The Occurrence Of Shear-Torn Palynomacerals In Middle To Early Late Devonian Strata Of Southern Subandean Bolivia: Their Geological Significance

Palynomacerals damaged by shear, displaying characteristic tension gashes (subparallel tearing pattern), are reported from Middle to early Late Devonian shales of the Los Monos Formation, in the subsurface of Southern Subandean Bolivia. Shear has affected not only marine and terrestrial palynomorphs (trilete spores, organic-walled microphytoplankton, chitinozoans) but also land-derived phytoclasts. Sheartorn palynomorphs have been previously recorded in oilproducing sedimentary basins of Brazil. Here, their origin was attributed to faulting because of the following facts: (1) the shear-torn palynomorphs were recovered from wells drilled in areas infested by growth faults; (2) they occur mainly in Albian to Paleogene strata that immediately overlie the Aptian salt, and thus are severely affected by salt tectonics; (3) the subparallel tearing pattern of palynomorphs is suggestive of deforming processes that took place when the host sediment was still in a rather plastic (partly unconsolidated) state. In the investigated Los Monos sections, several samples present a considerable proportion of the palynomacerals sheared to varied degrees. The frequency of shear-torn palynomacerals is especially high in middle to upper parts of the formation. This suggests that thrusting, faulting and other deformation processes could have affected more plastic (pelitic) strata in those intervals. Tectonic disturbances of the normal stratal succession are not always distinguished on the basis of strictly biostratigraphic evidence. However, other apparent palynological anomalies (involving abrupt changes in composition, nature and/or preservation of the organic residues) occur in different intervals of the investigated wells. Some of these match intervals where palynomorph shear becomes more frequent and intense, thus pointing out to possible tectonic controls. Other palynological anomalies, dissociated from shear-torn palynomorphs, are more probably related to faciological causes.