The lithostratigraphy of the upper Ordovician of central Portugal

Abstract

A lithostratigraphic scheme for the upper Ordovician of central Portugal is described incorporating previously-erected local lithostratigraphic terms and new units. Thirteen formations (10 new), including 11 members (eight new), with one named bed, are arranged into four groups (three new). A description of each unit is given, together with comments on its age and correlation. Deposition in a storm-dominated, shallow-shelf environment through Llandeilo and Caradoc time is indicated. Differential subsidence occurred to the NE and SW of the Dornes/Maçâo ‘rise’ particularly during the Caradoc. Lower to mid-Ashgill sequences in the north are dominated by volcanic rocks with subordinate carbonate deposition. Farther south a hiatus preceded the regressive deposits associated with the onset of the upper Ordovician glaciation. Hirnantian glaciomarine deposits occur across most of the area, except for the regions of carbonate and volcanic build-ups, and appear to represent the period of waning of the glaciation. The Ordovician/Silurian boundary is difficult to place, but probably lies close to or within a sheet sand-body which underlies a condensed Llandovery sequence. Lithostratigraphic correlation of the study area with W. Brittany, France, is generally good; the relationship of some of the upper Ordovician units in central Portugal aids the interpretation of the relationships of the equivalent deposits in Brittany.