Carbonate-siliciclastic interactions : tertiary examples from Spain
It is a common misconception that shallow water biogenic carbonate development is inhibited in areas of active siliciclastic input. However, an increasing number of examples of ancient and modern coral communities are being identified which are affected by siliciclastic input and are developing in areas traditionally regarded as unfavorable. Corals can develop in nearshore settings affected by high sedimentation rates, turbidity, mobile substrates and episodic (storm-related) discharges of freshwater and terrestrially-derived sediments. Coral reefs in these systems are not necessarily impacted reefs, and represent natural states of development with coral abundance and diversity comparable to clear-water systems. Accepted models of shallow water carbonate production in clear water conditions only represent one end-member in a diverse range of shallow coral-dominated communities. In order to investigate the development of shallow water, biogenic carbonate development under the influence of siliciclastic input, two mixed carbonate-siliciclastic successions have been studied from the Tertiary of Spain. The Vie Basin (NE Spain) and the Fortuna Basin (SE Spain) provide contrasting examples of coral reef development within siliciclastic shallow marine shelf environments bordering temperate-humid and semi-arid land areas respectively. The methods employed in this investigation were 1) high resolution sedimentary logging and sample collection, 2) petrographic and palaeontological analysis of samples and 3) quantification of non-carbonate content through acid digestion. The correlation of logged sections, and development of a facies scheme for each study area, has enabled the temporal and spatial relationships between carbonate development and siliciclastic sedimentation to be deciphered. The Calders study area is situated within the Vie Basin. The broad environment of deposition was a moderate energy, northward-prograding siliciclastic shelf where high sediment input and unstable substrates inhibited the development of sessile calcareous biota. Carbonate development occurred following abandonment of the siliciclastic substrate. Abandonment facies, which developed on dune foresets and topsets, are dominated by large benthic foraminifera and coralline algae. Carbonate development, as high-energy foralgal shoals and muddy coral-dominated sediments, occurred down-slope. Carbonate units developed as very low angle clinoforms. Coral development was variable, existing as metre-scale patch reefs with associated debris and protected low-energy environments. Coral framework was only locally evident. Six isolated carbonate intervals are identified within the siliciclastic-dominated succession. The change from carbonate development to siliciclastic deposition is abrupt. It is proposed that sediments situated stratigraphically above the main Calders section in the Sant Amanc area are part of the Terminal Complex. The broad environment of deposition was a protected, inner-shelf that developed in the latest stages of marine sedimentation in the Pyrenean foreland prior to regional deposition of continental sediments. Nutrient-rich conditions persisted due to terrigenous input and terrestrial run-off. Metre-scale patch reefs developed through the baffling effect of the perennial seafloor vegetation that acted as a substrate for large benthic foraminifera. The Altorreal study area is situated within the Fortuna Basin. The environment of deposition was a marginal marine, high-energy fan delta system. Only robust organisms such as oysters were able to tolerate the periodic high-magnitude input of coarse-grained siliciclastics. The temporary abandonment of fan-delta lobes provided a site for coral colonization. Abandonment facies are siliciclastic packstones with laminar stromatolites in up-slope areas. Carbonates contain minor siliciclastic material and form laterally restricted sigmoidal units. Corals formed a framework, with variations in coral morphology a function of water depth. Carbonate production was halted through the combined affects of freshwater input, emergence and erosion. Eroded carbonate bodies were buried by fan-delta sediments as the locus of sedimentation changed. The main impacts of sediment input on photoautotrophs are physical burial, reduction in rates of photosynthesis though increased turbidity and changes in seawater chemistry, particularly salinity and nutrients. The amount and grainsize of siliciclastic sediment input has influenced the biota in both study areas. Siliciclastic-dominated sediments contain a low diversity fauna dominated by echinoids, molluscs and burrowing organisms. The reduction of siliciclastic input created a new environment that was initially colonized by organisms acting as r-strategists. These transitional settings are dominated by larger benthic foraminifera at Calders. At Altorreal, a prolonged period of non-deposition led to the formation of a hardground. Coral development at Calders occurred during a constant input of clay to silt-grade siliciclastics. In low-energy areas where sediment input was particularly high, constratal growth of branching corals is inferred. In marginal marine reef areas at Altorreal, coarse lithoclastic grains supported stick-like coral branches. Demise of coral communities is attributed to a number of factors. At Altorreal, emergence and erosion of the reef is inferred from erosional contacts. The development of columnar stromatolites is associated with a prolonged period of non-deposition and possibly a change in seawater chemistry that was detrimental to corals. The development of coral communities in siliciclastic settings can be aided through the existence of a protection mechanism. At Calders and Altorreal, temporarily abandoned siliciclastic substrates provided sites away from silts of high siliciclastic input. Autogenic factors such as delta lobe switching were the most important controls on coral development in the studied areas although allogenic factors such as climate and the tectonic regime influenced rates, magnitude and grainsize of input were also important.