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Carbonate ramps: an introduction

V. P. WRIGHT 1 & T. P. BURCHETTE 2 1Department of Earth Sciences, Cardiff University, Cardiff CF1 3YE, UK and BG Exploration and Production, 100 Thames Valley Park Drive, Reading RG6 1PT, UK 2Bp Exploration, Building 200, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, UK

Carbonate ramps are carbonate platforms which history. The term shelf is, however, most widely have a very low gradient depositional slope used in the geological context for any broad, (commonly less than 0.1 ~ from a shallow-water gently-sloping surface, clastic or carbonate, shoreline or lagoon to a basin floor (Burchette & which has a break in slope in deeper water, and Wright 1992). A large proportion of carbonate is typified by usage of the term 'continental successions in the geological record were shelf' (e.g. Bates & Jackson 1987). In addition, deposited in ramp-like settings. Nevertheless, the term ramp is now also widely used by clastic ramps remain one of the more enigmatic sedimentologists for low-gradient submarine carbonate platform types. In contrast to steeper- slopes, particularly on continental shelves. sloped rimmed shelves and isolated buildups, Where the dominant sea-floor sediments are of where the factors which have controlled their carbonate mineralogy, however, such a configu- location and development are commonly quite ration has become known as a 'distally steep- evident, the controls on ramp development have ened ramp', a morphology which in carbonate seldom been clearly demonstrated. In order to settings is more often than not inherited from an shed new light on this topic, and related aspects antecedent morphological feature. The dis- of ramp development, this volume addresses a cussion is continued in two papers in this number of key issues including: terminology volume, by Testa & Bosence and by Light & (can we classify ramps?), processes (how impor- Wilson, who document present-day carbonate tant are water temperature controls, produc- sedimentation on two continental shelves, tion-depth profiles); tectonic controls (are environments which are effectively distally- ramps by necessity restricted to relatively steepened ramps. shallow, flexural basins?); sedimentary pro- The term carbonate 'platform' has become cesses (e.g. sediment dispersal versus in situ pro- generally applied to any thick, more or less flat- duction in maintaining the ramp profile); the topped carbonate depositional system and dis- origins of ramp reef-mounds; and the reasons for tinguishes such features from the much more the evolution of ramps into other carbonate general and widely applied concept of a 'shelf'. platform types (see e.g. Read 1985). Many ancient ramp systems appear to have developed into flat-topped 'rimmed' carbonate Terminology platforms and an example is described here by Hips. A prominent school of thought holds that The appropriateness of the term 'ramp' con- ramps might merely represent the incipient, tinues to be an area of strong debate. General catch-up stages of rimmed platforms (where concensus in this volume is that the currently carbonate supply had not filled available applied terms and concepts are indeed useful, accommodation space) while carbonate shelves and the historical basis for this debate is represent true keep-up systems (where the reviewed in this volume by Ahr, who introduced carbonate pile has built up to sea level and keeps this term more than 25 years ago. Nevertheless, pace with subsequent sea level changes). even after all this time, a review of the literature However, from the numerous examples in the shows that confusion in terminology between geological record, it is clear that ramps can rep- the concepts of carbonate 'shelf', 'ramp', and resent keep-up depositional systems in their own 'platform' is common. right and commonly developed into flat-topped Our view is that carbonate shelves are platforms even while maintaining their low-gra- shallow, flat-topped structures with a clearly dient seaward margins. defined margin determined by a steep slope Understanding modern analogues is a key down to the adjacent basin. A modern example factor in deciding whether or not ramps are enti- is the east Florida shelf, and many are known ties in their own right, or are simply transient from the geological record, the Permian Capitan phenomena in the evolution of rimmed plat- Shelf being a good example for much of its form. A classic modern ramp analogue is the

WRIGHT, V. P. & BURCHETTE,Z. P. 1998. Carbonate ramps: an introduction. In: WRIGHT,V. P. t~; BURCHETTE,Z. P. (eds) Carbonate Ramps. Geological Society, London, Special Publications, 149, 1-5. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 V.P. WRIGHT & T. P. BURCHETTE southern margin of the Arabian Gulf. In this receive more sediment from the platform-top volume, Walkden & Williams question the valid- factory, but Pedley highlights the paradox of ity and usefulness of the analogy. These authors high progradation rates in windward ramps com- stress the structural, stratigraphic and geomor- pared to leeward ramps in the mid-Tertiary of phic complexity of the substrate upon which the the central Mediterranean. present thin veneer of ramp sediments lies, and The role of longshore and contour currents in regard the ramp profile as unstable. They argue shaping sediment bodies on ramps on the Rio that a true ramp is self sustaining and can be Grande do Norte Shelf of north east Brazil is maintained over geologic time intervals. illustrated by Testa & Bosence. Such currents Perhaps the answer lies in trying to improve our can be active in both shallow and deep-ramp set- understanding of the process of