Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 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.