Profil, Vol.6, 1993
Leinfelder, R. R. (1994a):
Karbonatplattformen und Korallenriffe innerhalb siliziklastischer Sedimentationsbereiche (Oberjura, Lusitanisches Becken, Portugal).
Carbonate Platforms and Coral Reefs within a Siliciclastic Setting (Upper Jurassic, Lusitanian Basin, Portugal).-
Profil, 6, 1-207, Stuttgart (Inst. Geol. Pal. Univ.)
Conclusions
Resumo portugues
Abstract:
During the Late Jurassic, the Lusitanian Basin of Portugal experienced an intensive rifting phase which caused pronounced bathymetric and, hence, facies differentiation. Particularly during the Kimmeridgian and Tithonian, siliciclastics were fed into the basin, resulting in a mixed carbonate-siliciclastic basin fill. Carbonate platforms and isolated coraliferous reefs of different dimensions and composition frequently developed within this setting.
A shallow-water carbonate platform exhibiting distinct facies zonation is represented by the narrow Ota Platform (Kimmeridgian). The buildup exhibits an aggradational architecture and is rimmed by a high-energy, high-diversity coral reef. In contrast to most other Upper Jurassic high-energy reefs, the Ota coral reef contains abundant microbial and algal crusts. This was due to the achievment of equilibrium conditions between production and export of debris, which can be explained by the existence of a tectonically caused, steep by-pass margin. Sedimentation in interior platform settings is mostly characterised by stacked, autocyclic, small-scale shallowing-up sequences. The narrow Ota buildup developed over a basement horst and was protected from surrounding siliciclastics by its elevated position and a strong longshore current.
The Castanheira slope-type fan delta (Kimmeridgian) also formed at the strike-slip margin of a continental pull-apart subbasin. The fan sediments are dominated by coarse arcosic conglomerates. Coral-microbial reefs grew on deactivated fan areas during two phases of relative sea-level rise. Collapse events in the course of sea level falls led to resedimentation of allochthonous limestones in more distal fan areas.
During transgressive phases coral biostromes and ooid bars developed on top of a fine-grained siliciclastic slope system which formed coevally with parts of the Ota Platform and the Castanheira Fan (Kimmeridgian Amaral-Abadia ramp system). The Alrota patch reefs (Tithonian), the Caldas-Bolhos carbonate unit (Kimmeridgian) and other structures also developed within siliciclastic settings. Diversity of coral faunas decreased where elevated sedimentation rates occurred. Functional parameters such as number of septae, calical type and general growth form of corals, as well as incrustation rates enable to estimate the rate of sedimentation.
Comparison with carbonate buildups developing distant from siliciclastic influence (Montejunto Platform, Oxfordian; Sintra Ramp, Kimmeridgian to Tithonian) shows that carbonate production and faunal diversity within siliciclastic settings need not be inhibited, as long as efficient siliciclastic fences and traps are active (e.g. rising salt diapirs, structural highs, subsidence traps, longshore currents, oolite filters). Relative sea-level rise additionally facilitates the growth of reefs and carbonate platforms. However, it is not a general necessity for carbonate production within siliciclastic settings. Well adapted corals may produce carbonate structures even directly within a terrigeneous environment.
The Upper Jurassic mixed carbonate-siliciclastic sediments of the Lusitanian Basin are characterised by rapid facies transitions. Third-order sea-level changes only modified the principal depositional systems rather than entirely changing them. These principal systems (particularly siliciclastic prograding slope, siliciclastic fan, carbonate platforms) were determined by pre- and synsedimentary rift tectonics and persisted over several third-order sea-level cycles. Oil plays were common within the Jurassic of the basin, although former fossil fuels were largely destroyed by later inversions and subsequent microbial decay.
The analysis of the architecture of Upper Jurassic deposits in the Lusitanian Basin shows the complexity of such ocean marginal basins, most of which today represent poorly accessible offshore basins. The interpretation of factors leading to the development of carbonate platforms and coral reefs within a dominantly siliciclastic realm is a powerful tool for reconstructing basin development and for assessing hydrocarbon potential.
Please note: This study also includes figure labelling, figure captions and ca. 8 pages of chapter summaries (see table of contents) in english language.
