Laminated Eocene maar-lake sediments from Eckfeld (Eifel region, Germany) and their short-term periodicities

The Eocene age Eckfeld/Eifel maar lake is interesting both because of the excellently preserved fossil remains and also for the laminated sediment sequence deposited under subtropical conditions. The latitude, however, has a higher seasonal insolation difference than comparable recent environments due to the northward shift of climatic zones during the Eocene. The overall aim of this investigation was to compare sediment types and lake development of a Tertiary (subtropical) maar lake with recent maar lakes, to discuss the origin of laminations, and search for their possible periodicities. The main sediment types, which were exposed during recent paleontological excavations, are laminated oilshales, partly laminated bituminous siltstones, and graded beds (mostly turbidites), which sometimes include laminated clay tops. Some deeper parts of the profile drilled in 1980, are dominated by diatomaceous laminites. The amount of organic carbon (around 8.5%) is comparable with recent lakes with low sedimentation rates. The high C/N ratios (25-30) of the Eckfeld sediments, as well as the fact that the highest C-org values are found in the graded beds, shows the importance of terrestrial plant debris for these lake sediments. Lamination of the oil shales was caused by the complex interrelationship of elastic supply (mineral and plant detritus), autochthonous organic deposition (mainly green algae), and early diagenetic siderite formation. The darker sublaminae represent the coarser elastic (organic and inorganic) input together with the autochthonous supply from the lake (green algae), whereas the lighter sublaminae are composed of settled fine-grained, mostly siliciclastic detritus derived from the probable yearly turnover of the density stratification of the lake. The thickness of the fine siliciclastic and organo-clastic layers, the morphology and palaeogeographical situation of the maar lake, and the occurrence of algal layers, strongly suggest that the laminated sediments are varves (although there is no definitive proof of this). Thickness variations from ca. 700 oil shale laminae were measured and computed. Time series analysis yielded at least one certain period of 5.5 yr, accepting that the laminae are indeed varves. The duration of the laminated lake sequence from Eckfeld maar, as deduced from sedimentation rates of the different sediment types is ca. 82,000 yr and, thus, within the typical life time of other small maar lakes. (C) 1998 Elsevier Science B.V. All rights reserved.