Sediment cores from southeastern Tanzania contain exceptionally well-preserved calcareous and organic-walled microfossils in numerous samples spanning from the Aptian to the Miocene. The unusually high quality of preservation is commonly attributed to shallow burial and high clay content of the host sediments. However, such attributes apply to many deposits that are not characterized by exceptional preservation, and, thus, invoking only grain size and burial depth is clearly an incomplete explanation for the exceptional microfossil preservation. In an attempt to better characterize additional factors that were important in creating the Tanzanian microfossil lagerstätte, we integrated a wide range of paleontological, geochemical, and sedimentological observations to constrain the paleoenvironmental and early diagenetic conditions of a Turonian interval where exceptional preservation is common. Planktic microfossil assemblages suggest that open-ocean surface-water conditions prevailed at the site, but, despite excellent organic matter preservation, marine biomarkers are rare. The sediments are dominantly composed of terrigenous silts and clays and terrestrially derived organic matter. Paleontological and geochemical observations record a remarkably stable interval dominated by excellent preservation through the Lower–Middle Turonian. During the Middle–Upper Turonian, though, the preservation quality declines, and this is associated with notable shifts in foraminiferal assemblages, palynological species diversity, carbon and sulfur isotope compositions, and biomarker distributions. The integrated data suggest that an increase in subsurface microbial activity and associated changes in pore-water chemistry were important proximate variables that led to the decline in the quality of microfossil preservation up section within these shallowly buried, Turonian silty claystones from Tanzania.