STRATIGRAPHY AND PALEOECOLOGY AT THE WENLOCK/LUDLOW BOUNDARY ON SAAREMAA ISLAND, ESTONIA

The boundary between the Wenlock Series and the Ludlow Series can be easily observed on the island of Saaremaa in western Estonia. Here, the boundary is distinguished by a major disconformity that can be correlated to a regional regression described in several previous studies. During this time, western Saaremaa was a lagoonal facies that reflected sea-level changes within the Baltic Basin. We measured and described this Wenlock-Ludlow boundary interval at Soeginina Cliff on the western shore of Saaremaa. Here this boundary consists of the Vesiku Beds of the Rootsiküla Formation (Wenlock) overlain by the Soeginina Beds of the Paadla Formation (Ludlow). The Vesiku Beds (Wenlock) record a carbonate lagoonal environment with finely laminated beds and Thalassinoides burrows (indicating oxygenated bottom conditions). The fauna is much less diverse than that in normal marine sediments of the Wenlock. The top surface of these beds (the primary discontinuity surface) shows a microtopography and dissolution consistent with exposure and abrasion. The top 20 centimeters also show diagenetic alteration of the laminated sediments, probably from fluids traveling through the Thalassinoides burrow systems. The Soeginina Beds (Ludlow) show pulsating transgressive sediments with multiple discontinuity surfaces. Large oncoids are common in these beds. They have distinctive shapes because they were initially spherical and later stabilized and grew like small stromatolites upwards. These forms may indicate periodic energy reductions in these transgressive waters. There are also storm beds with biogenic debris including oncoids nucleated on gastropods. This boundary interval is topped by thin dolomites and stromatolites. This example of the Wenlock-Ludlow boundary can be correlated with other such disconformities recorded in a variety of depositional environments, such as in the equivalent reef complexes of Gotland, Sweden.