Scope:

To examine, on the field, the architecture and internal heterogeneities of depositional facies, as well as diagenetic patterns, in two different types of carbonate platforms:

    - a distally steepened ramp, and
    - a reef-rimmed shelf,

that are exquisitely exposed, from shallow-water to basinal (shallow basin) settings, on the sea cliffs of the Balearic Islands (Mallorca and Menorca).

Argument:

Predictive models for inter-well-scale variations in heterogeneous carbonate rocks are best made from outcrop studies of well-exposed examples, and the accuracy for prediction of these models highly depends on the detailed understanding of the genetic factors controlling facies architecture.

Excellent exposures of the Upper Miocene platforms along continuous outcrops on the sea cliffs of the Balearic Islands, as well as abundant water-well data (Figure 1), reveal in detail the 3-D facies belts distribution in two types of carbonate platforms. Oligophotic carbonate producing biota (red algae) dominates the Lower Tortonian platform and euphotic carbonate producing biota (coral reefs) dominates the Upper Tortonian-Lower Messinian platform. In these examples, most of the detailed stratigraphic heterogeneities are below the resolution of seismic and well-log analyses. Thus, they could aid in constructing realistic models for distribution, geometry, and volume of porous and permeable units of some shallow-water carbonate reservoirs, as well as models for fluid flow. The stratigraphic complexity, the heterogeneity in the lithofacies architecture and distribution of porosity in these carbonate platforms were the result of the type and amount of carbonate production, and the production/accumulation loci. These variables were related, in turn, to the interaction of:
(1) type of dominant carbonate-producing biota,
(2) accommodation changes controlled by sea-level fluctuations and
(3) basin-floor physiography.

Figure 1	Figure 2

The Lower Tortonian depositional sequence (Figure 2) is a 200 m-thick unit in Mallorca, mainly known from borehole data. On Menorca, however, it is up to 500 m thick in the subsurface and is well exposed. It is composed of two systems tracts. The transgressive systems tract is composed by large-scale cross-bedded pebbly sandstone units (foreshore), onlapping and backstepping onto Neogene and Mesozoic rocks. The highstand systems tract is composed of prograding and aggrading units, deposited on a high-energy distally-steepened ramp. On such a ramp, the slope steepens at the transition between the middle and outer ramp (basinal).

On this distally-steepened ramp in the HST (Figure 3), fan-delta and pebbly beach deposits pass into gently seaward-dipping inner-ramp lithofacies (seagrass meadow deposits), composed of burrowed foraminifer, lithoclast and mollusk dolopackstone. The inner ramp was in the euphotic (good light penetration) zone. They grade seawards into medium- to coarse-grained cross-bedded dolopackstone-grainstone of the middle ramp, with red algae, mollusks, bryozoans and some rhodoliths. The middle ramp itself passes seaward into a ramp slope facies with large-scale clinoforms dipping at 15º-20º. These are composed of rhodolithic rudstones to floatstones interlayered with coarse- to medium-grained grainstones, rich in red algal fragments, echinoids, bryozoans and foraminifera. The middle ramp and upper part of the ramp slope were in the oligophotic zone (poor light conditions) with carbonate production dominated by red algae and larger foraminifera. Water depth at the edge of the middle-ramp/ramp-slope is estimated to have been at about 30 to 40 m. Basinward, the ramp slope interfingers with thinly bedded and gently undulating fine-grained dolopackstone/wackestone containing planktonic foraminifera (outer ramp). The toe of the ramp slope was in the aphotic zone (no light), at more than 100-m water depth. Slide/slump scars on the lower slope are infilled by upslope backstepping cross-bedded dolograinstones (backsets, or antidunes) with very coarse intraclasts, mollusks, rhodoliths and, locally, Halimeda and oolites, interbedded with dolowackestones containing planktonic foraminifera. Large intraclasts are either derived from the littoral zone (bored clasts) or from the slope (fine grained dolowackestone).

In this ramp, coarse-grained grainstone, other than beach deposits, exist at four different settings (Figure 4), and they were deposited below the "wave-sweep base":
(1) Near the platform edge, where cross-bedded dolograinstone deposited by currents roughly parallel to the shoreline at 20-40-m estimated water depth.
(2) On the upper slope, where clinobeds are composed mostly of in-situ accumulation of rhodoliths and red-algae fragments.
(3) Lower on the slope, where very coarse intraclasts, mollusks, rhodoliths and other skeletal fragments infill slide/slump scars as upslope-backstepping bodies (backsets) and are incased in planktonic-foram dolowackestones.
(4) At the toe of the slope, where coarse-grained skeletal dolograinstones indicate bedform migration parallel to the platform margin, induced by currents at more than 100-m estimated water depth.

