PALYNOFACIES ANALYSIS OF NEOGENE SEDIMENTS IN THE LAKE IZABAL BASIN: INSIGHTS INTO DEPOSITIONAL PALEOENVIRONMENT AND ORGANIC MATTER DISTRIBUTION FROM THE COLORADO-1 WELL

The Lake Izabal Basin (LIB) in eastern Guatemala is a pull-apart basin that developed during the Seravallian and is one of the few basins in Central America that preserve Neogene sediments. The Colorado-1 well, drilled in 1991 in the eastern part of the basin, penetrated the oldest sediments mainly - sandstone, conglomerate, mudstone, and coal from the upper Miocene Carboneras and lower Pliocene Herreria formations. A previous palynostratigraphic study inferred a Tortonian age for the basal 1200 m sediment in the well. Here, we present a palynofacies analysis of sixty-five ditch-cutting samples from a depth range of 550-1200 m for the first time. Three major categories of particulate organic matter were identified: phytoclasts, palynomorphs, and amorphous organic matter (AOM); the AOM appears to be derived from freshwater phytoplankton. The organic matter components were further divided into eight subgroups: cuticles, structured phytoclasts (tracheids, parenchyma), degraded phytoclasts, lath-shaped opaques, equant-shaped opaques, pollen, spores, and fungal remains; the opaques are black woody debris. Point counting of 300 particles per sample indicates that phytoclasts constitute approximately 75% of the organic matter assemblage, followed by AOM, which accounts for just over 20%. The low percentage (<4%) of palynomorphs appears to be related to the masking effect of phytoclasts in the samples. The predominance of phytoclasts and the high ratio of equant- to lath-shaped opaques suggest proximity to the source area and a short transport distance. Increasing AOM with depth indicates deeper depositional conditions, and most of these samples indicate deposition in a sub-oxic shelf based on the AOM-Phytoclast-Palynomorph (APP) ternary diagram. The analysis reveals that all observed organic matter components are of terrestrial origin, and no marine influence is observed. The results provide valuable insights into the distribution of particulate organic matter and paleoenvironmental conditions as the LIB evolved.