The Applications of palynostratigraphy to the Devonian of Bolivia

Abstract

A continuous and unthrusted 1.2km section of latest Emsian to Frasnian sediments is exposed in the Rio Lajas at Bermejo, Santa Cruz Province, Bolivia. One hundred and forty-five samples were taken from this section to validate the cuttings-based Devonian subsurface palynostratigraphy. Similar palynological trends were identified in the same stratigraphic order as seen in the subsurface. A detailed study of the chitinozoan fauna was undertaken and 36 taxa were identified of which 9 are new but retained in open nomenclature pending formal description. Only 3 of the chitinozoan taxa are cosmopolitan. The palynostratigraphy from Bermejo for the first time is tied to accurate lithostratigraphy and this data was used to revise the Devonian stratigraphy at the Campo Redondo and Cha-Kjeri sections in Chiquisaca Province. Mid Devonian age assignments for the economically important Devonian Huamampampa Formation reservoir are not valid, since the new palynology indicates a mainly Early Devonian (Emsian) age.

There is sedimentological evidence in the basal Los Monos Formation for an early Eifelian marine transgression associated with the important acritarch Evittia sommeri. In Bolivia and South America, due to the near/effective absence of conodonts and goniatites, the Devonian Euramerican sea-level curve is relatively untested. However, a possible candidate for the Chotec Event is the basal Los Monos Formation transgression. This is the only major transgressive event during the early Eifelian. Assuming similar stratigraphic trends to Bolivia, attempts were made to find E. sommeri at the Chotec level in Euramerica. A total of 158 Emsian/Eifelian samples were collected and processed for palynology from the Czech Republic, Morocco, Spain and the UK with additional non-Euramerican samples from Russia . E. sommeri was not found. While 2 younger Devonian marine transgressions are recorded in Bolivia (mid-late Givetian and earlyFrasnian), the major flooding event around the Eifelia n/Givetian boundary in Euramerica (Kacák Event) is notably absent. This is important since the Kacák Event has been implicated in the break-up of the high latitude, cool water Malvinokaffric Realm that included Bolivia.

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ORCID for John Marshall: orcid.org/0000-0002-9242-3646

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