depositon and tings. They compare this system with distally controls on ramp development, and with only steepened ramps off NW Yucatan and western short-lived modern analogues available, it is Florida. All three are influenced by the Southern essential to integrate data from the Recent and Equatorial Current and its derivatives, the ancient by means of computer modelling. Yucatan and Loop Currents. Although most ramps have sufficiently low gradients that sediment gravity flows are not Processes active, exceptions exist and Pedley provides While acknowledging that relative sea level is examples from the middle Tertiary, as do one of the most significant controls on deposi- Giiham & Bristow from the lower Tertiary of tional systems, many now seem to regard it as north Spain. the only influence on deposition.Careful analy- sis of the factors which influence sedimentation Controls in active carbonate depositional systems is criti- cal. Depositional systems are inherently In contrast to siliciclastic sediment, the produc- complex, yet ancient successions are often tion of which is closely linked to hinterland tec- reduced to the simplest of models related to tonism or climate, coarser carbonate sediment is relative sea-level changes. Thus, a re-evaluation typically produced in or close to the environ- of the southern Arabian Gulf in terms of ment in which it is created. However, carbonate environmental controls is long overdue and sediment production rates are water-depth Kirkham reviews the 'classic' inner-ramp area of dependant and are highest in shallow water, the United Arab Emirates, emphasising the factors which make carbonate systems sensitive importance of wind control on geomorphology even to small amounts of subsidence and uplift. and sediment dispersal patterns in this regime. Subsidence regime is therefore of great import- The importance of tidal regime as a control on ance in controlling carbonate ramp styles and the architecture and geometries of inner-ramp drowning history. Carbonate ramp successions sandbodies is discussed in this volume by are prominent components of foreland basin fills Azeredo in a study of the Middle Jurassic of and two examples, presented by Sinclair et al. west-central Portugal. In mid-ramp settings the and Gilham & Bristow respectively, illustrate dominant events affecting sediment character sequence development in Eocene foreland are storms and Light & Wilson re-emphasize this basins of the French Alps and the south-eastern in their study of rocks of similar age in the NE Pyrenees. An example of ramp development in Atlantic province. The strong depth-zonation of an extensional setting is provided by Gomez- foraminifera on Tertiary ramps allows individual Perez et al. from the Lower Cretaceous of north- taxa to be used as 'tracers', and the importance of eastern Spain which emphasizes the complex storms in sediment redistribution on mid-Ter- facies patterns which can characterise such a tiary ramps is discussed by Pedley for successions system. in Italy and Malta. Moreover, computer simu- The influence of relative sea-level changes on lation has advanced to the stage where it is poss- ramp development is discussed throughout this ible to model the sediment dispersal role of volume. It is a common preconception that sea- storms, as demonstrated by Aurell et al. level changes simply shift facies belts up and The degree of exposure and orientation of a down ramps, but two examples are presented coastline to wind, wave and storm activity is a which demonstrate that facies partitioning is an critical control on ramp facies distribution. The important feature of carbonate ramp succes- effects of windward and leeward orientations on sions, just as it is in other carbonate platform isolated platforms are well known, but have types, leading to marked differences between been less clearly demonstrated for ramps. Intu- facies architectures of the different systems itively, leeward margins might be expected to tracts. Gomez-Perez et al. illustrate this Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 3 phenomenon for the Lower Cretaceous of Lasemi et ai. describe heterozoan-dominated northeastern Spain, while Bachmann & Kuss biotas from Carboniferous ramps of Illinois in provide an example from the Middle Cretaceous which the main faunal components are bry- of northern Sinai. ozoans, crinoids and brachiopods. Such organ- Environmental controls on sediment produc- isms are the main sediment producers in modern tion have been emphasised in our search to deeper, cooler waters, and a strictly uniformitar- understand ramp development. In flat-topped, ian approach would lead us to interpret similar photo- or mixotroph-dominated carbonate plat- ancient assemblages as cool-water faunas too. forms, sediment production is highest in very We might be in danger, though, of making over- shallow water. In many ancient ramp systems simplistic interpretations based in an implicit sediment production was less clearly biased faith in uniformitarianism. In fact, Taylor & towards narrow depth ranges, partly because of Allison (1998) have recently shown that bry- the more prominent involvement of het- ozoans experienced a major taxonomic turnover erotrophs in sediment production. This is shown at the end of the Palaeozoic. Direct comparison by Testa & Bosence in their Rio Grande de of latitudinal distribution between pre-Palaeo- Norte study in which they note the absence of zoic and post-Palaeozoic bryozoans thus any single locus of high sediment production in appears to be invalid. Post-Palaeozoic forms are an area lacking coral domination. most abundant in high latitudes while Palaeo- The importance of high rates of sediment pro- zoic bryozoans are