New U–Pb constraints identify the end-Guadalupian and possibly end-Lopingian extinction events conceivably preserved in the passive margin of North America: implication for regional tectonics

Abstract

The discovery and dating of a volcanic ash bed within the upper Phosphoria Formation in SE Idaho, USA, is reported. The ash occurs 11 m below the top of the phosphatic Meade Peak Member and yielded a 206Pb/238U date of 260.57 ± 0.07 / 0.14 / 0.31 Ma, i.e. latest Capitanian, Guadalupian. The stratigraphic position of this ash near the top of the Meade Peak phosphatic Member of Phosphoria Formation indicates plausible completeness of the sedimentation within the Guadalupian–Lopingian and probably at the Permo-Triassic (P-T) transitions. The new radiometric age reveals that the regional biostratigraphy and palaeontology of Phosphoria and Park City formations requires serious reconsideration, particularly in cool water conodonts, bryozoans and brachiopods. The new age proposes that the Guadalupian–Lopingian boundary (GLB) coincides with the Meade Peak – Rex contact and consequently with the end-Guadalupian extinction event. The lack of a major unconformity at the P-T transition suggests that the effects of the Sonoma orogeny were not as extensive as has been assumed.

Phosphorites, glass ramps and carbonate factories: The evolution of an epicontinental sea and a late Palaeozoic upwelling system (Phosphoria Rock Complex)

“…In the Davydov et al . (2018b) study area, the Franson, Retort, Tosi and Ervay are absent (far left side of Fig. A; McKelvey et al., ; Mytton et al., ; Maughan, ).…”

“…(B) The model of Davydov et al . (2018b) based on a U‐Pb age from an ash bed in the Meade Peak Member of southern Idaho combined with a modified biostratigraphic zonation scheme. Dinwoody age constraints are based on Paull et al .…”

“…Comparatively, Davydov et al . (2018b) combined a U‐Pb date from the first known and datable ash in the PRC (Meade Peak Member, south Idaho) with a revised biostratigraphic interpretation to propose that PRC sedimentation continued up until the Permo–Triassic boundary (Fig. B).…”

“…The model of Davydov et al . (2018b) suggests a relatively conformable contact between the PRC and overlying Triassic Dinwoody Formation (Clark & Carr ; Paull & Paull, ; Hoffman et al., ). Description and interpretation of the PRC–Dinwoody contact are ambiguous in supporting one model over the other.…”

“…However, it has also been interpreted as recording significant regional post‐PRC, pre‐Dinwoody (pre‐Triassic) erosion that led to absence of the Retort, Tosi and Ervay members in Idaho [the study area of Davydov et al . (2018b); Fig. A] (McKee & McKee, ; Schock et al., ; Maughan, ; Peterson, ; Paull & Paull, 1986b; Paull et al., ; Matheson, ).…”