approach is to first sum- however, the solution to the problem lies in regimes and a prime example of a "stratig- marize the results of this experiment and the geologic record, and careful analyses of raphic turning point" as defined by Schlager evaluate its significance for the geologic existing data are needed, as well as field and Schollnberger (1975). Selective drown- record, then compare the Holocene with the studies of drowned platforms and their dis- ing is ultimately responsible for the differ- more distant geologic past, and finally dis- tribution in time and space. The significance ence between an atoll and a guyot or be- cuss possible causes of platform drowning. of platform drowning justifies a substantial tween an array of platforms, such as the The rapid increase of our knowledge of effort of this kind. From point of view of Bahamas, and the Blake Plateau. Drowned Holocene reefs and of processes influencing depositional history of an area, the change platforms are also of economic significance. sea level now makes it possible to recognize from neritic carbonate deposition to Capped by shale or tight limestone and platform drowning as a problem and to bathyal deposition is one of the most commonly exposed and leached prior to identify possible explanations. Ultimately, dramatic changes possible in depositional drowning, these platforms possess all of the

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TOTAL CARBONATE REEF SKELETAL MUD Finally, a technical remark. Time- PRODUCTION GROWTH SAND standing crop of distance rates throughout this report are 100% mm/yr g/m^/yr algae/mz expressed as microns (/xm) per year, which equals millimetres per 1,000 yr or metres per m.y. Fischer (1969) introduced the name "Bubnoff ' for this unit. I believe that use of a standard unit for geological time- distance rates is extremely important for all comparative studies and should be encour- aged.

SOME PRINCIPLES OF PLATFORM GROWTH

In order to understand the causes of plat- form drowning, we need to remember cer- tain facts about the way they grow. First, carbonate sediment for the platforms origi- nates more or less in situ, that is, within the Bahamas depositional environment. The material is ® produced largely by organisms as a by- product of photosynthesis, the process is thus dependent on light and decreases Figure 2. Decrease of carbonate production rapidly with water depth. Prolific produc- with water depth. Note significant drop in tion of carbonate is limited to the upper 50 to 100 m of the water column, which can production at 10 to 20 m of water depth; that sustain abundant growth of photosynthetic is, within the euphotic zone. Sources: (1) organisms ("euphotic zone" in Fig. 1). Even Ginsburg in Wilson (1975), (2) Adey and within this favorable zone, however, pro- others (1977), (3) Smith (1972), (4) Neumann duction decreases significantly, as illus- and Land (1975). F lor Ida - trated in Figure 2. The narrow depth limits Bahamas of neritic carbonate production are an im- 100-1 portant prerequisite for drowning of plat- forms. ingredients for stratigraphie traps for hy- carbonate platforms in this respect. In the The basic growth anatomy of a modern drocarbons, and have actually become ex- Late Jurassic, the extensive and rapidly platform is that of a bucket, held together ploration targets in the continental margins subsiding platforms along the eastern mar- by stiff rims of "competent" material and on both sides of the Atlantic. gin of North America were using up cal- filled with "incompetent" lagoonal or tidal Large-scale drowning of platforms can be cium at a rate equal to 12% of the present- deposits (MacNeil, 1954; Klovan, 1974; expected to influence the balance in the day river input. Because large parts of these, Wilson, 1975; Fig. 3). This pattern is the re- global carbonate cycle by shifting the locus platforms were drowned and subsidence sult of organic frame-building along the of (organic) carbonate precipitation from decreased, this figure was reduced to less platform margin, reinforced by early ce- neritic to pelagic environments. Two figures than 1% by Cenozoic time (A. Droxler, mentation which selectively affects the reefs may serve to illustrate the importance of 1979, written commun.). and sand shoals at the rims of the platform and leaves the interior uncemented (Land and Goreau, 1970; James and others, 1976; James and Ginsburg, 1980; Shinn and others, 1981). The growth potential of a

Figure 3. Cartoon of the growth anatomy of carbonate plat- forms — a bucket, held together by stiff rims of reefs or rapidly cemented, stacked sand shoals, that is filled with unconsolidated deposits of lagoons and tidal flats. Growth potential of a platform 50FTFLANK\ \sand,mud. / is largely determined by growth potential of rim. mud., rabble \ \ / \ STIFF RIM / \ organic frame, / \ cement /

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accumulation rate in /um/yt 101 10* 10 10 10"

J> • Figure 4. Average growth potential of carbonate platforms es- Holocene sea le vel D-M timated from growth rates and accumulation rates during M Holocene transgression (open bars) and from accumulation rates of prograding platforms in the geologic record (triangles). Average P-P M Teef corals