more abundant in low lati- duction in mid-ramp settings is emphasized too tudes. In the context of this debate, Lasemi et al. by Pedley for middle Tertiary ramps. Rates of dismiss both a cool-water model for Carbonifer- production were high enough in these systems to ous low-latitude seas as well as any model invok- create a 'bulge' in the ramp pofile at estimated ing thermal stratification. Instead, they favour water depths of 40-60 m. The change in clino- upwelling as a major control, whereby the faunal form angle on some seismic profiles of ramps (e.g. composition of the sediments indicates that 'ramp slope crest' of Burchette & Wright, 1992) waters were nutrient-rich. could conceivably correspond to such 'bulges', particularly where, as on Tertiary ramps, organic Modelling banks developed in offshore settings. Carbonate successions in the geological One of the most effective methods of testing the record have been traditionally regarded as evi- efficacy of these multitude of factors in control- dence of warm water, but this view has been ling ramp development is to use computer questioned recently, with the re-interpretation models conditioned with appropriate geometric of some ramp successions as the product of cool, and stratigraphic data from outcrop and process or even cold-water environments. This is most data from modern carbonate depositional clearly advocated by James & Clarke (1997), environments. The effectiveness of this who take the view that ancient platform succes- approach is illustrated in two papers. Read, sions lacking corals and calcareous green algae, using Bowman's PhilR programme, models and dominated by heterozoan communities (viz. ramp development and architecture during a good number of ramp successions) were the greenhouse, transitional and icehouse intervals. products of cool-water seas. This view appears His models are tested and refined against many to be held by many workers in this field. examples, drawn largely from North America. However, Testa & Bosence show that corals are Aureli et al. use the programme Carbonate 6 to not major sediment contributors on the Rio analyse the controls on Kimmeridgian (late Grande de Norte Shelf of Brazil even though the Jurassic) ramp development in the Teruel region waters are sufficiently warm and appropriately of north east Spain. This model tests whether the low in nutrients, to allow their growth. The mud component of mid-ramp sediment was absence of corals from this system probably more likely to have been redeposited from reflects the predominantly unstable substrates inner-ramp sediment factories, or was the available. product of pelagic or hemipelagic supply. The Triassic ramps from Hungary, described by model with redeposition conforms most closely Hips and Torok in this volume were also hetero- to the actual facies distribution. zoan-dominated. These authors both favour the view that coral absence in these ramps was an Mud mounds evolutionary phenomenon related to the end- Permian extinction event, perhaps also coupled The enigmatic question of why mud mounds with other environmental factors, rather than a develop in many outer-ramp successions is simple temperature restriction. addressed here by three papers. Lasemi et al. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 V.P. WRIGHT & T. P. BURCHETTE document Lower Carboniferous mounds from events in the Earth's history. They consequently the Illinois Basin and Wendt & Kauffman and lack the continuity with precursor successions Kauffman provide separate studies of Devonian which is critical if such information is to be effec- mounds from Algeria and Morocco. All repre- tively applied to the interpretation of whole sent intra-shelf basinal settings. The Algerian ramp depositional sequences. Some useful mounds show local alignment with tectonic lin- additional information, on sediment dispersal eaments, but but do not appear to have been patterns for example, can be gleaned from associated with hydrothermal activity (cf. Belka modern shelfal siliciclastic regimes, although 1998). Some of the Moroccan mounds are coral- differences in sediment provenance and rich, suggesting that they formed in shallower, accumulation sites set limits to the analogies warm waters, in contrast to many ancient mud which are possible here. Review of the papers in mounds which are characteristic of deeper ramp this volume shows that, beneath their deceptive settings. simplicity, ramp depositional systems are every bit as diverse and complex as those of other Epeiric ramps carbonate platform types, with sediment trans- port paths which may trend for tens of kilome- Many of the more stylized ramp models from the tres in both onshore, longshore and offshore literature envisage shoal belts separating storm- directions. The implications for the interpre- dominated mid-ramp from lagoons and tidal tation of thick sedimentary accumulations con- flats in the inner ramp. However, there is no lack structed by ramp depositional processes are of examples in which no such belt can be identi- clear. fied. Choi & Simo provide one such from the Ideally, studies of ancient ramp depositional Upper Ordovician of Wisconsin. Should we con- systems should include a mechanism for com- sider designating such low-energy systems as bining the sort of process sedimentology derived 'epeiric ramps', or do we already have sufficient from the study of modern ramp environments terms to juggle? Nevertheless, this category of with the historical, sequential dimension gained ramps, apparently restricted exclusively to cra- from investigations of large outcrops. The use of tonic interiors, do require special consideration. increasingly sophisticated computer models pro- In the latest