Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 Carbonate platform systems: components and interactions - an introduction ENZO INSALACO 1, PETER SKELTON 2 & TIM J. PALMER 3 1 TotalFinaElfExploration UK PLC, Geoscience Research Centre, 30 Buckingham Gate, London, SW1E 6NN, UK 2 Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, UK 3 Institute of Geography and Earth Sciences, University of Wales Aberystwyth, Aberystwyth, Ceredigion SY23 3BD, UK Carbonate platforms are open systems with analyse their interactions, detect significant natural boundaries in space and time. Across differences as well as similarities between them, their spatial boundaries there are fluxes of and so explore the possible causes and effects of energy (e.g. light, chemical energy in com- changes through time. In short, our quest is for pounds, and kinetic energy in currents and mass the reality of variety, not imagined unity. flows) and matter (e.g. nutrients, dissolved gases The studies presented here concentrate on such as CO2, and sediment - especially, of shallow-water platform systems, with an occa- course, carbonates). Internally, these fluxes are sional nod towards fossil mudmounds of deeper regulated by myriads of interactions and feed- water origin, largely because these are the most backs (Masse 1995), and the residue is consigned accessible and best-understood examples of sub- to the geological record. The most distinctive stantial carbonate bodies. Nevertheless, we aspect of carbonate platforms is the predomi- should also note in passing the existence of sig- nant role of organisms in producing, processing nificant tracts of deep-water coral mounds today, and/or trapping carbonate sediment, even in around the North Atlantic (Mortensen et al. Precambrian examples. 1995), for example. Ongoing work on these Because of evolutionary changes in this strong should provide interesting comparisons with biotic input, it is harder to generalize about their shallow-water counterparts, especially with carbonate platforms than about most other sedi- respect to the effects on fabrics and facies of their mentary systems. Evolution has altered both the very different circumstances of development. constructive and destructive effects of platform- The geological outcomes of platform develop- dwelling organisms on carbonate fabrics, with ment depend upon a hierarchy of interactions. profound consequences for facies development. At the lowest level are those that structure com- Moreover, changing patterns in the provision of munities, often involving taphonomic feed- accommodation space (e.g. between greenhouse backs. Over larger scales of time and space, and icehouse climatic regimes) have also left tectonics, eustacy, climate and oceanographic their stamp on facies geometries, in turn feeding factors set their own imprints on the physiogra- back to the evolution of the platform biotas. phy and facies anatomy of platforms, including Hence simplistic analogies between modern and the determination of their beginnings and ends - ancient platforms may give rise to misleading the temporal boundaries of the platform interpretations of what the latter were like and systems. We have accordingly arranged the thir- how they formed. Although a number of carbon- teen papers presented in this volume in two ate platform and reef specialists have warned of parts, to reflect this hierarchical scaling. Part 1 the dangers of such misplaced uniformitarianism concerns community level aspects, from organ- (e.g. Braithwaite 1973; Gili et al. 1995; Wood isms and sediment production, to growth fabrics. 1999), it remains depressingly commonplace in Themes include: the literature on ancient carbonate platforms. (a) the ecology and palaeoecology of benthic The endless quest in the literature for an all- biotas, particularly factors influencing purpose definition of 'reefs' in the fossil record growth fabric genesis; is symptomatic of this delusion. Like the Holy (b) processes and rates of skeletal growth, bio- Grail of Medieval legend, the object of the erosion and sediment production; and search remains cloaked in the vagueness of (c) taphonomic and diagenetic influences. myth. The aim of this volume is more pragmatic - to present case studies that describe the com- Part 2 concerns larger scale aspects, from influ- ponents of some ancient and modern examples, ences on the growth and demise of individual From: INSALACO,E., SKELTON,P. W. & PALMER,T. J. (eds) 2000. Carbonate Platform Systems: components and interactions. Geological Society, London, Special Publications, 178,1-8. 0305-8719/00/$15.00 9The Geological Society of London 2000. