Late Tertiary floral assemblage from upland gravel deposits of the southern Maryland Coastal Plain

A diverse flora has been discovered in a dark clay lens in upland gravel in southern Maryland near Brandywine. More than 49 taxa have been identified in the assemblage, which includes leaves, seeds, fruits, pollen, and a Taxodium (bald cypress) trunk. The vegetation is dominated by deciduous trees and vines. Four taxa are now absent from North America but survive elsewhere; one is extinct. A late Miocene age and warm-temperate climate are inferred from the flora. The clay lens probably represents a cutoff distributary in the extensive braided stream system that covered the area and is unique in Maryland. Similar dark clays have been described from Miocene sands and gravels in New Jersey. The Brandywine flora is the first direct evidence of the Miocene age of part of the fluvial upland deposits of Maryland. On the basis of the age inferred from the flora, the Brandywine deposit is correlated with the St. Marys Formation or the Eastover Formation, which are upper Miocene shelly marine units south and southeast Of Brandywine. INTRODUCTION The Brandywine deposit, located 20 km southeast of Washington, DC. (Fig. 1), is geologically significant. The deposit is contained in a channel filled with clay that is part of a system of thin surficial sand and gravel sheets that bevel marine units of Cretaceous through late Miocene age. Before the discovery of the Brandywine fossils in 1986, little direct evidence was available with which to date these upland deposits, which compose the most extensive nonglacial gravel sheets in the emerged Atlantic Coastal Plain. Figure 1. Map showing location of deposit at Brandywine, Maryland (solid circle in lobe 2), in western part of Salisbury embayment, U.S. Atlantic Coastal Plain. Fall zone marks approximate inner edge of Coastal Plain. Numbered lobes show where sand and gravel accumulated in subaerial alluvial plain during early to middle Miocene (1), late Miocene (2), and Pliocene (3, 4). The clay lens contains the most diverse late Cenozoic terrestrial flora currently known from the Atlantic Coastal Plain, and the flora provides good evidence for the age and climatic conditions during deposition of the second oldest upland deposit of the Maryland Coastal Plain (lobe 2 in Fig. 1). The age and origin of the upland deposits throughout the Salisbury embayment of the Atlantic Coastal Plain have been debated for more than 100 years. Some workers have considered the surficial sand and gravel sheets to be Quaternary glacial outwash (McGee, 1888; Jordan, 1967) or interglacial marginal marine deposits (Cooke, 1958; Shattuck, 1906). Hack (1955), following the lead of Flint (1940), argued against a marine origin for any surficial deposits above 30 m and conclusively demonstrated the fluvial nature of the upland deposits at Brandywine. Hack (1955) and Glaser (1971) established the stratigraphic age of the upland deposits near Brandywine as post-middle Miocene because they bevel the middle Miocene Choptank Formation and probably pre-Pleistocene because of the deep weathering and their physical separation from dated Pleistocene units in the lowlands of southern Maryland. Lithologic evidence suggests a nonglacial climate during deposition. The few published reports of late Tertiary floral assemblages (Hollick, 1906; Berry and Hawkins, 1935; Berry, 1937; Leopold, 1969) were based on sparse and poorly preserved material and, in most cases, controversial lithostratigraphy. In contrast, the Brandywine flora is diverse and well preserved, and it GEOLOGY, v. 18, p. 311-314, April 1990 311 has aided significantly in establishing the correct stratigraphic interpretation of the host deposit. GEOLOGY OF THE BRANDYWINE DEPOSIT The Brandywine clay lens was probably about 30 m wide, 90 m long, and 6 m thick. It occupies an irregular channel remnant in a southward-accumulating alluvial plain (Fig. 1) composed largely of quartzose sand and gravel (Fig. 2). The clay occurs in the deepest part of the channel, which is lined by steeply dipping sand and gravel cross beds. The surface elevation is 64 m. Such channels are unusual features in the upland deposits of southern Maryland; most channels in this plain do not exceed 1-2 m in depth and are filled with coarse sediment. The upland deposits are mainly composed of interbedded and poorly sorted coarse sand and fine to medium gravel, massive to subhorizontally bedded medium gravel, and sparse lenses of troughcross-bedded to planar-bedded coarse sand. These features are characteristic of braided stream deposits in alluvial plains (Rust, 1978). The homogeneous fine-grained texture of the clay that fills the channel suggests that the cutoff remained connected to the main channel, at least during floods, throughout the filling process. The channel probably filled rapidly, because pollen data do not suggest the ecological changes exFigure 2. Diagrammatic map of channel that contained cutoff channel, and lithologic cross section reconstructed from outcrops and two drill holes (open circles); Taxodium (cypress) trunk not to scale. All material above Calvert Formation is inferred to be upper Miocene on basis of flora. No major lithostratigraphic breaks were found in section beyond edges of channel. Fresh clay was dark gray to black where wet, pale to medium gray where dry; weathered clay was gold. Inset: Dot pattern indicates coeval gravel surrounding channel fill; A-A is cross-section line; circles indicate drill holes; CN and CS are northern and southern edges of abandoned channel. 1—Fallen trunk of Taxodium, not to scale; 2—sediment removed by erosion or quarrying operations. Large arrow denotes direction of water movement inferred from sedimentary structures. pected of a slow transition from a pond surrounded by sparse vegetation to a swamp or pond in dense forest. The top of the clay is at the same elevation in the excavated pit and in two drill holes located about 12 m and 25 m to the southeast; this uniform elevation suggests that the top of the clay lens is level. No evidence of deformation caused by surface loading was observed in the clay, so it had time to dewater and become consolidated before the overlying 7.5 m of gravel was deposited. Gravel above and below the clay lens is similar, about 85% monocrystalline quartz, 10% chert, and 5% polycrystalline quartz.