A NEW LOOK AT SOUTHERN LAURENTIA IN THE LATE PALEOZOIC; RIFTING OF CUYANIA AND SABINIA, AND THE OUACHITA ‘SOFT CLOSURE’

Recent improvements in understanding of the Marathon area and of the Cuyania block, coupled with a close look at basin subsidence on the craton, lead to revisions in the accepted history of Laurentia rifting and Ouachita closure. Main points:

1) Marathon and Ouachita strata were deposited on Cambrian thinned continental crust (possibly oceanic) south of the craton edge. Marathon rocks show intrabasin horst sources and southern sources (including arc volcanics) for Cambro-Ordovician strata, indicating a two-sided basin.

2) Cambrian subsidence is local, not regional, suggesting that extension is variable and did not lead to extensive oceans near the craton margin.

3) Continued seismic activity in the Marathon area culminates with rifting of Cuyania in mid-Ordovician, as Dickerson has proposed. This history is not shared with the Ouachita strata, which formed in a stable environment. The two orogens have much more consistent, chert-rich stratigraphy from Late Ordovician on, with some craton subsidence.

4) Appalachian debris arrives in deep-water fans first in the Marathon area (Tesnus), only later in Ouachita (Stanley). This suggests a direct route south of the Sabine block (Sabinia continental fragment), and that Sabinia is a slightly displaced bit of Laurentia.

5) Hercynian compression shoved Yucatan northwest into Sabinia, and both towards Laurentia, everting the Ouachita and Marathon basins. But docking was not complete, as shown by lack of high-grade metamorphic rocks, limited clastic wedge and foredeep development, and presence of remnant oceanic crust.

6) A ‘hole’ west of Sabinia may have experienced minimal compression. Eastward escape of the Texas block into this hole may be responsible for Pennsylvanian strike-slip faulting in southern Oklahoma and in West Texas. Compression in these areas was NE-SW, perhaps due to Appalachian hard collision or to Pacific subduction.