RADIOLARIANS AND RADIOLARITES - PRIMARY P RODUCTION, DIAGENESIS AND PALEOGEOGRAPHY

Radiolarites are used as environment indicators, but indicators of whi ch environment? Moreover in literature, one commonly encounters opposi tion between calcareous and siliceous sediments. Is this correct for t he initial environment of deposition ? Backtracking from the geologica l message to the biological signal of the initial environment requires a comprehensive knowledge of the different filters which have tangled it and of the actors which generate the signal. Siliceous plankton Ra diolarians are planktonic organisms and as such obey the common rules of planktonic life: abundance/scarcity of nutrient. For decades one ha s evoked the necessity of a minimum silica content in sea water to all ow these organisms to precipitate their skeleton, to develop and to be profuse. Consequently geologists have considered it logical to find a bundant Radiolarians in rocks associated with ophiolites. However, the evidence to support this view, based on the study of Recent sediments , is lacking, indeed, Radiolarians are not most abundant along oceanic ridge areas but they are profuse in peripolar and equatorial belts an d on some western sides of continents. These are all regions where the overall plankton levels (phyto- or zoo-) are prolific and where nutri ent levels are high close to the water surface. There are no marked di fferences in the way oi life between siliceous and calcareous organism s. After death the only remnant of the organism is the test, as the cy toplasmic envelope soon disintegrates and exposes the mineral skeleton to the vagaries of nature. The dissolution of siliceous skeletons is more pronounced in surface waters. Moreover, the non-existence of a si lica compensation depth explains how a present day Radiolarian can occ ur in sediment at 1000 m as well as at 10000 m water depth: the critic al zone for its dissolution is within waters above c. 500 m. There is a ''behavioural'' difference between siliceous and calcareous organism s concerning their depth of dissolution. Siliceous sediments Siliceous skeletons are often very rare in sediments except in those deposited from high productivity zones. Indeed, if the original signal is reflec ted in the sediment, its variations are strongly enhanced and several times exaggerated. All occurrences indicate that the siliceous signal must be preserved beyond a critical threshold for tests to remain abun dant. In moderate productivity zones for instance, 1/3 to 1/2 of the s urface production reaches the ocean floor, while in zones of high prod uctivity, 2/3 is deposited. On a global scale, it is generally estimat ed that less than 1 % of the biogenous silica produced is found in the geological record. Siliceous rocks: example of Mesozoic radiolarites Radioiarian tests are made of opal-A which is highly instable and is s uccessively transformed into opal-CT and then into quartz. These two t ransformations occur in a liquid phase (one can here understand the ri sk that a Radiolarian with a delicate and fragile morphology will disa ppear before it reaches posterity) which explains both the relatively small number of well-preserved Radiolarians and the frequent concentra tion of silica as nodules. These transformations are associated with a considerable porosity reduction. This porosity variation is coupled w ith a major decompaction factor which has to be considered when recons tructions of sedimentary piles are put forward. One may generally esti mate a multiplication of the present thickness by a factor of 5. Appli cation of this factor to tethyan radiolarites leads to the following o riginal thicknesses for cherts: 20 cm (mean present thickness: 4 cm, f actor. 5) and for shares: 1 cm (mean present thickness: 0.5 cm, factor : 2). Using this formula, 100 m of radiolarites, a common thickness fo r this series in the Tethys, corresponds to an original thickness of a pproximately 450 m. The silica phase in most siliceous rocks is transf ormed from opal-A through opal-CT to quartz during progressive diagene sis. The transformations are principally controlled by temperature and time. This explains why quartz predominates in older rocks (Mesozoic and older) and opal-CT in the Cenozoic (porcelanites, tripoli,...). Ho st rock lithology, together with porewater chemistry, also plays an im portant role. The presence of primary carbonate tends to promote the o pal-A to opal-CT transformation, whereas the presence of clay tends to hinder it. For a primary alternation with slight lithological variati ons, an onset of the opal-A to opal-CT transformation can be different , depending on the lithologies. The beds with an early onset of this t ransformation import silica from the beds in which the transformation is delayed or inhibited. Local redistribution of silica between the co ntrasting lithologies is possible during the opal-A to opal-CT transfo rmation, whereas the redistribution seems more restricted during the o pal-CT to quartz transformation. Field evidence also suggests the form ation of chert nodules and pinch-and-swell beds by additional silica c ementation within the opal-A zone, especially within calcareous silice ous rocks. Silica redistribution during the opal-A to opal-CT transfor mation enhances the variations in composition between originally clay- rich and clay-poor layers. The majority of siliceous rocks lose their porosity by mechanical and chemical compaction and reprecipitation ass ociated with the opal-A to opal-CT transformation. One estimates that 30 to 40 My are necessary for a transformation from opal-A to quartz i n zones with a high sedimentation rate and 60 to 70 My for those with a moderate sedimentation rate. For the tethyan radiolarite, the quartz stage was thus reached around the Lower Cretaceous which explains: (1 ) irregular levels because of incomplete lithification, only the porce llanite stage being reached during the Tithonian tectonic phase (Helle nides structures); (2) the difficulty in dating cherts associated with ophiolites of inner zones, which were tectonically deformed before th e outer zones (i.e. Pindos-Olonos zone) where dating is relatively eas ier. Domains of deposition and age of radiolarites Radiolarites are Te thyan (those which are not of unknown origin !). They have been compar ed to the red clays of deep oceans. Current knowledge indicates that t his comparison is not valid, their only common characteristic being th eir colour (their deposition below the CCD not being systematic). Were they deposited in large or small basins? The problem with l