SEDIMENTARY FOLDS AND THE DEVELOPMENT OF LIMESTONE IN AN EARLY ORDOVICIAN SEA

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Abstract
In a vast region in Scandinavia, a monotonous and extremely slow deposition of anaerobic mud persisted through the Late Cambrian. After a long time with no or very little sedimentation, deposition was recommenced in the Early Ordovician at an equally slow rate, but with calciumcarbonate as the main component. pH at the bottom was so low that the deposition of the carbonate in the long run only just outweighed the solution, which occurred frequently and sometimes during long periods. By oscil‐ lating solution and precipitation in the pores, lime mud could set into limestone on the sea‐bed. By more definite phases of solution, most of the limestone could be reduced to a marl bed with remains of undissolved limestone. Single limestone beds formed over such marl were apt to glide and deform into anticlines, which rose from the sea‐ bottom to heights up to 20 cm. These folds were rarely destroyed mechanically, which speaks for a tranquil bottom environment. Their crests were, however, cheinically corroded in the same way as discontinuity surfaces, which were also formed at many intervals. The processes by which the discontinuity surfaces were formed never affected a fold mechanically. The corrosion of the fold crests and of the discontinuity surfaces took place on the bottom of the sea, at safe distance from the surface. The corrosion was most frequently associated with the formation, and maintenance, of a glauconite skin. The abundance of preserved fold structures indicates that the region was relatively deep under the sea for most of the Early Ordovician. Later on, already in the middle‐Late Arenigian, the sea may have become shallower. The findings are extrapolated, with some probable interpretations and new observations, to include a hypothetical explanation for the passage from black shale to limestone facies.

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