Quantitative variations in the size, shape, and preservation state of microscopic plant debris assemblages correlate with the actual distance of the sample site from the coastal siliciclastic sediment source. A case study from a shelf to basin transect in the Upper Cretaceous of the Spanish Pyrenees reveals that (1) the diameter of equant, black wood phytoclasts, (2) the ratio of equant- to lath-shaped, black wood phytoclasts, and (3) several other parameters show predictable changes over a 55 km onshore-offshore transect. These parameters primarily reflect sedimentological sorting based on particle size and density, rather than paleoecological gradients. When only samples from the transgressive systems tract are considered, multiple regression models predict the distance from the source with a standard error of ±5 km (r2 = 0.84–0.93). These parameters also allow the maximum flooding surface to be more precisely located within mudstone-dominated sections. By applying the lateral regression relationship to stratigraphic data, a quantitative estimate can also be made for the scale of lateral facies shifts through depositional sequences (here ∼10 km).