Palynological record of Devonian and Pennsylvanian units in the Espejos Range, western Santa Cruz de la Sierra, central Bolivia

DI PASQUO, M.M.; ANDERSON, H.

Lexington

Encuentro; 45 Annual Meeting of AASP (The Palynological Society) and CIMP (Commission Internationale de la Microflore du Paléozoïque Subcommissions),; 2012

University of Kentucky

As part of a major Carboniferous-Permian project in Bolivia between 2007 and 2009 (undertaken by geologists from different countries and funded by Spain), palynological investigations were carried out to test the Pennsylvanian biostratigraphic scheme (KA, RS, BC, MR and TB zones) in the central portion of the Tarija-Chaco Basin and its aid in stratigraphic discrimination. This contribution presents the palynological results of several sections located in the Espejos range along the La Angostura-Bermejo-Samaipata-Mairana highway, west of Santa Cruz de la Sierra. The geology of the Espejos range is tectonically complex consisting of faulted anticlines and synclines of Devonian to Permian and Mesozoic and Cenozoic strata. Cenozoic and Mesozoic strata crop out on the east side of the range, whereas the Paleozoic succession appears after La Angostura. Different Devonian, Carboniferous and less frequently Permian rocks (at La Angostura) crop out to the west up to Mairana. The stratigraphic units are generally in faulted contact and structural repetition of units is quite common.  Pennsylvanian units are represented by the Macharetí (McG) and Mandiyutí (MdG) groups which consist of interbedded sandstone and diamictite units with minor shale and mudstone beds. Each group has traditionally been divided into formations based on the first occurrence of sandstone within mudstone- or diamictite-dominated units. The McG is divided into five formations in Bolivia (oldest to youngest): Tupambí, Itacuamí, Chorro, Tarija and Taiguati. The MdG consists of the Escarpment and San Telmo formations. This lithostratigraphic scheme is well applied in southern Tarija-Chaco Basin where contacts between sandstone-dominated (Tupambi, Chorro) and mudstone-diamictite-dominated (Itacuamí, Tarija/Tupambi) units is clear. However, to the north unit contacts are difficult to establish due to increased interbedding of sandstone and diamictite units, reinforced by tectonic effects. This study focuses largely on the McG because the MdG in this area, mainly exposed in the upper part of the mountains, is dominantly red in color (four samples were barren).  Twenty six samples were taken from the McG at five different locations between Mairana and Bermejo. From west to east, near Mairana one productive sample was taken from a grey muddy diamictite whilst other seven from reddish and greenish grey muddy diamictites were barren. Two productive samples were taken from grey diamictites 24 km from Samaipata toward Bermejo (Point 1). Fourteen kilometers ahead (Point 3), one barren sample taken from light grey very fine-grained sandstone over Devonian rocks (faulted contact) and up-section, two productive samples were collected from grey diamictites. Thirteen kilometers ahead, a thick McG section (Ginger’s Paradise, ca. 500 m) was sampled. From this area, eleven grey and greenish-grey mudstone and diamictites were productive whislt two reddish brown mudstones were barren. The palynoassemblages recovered from McG are composed of biostratigraphically important indigenous species such as Cristatisporites chacoparanensis, Cristatisporites spp., Dictyotriletes bireticulatus, Granulatisporites varigranifer, Verrucosisporites morulatus, V. patelliformis, V. quassigobbettii, Endosporites rhytidosaccus, Vallatisporites arcuatus, Costatacyclus crenatus, Potonieisporites magnus. Abundant reworked palynomorphs from Devonian (e.g., Umbellasphaeridium saharicum, Maranhites spp., Retispora lepidophyta, Grandispora pseudoreticulatus) and Mississippian (e.g., Pustulatisporites gibberosus, Cristatisporites peruvianus, Cordylosporites magnidictyus) rocks are present, as well as from the underlying Bashkirian KA-RS zones in agreement with previous works (e.g., Schopfipollenites ellipsoides). The stratigraphic distribution of the indigenous species supports the correlation of all assemblages to the Bashkirian-Moscovian D. bireticulatus–C. chacoparanensis (BC) Zone, reinforced by the presence of exclusive forms such as C. chacoparanensis, D. bireticulatus, E. rhytidosaccus, and V. morulatus. It corresponds to the Tarija Formation and the base of the Escarpment Formation in northern Argentina and southern Bolivia (Tarija-Chaco Basin). The diamictitic units bearing the BC Zone in the studied area are similar lithologically to the Tarija Formation in southern part of the basin, thus confirming their stratigraphic attribution. Three productive samples were collected from Los Monos/Iquirí mudstones (Givetian-Frasnian) under a faulted contact with the Carboniferous (Point 2, ten km from previous Point 1), which yielded abundant and diverse well preserved palynomorphs (spores, acritarchs, prasinophytes and chitinozoans). Species such as Lunulidia micropunctata, Maranhites spp., Samarisporites triangulatus, Angochitina mourai, Lagenochitina avelinoi, and Fungochitina pilosa allowed the attribution of these assemblages to the late Givetian - late Frasnian. Identification of palynomorphs of the BC Zone in diamictites below the Escarpment Formation in the central Tarija-Chaco Basin supports the previous biostratigraphic scheme for the Macharetí Group developed from deposits in the northern Argentina and southern Bolivia. Even though tectonic complexity disrupts the succession, this work demonstrates that the biostratigraphic zones follow lithostratigraphic boundaries across the basin and that the lithologic changes from south to north in the Tarija-Chaco Basin are related to major shifts in paleoclimate during the Pennsylvanian.