Palynomorph preservation in volcaniclastic rocks of the Miocene Tepoztlán Formation (Central Mexico) and implications for paleoenvironmental reconstruction

Palynomorph preservation in sedimentary rocks is strongly affected by various taphonomic factors related to transport, deposition, diagenesis, and preservation potential. The palynological record may help distinguish different taphonomic factors and also displays changes in paleoenvironment, especially in volcanic settings where a very complex interaction of eruptive, gravitational, and fluvial processes in time and space can be observed. Herein, we report on new palynological data from the Miocene Tepoztlán Formation. The 800-m-thick formation mainly consists of pyroclastic rocks, mass flow units (lahars) and fluvial deposits. It is part of the southern Transmexican Volcanic Belt, cropping out south of the Valley of Mexico and within the two states of Morelos and Mexico State. The volcaniclastic succession records various stages of recovery of vegetation related to a wide variety of disturbance factors and mechanisms. During the entire period of deposition, mixed mesophytic forests appear to have been widespread in the lowlands along streams and midaltitude uplands surrounding the valley. Pollen assemblages were repeatedly reset by volcanic eruptions or their secondary effects (lahars) to more limited assemblages with gradual recoveries to the initial stages before the eruption. A clear distinction can be made between samples taken from different transport regimes (fluvial, lahar and pyroclastic flow transport). The highest percentages of well-preserved, amorphous, and crumpled palynomorphs can be found in fluvial sediments while the highest percentage of fragmented palynomorphs is characteristic of lahar deposits. In contrast, the highest percentage of corroded palynomorphs can be found in deposits originating from pyroclastic flows.