Triassic, early Jurassic, and early to vides one obvious route to test ideas on ramp mid-Cretaceous, such ramps were widespread in dynamics formulated from outcrop studies. what is now Iberia, northwestern Europe and Sediments on ramps appear to have three main the Middle East. As with their Ordovician origins: in situ production, redeposition in the equivalents in Wisconsin, such ramps were inner and mid-ramp by storms or contour cur- characterized by low wave energy and low tidal rents, and pelagic fallout on the mid- and outer ranges, such that facies transitions are very ramp. Using computer models, the relative gradual and facies belts are broad, while distinct volumes of these three sources, and variations in shoal deposits are rare. In these Lower to sediment dispersal patterns, can be modelled Middle Jurassic 'Lias'-type successions diage- and tested against documented ancient netic bedding and nodular limestones are domi- examples. nant features, pointing to low sedimentation Complexity is further compounded if biotic rates and extensive, early diagenetic remobilisa- evolution is introduced into the modelling tion of carbonate in these calcitic seas. process. Many Palaeozoic, and possibly early Tertiary, ramps were clearly strongly influenced Future initiatives by abundant sediment-producing organisms in offshore settings and may represent systems The fact that modern oceans provide no good dominated by in situ production of organic par- analogues for large, mature ancient ramp suc- ticulate (rather than framework) sediment in this cessions of the sort discussed in many of the location. In contrast, the offshore environments papers in this volume, and which are so abun- of many early to mid-Mesozoic ramps are strik- dant throughout the geological record, makes it ingly muddy and may represent systems in which difficult to effectively apply uniformitarian prin- offshore sediment transport was the most impor- ciples to the interpretation of these ancient tant process, as suggested by Aurell et ai. From structures. The few modern, incipient ramps to the early Cretaceous onwards, outer-ramp sedi- which we do have access can provide an instant- ments have been dominated by the remains of in-time view of sediment dynamics in this sort of planktonic organisms, such as foraminifera, cal- system (although little studied), but they actu- cispheres and coccolithophorids. Naturally, sedi- ally represent only the recovery stage following ment is contributed to some extent from most of one of the most pronounced global sea-level these sources on most ramps, but variations in Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 5 the proportional contribution of carbonate sedi- over-produced carbonate sediment from these ment due to evolutionary changes in the sedi- settings have gone? Ancient epeiric ramps of ment producing organisms during the this sort persist in presenting us with major prob- Phanerozoic may have influenced ramp profiles lems in applying current ideas to these unusual and their response to relative sea-level changes. systems. This hypothesis could also be tested by means of computer simulations using multiple realisations. We sincerely thank all authors of papers in this volume The role of temperature as a control on ramp for their patience during the reviewing and editing biotic patterns is likely to be another productive process and also the many reviewers for their time and avenue for research and has been neglected his- effort. This set of papers derives from a conference held at the Geological Society on 2-3 July 1996, torically. Once more, overstrict adherence to entitled Carbonate Ramps: oceanographic and bio- uniformitarianism when assessing the biotic logical controls, modelling and diagenesis. The compositions of ancient platform limestones meeting was sponsored by the British Sedimentologi- seems unwise. New ideas on cool-water carbon- cal Research Group, the Geological Society of ate deposition have stimulated attempts to London and BP Exploration. understand which factors really controlled ancient biotic distributions and we are being References prompted to ask more appropriate questions as to why ancient carbonate systems differed from BATES, R. & JACKSON,J. A. 1987. Glossary of geology. the modern. One possible research direction American Geological Institute, Alexandria. might be to more closely correlate palaeolati- BELKA, Z. 1998. Early Devonian Kess-Kess carbonate tude with the nature of carbonate deposystems mud mounds of the eastern Anti-Atlas (Morocco), and their relation to submarine within discrete time intervals in order to assess hydrothermal venting. Journal of Sedimentary likely temperature controls. It would also be Research, 68, 368-377. appropriate to link this to studies of ancient sea- BURCHETrE, T. P & WRIGHT, V. P. 1992. Carbonate water temperatures using stable isotopes. ramp depositional systems. Sedimentary Geology, Low-energy, cratonic-interior, or epeiric 79, 3-57. ramps are a special category of carbonate plat- JAMES, N. R & CLARKE, J. A. D. (eds) 1997. Cool-water form and deserving of more intensive investi- carbonates. Society for Sedimentary Geology & gation. Were they low-energy or low- Special Publication 56. productivity systems - or both? Many show READ, J. E 1985. Carbonate platform facies models. Bulletin of the American Association of Petroleum abundant evidence for condensation and Geologists, 69, 1-21. reworking of sediments. Does this just reflect TAYLOR, P. D. (~; ALLISON, P. A. 1998. Bryozoan car- low rates of accommodation-space creation, e.g. bonates through time and space. Geology, 26, due to locations in slowly subsiding cratonic 459-462. interiors? If this were the case, where might any