growth potential is probably in the l,000-/u.m/yr range. Sources P • i listed in Table 2 and Figure 5. reefs <5~m reefs 10-20 m s • oolites modern platform is thus determined by the iidalites growth potential of its rim and drowning of O D • J the platform means, first and foremost, X drowning of the rim as the competent ele- t prograding Holocene growth ment in the system. During rapid relative average growth platforms potential ofplatf. rates rises of sea level, the rim may keep pace while the platform interior is lagging be- fimlyc and has slowed to about 500 /xm/yr magnitude and grow linearly at rates of hind. Atolls with deep lagoons are an in the past 1,000 yr (Neumann, in Macln- more than 100,000 /um/yr (Buddemeier and example in question. Likewise, we may tyre and others, 1977; Adey, 1978). Pacific Kinzie, 1976). Reefs grew considerably consider the Australian shelf with the Great sea level may have risen somewhat slower more slowly than individual corals, as ex- Barrier Reef a carbonate platform where overall and seems to have passed its climax pected, because their vertical growth is a only the rim kept pace with the rapidly ris- and is now dropping (Bloom, 1977, for function of total mass balance (including ing sea. For modern platforms and for time summaries; Adey, 1978). loss through erosion) and not just a func- spans on the order of 10,000 to 100,000 yr, Reef-building scleractinian corals can ex- tion of the upward growth of reef builders. it thus seems logical to equate the growth ceed the rate of sea-level rise by an order of Furthermore, upward growth is, of course, potential of a platform with the growth po- tential of its rim. 1 The preferential occurrence along the 10' 10' 10 10 10" 1- platform margins of organic framework desiccation of basins and early cement suggests that many an- faster cooling crust, cient platforms, too, were built by the sea- floor spreading glacio-eustacy © bucket principle. Elevated reef rims on the Holocene Pacific's oldest atoll, Cretaceous Darwin long-term subsidence (§) guyot (Ladd and others, 1974), indicate a bucket-structure very similar to that of to- day. Recent reef corals (2) The "carbonate ramp" (Ahr, 1973) is a ¡P Alps facies model that does not obey the bucket ® reefs <5m deep principle. Instead of a well-defined margin Apennine platforms reefs 10-20m„ defended by reefs or sand shoals, the ramp ¡F-7"e © | shows a seaward-dipping surface without a break in slope. Ramps are particularly -Bahamas J-Hoi oc. oolites common in epeiric seas and often mark the tidalites • a ' ' early stages of carbonate deposition during

a transgression. Drowning of a ramp leads 1 10 10' 10 10' 10' to a landward shift of facies belts but the rates in ^im/yi (Bubnoff units) • change from neritic to bathyal deposition is less abrupt than on platforms and lacks the Figure 5. The paradox of platform drowning is illustrated by a comparison of rates of hiatuses so common on drowned platforms. the relevant processes. Rates of relative rise of sea level produced by various processes in upper part of graph, rates of growth and sediment accumulation in lower part. Holocene THE HOLOCENE TRANSGRESSION: rates = open bars; distant geologic past = black bars. Holocene accumulation matches or A GLOBAL EXPERIMENT exceeds glacio-eustatic Holocene rise of sea level, all Holocene rates are one to several IN PLATFORM DROWNING orders of magnitude faster than those of the geologic record. Sources: (1) Adey (1978), Neumann in Maclntyre and others (1977); (2) Buddemeier and Kinzie (1976); (3) Adey Figures 4 and 5 show rates of the most (1978), Adey and others, (1977), Shinn and others (1977), Maclntyre and Stuckenrath important processes involved in upbuilding (1978), Land (1974), Lighty (1977) Davies and Marshall (1979); (4) Wilson (1975), Harris and drowning of platforms during the latest (1979); (5) Meyerhoff and Hatten (1974), Paulus (1972); (6) D'Argenio and others (1975); Pleistocene-Holocene glacio-eustatic rise of (7) Schlager and others, unpub. data; (8) Fischer (1975), Schwab (1976) (subsidence = the sea. In the Atlantic-Caribbean area, the driving component plus sediment loading; only basins with negligible water depth have sea rose at a maximum rate of about 8,000 been considered); (9) Sclater (1978); (10) Pitman (1978); (11) Hsu and Winterer (1980).

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limited by the relative rise of sea level. Even thousand years BR • 5 10 10 10 so, with maximum growth rates of 12,000 . 1 i . . 1 to 15,000 /um/yr (Maclntyre and others, \ \ 1977; Adey, 1978), many Holocene reefs \ \ rubble \ \ fv \ I can be shown to have outpaced even the \ I fastest sea-level rise. Platform sediments such as oolite sands and tidal deposits show 10 \ \ \ ^ \\ \ \ \ \ I slower rates of accumulation mainly be- A.cervicof- | * cause they formed during the late Holpcene, nis facies • \ \ 'IS Gt.Barrier Reef \ when sea-level rise was slower. V \ ' \ In spite of the rapid growth rates of cer- \ coral head factes\ * \ \ tain reefs, a large number of reefs and plat- —Ò \ * forms could not keep pace with the rising \ \ sea during the early Holocene. Some plat- y. I forms, such as the Campeche Bank (Logan coral rubble * 50 and sand factes ^ • T and others, 1969), the eastern Australian • \ shelf landward of the Barrier Reef (Max- Alacron, Yucatan St. Croix, Virgin is. \\ well, 1968) or parts of the Indonesian reef belt (Kuenen, 1950, p. 450—467), were Figure 6. Growth history of Holocene reefs. Examples illustrated have been drowned; other platforms have lost some documented by drilling or excavating and radiocarbon dating. Note start-up phase where ground along their seaward margins — for reef growth lags behind sea level, catch-up phase where growth exceeds sea-level rise, and example, the Bahama Banks and Caribbean keep-up phase when growth equals sea-level rise. Sources: (1) Adey (1978); (2) Maclntyre platforms (Figs. 6, 7, 8). Several environ- and Stuckenrath (1978); (3) Davies and Marshall (1979). mental factors plus topography seem to have determined which platforms kept pace suggested by observations on Holocene growth rates at 20 to 40 m water depth are with the Holocene transgression and which reefs (Fig. 2; Land, 1974; Adey and others, still on the order of hundreds of microns per were drowned. Three environmental fac- 1977). Vertical growth rates of reefs below year. The decrease in growth rates is ac- tors, (1) inimical bank water, (2) decrease 10 to 15 m decrease in what seems to be a companied by changes in coral com- of growth with water depth, and (3) initial nonlinear, possibly exponential, fashion. munities and shape of coral colonies (Adey, lag period of slow growth, seem especially The data are scarce and considerably scat- 1978). These features should also be rec- important and are discussed in more detail tered, and they provide only a crude esti- ognizable in the geologic record (James, below. mate of the trend; it seems, however, that 1978, his Fig. 12). 1. Inimical Bank Water. Reef growth is easily disturbed by changes in the environ- ment. Modem reefs are interrupted where JAMAICA land 19 tidal passes funnel lagoon waters of highly variable temperatures, salinity, and sedi- ment load across the bank margin (Gins- S FLORIDA Lighty 1977 burg and Shinn, 1964). In the Holocene record, the effect of this inimical bank water is particularly prominent when the -15oom"'tö shò'rie rising sea first flooded the platform tops, ST. CI?OIX Adey and others 1977 washed off the soil, and formed extensive but very shallow lagoons (Adey and others, 1977; Lighty and others, 1978). At this stage, reefs at the platform margin were 5o m TOTO N Schlager unpubl. sometimes killed or reduced in size. Com- monly, this crisis was only temporary and TOTO S Schlager unpubl. f the environment improved as water volume and circulation in the lagoons increased and soil influx decreased. At this point, reef growth commonly was resumed, albeit with 5om L a slower growing, deeper water coral com- loom 2oo rr. munity (see below). It seems that the inimical-water effect is strong where ex- posed hinterland provides closure for la- Figure 7. Holocene platform margins in the Bahama-Caribbean area. Modern, goons and terrigenous influx. I doubt that shallow-barrier reefs (cross-hatched) occupy positions landward of the original steep bank this effect should be counted on when it margins. Reefs seaward of the modern barriers (black) are now inactive or occupied by comes to drowning atolls or open, com- relatively slow growing, deeper-water communities. Most of them were initiated as shal- pletely submerged platforms. low fringing reefs during the earlier Holocene, but they could not keep up with sea-level 2. Decrease of Growth Rates with Water rise because they were flushed with inimical bank water of highly variable salinity, tem- Depth. This is another characteristic perature, and sediment load (see text).

