SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY OF REEFS AND CARBONATE PLATFORMS A Short Course
by
Wolfgang Schlager Free University, Amsterdam
Continuing Education Course Note Series #34
Published by The American Association of Petroleum Geologists Tulsa, Oklahoma U.S.A.
PREFACE
Classical sequence stratigraphy has been developed primarily from siliciclastic systems. Application of the concept to carbonates has not been as straightforward as was originally expected even though the basic tenets of sequence stratigraphy are supposed to be applicable to all depositional systems.
Rather than force carbonate platforms into the straightjacket of a concept derived from another sediment family, this course takes a different tack. It starts out from the premise that sequence stratigraphy is a modern and sophisticated version of lithostratigraphy and as such is a sedimentologic concept. "More sedimentology into sequence stratigraphy" is the motto of the course and the red line that runs through the chapters of this booklet.
The course sets out with a review of sedimentologic principles governing the large-scale anatomy of reefs and platforms. It then looks at sequences and systems tracts from a sedimentologic point of view, assesses the differences between siliciclastics and carbonates in their response to sea level, evaluates processes that compete with sea level for control on carbonate sequences, and finally presents a set of guidelines for application of sequence stratigraphy to reefs and carbonate platforms.
In compiling these notes, I have drawn not only on literature but also on as yet unpublished materials from my associates in the sedimentology group at the Free University, Amsterdam. I acknowledge in particular Hemmo Bosscher, Ewan Campbell, Juul Everaars, Arnout Everts, Jeroen Kenter, Henk van de Poel, John Reijmer, Jan Stafleu, and Flora Vijn.
The Industrial Associates Program in Sedimentology at the Free University was instrumental in getting this course underway-through stimulating discussions as well as by providing financial support.
CHAPTER 1 PRINCIPLES OF CARBONATE SEDIMENTATION
In order to prepare ourselves for application of latitude carbonate environments, the base of the sequence stratigraphy to reefs and carbonate euphotic zone usually lies at 50-120m (Figure 1-3). platforms, we first have to understand how shallow Temperature is the second dominant control on water carbonate accumulations are formed. We carbonate production. Generally, warmer is better, but therefore start out with an overview of basic rules that there exists an upper temperature limit for all govern production, distribution and deposition of carbonate organisms that sets important limits to carbonate material in the marine environment. This carbonate production, particularly in restricted overview is selective and focuses on those principles lagoons. The most important effect of temper-ature, that are immediately relevant for the large-scale however, is the global zonation of carbonate deposits anatomy and seismic signature of reefs and platforms. by latitude (Figures 1-4 through 1-7). In spite of what Three basic rules capture the peculiar nature of has just been said about the role of light, the carbonate depositional systems-carbonate sediments boundary of tropical and temperate carbonates seems are largely of organic origin,the systems can build to be largely a function of winter temperature rather wave-resistant structures and they undergo extensive than radiation.Throughout the Phanerozoic, this 0 diagenetic alteration because the original minerals are boundary lay at approximately 30 latitude. metastable. The implications of these rules are Nutrients. Contrary to common expectations, high pervasive. We will encounter them throughout the nutrient environments are unfavorable for carbonate chapters of this booklet, starting with this first systems (Figure 1-8). Nutrients, to be sure, are section, which briefly reviews the principles of essential for all organic growth, including that of growth of reefs and production of sediments as well carbonate-secreting benthos. However, the carbonate as basic patterns in the anatomy of carbonate communities, particularly reefs, are adapted to life in accumulations. submarine deserts. They produce their organic tissue with the aid of sunlight from the dissolved nitrate and phosphate in sea water and are very efficient in Growth and production recycling nutrients within the system. In high nutrient settings, the carbonate producers are "Carbonates are born, not made." This little phrase outpaced by soft-bodied competitors such as fleshy by Noel P. James illustrates the fact that carbonate algae, soft corals or sponges. Furthermore, the growth and production are intimately tied to the destruction of reef framework through bio-erosion ocean environment with light, temperature and increases with increasing nutrient supply. nutrients exerting the most dominant controls. Growth potential of carbonate systems. Siliciclastic Light intensity decreases exponentially with water depositional systems depend on outside sediment depth. Growth of organic tissue and carbonate supply for their accumulation. In carbonate settings, the fixation can be related to this exponential decay via a ability to grow upward and produce sediment is an hyperbolic tangent function (Figures 1-1, 1-2). This intrinsic quality of the system, called the growth implies that there is a shallow zone of light potential. Conceptually, one should distinguish saturation, usually 10-20m, where light is not a between the ability to build up vertically and track sea growth-limiting factor; below it, light is the level, the aggradation potential, and the ability to dominant control. The base of the euphotic zone is produce and export sediment, the production potential. defined by biologists as the level where oxygen The aggradation potential is particularly critical for production by photosynthesis and oxygen survival or drowning of carbonate platforms (Figure 1- consumption by respiration are in balance. In the 9); the production potential is a crucial factor for the geologic record, all environments with abundant progradation and retreat of carbonate platforms and growth of photosynthetic organisms, such as green the infilling or starvation of basins. algae or corals, are considered euphotic. In low- The growth potential is different for the different
1 2 Schlager facies belts of a platform. Most important is the fact much of this growth in place, enables carbonate that the growth potential of the rim is significantly systems to build rather steep relief. Depth-dependent, higher than that of the platform-interior facies. When in-situ growth may be an explanation for the confronted with a relative sea-level rise that exceeds formation of steep forereef walls (Figures 1-14 the growth potential of the interior, a platform will through 1-16). raise its rim and leave the lagoon empty ("empty Despite the fact that platform rims can aggrade bucket"). faster than lagoons, many platforms backstep when Carbonate production commonly follows the law they come under stress. Figure 1-17 shows why of sigmoidal growth shown in Figure 1-10. backstepping can be advantageous for platforms. The Communities respond to opening up of new living basic principle is that the platform moves the margin space with a sigmoidal growth curve. Growth is slow to a position where growth potential and relative sea at first, then accelerates, often exceeding the rate of level rise are again in balance. change in the forcing function; finally, growth Platforms built from loose sediment without reef decreases as the systems reaches the limits of the construction or lithification at the shelf break are newly formed niche. In carbonate sedimentology, called carbonate ramps (see Read, 1985 for review). this pattern is known as the start-up, catch-up, keep Ideally, ramp profiles are in equilibrium with the up stages of growth (Neumann & Macintyre;1985) seaward-dipping wave base and parallel those of and reef response to the Holocene sea-level rise is a sediment-stuffed siliciclastic shelves (Figure 1-12a). typical example of this rule (Figure 1-11). The law of However, the term is often used loosely and some sigmoidal growth implies that the growth potential rim-building is tolerated by most authors. of the system is significantly lower in the start-up Slopes and rises. Slopes and debris aprons around phase. In the Holocene, the effect lasts only 2000 to platforms are important elements of the edifice and 5000 years. However, much longer lag effects have largely determine the extent and shape of the top. been postulated after mass extinctions (e.g. These areas also act as sinks for much of the excess Hottinger, 1989, p.270). sediment produced by the platform top. In sequence stratigraphy, platform margins and slopes play a crucial role as they hold much of the information on Anatomy of reefs and carbonate platforms lowstands of sea level. Geometry and facies of slopes and rises are In-situ production is the hallmark of carbonate governed by several rules that are summarized platforms. Any parcel of sea floor in the photic zone below. acts as a source of sediment. Since production is a) The volume of sediment required to maintain a highest in the uppermost part of the water column slope as the platform grows upward increases as a and the overlying terrestrial environment is inimical function of platform height. The increase is to carbonates, platforms have a very strong tendency proportional to the square of the height for conical to develop flat tops at sea level. slopes of isolated platforms, such as atolls, and it is Rim-building. The tendency towards flat tops is proportional to the first power of the height for linear enhanced by the ability of tropical carbonate systems platform slopes. to build wave-resistant structures, particularly at the b) The upper parts of platform slopes steepen with platform margins. Reefs are the most common the height of the slope, a trend that siliciclastics building blocks of platform rims, but carbonate sand abandon at early stages of growth. As a consequence, shoals, too, are able to build stable barriers by the the slopes of most platforms, notably the high-rising interplay of storm deposition and rapid lithification. ones, are steeper than siliciclastic slopes (Figures 1-19, Thus reefs and shoals may alternate in the 1-20). Changes in slope angle during platform growth construction of a platform rim. The presence of a rim, change the sediment regime on the slope, shifting the often built to sea level, disrupts the seaward-sloping balance between erosion and deposition of turbidity surface normally developed by loose sediment currents. This in turn profoundly influences sediment accumulations on a shelf. Rimmed platforms differ geometry on the slope and the rise (Figure 1-21). fundamentally from siliciclastic shelves in this respect c) The angle of repose of loose sediment is a (Figure 1-12). The growth anatomy of a rimmed function of grain size. Engineers have quantified this platform is that of a bucket-a competent, rigid rim relationship for man-made dumps (Figure 1-22) and protects the loose sediment accumulation of the the same numerical relationships have recently been platform interior (Figure 1-13). Because of the higher shown to apply to large-scale, geological features growth potential of the rim, "empty buckets," i.e., (Figures 1-23 through 1-26; Kenter, 1990). The raised rims and empty lagoons, are cohunonly found important property seems to be the degree of in the geologic record. cohesion of the sediment. Changes in composition of The rapid decrease of carbonate production with the sediment dumped on a slope may produce depth, in conjunction with the ability of reefs to hold unconformities as shown in Figure 1-27. Principles of Carbonate Sedimentation 3
00~ 0 0 i 0 ~ 00 0 0 1 0 0 0 0: 00 I ~ 8 I
100 --1
Fig.l-3. Depth limits of reef growth in the Caribbean. active reef growth m Reef growth and growth of green algae are commonly strongly reduced growth Cl used to define euphotic zone in a geologically 150 max1mum depth • reproducible way. After Vijn &: Bosscher, written comm. 4 Schlager
Fig.l-4. Coral growth versus solar radiation (a) and water DATA FROM WEBER and WHITE 1974 temperature (b). Note increase of growth with temperature and poor correlation of coral growth with radiation. After Bosscher & Schlager, written comm.
4~------~-- 420 450 480 510 AVERAGE SOLAR RADIATION IN G CAUM2/DAY
8 ..
IIt U'l 6 z ::J
Fig.l-5. Distribution of recent coral reefs is limited in the 23 24 25 26 27 28 29 30 north and south by minimum winter temperatures. After AVERAGE SURFACE WATER TEMPERATURE IN'C Bosscher & Schlager, written comm.
