K/AR DATING OF BLUE RIDGE THRUSTING AND OF ITS INFLUENCE ON SEDIMENT INPUT TO TWO PENNSYLVANIAN-AGE DEPOCENTERS IN THE APPALACHIAN BASIN

We constrain the timing of the emplacement of the Blue Ridge-Piedmont terrane by K/Ar dating of the illitization of mid-Ordovician K-bentonites along the eastern edge of the Basin in front of the Blue Ridge Thrust in northeastern Georgia and northern Virginia. The bentonites have converted into illitic K-bentonites because of their tectonic involvement in folding and deep burial in front of Alleghanian thrusting. The Horseleg Mountain, GA locale lies in a window beneath the Rome Thrust, the eastern-most imbricate thrust in front of the Blue Ridge Thrust. K/Ar dates versus grainsize of this K-bentonite all concur at 273 Ma ± 7 Ma. and indicate this early Permian age is when over-thrusting occurred here. The northern Virginia K-bentonite is from the Martinsburg Formation only 1.5 km west of the Blue Ridge on the Massanutten Syncline where the folded Martinsburg has a well developed axial plane slaty cleavage. However, the pattern of increasing K/Ar dates of the K-bentonite versus grain size indicates this bentonite was contaminated by Taconic or older detrital illite during deposition. At 282 Ma the youngest, least contaminated fraction is the finest (<0.2 ìm), and close to the concordant GA dates.

K/Ar dates of detrital muscovites from Pottsville Formation sandstones of the north central Appalachian basin give very uniform Acadian Orogen dates of 360 ± 10 Ma (two size fractions each from 5 localities near Carbondale, PA) suggest significant, uniform input of sediment from Acadian terranes to the northeast, which have been previously demonstrated to be the source of Pennsylvanian strata throughout the northern and central Appalachians. By contrast muscovite dates from Pottsville sandstones in the Black Warrior Basin, AL appear to be unique the Appalachian Basin in showing provenance from uplifted parts of the Alleghanian Orogen that are both older and younger then the Acadian Orogeny. These include the first appearance of detrital muscovites in the Appalachian Basin derived from the Alleghanian-age metamorphic rocks in the Orogen.