Tectonic Control on Stratigraphic Sequences deposited in the Taconic Foredeep
MITCHELL, C. E., R. D. JACOBI, M. P. JOY, and D. GOLDMAN
Several recent interpretations of the Middle Ordovician succession in the
central and southeastern U.S. posit the existence of a series of stratigraphic
sequences and parasequences. Similarly, rocks of the Taconic foredeep in New
York State exhibit a series of unconformity-bounded, pulsed shallowing-up cycles
that, within available biostratigraphic resolution, are synchronous with those
identified elsewhere. For instance, the Sugar River Limestone of the lower
Trenton Group may correspond to transgressive systems tract deposits of the MS
Sequence and are laterally equivalent in part to a marine hiatus and maximum
flooding surface within the deep basin. The overlying highstand deposits
(Denmark Formation) include a series of four parasequences that can be
correlated precisely with carbonate/clastic cycles in the offshore Flat Creek
Shale based on K-bentonite time lines and graphic correlation. However, patterns
of facies distribution and rock accumulation rates reveal major contributions
from local growth faulting to cycle and unconformity development. Similar fault
controlled features of the bounding unconformities suggest that the Taconic
basin succession primarily records a history of tectonically driven basin
flexure rather than eustatic sea level changes. If flexural forcing in New York
is correct, it raises questions about the eustatic origin of the sequences
elsewhere. This interpretation can be tested by expansion of the K-bentonite
chronostratigraphic framework to regions in eastern North America that
experienced lithospheric flexure at different times during the Taconic
Orogeny.