DISTRIBUTION OF CORAL REEFS
40 40
20 20
···I'· ... ·-:.•. ·:· 0 ··. '\ ··. ···:-~ ... ~·!· ·: 20 ·~ .. ·..· ...... 20
40 C)'t? 40 Q CORAL REEFS 6J 20° ISOTHERM Principles of Carbonate Sedimentation 5
A ..... z <{ 0z w :::> u m z <{ oo 20° LATITUDE B REEF GROWTH I-----OPTIMUM--- •MARGINAL- -RARE----..! MASSIVE CORAL-ALGAL REEF REEF COMPLEXES BEDDED BRYOZOAN-ALGAL COMPLEXES INTERMIXED WITH BIOCLASTIC DEBRIS ~ REEF (/) BRYOZOAN- ALGAL GROWTH DURING CLIMATIC U) BIOSTROMES, SANDS, OPTIMA w z BIOCLASTIC DEBRIS ~ u J: 1- w > 1- o 1oo zo• 3o• 4o• LATITUDE INDONESIA PHILIPPINE-MARIANAS TAIWAN RYUKYUS -JAPAN- CORAL-ALGAL -fy.(y~y BRYOZOAN-ALGAL REEF ' ... ·':·· ... ·. -' BIOCLASTIC DEBRIS Fig.l-6. Latitudinal change from tropical to temperate carbonate facies in the northern Pacific (after Schlanger, 1981). Note disappearance of reefs and decrease in thickness, indicating reduced production in temperate climate. (Reproduced with permission of Society for Sedimentary Geology) 6 Schlager Fig.l-7. Carbonates in temperate latitudes and tropical latitudes - a comparison. Figs A,B,C illustrate changes in environmental conditions; Fig.D illustrates difference in skeletal carbonate - temperate carbonates dominated by benthic foraminifers and molluscs ("foramol" association), tropical latitudes by green algae and corals ("chlorozoan" association); Fig.E shows that non-skeletal grains are absent in temperate-water carbonates. After Lees (1975). (Reproduced with permission of Elsevier A Salini1y. ~ B Maximum temperature. •c Science Publishers) D Skeletll associations C Minimum tamper~ture, "C a) b) PHYSICAL WATER VARIABLES vs. LINEAR GROWTH FOR MONT ASTREA ANNULARIS DATA FROM TOMASCIK aM BANDER 196!1 DATA FROM CORTES and RISK 1985 e DODGE, ALLER and THOMSON 1974 b. 2 0~--~------~------~--~~~------.1 .2 2 10 20 100 200 2 RESUSPENBION MGICM210AY B.P. M. MG/1. Fig.l-8. Coral growth versus amount of resuspended sediment (a) and amount of suspended particulate organic matter (b). Both variables are negatively correlated with reef growth. Suspended matter reduces light and, in the case of suspended organic matter, also stimulates growth of organisms that compete with carbonate benthos. After Bosscher & Schlager (written comm.). Principles of Carbonate Sedimentation 7 FRI!QUENCY Of PLATFORM ACCUMULATION RATES : i " 1 10 ···j~, cooling oru11 ..• Hatooane ••• tewel 0 .,,•• dtnt Q 0 c L I lont-tlf'IR IMbllde•c•·····- Recent ra•t a.rala Tr Alpl lHotoc ... ,.... ( .L a Apan•tn• 'lalf. ·- c H.ree I 10-2010 0 () ..c eatta ..aa .Jur.-Rec.I oJHio """':;9 RU!S IN MM/IOOOYR ISUSNOI"I" UNITII Fig.1-9. Carbonate growth rates and rates of (relative) sea-level rise. Open bars- Holocene, black- distant geologic past. Short-term Holocene rates exceed long-term Phanerozoic rates by one order of magnitude, in agreement with the rule that sedimentation rates increase as the duration of the observation interval decreases (e.g. Sadler, 1981). Based on the Holocene and cyclostratigraphy in ancient formations, Schlager&: Bosscher (written comm.) estimate the average growth potential of platforms in the time range of 1ol-1oS years at 300 -lOOOm/Ma. This short-term growth potential is relevant for platform drowning: whether a platform drowns or survives a relative sea-level rise is determined in a short-term race through the photic zone, whereby the zone of light-saturation in the top 10-20 m is most critical. It is, therefore, unlikely that long-term, steady subsidence and third-order sea-level cycles, both typically at 10 -100 m/Ma, can drown healthy reefs and platforms. Modified after Schlager (1981), Bosscher & Schlager (in press,a). LAW OF SIGMOIDAL GROWTH ,~------·:-::-:.:-::-:SI!!k~•..,~up~ Fig.l-10. Populations of organisms respond to the ,/ opening up of new living space in three steps - first, , growth lags behind the creation of living space, second, I ,, population growth exceeds the rate of change in space, rapid log finally, population growth is limited by the rate of creation phase c.tch up of niche ,/ growth in living space. Most carbonate systems are , controlled by organic growth and thus follow this law. J: I ie 0 I CD > Q) 5 10 5 10 as ... _.. 1!!. \ CD \ C/) llal \ ' ' \ rubble ' ' \ -c ' ' \ CD ' ' \ C/) ' ' \ \ Q) ' ' ' \ .... ' \ ' \ \ Q 10 ' \ \ '\ I ~ ' \ \ I 0 \ \ \ \ t Q) ,_, ...,j \ \ t .D nis lacles • \ \ \ \ t C/) Gt.Barrier Reef ..... \ \ Q) \ \ \ ' \ \ -Q) 20 \ \ E \ coral head\ • \ \ \ lacies \ \ \ \ ! \ \ \ '1' \ coral rubble and \ \ \\ 30 sand lades ' "'' Alacran, Yucatan ' St.Crohc, Virgin Is. \\ Fig.1-11. Growth history of Holocene reefs. Note start-up, catch-up, and keep-up phase of growth, following the law of sigmoidal growth. After Schlager (1981), Neumann & Macintyre (1985). a) /high onergy deposits, close to shore SILICICLASTICS b) moderato to high on orgy high-energy sand shoals and reels elevated by organic framebulfdlng I shore (dependant on width 1 and depth of lagoon) protocted lagoon and ayndepositionallithification ··· ' \ soFrnu ' ····· ...... · SOFTFtANK\ \sand,mud. ,' ! \ ' I I mudJ rubble \ ',,__ ...... ' / I I \ STIFF~IM 1 \ organic: frt:IJI't&, / RIMMED PLATFORMS ' cement ' Fig.1-12. Shore-to-slope profiles in carbonates and Fig.1-13. Bucket principle. The growth anatomy of siliciclastics. (a) Siliciclastics, abundant sediment supply. rimmed carbonate platforms resembles a bucket, held Result is seaward dipping surface in equilibrium with together by stiff rims of reefs or rapidly cemented sand deepening wave base. (b) Rimmed carbonate platforms. shoals, and filled with less consolidated sediments of Wave-equilibrium profile grossly distorted by lagoons and tidal flats. Growth potential of a platform is construction of wave-resistant reefs and quickly largely determined by the growth potential of the rim. lithifying sand shoals, mainly at platform margin but also After Schlager (1981). (Reproduced with permission of elsewhere on platform (e.g. lagoonal patch reefs). Geol. Society of America) Platforms are basically dish-shaped and equilibrium profiles develop only locally in parts of the lagoon. Principles of Carbonate Sedimentation 9 • • • ' ' Fig.1-14. Reef wall of the barrier reef in Belize. It is unclear whether these walls are Pleistocene sea cliffs mantled by Holocene reef or whether they are constructional features, formed by upward and outward growth of corals. In any case, the walls illustrate the tendency of platforms to oversteepen their upper slopes. After James & Ginsburg (1979). 10 Schlager 200~------~------~ 0 years B.P. 80000 Belize Reef Wall k:O.OS;Ik:250;10:2000 Om:0.005 Rllllime 110000 yn SUrfoce plocted ..ay 1000 yn 20.0 40.0 60.0 IO.U 100.0 120.0 140.0 meten Fig.l-15. The rapid decrease of reef growth with depth may explain the near-vertical reef walls. This computer simulation attempts to duplicate the reef wall of Belize as a series of reef shoulders formed during minor highstands of sea level. After Bosscher & Schlager, in press. ~ YEARS B.P. &10 0 10 20 30 40 50 eo I , 7{l 0 ,.., I I I I SeaLevel~ 40 ... •• •• t• • fl) f\ ~ I • 160 160 Fig.1·16. Conceptual model of James & Ginsburg (1979) that formed the basis for computer simulation in Fig.1·5. Fig.l-17. Backstepping is a typical response of carbonate platforms to a relative rise of sea level that slightly exceeds the platform's growth potential. There are several reasons why backstepping may be advantageous for a platform under stress. (a) Backstepping reduces wave destruction of the margin, making it easier for the platform rim to grow upward. (b) Platforms may step back to higher ground, such as an old shoreline. (c) Differential subsidence can be a reason for backstepping of platforms when the outer, most rapidly subsiding part of the platform can no longer cope with sea-level rise. Principles of Carbonate Sedimentation 11 A) BACKSTEPPING/WAVE POWER Ga •C-D C construction D destruction G reef growth t?i:dZiSMHi:'1~:;,\(ittr!(t1~i~~f!!.-i\}J/XC-''??/!.!~/):\/:UJ(/:~~\i(i!_1 ';, ... =decay of wave power by bottom friction · -:::;·,;:··<·:·.:·:·:,.::·:. Airy waves, depth constant 1 dH H wave height (1) =-gH-V 4 dx v wave phase velocity g gravity acceleration B) BACKSTEPPING ONTO HIGHER GROUND sealevel rise reefs beach ridges lagoon &dunes reef C) BACKSTEPPING TO AREA OF LOWER SUBSIDENCE f otential CD ...en -0 distance from hinge line --+ hinge zone lws 1982 12 Schlager Atoll "coM" Platform "prJsm" tana W'Jlume y - ta':.a- .oV,,.o~oincrrmtnfJ in •olum• ctlU5«1 by ~rtical growth All of platform or atoll rolum~ofaloi/COM V• 7Th' oolumtof5i01Wpri•m y• ...l!.:...l Jiiin1ii of Jtngth "} 21CIIIfl growth a1 rolumo IH •ik h•l QroW/11 tJf ro/utM If* •:;;;k-hj Fig.1·18. Upward growth of carbonate platforms with constant slope requires deposition of ever larger volumes of sediment on the flanks. In the case of a cone-shaped ..._., __ atoll, the growth of volume (V) is proportional to the square of the height of the cone. In case of a linear platform margin, the growth of V is proportional to the Fig.1·19. Modem submarine slope angles of carbonate height of the platform. Real platforms probably fall platforms and siliciclastic systems in Atlantic and between these two extremes. The steady increase in slope Pacific. Contours of 1, 2 and 4% of total sample (N) in unit volume may limit the growth of very tall platforms such area. N (carbonates) = 413, N (clastics) = 72. Carbonates as oceanic atolls. Smaller platforms seem to compensate steepen with height by building slump-resistant slopes. for this effect by increase in slope angle and/or rapid After Schlager &c Camber (1986). (Reproduced with filling of the basin. Schlager (1981). (Reproduced with permission of Society for Sedimentary Geology) permission of Geol. Society of America) pic.lo pordol aaa pordol A • • 600 2600 orm 400 2400 200 2200 ., 0 2000 .,... alOO 1500 1000 500 0 ., -E Ia locomotive plz clavaces t.re del sella .E • .. .. .z; B -tl 600 2600 "i platform .z; 400 2400 200 2200 basin 0 2000 2500 2000 1500 1000 500 0 distance In meters Fig.1-20. Decrease in slope angle with decreasing slope height in a shoaling basin (Triassic, Southern Alps, Italy). This fossil example confirms the trend observed in modem slopes (Fig.1-18). (Reproduced with permission of Blackwell Scientific Publ.) Principles of Carbonate Sedimentation 13 Angle vs. depositional regime EROSION Bahama Escarpment > 25° ~====~Jt----~~~---7\' ,~~0 \ \ \ \ NEUTRAL (sand bypassilg) .?"" Exuma transect ODP gravel sand/silt 10-12° Fig.1-22. Grainsize vs. angle of internal friction for dry sediments. (Angle of repose is considered equal to angle of internal friction at zero confining pressure). This relationship explains much of the difference in slope angle of coarse, mud-free slope sediments and muddy slopes. After Kirkby (1987), modified. ACCRETION Blake transect ODP 2-3° 0 WELL DOCUMENTED EXAMPlES (GROUP A) 0 EXAWPLES LACKING PRECISE CONTROL ON GEOMETRY (GROUP B) 6, FLANKS STABILIZED BY ORGANIC FRAMEBUILDING OR CEMENTAT;~:OUP Fig.l-21. Slope angle and the balance of erosion and deposition on slopes. As slope angle increases the vigor of turbidity currents increases too and changes the depositional regime on the slope from accretion to erosion. By-pass slopes represent an intermediate stage. They receive mud from the perennial rain of sediment, whereas coarse material travelling in large turbidity currents is swept farther into the basin. After Schlager & Camber (1986). DOMINANT SEDIMENT.FABRIC Fig.l-23. Sediment composition strongly influences slope angles of carbonate platform flanks. Cohesionless sediments, such as clean sand and rubble, build up to angles of over 40°. Muddy, cohesive sediments tend to develop large slumps that maintain a low slope angle. After Kenter (1990). (Reproduced with permission of Blackwell Scientific Publ.) 14 Schlager .------~~~ akm common pOinla f---1---+-~·-il dopth LIIB-111 A modern gully erosion surface slump mass Fig.1-24. Profile of slope with muddy cohesive sediment. Characteristics are gentle slope angle and large-scale slumps with toe thrusts at the distal end of the slope. Northern flank of Little Bahama Bank, after Harwood &: Towers (1988) and Kenter (1990). (Reproduced with permission of Ocean Drilling Program and Blackwell Scientific Publishers) . . ~ .. ; !. blae-a-Mo~M· ·, Mdlmlnt ipfon mid-slope ,.... Fig.l-25. Large-scale slumps on muddy slope in plan view. Note combination of slump scars on the slope and turbidite aprons at the basin margin. After Harwood & Towers (1988). (Reproduced with permission of Ocean Drilling Program) Principles of Carbonate Sedimentation 15 NNE 024 INTERPRETATION CROSS-SECTION VAL DE MESOI, NORTHERN SELLA ssw 204 platform atructureleaa zone meters 200 100 500 300 200 100 0 Fig.1-26. Profile of platform slope composed of non-cohesive sediment, i.e. sand and rubble with little or no mud. Slumps are small and express themselves as numerous small-scale truncations. Overall bedding remains steep and sub-parallel. Triassic Sella platform, Dolomites, Italy. After Kenter (1990). (Reproduced with permission of Blackwell Scientific Publ.) 16 Schlager a CARBPLAT runtime: 2000 yrs timestep: 100 yrs platform height: 50 m platform width: 400 m Gmargin: 10 mmtyr Ginlerior: 4 mm/yr k: 0.2/m initial depth: 2 m linear sea-level rise: 3 mm/yr wavebase: 10 m minangle/maxangle: 5135 degrees width of section: 330 m unconformity caused by change in sediment composition b Fig.l-27. Grain-size-related unconformity in model and sea level outcrop. (a) Computer program CARBPLAT (Bosscher & Southam, 1992) takes results of Kenter (1990) into account and assigns different angles of repose to rubbly sand and mud. This produces an unconformity on the slope of a continuously growing platform. At first, the platform grows with an empty lagoon and thus sheds only sand and rubble from its reef margin. Subsequently, the lagoon fills up and exports large volumes of mud; this mud buries the reef talus at a more gentle slope angle. (b) windward slope leeward slope Sediment geometry of the modem fore-reef slopes of Bahamian platforms as observed in submersible dives by Grammer et al. (1990) and Grammer (1991). Mud wedge unconformably overlies reef rubble and the situation closely resembles the model predictions of Bosscher & Southam (1992). CHAPTER2 THE STANDARD FACIES BELTS Irrespective of geologic time and setting, shallow gravity transport at the toe-of-slope in an oceanic or water carbonates have a strong tendency to develop cratonic basin or on the proximal part of a deep shelf. similar facies belts. These facies can be grouped into a Sediments: Lime mud, somewhat siliceous, with platform model (Figure 2-1), best summarized by Wil intercalations of debris flows and turbidites in the son (1975). Important specifics on particular facies form of microbreccias and graded beds of lime rubble, have been added from other authors. sand and silt. 1A) DeepSea Biota: Mostly re-deposited shallow-water benthos, Setting: Below wave base and below euphotic zone; some plankton and deep-water benthos. part of deep sea, i.e. reaching through the thermocline 4) Slope (Figures 1-18 through 1-27) into the realm of oceanic deep water. Synonym: foreslope of platform Sediments: Entire suite of deep-sea sediments such Setting: Strongly inclined sea floors (over 1.4°) sea as pelagic clay, siliceous and carbonate ooze, ward of the platform margin. hemipelagic muds including turbidites; adjacent to Sediments: Mostly pure carbonates, rare intercala platforms we find mixtures of pelagic and platform tions of terrigenous mud. Grain size highly variable derived materials in the form of peri-platform oozes from mud to rubble (end members are gentle muddy and muds. slope with much slumping and sandy slope with Biota: Predominantly plankton, typical oceanic steep, planar foresets). associations. In peri-platform sediments up to 75% Biota: Mostly redeposited shallow-water benthos, shallow-water benthos. some deep-water benthos and plankton. 