New results on carbonate sedimentology based on the Ota-Platform and other 'buildups':
- Almost the entire narrow Kimmeridgian Ota Platform, today of max. 11 x 2 km extension, is preserved. Good exposure conditions allow a three-dimensional analysis of the strucuture of this remarkable carbonate platform. It is particularly characterised by well developed facies belts forming an aggradational architecture. From West to East follow a reef, back reef, peritidal and lagoonal belt. In the East, cross-bedded grainstones can be interpreted as relics of a shoal situated at the landward margin of the platform. More general results of facies analysis include:
- Secondarily irregularly blackened sediment horizons and blackened karstic cave fills occur in association with black pebble development in both Ota and Montejunto-Platform. Both blackened horizons and black pebbles may be rich in organic matter. This proves the existence of host rocks for black pebbles and of migrating organic substances, both of which was doubted by Shinn & Lidzt (1988) who rather see black pebble formation related only to fossil fires. The existence of entire sediment layers and the richness in bituminous matter proves the general correctness of the models of black pebble formation by migrating organic substances of various origin (microbial or algal matter, higher land plant material, partly charcoal), as expressed by Barthel (1974), Strasser & Davaud (1983) or Leinfelder (1987b) (see Chap. 2.2.6).
- Marine giant oncoids up to 15 cm in diameter occur within the Ota Limestone and the Caldas-Bolhos unit. Birdseyes and Bacinella structures indicate their very shallow origin. They grew within tidal channels. Amalgamation of several oncoids and growth of cortical protrusions into interparticle cavities proves the in-situ growth of at least parts of them. In-situ growth of oncoids is well known from modern freshwater environments, and can be deduced for some ancient freshwater environments by a set of criteria (Leinfelder & Hartkopf-Fröder 1990), whereas in-situ growth is rarely discussed for marine oncoids so far (see Chap. 4.1.2).
- Autocyclic small scale shallowing up sequences are widespread in the interior parts of the Ota-Platform. Their character and lateral non-persistence reveal that they were formed by autocyclic shifts of depositional subsystems. Models for their development comprise: Progradation-cementation-erosion system in the back reef zone; progradational wedge and tidal channel system in the peritidal belt, and autocyclic mud ridge development in the lagoonal area (see Chap. 2.2, 2.4).
- Early karst features of the Ota Limestone include marginal microsparitisation of carbonate grains, early diagenetic solution pipes, honicomb structures and karst cavities. The latter are often filled with residual sediment which became secondarily enriched in echinoderms by selective dissolution of aragonite particles. This is evidence of the very early character of these features, which are thought to be due to mixed-water diagenesis. Partly amalgamated subaerial exposure horizons and karst cavities allow to decipher 14 phases of intraformational and postformational development of the Ota-Platform.
- The aggradational Ota-Platform is a good example for the inhibition of progradation. Carbonate productivity was doubtlessly high, which is shown by the high rate of infill of the very shallow lagoon and particularly by the position of the intertidal belt directly behind the reef. It is assumed that the tectonic origin of the eastern plarform margin resulted in the development of a steep by-pass escarpment, preventing progradation. This interpretation is in accordance with thewinnowed character of the reef rim (see below) and the detection of marginal faults in seismic lines. In this example, sea level change had no influence on the general architecture (progradation or aggradation) of the buildup. Base-of-slope debris aprons can be postulated but are not exposed. If positioned in an offshore basin and drilled by only one core, as common in offshore basin analysis, the character of this narrow buildup, which does not follow 'Waltherian' rules, would certainly have been misinterpreted.
Comparison between Upper Jurassic buildups from the Lusitanian Basin
Three types of buildups exist:
- Halokinetically controlled buildups (type A) developed away from rising salt pillows into the salt rim structures. They hence are mostly of large dimension and show progradational ramp-type to shelf-with-depositional-margin development. The standard example is the grain-dominated Oxfordian Ramalhal buildup. Its Kimmeridgian relics are the Vimeiro and Caldas-Bolhos buildups, which developed close to the structural axis of the salt pillow and show an additional control by tectonics.
- Most medium sized buildups of the basin fall into the category of tectonically controlled buildups (type B). These are mostly lime-mud dominated or, at least, rich in micritic sediments. According to the degree of structural control they range from ramp to bypass-margin types. Examples are the Sintra, Barreiro, Montejunto and Ota buildups.
- Transgressive buildups (type C) occur in correlatable levels or units characterised by a reduction in background sedimentation, which came along with sea level rise. The buildups are grain- or mud-dominated or mixed, and may exhibit a clay-rich matrix. They are mostly lensoid, including medium to small-sized bioherms as well as biostromes and marly coral meadows. Examples comprise the Monte Gordo and Castanheira reefs, the Alrota reefs and similar coral associations in the Sobral beds as well as at Consolação and in the western Serra da Arrábida. Transgressive buildups may also be laterally extensive as in the case of the Amaral formation.