Figure 3	Figure 4

According to this model, the toe-of-slope grainstones may form significant reservoirs (in thickness and extension) and occur close to potential source rock. Grainstones at the outer edge of the platform may also form good reservoirs. Backsets on slope slide/slump-scars can form limited reservoirs, but they may also act as conduits connecting toe-of-slope grainstones with those at the platform outer edge. No grainstones related to shoals exist in this example.

The Lower Tortonian Ramp is considered a third-order depositional sequence according to stratal patterns and facies architecture (Figures 2 to 4). The transgressive systems tract (TST) is mainly composed of nearshore large-scale cross-bedded pebbly siliciclastic sandstone units, that onlap and backstep onto Neogene and Mesozoic rocks. The highstand systems tract (HST) is aggrading and prograding and corresponds to the high-energy, distally steepened carbonate ramp here described.

This 3rd-order highstand systems tract was deposited under conditions of high-frequency/moderate-amplitude sea-level fluctuations, driven by glacio-eustasy, which dominated the Late Miocene. Nevertheless, the architectural expression of these high-frequency sea-level cycles in the distally steepened ramp is not very apparent. Inner- to middle ramp settings are characterized by aggradation, with thin rhodolith-rich layers being the only expression of some kind of high-frequency cyclicity, with a subtle deepening-upward trend. The subtle deepening-upward trend observed in the inner/middle-ramp deposits can be related to the progressive landward expansion of the middle ramp, and reduction of land surface, which accompanied any amount on increase of accommodation. Ramp-slope settings are characterized by an alternation of rhodolithic rudstone/floatstone and grainstone intervals within the progradational package of the ramp. This alternation is possibly the expression of the high-frequency sea-level cycles, reflecting base-level fluctuations. The distally steepened ramp represents accumulation of loose grains that were produced both in the euphotic zone and in the oligophotic zone. During stillstands of sea level, basinward transport shed sediment from the inner- and middle ramp onto the ramp slope (Figure 5 A). During rise of sea level, elevation of base level produced aggradation of the system (Figure 5 B). During falls in sea level, slope progradation increased as result of basinward-sweeping of sediments from the inner and middle ramp settings remobilized by lowering of base level, and additionally by a basinward shift of the carbonate-production areas (Figure 5 C).

Figure 4	Figure 6

Tortonian ? Messinian Reef Complex

The Upper Tortonian-Lower Messinian depositional sequence (Figure 2) consists of progradational reef-rimmed platforms on all the Balearic islands. On Mallorca, it is exquisitely exposed in sea-cliff outcrops and well known from numerous boreholes. It is composed of several facies associations (Figures 2, 6 and 8): off-reef open-shelf lithofacies overlie the HST of the Lower Tortonian ramp and, in turn, are overlain by progradational forereef-slope and reef-core and, locally, by back-reef lagoon lithofacies.

In this Reef Complex, both stratigraphic and diagenetic heterogeneities derive from the complex hierarchical stacking patterns of high-frequency (4th- to 7th-order) accretional units (Figure 6). The basic accretional unit or building block of this prograding platform is the "sigmoid". Sigmoids stack into progressively larger-scale accretional units of sets, cosets, and megasets of sigmoids, which reflect hierarchical orders of sea-level cycles. Estimated amplitudes of these cycles are of less than 15 m, 20-30 m, 60-70 m, and about 100 m, respectively, and are thought to be glacioeustatic in origin. All these accretional units, which represent high-frequency depositional sequences (7th- to fourth-order), have similar characteristics in stratal geometries, bounding surfaces, and facies architecture (Figure 6). All of them are composed of horizontal lagoonal beds passing basinward into reef-core lithofacies with sigmoidal bedding, then into fore-reef slope clinobeds, and then into flat-lying open-shelf (or shallow basin) beds. For the lagoonal and reef-core units, boundaries are erosion surfaces (submarine and subaerial), that pass basinward into correlative conformities.

Figure 7	Figure 8

The Llucmajor platform exemplifies how sea-level change not only determines the relative hierarchy of the accretional units, but also their relative position and the facies belts developed within them. Up to four bundles (or systems tracts), which are related to specific parts of the sea-level cycle, can be defined from characteristic changes in the hierarchical stacking patterns among these accretional units (Figure 7). The "lowstand" systems tract (LST) formed during the initial sea-level rise, after the lowest point of the sea-level cycle. It mainly consists of prograding reef-core, with thin forereef-slope and open-shelf lithofacies without significant lagoonal beds. The aggrading systems tract (AST) corresponds to the most rapidly rising part of the sea-level curve, and it is volumetrically the most important. The AST is characterized by well-developed barrier reefs and thick aggradation without backstepping in all depositional systems, from the lagoon to the open shelf (shallow basin). The AST lagoonal lithofacies overlies the LST and consists of landward onlapping strata. The highstand systems tract (HST) is related to the highest part of sea-level cycle. It consists of prograding reef core, with forereef-slope lithofacies wedging out basinward and volumetrically condensed open-shelf lithofacies. Lagoonal beds commonly are absent (because of non-deposition or erosion during subsequent fall of sea level). The offlapping systems tract (OST) formed during falling sea level. It consists of prograding and downstepping reef lithofacies (fringing reefs without significant forereef-slope lithofacies), which downlaps on to the distal-slope and open-shelf lithofacies of the previous HST. There is no lagoonal lithofacies, and the open-shelf lithofacies is volumetrically condensed.

