资源环境科技发展态势分析平台

Current models of the terrestrial biosphere's response to the end-Permian crisis are based largely on lithostratigraphic, magnetostratigraphic, and biostratigraphic records obtained from sedimentary successions in the Karoo Basin, South Africa. These successions have been interpreted to represent continuous sedimentation across the upper Daptocephalus and lower Lystrosaurus vertebrate-assemblage zones, assigned to the Elandsberg Member and lower Palingkloof Member, and upper Palingkloof Member and overlying Katberg Formation, respectively. The stratigraphic section at Old (West) Lootsberg Pass, Eastern Cape Province, is used as a cornerstone for the currently accepted model correlated with the marine extinction event, in part, based on a magnetic polarity stratigraphy of limited documentation. Our current multidisciplinary effort provides a refined stratigraphic framework, encompassing over 740 m of measured section, compiled from 12 closely spaced and physically correlated localities across a northwest-to-southeast distance of less than 2 km. This framework is placed into magnetostratigraphic context and constrained by U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) results for two horizons.

The Old Lootsberg Pass succession does not record continuous sedimentation, as previously argued. Rather, the presence of several intraformational pedogenic-nodule conglomerate-lag deposits, a character used by other workers as diagnostic of the Katberg Formation, occurs in the Elandsberg Member -100 m stratigraphically below the Katberg Formation. An interlaminated lithofacies, previously used as a diagnostic feature of the terrestrial extinction event, occurs isolated at several stratigraphic positions, indicating that it is neither unique nor mappable. A dominant signature of normal-polarity magnetozones in the section is interrupted by two, short, reverse-polarity intervals that are restricted to siltstone beneath erosional contacts with fluvial sandstones, indicating their cryptic nature in the succession. In addition, we report a Wuchiapingian maximum-age estimate for a detrital zircon population from a bed stratigraphically above a previously reported Early Changhsingian age estimate for these rocks. A comparison of the sedimentologic features of these deposits reveals that the bed from which the Changhsingian age is derived supports an ashfall interpretation rather than a reworked deposit from which the Wuchiapingian depositional age is derived. A synthesis of our observations from Old (West) Lootsberg Pass with global patterns reported from the marine realm leads to the conclusion that the turnover from the Daptocephalus to Lystrosaurus Assemblage Zones is not coincident with the end-Permian marine event. Hence, our results demonstrate that geochronometric constraints, where available, and a high-resolution sampling strategy for paleomagnetic context are essential to any research program when attempting to interpret patterns of biological turnover, replacement, and extinction in continental successions.