Geological Society, London, Special Publications 1998 Wright 1 5

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1999.149.01.01p1-5.

1998, v.149;Geological Society, London, Special Publications

V. P. Wright and T. P. Burchette

Carbonate ramps: an introduction

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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 have a very low gradient depositional slope (commonly less than 0.1 ~ from a shallow-water shoreline or lagoon to a basin floor (Burchette & Wright 1992). A large proportion of carbonate successions in the geological record were deposited in ramp-like settings. Nevertheless, ramps remain one of the more enigmatic carbonate platform types. In contrast to steeper- sloped rimmed shelves and isolated buildups, where the factors which have controlled their location and development are commonly quite evident, the controls on ramp development have seldom been clearly demonstrated. In order to shed new light on this topic, and related aspects of ramp development, this volume addresses a number of key issues including: terminology (can we classify ramps?), processes (how important are water temperature controls, production-depth profiles); tectonic controls (are ramps by necessity restricted to relatively shallow, flexural basins?); sedimentary processes (e.g. sediment dispersal versus in situ production in maintaining the ramp profile); the origins of ramp reef-mounds; and the reasons for the evolution of ramps into other carbonate platform types (see e.g. Read 1985).

Terminology The appropriateness of the term 'ramp' con- tinues to be an area of strong debate. General concensus in this volume is that the currently applied terms and concepts are indeed useful, and the historical basis for this debate is reviewed in this volume by Ahr, who introduced this term more than 25 years ago. Nevertheless, even after all this time, a review of the literature shows that confusion in terminology between the concepts of carbonate 'shelf', 'ramp', and 'platform' is common.

Our view is that carbonate shelves are shallow, flat-topped structures with a clearly defined margin determined by a steep slope down to the adjacent basin. A modern example is the east Florida shelf, and many are known from the geological record, the Permian Capitan Shelf being a good example for much of its

history. The term shelf is, however, most widely used in the geological context for any broad, gently-sloping surface, clastic or carbonate, which has a break in slope in deeper water, and is typified by usage of the term 'continental shelf' (e.g. Bates & Jackson 1987). In addition, the term ramp is now also widely used by clastic sedimentologists for low-gradient submarine slopes, particularly on continental shelves. Where the dominant sea-floor sediments are of carbonate mineralogy, however, such a configu- ration has become known as a 'distally steepened ramp', a morphology which in carbonate settings is more often than not inherited from an antecedent morphological feature. The dis- cussion is continued in two papers in this volume, by Testa & Bosence and by Light & Wilson, who document present-day carbonate sedimentation on two continental shelves, environments which are effectively distally- steepened ramps.

The term carbonate 'platform' has become generally applied to any thick, more or less flat- topped carbonate depositional system and dis- tinguishes such features from the much more general and widely applied concept of a 'shelf'. Many ancient ramp systems appear to have developed into flat-topped 'rimmed' carbonate platforms and an example is described here by Hips. A prominent school of thought holds that ramps might merely represent the incipient, catch-up stages of rimmed platforms (where carbonate supply had not filled available accommodation space) while carbonate shelves represent true keep-up systems (where the carbonate pile has built up to sea level and keeps pace with subsequent sea level changes). However, from the numerous examples in the geological record, it is clear that ramps can represent keep-up depositional systems in their own right and commonly developed into flat-topped platforms even while maintaining their low-gradient seaward margins.

Understanding modern analogues is a key factor in deciding whether or not ramps are enti- ties in their own right, or are simply transient phenomena in the evolution of rimmed platform. 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.


2 V.P. WRIGHT & T. P. BURCHETTE

southern margin of the Arabian Gulf. In this volume, Walkden & Williams question the valid- ity and usefulness of the analogy. These authors stress the structural, stratigraphic and geomor- phic complexity of the substrate upon which the present thin veneer of ramp sediments lies, and regard the ramp profile as unstable. They argue that a true ramp is self sustaining and can be maintained over geologic time intervals. Perhaps the answer lies in trying to improve our understanding of the process of depositon and controls on ramp development, and with only short-lived modern analogues available, it is essential to integrate data from the Recent and ancient by means of computer modelling.

Processes

While acknowledging that relative sea level is one of the most significant controls on depositional systems, many now seem to regard it as the only influence on deposition.Careful analy- sis of the factors which influence sedimentation in active carbonate depositional systems is critical. Depositional systems are inherently complex, yet ancient successions are often reduced to the simplest of models related to relative sea-level changes. Thus, a re-evaluation of the southern Arabian Gulf in terms of environmental controls is long overdue and Kirkham reviews the 'classic' inner-ramp area of the United Arab Emirates, emphasising the importance of wind control on geomorphology and sediment dispersal patterns in this regime.