1B) Cratonic deep-water basins 5) Reefs of platform margin (Figures 2-2, 2-3) Setting: Below wave base and below euphotic zone Setting: (I) Organically stabilized mud mounds on but normally not connected with the oceanic deep upper slope or (II) ramps of knoll reefs and skeletal water body. sands or (III) wave-resistant barrier reefs rimming the Sediments: Similar to 1A but in Mesozoic-Cenozoic platform. rarely ever pelagic day; hemipelagic muds very com Sediments: Almost pure carbonate of very variable mon; occasionally anhydritic; some chert; anoxic con grain size. Most diagnostic are masses or patches of ditions common (lack of bioturbation, high organic boundstone or framestone, internal cavities with fill content). ings of cement or sediment, multiple generations of Biota: Predominantly nekton and plankton, construction, encrustation and boring and destruction. coquinas of thin-shelled bivalves (Posidonia type), Biota: Almost exclusively benthos. Colonies of sponge spicules. framebuilders, encrusters, borers along with large vol 2) Deep shelf umes of loose skeletal rubble and sand. Setting: Below fair-weather wave base but within 6) Sand shoals of platform margins (Figure 2-3) reach of storm waves, within or just below euphotic Setting: Elongate shoals and tidal bars, sometimes zone; forming plateaus between active platform and with eolianite islands; above £air-weather wave base deeper basin (these plateaus are commonly estab and within euphotic zone, strongly influenced by tidal lished on top of drowned platforms). currents. Sediments: Mostly carbonate (skeletal wackestone, Sediments: Clean lime sands, occasionally with some grainstone) and marl, some silica; well biotur quartz; partly with well-preserved cross bedding, bated, well bedded. partly bioturbated. Biota: Diverse shelly fauna indicating normal Biota: Worn and abraded biota from reefs and asso marine conditions. Minor plankton. ciated environments, low-diversity in-fauna adjusted 3) Toe-of-slope apron to very mobile substrate. Synonyms: lower slope, deep shelf margin. 7) Platform interior-normal marine Setting: Below £air-weather wave base, below Setting: Flat platform top within euphotic zone and euphotic zone; debris apron formed by sediment normally above fair-weather wave base; called lagoon 17 18 Schlager when protected by sand shoals, islands and reefs of Biota: Shallow-water biota of reduced diversity, but platform margin; sufficiently connected with open sea commonly with very large numbers of individuals; to maintain salinities and temperatures close to those typical are cerithid gastropods, miliolid foraminifers. of adjacent ocean. 9) Platform interior-evaporitic Sediments: Lime mud, muddy sand or sand, Setting: as in facies 7 and 8, yet with only episodic depending on grain size of local sediment production influx of normal marine waters and an arid climate so and the efficiency of winnowing by waves and tidal that gypsum, anhydrite or halite may be deposited currents; patches of bioherms and biostromes. Terrige besides carbonates. Sabkhas, salt marshes and salt nous sand and mud may be common in platforms ponds are typical features. attached to land, absent in detached platforms such as Sediments: Lime mud or dolomite mud along with oceanic atolls. nodular or coarse-crystalline gypsum or anhydrite; Biota: Shallow-water benthos with bivalves, gas intercalations of red beds and terrigenous eolianites in tropods, sponges, arthropods, foraminifers and algae land-attached platforms. particularly common. Biota: Little indigenous biota except blue-green 8) Platform interior-restricted algae, occasional brine shrimp, ostracodes, mollusks. Setting: As for facies 7, but less well connected with The standard model says nothing about windward open ocean so that large variations of temperature and leeward differentiation. Most platforms develop salinity are common. asymmetries in response to dominant wind directions Sediments: Mostly lime mud and muddy sand, (Figures 2-4, 2-5). 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I UAIISTOIE I 'IUPUIT[ 1 PGIIIOSTIM SHAlE lUCillE PACISTOIII. e Ul IIIECCIA 711APUTOII IJCIIIU SELUITE ! FlOAT STOll. S OOliTE OIIOJDS II J 101 lAIIIIAT£ ILADES IN IIICIIf£ PUll[ IIICIITI IIICIITI. I IEIF .,. .... , UllOIIAl ! RUDSTOII. MSYCUIIM:EM IIICIITE ~ CUINSTOIIE .."' Fi~ure 2-1. Synopsis of standard facies belts reviewin~ second-order bodies of sediment and standard m1crofacies associated with each belL (Reproduced with permission of Springer Verlag) Figure 2-3. Rimmed margins are common in carbonate platforms and re~uire certain adjustments in seismic statigraphic techniques. Reefs are important rim builders (a), but early-hthified stacked sand shoals can be equally efficient in defending platform margins (b). (After James & Macintyre, 1985. Reproduced with permission of Colorado School of Mines Press) The Standard Facies Belt 19 a) REEF MOSAIC b) BAFFLES TONE BINDSTONE FRAMESTONE CAVITIES ATTACHI!D II!GIIENTI!D WITH INTE-L CAI.CAJIIEOUI IIENTHOI llf:DIIIENT OOIIIAL a IIAIIIYI III!TAZOA REEF LIMESTONE FLOATSTONE RUDSTONE Figure 2-2. Recognition of ecologic reefs in the geologic record depends on a variety of features; most of them require outcrops or cores as an observational basis. This sketch illustrat~s characteristic aspects of reefs. Note particularly the multiple successions of different encrusters and frame builders as well as cavities with mtemal sediment and cement. Obviously, outcrops or cores, not seismic profiles, are required to apply these criteria. (After James & Macintyre, 1985. Reproduced with permission of Colorado School of Mines Press) SHOAL - RIMMED PLATFORM REEF - RIMMED PLATFORM REEF - RIMMED PLATFORM 20 Schlager WINDWARD z I!DGI! PRO FILl! A -0 I GEN(RAI..ISEDI Figure 2-4. Sediment tvpes and patterns on Bu Tini Shoal. (From Purser et al., 1973). Windward-leeward asymmetries such as this one probably are common in the geologic record. Seismic grids are one of the best techniques to reveal them. (Reproduced with permission of Springer Verlag) The Standard Facies Belt 21 ROCK RIDGE (EOLIAN?) D HCX...OCEf\E SAND g HOLOCEI\E REEF ~ PLEISTOCENE ROCK 500M~: I}}DI HOLOCENE SAIII> ~ HOLOCEI\E REEF ~ PlEISTOCENE ROCK Figure 2-5. Windward and leeward platform mar~ins in the Bahamas-a comparison. Note active reef growth along windward margin (upper panel), reefs buned in sediment on leeward margin (off-bank sediment transport). After Hine & Neumann (1977). CHAPTER3 RHYTHMS AND EVENTS IN CARBONATE DEPOSITION Shallow water carbonates rarely ever accumulate in in many instances (e.g., Fischer, 1964; Read & Gold a uniform, steady fashion. Rather, the record shows a hammer, 1988; Goldhammer et al., 1990; ). These obser hierarchy of rhythms on time scales of thousands to vations imply that the frequency bands of orbital cycles hundreds of millions of years. These rhythms are and stratigraphic sequences broadly overlap and the punctuated by singular events and overprinted by the two approaches complement each other. unidirectional changes of organic evolution. Below we briefly discuss some of these patterns and processes. They fall outside the domain of classical sequence Autocycles stratigraphy but we believe that the seismic stratigra pher should be aware of them as they may well influ Under most circumstances, depositional systems ence sediment production and sequence anatomy in respond rhythmically to linear forcing, i.e. they fall reefs and carbonate platforms. behind, then catch up, overshoot, and fall behind again (Figures 3-2 and 3-3). Besides this non-linear Milankovitch cycles response of an entire system in the time domain, we also find space rhythms within a system-mud banks, tidal passes, delta lobes etc. typically form rhythmic The perturbations of the Earth's orbit, their influ patterns in space. Migration of these spatial patterns ence on climate, sea level and the sediment record can also produce a rhythmic sediment record (see Fig have received much attention recently. Reefs and plat ure 3-4). Both effects have been invoked to explain forms are particularly sensitive to Milankovitch forc rhythmicity in the geologic record. The notion of auto ing because they respond to both sea-level cycles as cyclicity is !Particularly popular for short time periods well as environmental change driven by the orbital of 103-10 years, the typical domain of parase perturbations. Sea-level signals are best recorded in quences. However, there is no reason to dismiss auto the shallow lagoons and tidal flats of the bank tops. cyclicity as an explanation for longer-term rhythms in Figure 3-1 illustrates that this record strongly depends the geologic record. The emphasis on short-term auto on the interference of various frequencies (as well as cyclicity is probably an artefact of our strong observa on subsidence). At slow subsidence (and large ampli tional bias towards the Holocene. tudes in the low frequencies) the record may be erratic in spite of regular orbital forcing. The most pro nounced orbital cycles have periods of ca. 20,000 to Organic evolution 400,000 years, but the full spectrum of orbital pertur bations ranges from 101 to 106 years. The organic origin of most carbonate sediment Orbital cyclostratigraphy is a rather sophisticated makes carbonate depositional systems especially sen technique and we lack the time to adequately cover it in sitive to evolutionary changes in the biota. Figures 3-5 this course (see Fischer & Bottjer, 1991 and Goldham and 3-6 depict some of these changes for reef biota. It mer et al., 1991 for overviews). However, the approach is likely that they modulate the growth potential and has great potential in stratigraphy and is rapidly the growth geometry of reefs and platform margins, expanding into the realm of sequence stratigraphy. governing the distribution of, for instance, drowning Recently, many sequence stratigraphers reported much events or progradation phases through time. Bosscher higher numbers of sequence boundaries than previous & Schlager (in press) found that the observed maxi ly observed. For instance, Van Wagoner et al. (1990, mum accumulation rates of platforms go through a p.52) estimate that type-1 unconformities are spaced at minimum in the wake of mass extinctions of reef intervals of 100,000-150,000 years and that global biota, suggesting some evolutionary control on the curves such as the one by Haq et al. (1987) refer to sets carbonate growth potential. James & Bourque (in of sequences. The carbonate record confirms this notion press) emphasize the difference between reef-building 22 Rhythms and Events in Carbonate Deposition 23 and mound-building platforms and their irregular warm poles and reduced temperature gradients. Fis distribution through time. cher argued for an endogenic control of the ice house-greenhouse cycle, because it seems to correlate with variations of magmatic activity and sea-floor Chem~cal evolution spreading. Figure 3-7 shows that the calcite/aragonite ratio in carbonates also seems to vary in accordance The composition of the ocean and atmosphere is with the icehouse-greenhouse cycle. In this way, the determined largely by the rates of recycling and plate growth potential of carbonate systems as well as their motion. Fischer (1982) proposed that during the potential for diagenetic alteration may be tied to Phanerozoic, the Earth has oscillated between an ice changes in the ocean environment. (See Chapter 6 for house state with cold poles and maximum latitudinal follow-up.) temperature gradients and a greenhouse state with Fl SCHER CYCLES Figure 3-1. Scenarios for deposition of carbonate PERIODIC PLATfORM CARBONATE DEPOSITION cycles dictated by Milankovitch rhythms. The top IN TUNE TO MILANKOVITCH RHYTHMS panel shows the drowning and exposure of a carbonate platform due to glacio-eustasy produced by superimposition of 20,000-year and 100,000-year Milankovitch pulses. Small variations in the amplitude of the 100,000-year pulse produce bundling of tidal-flat cycles into clusters of five. With large variations in the amplitude of the £> P SUAE DEPOSITION 100,000-year pulse, as in the Pleistocene, deposition Of PLAlFOIIM ON PLATfORM on carbonate platforms is likely to be erratic and will not easily reveal the Milankovitch control, as depicted in the lower panel. (After Hardie & Shinn, 1986. Reproduced with permission of Colorado APERIODIC PLATFOIU4 CARBONATE DEPOSIT I ON WITH MILANKOVITCH SEA LEVEL OSCILLATIONS School of Mines Press) TIME --200,000 Vll---1•00,000 Yll---- 0 VII- 24 Schlager Q) DROWNING (SUBSIDENCE »CARBONATE PRODUCTION) <:;:::J SHORE LINE TRANSGRESSES @ PROGRADATION (CARBONATE PRODUCTION > SUBSIDENCE) .. , ...···· LAGOON CARBONATE FACTORY AND DISPERSAL SYSTEM IN FULL OPERATION w u 2 w @PROGRADATION STALLED !CARBONATE PRODUCTION~ SUBSIDENCE) 0 SUPRATIDAL CAP CARBONATE FACTORY TOO SMALL (f) FF'I lENT OPERATION CD ..... ·. ,• ·.··.···~-. w ~ (f) ~ 1- (f) A-B-C SHALLOWING UPWARD SEQUENCE :J 0 ~ 2 i= @ DROWNING (SUBSIDENCE »CARBONATE PRODUCTION) z 0 ~SHORELINE TRANSGRESSES u WATER TOO SHALLOW FOR EFFICIENT OPERATION OF' CARJ_ONATE FACTORY @PROGRADATION (CARBONATE PRODUCTION > SUBSIDENCE) ~ SHORELINE PROGRADE$ OMISSION SURFACE (SUBTIDAL L_ DIRECTLY ON SUPRATIDAL) _.. •.. 