Collapse/Sag Features in Northern Fort Worth Basin, Texas: Suprastratal Deformation Associated with Coalesced Paleocave System Collapse or Wrench Fault Sags?

Angela McDonnell¹, Tim Dooley¹, and Robert Loucks^2
1 Bureau of Economic Geology, Jackson School of Geosciences, Austin, TX
² The University of Texas at Austin, Austin, TX

A 60-km² 3-D seismic volume from the Boonsville field has been reexamined (following 90o phase shift and frequency enhancement). More than 80 subcircular geometries (143-1,188 m diameter) are observed extending vertically (180-380 m) from the Lower Ordovician Ellenburger Formation. Sag impact envelopes broaden upward into the Pennsylvanian. Coherency slices show random distribution to occasional NW-SE alignment. Some features amalgamate upward into broader linked zones. Several geologic processes could form these features and geometries (e.g., karsting, local transtension, or facies interrelations).

The circular structures are interpreted as cylindrical faults reflecting incremental overburden failure from collapse of coalesced paleocave systems within the Ellenburger. Broad structural sags overlie these features resulting from this suprastratal-deformation. Lateral dimensions and height of vertical collapse/sag are consistent with measured Ellenburger karst-related collapse in outcrop. In cross section, internal geometries mimic fault systems observed over collapsed mines and/or salt withdrawal resulting in coherent down-faulted roof blocks surrounded by inward/outward-dipping faults.

Given initiation of these structures within the karsted Ellenburger, their infrequent alignment, geometries, density, and local amalgamation, it is unlikely that wrench faulting controlled karst development in this area. Although alignment is locally observed, there is a distinct absence of regional linear faults, either on seismic data or its derivatives. This leads to the conclusion that any alignment of the karst features results from exploitation of earlier fracture trends in the host rock, rather than from “active” regional faulting during formation of the features. Understanding these features origins has implications for development within these and overlying deposits.