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Figure 8. The flat tops of the Bahama Banks, subdivid- ed into areas of water depth of 10 m or less (dotted) and areas deeper than 10 m (black). The deeper areas are close to or below the zone of maximum carbonate production. Accumulation there has obviously not kept pace with the rising sea. Were sea level to continue its rapid rise, these parts of the platforms would almost certainly be drowned.

3. Slow Growth during Reef Initiation. Most reefs show a characteristic accumu- lation curve consisting of three phases: a start-up phase during which the reef lagged behind sea level, a catch-up phase when upward growth often exceeded sea-level rise, and a keep-up phase when reef growth closely followed sea levels as it slowed toward its present high stand (Fig. 5; Maclntyre and others, 1977; Adey, 1978; Davies and Marshall, 1979; Kendall and .Schlager, 1981). During the start-up phase, reefs are apparently more prone to drown- ing than after a period of sustained growth. The fact that the rapid Holocene sea-level rise followed a long period of subaerial ex- posure made the rise more efficient in drowning. While the early Holocene sea-level rise of 8,000 to 10,000 /nm/yr was clearly too fast for the majority of reefs and platforms, the TABLE 2. PROGRADING PLATFORMS IN THE GEOLOGIC RECORD late Holocene rate of 500 to 3,000 /xm/yr in AND THEIR RATES OF ACCUMULATION the Atlantic-Caribbean area (A. C. Neu- Time Platform Rate Source mann, in Macintyre and others, 1977; Adey (pt-m/yr) and Burke, 1976) was commonly matched by reef growth and sedimentation. At this Devonian Canning Basin 30 Playford and Lowrie (1966) stage, most margins recovered, reefs built (Givetian/Frasnian) up to sea level and developed intertidal Devonian-Mississippian Rocky Mountains 50-80 Rose (1976) algal ridges (Adey and Burke, 1976), sand (Kinderhookian-Meramecian) shoals filled up (Ball, 1967; Harris, 1979), Mississippian Rocky Mountains 100-150 Rose (1976) and tidal flats started to prograde (Shinn (Meramecian-Chesterian) and others, 1969; Gebelein, 1974). For Pennsylvanian Sverdrup Basin 30-40 Davies (1977) further discussion, we will assume the aver- Permian (Nansen Fm.) age growth potential of modern carbonate Permian Delaware Basin 75 Harms (1974) platforms to be on the order of 1,000 (Capitan Fm.) (Guadalupian) ¿im/yr, probably a rather conservative esti- Triassic Northern Limestone 100 Ott (1967) mate. (Late Anisian-Ladinian) Alps Dolomites Schlager and others, (Early Carnian) 300-500 LATERAL PROGRADATION OF (Picco di Vallandro) unpub. data REEFS AND PLATFORMS: Late Jurassic Southern Alps 30-45 Winterer and AN INDEPENDENT MEASURE OF (Friuli Platform) Bosellini (1981) GROWTH POTENTIAL Cretaceous Tampico Embayment 60-90 Enos (1977, p. 279-286) (Late Albian-Cenomanian) The Holocene test shares the fate of many Note: Calculated from stratigraphie age bracket reported for the formation, applying absolute time laboratory experiments. It produced a rea- spans indicated in the Phanerozoic time scale, 1964; Cohee (1978); accumulation rates are not sonably clear answer, but its application to corrected for compaction. the real world, in our case, the more distant