'I'. . .. STACKING OF A-8-C SHALLOWING UPWARD SEOUEttCES Figure 3-2. The autocyclic model of R.N. Ginsburg. Ginsburg (1971) proposed an autocyclic model that elegantly explains superimposed A-B-C shallowing-upward sequences m platform carbonates. The model calfs on continuous, steady basinal subsidence coupled with carbonate sediment supply rates that are self regulated by the extent of progradation. The crux of the model lies in the recognition that carbonate sediment production rate will fall behind subsidence rate as progradation drastically reouces the size of the shelf lagoon "factory'' (see above), and will not recover until the lagoon once again becomes deep enough (due to subsidence) to allow efficient carbonate production and sediment dispersal (perhaps 1-2 meters). This factor introduces an automatic lag time in deposition so that subtidal sediments lie directly on supratidal sediments. (Hardie & Shinn, 1986. Reproduced with permission of Colorado School of Mines Press) Rhythms and Events in Carbonate Deposition 25 AUTOCYCLES OF SEDIMENT SYSTEMS linear forcing by t sea level \\ rythmlc response of sedimentation horizontal distance -. E veRAARs et at. time-. Figure 3-3. Examples of autocycles on platforms generated by the computer program MAPS of Demicco & Spencer (1989). C)_•cles consist of shoaling-upward sequences with supratidal flats (dark) and subtidal (light gray). Autocyclictty is poorly understood and may operate also on time scales of millions rather than thousands of years. Erosion of foreshore & shoreface ___ Newly prograded mud coastline 5m layers Discontinuities VERTICAL IXAOGf."'ATtON 10 0. Figure 3-4. Autocycles generated by migrating mud banks on the coast of Suriname. Stacking of sediment from migrating mud banks creates on the shoreface a vertical sequence of laminated and bioturbated muds with discontinuity features and on the coastal plain a horizontal sequence of mud marshes and sand cheniers. Average duration of a cycle is 30 years. (After Rine & Ginsburg, 1985. Reproduced with permission of Society for Sedimentary Geology) 26 Schlager PERIODS BIOHERMS MAJOR SKELETAL ELEMENTS 6 CORALS T I tn z 0 5 I -..J ..J 4 -~ 3 I -a:ztn 4( ~~ 300 REEFS -t-~o~. uo 2 I -CJ 0 ..J 1 I 0 REEF UJ C) OUNDS PRECAMB Figure 3-5. An idealized stratigraphic column representing the Phanerozoic and illustrating times when there appear to have been no reefs or bioherms (gaps), times when there were only reef mounds,· and times when there were both reefs and reef mounds. Bars on right side indicate occurrence of "empty buckets.'' They tend to coincide with intervals of full reef development. Arrows represent times of important faunal extinctions with the potential to generate sequence boundaries (after James, 1983). CYCLE II Figure 3-6. A generalized plot of the main biotic constituents of carbonate buildups against time for CYCLE I the Phanerozoic. The balloons represent the relative abundance and importance of the different taxa (from James, 1983). (James and Macintyre, 1985. Reproduced with permission of Colorado School of Mines Press) UOISTS Rhythms and Events in Carbonate Deposition 27 ICEHOUSE I GREENHOUSE "- CLIMATIC EPISOOES '-.:.FISCHER SEA LEV£L FLUCTIJA TIONS HALLAM VAIL. ET AL. ~ Hlgh-Mg Calcl11 and, 1111 abvndatttly. Aragonite II Cttclle; Mg content oenarelly lower. lncraa••no towtrd "ThtllhOid" Figure 3-7. Icehouse-greenhouse cycles of the Phanerozoic are correlated to change in composition of inorganic carbonate precipitates and to mineralogy of organic carbonate production. This illustrates the close connection between carbonate production and ocean environment. (After Fischer, [1982], Sandberg [1985]. Reproduced with permission of Macmillan Magazines Ltd.) CHAPTER4 SEQUENCES AND SYSTEMS TRACTS A SEDIMENTOLOGIC VIEW Stratigraphic sequences and systems tracts as formities." Sequence boundaries are defined as defined in sequence stratigraphy (Figure 4-1) are "observable discordances in a given stratigraphic sec lithostratigraphic concepts and as such interpretable tion that show evidence of erosion or nondeposition in sedimentologic terms. We believe that sedimento with obvious stratal terminations, but in places they logic analysis is the most appropriate way to reveal may be traced into less obvious paraconformities rec the primary controls behind a certain feature of ognized by biostratigraphy or other methods." sequence stratigraphy. This, in turn, allows one to At this point, it is again useful to consider the assess the role of sea level and other factors in build basic sedimentologic meaning of seismic unconfor ing the sequence record. mities: they represent changes in the pattern of sedi When sedimentologic analysis is performed, it ment input and dispersal in the basin (Schlager et al. turns out that very often sea level is not the only pro 1984, p.725; Schlager 1989; see Galloway, 1989, for cess that may generate sequences and systems tracts. similar emphasis on "reorganization"). Sea level is The classical interpretation of sequences and systems an important control on input and dispersal patterns, tracts in terms of sea-level cycles (Figure 4-2) is a pos but so are tectonic changes in sea-floor topography, sible, but generally not a unique solution. In particu tectonic changes in the hinterland [drainage pat lar, changes in volume and composition of sediment terns], Figure 4-6), changes in ocean environment will often have similar effects as sea-level fluctuations (Figures 4-5, 6-6) and autocyclic processes in sedi (Figure 4-3). Whether these alternative controls do mentation (Figures 3-3, 3-4). apply, has to be determined specifically for each This sedimentologic interpretation of sequence instance. Chapter 6 discusses examples of sequences boundary is compatible with the definition of created not by sea level but by other processes. Here, sequence and sequence boundary by Vail et al. (1977, we will examine some pivotal concepts and terms of p.53). It is meant as a complement rather than a sequence stratigraphy and seismic stratigraphy to replacement of the original definition. A growing improve our understanding of their genesis. number of basin studies reveal a rather complex inter play of eustasy, regional tectonics and sedimentologic processes as control on depositional sequences (e.g. The sequence boundary Underhill, 1991). The original definition along with the above interpretation are broad, yet specific Despite the subdivision of sequences into systems enough to address this superposition of effects. tracts and the recognition of other surfaces (such as Van Wagoner et al. (1988) restrict the term uncon the maximum flooding surface), the unconformable formity and sequence boundary to surfaces that sequence boundary has remained the key element in include significant subaerial exposure. This burdens sequence stratigraphy. The limited resolution of seis the term with a genetic connotation that is often diffi mic profiles very often does not allow one to identify cult to verify in the field and nearly impossible to veri the systems tracts. What generally can be done, how fy in seismics. Furthermore, this restrictive definition ever, is to recognize sequences and sequence bound of sequence boundary "defines away" any alternative aries as unconformities and sediment packages with to sea level in the interpretation of sequences and internally coherent bedding patterns respectively. sequence boundaries (see Schlager, 1991, for discus We strongly recommend using the original defini sion). The successful application of sequence-strati tion of sequence and sequence boundary by Mitchum graphic techniques to terrestrial deposits far removed in Vail et al. (1977, p.531). Mitchum defines sequence from fluctuating sea level or lake levels, merits men as "a stratigraphic unit composed of a relatively con tion in this context (e.g. Hanneman & Wideman, formable succession of genetically related strata and 1991). It illustrates that packaging of sediment accu bounded by unconformities or their correlative con- mulations into unconformity-bounded sequences is a 28 Sequence and Systems Tracts 29 general principle that only happens to have been dis Ecologic reef, stratigraphic reef, covered in the sea-level domain. seismic reef "Was this reef really a reef?" This question contin Seismic unconformity .vs. outcrop ues to fuel heated discussions among geologists (see unconformity James & Bourque, in press, for review). Dunham (1970) introduced a novel perspective in this debate Perhaps the most significant contribution of when he proposed to qualify the term and distinguish sequence stratigraphy is the discovery that sedimenta between ecologic reefs, built and bound by organisms, ry basin fills are dissected by unconformities that can and stratigraphic reefs where the binding is done by be used to subdivide the sediment accumulations into cementation. This distinction has proved very useful mappable, genetically coherent units. Unconformities, but the increasing use of seismics to identify reefs has of course, have long been recognized in outcrop. One complicated matters again. The "seismic reefs" must emphasize, though, that seismic unconformities defined by geometry and reflection character do not and outcrop unconformities do not match. Three dif correspond to either ecologic reefs or stratigraphic ferent situations may occur: reefs. Seismics tends to overlook certain reefs and, in (1) Boundaries where in outcrop and seismic profile other circumstances, add non-reef deposits to the reef. the time lines abut against a surface. This type is Living parts of reefs are small by seismic standards, unproblematic because both the field geologist and easily destroyed and quickly buried by their own the seismic interpreter will recognize this feature as an debris. The limited resolution of the seismic tool unconformity. reveals reefs only where many generations of reef (2) There are outcrop unconformities that cannot be growth were stacked vertically to thicknesses of hun seen by the seismic tool because their geometric dreds of meters. Where the locus of reef growth has expression consists of a microrelief on the scale of cen been forced to shift laterally, reefs form small lenses timeters or decimeters, while the large-scale bedding embedded in detritus sediment. These lenses will be of the two units remains parallel. This is common in seen by the field geologist but not by the seismic tool. deep-sea sediments. (Where porous, these reefs will also be "seen" by (3) Finally, the seismic image may show unconfor migrating hydrocarbons, of course.) mities that correspond to transitional boundaries in Equally important are those situations where the outcrop. Here, two radically different situations must seismic tool shows more "reef" than is actually there be distinguished. in a stratigraphic or ecologic sense. Reefs defined by (3a) Seismic unconformities where time lines con their geometry and the incoherent reflection character verge into an interval of continuous, but very slow normally include, besides ecologic reefs, also the reef sedimentation. The limited resolution of the seismic aprons on the landward side as well as the coarse fore tool may portray this thin interval as a lap-out surface. reef debris where it is unstratified (Figure 4-14). In The condensed interval meets the definition of uncon addition, sand shoals interfingering with reefs may be formity by Vail et al. (1977) in so far as all deposits included in the seismic unit. below it are older than the oldest deposits above the interval. The difference with this definition lies solely in the fact that "non-deposition" needs to be replaced Valley cutting, fan building by "slow deposition". For the seismic interpreter this is a minor difference, but one that has to be kept in The model of systems tracts assumes that sea level mind when seismics is tied to boreholes or outcrops. controls the presence or absence of submarine fans as The deepening-upward succession on many drowned well as their position higher or lower on the toe-of platforms is an example of this "transitional seismic slope. A view of modern oceans and hydrodynamic unconformity" (Figure 5-10; Erlich et al., 1990, figure theory indicate that fan development is largely a func 11; Campbell, in press). tion of the transport capacity and competence of tur (3b) The most disturbing kind of seismic unconfor bidity currents. These parameters, in turn, depend on mity is the pseudo-unconformity, where a rapid facies sediment supply and topography (slope angle, size of change occurs in each bed at a similar position and the valleys to channel the flows etc.). Sea level can influ seismic tool merges these points of change into one ence sediment supply and, to a lesser extent, topogra reflection. Time lines cross this reflection, thus it is not phy but in doing so it must compete with other pro an unconformity in the sense of Vail et al. (1977) and cesses. needs to be recognized and eliminated before For carbonate platforms, Schlager & Camber (1986) sequence analysis is done (Figures 4-8 through 4-13). have proposed a model of slope evolution as a func Carbonate platforms are particularly prone to this tion of the height of the platform (Figure 1-21). This effect because rapid facies changes may occur on sev model predicts that the depocenter will shift basin eral places in the edifice and may go hand in hand ward as the slope angle increases. Steep slopes can be with significant changes in physical properties (e.g. bypassed and even eroded by turbidity currents and carbonate-shale transition at the toe of slope, or the fans can onlap these slopes irrespective of sea-level boundaries of reefs). position. The position and shape of the gravity-dis- 30 Schlager placed sediment bodies is largely a function of topog favor the development of erosional breaks that, in raphy (Figures 4-15, 4-16). Carbonate sediments, in turn, generate asymmetries by preferentially destroy particular reefs, are known for their ability to rapidly ing one limb of a cycle (e.g. Kauffman, 1984 on the build steep relief (e.g. Figures 1-14, 1-15). Depocenters Western Interior Basin). Recently, the case for gradual of gravity-displaced sediments will shift equally shifts and symmetrical cycles has been strengthened rapidly in response to the changes in sea-floor relief. by detailed studies on stacking patterns within low Thus, fans and submarine canyons in carbonate ter order cycles. It turns out that the stacking patterns in rains are probably rather unreliable indicators of sea low-order cycles often reveal gradual changes even level movements. where the individual high-order cycles are punctuat ed and asymmetric (Figure 4-18; Read & Goldham mer, 1988; Goldhammer et al., 1990; Goldhammer et Abrupt breaks versus gradual shifts in al., 1991). sequences It seems that the sediment record in the million year domain is less punctuated and more symmetric The classical sequence model is characterized by than the cycles in the ten-thousand year domain. In abrupt breaks at the sequence boundary, and, to a view of what has been said above about the seismic lesser extent, at the transgressive surface and the tool displaying rapid transitions as unconformities, maximum flooding surface. The punctuation of the one may ask to what extent the punctuated nature and record and the concomitant asymmetry is well dis pervasive asymmetry of the sequence-stratigraphic played in the sawtooth pattern of the coastal onlap models is an exaggeration inspired by the peculiarities curve (Vail et al. 1977) as well as the pattern of sys of the seismic tool. tems tracts (Figure 4-1). There can be no doubt that Sequence stratigraphy, to be sure, can be practiced these punctuations and asymmetric patterns exist with symmetric cycles too. Goldhammer et al. (1991) (e.g. Van Wagoner et al., 1990). However, there also have demonstrated this in exemplary fashion. How exist many well-documented examples of near-sym ever, in the absence of drastic events, correlation of metrical cycles and gradual shifts of facies belts in the sequence boundaries and systems tracts becomes 105-106 year domain, e.g. Figures 4-10a, 4-17, or much more arbitrary and sequence-stratigraphic anal Kauffman et al., 1977 on the Cretaceous of the West ysis converges with the tried and true sedimentologic ern Interior. The tendency towards gradual shifts and practice of delineating shoaling, coarsening and thick symmetrical cycles is most pronounced in deeper ening trends or their reverse, in a section (e.g. Gold water settings, whereas near-shore environments hammer et al., 1991). Sequence and Systems Tracts 31 INCISED \lALLEY livfl A) IN DEPTH CORRELATIVE CONFORMITY (SEQUENCE BOUNDARY) UNCONFORMITY HSTI CONDENSED SECTION TSTI ~ .... , SUBAERIAL HIATUS ,.,. : UNCONFORMITY~ ., ,.,,. '--- ;: cy: ' ;~-zfii3B SBI- .r j;* DISTANCE CORRELATIVE CONFORMITY BJ R GEOlOGIC TIME (SEQUENCE BOUNDARY) LEGEND t,:;·::;,.j ALLUVIAL l ,JJN) MARINE SILT, MUDSTONE if!l COASTAL PLAIN f=::.::=:=J MARINE SHALE C) ESTUARINE/FLUVIAL (:·:·:{{!;:) DEEP-WATER SANDS ( i J SHOREFACE/DELTAIC SANDS Figure 4-1. Stratigraphic sequences and their systems tracts as defined in sequence stratigraphy. (After Vail, 1987) 32 Schlager TIME --.....