The importance of tidal regime as a control on the architecture and geometries of inner-ramp sandbodies is discussed in this volume by Azeredo in a study of the Middle Jurassic of west-central Portugal. In mid-ramp settings the dominant events affecting sediment character are storms and Light & Wilson re-emphasize this in their study of rocks of similar age in the NE Atlantic province. The strong depth-zonation of foraminifera on Tertiary ramps allows individual taxa to be used as 'tracers', and the importance of storms in sediment redistribution on mid-Ter- tiary ramps is discussed by Pedley for successions in Italy and Malta. Moreover, computer simu- lation has advanced to the stage where it is possible to model the sediment dispersal role of storms, as demonstrated by Aurell et al.

The degree of exposure and orientation of a coastline to wind, wave and storm activity is a critical control on ramp facies distribution. The effects of windward and leeward orientations on isolated platforms are well known, but have been less clearly demonstrated for ramps. Intu- itively, leeward margins might be expected to

receive more sediment from the platform-top factory, but Pedley highlights the paradox of high progradation rates in windward ramps com- pared to leeward ramps in the mid-Tertiary of the central Mediterranean.

The role of longshore and contour currents in shaping sediment bodies on ramps on the Rio Grande do Norte Shelf of north east Brazil is illustrated by Testa & Bosence. Such currents can be active in both shallow and deep-ramp settings. They compare this system with distally steepened ramps off NW Yucatan and western Florida. All three are influenced by the Southern Equatorial Current and its derivatives, the Yucatan and Loop Currents.

Although most ramps have sufficiently low gradients that sediment gravity flows are not active, exceptions exist and Pedley provides examples from the middle Tertiary, as do Giiham & Bristow from the lower Tertiary of north Spain.

Controls

In contrast to siliciclastic sediment, the production of which is closely linked to hinterland tec- tonism or climate, coarser carbonate sediment is typically produced in or close to the environ- ment in which it is created. However, carbonate sediment production rates are water-depth dependant and are highest in shallow water, factors which make carbonate systems sensitive even to small amounts of subsidence and uplift. Subsidence regime is therefore of great importance in controlling carbonate ramp styles and drowning history. Carbonate ramp successions are prominent components of foreland basin fills and two examples, presented by Sinclair et al. and Gilham & Bristow respectively, illustrate sequence development in Eocene foreland basins of the French Alps and the south-eastern Pyrenees. An example of ramp development in an extensional setting is provided by Gomez- Perez et al. from the Lower Cretaceous of northeastern Spain which emphasizes the complex facies patterns which can characterise such a system.

The influence of relative sea-level changes on ramp development is discussed throughout this volume. It is a common preconception that sea- level changes simply shift facies belts up and down ramps, but two examples are presented which demonstrate that facies partitioning is an important feature of carbonate ramp successions, just as it is in other carbonate platform types, leading to marked differences between facies architectures of the different systems tracts. Gomez-Perez et al. illustrate this


INTRODUCTION 3

phenomenon for the Lower Cretaceous of northeastern Spain, while Bachmann & Kuss provide an example from the Middle Cretaceous of northern Sinai.

Environmental controls on sediment production have been emphasised in our search to understand ramp development. In flat-topped, photo- or mixotroph-dominated carbonate platforms, sediment production is highest in very shallow water. In many ancient ramp systems sediment production was less clearly biased towards narrow depth ranges, partly because of the more prominent involvement of het- erotrophs in sediment production. This is shown by Testa & Bosence in their Rio Grande de Norte study in which they note the absence of any single locus of high sediment production in an area lacking coral domination.

The importance of high rates of sediment production in mid-ramp settings is emphasized too by Pedley for middle Tertiary ramps. Rates of production were high enough in these systems to create a 'bulge' in the ramp pofile at estimated water depths of 40-60 m. The change in clino- form angle on some seismic profiles of ramps (e.g. 'ramp slope crest' of Burchette & Wright, 1992) could conceivably correspond to such 'bulges', particularly where, as on Tertiary ramps, organic banks developed in offshore settings.

Carbonate successions in the geological record have been traditionally regarded as evidence of warm water, but this view has been questioned recently, with the re-interpretation of some ramp successions as the product of cool, or even cold-water environments. This is most clearly advocated by James & Clarke (1997), who take the view that ancient platform successions lacking corals and calcareous green algae, and dominated by heterozoan communities (viz. a good number of ramp successions) were the products of cool-water seas. This view appears to be held by many workers in this field. However, Testa & Bosence show that corals are not major sediment contributors on the Rio Grande de Norte Shelf of Brazil even though the waters are sufficiently warm and appropriately low in nutrients, to allow their growth. The absence of corals from this system probably reflects the predominantly unstable substrates available.