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geologic past, faces considerable uncer- growth rates and growth potential of car- taken in the million-year domain) are far tainty. The most significant challenge to the bonate platforms. Accumulation rates of less than what seems necessary to drown general application of the Holocene results carbonate platforms commonly range from healthy reefs and platforms. lies in evolutionary changes of carbonate 10 to 100 /im/yr, one or two orders of It seems, for causes of platform drown- benthos. There can be no doubt that the magnitude down from what I estimate to be ing, one must not search among the long- reef communities have changed dramati- the average growth potential of carbonate term processes in the geologic record. cally through time (for reviews, see Heckel, platforms. The figures of platform accumu- Rather one should look for short-term 1974; James, 1978) and that the growth lation are in line with rates of over-all sub- pulses of relative rise of sea level or for potential of reefs and carbonate platforms sidence in sedimentary basins, which range (ecologic) factors that will depress the has varied through time. The question is from few tens to few hundreds of /xm/yr, growth potential of the carbonate benthos only if the Holocene growth potential is ex- with extreme values of 300 to 500 /xm/yr, in to levels where long-term processes can ac- cessively high and if the potential rate of tectonic grabens (Fischer 1975; Schwab, complish the drowning. reef growth and platform building was in- 1976). Thus, rates of long-term subsidence, deed an order of magnitude less than today. commonly invoked as a cause of platform Pulses of Relative Rise of Sea Level The strongest argument against this as- drowning, fall short of the average growth sumption and in support of the Holocene potential of platforms by at least one order Relative rises of sea level that are able to results is the progradation of platform of magnitude. Even the peak of thermal drown healthy carbonate platforms must be margins so commonly observed throughout subsidence, 250 /xm/yr for the first 2 m.y. of short pulses. This conclusion is based not the geologic record. newly formed ocean crust and adjacent only on the vain search among the long- A platform with a sediment-covered flat continental margins (Sclater, 1978; Watts term processes in the geologic record, but top that simultaneously builds upward and and Ryan, 1976, p. 31), is at least four also on considerations of the required min- progrades basinward must produce more times slower than average Holocene plat- imal and the maximal possible amplitudes sediment than it needs to match the relative form growth. More importantly, cooling of sea-level fluctuations. rise of sea level. Consequently, its growth crust subsides steadily, without pulses, and A platform can be drowned by a minimal potential must be greater than its vertical the rate decreases with time. If anything, sea-level rise equal to the height of the growth rate. Prograding platforms thus thermal subsidence may prevent the initia- euphotic zone, that is, about 100 m. At a provide a rough estimate of the growth po- tion of platform growth on new ocean crust rate of 1,000 to 10,000 /xm/yr, it would tential totally independent of the Holocene or on the adjacent continental margin. take sea level only 10,000 to 100,000 yr to record. Table 2 lists platforms that meet the Once platforms are established, however, rise 100 m. Even if one assumed that part of above criteria. Their accumulation rates the steadily decreasing thermal subsidence this rise will be compensated by sediment range from 30 to 500 ¡jumlyr, again indicat- presents no obstacle to their growth unless accumulation, a rise of 150 m (that is, an ing growth potential clearly exceeding the other adverse effects are added. event of 15,000 to 150,000 yr) will be real growth rates observed in the geologic Eustatic rises of sea level caused by in- sufficient to submerge the platform below record. creases in the rate of sea-floor spreading or the euphotic zone and drown it. Data from the Holocene "test" and from changes in subduction and spreading pat- The maximum duration of a drowning progradation of ancient platforms (Fig. 4) terns frequently have been used to explain pulse can be estimated from the rate of rise strongly suggest growth potential of reefs platform drowning (Heezen and others, required, let us assume 1,000 /xm/yr in our and platforms in excess of 100 /xm/yr, most 1973; Bechstaedt and others, 1976; Eliuk, case, and the maximum amplitude of sea- probably on the order of several hundreds 1978). However, the rates of sea-level rise level excursions documented from the to thousands of /xm/yr. Although these produced by these processes appear to be geologic record. This latter figure is still figures represent only a rough estimate, and far too slow for drowning. Berger and rather poorly known. Vail and others although we have, at present, no data on Winterer (1974, p. 26) and Pitman (1978) (1977), postulated 500 m as the total range, variation of growth potential through time, estimated that the most rapid of these ef- Hancock and Kauffman (1979) estimated a the above figures considerably narrow the fects, changes in the rate of sea-floor spread- rise in excess of 600 m for the Late Creta- options in search of possible causes of plat- ing, would cause sea level to fluctuate at ceous. At a rate of 1,000 /xm/yr, it would form drowning. rates of less than 10 /xm/yr, two orders of take sea level only 1 m.y. to rise 1,000 m, magnitude less than average accumulation and I consider this to be the maximum du- POSSIBLE CAUSES OF DROWNING rates of Holocene platforms, and still one ration of a drowning event caused solely by order of magnitude below the sustained ac- rise of relative sea level. Long-Term Geologic Processes cumulation rates of prograding platforms in The high rate of rise required for drown- Are Too Slow the geologic record. ing seriously limits the number of processes Other eustatic rises of sea level were to be considered. In the following, 1 will Figure 5 shows a comparison of the demonstrably much faster than the ones at- briefly discuss processes that can be shown Holocene rates and the assumed average tributed to changing spreading rates. Han- to cause relative rises of sea level of 1,000 growth potential of reefs and platforms of cock and Kauffman (1979) calculate rates /xm/yr or more. 1,000 /xm/yr with long-term rates of proc- of 10 to 90 /xm/yr for some Late Cretaceous Pulses of Tectonic Subsidence. Move- esses in the geologic past. There is a gap of rises, and similar rates can be calculated ment along transcurrent faults commonly one or two orders of magnitude between from the eustatic sea-level curve of the Ter- proceeds in episodic pulses (Dead Sea Rift rates in the geologic record, all averaged tiary (Vail and Hardenbol, 1979). How- in the Pleistocene; see Freund and others, over millions of years, and Holocene ever, even these figures (again averages 1968; San Andreas fault in the Pliocene-

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Pleistocene; see Nilsen and Clarke, 1975). hood of synchronous volcanism in the have been suggested to explain world-wide These pulses appear also in vertical move- world ocean with concomitant uplift opens extinctions in marine biota — for example, ments realted to wrench-tectonics (Neogene a new possibility for rapid and world-wide the hypothesis of fresh-water incursions at pulse in basin subsidence in California; see rises of sea level (Schlanger and others, the end of the Cretaceous (Gartner and Yeats, 1978; or, as an example from a 1981). Mass extinctions of platforms dur- Keany, 1978; Thierstein and Berger, 1978) carbonate-rock province, the 400- to 500-m ing exactly this time span in the Cretaceous or the concept of a Permian salinity drop in uplift of Pleistocene reefs on Tiran Island, make this concept even more attractive. the world ocean caused by extensive evap- Red Sea; Brown, 1970, p. 84). Conceivably, In the geologic record, drowning by orite deposition (Fischer, 1964b; Lantzy similar pulses in subsidence may occur with pulses of tectonic subsidence would follow and others, 1977). Neither time period is fault blocks of a segmented continental a regional pattern, whereas the other effects associated with major drowning, however. margin, a setting where drowned carbonate listed above would cause world-wide rises platforms commonly occur. of sea level and world-wide effects on car- Increase in Slope Height Glacio-Eustatic Rises. The Pleistocene bonate platforms. and Holocene record upholds ample proof For a platform to grow upward at con- for sea-level rises at rates of 5,000 to Reduction of Benthic Growth Potential stant slope angle requires deposition on the 10,000 /um/yr. Controlled by the Earth's slope of ever larger volumes of sediment orbital parameters, one can expect these Rather than assuming pulses in relative (Fig. 9). As most slope sediment stems from fluctuations to appear as soon as major rise of sea level, we can invoke drastic re- the same source as the material for upward polar ice caps are established (Imbrie and duction of growth potential of platforms to growth, namely the platform top, increase Imbrie, 1979). These cycles, however, will values where long-term rises of relative sea of slope height must reduce the potential for only drown platforms if they are sufficiently level may finally cause drowning. Adverse vertical growth of a platform. I believe that asymmetrical to produce a undirectional -changes of the environment may occur on a for platforms up to several hundreds of excursion on the order of 100 m, which re- local, regional, or global scale. The effect of metres or even 1,000 m in height this is not quires a special and uncommon superposi- inimical bank water mentioned earlier is an significant, because the platform can re- tion of the three components. A symmetri- example of local disturbance. spond to rapid rises of relative sea level by cal cycle may drown the platform tem- Regional deteriorations of the environ- steepening the slopes. The modern Baha- porarily but will expose it again during the ment can be expected, for instance, from mas are a point in case. Slope heights in- following low stand. To permanently plate motion, as carbonate platforms drift crease from 500 m in the Straits of Florida drown the platform, the fall of sea level has to higher latitudes and away from the to nearly 5,000 m along the ocean-facing to be delayed until subsidence has lowered favorable belt of carbonate production. Bahama escarpment in the east. Concomit- the platform top below the bottom of the Global crises in the ocean environment antly, slope angle increases from 2° to euphotic zone during the next low stand. Rises by Dessication of Ocean Basins. Nearly isolated ocean basins prone to de- Atoll, cone" Platform „prism" siccation seem to have existed frequently during the present cycle of sea-floor spread- t ùh ing fragmentation of continents. This is in- 1 \ dicated by evaporites in Atlantic-type con- T tinental margins and in small ocean basins ah such as the Gulf of Mexico and the Mediterranean. Berger and Winterer (1974, \ p. 27) calculated a rise of world sea level of 10 m for desiccation of the present-day

Mediterranean, a rise of 50 m for emptying « X X \ the much larger South Atlantic basin during / volume V volume V the Early Cretaceous. Once isolated, these tana tana basins would dry up within thousands of AV^AV^increments in volume caused by vertical growth Ah of platform or atoll years. Desiccation rises, like glacial rises, Wh3 volume of slope prism h can thus be expected to easily exceed the volume of atoll cone V 3tan2a of length.r 2tana critical 1,000 /xm/yr limit. Hsu and Win- dV ïï àV=_ _1 growth of volum e 2 growth of volume terer (1980) estimate them to be on the dh tan\ a. dh ~tan

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Figure 10. Influence of steady subsidence on drowning by eustatic rises of sea level. Heavy line is eustatic sea-level curve for Tertiary (Vail and Hardenbol, 1979). Straight lines are hypothetical subsidence paths of platforms that built up with the rising sea and became exposed during eustatic falls of sea level. The number of pulses actually able to drown depends on 200 60 50 W 30 20 10 C the rate at which a platform subsides. A platform that grew Paleocene & with sea level to its Oligocene high and subsided at 10 fj-mlyr Eocene 1 Oligocene Miocene S s; had no chance of being drowned throughout the rest of the Cenozoic; its top was mostly emergent. At 50 /u-m/yr subsidence, the "pro- tective shadows" of previous high stands are much shorter, and several Neogene rises of sea level had the potential to drown the platform.

