Offshore extent of Gondwanan Paleozoic strata in the southeastern United States: The Suwannee suture zone revisited

Abstract

The recognition of the Suwannee terrane in southeastern North America was a critical step in advancing our current understanding of the tectonic evolution of ocean basins. Unfortunately, further clarification of the boundaries of the Suwannee terrane has proven difficult due to the thick coastal plain cover, limited well log analyses, and a paucity of seismic reflection data onshore. We present the results from a new compilation and analysis of legacy marine seismic reflection, refraction and well data from offshore the southeastern United States, which reveals in the subsurface a previously unmapped lower Paleozoic sedimentary section spanning the continental shelf from Florida to North Carolina. Previously only identified in two deep offshore wells (COST GE-1 and Transco 1005-1), this Paleozoic platform sequence is identified in seismic reflection profiles as a package of low-frequency, sub-horizontal, laterally-continuous reflectors which are clearly discordant with and distinct from the overlying Mesozoic and Cenozoic stratigraphy above the post-rift unconformity (PRU). The inferred base of the Paleozoic sequence is marked by the downward diminution of parallel seismic reflectors, as well as an increase in seismic velocities to >6km/s observed on numerous offshore seismic refraction surveys. While clearly deformed in some areas, these Paleozoic strata are generally sub-horizontal, range in thickness from 4 to 6km, and can be mapped continuously over an area in excess of 130,000km2. Similar sedimentary rocks have been recognized from onshore exploration wells in Florida since the 1930s, and were subsequently identified to be part of the Suwannee basin within the larger exotic Suwannee terrane of Gondwanan affinity. Recognition of the presence and extent of these Gondwanan strata offshore implies: (1) the inferred position of the Suwannee suture zone offshore lies >200km further north, approximately along the boundary between the Carolina terrane and the Charleston terrane; (2) previously identified terranes (Brunswick, Charleston, Suwannee, Northern Florida) are likely distinct crustal blocks, but their amalgamation predates deposition of the overlying Paleozoic section of the Suwannee basin; (3) collectively, this crust represents the Gondwanan continent based on the size and presence of a stable platform stratigraphy, nominally doubling the size of the last sutured terrane, the Suwannee terrane.