Abstract

Contemporaneous siliceous stratigraphic units of Early Ordovician to Early Permian ages in north-central and northeastern Nevada and equivalent units in the Marathon uplift of Texas and the Ouachita Mountains of Arkansas and Oklahoma are broadly similar in lithic properties and inferred depositional environments. In all four areas, the lower half of the Ordovician consists of coarser grained sediments than the upper half and includes a large proportion of deposits such as mature quartz arenite, calcarenite, or limestone-gravel conglomerate. The upper half of the Ordovician is mainly fine grained and in each area consists of a thick shaly unit overlain by a unit composed mainly of bedded chert. The Silurian in Nevada and the Ouachitas consists in large part of turbidite sequences of arkosic sandstone. Beds inferred to be at least partly Silurian and immediately overlying the Upper Ordovician in both northeastern Nevada and the Marathon area are novaculite. Devonian rocks of all areas and the lower Mississippian in the Ouachitas, tend to be fine grained and highly silicic. Chert predominates in the Devonian of north-central Nevada, is an important constituent in northeastern Nevada, and, including novaculite, predominates in the Marathon and Ouachita areas. Mississippian to Middle Pennsylvanian sequences are mainly siliceous “flysch” sediments rapidly deposited in deep troughs. Upper Pennsylvanian and Permian clastic rocks lie unconformably on beds tectonically deformed in Middle Pennsylvanian to very early Permian time.

This stratigraphic and depositional symmetry across the southern part of the Paleozoic continent implies a large degree of continuous tectonic symmetry as well. The two principal tectonic events took place on both sides of the continent at about the same time, but differed somewhat in intensity and details of structural style. The mid-Paleozoic Antler orogeny in Nevada involved the formation of an outer (with respect to the craton) highland provenance terrane and an inner structurally deep, narrow trough. The corresponding event in the Marathon and Ouachita areas also involved the formation of an outer highland provenance terrane and an inner structurally deep, narrow trough. Late Paleozoic tectonism in Nevada began in Middle Pennsylvanian and consisted principally of regional uplift, the deposition of thick sediments in flanking basins, local folding, and local thrust faulting. The Ouachita orogeny also began in Middle Pennsylvanian and consisted principally of intense folding and thrust faulting. On both sides of the continent, contemporaneous satellitic foreland basins were formed on the craton and were rapidly filled with thick “flysch” deposits in late Pennsylvanian.

The stratigraphic symmetry across the southern part of the continent implies that the allochthonous parts of these sequences could hardly have been gathered from distant random sources and accreted to the continent by means of plate convergence. On the contrary, they must have been deposited around the southern end of the continent under very similar conditions and perhaps in the same seaway. The tectonic symmetry suggests that the entire southern end of the continent was responding simultaneously and in much the same manner to plate interactions.