Abstract: Ordovician-Silurian Glacio-Eustatic Sea-Level Changes in Easternmost Appalachians (Southern Nova Scotia)
Mixtite and quartz arenite rock types appear in an isolated position between Ordovician and Upper Silurian black graptolitic slates in the White Rock Formation. Their presence is related to extreme changes in geography and sedimentation at the end of the Ordovician.
Mixites constitute part of a shallow glaciomarine sedimentary rock assemblage which includes slates, storm-bed arenites, rhythmites, shelly fauna, pillow basalts, and ash flow felsic volcanic rocks. The Halifax Formation of slate, directly below, lacks evidence of shallow-marine origin. The mixtites contain extrabasinal clasts of metaquartzites and metavolcanic rocks enclosed in poorly sorted arenaceous slates. Finely laminated rhythmites of pure silt and clay were deposited under unusual conditions such as an ice-covered sea.
The quartz arenites appear in most places as amalgamated beds, 10 to 30 m thick which regionally are a minimum of 150 km long by 20 km wide. The lower sheet abruptly overlies the glaciomarine rock assemblage, and is separated by the upper "sheet arenite" from a black slate unit. The arenites are considered to be the products of large-scale erosion and reworking of sand that occurred during major postglacial sea transgressions in the Early Silurian.
The mixtites and "sheet arenites" are significant time-rock units which reflect important events of widespread ice (glaciation) and major marine transgression during postglacial eustatic readjustment. A similar stratigraphic record of events is preserved in other areas where the Ordovician-Silurian is present.
Also, cold climate rock types, such as mixtites, aid in defining the close paleogeographic association between Saharan Africa and the eastern Appalachians during the Ordovician.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma