J2 - Mississippian Calm


 
The Mississippian Orogenic Calm
 
Mississippian; 350 - 320 mya

 
Introduction
    
In the European geologic time scale the Mississippian or Pennsylvanian are not recognized as a geologic periods but in this country we separate these rocks because they have such distinctive records. The Mississippian is known for its Carbonates and the Pennsylvanian for its Coal deposits. 

In the Mid-Atlantic, the Mississippian is divided into three portions; a lower portion dominated by clastics (the Pocono, Stage J), a middle Carbonate portion, the Greenbriar group, and a top portion, the Mauch Chunk. 
     
The Pocono is found in the Valley and Ridge, but the Greenbriar Carbonates and Mauch Chunk clastics are only exposed in the Allegheny plateau and west. The picture at right is of a quarry along Rt 33 at the east entrance to Canaan Valley, West Virginia. Below are the resistant Greenbriar Carbonates (reddish), and above them Shales of the Mauch Chunk (brownish); the beginning of the Alleghenian orogeny.

The picture to the right is an outcrop of the Greenbriar along Rt 33 east of Elkins W.Va. (located just about at the eastern end of the 4 lane portion of the highway.) Further outcrops are found west down the highway. Notice here that there are some interbedded red Shales (the Taggert), typical of the Greenbriar but up close you could see mega-crossbedded (6+ feet high) oolitic and fossiliferous Carbonate sands.
     
Good exposures of the Greenbriar are also found along I64 west of White Sulphur Springs, W.Va. These include not only off shore Limestones but also tidal flat Carbonates. An alert traveler will see through this country the numerous sinkholes dissolved into the Carbonates, surface expressions of the caverns also in the region.
     
Virginia has virtually no record of these Carbonates. It is possible that a good record existed east of the Allegheny front and it has been since eroded, but more likely it never really got this far east, being kept at bay by the land built out by the prograding clastic wedge of the Acadian orogeny. Carbonates have been around for a long time, and we have encountered them in Stage E (tidal Limestones and Dolomites), Stage G (fossil rich Limestones), and Stage I (tidal, again; and fossiliferous Limestones, again). But Carbonates have also evolved through time. We cannot explore this evolution here, but by the Mississippian, Carbonate deposition takes a unique turn in world history before it changes into something different again.
 
The Mississippian is a period of transition. The transition is both biological and geological and give the Mississippian a unique character. From a biological perspective the end of the Devonian brought about the second of the five major extinctions. Notice in the Graph the sharp drop in number of fossil families under the arrow labeled "2". Some groups that were common in the past were completely wiped out, while others were severely cut back in number and diversity, never to recover. The Mississippian is the period during which life in general recovers and branches out in new directions. Diversity and abundance are not the same thing, however. It is possible for a group to have many species, but each species has only a few living forms. Very successful groups tend to be both diverse and abundant, and to evolve rapidly. Several Mississippian groups are especially good at the diversity/abundance combination, including echinoderms (crinoids and blastoids), and lacy bryozoans. These groups existed in large numbers in the Mississippian and dominated the fossil record, at least for a time.

From a geological perspective, the Mississippi underwent two major orogenies; at the beginning of the period was the Acadian orogeny and the end of the period was the beginning of the Alleghanian orogeny. 

The Greenbriar
     
In Virginia and West Virginia the most characteristic Mississippian rocks are in the Greenbriar Formation. The Greenbriar formation indicates a very warm, shallow, clear, highly agitated sea with an abundance of invertebrate life. Important sediments include Oolites, and coarse crystalline Limestones composed almost completely of crinoidal fossil fragments.

Oolites are tiny spheres of Calcite that look like miniature pearls. The right picture is of the same quarry as the top picture, only here the large block of Limestone in the foreground is almost all Oolites, with a few fossils scattered in. Blocks and pieces like this can be found throughout the quarry, and along Rt. 33 down the hill where the rest of the Greenbriar is exposed. Oolites form only in very warm, clear shallow water that is highly agitated by waves or tidal currents. The Oolites form in large underwater dunes that continuously shift about on the sea floor. Oolites, while forming, act like individual sand grains but in the Greenbriar are cemented together as a rock.
    
Other Limestones are made mostly of crinoid fragments, with lacy bryozoans and other fossils mixed in, and are called encrinoidal Limestones. The number of individual crinoid animals required to form some of these Beds is unimaginable. For one Formation in Kentucky it has been estimated 28 x1016 (280,000,000,000,000,000) individual crinoids were required.  Among all these crinoids were corals, blastoids, and brachiopods. White Carbonate beach sands and tidal flat deposits were present in eastern outcrops indicating the presence of Shorelines, but how far east into Virginia the shore came is unknown.

No other geologic record exists of what the rest of Virginia was like during deposition of the Greenbriar. It must of had some relief at times since clastics occasionally enter the Greenbriar sea. Yet for the most part Virginia was, again, at the end of the Acadian orogeny, a peneplain, from near its western boundary all the way east to Richmond, and beyond that the Rheic Ocean. At this time the supercontinent Gondwana was closing fast. 
    
These shallow water Carbonates, Oolites and encrinites, are not just found in the Mid-Atlantic region, but were deposited across most of North America during this time, all the way out to the southeast and up through Nevada. The vast extend of these clear water environmental conditions is in sharp contrast to the earlier Chattanooga Shale deposited during the Devonian Acadian orogeny when the epicontinental seas were under the influence of very stagnant conditions across most of the continent.
     
To the northeast, away from the Craton, Mississippian Carbonates become more and more scarce, and disappear. Instead, clastics prevail almost without interruption from the end of the Acadian to the beginning of the next orogeny. In fact, it is likely that these two orogenies in the northeast run overlap in time. The implication is that no sooner had the Avalon terrane collided, built a mountain range and then mostly eroded, than the supercontinent Gondwana began its collision to start the next orogeny, an event to profoundly rearrange the geologic record of the Mid-Atlantic, the Alleghenian Orogeny.
Tuesday, August 05, 2014
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