Geological Survey Bulletin 1265 Ancient Lavas in Shenandoah National Park Near Luray, Virginia

The Catoctin Formation unconformably overlies the granitic basement rocks in the Luray area and is not intruded by them as was suggested by some earlier workers. The basaltic Catoctin lavas poured out on land over an erosion surface of considerable relief. The lava drowned the valleys and, at least in the earlier stages of the volcanic activity, flowed around and isolated hills of granitic rocks. Several thousand feet of lava flows, volcanic ash, and sedimentary material accumulated before volcanic activity ceased and the entire area was submerged beneath the sea in which the sedimentary rocks of the Chilhowee Group were deposited. A period of weathering which formed a saprolite as much as 150 feet thick may have followed the end of Catoctin volcanism, but no proof of a major unconformity between the Catoctin and the Chilhowee has been found.

The greenstone of the Catoctin Formation is sometimes mentioned as marking volcanic activity associated with the early stages of formation of the Appalachian geosyncline (Bucher, 1933). The present study indicates, however, that the Catoctin lavas resemble basalt accumulations characteristic of stable areas and that they should not be considered as belonging to the Appalachian geosynclinal cycle. The Catoctin greenstones are probably remnants of a great basaltic plateau which foundered beneath the advancing Early Cambrian seas during the initial stages of formation of the Appalachian geosyncline.