Palynology of latest Neogene (Middle Miocene to late Pliocene) strata in the Delmarva Peninsula of Maryland and Virginia
Palynology of Miocene and Pliocene formations in the Delmarva Peninsula of Maryland and Virginia reveals a significant representation of exotic pollen interspersed in pollen assemblages that are otherwise comparable to those from the modern vegetation of the Mid-Alantic coastal plain region. The late Tertiary arboreal pollen (AP) assemblages are dominated by oak, hickory, pine, birch and alder with minor amounts of mid- and southern coastal tree taxa, as well as minor spruce and hemlock and a trace of fir. Nonarboreal pollen (NAP) include grass, sedge, composite and aquatic taxa. Exotic pollen in these assemblages represent plants now foreign to this region. They may be placed in three categories. First, there are extinct forms, such as Labrapollis, Plicatopollis, and Multiporopollenites, that can be traced from the Cretaceous or Early Tertiary into the Late Tertiary. The second group includes forms, such as Podocarpus, Engelhardtia, Pterocarya, Ephedra, Eucommia, Ulmus-Zelkova, Glyptostrobus, Palmae, and Cyathea, that are not found in this region today and not found in early Pleistocene sediments in the eastern United States. Many of these taxa are subtropical or greatly restricted in geographic range. A third group of exotics, mainly Cyrilla, Planera, Gordonia, Jussiaea, and Sapotacaea, including Minusops, are generally found south of the study area or have their northern limit here at this time. The lack of the extinct or distant exotics in early to mid-Pleistocene sediments in the mid-Atlantic coastal plain and the last appearance of Pterocarya, as the last exotic taxon in the early Pleistocene of western Europe, support the stratigraphic assignment of the Pliocene units. The number of exotic taxa diminish markedly between the Miocene pollen assemblages and those of the Late Pliocene. Climatic fluctuations characterize the Late Tertiary environments. The Miocene, for example, incorporates a warming trend between the upper, middle Miocene and the Manokin beds and the late Miocene of the Pokomoke beds. The late Miocene was probably somewhat warner than the present climate in the Delmarva region. This trend is based on the presence of colder climate indicators, mainly spruce and hemlock, in the Manokin pollen record. The two distinct pollen assemblages constitute two pollen zones. Similarly, the Pliocene pollen record also shows a warming trend. The pollen zone of the Yorktown Formation of the early Pliocene age contains the colder climate indicators spruce and hemlock. The Beaverdam and Walston formation of late Pliocene age contain pollen assemblages that reflect climatic conditions warmer than the present time.