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Dislocations in the Wilcox/Carrizo Sediment-Routing System: Emerging Evidence From Central Texas


In modern depositional systems, source-to-sink relationships can often be traced continuously from fluvial/alluvial hinterland source areas, through coastal and nearshore marine depositional areas, and ultimately into deepwater depositional basins. The rock record of the sediment-routing system, from erosional through depositional areas, has broken links of various types, and in many cases, arguably more missing links than preserved connections. Wilcox/Carrizo outcrops in Central Texas have been conceived as being part of a more continuous sediment-routing system than is actually present, based on questionable interpretations of depositional environments and a poorly constrained age framework. In particular, the Simsboro sandstone in the Wilcox Group and the Carrizo sandstone in the Claiborne Group have been interpreted as fluvial channel systems that fed sediment to shelf-edge deltas and beyond. Current studies of sedimentary structures and trace fossils show both of these to be marine deposits, possibly tidal deltas, and not sediment conduits. Additional evidence for shallow marine, tidally-influenced or -dominated deposition in other formations of the Wilcox Group suggest that the coastline was to the northwest of the current outcrop belt for much of its depositional history: subsurface shelf-edge deltas cannot be a coeval, distal part of this coastal complex. On-going research is revealing the fragmentary nature of the outcrop succession, which has inter- and intra-formational hiatuses. One such hiatus is an erosional surface at the Wilcox/Carrizo boundary, potentially representing as much as 4 myr of missing time. We hypothesize that during such time gaps, sediment was by-passing across a coastal plain to feed shelf-edge deltas. This part of the sediment-routing system is not preserved at outcrop and has yet to be identified in the subsurface.