Stratigraphic, paleoclimatic, and paleobiogeographic significance of Tertiary sporomorphs from Massachusetts

Little has been published on the Tertiary sporomorph (spore and pollen) floras of New England. For this paper, I have examined 24 sporomorph samples from six localities in Plymouth County and Martha's Vineyard, Mass. Two independently dated samples, from the middle Miocene and Pliocene, respectively, of Gay Head, Martha's Vineyard, show that the Miocene flora of eastern Massachusetts represents rich subtropical forest, whereas the Pliocene flora reflects a relatively depauperate cool temperate forest. This middle Miocene sporomorph assemblage is compared with Tertiary assemblages from five other localities in eastern Massachusetts, and these six assemblages are compared with Miocene assemblages, most of them previously described, from Maryland and New Jersey. All six Massachusetts assemblages are similar to those from the Miocene of the Middle Atlantic States, but because the obvious qualitative (presence-absence) changes were few in the sporomorph floras of the Middle Atlantic States during the Miocene, it cannot be determined from the taxon lists whether all the Massachusetts samples are middle Miocene like the independently dated sample from Gay Head. Climatic differences among Miocene samples in New Jersey have been determined on the basis of relative frequencies of the main sporomorph taxa However, taxon percentages could not be found for the Plymouth County samples because they contain abundant reworked sporomorphs. The reworked sporomorphs show that Miocene strata of Plymouth County were derived at least in part from sediments of Late Cretaceous, Paleocene, and early Eocene to earliest middle Eocene ages. Most of the reworked sporomorphs are probably early Eocene to earliest middle Eocene in age, and all these taxa are also found in the Gulf Coast, showing that the Atlantic and Gulf Coastal Plains seem to have formed one floristic and climatic province in the early Eocene to early middle Eocene. At that time, the climate of coastal Massachusetts was probably winter-dry tropical. Miocene samples of Martha's Vineyard probably lack reworked palynomorphs. Counts of sporomorph taxa from these samples, and the taxon lists for all the samples, suggest that, in the Miocene, Massachusetts was covered with dense Mixed Mesophytic coniferhardwood forest that included several exotic elements such as Sciadopitys, Podocarpus, Engelhardia group, and perhaps Gleichenia-Dicranop teris. INTRODUCTION Tertiary deposits in eastern Massachusetts have been known at least since 1824 (Hitchcock, 1824; see White, 1890), but the ages of the scattered Tertiary outcrops in the region have been a matter of debate. The main purpose of this paper is to correlate the best known outcrops with well-dated sections farther south on the Atlantic Coastal Plain and, where possible, to determine correlations among the individual Tertiary outcrops of eastern Massachusetts. The sporomorph assemblages also have significant paleofloristic, paleobiogeographic, and paleoclimatic implications because little has been published on Tertiary floras of Northeastern United States except on the Oligocene(? ) Brandon lignite flora of Vermont (fig. 1; Traverse, 1955; many papers by B. H. Tiffney). Finally, the reworked sporomorphs in many of the Massachusetts samples offer information about cycles of reworking, about the provenance of the Neogene sediments, and about Paleogene biogeography and climates. The 24 Massachusetts samples that I studied are listed in table 1. They were prepared by means of standard acid maceration as well as by swirling (Funkhouser and Evitt, 1959) and (or) ZnBr2 separation. Residues were mounted in glycerine jelly or vinylite AYAF (vinyl acetate resin). Coordinates listed in table 6 at the end of the paper locate the illustrated specimens on Leitz microscope 871956 at the U.S. Geological Survey, Reston, Va. , where slides used in this study are stored; for methods of relocating specimens on other microscopes, see Frederiksen and Christopher (1978) and Frederiksen (1978b). 80° SIGNIFICANCE OF TERTIARY SPOROMORPHS FROM MASSACHUSETTS