Devonian sea-level change in Bolivia: A high palaeolatitude biostratigraphical calibration of the global sea-level curve

Abstract

During the Devonian Period the cool water Malvinokaffric Realm was located at high palaeolatitudes in the southern hemisphere. The Realm is defined by its highly endemic marine benthic fauna, which makes extra-Malvinokaffric correlations problematic. Investigation of the Devonian palynomorphs from an extensive regional sub-surface dataset (24 wells) in the Chaco Basin, Bolivia, reveals the presence of a number of stratigraphically restricted and regionally correlative epiboles (peak abundances) of the distinctive palynomorphs Bimerga bensonii, Crucidia camirense, Evittia sommeri, Petrovina connata and Ramochitina magnifica. These palynomorph epiboles were then located at outcrop by a high-resolution palynological investigation (225 samples) of two Bolivian outcrop localities in the Chaco Basin: Bermejo, Santa Cruz Department and Campo Redondo, Chiquisaca Department. The important Evittia sommeri epibole in the basal Los Monos Formation is related to a marine transgression that is present in both these outcrop sections. Additional chronostratigraphic control on this marine transgression comes from the occurrence of rare goniatites in the base of the Huamampampa Formation at Campo Redondo, which are at least early Eifelian in age (post Chotĕc Event). Based on the goniatite together with spore data, the marine transgression with Evittia sommeri in the basal Los Monos Formation can be tentatively assigned to the Mid Devonian (mid-late Eifelian) Kačák Event. The presence of a datable Malvinokaffric goniatite has shown that key spore taxa which are used zonally in both Laurussia and Gondwana do not all have coincident first occurrences in both areas. Furthermore, despite the appearance of some cosmopolitan elements in the microflora, Mid and Late Devonian spore assemblages in Bolivia are distinct from other regions in being relatively impoverished in both progymnosperm and lycopod spores. This floral difference is attributable to the Malvinokaffric Realm continuing to retain a distinctive cool climatic signature throughout this interval. However, the sporadic occurrences of extra-Malvinokaffric macrofauna in restricted stratigraphic intervals of the Middle Devonian in South America and South Africa are significant. The oldest known occurrence of the extra-Malvinokaffric brachiopod Tropidoleptus in Bolivia is coincident with the late Eifelian basal Los Monos Formation transgression. Hence there was a relationship between influxes of this fauna and marine transgressions, i.e. temporary periods of reduced climatic gradient. The migration of Devonian spores, and particularly heterosporous spores, between Gondwana and Laurussia clearly occurs during the Mid and Late Devonian. This is at variance with models claiming a wide Rheic Ocean during much of the Devonian.