Reports
The spores and pollen of the Potomac Group of Maryland
1963, Brenner, G.J.
Bulletin 27
Introduction
The Potomac Group forms the oldest unit of the Maryland Coastal Plain. It consists of a series of interlensing gravels, sands, silts, and clays deposited in a continental environment. The Potomac Group is of extreme interest to paleobotanists, for in the upper part, the Patapsco Formation, the first undoubted remains of angiosperms are found. The introduction of angiosperms in Patapsco time is an important phase in plant evolution, because in the Raritan, which overlies the Potomac Group, the angiosperms have, for the first time, become the dominant group in the vegetation.
In the area between Washington, D. C. and Baltimore, Maryland, the Potomac Group is divided into three formations: the Patuxent, Arundel, and Patapsco. Outcrops of these three formations are recognizable in this area, although mapping the boundaries presents frequent difficulties. To the northeast towards Delaware and to the southwest in Virginia rapid lateral and vertical lithologic changes, so typical of continental sedimentation, make correlation with the Potomac area extremely difficult. The problem is further complicated by the fact that in Delaware and New Jersey the overlying nonmarine Raritan Formation is lithologically indistinguishable from the Patapsco Formation. Although plant megafossils can be used to recognize the formations, they are extremely sparse, and, therefore, not practical as a stratigraphic tool.
In the summer of 1960, the writer examined several samples from the Potomac Group of Maryland for palynological content. The carbonaceous silts and clays scattered throughout the section proved to be highly productive, containing numerous spores and pollen. It then became apparent that the microflora might be used to develop an effective zonation of the Potomac Group, regardless of geographic differences in lithology. It was also believed that a study of the spores and pollen would contribute to an understanding of such topics as the nature and time of the arrival of angiosperms into this region, the age of the sediments, and vegetational changes during Potomac time.
Until now no detailed study of Lower Cretaceous spores has been published in the United States. It was thought, therefore, that a detailed account of the spores and pollen from the Potomac group would be a contribution to the palynology of this part of the geologic column.
