The geology and hydrology of the area between Wrangle Hill and Delaware City, Delaware, have been the focus of numerous studies since the 1950s because of the importance of the local groundwater supply and the potential environmental impact of industrial activity. In this report, 490 boreholes from six decades of drilling provide dense coverage, allowing detailed characterization of the subsurface geologic framework that controls groundwater occurrence and flow.
The region contains a lower section of tabular Cretaceous strata (Potomac, Merchantville, Englishtown, Marshalltown,and Mount Laurel Formations in ascending order) and a more stratigraphically complex upper section of Pleistocene-to-modern units (Columbia, Lynch Heights, and Scotts Corners Formations, latest Pleistocene and Holocene surficial sediments and estuarine deposits). The lowermost Potomac Formation is a mosaic of alluvial facies and includes fluvial channel sands that function as confined aquifer beds; however, the distribution of aquifer-quality sand within the formation is extremely heterogeneous. The Merchantville Formation serves as the most significant confining layer. The Columbia Formation is predominantly sand and functions as an unconfined aquifer over much of the study area.
To delineate the distribution and character of the subsurface formations, densely spaced structural-stratigraphic cross sections were constructed and structural contour maps were created for the top of the Potomac Formation and base of the Columbia Formation. The Cretaceous formations form a series of relatively parallel strata that dip gently (0.4 degrees) to the southeast. These formations are progressively truncated to the north by more flatly dipping Quaternary sediments, except in a narrow north-south oriented belt on the east side of the study area where the deeply incised Reybold paleochannel eroded into the Potomac Formation.
The Reybold paleochannel is one of the most significant geological features in the study area. It is a relatively narrow sandfilled trough defined by deep incision at the base of the Columbia Formation. It reaches depths of more than 110 ft below sea level with a width as narrow as 1,500 ft. It is interpreted to be the result of scour by the sudden release of powerful floodwaters from the north associated with one or more Pleistocene deglaciations. Where the Reybold paleochannel cuts through the Merchantville confining layer, a potential pathway exists for hydrological communication between Columbia and Potomac aquifer sands.
East of the paleochannel, multiple cut-and-fill units within the Pleistocene to Holocene section create a complex geologic framework. The Lynch Heights and Scotts Corners Formations were deposited along the paleo-Delaware River in the late Pleistocene and are commonly eroded into the older Pleistocene Columbia Formation. They are associated with scarps and terraces that represent several generations of sea-level-driven Pleistocene cut-and-fill. They, in turn, have been locally eroded and covered by Holocene marsh and swamp deposits. The Lynch Heights and Scotts Corners Formations include sands that are unconfined aquifers but complicated geometries and short-distance facies changes make their configuration more complex than that of the Columbia Formation.
Slightly calcareous, glauconitic, quartz sand that is medium to fine grained. Contains about 3 to 5 percent glauconite. Sand is subrounded to subangular and slightly silty with a few moderately silty zones. Scattered belemnites are present as well as a few scattered shell fragments or thin shell beds. Uniform dark olive gray or yellowish-brown where weathered. In outcrop, reported to be extensively burrowed (Owens, et al., 1970). Where it is the surficial deposit south of the Chesapeake and Delaware Canal, the Mt. Laurel can be confused with the Columbia Formation, especially where the color is similar. Can be differentiated by ubiquitous presence of glauconite and generally better sorted sands of the Mt. Laurel. Marine in origin. Ranges from 30 to 100 ft in thickness.
This map shows the saturated thickness of the water-table aquifer. This aquifer consists of the deposits of the Columbia Formation and those portions of the Magothy and Englishtown-Mt. Laurel formations, and Rancocas Group that are hydraulically connected with the Columbia deposits (see Groot, Demicco, and Cherry, 1983). For example, large, saturated thicknesses in the zone trending northeast-southwest near Townsend reflect the addition of the sands of the Rancocas Group to the total thickness of the sands of the overlying Columbia Formation.
This Bulletin presents the subsurface stratigraphy of the post-Potomac Cretaceous and Tertiary rocks of the Atlantic Coastal Plain of central Delaware, between the Chesapeake and Delaware (C & D) Canal and Dover. Geophysical log correlations supported by biostratigraphic and lithologic data from boreholes in Delaware and nearby New Jersey provide the basis for the report. The stratigraphic framework presented here is important for identifying subsurface stratigraphic units penetrated by the numerous boreholes in this part of Delaware, particularly those rock units that serve as aquifers, because such knowledge allows for better prediction at ground-water movement and availability. Also, accurate stratigraphy is a prerequisite for interpreting the geologic history of the rocks and for the construction of maps that depict the structure and thickness of each unit.
Fossil collectors have been attracted to Delaware since the late 1820s when the excavation of the Chesapeake and Delaware (C&D) Canal first exposed marine fossils of Cretaceous age. Since then, many technical and nontechnical works have been written about the fossils. However, there has not been a single source for casual or student collectors to turn to for help in the identification of typical finds. This paper is written to fill that void as well as provide general information about fossils and specific information on geologic formations and collecting localities at the Canal. This report is not designed to be an encyclopedia on the fossils of the Canal but focuses on those fossils found most frequently. The majority of the fossils collected today are from the spoil areas in the vicinity of the Reedy Point Bridge. Thus, the chapter on classification concentrates on the fossils of the Mount Laurel Formation, the stratigraphic unit dredged in that area.