A Lithostratigraphic and Magnetostratigraphic Framework in a Geochronologic Context for a Purported Permian–triassic Boundary Section at Old (West) Lootsberg Pass, Karoo Basin, South Africa

Abstract

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 succ...

The base of the Lystrosaurus Assemblage Zone, Karoo Basin, predates the end-Permian marine extinction

“…A rich vertebrate assemblage spanning the uppermost Daptocephalus and lowermost Lystrosaurus AZs recovered from the slopes of two koppies (hills), referred to as Loskop and Spitskop ( Supplementary Fig. 1), are inferred to represent a complete Permo-Triassic Boundary (PTB) sequence 29 -but see geologic evidence for extensive unconformities contained in this stratigraphy 20,21,30 -and the locality has been utilized in studies on the extinction dynamics in the basin 29,31 . A rich assemblage of Lystrosaurus maccaigi, considered to be diagnostic of the pre-extinction fauna 4,29,32,33 , is overlain by two "marker" taxa, Lystrosaurus curvatus and Moschorhinus kitchingii, that have been used to support the placement of the "PTB" at the top of a so-called heterolithic facies 18,29,31 .…”

“…fine-grained feldspathic wacke in which intraformational conglomerate lags occur infrequently 21,30 . The first thick sheet sandstone occurs approximately 12 m above the proposed "PTB" 29 in our section (Fig.…”

“…The Karoo model holds that changes in fluvial architecture, from broad and meandering channels to "braided" regimes 19 , resulted from the loss of wetland vegetation 16 which, in turn, reduced resource availability for late Permian vertebrates leading to rapid extinction and turnover 3,4 . These reported Karoo patterns have very limited geochronometric context 20,21 and are not without controversy.…”

“…In contrast, the reported turnover from the Daptocephalus (= Dicynodon 27 ) AZ to the Lystrosaurus AZ, long purported to be coeval with the end-Permian marine extinction interval, has never been well-constrained temporally, although an early Triassic detrital zircon age recently was reported for the upper Daptocephalus AZ 18 . The postulated correlation with the marine record currently rests on stable carbon-isotope and magnetic polarity data 3,4, but see 21 . An early Changhsingian age, 1.5 million years older than the crisis, obtained from an ash fall deposit in magnetostratigraphic context is reported from the upper Daptocephalus AZ at one classic Karoo locality 20,21 ( Fig.…”

“…The postulated correlation with the marine record currently rests on stable carbon-isotope and magnetic polarity data 3,4, but see 21 . An early Changhsingian age, 1.5 million years older than the crisis, obtained from an ash fall deposit in magnetostratigraphic context is reported from the upper Daptocephalus AZ at one classic Karoo locality 20,21 ( Fig. 1).…”