-:-t---t:'-r-~ EUSTACY ~ Lowstand SystarM ll'act (lST) [;j lowstand Basin Floor Fan (bf) !]] Lowstand Slope Fan (If) ~Lowstand Wedge-Prograding Compte• (law) TECTONIC SUBSIDENCE ~ Transgressive SyaterM ll'act (TST) 0 HlgMtand SyaterM ll'act (HST) D Shelf Margin Systenw Tract (SMST) RELATIVE CHANGE OF SEA LEVEL LST TST HST SMST LEGEND SURFACES SYSTEMS TRACTS (SB) SE~NCE BOUNDARIES HST = HIGHSTAND SVSTEMS TRACT (SB 1) =TYPE 1 TST ~ TRANSGRESSIVE SVSTEMS TRACT (SB 2) ~ TYPE 2 lvf = lnciMd wiley fll (Dl.S) DOWNLAP SURFACES LST = LOWSTAN) SVSTEMS TRACT (mts) ~maximum flooding -lac:• 1¥1 = Incited wiley .. (Ibis) ~ top 118M\ noor tan -tac• llw = lowaUnd wedge-prograding complex (tats) = top slope tan ...race If =lowetand elope f8n (TS) TRANSGRESSIVE SURFACE bl =lowetand baetn floor tan (First lloodlng -tac• above fc =tancMnnela m.. lmum progr..s.tlon} " ~ f8n Iobei SMST = SHELf MARGIN SVSTEMS TRACT Figure 4-2. Relation of systems tracts to sea-level cycle as proposed by sequence stratigraphy. (After Vail, 1987) a)o bP E ,.,_ ,.,_E c: c .c a. .cc. Cl) Cl) lJ lJ L L distance in basin (kml- 0 90 Figure 4-3. Variations in sediment supply and sea level fluctuations have a similar effect on sequence patterns. This example shows the synthetic strati~raphy of the Permian-Triassic Akhdar Group in Arabia. (After Aigner et al., 1991, modified.) (a) Synthetic stratigraphy generated by sinusoidal variations in sediment supply. (b) Synthetic stratigraphy generated by sinusoidal fluctuations of sea level. Sequence and Systems Tracts 33 - - Figure 4-4. Sequence boundary in deef Gulf of NW GULF OF MEXICO Mexico related to change in sediment mput and UTIG Line GT2-3b dispersal. The carbonate platform on the right was drowned in the wake of a mid-Cretaceous anoxic event and later covered with pelagics. As carbonate 4.0 input ceased, the debris apron of platform was gradually buried by clashc sediments transported at right angle to the profile. (Campeche Bank, Gulf of Mexico, line courtesy of R.T. Buffler, Texas Institute for Geophysics.) 8.0 sec twt .1 .2 .3 .4 CONTOURS post-MCU siliciclntics Canom-l.l'lleacane pre-MCU Cllrbonlltes Albian-early Cenom. FLORIDA PLATFORM 0 60 KM CAMPECHE PLATFORM CUBA .3 .2 .1 0.0 1.0 Figure 4-5. Isopachs above and below the mid-Cretaceous unconformity-a major sequence boundary in the Gulf of Mexico characterized by drowning of most platforms in the Gulf rim. Pre-MCU isopachs reflect the input of carbonate debris from the platforms, post-MCU isopachs reflect the growth of a flysch wedge, deposited in front of the advancing Cuban island arc. The unconformity represents a fundamental change in the sediment-input pattern of the Gulf. (Modified after Phair in Schlager, 1989.) 34 Schlager "ttlckness• in two-way traveltime (seconds) SIGSBEE Pleistocene CNCO DE MAYO late Mocene tiYOI.Ul Plocene lPPER I'.EXICAN RDGES midde Tertiary (?) to late IViocene LOWER I'.EXICAN RIDGES earty(?) to mldde Tertiary(?) after ShatJ) et al, 1984 Figure 4-6. Isopachs of Cenozoic seismic sequences in the Gulf of Mexico (After Shaub et al., 1984). Sequences were defined and mapped following the principles of Vail et al. (1977). Note that each sequence shows a different pattern of sediment input. The number of sequences and their ages do not correspond well with the Cenozoic sea-level curve of Haq et al. (1987). We postulate that they are controlled by changes in sediment input that reflect tectonic events in the hinterland of the Gulf. Sequence and Systems Tracts 35 WEST RIMBU EAST WEST MUDOIVIROO« ,, ... ,...,.. CH,ji#J ALBERTA "DEEP BASIN"" WEST SHALE BASIN EA$1 SHALE BASIN Fi~ure 4-7. Generalized sequence stratigraphic cross section of Upper Devonian deposits in central Alberta. Dtrection and location of clinoforms as well as distribution of reefs, platforms and shale wedges indicate that all sequence boundaries shown represent important changes in sediment input and dispersal patterns. This does not exclude sea level as a possible control, but it strongly suggests that other factors besides sea level strongly influenced the sequence pattern. (After Stoakes & Wendte, 1988. Reproduced with permission of Canadian Society of Petroleum Geologists.) Figure 4-8. The difference between seismic unconformity and outcrop unconformity in the Sella mountains of the Southern Alps. (After Stafleu & Schlager [1991] and J. Stafleu, written communication.) The Late Triassic platform of the Upper Schlem Fm. prograded by many hundreds of meters while aggrading by tens of meters only. Platform growth was ultimately terminated by terrigenous deposition of the Raibl Fm. This situation was surveyed in the field and subsequently modelled seismically (see Figure 4-9). 36 Schlager a N ~ ~ 0 0 0 llll. 500. IIIII. IIIII. lllll. 1500. mi. lllll. 15111. 2500. m. .Jill. lilll. MI. 4GIIl. 41111. b 0 511. 500. IIIII 1110. Ill!. 1500. 2IJI) 2IJI) Zlll. 2m .1111. Jill Jill. ». mi. 41111. Figure 4-9(a). Outcrops shown in Figure 4-8 were used to construct a lithologic model indicating rapid progradation and an alternative model based on the unlikely assumption of genuine toplap. (After Stafleu & Schlager [1991] and J. Stafleu, written communication.) Figure 4-9(b). Synthetic seismic profiles of the progradation model and the toplap model, using vertical incidence and 20Hz wavelet (top right). Note that the thin undaform beds (=topsets) are not resolved and the synthetic of either model portrays the clinoforms toplapping against the terrigenous unit. (After Stafleu & Schlager [1991] and J. Stafleu, written communication.) Sequence and Systems Tracts 37 Figure 4-9(c). Synthetic profile of the progradation model (time section) at 20Hz (top) and 50 Hz (bottom). At 50 Hz the transition from clinoforms into undaform beds is visible as a separate event below the base of the terrigenous umt. However, the relationship between dinoform and undaform bedding is shown as toplap, i.e., as a pseudo unconformity. (After Stafleu & Schlager [1991) and J. Stafleu, written communication.) 38 Schlager a RAIILGROUP 0 fl) fl) zw ~ 0 :E a: 5 1- 1- DISTANCE km b 0.1 w :& ~ 0.2-- >--c ~ I 0 0.3 3: 1- Cl) Qz 0 o;:; 0w (I) 0.5 Figure 4-10. Pseudo-unconformities in seismic models of outcrops (after Rudolph et al., 1989; Schlager et al., 1991; Biddle et al., 1992). (a) Platform-to-basin transition, Picco di Vallandro, Triassic, Southern Alps. (b) Seismic model of platform-basin transition, showing several unconformable relationships. Only one of these seismic unconformities (open arrows) was portrayed as an unconformity in the lithologic model; all other lap out patterns correspond to rapid lateral facies changes and represent pseudo-unconformities (black arrows). Sequence and Systems Tracts 39 F 100M B G F .650 D ...... fl) -w .7001 ~ i= .750 '.· f.'. i 100 M Figure 4-11. Example of pseudo-downlap surface in the Triassic platform-basin transition. Change from carbonate muds and breccias into (mostly terrigenous) basin sediments at base of a prograding platform slope is shown as pseudo-downlap. (After Rudolph et al., 1989. Reproduced with permission of Geological Society of America) 40 Schlager D o- 200m A - IOULDERSI"--J DOLOMITE 'r-:i. LIMESTONE ~ INTERBEDDED LIMESTONE, ARGILlACEOUS LIMESTONE, AIIIO MARL ~ 100M Figure 4-12. Example of pseudo-onlap in the Triassic platform-basin transition. Toe-of-slofe retreats but keeps shedding debris in the form o meter-thick tongues of megabreccias that extend into the basin. The seismic model images this situation as onlap of basin sediments on the clinoforms. (After Rudolph Figure 4-13. Increased seismic resolution can solve et al., 1989. Reproduced with permission of the problem of pseudo-unconformities. When Geological Society of America) frequency is increased from 25 Hz to 100Hz, the pseudo-onlap of the Triassic example is correctly displayed as an interfingering pattern. (Mter Staileu & Schlager et al., 1991) Sequence and Systems Tracts 41 AG-4 sw NE "'u 2 w "'w ::1! f: 3 z 0 f: ~ 4 ..J IL w a: ·~-- ISQm 5 ns 8 U 300 Fi~ure 4-14. Margin of the West Florida :platform in seismic profile. The margin shows a "seismic reef" (labeled as "margin facies") with elevated top and Figure 4-15(a). Holocene highstand wedge on a incoherent reflections, but dredge hauls and gentle, accretionary slope. Note maximum thickness submersible dives revealed this "reef'' to consist of on upper slope. (Western margin of Great Bahama platform-interior deposits (after Corso et al., 1988). Bank, from unpublished surveys by W. Schlager Seismic reefs are not necessarily ecologic reefs in and A.W. Droxler, interpretation as in Wilber et al. outcrop. 1990.) Figure 4-15(b). Holocene highstand wedge at the foot of a steep, erosional slope. Pedro Bank on Nicaragua Rise. (After Glaser et al., 1991). In comparing (a) and (b), note that position of , ...... 1... sediment wedge is a function of slope angle rather t3o~---SLOPE -;.. BASIN ,,...... 1I than sea-level position. Both wedges formed in the ~,~':______...... _ last 8000 years. Reproduced with permission of 120 Elsevier Science Publishers. . ,~--- >: 110 ,., • eE &,,,,•• ,, : c: tOO fl·~;, : ...__.LIB TRANSECT, 2-3• .§ ,,' : •--•I!XUMA TRANIECT, 10-11° !i 10~ ~ j :: g,~~~...~. ~ \ ------· ~~~!!!------. -· ~-~.. ------10 • • • Km Figure 4-16. Accumulation rates of various Neogene time intervals of the Little Bahama Bank and Exuma Sound slope transects. Note that on the gentle slope (i.e., LBB), sedimentation rates are highest on the slope and decrease basinward. On the steep Exuma slope, rates in the basin are higher than on the slope, which is interpreted as evidence for more efficient basinward transport by turbidity currents with increasin~ slope angle. This effect explains the different positions of the highstand wedges in Figure 4-12. (After Austin, Schlager et al., 1988. Reproduced with permission of Ocean Drilling Program) 42 Schlager FACIES MOSAIC ACROSS DEPOSITIONAL STRIKE LOWER ORDOVICIAN, CENTRAL APPALACHIANS EAST 275 KM ------~ (RESTORED) v; 500 0:: w 1- TIDAL w ~ 400 (/) (/) w (/) ~ 300 w u u J: I- ~ I- 200 z z w (/) w :f I 00 CLEAR SPRING HAGERSTOWN FREDERICK MD MD MD Figure 4-17. Gradual facies shifts in Early Ordovician carbonates of the Appalachians. Note absence of abrupt breaks that could represent sequence boundaries. The gradual shifts in the million-year domain contrast with the largely asymmetric, punctuated cycles in the ten-thousand-year domain. (After Hardie &: Shinn, 1986. Reproduced with permission of Colorado School of Mines Press) Sequence and Systems Tracts 43 b history of sea level sedimentation subsidence synthetic stratigraphy Ill -..~ TST .,... 0 Ill c -sb•!- ~ 0 i i :J Condensed 1 I Megacycle E II> E Rhythmic =: J Megacyle HST - J Amalgamated - I'QfS- - Megacycle TST 0~~ -25m +25m Sea Level Amplnude Figure 4-18(a). Gradual and largely symmetrical changes in cycle-stacking patterns, Triassic Latemar platform, Dolomites, Northern Italy. (After Goldhammer et al., 1990.) Cycles change from mainly subtidal deposits at the bottom (LPF) to subtidal-supratidal cycles (LCF) to tepee-rich cycles with thin subtidal members (TF) and back to subtidal-supratidal cycles (UCF). The pattern shows a rather symmetrical swing from subtidal to exposure-dominated facies and back. Figure 4-18(b). Model of the Latemar cycle stacking produced by computer program "Mr. Sediment" (after Golhammer et al., 1990). Synthetic stratigraphy on the left and model of sea level, subsidence and sedimentation on the right. Sequence-stratigraphic classification of synthetic stratigraphic column: TST, HST =transgressive and higlistand systems tract, respectively; sb =sequence boundary; mfs =maximum flooding surface. Note that with gradual change such as these, the sequence boundary and the maximum flooding surface are no longer unique horizons, their position becomes to some extent arbitrary. CHAPTERS CARBONATES VERSUS SILICICLASTICS IN SEQUENCE STRATIGRAPHY The principle of depositional bias leeward margin in Figure 5-1). However, even in these loose-sediment dominated prograding margins, one In chapter 4 we concluded that sea level fluctua finds reefs or sand rims. We believe that the shelf-mar tions are but one of several ways to generate gin elevation of these prograding platforms yields the sequences and systems tracts. This claim is based on a most reliable sea-level record in carbonate seismic sedimentologic evaluation of sequence anatomy. stratigraphy (e.g. Eberli & Ginsburg, 1988; Sarg, 1988). Where sea level does exert the dominant control on sequence stratigraphy, sedimentology must be called upon again, this time to assess the differences of the Highstand shedding sea-level records of depositional systems. Siliciclastics and carbonates are the two most prominent examples It is a well-established fact that in the Pleistocene, that come to mind, but evaporites, too, have their spe siliciclastic sediment supply to the deep sea was at its cific ways of recording sea level. Depositional systems maximum during glacial lowstands of sea level. resemble newspapers that all report on the events of Rimmed carbonate platforms were in antiphase to this the day, but each with different editorial bias. It rhythm. They produced and exported most sediment behooves the reader to learn about the editorial bias of during interglacial highstands when the platform tops his paper. Similarly, the geologist ought to know were flooded. This "highstand shedding" is best doc about the bias of depositional systems in recording umented for the Bahama Banks (Figure 5-2; Droxler et changes of sea level (and other environmental fac al., 1983; Mullins, 1983; Reijmer et al., 1988). The pat tors). Below, we discuss some peculiarities of carbon tern also appears in platforms of the Caribbean, the ates vis a vis siliciclastics. Indian Ocean (Droxler et al., 1990) and the Great Bar rier Reef (Davies et al., 1989). See Schlager et al. (in press), for review. Defended platform margins Highstand shedding is a general principle of car bonate sedimentation because of the combined effect We have mentioned above that carbonate platforms of sediment production and diagenesis. Sediment pro have a strong tendency to form elevated margins that duction of a platform increases with its size, and the build to sea level right at the shelf break, protecting a production area of a platform with its top exposed is slightly deeper lagoon that gently rises to the littoral normally smaller than the production area of a flood zone farther landward. When these rims are over ed platform. In the Bahamas, the flooded top is one whelmed by sea level, they cause facies belts to jump order of magnitude larger than the belt of shallow and interrupt the gradual shift in onlap. This jump water production during lowstands (Droxler et al., must not be confused with a sea-level fall and 1983; Schlager et al., in press). The effect of increased