Abstract

Changes in the record of vertebrate-assemblage zones in the Karoo Basin, South Africa, serve as a model for the response of terrestrial ecosystems to the end-Permian biotic crisis. Vertebrate turnover from the Daptocephalus to Lystrosaurus Assemblage Zone is believed to have begun with the appearance of subtle siltstone-color variance and is coeval with a unique, laminated lithofacies, both of which are interpreted by other workers to represent evidence for an aridification trend. The laminated facies consists of interbedded greenish-gray and dark reddish-gray siltstone, with coloration of the latter reported as a function of an increasing eolian component. The current study combines stratigraphy, petrography, geochemistry, and rock magnetic properties of an interlaminated succession and lateral equivalents at Old Lootsberg Pass in the Eastern Cape Province to determine whether such subtle color variance is a function of aridification and, hence, climate change.

Siltstones were evaluated from intervals both below and above a pedogenic nodule-bearing feldspathic wacke that is physically correlated over a > 0.5 km distance. The lower succession consists of interbedded greenish-gray (sensu latu) and reddish-gray (sensu latu) siltstone in which undisturbed and minimally disturbed primary sedimentary, along with few secondary biogenic structures, are interpreted to represent deposition in an abandoned channel-fill complex. Massive greenish-gray and dark reddish-gray siltstone above the channel complex demonstrate that coloration is variable across laterally equivalent strata. Both lithologies were assessed using X-ray fluorescence, X-ray diffraction, Mössbauer spectroscopy, scanning electron microscopy with EDS, and bulk magnetic susceptibility, stepwise acquisition of an isothermal remanent magnetization (IRM), and backfield direct field demagnetization.

Our data indicate that there is no statistically significant difference in major-element concentrations between lithologies and that subtle color difference is a function of the presence of fine (< 1 wt. %) hematite in the reddish-gray rocks. Hence, the abundance of Fe is not responsible for either color. Rather, hematite is found to be localized as reticulate, surficial coatings on both illite and chlorite flakes, the only two clay species identified in the study. Similarly, there is no clear distinction in the magnetic mineralogy of sample suites; both magnetite and maghemite vary in abundance. These data lead to an interpretation that the presence of hematite is early diagenetic and the landscape in the earliest part of the Lystrosaurus Assemblage Zone was not arid. In contrast, the landscape was wet, that allowed for the continued growth of a Permian-age flora under conditions of high water table that experienced temporal fluxes in the hydrological regime promoting spatial color modification of sediment.