SOUTHERN AND CENTRAL APPALACHIANS 



99 



Fig. 8.2. Stratigrauhic relations of Late Precambrian and Early Cambrian formations in Blue Ridge 

 of Virginia. After Bloomer and Werner, 1955. 



or fan was about 8000 feet thick near its apex but thinned toward its 

 edges. Beds representing the Middle Ordovician, as well as the Upper, 

 are only 500 feet thick to the southwest in Alabama, and are carbonates. 

 Likewise to the northeast in Pennsylvania Middle Ordovician beds are 

 carbonates and only 700 feet thick. 



Late Ordavician-Devonian Clastic Wedge. Apexing in east-central 

 Pennsylvania is another great wedge of clastic sediments which began 

 to accumulate in Late Ordovician time and continued through the 

 Silurian and Devonian. The greatest subsidence and sediment accumula- 

 tion occurred during the Late Devonian, which deposit is commonly 

 referred to as the Catskill delta. It has a maximum thickness of over 

 8000 feet. Isopach maps of the Late Ordovician and Silurian deposits are 

 shown in Fig. 8.6, and detail of facies relations in Fig. 8.7. A cross 

 section of the Devonian wedge is given in Fig. 8.8, and a map of the 

 deposit in Fig. 8.9. Further detail on the stratigraphy may be found in 

 publications by Willard (1936). 



Mississippian Deposits. In eastern Tennessee in the Great Valley 

 a sheet of black shale may be seen transgressing across the Silurian strata 

 and on the southeast side of the Valley to be resting on Middle Ordovician 

 rocks. See Fig. 8.5. It is known as the Chattanooga shale and probably 

 ranges in age from latest Devonian to earliest Mississippian (Rodgers, 



1953). It thickens northeastward and eastward to a maximum of 400 feet 

 at Cumberland gap. The Chattanooga shale is extremely widespread in 

 the Nashville and Cincinnati arch areas and represents a marine facies 

 of the upper continental beds of the Catskill delta. 



The Mississippian above the Chattanooga in eastern Tennessee may 

 have attained a maximum thickness of 6000 feet at the time of deposi- 

 tion near the Blue Ridge source region, but is generally much thinner 

 than this in sections now preserved. It consists of three units each ex- 

 hibiting a parallel gradation from finer, thinner, and less detrital — 

 more carbonate sediments on the northwest side of the Great Valley to 

 coarser, thicker, and more detrital sediments on the southeast side. 



In Alabama, the thin Mississippian limestones of the foreland change 

 toward the southeast into 5000 feet of sandstones and shales. In north- 

 ern Virginia, Maryland, and Pennsylvania, the lower 1000 to 2000 feet 

 of the Mississippian is shale and sandstone, the middle formations are 

 limestone with a maximum thickness of 4000 to 5000 feet, and the upper 

 formations are calcareous shale, red mudrock, and red and gray sand- 

 stones. The Pocono and similar sandstones of the lower division are thick 

 bedded and conglomeratic. The thickening of most all units of the 

 Mississippian from the western shelf to the eastern geosynclinal trough is 

 conspicuous, and the coarsest material occurs where the section is thickest. 