The relative volume of these high-frequency depositional sequences was basically dependent on the amount of carbonate production and sedimentation, which was directly related to accommodation changes controlled by sea-level fluctuations. The depositional profile was another important factor in controlling carbonate production. Areal extension of the lagoon was dependent upon the previous floor morphology and on the changes of relative sea level (Figure 8). Maximum lagoonal extension took place behind barrier reefs during rises of sea level on gently inclined surfaces; areas with steeper inclined surfaces had narrower lagoons. Volume of the forereef-slope sediments, however, was directly related to how extensive were the existing lagoons. Fringing reefs with little or no lagoons predominated in all areas during falls of sea level when coral reefs shifted downward and basinward as a result of the decrease in accommodation.

During major lowstands of sea level, carbonate production in shallow-basin settings was significant over a large area (Figures 8 and 9). Only coarse red-algae-rich sediment remained on the shallow open shelf, because wave action winnowed finer material and transported it to the deeper open platform. The red-algal biostromes of this setting interfingered with the fore-reef slope of the LST on the shallower shelf (Figure 8). When sea level rose, most of the production of sediment shifted to the shallower shelf, where extensive lagoons developed behind reefs, and the red-algal deposits were not produced because the basin floor was too deep to allow light penetration. Maximum progradation rates occurred across gently inclined areas, not only because accommodation was less, but also because carbonate production was greater.

In summary, heterogeneity in the lithofacies architecture in this type of prograding platform was controlled by the high-frequency changes both in accommodation and in carbonate production, in the absence of significant compaction and subsidence. Carbonate production was not an independent factor with respect to changes in accommodation; production varied according to changes in relative sea level and to the depositional profile of the basin floor

Figure 9	Figure 10

This carbonate complex also was overprinted by complex patterns of dolomite and secondary porosity that are related to probable 3rd- and 4th-order sea-level fluctuations. Distribution of porosity and permeability are functions not only of depositional textures, but also original mineralogy. Reef, upper-slope, and outer-lagoon units of the aggradational packages contain the thickest sections of permeable and porous rocks. Barriers of permeability mostly correspond to horizontal inner-lagoonal beds and condensed intervals in the fore-reef slope and basinal deposits. These stratigraphic and diagenetic heterogeneities are inherent in this type of prograding reef- rimmed carbonate platform as a consequence of the direct relationship between sediment input (carbonate production) and changes in accommodation (changes in relative sea level and basin-floor morphology). Heterogeneity such as this, therefore, should be considered in models for development of reservoirs in rimmed-shelf carbonate rocks, especially where subsurface data are insufficient to define the details of heterogeneity.
 

CONCLUSIONS

Process-oriented analysis of sedimentary rocks is shown to be fundamental in sequence-stratigraphic interpretation. This is particularly significant for carbonate rocks because of the complex biological response to environmental changes while siliciclastic systems show primarily a physical response.

Stratigraphic architecture of depositional sequences depends on the balance between changes in accommodation and sedimentation rate. However, these factors are interdependent in carbonates, because biological systems are involved. Relative sea-level changes and sea-floor topography determine the size (area) and efficiency of the carbonate factory. Other factors such as nutrients and temperature are fundamental in controlling carbonate production. Moreover, the type and loci of carbonate production influence the base level for sediment accumulation. Consequently, in carbonates, accommodation changes may result from a change of the predominant type of carbonate-producing biota (i.e., in ecological terms, from a change in community structure).

Upper Miocene platforms of the Balearic Islands illustrate this interdependence between accommodation and carbonate production in controlling both the internal architecture and the stacking pattern of carbonate depositional sequences. A distally-steepened ramp resulted from a biotic system producing loose grains throughout the photic zone and particularly in the deeper oligophotic zone. A rimmed platform resulted from euphotic carbonate production in a framework-dominated reef system. These two types of platforms exhibit different facies belts, internal architecture and distribution of heterogeneities despite being deposited under similar conditions of high-frequency sea level fluctuations.

