Paleocollapse megastructures (suprastratal deformation) related to Lower Ordovician Ellenburger coalesced, collapsed-paleocave systems in the northern Fort Worth Basin, Texas

Angela McDonnell, Bob Loucks, and Tim Dooley, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Texas

This study aims to understand the genesis of numerous subcircular structures identified on 3D seismic, in the north-east Fort Worth Basin. They have been described as either karst-collapse chimneys or pull-apart basins. In this study, quantitative assessment of the geometries and seismic infill character of these structures further constrains their origin. Concentric faults extend vertically (760 to 1060 m) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the early Desmoinesian Strawn. Geometries are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently collapses and sags (e.g. mine collapse and dissolution processes). The circular features are interpreted to result from incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed-paleocaves within the Ellenburger Group. This interpretation honors the observations and geometrical characteristics of the circular structures analyzed. It is also consistent with regional studies of Ellenburger karsting. A rectilinear paleokarst system is identified, with coalesced passage and cavern systems aligning along NW-SE and NE-SW trends. This orthogonal trend reflects a rectilinear joint pattern that existed within the Ellenburger Group and was preferentially exploited by karst processes. The data do not support a strike-slip pull-apart origin for the depressions. Space creation in pull-apart basins is governed by extension, accommodated along a linked system of strike- and oblique-slip faults. In cross-section pull-apart basins broaden upward forming a typical flower-structure geometry  In the study area the collapse zones narrow, rather than broaden, upward. In addition there is a lack of seismically resolvable fault lineations connecting the circular structures.