nearly 30° (Schlager and Ginsburg, 1981) a steady process with a pulse of another and episodic crises in benthic growth. A and for slopes up to 150 m in height, nearly process. This combination, too, can pro- third pattern includes superposition of sud- vertical walls can be maintained. A change duce some interesting effects. Figure 10 den events with gradual decrease in growth in sedimentary environment explains the shows the possible interplay of different potential by increasing height of slope or by stability of the steep slopes; although the rates of steady subsidence and pulses of eus- gradual deterioration of the environment — gentle slopes are simply piles of loose sedi- tatic sea level in the Tertiary. It appears that that is, by drift to higher latitude. Here ment, the steeper slopes are being bypassed changes in the rate of subsidence change the again, the long-term reduction of growth and scoured by turbidity currents and con- likelihood of platform-drowning by eustacy. potential predetermines which platforms tour currents; nondeposition leads to wide- A platform that built up with the rising sea will ultimately be drowned by a sudden spread submarine lithification of slope sed- to its early Oligocene high stand cannot be event. iment, and the resulting hardgrounds, in drowned during the Neogene if it subsides turn, attract encrusting and binding or- at a rate of 10 jum/yr because it will never be EXAMPLES IN THE GEOLOGIC ganisms that further stabilize the slope sufficiently submerged. The subsequent eus- RECORD (Mullins and Neumann, 1979b). Only the tatic pulses just barely reached the platform highest, steepest, and most deeply eroded top. At 50 |u.m/yr, on the other hand, the Global Drowning slopes along the Bahama escarpment may "protective shadow" cast by the Oligocene have reached the limits of growth and start high stand is much shorter, and several A brief glance at the geologic record pro- to collapse (Ryan, 1978). I expect, there- Neogene pulses have the potential to duces evidence for global mass extinctions fore, that for the majority of reefs and plat- drown. Subsidence, although not directly of reefs and platforms besides what appear forms in the geologic record the increase in the cause of drowning, governs to some ex- to be local and regional patterns of drown- slope height was compensated by a con- tent the "where" and "when" of drowning ing. One of the most convincing examples comitant increase of declivity and did not by eustacy. A similar interplay is conceiva- of this kind is the drowning in middle Cre- significantly reduce growth potential. ble between, for instance, steady subsidence taceous (Albian-Cenomanian) time (Fig. Drowning and cessation of progradation of these relatively low buildups must have had other causes. Ultimately, however, increase in slope height must set a limit first to pro- gradation and eventually to upward growth of a carbonate platform. Platforms rising 5,000 m from abyssal plains, such as the southeastern Bahamas or some Pacific atolls, may be close to this limit. These plat- forms may be prone to drowning by moderate rises of relative sea level, because their rims slump and collapse and leave the interiors unprotected.

Superposition of Several Effects

The processes listed are not mutually ex- clusive and can become superimposed on one another. Because drowning of a reef or platform requires an excursion by one of the relevant processes from its average rate, Figure 11. Platforms drowned during the middle Cretaceous (Albian or Cenomanian). by at least one order of magnitude, it is un- The world-wide distribution suggests a eustatic cause. Sources: (1) Heezen and others likely that two such excursions will coincide (1973), Matthews and others (1974); (2) Young (1977), Bebout and Loucks (1974); (3) to produce a particularly efficient pulse. Worzel, Bryant, and others (1969); (5) Montadert, Roberts, and others (1979), (6) More common will be the superposition of Tucholke, Vogt, and others (1979).

