SHALES, CARBONATES, AND STRUCTURE: EVIDENCE FOR TIMING OF TACONIAN OROGENESIS ALONG THE OUTER PORTIONS OF THE CAMBRO-ORDOVICIAN SHELF IN THE CHAMPLAIN VALLEY AND SOUTHEASTERN PENNSYLVANIA

Although the transition from carbonate to shale has generally been used to date the onset of Taconian orogenesis in the northern and central Appalachians, recent work in the central and southern Champlain valley has found that this approach leads to incorrect results. The carbonate shelf surface was deeply incised during the Middle Ordovician eustatic lowstand that created the Knox unconformity, and subsequent carbonate platform development was limited to the remnant highs between the submarine canyon successors to the lowstand valleys. The mid-to-late Ordovician shales, representing the distal edge of foreland deposition ahead of the advancing orogenic wedge (accretionary prism?) were deposited first in the canyons (probably beginning shortly after the Knox unconformity) and only later topped the terminal carbonate platforms. The carbonate-block mélanges within these shales formed by slumping of canyon-wall material into the filling canyons and have no connection with the onset of orogenesis – nor do the fossils in these carbonates have any temporal correlation with the enclosing shales.

The deformational history of local areas in the Champlain valley has been worked out in sufficient detail that we are now able to distinguish at least three episodes of Paleozoic contraction and two episodes of Paleozoic extension, and constrain their timing with available stratigraphy. There is one minor contractional event that occurs before the canyons are mostly filled with shale, but no earlier than the end of carbonate platform development in the eastern and central Champlain Valley; this event must have happened in latest Black River to early Trenton time. The primary Taconian orogenesis, including emplacement of the Taconic and Champlain thrust sheets, occurred much later after the deposition of the entire shale sequence; thus, it occurred no earlier than latest Cincinnatian time.

Although the overprint of Alleghanian structures in southeastern Pennsylvania has obscured much of the structural evidence for earlier events, the stratigraphic sequence shows many of the same indicators for the timing of Taconian events. Based on this evidence, orogenesis did not affect the shelf until at least mid-Trenton time, and more probably occurred in conjunction with the development of the Queenstown clastic wedge.