AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
(1) Institute of Tectonic Studies, New Mexico State University, Las Cruces, NM.
(2) Rown Consulting, Boulder, CO.
(3) Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
Stratigraphic and facies analysis of the Neoproterozoic Wonoka Formation and Patsy Hill Member of the Bonney Sandstone that surround Patawarta allochthonous salt sheet permit identification of an isolated suprasalt carapace section of the Wonoka Fm that is condensed and lithologically distinct from the correlative minibasin section. The two sections are spatially separated by a 3.8 km wide zone of outcropping Callana Group in the Patawarta diapir.
The Wonoka Fm carapace section displays a uniform 14 m thickness of parallel strata over a distance of 2.5 km and lies unconformably above the Patawarta salt sheet. The lower 7 m comprises upper-shoreface to foreshore silty lime mudstone and the upper 7 m comprises debris-flow facies interbedded with peritidal sandstone and shale capped by lagoonal stromatolitic mudstone. Debris-flow clasts were derived from older Wonoka Fm units and the Callana Grp. Equivalent strata in the adjacent minibasin comprise outer-shelf to upper-shoreface lime mudstone, siltstone and shale with minor sandstone. These strata form the bulk of a tapered composite halokinetic sequence (CHS) that thins (975 m to 117 m) and turns upward (<86 degrees) toward the diapir over a distance of 457 m. The uppermost shale unit in the minibasin contains 12 thin, sandy, pebble conglomerate beds, also sourced from older Wonoka Fm units and the Callana Grp, that display a progressive unroofing sequence.
The carapace and correlative minibasin section record the highstand systems tract (HST) of a 3rd-order depositional sequence. The trangressive systems tract (TST) and early HST formed by the lower Wonoka Fm units in the minibasin are not preserved in the carapace section. The top of the Wonoka Fm carapace is a sequence boundary (SB) that correlates to a SB in the minibasin formed at the contact between the Wonoka Fm and overlying Patsy Hill Member of the Bonney Sandstone.
The debris flow facies in the Wonoka Fm carapace and the correlative conglomerate beds in the minibasin are interpreted to be locally derived from strata that were originally deposited atop the ramping Patawarta salt sheet between the carapace and the minibasin. We infer that during the process of salt sheet breakout, the tip of the Patawarta sheet became a zone of diapiric inflation forming a local topographic high in the margin area, which was eroded during the later part of the HST and shed clasts onto both the carapace and the minibasin.