The significance of the buildups from central Portugal for Upper Jurassic reef models
- Most reefs from central Portugal match, to a large degree, the Upper Jurassic reef models as given by Crevello & Harris (1984). An exception is the Ota reef, which despite its high-energy, shallow-water character contains abundant microbial crusts. This is explained by the wave-washed character of the reef, which was facilitated by the existence of a steep bypass margin. This prevented excessive accumulation of carbonate debris within the reef belt, a fate which afflicted most other Jurassic high-energy reef, leading to their eventual suffocation within debris piles.
- Thrombolite reefs with partial participation of siliceous sponges and corals, as occurring in the Serra Isabel level and in the topmost part of the Abadia formation, represent another, previously largely unknown Upper Jurassic reef type. Its development is related to fluctiations in oxygen/nutrient concentrations (Leinfelder et al. 1993a,b). For a more complete discussion of Upper Jurassic reef types and models see Leinfelder (1992, 1993a, in english language).
The mixed carbonate-siliciclastic depositional models
During the Kimmeridgian and Tithonian coral reefs and carbonate buildups of the Lusitanian Basin largely grew within siliciclastic settings. The carbonates were largely or completely protected from terrigeneous influence by a variety of sheltering mechanisms. These represent a variable combination of factors, such as elevated position, longshore currents, filtering of quartz by ooid formation, local subsidence traps and sea level rise. In incompletely protected areas, adapted corals could form local patch reefs. The degree of adaption of corals can be estimated by several morphological criteria (number of septae, calical type, growth form) (see Chap. 6.2).
The principal mixed depositional system models are:
- (1) Intrabasinal platform horst: The Ota-Platform, situated on a basement uplift, is protected from terrigeneous clastics by this elevated position and by coastal longshore currents, the latter explaining the screening-off of suspended clay and, therefore, the purity of carbonates. Model criteria are: Coeval sedimentation of siliciclastics and carbonates; deep marine character of surrounding siliciclastics, development of aggradational architecture with facies zonation spanning several sea level cycles (indicating tectonic nature of platform uplift), episodic contamination of carbonates by aeolian quartz silt and terrigeneous driftwood pebbles. The Boldos-Caldas buildup was controlled by tectonic and halokinetic uplift, and was additionally protected from siliciclastics by an island chain.
- (2) Mixed siliciclastic-carbonate fan delta: The calcarous coral reefs of the arkosic Castanheira slope-type fan delta grew on deactivated fan lobes (see before). Among the sreening-off facors are sea level rise and longshore currents, whereas climate did not play a major role in reducing siliciclastic influx. Diagnostic for the model are: fan-delta sedimentology and architecture; isolated reefs within distinct levels; deepening-shallowing trend of reefs; termination of reef growth by karst phases; allochthonous carbonates.
- (3) Prograding, fine-siliciclastic slope system capped by reefal to oolitic carbonates: On top of the Abadia slope system, the Amaral carbonates were formed during reduced siliciclastic input caused by growing distance from the terrigenous source area due to slope progradation and sea level rise. Remaining siliciclastics were partly encrusted by ooids or channelled downslope. Model criteria include: shallowing-up siliciclastic slope succession, with slight deepening trend in topmost part; crust-rich carbonates in transition zone, evidencing strong reduction of sedimentation rate; shallowing-up trend within carbonates, with a final deepening into terrigeneous prodelta sediments, bringing a return to siliciclastic sedimentation. Ramp type carbonates show a general large-scale uniformity, superimposing a small-scale mosaic distribution of reefal subfacies, due to rapid retrogradation and back stepping during sea level rise.
- (4) Lagoonal patch reefs within shallow siliciclastic setting are represented particularly by the Alrota coral reefs and biostromes. Sea level rise resulted in general reduction of terrigenous influx, although fine siliciclastics did reach the depositional setting in considerable amounts. Model characteristica are: Lensoid character of reefs; occurrence of reefs within a distinct time interval; reef organisms adapted to background sedimentation; local catch-up trends due to positive feedbacks; frequent termination of reef growth by clayey sedimentation; general reduced sedimentation rates of the low-energy, shallow lagoonal sediments, indicated by a high amount of burrows and other criteria.
