96-13 Am I disturbed?! Expanding the paradigm of subsalt “deformation” in the Flinders Ranges of South Australia

Session: Twenty-Seven Years of Advances in Understanding Salt-Sediment Interaction: A Legacy of Katherine A. Giles

Presenting Author:

Mark Fischer

Authors:

Fischer, Mark P.¹, Schwartz, Joshua M², Gannaway Dalton, C Evelyn³, Williams, Nicholas J⁴, Canova, David Paul⁵, Lueck, Lillian R⁶

(1) Earth, Atmosphere and Environment, Northern Illinois University, DeKalb, IL, USA, (2) Earth, Atmosphere and Environment, Northern Illinois University, DeKalb, IL, USA, (3) Geosciences, Utah State University, PRICE, UT, USA, (4) Earth, Atmosphere and Environment, Northern Illinois University, DeKalb, IL, USA, (5) Earth, Atmosphere and Environment, Northern Illinois University, DeKalb, IL, USA; Universitat de Barcelona, Barcelona, Spain, (6) Earth, Atmosphere and Environment, Northern Illinois University, DeKalb, IL, USA; Nevada Gold Mines, Elko, NV, USA,

Abstract:

Well, drilling and seismic data collected over nearly three decades has recognized that strata beneath allochthonous salt sheets are often “disturbed.”  Such disturbance can take the form of brecciation, mixing of strata of different ages, overturning of strata, repeated section, overpressure, lower than expected temperature gradients, changes in stress state, or a misidentification of concentration of intra-salt inclusions near the base of the salt sheet due to poor seismic imaging.  Disturbed zones beneath allochthonous salt sheets have been referred to as smeared zones, rubble zones, shear zones, gumbo zones, gouge zones and brecciated zones, and they have been attributed to both structural and sedimentary processes.  An assessment of which processes dominate has been examined through geomechanical and kinematic modeling, as well as field observations.  For a decade, the paradigm for allochthonous salt emplacement in the Flinders Ranges of South Australia has been that Neoproterozoic salt sheets there were emplaced without causing significant deformation of the strata over which they were emplaced.  Instead, disturbed subsalt strata are typically interpreted as the result of salt sheets overriding slumped carapace that was originally deposited on top of the salt, such as at Beltana diapir.  In these circumstances the subsalt strata are mixed and often contain detritus that originated from within the salt sheet.  Our detailed field observations and mapping at three other diapirs in the Flinders suggest that this paradigm may need to be expanded to include folding, and to a lesser degree, faulting of subsalt strata during sheet emplacement. Beneath salt sheets at the Witchelina and Tourmaline Hill diapirs, deformation of the Skillogalee Dolomite suggests that there was significant strain extending up to 100 m into the subsalt strata.  Similar magnitudes and styles of strain are not observed in suprasalt strata.  Fold wavelengths in the subsalt disturbed zones range from centimeters to hundreds of meters and tightness ranges from open to isoclinal.  Folds often show consistent orientations, although this is not universal.  Similar structures occur in the Ulupa Siltstone at the Loch Ness diapir, where localized zones of quartz and dolomite mineralization are also preferentially developed in the subsalt strata.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9365

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