69-7 New studies in the Los Monos, Iquiri, and Itacua formations, Chaco Basin, Bermejo (Lajas) section, Devonian, District of Santa Cruz, Bolivia

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change (Posters)

Poster Booth No.: 168

Presenting Author:

Ryan Britt

Authors:

Britt, Ryan¹, Gilfus, Ethan², McCarthy, James³, Gilleaudeau, Geoffrey J.⁴, Over, D Jeffrey⁵, Remirez, Mariano N.⁶, Strandberg, Bastian⁷, Ortiz, José Luis⁸, Suarez Riglos, Mario⁹, Dalenz Farjat, Alejandra¹⁰, Segessenman, Daniel¹¹, Nana Yobo, Lucian¹²

(1) Geological, Environmental, and Planetary Sciences, SUNY College at Geneseo, Geneseo, NY, USA, (2) SUNY College at Geneseo, Geneseo, New York, USA, (3) SUNY College at Geneseo, Geneseo, NY, USA, (4) George Mason University, Fairfax, VA, USA, (5) SUNY College at Geneseo, Geneseo, NY, USA, (6) Kobenhavns Universitet, Kobenhavn, Denmark, (7) Kobenhavns Universitet, Kobenhavn, Denmark, (8) Museo Noel Kempff Mercado, Santa Cruz de la Sierra, Bolivia, (9) Museo Noel Kempff Mercado, Santa Cruz de la Sierra, Bolivia, (10) GEOMAP S.A., Salta, Argentina, (11) George Mason University, Fairfax, VA, USA, (12) Texas A&M University, College Station, TX, USA,

Abstract:

In the tropical to sub-tropical epeiric seas of North America and Europe, the Devonian-Carboniferous transition is recorded by metal- and organic-rich transgressive black shales corresponding to the Hangenberg mass extinction event overlain directly by coarser-grained, organic-poor strata deposited during eustatic lowstand, marking a major climatic shift at the onset of the Late Paleozoic Ice Age. Little is known, however, about this key transition of extinction and glaciation at high paleo-latitudes, which would be expected to respond earlier than equatorial basins to a cooling climate. Here, we investigate dark gray and black shale of the Los Monos and Iquiri formations overlain by glacial diamictites of the Itacua Formation in the Chaco Basin of southcentral Bolivia, collected in the Bermejo (Lajas) section. Preliminary observations and magnetic susceptibility (MS) results from the Iquiri-Itacua transition show decimeter- and meter-scale cyclic patterns in the silty gray shales, while clay-rich black shales, presumably deposited farther offshore, have remarkably uniform MS values. The overall MS values in the shales and diamictites are positive, similar to values from the Pando X-1 core from similar age strata in northern Bolivia. This is in contrast to Upper Devonian black shale strata in Alberta and Oklahoma which are generally negative, approximately 1 to 1.5 lower in δMS values, indicating distinct sediment sources and local climatic conditions between these equatorial and high-latitude sites. Black shale of the Iquiri Formation contains numerous dropstones indicative of sea ice prior to the Devonian-Carboniferous boundary. A tephra bed in the upper Los Monos Formation yielded numerous zircons which, when dated, may better resolve the absolute age of the formation. Future work will involve high-resolution redox geochemistry of Iquiri Formation black shales, as well as Re-Os geochronology to determine if the Iquiri-Itacua transition is equivalent to the Hangenberg Event as recorded in North America and Europe.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9807

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