Triassic ramps from Hungary, described by Hips and Torok in this volume were also heterozoan-dominated. These authors both favour the view that coral absence in these ramps was an evolutionary phenomenon related to the end- Permian extinction event, perhaps also coupled with other environmental factors, rather than a simple temperature restriction.

Lasemi et ai. describe heterozoan-dominated biotas from Carboniferous ramps of Illinois in which the main faunal components are bryozoans, crinoids and brachiopods. Such organisms are the main sediment producers in modern deeper, cooler waters, and a strictly uniformitarian approach would lead us to interpret similar ancient assemblages as cool-water faunas too. We might be in danger, though, of making over- simplistic interpretations based in an implicit faith in uniformitarianism. In fact, Taylor & Allison (1998) have recently shown that bryozoans experienced a major taxonomic turnover at the end of the Palaeozoic. Direct comparison of latitudinal distribution between pre-Palaeo- zoic and post-Palaeozoic bryozoans thus appears to be invalid. Post-Palaeozoic forms are most abundant in high latitudes while Palaeo- zoic bryozoans are more abundant in low latitudes. In the context of this debate, Lasemi et al. dismiss both a cool-water model for Carbonifer- ous low-latitude seas as well as any model invok- ing thermal stratification. Instead, they favour upwelling as a major control, whereby the faunal composition of the sediments indicates that waters were nutrient-rich.

Modelling

One of the most effective methods of testing the efficacy of these multitude of factors in controlling ramp development is to use computer models conditioned with appropriate geometric and stratigraphic data from outcrop and process data from modern carbonate depositional environments. The effectiveness of this approach is illustrated in two papers. Read, using Bowman's PhilR programme, models ramp development and architecture during greenhouse, transitional and icehouse intervals. His models are tested and refined against many examples, drawn largely from North America. Aureli et al. use the programme Carbonate 6 to analyse the controls on Kimmeridgian (late Jurassic) ramp development in the Teruel region of north east Spain. This model tests whether the mud component of mid-ramp sediment was more likely to have been redeposited from inner-ramp sediment factories, or was the product of pelagic or hemipelagic supply. The model with redeposition conforms most closely to the actual facies distribution.

Mud mounds

The enigmatic question of why mud mounds develop in many outer-ramp successions is addressed here by three papers. Lasemi et al.


4 V.P. WRIGHT & T. P. BURCHETTE

document Lower Carboniferous mounds from the Illinois Basin and Wendt & Kauffman and Kauffman provide separate studies of Devonian mounds from Algeria and Morocco. All represent intra-shelf basinal settings. The Algerian mounds show local alignment with tectonic lin- eaments, but but do not appear to have been associated with hydrothermal activity (cf. Belka 1998). Some of the Moroccan mounds are coral- rich, suggesting that they formed in shallower, warm waters, in contrast to many ancient mud mounds which are characteristic of deeper ramp settings.

Epeiric ramps

Many of the more stylized ramp models from the literature envisage shoal belts separating storm- dominated mid-ramp from lagoons and tidal flats in the inner ramp. However, there is no lack of examples in which no such belt can be identi- fied. Choi & Simo provide one such from the Upper Ordovician of Wisconsin. Should we con- sider designating such low-energy systems as 'epeiric ramps', or do we already have sufficient terms to juggle? Nevertheless, this category of ramps, apparently restricted exclusively to cratonic interiors, do require special consideration. In the latest Triassic, early Jurassic, and early to mid-Cretaceous, such ramps were widespread in what is now Iberia, northwestern Europe and the Middle East. As with their Ordovician equivalents in Wisconsin, such ramps were characterized by low wave energy and low tidal ranges, such that facies transitions are very gradual and facies belts are broad, while distinct shoal deposits are rare. In these Lower to Middle Jurassic 'Lias'-type successions diagenetic bedding and nodular limestones are dominant features, pointing to low sedimentation rates and extensive, early diagenetic remobilisa- tion of carbonate in these calcitic seas.

Future initiatives

The fact that modern oceans provide no good analogues for large, mature ancient ramp successions of the sort discussed in many of the papers in this volume, and which are so abundant throughout the geological record, makes it difficult to effectively apply uniformitarian prin- ciples to the interpretation of these ancient structures. The few modern, incipient ramps to which we do have access can provide an instant- in-time view of sediment dynamics in this sort of system (although little studied), but they actu- ally represent only the recovery stage following one of the most pronounced global sea-level

events in the Earth's history. They consequently lack the continuity with precursor successions which is critical if such information is to be effectively applied to the interpretation of whole ramp depositional sequences. Some useful additional information, on sediment dispersal patterns for example, can be gleaned from modern shelfal siliciclastic regimes, although differences in sediment provenance and accumulation sites set limits to the analogies which are possible here. Review of the papers in this volume shows that, beneath their deceptive simplicity, ramp depositional systems are every bit as diverse and complex as those of other carbonate platform types, with sediment transport paths which may trend for tens of kilome- tres in both onshore, longshore and offshore directions. The implications for the interpretation of thick sedimentary accumulations con- structed by ramp depositional processes are clear.

