Reports
Summary of geologic data from three core holes drilled through the Potomac Group in the Coastal Plain of Cecil County, Maryland
2020, Quinn, H.A.
Report of Investigations 87
Abstract
Three core holes were drilled to gain additional insight into the stratigraphic and hydrogeologic framework of the Potomac Group, which contains important water-bearing units in the coastal plain of Cecil County, Maryland. Core holes were located on the Elk Neck peninsula (CE Cd 91) and the west and east sides of the Town of Elkton (CE Be 155 and CE Bf 156, respectively). Each hole was continuously cored through coastal plain sediments into variably weathered crystalline basement rock. Geophysical logs were run in each hole to obtain additional information on geologic and hydrogeologic properties. Analyses of fossil pollen and spores were performed on core samples primarily to obtain information on the ages of the strata and assist with correlation. This report presents lithologic descriptions of the cores, compilations of the geophysical logs, results of the palynological analyses, and summarizes data collection, compilation and interpretive activities conducted.
The Potomac Group is present in the Coastal Plain Province of Maryland, Delaware and New Jersey and appears to extend some distance off-shore. In many areas of the Maryland Coastal Plain, the Potomac Group can be divided into three formations – Patapsco, Arundel and Patuxent Formations, in descending order – within which are several aquifers and confining units. In the upper Chesapeake Bay area, including portions of Cecil County, these formations and the related aquifers and confining units are not as well-defined or as clearly distinguishable from each other as they are in the central and southern (western shore) portions of the Maryland Coastal Plain.
Information obtained from these core holes helped fill data gaps on the east side of the Chesapeake Bay between Turkey Point and the Maryland-Delaware line. As a result, previous conflicting stratigraphic and hydrostratigraphic correlations in the area and the assumptions in the interpretive techniques used could be evaluated. Data collected and analyzed for this project underscore the geologic complexity of the Potomac Group even over relatively short distances and emphasize the importance of palynology as a key component to be used with other geologic and geophysical data in correlating strata. Results of this study yielded some significant new information about, and greater understanding of, the subsurface geology and hydrogeology of the area.
The thickness of the coastal plain sediments differs among the core holes, ranging from approximately 489 feet at central Elk Neck peninsula to approximately 200 feet or less near Elkton. Within the Potomac Group, the lateral extent of a particular water-bearing sand interval can vary greatly. Notably a 40-foot-thick sand recorded at depth in a well about 0.52 miles from CE Cd 91 on the Elk Neck peninsula appears to be absent in CE Cd 91, yet a shallower, approximately 50-foot-thick, sandy interval is present in both. There is also considerable variation among the three core holes in the type of basement rock and the thickness of saprolite and weathering of the basement rock below the Potomac Group.
Stratigraphic correlations made in this study, supported by palynological data, suggest that the upper portion of the Potomac Group (the Patapsco Formation) in the Elkton area lies directly on weathered basement rocks and the older portions of the Potomac Group are absent there. In contrast, in the central part of Elk Neck a relatively thick interval (possibly 48 to 100 feet or more) of the older portion of the Potomac Group (Patuxent-Arundel Formations) is present above basement rock and is overlain by the Patapsco Formation. This differs significantly from some previous interpretations and indicates that the Patuxent-Arundel interval extends farther updip in the subsurface of the central part of Cecil County. In addition, the boundary between palynological Zones II and III (within the Patapsco Formation) may be higher in the subsurface below the Elk Neck peninsula than indicated in previous interpretations for that area.
Palynological information from these three core holes suggests that stratigraphic correlations within the Potomac Group that are completed primarily using geophysical logs – especially where logs are widely separated and lack age-datum control – need to be revisited. Often no single geologic investigative method or technique is sufficient to define or refine the stratigraphy (or hydrostratigraphy) of the Potomac Group sediments so multiple lines of evidence are needed. Because stratigraphic relations play a role in developing a hydrostratigraphic framework and aquifer models, which in turn are used to estimate aquifer recharge and groundwater resources, errors in the extent and relation of strata can have an impact on groundwater resource assessments.