The change from a distally-steepened ramp to a rimmed shelf produced two quite different 3rd-order depositional sequences. This change resulted from the replacement of a system with a physically-dominated base level, which was dependent on a loose-grain-producing biota, by a system with a biologically-dominated base level, which was characterized by an euphotic, framework-producing biota. While mean relative sea level remained approximately static, although punctuated by high-frequency (4th-order and higher) cycles (Figure 10), the ability of the system to build-up was enhanced by the change in the biotic system. Base level for sediment accumulation for the loose foramol-rhodalgal sediment associations of the ramp was related to wave base and associated currents, whereas base level for the framework-dominated reef complex was sea level. Thus, in carbonate rocks, sea-level cycles controlling depositional sequence development cannot necessarily be deduced directly from bedding geometries and lap-out patterns.

KEY  REFERENCES

2005, Pomar, L., Westphal, H. and Obrador, A., Oriented calcite concretions in upper Miocene carbonate rocks of Menorca, Spain: evidence for fluid flow through a heterogeneous porous system. Geologica Acta, 2 (4): 271-284.
    http://www.uib.es/depart/dctweb/LuisPomar/GeolActa.pdf

2004, Pomar, L., Brandano, M. and Westphal, H., Environmental factors influencing skeletal grain sediment associations: A critical review of Miocene examples from the Western Mediterranean. Sedimentology, 51 (3), 627-651.
    http://www.uib.es/depart/dctweb/LuisPomar/Sed-2004.pdf

2002, Pomar, L., Obrador, A. and Westphal, H., Sub-wavebase cross-bedded grainstones on a distally steepened carbonate ramp, upper Miocene, Menorca, Spain. Sedimentology, 49: 139-169.
    http://www.uib.es/depart/dctweb/LuisPomar/Sedim.pdf

2001, Pomar, L. Types of carbonate platforms, a genetic approach. Basin Research, 13: 313-334.
    http://www.uib.es/depart/dctweb/LuisPomar/Platforms.pdf

2001, Pomar, L. Ecological control of sedimentary accommodation: evolution from a carbonate ramp to rimmed shelf, Upper Miocene, Balearic Islands. Palaeogeography, Palaeoclimatology, Palaeoecology, 175: 249-272
    http://www.uib.es/depart/dctweb/LuisPomar/Palaeo3.pdf

2000, Robledo, P. and Pomar, L., The karst collapse structures in the Upper Miocene of the East coast of Mallorca: genetic model. XVIIth Edition Theoretical and Applied Karstology and IGCP 448. (World correlation on karst geology and relevant ecosystems) Cluj, Hungary (19-22 July, 2000). POSTER  
    http://www.uib.es/depart/dctweb/LuisPomar/PaleocollapsesPOSTER.pdf

1999, Pomar, L. and Ward, W. C., Reservoir-scale Heterogeneity in depositional Packages and Diagenetic Patterns on a reef-Rimmed Platform, Upper Miocene, Mallorca, Spain, A.A.P.G. Bulletin, 83, 1759-1773.
    http://www.uib.es/depart/dctweb/LuisPomar/aapg.AAPGbull99.pdf

1996, Pomar, L., Ward, W. C., Green, D.G., Upper Miocene reef complex of the Llucmajor area, Spain; in Franseen, E., Esteban, M., Ward, W.C. and Rouchy, J.-M., eds., Models for Carbonate Stratigraphy from Miocene Reef Complexes of Mediterranean Regions: S. E. P. M. Concepts in Sedimentology and Paleontology No. 5, pp. 191-225.

1995, Pomar, L. and Ward, W.C, Sea level change, carbonate production and platform architecture, in B. Haq ed., Sequence stratigraphy and depositional response to eustatic, tectonic and climatic forcing, Kluwer Academic Press. p. 87-112
    http://www.uib.es/depart/dctweb/LuisPomar/PomarWard95.pdf

1994, Pomar, L. and Ward, W.C, Response of a late Miocene Mediterranean reef platform to high-frequency eustasy, Geology, v. 22, p. 131-134.
    http://www.uib.es/depart/dctweb/LuisPomar/Geology94.pdf

1994, Bosence, D. W. J., Pomar, L., Waltham, D. A. and Lankaster, H. G, Computer modelling a Miocene carbonate platform, Mallorca, Spain, A.A.P.G. Bull., v. 78, p. 247-266.
    http://www.uib.es/depart/dctweb/LuisPomar/AAPGbull94.pdf

1993, Pomar, L., High-Resolution Sequence Stratigraphy in Prograding Miocene carbonates: Application to Seismic Interpretation, in R.G. Loucks and J.F. Sarg, eds., Carbonate Sequence Stratigraphy, Recent Developments and Applications, AAPG Memoir 57, p. 389-407.
    http://www.uib.es/depart/dctweb/LuisPomar/Seismic-93.pdf

1991, Pomar, L., Reef geometries, erosion surfaces and high-frequency sea-level changes, Upper Miocene Reef Complex, Mallorca, Spain: Sedimentology, v. 38, p. 243-269.

Home | Technical Program | Field Trip | General Information