Ideally, studies of ancient ramp depositional systems should include a mechanism for com- bining the sort of process sedimentology derived from the study of modern ramp environments with the historical, sequential dimension gained from investigations of large outcrops. The use of increasingly sophisticated computer models provides one obvious route to test ideas on ramp dynamics formulated from outcrop studies. Sediments on ramps appear to have three main origins: in situ production, redeposition in the inner and mid-ramp by storms or contour currents, and pelagic fallout on the mid- and outer ramp. Using computer models, the relative volumes of these three sources, and variations in sediment dispersal patterns, can be modelled and tested against documented ancient examples.

Complexity is further compounded if biotic evolution is introduced into the modelling process. Many Palaeozoic, and possibly early Tertiary, ramps were clearly strongly influenced by abundant sediment-producing organisms in offshore settings and may represent systems dominated by in situ production of organic par- ticulate (rather than framework) sediment in this location. In contrast, the offshore environments of many early to mid-Mesozoic ramps are strik- ingly muddy and may represent systems in which offshore sediment transport was the most important process, as suggested by Aurell et ai. From the early Cretaceous onwards, outer-ramp sediments have been dominated by the remains of planktonic organisms, such as foraminifera, cal- cispheres and coccolithophorids. Naturally, sediment is contributed to some extent from most of these sources on most ramps, but variations in


INTRODUCTION 5

the proportional contribution of carbonate sediment due to evolutionary changes in the sediment producing organisms during the Phanerozoic may have influenced ramp profiles and their response to relative sea-level changes. This hypothesis could also be tested by means of computer simulations using multiple realisations.

The role of temperature as a control on ramp biotic patterns is likely to be another productive avenue for research and has been neglected his- torically. Once more, overstrict adherence to uniformitarianism when assessing the biotic compositions of ancient platform limestones seems unwise. New ideas on cool-water carbonate deposition have stimulated attempts to understand which factors really controlled ancient biotic distributions and we are being prompted to ask more appropriate questions as to why ancient carbonate systems differed from the modern. One possible research direction might be to more closely correlate palaeolati- tude with the nature of carbonate deposystems within discrete time intervals in order to assess likely temperature controls. It would also be appropriate to link this to studies of ancient sea- water temperatures using stable isotopes.

Low-energy, cratonic-interior, or epeiric ramps are a special category of carbonate platform and deserving of more intensive investi- gation. Were they low-energy or low- productivity systems - or both? Many show abundant evidence for condensation and reworking of sediments. Does this just reflect low rates of accommodation-space creation, e.g. due to locations in slowly subsiding cratonic interiors? If this were the case, where might any

over-produced carbonate sediment from these settings have gone? Ancient epeiric ramps of this sort persist in presenting us with major prob- lems in applying current ideas to these unusual systems.

We sincerely thank all authors of papers in this volume for their patience during the reviewing and editing process and also the many reviewers for their time and effort. This set of papers derives from a conference held at the Geological Society on 2-3 July 1996, entitled Carbonate Ramps: oceanographic and bio- logical controls, modelling and diagenesis. The meeting was sponsored by the British Sedimentologi- cal Research Group, the Geological Society of London and BP Exploration.

References

BATES, R. & JACKSON, J. A. 1987. Glossary of geology. American Geological Institute, Alexandria.

BELKA, Z. 1998. Early Devonian Kess-Kess carbonate mud mounds of the eastern Anti-Atlas (Morocco), and their relation to submarine hydrothermal venting. Journal of Sedimentary Research, 68, 368-377.

BURCHETrE, T. P & WRIGHT, V. P. 1992. Carbonate ramp depositional systems. Sedimentary Geology, 79, 3-57.

JAMES, N. R & CLARKE, J. A. D. (eds) 1997. Cool-water carbonates. Society for Sedimentary Geology & Special Publication 56.

READ, J. E 1985. Carbonate platform facies models. Bulletin of the American Association of Petroleum Geologists, 69, 1-21.

TAYLOR, P. D. (~; ALLISON, P. A. 1998. Bryozoan carbonates through time and space. Geology, 26, 459-462.


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