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 2 ENZO INSALACO, PETER SKELTON & TIM PALMER platforms, to global patterns of change. Themes conceptual dynamic model that integrates these here include: processes. When viewed in this fashion it is clear that changes in one component can result in (a) factors influencing the establishment and positive or negative feedbacks from others. demise of carbonate platform systems; The rates at which these component processes (b) the sequence stratigraphy of carbonate plat- operate, over very short temporal scales forms and its bearing on the development of (seasons to decades), and their effects and link- biogenic lithosomes within them; and ages with other factors are variously examined (c) global estimates of carbonate production by in this first part of the book. Their geological various carbonate platform systems through implications and the degree to which they can be the Phanerozoic. inferred from the geological record are also con- sidered. There is a huge literature on reef Part 1: Community level processes and science spread across the biological, palaeonto- logical and sedimentological domains, and a products comprehensive review is beyond the scope of Growth fabrics (sensu Insalaco 1998) emerge this book. Recent reviews of the general subject from tightly integrated systems of abiotic and area are given in Fagerstrom (1987), Birkeland biotic components that display a variety of (1997) and Wood (1999). organism - environment feedbacks. Processes These themes are discussed with respect to that have a bearing on what type of fabric is pre- corals (Riegl & Piller), coralline algae (Nebel- served include: recruitment, growth and repro- sick & Bassi), cyanobacteria (Wright & Alter- duction of skeletalized organisms; bioerosion: mann), rudist bivalves (Steuber, Gili & Skelton), secondary encrustation; sediment autoproduc- polychaetes (Naylor & Viles) and macroborers tion by mechanical and biological erosion in situ: (Perry & Bertling), and with reference to differ- sediment import (both siliciclastic and carbon- ent time periods from the Precambrian to the ate) and export (including dissolution); and present day. The different temporal scales of early marine cementation. Figure 1 presents a processes are also addressed, ranging from those GI" *-- 9''" t 7 EX ~( ........En ,m Fig. 1. Conceptual diagram illustrating the biological and sedimentological components and interactions involved in growth fabric production by shelly benthos on carbonate platforms. (Ac) sediment accumulation rate; (Au) Autoproduction (mechanical and biological erosion); (Bi) bioerosion; (En) encrustation; (Ex) sediment export (including dissolution); (Gr) skeletal growth rate; (Im) sediment import; (Mo) mortality; (Re) recruitment; (SAZ) sedimentologically active zone; (T n) successive time slices; (TAZ) taphonomically active zone. Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 CARBONATE PLATFORM SYSTEMS - AN INTRODUCTION 3 operating within the lifetime of an individual system of feedbacks between shell growth, (Naylor & Viles, Steuber, Riegl & PiUer) to the packing density and current activity, which time involved in the formation of entire biogenic yielded a highly efficient, if ephemeral, kind of lithosomes (Gili & Skelton). The studies also carbonate factory in propitious conditions. concern a variety of depositional contexts In modern platform environments, organisms (ramps, shelves, mixed carbonate-siliciclastic also play a major role in skeletal breakdown systems, and both tropical and temperate (Bromley 1994). Such bioerosional processes regimes). are considered in the paper by Perry & Bertling, The first two papers look at the role of skeletal who have assessed the impact of boring by accretion (either by sand-trapping or carbonate macroscopic organisms on Mesozoic and Ceno- shell growth) and environmental consequences. zoic coral reefs. Macroboring of coral reefs has Though set in a siliciclastic context, the study varied significantly through time, with the of Naylor & Viles illustrates the environmental modern intensity and producer composition a feedbacks of bioconstructions, with reference to relatively recent phenomenon. Given the impact Sabellaria alveolata (Linn6), a sedentary poly- of bioerosion on framework morphology, com- chaete that builds significant, but poorly docu- munity composition and sediment production mented, wave-resistant reefs from sand particles. today, Neogene reef systems make poor ana- Reefs can form quite rapidly in temperate waters logues for understanding the structure and with a large, continuous supply of sand-grade dynamics of older examples. Sponges (dominant sediment and turbulent water in the lower eulit- now) appear to have played a subordinate role total zone. They have three