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11). In this period, numerous seamounts in drowning event in the Aptian (Arthur and water incursions as possible causes. Eus- the Geisha and mid-Pacific mountains were Schlanger, 1979, p. 876). tacy, that is, a regression followed by rapid drowned (Matthews and others, 1974); the The cause of the middle Cretaceous transgression, is mentioned by McLaren Campeche platform margin retreated sub- drowning is very much open to debate. (1970) as another, remote possibility. stantially, as indicated by seismic profiles; Contrary to Heezen and others (1973), I Whatever its precise origin, the Late Devo- and DSDP site 95, south Florida, the west believe we must rule out increase of the rate nian drowning seems to have been a global Florida shelf, and Jordan Knoll experienced of sea-floor spreading as an explanation. event with strong circumstantial evidence of an abrupt change from shallow-water car- Rapid eustatic rises due to large-scale sub- enviromental causes. bonte to pelagic chalk deposits (Bryant and marine volcanism (Schlanger and Premo- Regional Tectonic Drowning. This is others, 1969); on the Comanche shelf of li-Silva, 1981; Schlanger and others, 1981) well documented for the Mesozoic margins Texas, the Edwards-Stuart City reefs gave or due to desiccation of a young ocean of the Tethys Sea. Occurrence in space and way to dark, pelagic deposits of the basin such as the South Atlantic (Berger and time of drowned platforms along these Georgetown Limestone (Young, 1977; Be- Winterer, 1974; Hsu and Winterer, 1980) margins ties in well with evidence for bout and Loucks, 1974); and shallow-water are distinct possibilities. There is little evi- block-faulting, volcanism, and tectonically carbonate deposition ended on J-anomaly dence for a global crisis in shallow-water controlled changes in paleogeography (Ber- ridge in the North Atlantic (Tucholke, environments. The coincidence of platform noulli and Jenkyns, 1974; Schlager and Vogt, and others, 1979). Probably some- drowning and Cretaceous anoxic events has Schoellnberger, 1975; Bechstaedt and what earlier than the above events was the been noticed by Arthur and Schlanger others, 1978). Along the eastern margin of drowning of carbonate banks along the Bis- (1979); exactly how peaks in anoxia should the Ligurian Ocean, the timing of drowning cay continental margin, where shallow be linked with platform drowning, how- correlates with the landward progression of water calcarenites are abruptly overlain by ever, is not at all clear. I suspect they may blockfaulting as the ocean opened during Aptian pelagic sediments in DSDP Site 401 have a common cause rather than a direct the Jurassic (Fig. 13). The pattern is particu- (Montadert, Roberts, and others, 1979). cause-and-effect relationship. As Fischer larly clear in the Apennines, but it is still The middle Cretaceous example also il- and Arthur (1977, p. 22) pointed out, visible, albeit more complex, in the South- lustrates the difficulty of precisely dating "oceanic and atmospheric behavior appear ern Alps (Bosellini, 1973; Winterer and the drowning events. The error bars given linked with eustacy, in ways which remain Bosellini, 1981). in Figure 12 range from 5 to 30 m.y. It may obscure." Regional Drowning due to Environmen- be possible to reduce them considerably, The Late Devonian is another period of tal Changes. Regional deteriorations of but there are limits set by the resolving widespread demise of reefs and platforms, the environment can be expected, for power of stratigraphy. If we assume the du- for example, in Canada, western Europe, example, during plate motion, as reefs and ration of biozones in the Cretaceous to vary and western Australia. Evidence for drown- platforms drift to higher latitudes and away between 1 to 6 m.y. (Van Hinte, 1976), one ing is particularly impressive in Germany from the favorable carbonate belt. Before a would still be left with a gap of one order of where all of the then-thriving reefs and plat- platform arrives at a latitude where reef magnitude between the duration of the forms were turned into submarine plateaus growth is completely impossible, it will pass drowning event of abour 100,000 or and seamounts with thin veneers of pelagic through a transitional belt where growth is 200,000 yr and the accuracy with which limestone. Eder and Franke (1979) pointed possible but under additional stress. In this one can date the event. Additional criteria out the global synchroneity of the event, belt, platforms will be more prone to are needed to narrow down the options. which happened within one conodont zone, drowning and reef organisms may be killed Figure 12 illustrates this additional restric- and suggested an environmental cause in more easily than at lower latitudes. One tion of options under the assumption that the form of widespread anoxia. The drown- possible example for stepwise drowning in the middle Cretaceous event was caused by ing event seems to have coincided with a the course of drift to higher latitudes is the eustacy. In this case, the possible drowning major extinction of shallow-water biota. In Atlantic margin of North America (Fig. 14). events are reduced to four rapid rises of sea a thorough analysis, McLaren (1970) con- The Abenaki carbonate platforms were level, in the early and late Albian and the cluded that all of the evidence pointed to a drowned in the Early Cretaceous (Eliuk, early and late Cenomanian. The middle very sudden external event, and suggested 1978); those on the Blake Plateau largely in Cretaceous pulses were preceded by a turbitidy due to meteorite impact or fresh- the Late Cretaceous (Shipley and others,

m 10 60 m.y. _i I Qetsna seamounts 0 Mid-Pacific seamounts Comanche shelf ® Campeche scarp g) Ionian Knoll © Figure 12. Stratigraphie dating of middle Cretaceous drown- V/Florida ® ing. Time brackets (black bars) are relatively wide compared to Say of Biscay @ duration of drowning event itself because of limited stratigraphie • W NOM-Mlanttc © data and because periods of nondeposition occur immediately after drowning. If one accepts eustacy as the cause of drowning, then the options can be narrowed to periods of rapidly rising sea level (shaded gray). Even then, however, it seems likely that sev- eral drowning pulses during the Albian-Turonian interval con- tributed to the mass extinction of platforms. Sea level after Han- cock and Kauffmann (1979); other sources, same as Figure 11.

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Figure 13. Drowned platforms along the margin of the Ligurian Ocean (northern Italy) in the Jurassic. Successive drowning of plat- forms as block-faulting progresses from ocean toward continent suggests a tectonic cause of drowning. After Bosellini (1973).

