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Quantifying the origin and geometry of circular sag structures in northern Fort Worth Basin, Texas: Paleocave collapse, pull-apart fault systems, or hydrothermal alteration?

¹ Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924; angela.mcdonnell{at}beg.utexas.edu
² Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924
³ Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924

Angela McDonnell received her Ph.D. and her M.Sc. degree from the University College Dublin and her B.Sc. degree from the University College Cork, Ireland. She was a consultant geophysicist for Robertson Research International prior to joining the Bureau of Economic Geology, University of Texas at Austin, as a research associate. Her present research focuses on the seismic sequence stratigraphy of the Gulf of Mexico and onshore Texas.

Bob Loucks received his B.A. degree from the State University of New York at Binghamton in 1967 and his Ph.D. from the University of Texas at Austin in 1976. He is a senior research scientist at the Bureau of Economic Geology, University of Texas at Austin. He as been involved in karst and paleocave research for 30 years.

Tim Dooley received his B.A. degree from Trinity College, Dublin, and his Ph.D. from the University of London. He is a research scientist at the Bureau of Economic Geology, where he manages the physical modeling laboratories. Prior to 2003, he worked at Royal Holloway, University of London. His current research focuses on physical modeling of salt tectonics.

Three-dimensional seismic data reveal numerous subcircular sag structures in the northern Fort Worth Basin. The structures are defined by concentric faults that extend vertically upward 760–1060 m (2500–3500 ft) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the lower Desmoinesian Strawn Group. Criteria are outlined for defining seismically resolvable sag structures, and a detailed quantitative analysis of the geometries of these circular features was undertaken. Results are compared and contrasted, with reviews of subsurface collapse mechanisms and strike-slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view. In this manner, we develop several constraints for differentiating collapse-related sag structures from strike-slip–related sag structures. Qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently sags and collapses. Quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed paleocaves within the Ellenburger Group. Observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins.