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 In compiling these notes, I have drawn not only on literature but also on as yet unpublished materials from my associates in the sedimentology group at the Free University, Amsterdam.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