The effect of platform topography on drowning is best illustrated by a compari- son of carbonate platforms and siliciclastic shelves. The classical model of siliciclastic sedimentation on continental shelves and eperic seas assumes a basinward-sloping sea floor. Facies changes in this setting are tirne-transgressive and come about by lat- eral migration of depositonal environments, including the strand line, in response to changes in relative sea level. This model has been confirmed for at least the rises of rela- continental crust tive sea level (Vail and others, 1977). Car- pelagic with carbonate bonate platforms do not conform well with sediments oceanic crust platforms this model, because they tend to develop 1978; Benson, Sheridan, and others, 1979); large areas because the drowning events elevated rims and flat or dish-shaped in- those even farther south, in the Bahamas, were rapid and thus of short duration, of teriors. Because of the lack of consistent are still active. The actual drowning periods unusual nature, thus rare, and because flat seward dip of the platform surface, the rim within this trend seem to correlate again topography of platform tops hampered lat- cannot simply retreat upslope. It has to with global events (Valanginian and eral migration of facies belts. grow vertically upward to cope with a rise Ablian-Cenomanian). Drift to high latitude The duration of a drowning pulse by rel- of the sea. If this race is lost, large areas of thus appears to only modify rather than ative rise of sea level we have already esti- the platform will be drowned simultane- replace the effects of global drowning mated to be 1 m.y. at the very most; more ously. events. An exception in the trend is DSDP commonly it is on the order of 10,000 to Site 384, which lies at the same paleolati- 100,000 yr. Even where platforms were CONCLUSIONS tude as the Abenaki carbonate platforms drowned by a combination of environmen- but which was drowned later than those tal stress and more normal rates of subsi- On the basis of their response to the (Tucholke, Vogt, and others, 1979). dence, the event was probably shorter than Holocene transgression, the average growth most biozones in stratigraphy. At a rate of potential of modern reefs and carbonate DROWNED PLATFORMS AS 50 txmlyr, it would take only 2 m.y. to shift platforms is on the order of 1,000 //.m/yr or TIME MARKERS the platform through a 100-m-wide eupho- more. Independently, accumulation rates of tic zone. Thus, whatever the cause of fossil platforms that simultaneously built I expect the tops of drowned platforms to drowning, the event was probably nearly upward and outward, suggest growth po- be time markers, that is, synchronous over instantaneous by stratigraphic standards. tential in excess of several hundreds of mi- crons per year. This growth potential is orders of mag- ¿ate Cretaceous ¿ate Jurassic — nitude higher than the rates of relative rises 80 m. y. Early Cretaceous of sea level caused by long-term geologic IbOm.y. processes, such as subsidence or increase in rate of sea-floor spreading. It is extremely unlikely that these processes could outpace the growth of healthy reefs and carbonate platforms. Possible causes of platform drowning include (1) reduction of benthic growth due to environmental stress, such as Scotia shelf (a) global salinity drops due to fresh-water injections or excessive evaporite deposition or (b) regional deterioration during drift to higher latitudes; or (2) rapid pulses of rela- Figure 14. Timing of platform drowning on the margin of North America roughly tive sea level, such as regional downfaulting parallels the drift to higher latitudes caused by sea-floor spreading in the Atlantic. How- or global rises due to desiccation of small ever, the drowning still appears to have occurred in major steps in the response to eüstatic ocean basins, submarine volcanic outpour- or tectonic events. Sources: Scotia shelf — Eliuk (1978); Blake Plateau — Benson, Sheri- ings, or glacio-eustacy. dan, and others (1979). Continent positions after Barron and others (1981). Dotted line is The geological record shows both global 2,000 m contour. mass extinctions of reefs and platforms as a

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result of eustacy or world-wide environ- bonate platform: Gulf Coast Association of D'Argenio, B., Pescatore, T., and Scandone, P., mental crises (as, for example, in the Late Geological Societies Transactions, v. 17, 1975, Structural pattern of the Campania- Devonian and the middle Cretaceous), as p. 265-267. Lucania Appennines: Quaderni de 'La Bandel, K., 1974, Deep-water limestones from richerche scientifica, Roma, Consiglio well as regional tectonic drowning (as, for the Devonian-Carboniferous of the Carnic Nazionale delle Richerche, no. 90, example, in the Tethyan Mesozoic). The Alps, Austria, in Hsu, K. J., and Jenkyns, p. 313-327. drowning of platforms along the western H. C., eds., Pelagic sediments: On land and Davies, G. R., 1977, Turbidites, debris sheets, North Atlantic seems to follow a regional under the sea: International Association of and truncation structures in upper Paleozo- Sedimentologists Special Publication No. 1, ic deep-water carbonates of the Sverdrup pattern controlled by plate motion to higher p. 93-115. Basin, Arctic Archipelago: in Cook, R. E., latitudes. Barron, E. J., Harrison, C.G.A., Sloan, J., and and Enos, P., eds., Deep-water carbonate In order to succeed, the drowning process Hay, W. W., 1981, Paleogeography of the environments: Society of Economic Paleon- had to be rapid, and thus short, probably globe, 180 m.y. to the present: Ecologae tologists and Mineralogists Special Publica- between 100,000 yr and 1 m.y. Because of Geologicae Helvetiae (in press). tion 25, p. 221-247. Beall, A. O., and Fischer, A. G., 1969, Sedimen- Davies, P. J., and Marshal, J. F., 1979, Aspects of the subdued relief on carbonate platforms, tology, in Ewing, M., Worzel, J. L., and Holocene reef growth: Substrate, age and large areas were downed at once; because others, Initial reports of the Deep Sea Drill- accretion rate: Search, v. 10, p. 276-279. of the unusual conditions required, drown- ing Project, Volume 1; Washington, D.C., Davies, T. A., Wilde, P., and Clague, D. A., 1972, ing events occurred infrequently. Drowned U.S. Government Printing Office, p. 521- Koko Seamount: A major guyot at the platforms can thus be used as stratigraphic 593. southern end of the Emperor Seamounts: Bebout, D. G., and Loucks, R. G., 1974, Stuart Marine Geology, v. 13, p. 311-321. markers. City trend, Lower Cretaceous, South Texas: Eder, W., and Franke, W., 1979, Das Ende des Austin, Texas, University of Texas Bureau devonishcen Riff-Wachstums—diskutiert ACKNOWLEDGMENTS of Economic Geology Report of Investiga- anhand der Entwicklung der Akkumula- tions 78, 80 p. tionraten labs.]: Nachrichten Deutsche Bechstaedt, T., Brandner, R., Mostler, H., and Geologische Gesellschaft, 1979, p. 7-8. I thank R. N. Ginsburg, A. G. Fischer, Schmidt, K., 1978, Aborted rifting in the Eliuk, L. S., 1978, The Abenaki Formation, Nova C.G.A. Harrison, J. van de Kreeke, and C. Triassic of the Eastern and Southern Alps: Scotia Shelf, Canada — A depositional and Klapa for valuable suggestions; and S. O. Neues Jahrbuch Geologie Palaentologie diagenetic model for a Mesozoic carbonate Schlanger and I. Premoli-Silva for providing Abhandlungen, v. 156, p. 157-178. platform: Canadian Petroleum Geologists an important manuscript on mid-plate vol- Benson, W., Sheridan, R. 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