SUBSURFACE MIOCENE PALYNOMORPHS FROM SANTIAGO DEL ESTERO, ARGENTINA. The palynological assemblage recovered from the YPF.SE. LH.x-2 Los Horcones well, Santiago del Estero Province, is dominated by Amaranthaceae/ Chenopodioideae and freshwater green algae, together with subordinate Nothofagaceae and Malvaceae. Marine dinoflagellates and foraminiferal linnings are also present in the upper half of the sampled section. This assemblage may have developed in a lacustrine environment. A mostly herbaceaous and shrubby vegetation may have surrounded the water-body. The climate may have been warm—temperate and relatively arid. The palynoflora could be related to the marine ingression that affected northwestern Argentina during the middle Miocene (15–13 My).