| Résumé en anglais |
Nannoconids are marine, calcareous organisms which played a paramount, lithogenetic role in the formation of Late Jurassic and Early Cretaceous fine-grained, pelagic limestones. These limestones occur as homogeneous units or as successive, rhythmic beds with or without interbedded marly layers. This paper is based upon: a) our own studies of limestones and limestone-marl alternations which have been sampled both in land sections (South-East of France, Central Italy) and in oceanic cores (DSDP leg 1-40-41); b) data from an extensive literature dealing with these facies; c) data from literature on living Dinoflagellates. Most Nannoconids are observed as isolated bodies in sediments. Then their biological affiliation is not evident. We no longer consider them to be Coccolithophorids. We propose to compare them with calcareous Dinoflagellates, the life and ecological behaviour of which could allow explanation of the unusual characteristics of accumulations of Nannoconids. 1) Several examples of limestone/marl sediments lead us to confirm that Nannoconids proliferated in marine waters having reduced land derived imput: then fine-grained limestones were deposited. These Nannoconids coincided with an inhibition of Coccolithophorids. Nannoconids disappeared when detrital imput was high. Coccolithophorids replaced them and their calcareous plates constituted the fine calcareous component of marls. 2) Proliferations of Nannoconids apparently inhibit also the development of planktic foraminifera which are rather rare in many non argillaceous, marine limestones. They developed more commonly and were more diversified -along with Coccolithophorids- when marine waters were richer in clay material and marls were deposited. Benthic foraminifera also are sparse and less diversified in limestones beds than in marls. Besides Nannoconids, planktonic organisms are only represented by Radiolarian faunas in which species diversity follows the lithologic changes from limestones to marls. To explain the compatibilities between flora and fauna, we argue that selective dissolution, that would have dissolved only coccoliths and not the Nannoconids, is not reasonable. We favour a likely poisoning both of the surface marine waters, as happens when some modern Dinoflagellates develop red tides, and of the deep marine water because Nannoconus could have blanketed the sea floor impeding colonization by benthic life. 3) For a long time Nannoconids were thought to be confined to the Tethyan realm. Rather they are cosmopolitan and in noway limited to tropical areas. They are not absent from oceanic, Early Cretaceous deposits but less abundant and scattered in the sediment. Most of the time they are also badly preserved because of only slight lithification. They did not build up massive, extensive, thick deposits as they did in coeval epicontinental areas. Likely because water depth was greater in oceans than in epicontinental basins. 4) In epicontinental environnements, Nannoconus-rich sediments are located near the continents. It is an unlikely explanation that such localization is solely due to nutrient imput which would have been higher near the continents because Nannoconus accumulations always coincide with lowest continental alluviation. We show that proliferation of Nannoconids should have been impeded by a great water depth and in some cases by a more or less hypoxic or anoxic environment. From the literature dealing with modern, marine Dinoflagellates we have learned the following: 1) the formation of cysts, that are able to be fossilized, represents a reproductive system which greatly helps diffusion and migration of species. But such cysts can germinate and re-sow the surface waters only when they have settled in shallow water, in a thermic fluctuation zone, above the thermocline; 2) Dinoflagellates are able to develop in nutrient depleted waters; 3) blooms occur and persist during periods of stability of the water column. This set of data suggest the following interpretation of Nannoconus-rich limestones: a) sea waters with low, land derived argillaceous content and low nutrient content were favourable to the growth of Nannoconids; b) their blooms, like the red tides of some modern Dinoflagellates, can explain the exclusion of most of the other living organisms, not only the planktic ones (Coccolithophorids, some Radiolaria, planktic Foraminifera) but also at times the benthic ones; c) we favour the hypothesis that Nannoconids could have been meroplanktonic organisms, as are some modern Dinoflagellates. Below a critical depth, cysts which have been formed by these organisms and settled on the bottom lose any possibility of transport to the surface again and thus to germinate. This hypothesis can also explain their absence in hypoxic or anoxic facies such as black shales; d) the differentiation between tethyan realm and boreal realm is also evoked in the light of possible differences in volumetric detrital influx originating from significantly different hinterlands. Such different alluviations control, in turn, the proliferation of Nannoconus and thus the genesis of fine-grained limestones. |
