Orogenic Calm in the Central Appalachian Basin
Silurian and Early Devonian; 435 - 370 mya
By the beginning of the Silurian, the mountains that formed as a result of the Taconic collision were eroded nearly to sea level (cross section above). The central portion of Virginia where the mountains once stood (Blue Ridge and Piedmont provinces today) was a smooth, low, gently domed strip of land, or a peneplain. Active erosion of sediment from the mountains became scarce compared to the large volumes of sediment supplied during the Taconic. To the west, in western Virginia and West Virginia, the old foreland basins of the Taconic orogeny were completely filled with sediment. All that remained was the Tippecanoe, a shallow inland epicontinental sea.
Below the surface of the Tippecanoe, distinct basins were separated by arches. Basins typically have a thicker accumulation of sediment and tend to be deeper while arches remain near or slightly above sea level and have thin accumulations of sediment.
The Basin in eastern North America was called the Central Appalachian Basin (CAB). It was an area of subsidence trending NE to SW and was bordered on the west by the Cincinnati arch in central Ohio and Kentucky. The eastern shore lay somewhere in today's Shenandoah Valley, lapping up onto the edges of the old mountains.
Except when sea level was high, the CAB was mostly a closed inlet, surrounded by low land areas, and isolated from the rest of the Tippecanoe sea. The CAB opened to the rest of the Tippecanoe in the deep south, however, a connection that seemed to persist throughout the period allowing the sea to flow in and out.
Events that occurred during the Silurian are unique in Virginia history, and are considered as important as the Taconic orogeny. What is more, these rocks played an important role for both native Americans and European settlement in the region. The Silurian and early Devonian record of events is divided into three phases.
~Iron in the Clinton and Cayugan~
The Clinton/Cayugan formation was deposited just after the Taconic orogeny came to a close, and as a result contains the last of the clastic sediments to be supplied by the eroded sourceland. The sediments are mostly Quartz rich sandstones and Shales and, as is typical of non-orogenic periods, these do not blanket the CAB but are thin, scattered, patchy deposits that change rapidly from place to place and become more carbonate-rich to the west. The Cincinnati arch almost never received clastics during this time as it was located too far west.
To this point, these features are typical of almost any post-orogenic period, but for reasons not yet clearly understand, a very large volume of iron also was deposited with the sandstones and Shales, turning many of the rocks an intense red, a trait representative of the Silurian everywhere. This iron emplacement played an important role in European settlement of the region. For the last 2 1/2 millennia civilization has been dependent on iron, and its importance for Colonial America is indicated by the fact that one of the first industries in New England in the 1600's was the Saugus iron works. Iron was essential, and since it was heavy to transport, local sources were sought. Scattered all up and down western Virginia in the Valley and Ridge Province are the remains of iron foundries, such as the Elizabeth Furnace and the Katherine Furnace. Like the early mills, most of these were destroyed during the Civil War, but for a time they were an important local source of iron and virtually all of them were based on the Silurian Clinton/Cayugan deposits. These deposits still support and iron and steel industry further to the south (Birmingham, Alabama).
During the Cambrian and Ordovician (Stages C and H) the Mid-Atlantic region lay in the trade wind belt, approximately 10-20 degrees below the equator. Places like this today are tropical, usually with steady winds and plenty of rainfall. But by the Silurian, North America had drifted further south and the Mid-Atlantic region now lay in the region 25-30 degrees south, or in the zone of tropical deserts such as the Sahara.
The combination of arid climate, tectonic stability, and narrow inland sea partially restricted in its circulation, set up special conditions for sedimentary rock Formation. Carbonate deposition dominates around the edges of the basin. The Tonoloway Formation is a well known, widespread deposit with abundant tidal features. The Tonoloway contains may of the tidal features we noticed in the Cambrian divergent continental margin tidal Formations (Stage E), but here they are much more diminutive, indicating very low, weak tides. These tidal flats extend in a horseshoe shape around the edge of the CAB basin, passing through western Virginia, swinging around in an arc through northern West Virginia, and then coming back down through central Ohio on the Cincinnati arch (CAB Map). And as time goes by, and sea level rises, this tidal horseshoe and the sea will transgress northward until it reaches upstate New York. Small patch reefs also dot the area, just off shore from the tidal flats, and the Beds around the reefs are rich with fossils (crinoids, corals, brachiopods, stromatoporoids, etc.)
In the center of the basin, evaporation rates were very high, and the reefs prevented circulation of sea water so a salt brine accumulated. During the Salina salt deposition, the salt water in the CAB is salty enough that salt crystals grow in crusts on the Shoreline and form salt casts in the Tonoloway Formation. Salt casts are cubic impressions left by Halite crystals in the old mud on the sea bottom. As the salt concentration increases, it begins to precipitate. The result is a widespread 300-900 foot thick layer of salt deposited in the center of the Central Appalachian Basin. These salts are mostly Halite (table salt) and anhydrite (a mineral similar to Gypsum) and together are called evaporites. Some of these salt deposits also spread south where today the salt is exposed at the surface around Saltville, Virginia. These salt deposits have been important to animals, native Americans, and European settlers.
Salt deposition lasts only through the end of the Silurian. By the early Devonian the Tippecanoe sea begins a major Transgression. It spreads up the axis of the CAB toward upstate New York, and no spills over the Cincinnati arch into the epicontinental sea on the other side. This opening breaks the Cycle of isolated evaporation in the CAB, and as salinity returns to normal salt deposition stops and Carbonate deposition expands.
The Helderberg is a richly fossiliferous deposit stretching from Virginia north through Pennsylvania and into New York. In New York it is well known because it forms the Helderberg escarpment, a distinctive topographic feature. Through far western Virginia and eastern West Virginia the Helderberg is frequently mined for lime and is exposed in numerous quarries. It also has numerous caves in it, avidly explored by generations of spelunkers.
By the end of the early Devonian sea level had dropped, resulting in a major Regression of the Tippecanoe sea. All of Virginia and North America lay exposed, much as it does today, and erosion began to remove some of the rocks deposited in the central Appalachian Basin, creating the Wallbridge Unconformity.
The Wallbridge Unconformity is the boundary between the Tippecanoe sea and the Kaskaskia sea that will begin its Transgression, again to put the entire North American continent under a shallow sea, by the end of the Devonian.
The end of this phase of Virginia's history is marked by the Oriskany Formation, a pure Quartz Sandstone rich in brachiopod fossils. The Oriskany is one of the few times in the history of the Mid-Atlantic when pure Quartz sand spreads like a blanket across the region as an indicator of tectonic stability. We have seen two previous episodes already, the Antietam in the early Cambrian (Stage C), and the Tuscarora at the end of the Taconic orogeny (Stage H). Sometimes these pure concentrations of Quartz are valuable, and the Oriskany is mined throughout the Appalachians for glass production.
Overlaying the Oriskany is the Needmore Formation (Link To Stratigraphic Section), a black, deep- water Shale. Evidence indicates that, geologically, the earth drops right out from under the Mid-Atlantic region, collapsing into a deep water basin. The second of the Appalachian orogenies, the Acadian, is about to begin (Stage J).
Contributed by Lynn Fichter