sequence boundary. Furthermore, elevated rims have highstand production is enhanced by the rapid lithifi a strong tendency to stack vertically, putting reef on cation of carbonates during lowstands. Siliciclastics reef, sand shoal on sand shoal, because the environ owe part of the high sediment input during lowstands mental conditions favor the establishment of a new to erosion of the preceding highstand deposits. Car reef on top of an old reef, an oolite shoal on top of an bonate diagenesis largely eliminates this effect. In the older shoal. In this way, the lateral migration of sys marine environment, there is a strong tendency for the tems tracts in response to sea level is interrupted (e.g. sea bed to lithify and develop hardgrounds wherever windward margin in Figure 5-1). Prograding margins waves and currents interrupt continuous sedimenta dominated by offshore sediment transport more close tion (Schlager et al., in press). Thus, widespread hard ly resemble the classical sequence model. They are grounds will protect the highstand deposits when sea controlled by loose sediment accumulation and level starts to fall and wave base is lowered. When approach the geometry of siliciclastic systems (e.g. sediments finally become exposed, they tend to devel- 44 Carbonates Versus Siliciclastics 45 op an armor of lithified material within hundreds to a exposed platforms. Reefs are "born" as rock-hard few thousands of years as borne out by the numerous structures, other platform deposits usually lithify rocky Holocene islands on extant platforms (e.g. Hal within a few thousand years when exposed. Subse ley & Harris, 1979). Subsequent erosion acts largely quent erosion is largely chemical and directed into the through chemical dissolution, enlarging porosity and rock rather than at its surface (see above). Surface creating cave networks but keeping surface denuda denudation is generally less than in siliciclastics and a tion lower than in siliciclastics. reasonable sea-level record can be gleaned from plat Highstand shedding creates highstand wedges of forms by determining overall subsidence and measur sediment, displayed in exemplary fashion by the ing the thickness of marine intervals plus position and Holocene sediment of extant carbonate platforms timing of exposure horizons (see Figure 5-5 and Lud (Figure 4-15). Furthermore, turbidites are more fre wig et al., 1988; McNeill et al., 1988). quent in highstand intervals, forming highstand bun The combination of defended margins and dles, (Figure 5-6). enhanced resistance to erosion creates some special Platforms are not completely shut down during opportunities for sequence stratigraphy. Rapidly pro lowstands. Sediments continue to be produced from a grading platform margins tend to preserve the origi narrow belt of sands and fringing reefs, and from nal elevations of the shelf surfaces particularly well, eroding sea cliffs that cut into the exposed platform. including the very important lowstand systems tracts. There is no evidence, however, that the lowstand Eberli & Ginsburg (1988) and Sarg (1988) have con input reaches anywhere near the volume of highstand tributed excellent examples of sea-level curves sediments and the postulated apron-building around gleaned from carbonates using the technique of the Pacific atolls during lowstands of sea level has not fluctuating shelf surface (Figure 5-6). withstood close scrutiny (see Thiede et al. [1981] vs. Recently, sea-level studies of platform tops have Dolan [1989]). Furthermore, lowstand input is compo been complemented by compositional analysis of sitionally different and can be recognized by petro- platform-derived turbidites on the platform flanks. graphic analysis (e.g. Everts, 1991). . The depositional environment of these calciturbidites The limitations of highstand shedding can be is below the range of sea-level fluctuations so that sed deduced from the above discussion of its causes. The imentation is not interrupted during lowstands. High difference between highstand and lowstand prod uc stand turbidites differ not only in abundance but also tion depends on the hypsography of the platform. in composition from their lowstand counterparts as Flat-topped, rimmed platforms with steep slopes they contain more ooids, pellets, and show more pronounced highstand shedding than grapestones-grains that require flooded bank tops ramps. If there is no flat top at all and sea-level simply for their formation. This has been well documented moves up and down an inclined plane, the difference for Pleistocene turbidites in the Bahamas (Figure 5-7). between highstand and lowstand production will be Reijmer et al. (in press) report on a Triassic example zero. Figure 5-4 shows that besides platform morphol (Figure 5-8), Everts (1991) on a Tertiary one. ogy, the duration of sea-level cycles is important. Carbonate petrography reveals not only changes in When sea-level cycles are long (e.g. millions of years) the spectrum of contemporary grains. Lithoclasts and flanks gentle, the platform can build a lowstand derived from erosion of older, lithified parts of the wedge that partly substitutes for the production area platform are easily recognized. We must emphasize, lost on the platform top (see next section for further however, that erosion may cut into older material on discussion). Finally, lithification and the resistance the flanks at any time during platform history, regard against lowstand erosion vary with latitude and less of sea-level position (see Figure 1-21). During the possibly with age. Figure 5-3 illustrates the decrease present highstand, old limestone is exposed and erod of marine cementation with latitude. Cool-water car ed in many places on the Bahamian slopes and rim bonates are less prone to submarine lithification than collapse caused high admixture of lithoclasts in a their tropical counterparts. Lithification upon expo debris flow from the Sangamonian highstand (Crevel sure, too, is reduced because of the low content of lo & Schlager, 1980). metastable aragonite and magnesian calcite. It is pos Limitations on the compositional tool are the same sible that Paleozoic carbonates with their generally as for highstand shedding. A platform that builds a lower content of metastable minerals are also more lowstand wedge during an extended sea-level cycle prone to lowstand flushing. Lowstand shedding of may well be able to produce highstand material on Tertiary cool-water carbonates has been described by this auxiliary platform top. Driscoll et al. (1991). Drowning and drowning unconformities Sea-level records specific to carbonates Siliciclastic systems can build to sea level from any With their flat tops built to sea level and their biota depth, the rate of aggradation being simply a function very sensitive to water depth, carbonate platforms are of sediment input. For reefs and carbonate platforms, one of the most reliable dip sticks in the ocean. This on -the other hand, there exists a maximum depth from quality is enhanced by the resistance to erosion of which they can rebound. This maximum depth of 46 Schlager rebound is the limit of the photic zone because carbon Many drowning events are represented by a grada ate production in the aphotic depths of the ocean is tional succession of facies that record the increasing negligibly small. Thus, platform growth can be termi water depth on the platform top. A characteristic ele nated by a relative rise whose rate exceeds the growth ment of this drowning succession is high-energy potential of the platform in question. A sequence grainstones, often rich in echinoderm debris, that boundary will be formed by the drastic change in sedi appear sandwiched between muddy lagoonal ment composition and in sediment dispersal patterns deposits below and deep-water muds above. Drown that accompany platform termination. Geometrically, ing succession may include numerous small hiatuses this sequence boundary resembles a lowstand uncon but the overall pattern is one of gradational, albeit formity. In reality, however, it must form during a rise rapid, change from platform interior to pelagic or or highstand of sea level because drowning can occur hemipelagic deposition. Erlich et al. (1990) illustrate only when the platform top is flooded (Schlager, 1989). how recognition of the drowning succession critically Drowned platforms and drowning unconformities are depends on seismic resolution (Figure 5-10). common in the geologic record (Figures 5-9, 5-10, 5-12). At least some of them have been interpreted as the result of major lowstands and this may explain some Structural control on reefs and platforms discrepancies between the sea-level curve from sequence stratigraphy and curves derived by other There are numerous examples for structural control techniques (Figure 6-5). Raised rim and empty bucket of the position of reefs and platform margins. Rather are common with drowning unconformities and pro than adding to these examples, we would like to vide a good criterion for their recognition. One prob emphasize the opposite quality, the "healing power" lem with the seismic interpretation of carbonate-silici of reefs and platforms. Through their vast in-situ pro clastic boundaries is the question of relief on the basin duction of sediment, platforms are able to quickly fill ward side. Interfingering and onlap relationships, i.e. in structural relief and prograde away from inactive pseudo-unconformities and genuine unconformities, structures, responding to environmental factors such are sometimes extremely difficult to distinguish (Fig as wind and currents. In this way, structural control ures 5-9 through 5-12). soon is lost unless continually renewed by synsedi The sediment record of platform drowning is high mentary movements (Figure 5-13). ly variable. Campbell (1992) pointed out that drown After deposition, reefs and platforms often attract ing unconformities, too, illustrate the mismatch deformation along their flanks and over their margins between outcrop unconformities and seismic uncon because of preferential compaction of the surrounding formities. Only a fraction of the drowning unconfor sediments. (Figures S-9, 5-10,5-12). This phenomenon, mities includes a significant hiatus and thus meets the caused by the extensive early lithification of platform definition of unconformity in the classical sense. rims, is a specific attribute of rimmed platforms. Carbonates Versus Siliciclastics 47 Fi9ure 5-1. Windward (left) and leeward (right) bank margins in the Bahamas. Note strong evidence for se1smic reefs where margin is stationary and reefs are allowed to stack vertically for extended periods. Apparent lack of reefs on the rapidly migrating leeward margin is probably caused by rapid shift of reef growth and frequent interruption by sediment dumping (compare with the leeward margin of Figure 2-5). Relative elevation of the shelf edge in the prograding margin is probably the best seismo-stratigraphic sea level indicator in carbonate platforms. The profile also illustrates the effect of sediment supply on sequence geometry. Supply is hi~h on the leeward margin (right) and therefore the margin progrades. The windward margin (left) receives httle sediment and builds up vertically. (Line courtesy of Texaco Inc.; interpretation after Eberli & Ginsburg, 1988) 48 Schlager Figure 5-2(a). Sedimentation rates during a highstands and lowstands of sea level in various Peri-platform mud Carbonate depositional systems. In terrigenous muds from the Bahamas continental rise, rates are low and uniform during 30 inter~lacial highstands and high and variable (turbtdite-controlled) during glaciallowstands ("lowstand sheddin§"). In peri-platform muds, the pattern is reversed ( 'highstand shedding"). In pelagic carbonate ooze, rates remain low and uniform in both highstands and lowstands. (After Droxler & Schlager, 1985, modified. Reproduced with permission of Elsevier Science Publishers) en Terrigenous mud w 30 A MARINE PRECIPITATES (..) N Atlantic z w 20 20 N-56 a: a: 10 ::> (..) (..) -60 -40 -20 0 20 40 60 at Glacial 0 PALEOLATITUDE Figure 5-3. Inorganic carbonate precipitation as estimated by the formation of ooids and submarine cements is most intensive in the low latitudes; 0 50 submarine cementation of cool-water carbonates is Sedimentalion rates In mm/ky - only a fraction of the cementation in tropical carbonates. This pattern has persisted thoughout the Phanerozoic. Rapid cementation reduces b reworking and sediment export from the platforms during lowstands of sea level and is partly responsible for the pronounced highstand shedding of tropical carbonate platforms. (After Opdyke & Wilkinson, 1990) Reproduced by permission of Elsevier Science Publishers• ...... mmlka 40 0...... _~~40::...... _,__....;8::.:0::...... _.~0- 40 Figure 5-2(b). Highstand and lowstand sedimentation rates during the late Quaternary in the Bahamas (Atlantic) and Maldives (Indian Ocean). In both instances, rates during glacial lowstands (shaded) are generally lower than during the interglacial highstands of sea level. Other effects may modulate this pattern (e.~. upward decreasing rates in the Bahamian basm site), but they do not erase it. Bahama store =ODP Site 633, Bahama basin =ODP 632, Maldtves slope =ODP 714. Solid bars indicate means with 90% confidence intervals. (After Reijmer et al. [1988], Droxler et al. [1990), modified.) Carbonates Versus Siliciclastics 49 a) STEEP SLOPE, SHORT CYCLES (modern Bahamaa) a) TIME (M.Y. belore present) ~~~;-.--,...... !;'15~~"T"""""""'.---!l!IO--..--.--.,...... ,--:r5-.--.--,...... ,.-;0I.O 100m Enewetak plat'f'orm 1.5 30-90. 40 upper elope 811o -40 -80 2.5 -120 b) GENTLE SLOPE. LONG CYCLES (Neogene Bahamas) oxyaen-1sotope sea level b) TIME (M.Y. before present) 20 15 10 5 0 sequence 200M -... 100 low-atand wedge '...... i Figure 5-4. Highstand sheddin~ is expected to be most pronounced with steep-stded platforms and short cycles (a). Gentle flanks and long cycles probably lead to growth of a broad lowstand wedge that produces bank-top sediment and may dampen the effect of highstand shedding. Figure 5-5. Sea-level record of carbonate r.latform in the Pacific compared with two other sea- eve) indicators: the eustatic curve of Haq et al. (1987) based on sequence stratigraphy and the oxygen isotope ratio, in this time interval largely a eustatic sea-level signal reflecting changes in ice volume. The platform record consists of the subsidence path of exposed bank top (dotted) and short intervals of marine sedimentabon during highstands (arrows). The overall similarity of all three curves points to a strong eustatic signal in all of them, but correspondence between platform stratigraphy and isotopes is significantly better than with the sequence-stratigraphy curve. (After COSOD Report, 1987, modified) 50 Schlager sea-level sea-level Fall----· b) Figure 5-6. Sea-level changes recorded in the fluctuating shelf surface of a prograding Bahama platfonn in the Tertiary. The fact that many platfonns develop hard, erosion-resistant rims makes this classical sequence-stratigraphic technique especially useful in carbonates. (After Eberli & Ginsburg, 1989, Cenozoic progradation of northwestern Great Bahama Bank, a record of lateral platfonn growth and sea-level fluctuations. In Crevello, P.D., Wilson, J.L., Sarg, J.F., & Read, J.R (eds.), Controls on carbonate platforms and basin development, SEPM Special Publication, v. 44, p. 33-351). Carbonates Versus Siliciclastics 51 a 12 13 14 ! :r ...ti: ...0 Q: 0 u 8 GRADED TURBIDITE~ PERIPLATFORM OOZE UNGRADED TURBIDITE Figure 5-7. Highstand bundling and compositional signals in calciturbidites. Quaternary cores from Tongue of the Ocean, a basin surrounded by the Great Bahama Bank. Turbidites are most abundant during interglacial highstands of the sea, when bank tops are flooded and produce sediment; turbidites are thin and scarce during slaciallowstands. Turbidites also vary in composition: Hi~hstand layers are rich in pellets and ooids-t.e., grains that form on the shallow tianks by the interaction of tidal currents and winnowing from waves. Lowstand turbidites consist mainly of skeletal material, includin~ reef detritus, because fringing reefs and skeletal sand can migrate down-slope with fallin~ sea level. Glacial-mterglacial strati~aphy is provided by variation in aragonite/calcite ratio, a property that ts closely correlated witli the oxygen tsotope curve. (After Haak & Schlager, 1989. Reproduced with permission of Geologische Vereinigung.) 