Bathymetric co-ordination of proximality trends and level-bottom communities; a case study from the Lower Silurian of Norway

Both level-bottom communities (Ziegler, 1965) and proximality trends (Aigner, 1985) may be used to interpret relative bathymetry. How do the two methods compare? Lower Silurian strata of the Oslo Region were deposited in a trough on a storm-dominated epicontinental shelf In the Lower Llandovery both level-bottom communities and proximality-trend analysis can be used to infer an onshore-offshore gradient out from a barrier system at two different sites for construction of two separate paleobathymetrical curves. There is good biostratigraphic correlation and the offshore and nearshore bathymetrical curves compare well. The curves may also be correlated with eustatic sea-level changes found in North America and elsewhere. In addition, the analysis reveals local tectonic movements. There was a general, slow tilting of the basin through Rhuddanian and Aeronian times that diminished the gradient of the slope. Use of proximality trends has some limitations in tectonically active areas. Use of level-bottom communities requires a fossiliferous sequence. Using some care, however, both methods may be used, preferably in combination. Some of the traditional Lower Silurian level-bottom communities (Ziegler, 1965) can be placed in the shelf zones of Aigner and Reineck (1982). The Clorinda and the Stricklandia communities occurred in the shelf mud zone, the former mainly under storm-wave base and the latter from slightly over storm-wave base to the transitional zone. The Cryptothyrella community began in the lower transitional zone, but probably continued farther landward. A new transitional formation, the Saelabonn-Solvik Formation, and a new member, the Sylling Member, are defined.