D - Early Divergent Margin


 
Early Divergent Continental Margin
 
Early Cambrian; 560 - 550 mya

     
Rifting can form an ocean basin hundreds to thousands of kilometers wide in as little as tens of millions of years. As this occurs, the source of heat that initially lifted, stretched, and separated the region to form the individual horsts and grabens remains in the center of the newly forming ocean basin, and thus moves farther and farther away from the new divergent continental margin (DCM). This can be best seen in the Third Rift Cross Section. As a result the DCM cools, becomes denser, and sinks. Soon the rugged topography of the rift system smooths out, both by erosion of the horsts and the filling of the graben. The transition from rugged horst mountains to sinking of the continental terrace takes less than 5 million years.
 
The final smoothing out of the rift margin is preserved in the Chilhowee group which contains three formations; the Weverton, Harpers, and Antietam. These are seen on both flanks of the Blue Ridge (NW Cross SectionSE Cross Section). Note that these formations, sometimes with different names, are found throughout the Mid-Atlantic region. The Weverton and Harpers Formations contain evidence of river and submarine fan deposits and represent the final filling stages of the graben.

The early DCM officially begins when the edge of the continent subsides below sea level. This transition is marked in the stratigraphic record by the Antietam Formation, a beach deposit of Quartz Sandstone, that began to migrate, or transgress, across the continent with the subsidence of the continental edge. The transgression will end in Wisconsin in about 100 million years. In the Third Rift Cross Section the Antietam is represented by the thin yellow bed labeled "transgressive beach sand." You can see here that it is already below sea level. 
     
In this more detailed reconstruction of the Proto-Atlantic DCM, the continental terrace has just subsided below sea level and the Antietam transgressive beach is being actively deposited.
     
So, at the end of the rifting, and the beginning of the DCM, a broad, flat, featureless continent would stretch off to the west while and the broad Proto-Atlantic ocean would stretch to the east (D2). This is the picture in the DCM 3rd Rift Cross Section. Notice that the axial rift is now below sea level, buried under a thin accumulation of sediment. Also notice that directly on top of the Antietam Formation are Carbonate rocks of the Shady Formation. This rapid transition from pure quartz sandstone to carbonates, without intervening shale, is an indication of how quickly the rift stage stabilized into the early divergent continental margin.
 
This stabilization will continue for the next hundred million years or so in Stage E.
 
Contributed by Lynn Fichter 
Saturday, August 02, 2014
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