52 Schlager a 1. Biota, non-specified 2. Platform-interior 0.4 reef 0 0 3. Shallow 0 4. Deep reef or forereef .0 .. 5. Open ocean biota .. . . •• t- , • '., 00 8. Embedding sediment ' . . . . Oo .. 0-2 0 .: e· •. . • .• •. I • :·.:•• :• • 0 .. .~ . . ·.. .. ., .. -.. : . . ,~ ... ·.··' . > • • .: • : •, • •, II • .. I .• "- • o.o 0 · . ~1 .. 0 ...... • .I • . ""'0 . 0 0 :rJ • •• • • " • 'It •• . . . . . 1\) ... . .!3·. . ··. .. ~ . . . . . 0 . .. : . : . -0.2 ... .0 . . . .· . 0 . . . . : . •5 . . . -0.4 ·4 . . ~----~----~----~----~--~~-----T----~----~-----;----~----·~-0-6 -0.9 -0.7 -o.s -0-3 -0 .} o.J 0.3 o.s 0.7 0.9 1 • I }.3 FACTOR 1 b Figure 5-8. Compositional signals in 1. PLATFORM 2. BASIN calciturbidite-a Triassic example from the northern TOTES GEBIRGE LACKE, GOSAU Alps (after Reijmer et al., 1991; Reijmer, 1991). Point Wavelength Ratioa Wavelength counts and subsequent cluster and factor analysis in m in m on 277 turbidites revealed that turbidite composition varies between two extremes: (1) high content of platform material (grains from the 93.87 24.0 platform interior and shallow reefs) and (2) high 13.0 23.68 content of open-ocean biota such as radiolarians 6.5 11.84 and halobiid bivalves. The variations of turbidite 18.17 ------· 4.7 ------·8.61 composition display a hierarchy of cycles and the 17.07 4.4 inferred periods of the cycles correspond very well 4.0 7.28 with the exposure cycles of the adjacent platform 14.44 3.7 (the Lofer cycles of Fischer, 1964). Reijmer et al. 13.74 3.5 conclude that the variation of turbidite composition 2.7 4.98 4.11 is driven by the flooding and exposure of the 8.94 2.3 ------tops. The inferred duration of the most 8.66 ------2.2 platform 5.03 1.3 2.42 prominent cycles is consistent with orbital control. 4.54 ------1.2 ------2.12 (a) Factor analysis of the compositional data of 277 4.27 ------1.1 ------turbidites. Factor 1, explaining 56% of total 3.91 1 1.82 variance, represents an oscillation between ------platform input (during highstands and platform flooding) and open-ocean input (during lowstands and platform exposure). (After Reijmer et al., in press.) (b) Ratios of the wavelengths of major cycles on the platform and in the basin as determined by Walsh spectral analysis. Wavelengths are expressed as multiples of the shortest cycle, that is, 3.91 m on the platform and 1.82 min the basin. Sedimentation rates ~leaned from biostratigraphy indicate a duration of 15,000....18,000 years for this basic cycle, compatible with its interpretation as the short cycle of the Earth's orbital precession. The similarity of the two spectra supports the notion that turbidite composibon varies m step with the flooding and exposure of the platform source area. (Reijmer et al., written communication.) Carbonates Versus Siliciclastics 53 a WILMINGTON PLATFORM silici clastics Cret.-Tert. plat for• Late Jur . ... B. Cret. elevated platro~m margin with drowning unconformity on seaward side b Figure 5-9(a). Drowning unconformity on Wilmington Platform accordin9 to Meyer (1989) and Schlager (1989). The seaward flank is over 2500 m high and is onlapped by more gently dtpping, almost transparent siliciclastics. Clastics prograde over platform after drowning has shifted the point of sediment onlap by tens of kilometers shoreward. Erlich et al. (1990) propose a much-reduced relief and interfingering of carbonates and siliciclastics. The debate illustrates a typical problem with drowning unconformities. (Profile courtesy of Shell Oil Co.) Figure 5-9(b). Deep lagoons and raised rims (empty buckets) often precede final drowning of a platform and can be used to distinguish drowning unconformities from lowstand unconformities. In the final stages of growth, the rim of the Wilmington platform rose 200 m above lagoon floor. 54 Schlager a) 1000 METERS 1.4 Cii 0z 0 0 w (/) w ~ i= 1.6 Platform 3 (Late·growth buildup) b) I Platform 2 A€; : : : : : : :.:--~..-!r;:...--L-r--J-;--.--JIL-.--=0~, Figure 5-lO(a). Seismic profile of a "late growth reef" rising above the drowned platfonn top. Miocene, Pearl River Mouth Basin, South China Sea. In this instance the amount of relief at the time of drowning was confinned by drilling. Figure 5-lO(b). Successive stages of drowning and backstepping in Miocene platfonn, Pearl River Mouth Basin. (After Erlich et al., 1990). Successive backstepping of platfonns prior to complete drowning is a very common pattern of carbonate platfonns and commonly produces excellent stratigraphic traps. Carbonates Versus Siliciclastics 55 N.W. S.E. "JOB CREEK" WF19 G.A.-6 G.A.·3 WF 16 G.A.-4 G.M.-1 WF 14 D.J.-1 0 1mi G.M.-2 500m ALEXO I 1 1:1 l:J· Job Creek reef-edge (Shell fieldwork 1979), incorporating unpublished Shell measured sections. Figure 5-11. Margin of Devonian Leduc platform, showing back-stepping mar~in onlapped by shale below and carbonate-shale interfingerin~ at the top (Alberta, Canada). Thts example ts based on outcrop observations. In seismic profiles, mterfingering and onlap relationships may be virtually indistinguishable. (After Andrews, 1988; published with permission of Canadian Society of Petroleum Geologists) 011 Morocco 10 km line cour1esy Euon Co. Figure 5-12. Unconformity on a Cretaceous-Jurassic platform off Morocco (dotted). This se9-uence boundary was attributed to a Valangiman lowstand by Vail et al. (1977). Schlager (1989) proposed an alternative interpretation as a drowning unconformity. Note the similarity with the coeval Wilmington platform in the nearly conjugate American margin. Note furthermore the raised rim and differential compaction, causing folds in the overlying siliciclastics. Arrows mark the position of boreholes. 56 Schlager -- / Ortist-i 0 ----- 0 Cortina d'Ampezzo Platform margins ~ Latest Anisian ~ Early Ladinian 10km Figure 5-13. Synsedimentary fault control on platforms is quickly modified and obliterated by progradation and basin-filling when fault movements cease. Triassic, Southern Alps, Italy. (After Bosellini, 1984, modified) CHAPTER6 ALTERNATIVES TO EUSTASY IN SEQUENCE CONTROL Classical sequence stratigraphy holds that succes ment are particularly important for reefs and carbon sion and geometry of sequences are controlled by rela ate platforms and will be discussed here. tive changes of sea level (e.g. Vail et al., 1977; Haq et Marine carbonate sediment is extracted directly al., 1987). The theory postulates furthermore that a from the dissolved load of sea water. Thus, carbonate global stack of relative sea-level curves from sequence production is intimately tied to the chemistry and stratigraphy reveals a strong eustatic signal that domi other environmental conditions of the ocean. This is in nates the record even in tectonically mobile settings stark contrast to siliciclastics, for which the ocean is such as convergent ocean margins. Consequent appli merely a transport medium of material ultimately cation of the above concepts has produced the most derived from land. widely cited eustatic sea level curve to date (Haq et The rate of carbonate production is an important al., 1987). This curve has proved very useful in many control on sequences and systems tracts. Changes in instances, particularly in periods with independent ocean environment can directly modulate the growth evidence for strong glacio-eustasy. However, the and thus the sequence geometry of reefs and carbon curve has also generated a growing wave of concern ate platforms. Environmental change may, but need and reports of serious discrepancies. We refer to Miall not, operate in tandem with sea level. Platform (1986), Hubbard (1988) and Underhill (1991) for cri drowning is a case in point. tique of the general concept. This chapter restricts Drowning requires that the reef or platform be sub itself to questions specifically related to sequence merged to subphotic depth by a relative rise that stratigraphy in reefs and carbonate platforms. exceeds the growth potential of the carbonate system. We contend that, particularly in carbonates, eustasy The race between sea level and platform growth goes is less dominant than postulated by classical sequence over a short distance, the thickness of the photic zone. stratigraphy and that other processes contribute signifi Holocene systems indicate that their short-term cantly to the formation of stratigraphic sequences. The growth potential is an order of magnitude higher than alternatives are of two kinds: First, many sea-level fluc the rates of long-term subsidence or of third-order sea tuations reflected in sequence stratigraphy are related level cycles (compare Figure 1-15 and Schlager, 1981). to local or regional tectonics. Second, there are controls This implies that drowning events must be caused by beyond the realm of sea level that can, at times, domi unusually rapid pulses of sea level or by environmen nate sequence anatomy. This line of reasoning builds on tal change that reduced the growth potential of plat the arguments presented earlier that the sedimentolog forms. With growth reduced, drowning may occur at ic interpretation of sequences and systems tracts leaves normal rates of rise. Examples from the Holocene and room for other controls besides sea level. the Cretaceous will be called upon to illustrate the role In the first category, regional changes instead of of environmental change. eustatic ones, we mention intraplate deformation as a In the Holocene, many reefs drowned at a time particularly attractive alternative to eustasy (Cloet when the rate of sea-level rise had already passed its ingh, 1988). The past decade produced mounting evi maximum and was slowing down. The drownings dence that the model of rigid, internally undeformed occurred when the sea started to invade the flat bank plates is inadequate. It has been shown how plates tops, generating shallow lagoons with highly variable buckle and warp in response to changes of the temperatures and salinities as well as high suspended intraplate stress field. The resulting sea-level changes loads due to soil erosion. The ebb flow carried these are not global but super-regional, i.e., they can imprint "inimical bank waters" seaward and killed the reefs synchronous events on entire ocean basins including (Figure 6-1). Neumann & Macintyre (1985) coined the their margins. For carbonates, this would provide an phrase "reefs shot in the back by their own lagoons" for explanation for the similarity of sea-level curves from this example of drowning by environmental change. mid-ocean settings and ocean margins without neces A modern example of a reef crisis induced by envi sarily implying eustasy. ronmental change is the coral mortality in the wake of In the second category, changes in ocean environ- the El-Nifio event of the 1980s (Glynn, 1991). In this 57 58 Schlager instance, the damage can be traced to unusually high combined: Volcanism may have provided the energy water temperatures that exceeded the tolerance of source for open-ocean upwelling. This upwelling, in many corals. The El-Nino crisis in the eastern Pacific tum, could cause fertility pulses with blooms of non may not have a lasting effect on reefs. However, one carbonate organisms that damaged the reefs and plat can easily imagine that more frequent occurrences of forms. severe El-Nifio events may damage some reefs to The notion of oceanic events terminating platform extent where they loose the race with subsidence and growth and creating sequence boundaries may drown. explain some peculiarities of the curve by Haq et al. A Holocene example of coral reef growth limited (1987). Figure 6-5 shows the Early-Cretaceous curve to by ocean circulation has been described by Roberts & be out of step with other sea-level records. The dis Phipps (1988). They noticed that coral reefs in the east crepancies can be reconciled if one assumes that the ern Java Sea are relatively scarce and restricted to postulated lowstands really are drowning unconfor water depths of 15 m or less; below this depth, coral mities. A similar situation is seen on Figure 5-5, where reefs are replaced by bioherms of green algae that are the curve of Haq et al. (1987) postulates two abrupt devoid of hard framework. These bioherms produce sea-level falls, at 16.5 and 15.5 Ma respectively. Nei much sediment but would be unable to build truly ther the oxygen isotopes nor the platform stratigraphy wave-resistant structures. The limitation of coral of Enewetak confirm these events. growth is attributed to upwelling of nutrient-rich Ocean currents are yet another process by which waters that increase bioerosion and stimulate the environmental change exerts sequence control. Figure growth of competitors to scleractinian corals. 6-6 summarizes a rather instructive case study from The damaging effect of excess nutrients on coral the Blake Plateau. Pinet & Popenoe (1985) mapped a reefs has been documented in numerous involuntary number of well-defined sequences and related them experiments with sewage outfalls and fertilizer to variations in the course and current intensity of the runoff. Gulf Stream. These variations, in tum, were tentative In the Cretaceous, spectacular growth of platforms ly correlated to sea-level cycles of the curve by Vail et alternates with extensive drowning events (Arthur & al. (1977). O.D.P. drilling confirmed the strong influ Schlanger (1979); Schlager, 1981). These drownings ence of bottom currents on these sediments but found coincide in space and time with oceanic anoxic events a very poor correlation with the eustatic curve from (Figure 6-2). A similar coincidence was observed in sequence stratigraphy (Austin, Schlager et al., 1988). It the Devonian and the Liassic. In the Cretaceous, the seems that, at least in this particular case, the mean switch from carbonate to siliciclastic deposition pro dering Gulf Stream shapes sequences without demon duced some spectacular unconformities and sequence strable connection to sea-level cycles. Yet another boundaries (e.g., Figures 4-4, 5-12). There is, however, example of a current- controlled sequence boundary is no indication that sea-level rises at the time of drown shown in Figure 6-7 from the Gulf of Mexico (Mullins ing were unusually rapid. There seems to be no corre et al., 1987). lation between the rates of (relative) rise and the fate Finally, changes in sediment input merit mention as of the platform. Rates of drowned platforms show the important controls of sequence boundaries. These same broad scatter as platforms overall and platforms changes are typically linked to tectonic rearrange growing at 20 m/Ma were drowned just as well as ments of the hinterland and thus pertain to siliciclas others growing at 200 m/Ma (Figure 6-3). Further tics more than to carbonates. Examples include the more, third-order cycles, the prime cause of sequence Cenozoic sequences in the central Gulf of Mexico (Fig boundaries according to classical sequence theory, ure 4-6) and the Mesozoic sequences described by show rates of rise one order of magnitude below the Embry (1989) from the Sverdrup Basin. Carbonates growth potential of healthy platforms (compare Fig may be influenced indirectly by such changes in ter ures 6-4b and 1-9). We conclude that the majority of rigenous input as certain areas become favorable, oth Cretaceous drownings were not caused by abnormal ers inimical for platform growth. The Devonian of ly high rates of relative sea-level rise but rather by Western Canada reflects this interplay of shifting sili abnormal reduction of growth through environmental ciclastic supply and the carbonate response to it (see change. The coincidence of drowning with anoxia Figure 4-7 and Stoakes & Wendte, 1988). may point to a common cause of the two phenomena We feel that alternatives to eustasy deserve more or to a cause-and-effect relationship. Hallock & attention in sequence stratigraphy. The sedimentolog Schlager (1986) have argued for the latter. In analogy ical underpinnings of the concept clearly indicate that to the modem reef killings by sewage outfalls and fer first, relative and absolute (i.e. eustatic) changes of sea tilizer runoff, they speculated that the carbonate ben level have the same effect on sequences and, second, thos was overwhelmed by faster-growing het sequences can be generated by processes other than erotrophic biota when excess nutrients from the anox sea level. The fascination with, and exclusive empha ic depths of the ocean were swept to the surface in sis on, eustasy may have reduced rather than open-ocean overturn. Vogt (1989) proposed poisoning improved the role of sequence stratigraphy as a pre by hydrothermal plumes. The two concepts can be dictive tool. Alternatives To Eustasy In Sequence Control 59 a) lank 1101 ~ --~.··, .... ,.., llloll ~ .:::·.. ~:!.e.... - --~ .....Outer l'oeture Dro!IOff -- Hen Channel 0 .. prase11t b) .. !JOOOBP~ II 6000 BP • 10 ., 7000 BP c.: " LU 1- 8000 BP~ w " ~ ro II z ~"'' l: o ... , 1- ' ,, .. .,,. G.. .,Jll BRANCHING CORALS IIJ 0 ,. a MASSIVE CORALS Figure 6-1. Caribbean reefs drowned during the Holocene trans~ession. Rate of sea-level rise cannot be the cause of drowning, because the reefs drowned when sea-level nse was decreasing. The demise is related to reduction of growth potential by environmental stress: the crisis coincides with tlte flooding of the bank tops and ~eneration of lagoon waters with hidt suspended loads and extremely variable temperatures and salinities. (After Adey et al., 1977. Repro'ltuced with permission of Rosenstiel School of Marine & Atmospheric Science) (a) Map of St. Croix, Virgin Islands, showing drowned reef at shelf ed~e, labeled "shelf edge feature." (b) Cross section of drowned reef. Radiocarbon dates indicate drowning coincided with flooding of the shelf. 60 Schlager 140 120 100 SOMe a) ORO~ING BY SEA-LEVEL PULSF ? Numbers: ampfilude (In meters) at effeclive rise at subsidence 50 m/Ma ANOXIA N=39 10 5 b) Ratel of rise In Cretaceous 3.order cycles DROWNING N=34 !!! z&o ... ,,r=J .,r: tlo •.. rate o1 rise- Figure 6-4. Sea level pulse as cause of platform drowning-an evaluation. (a) Postulated eustatic sea level curve (Haq et al., 1987) in the mid Cretaceous, a time of global platform drowning. A platform subsiding at 50 m/Ma must have been exposed during the dotted intervals of the curve and experienced only the fluctuations drawn in full. Figure 6-2. Frequency distribution through time of Their amplitudes, shown in meters on the graph, oceanic anoxic events (top) and platform drownings are too small to move a platform out of the photic (bottom). The coincidence in time may well point to zone. (b) Rates of third order cycles on the curve of a causal link. This mar be one example of Haq et al. (1987) based on straight-line connections environmental contro on sequences and sequence of maxima and minima. The rates are one order of boundaries. (After Schlager & Philip, 1990. magnitude lower than the short-term growth Reproduced with permission of Kluwer Academic potential of healthy platforms (Schlager, 1991, and Publishers) Figure 1-15. Reproduced with permission of Elsevier Science Publishers) GROWTH RATES OF DROWNED PLATFORMS (CRETACEOUS) N:46 1"\, moving average (numerous sources) \Medlen g"' I ~ C" ~ ~----~~------~------~------~~ m/Ma- 50 100 150 200 Figure 6-3. Growth rates of drowned Cretaceous platforms show broad scatter and are similar to growth rates of platforms in general; drowning shows no bias towards fast-growing platforms, indicating that the rate of long-term subsidence had little influence on Cretaceous drownings. (After Schla~er, 1991. Reproduced with permission of Elsev1er Science Publishers) Alternatives To Eustasy In Sequence Control 61 Figure 6-5. Earll Cretaceous sea-level curves from Sea Level and Valanginian Drowning various parts o the world show strong similarity, IEDM.t.~~ ET AL H.t.Q ET AL. HARRIS KAUFFMAN indicating long-term eustatic control. The curve 125 from sequence stratigraphy is at variance with the conventional record, probably because widespread drowning unconformities in the Valan&inian were misinterpreted as lowstand unconformities (e.g. Figure 5-12). (After Schla~er, 1991. Reproduced with permission of Elsevier Science Publishers) uo Ma Tit h. W Atrlco World Aroblo N Amerlco se~ lewetrlslnq ~ N a) w 28 R•llttve c:N"'"'OI ...... b ) ~., .... Sri1mic Mq~Aneft aM hl.rus•~ ...... lhll27 Sitt Ill Figure 6-6. Environmental control of sequences. (a) Cretaceous and Cenozoic stratigraphic sequences on the Blake Plateau. (After Pinel & Popenoe, 1985). (b) Ages of sequences and hiatuses on Blake Plateau after calibration by Ocean Drilling Project Sites 627 and 628. Correlation with tl:ie sea-level curve of Haq et al. (1987) is poor. Sedimentology of ODP holes indicates that erosion and deposition on Blake Plateau are largely determined by the shifting course of the Gulf Stream, i.e. by changes in ocean environment rather than fluctuations of sea level. (After Austin, Schla~er et al., 1988.) Reproduced with permission of Ocean Drilling Program) 62 Schlager n' ... 14' u' 10' ... 0 400 km BATHYMETRIC CONTOUR: 200m Figure 6-7. Environmental control of sequences, Gulf of Mexico. (After Mullins et al., 1987.) (a) Study area on West Florida Shelf is touched by the "Loop Current," a segment of the Gulf-Stream system. (b) Several sequence boundaries can be mapped in the Cenozoic. (c) Boundary 1/11 does not correspond to a significant event on the curve of Haq et al. (1987). Rather, it correlates with the closing of the Straits of Panama and the establishment of the Loop Current-a tectonic event that induces environmental change. Alternatives To Eustasy In Sequence Control 63 b PROFILE ~2-44 PROGRADING ' CLINOFORMS MID-MIOCENE UNCONFORMITY :u w :1 0 .,"' ,...... ;. X 0 ! 1-• I TIME GlOBAl ONlAP AGEIIII.J.I SEA-lEV£L W.l'lORIOA 0 c) 0 HIGH f- -=-_.:_;s-t="-- ..:- ~ ~ ~ ...... ~. ... \ I ...c ~ \ II...... \. z ~ ... 0 \.. u g \... ~~ ...... 5 2 ,.. ' .,~ ..... \ \ n CHAPTER7 CONCLUSIONS Accommodation and sediment (1987). Relative sea level enters into this system supply-two basic controls on sequences because it controls accommodation. However, sys tems tracts are faithful recorders of sea-level only Application of sedimentologic principles to where variations in sediment supply are small com sequence stratigraphy allows one, among others, to pared to variations in accommodation. In all other sit better understand the various factors that cause depo uations, the distribution of exposure features in the sitional systems to generate sequences. Recognition of sediment record provides a more reliable record of direct and indirect controls is of the utmost importance sea-level positions than systems-tract geometry. here. For instance, relative sea level has direct control In carbonates, the situation is analogous, except on the shifting strand line and the exposure and flood that outside sediment supply has to be replaced by G, ing of depositional surfaces; it has indirect control the in situ growth and production of carbonate materi (namely via changes in accommodation) on prograda al. Sediment geometry and systems tracts are again tion and retreat of clinoforms. Indirect controls can be controlled by two a priori independent rates: very powerful but more often than not they must com A' = the rate of change in accommodation as pete with other factors that have a similar effect. Con defined above, and trol on depositional sequences is no exception. G' = dG/dt, the rate at which a platform produces Classical sequence stratigraphy postulates that sys sediment and builds wave-resistant structures. The tems tracts are essentially controlled by sea-level maximum rate of growth that the system can sustain, change. Sedimentologic analysis leads to the conclu the growth potential, varies across the platform. sion that direct control of sea level extends only to a Therefore, we must distinguish between small portion of the features in the sequence and sys Gr'= the growth rate of the platform rim, and tems tract model, such as exposure and flooding phe G '= the growth rate of the platform interior. nomena. For most other features, sea-level control is Bfsed on the relationship between A' and G', five exerted indirectly, primarily by changing the accom common patterns can be distinguished; they are illus modation. And here sea level faces competition from trated in Figure 7-2. another factor-sediment supply. Progradation and The term A' is very closely tied to relative sea-level retreat of shelf margins, occurrence of condensed inter change. One caveat must be made, though: A' as well vals as well as development of turbidite accumulations as S' represent changes of volume in time, the deriva are influenced at least as much by sediment supply as tive dV I dt. Sequence stratigraphic models consider they are by changes in accommodation. Below we either the change in the vertical dimension only, the summarize the "double-control concept of systems partial derivative (}z/ at, or the change in two dimen tracts" first for siliciclastics, then for carbonates. sions, ((}z/(}t).(()xj()t). One tacitly assumes these par Siliciclastics. We define S as the volume of sediment tial derivatives to be good approximations of the in the system and A as the accommodation space, i.e., change in accommodation volume-a premise that the space available for sedimentation. Then sequences does not always hold. and systems tracts are primarily controlled by two The term S', rate of sediment supply, is only rates, A' and S'. remotely related to sea level and is a major cause of A'=dA/ dt, the rate of change in accommodation, extraneous controls on sequences, independent of sea i.e. the sum of the rates of subsidence by crustal cool level. In siliciclastic systems, sediment supply is gov ing, sediment loading, sediment compaction, struc erned largely by conditions in the hinterland; the tural deformation, eustasy etc. growth term G' of carbonates is intimately tied to the S' = the rate of sediment supply. ocean environment and to organic evolution. Figure 7-1 summarizes the relationship of A' and S' The essence of the above considerations is that for the systems tracts and major surfaces of the classi depositional sequences represent a composite record cal sequence stratigraphic model as described by Vail of eustasy, tectonically driven, regional change in sea 64 Conclusions 65 level and environmental change. This last term stands largely for the factors modulating sediment supply. The situation is analogous to that of the oxygen-iso tope record of pelagic sediments, known to reflect the combined effects of temperature, salinity, ice volume and biogenic fractionation etc. The succession of sequences, much like the oxygen isotope curve, is a proxy indicator of sea level only when all other effects are minor. However, even where this is not the case, sequence stratigraphy can be a powerful tool for stratigraphic correlation and prediction, offering information on eustatic and regional changes of sea level as well as events in the ocean environment. The record of environmental change again contains global events (such as certain mass drownings of platforms) besides regional ones. To study this composite record, the carbonate sequence stratigrapher should take a three-step approach: 1. Establish unconformity-bounded sequences and correlate them in a region, including platform-to basin transitions if at all possible. 2. Establish a calendar of regional events, based on Figure 7-1. Sedimentologic interpretation of the sedimentologic meaning of these sequences and systems tracts in siliciclastics. They are generated by the interplay of the rate of change in sequence boundaries. accommodation, A', and the rate orchange in 3. Interpret these events in the context of known (!) sediment supply, S'. Exposure surfaces that extend eustasy, regional tectonics as well as regional and beyond the shelf break are diagnostic of relative global environmental change. sea-level falls and cannot be generated by variations of supply. Messages for the seismic stratigrapher Unconformity-bound sequences are well developed in carbonates but eustatic sea level competes with tec tonics and environmental change for sequence control. Sequence boundaries are best defined as originally proposed by Vail et al. (1977). Sedimentologically, a;, GP >A' they represent important changes in the pattern of sediment input and dispersal in a basin. Optimum interpretation of sequences in carbonate platforms requires more ground truth than in silici clastics. (This situation is similar to interpretation of wireline logs, where carbonates require more core control for optimum results.) a;, a.;< A' Best seismic sea-level records on platforms can be gleaned from the change in deviation of the shelf break in prograding rimmed platforms. Vertically stacked rims yield poor seismic records but provide good sea-level curves via bore holes (from overall sub sidence plus position of exposure surfaces). Ideal carbonate ramps obey the same rules as silici clastics in sea-level response. However the ideal car bonate ramp is rare, and the transition to rimmed A' Rimmed carbonate platforms shed most sediment fied fore-reef debris as well as the back-reef apron. during highstands when the bank tops are flooded. Seismic unconformities are not congruent with out They form highstand wedges of sediment on the basin crop unconformities. The seismic tool commonly margins. 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