Aspects of pelagic sedimentation in the Devonian of Western Europe
Tucker, Maurice E.
Reading : Geological Institute
Other online version: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.737001
Tucker, Maurice E., 1971: Aspects of pelagic sedimentation in the Devonian of Western Europe. Reading : Geological Institute, 509 (405) S., DOI: 10.23689/fidgeo-3022.
During the Devonian of the Rhenich geosyncline (sediments exposed in the Rheinisches Schiefergebirge and Harz Mountains, Central Germany) condensed pelagic limestones, locally rich in cephalopods (Cephalopodenkalk), accumulated on submarine rises (termed Schwellen). These limestones pass laterally into shales with nodules and nodular limestones commonly involved in sedimentary slumping or reworking which were deposited on the slopes of the Schwellen. Silty shales, locally with turbidites were deposited in the deeper water areas (Becken) between the rises. The cephalopod limestones occur in three situations, above basement rises (geanticlines) submerged reefs and volcanic ridges. The Schwellen facies is present elsewhere in Europe, particularly during the Upper Devonian and has been examined in S.W. England and the Montagne Loire (S. France). The depth of deposition of the pelagic limestones probably did not exceed a few hundred metres and in some cases was 50 m or less. There is no evidence of emersion and all early diagenetic events took place subtidally. The basinal desiments probably accumulated at depths in the region of 1000 m. The Schwellen limestnes are fine grained carbonates with a dominantly pelagic fauna (cephalopods, thin-shelled bivalves, conodonts and cricoconarids) which have sufferedextensive recristallization and are now mostly homogeneous microsparites. Laminated carbonates and thin terrigenous units occur locally, the latter indicating deposition from low density suspension currents (or possibly nepheloid layers). Fossil concentrates of thin-shelled bivalves, cricoconarids and ostracods also indicate current activity. Lack of compaction and certain sedimentary structures suggest early lithification of the cephalopod limestones and hardgounds indicating synsedimentary cementationare locally developed. Planar corrasional hardground surfaces cut cavity-fill cements and skeletal material. Radiaxial calcite filling the truncated cavities has nucleated from the erosion surface indicating replacement of an earlier cement. Cryptohardgrounds having surfaces with an irregular relief and showing evidence of subsolution are encrusted by arenaceous foraminifera. Cricoconarid microcoquinas were cemented early through syntaxial overgrowths. The fibrous overgrowths crystals, showing some similarities with radiaxial calcite, are a replacement of an early acicular cement or the host sediment. Sheet cracks in the Schwellen limestones filled by microspatic internal sediment and radiaxial calcite are of variable size and shape and are considered to have formed by shear-failure. Neptunian dykes also occur. Ferromanganese encrustations associated with encrusting foraminifera (from the Montagne Louire) are detected in iron, manganese and nickel relative to modern manganese nodules. Chemically the Schwellen limestones are comparable with Recent pelagic sediments and are significantly different from limestones of other facies in being low in magnesium, high in iron and manganese. Differences exist between the Devonian Schwellen limestones, slope and basin nodules; the slope sediments with a more variable chemistry, tend to be enriched in magnesium, iron and manganese. The experiments are attributed to the effects of the slope and movement of connate waters. Contains: [Tucker, M.E., 1969. The sedimentological history of the Padstow area, North Cornwall (Abstract). Proc. Ussher Soc., 2, p. 111.] ; [Tucker, M.E., 1969. Crinoidal turbidites from the Devonian of Cornwall and their palaeogeographic significance. Sedimentology, 131, 281-290.] ; [Tucker, M.E., 1971. Devonian Manganese Nodules from France. Nature (Physical Science), 230, p. 116-117.] ; [Tucker, M.E. and Straaten, P. van, 1970. Conodonts and Facies on the Chudleigh Schwelle. Proc. Ussher . Soc., 2, p. 160-170.] ; [Tucker, M.E. and Straaten, P. van, 1970. Conodonts from the Upper Devonian of the Saltern Cove – Elberry Cove area (Abstract). Proc. Ussher Soc., 2, p. 159.] ; [Straaten, P. van and Tucker, M.E.. The Upper Devonian Saltern Cove Goniatite Bed is an intraformational slump. Palaeontology (pre-print).] .
- Geologie