AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Constraints on Deepwater Sedimentation in the Karoo Basin, South Africa, from U-Pb Geochronology of Ash Interbeds
(1) West Virginia University, Morgantown, WV.
(2) Univeristy of Liverpool, Liverpool, United Kingdom.
(3) Chevron Energy Technology Co., San Ramon, CA.
(4) University of Alabama, Tuscaloosa, AL.
To constrain patterns of deepwater sedimentation, U-Pb ages on zircon were obtained for ash beds in three stratigraphic sections of the well-studied Permian deepwater deposits of the western Karoo basin. Isotopic analysis was performed by SHRIMP-RG analysis at the Stanford-U.S.G.S. Microscopic Analytical Center, with 5-15 grains analyzed per sample. Sample age was determined from the youngest population of zircon ages using the TuffZirc algorithm of Isoplot.
In the Laingsburg depocenter near the town of Laingsburg, sections include the oldest turbidites in the Collingham Fm. through the basin plain Vischkuil Fm. and up into the overlying basin floor fan deposits (Unit A) and slope channel levee deposits (Units B and C) of the Laingsburg Fm. Six samples from the Geelbeck section show a younging-upward trend from the lowermost Collingham Fm. (276.9 +4.1/-2.4 Ma; n=4 of N=8). The 2 uppermost samples of the overlying Vischkuil Fm., LGV2 and LGV4, yielded a TuffZirc age of 262 +2.7/-2.0 Ma (n=3 of N=9) and 267 +1.5/-6.1 Ma (n=5 of N = 12) respectively; however, the youngest two zircons of the stratigraphically highest sample (LGV4) are 261.0±2.8 Ma and 261.1±2.6 Ma, which suggests analysis of additional zircons from this sample would ultimately support an age of ~261 Ma. If so, then the ~200 m of section between the lowermost and uppermost samples at Geelbeck was deposited in ~10-15 Ma. For another sample collected between Unit B and Unit C near Baviaans farm in the Laingsburg depocenter, two zircons were eliminated from consideration due to anomalous Th/U. Three other grains yield a weighted average of 265.0±2.1 Ma (MSWD = 2.1). For comparison, two ash samples from the Tierburg Shale at Bloukrans Pass were collected in the Tanqua depocenter to the north near the town of Calvinia. These samples produced ages of 273.9 +1.0/-0.7 (n=3 of N=10) and 275.2 +3.82/-0.2 (n=7 of N=16). These ages are statistically equivalent but the ash beds are separated by ~150 m of section; this implies a rapid sedimentation rate for the Tierburg Shale. Thus, the Tierburg Shale is not a condensed section but rather a sand-starved, mud-dominated depocenter that received a high influx of sediment during late Early Permian time. These data are consistent with prior results that suggest Vischkuil Fm. deposition likely extended through the Capitanian stage of the Middle Permian, implying temporal correlation with the Ft. Brown and/or Waterford Formations in the eastern Karoo basin.