Comparison of coral reefs and platforms from siliciclastic and carbonate settings
Mechanisms screening-off siliciclastics can be very effective, so that carbonate productivity and diversity of organisms often can be very high even within carbonate areas surrounded by siliciclastic sedimentation (e.g. Ota-Platform). Examples from pure carbonate settings (Sintra-Ramp, Montejunto-Platform) may even show lower diversity of reef organisms, caused by a deeper position of reefs and elevated sedimentation rate of lime mud. Rate of background sedimentation, both terrigeneous and calcareous, controls the diversity and structure of reef associations at least to, or higher than, the degree exerted by bathymetry/light, water energy and salinity.
Another interesting aspect is that nutrient input during times of dominance of terrigeneous material was probably higher than during times of carbonate dominance. The high diversity of reef organisms in the Ota reef (surrounded by siliciclastics) and their partial high diversity in the Alrota reefs (situated directly within a terrigeneously polluted setting) could indicate that during the Jurassic reef corals were not yet specialised to oligotrophic settings as are modern corals.
Sequence stratigraphy of the Upper Jurassic of the central Lusitanian Basin
- It was the rift activity of the basin which dominated the development of major depositional systems (siliciclastic fan delta, prograding slope system, carbonate buildups), part of which occurred penecontemporaneously, with siliciclastics being introduced from geographically different sources. Sea level changes of third order only caused modifications within these systems but did not generally transform them into other ones: Carbonate platforms developed karst and black pebble horizons, hardgrounds, or narrower spacing of small-scale cycles; within siliciclastic systems coraliferous reefs occurred during transgressive phases. Such local third-order cycles are mostly correlatable, despite the strong differences in sedimentary development. Furthermore, a composite sequence stratigraphic interpretation is correlatable with the redefined third order sequences of Ponsot & Vail (1991a,b). It is, however, assumed that Lusitanian Basin Upper Jurassic third order sequences are largely controlled by regional tectonics (see Leinfelder 1993b, Leinfelder & Wilson ,in prep.).
The discontinuous development of the Upper Jurassic of the Lusitanian Basin, and its significance for the analysis and hydrocarbon exploration of similar marginal basins
- Due to later Alpine tectonics the Mesozoic Lusitanian Basin is accessible on land, giving it a model character for most of the other Atlantic ocean marginal basins situated in today's offshore. Of particular importance are: the heterochroneity of many sedimentary units (including structurally controlled carbonate buildups), the narrowness of many buildups, and the development of local graben structures between the buildups, which are filled with siliciclastics.
- The great variety of sediment types, architectures and structural styles led to promising oil plays, due to a wealth of Upper Jurassic source, reservoir and seal rocks as well as structural traps and early maturation caused by local strong subsidence. However, later inversion tectonics destroyed most occurrences of exploitable petroleum. Nevertheless, the study of potential oil plays may provide a model for the style of petroleum formation in similar basins: The structure of the Barreiro buildup caused accumulation of petroleum in younger sediments, domed up by the buildup. The partially dolomitised Ramalhal grainstone buildup directly superimposes the dominant source rocks of the basin, the bituminous Cabaços beds, and is sealed by the fine grained Abadia beds. The poorly cemented arcoses of the Castanheira fan are still rich in microbially altered petroleum at places. They originally were sealed by onlapping fine-grained Abadia siliciclastics. Source rocks could be both the Cabaços beds, with good migration paths along deep synsedimentary rift faults, as well as parts of the fine grained Abadia beds, which at outcrop are rich in bitumen at places. The Castanheira conglomerates are an excellent example of a thick, promising reservoir rock developed in a graben, a structure which normally would not be drilled. Relics of bitumen are also visible in karst cavities and grainstones of the Ota and the Amaral buildups, both of which directly border the Abadia beds, as potential source rocks. The Ota-Platform is additionally connected to the Cabaços source rocks by syn- and postsedimentarily active normal faults. Furthermore, bituminous impregnations are common in the Sobral estuarine delta sediments and the Arranhó lagoonal sediments, both of which would show associated reservoir rocks within the same sedimentary systems (Sobral: delta front sands, sealed by prodelta and interbay clays and silts; Arranhó: level-constant, highly compartmentalised but numerous calcareous patch reefs) .
Resumo
Durante o Jurássico Superior, a Bacia Lusitânica (Portugal) foi sujeita a uma intensa fase de 'rifting' o que provocou uma acentuada diferenciação batimétrica e, consequentemente, de fácies. Particularmente durante o Kimeridgiano e o Titoniano, material siliciclástico invadiu a Bacia, daqui resultando o desenvolvimento de uma sedimentação mixta, carbonatada e siliciclástica. Neste contexto, formaram-se plataformas carbonatadas e recifes coralígenos de dimensões e composição variadas.
A Plataforma da Ota é o exemplo de uma plataforma carbonatada de pequena profundidade, que exibe uma pronunciada zonação de fácies (Kimeridgiano). Este edifício carbonatado revela uma arquitectura agradacionária e é bordejado por um recife de alta energia, composto por corais muito diversificados. Contrastando com a maioria dos outros recifes de alta energia do Jurássico Superior, o recife da Ota contém abundantes crostas algais e microbióticas. Isto deve-se ao equilíbrio estabelecido entre as condições de produção e de exportação do material biodetrítico, o que pode explicar-se por forte inclinação da margem da plataforma, facilitando o transporte 'off-bank'. A sedimentação no interior da plataforma é caracterizada por um empilhamento de sequências de pequena escala, batidecrescentes e autocíclicas. A estreita plataforma da Ota desenvolveu-se sobre um horst de soco e ficou protegida do material terrígeno circundante pela sua posição elevada e pela acção de forte corrente litoral.
O delta em leque de talude da Castanheira (Kimeridgiano) depositou-se na margem leste, tectonicamente activa, de uma subbacia de estrutura 'pull-apart' continental, sendo os sedimentos dominados por conglomerados arcósicos grosseiros. Recifes coralígeno-algais desenvolveram-se nas zonas desactivadas daquele leque submarino, durante duas fases de elevação relativa do nível do mar. Alguns episódios de colapso, no decurso da diminuição do nível do mar, provocou a resedimentação de calcários alóctonos nas áreas mais distais daquele leque submarino.
Durante as fases transgressivas, biostromas de corais e barras oolíticas desenvolveram-se no topo de um sistema siliciclástico fino, de talude, o qual é contemporâneo de parte da plataforma da Ota e do leque submarino da Castanheira (Fms de Amaral e de Abadia, Kimeridgiano). Os 'patch reefs' de Alrota (Titoniano), a unidade carbonatada de Caldas-Bolhos (Kimeridgiano) e outras estruturas coralígenas, desenvolveram-se, também, em contextos siliciclásticos. A diversidade das faunas coralígenas diminuia quando a taxa de sedimentação aumentava. Certos parâmetros funcionais, como o número de septos, tipo de cálice e a morfologia geral dos corais podem ser utilizado na avaliação da taxa de sedimentação.
Comparando aquelas estruturas com as plataformas e recifes que se desenvolveram longe das influências siliciclásticas (plataforma de Montejunto do Oxfordiano; rampa de Sintra do Kimeridgiano e Titoniano) conclui-se que a produção de carbonatos e a biodiversidade, nos domínios siliciclásticos, podem não ser afectados, desde que existam barreiras e armadilhas sedimentares activas. (por ex: relevos diapíricos, altos estruturais, 'armadilhas' por subsidência, correntes litorais, filtros de oólitos). A elevação relativa de nível do mar facilita e reforça o desenvolvimento dos recifes e das plataformas carbonatadas. No entanto, isto nem sempre é necessário para a produção de carbonatos em domínio siliciclástico. Corais bem adaptados podem produzir estruturas carbonatadas, mesmo em contacto directo com o ambiente terrígeno.
A sedimentação mixta carbonatada e siliciclástica do Jurássico Superior da Bacia Lusitânica carateriza-se por apresentar rápidas mudanças de fácies. Oscilações de 3a ordem do nível do mar apenas provocam modificações nos sistemas deposi-cionais principais, não conduzindo à sua mudança total. Estes sistemas deposicionais (em particular os de talude siliciclástico, e as plataformas carbonatadas) foram originados por tectónica pré e sinsedimentar ligada ao 'rifting' e persistiram durante vários ciclos eu-státicos de 3a ordem. Verificam-se numerosas ocorrências de hidrocarbonetos as quais, todavia, foram destruídas pela posterior tectónica de inversão e subsequente destruição microbiana.
A análise da arquitectura dos depósitos do Jurássico Superior da Bacia Lusitânica mostra a complexidade deste tipo de bacias marginais oceânicas, a maioria das quais situa-se na pouco acessível área do 'off-shore'. A interpretação dos factores que determinaram o des-envolvimento dos recifes e das plataformas carbonatadas no seio dos ambientes siliciclásticos é um instrumento essencial na reconstituição da evolução das bacias sedimentares e na avaliação do seu potencial petrolífero.
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Last changes Nov. 2004 by Reinhold Leinfelder