An investigation of a Devonian/Carboniferous Boundary section on the Bolivian Altiplano

Jon A Lakin,Ian Troth,John E.A Marshall

doi:10.1017/s0016756821000741

Jon A Lakin, Ian Troth + Show 1 more

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https://doi.org/10.1017/s0016756821000741

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Abstract

AbstractThe Devonian/Carboniferous Boundary (DCB) interval is associated with mass extinction, isotope excursions and a short glacial episode. This study investigates how boundary extinction and environmental change is expressed in the glacial high-palaeolatitudinal record of the Bolivian Altiplano (western Gondwana). A latest Devonian and early Carboniferous section has been investigated using sedimentology, palynology, total organic carbon and bulk δ13Corganic. The Colpacucho Formation is a Late Devonian shelfal–marine siliciclastic sequence. It is overlain in the study area by a unit of coarse sandstones and sandy diamictites, interpreted as glaciomarine. This distinctive glaciomarine unit is at least 7 km wide and 60–120 m thick with a variably incisive basal contact (<100 m). It is of very latest Famennian age and is a stratigraphic equivalent of proven glacigenic deposits across central South America. The offshore marine Kasa Formation overlies the glacigenic unit above a basal flooding surface. The DCB is 12 m above this flooding surface on the last occurrence of Retispora lepidophyta and significant palynological assemblage changes. This includes the loss of the Umbellasphaeridium saharicum phytoplankton bioprovince, endemic to Gondwana. Marine and terrestrial palynological extinctions are synchronous with a 2 ‰ positive carbon isotope excursion interpreted to be reflective of changes in organic matter delivery and preservation during an interval of environmental stress. These results inform wider debates on global environmental change and mass extinction at the DCB.

Highlights

End Devonian Mass Extinction (EDME) was a severe and distinct biotic crisis affecting terrestrial and marine ecosystems in the latest Famennian Stage (Fig. 1)
The phytoplankton fraction is relatively impoverished compared to Assemblage interval 1 (AI-1) and characterized by the high relative abundance of Gorgonisphaeridium spp., which accounts for 73 % of total identified phytoplankton
A prograding latest Famennian shoreface (Colpacucho Formation) is incised and overlain by a glacigenic unit consisting of coarse sandstones, diamictites and striated/grooved gravel beds (Cumaná Formation Equivalent Unit)

Summary

Introduction

End Devonian Mass Extinction (EDME) was a severe and distinct biotic crisis affecting terrestrial and marine ecosystems in the latest Famennian Stage (Fig. 1). It was coincident with a short glacial episode within the range of Retispora lepidophyta – a cosmopolitan miospore of latest Famennian age (Maziane et al 1999; Caputo et al 2008; Isaacson et al 2008; Lakin et al 2016). Proven glacigenic deposits are described in central South America (Díaz-Martínez & Isaacson, 1994; Cunha et al 2007; Vaz et al 2007; Wicander et al 2011; Caputo & Dos Santos, 2019) and the Appalachian Basin of North America (Brezinski et al 2008, 2010) These indicate a near-polar ice centre in western Gondwana and a temperate ice centre in the southern margin of Euramerica respectively (Fig. 2). Secondly, corresponding negative carbon isotope excursions (NCIEs) are recognized preceding the HBS in the upper praesulcata zone / lower costatus–kockeli interregnum zone and Devonian/Carboniferous Boundary (DCB) (Matyja et al 2020; Pisarzowska & Racki, 2020; Pisarzowska et al 2020)

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