State of Delaware DELAWARE GEOLOGICAL SURVEY David R. Wunsch, State Geologist Delaware GeoloGical Survey report of inveStiGationS no. 78 SubSurface GeoloGy of the area between wranGle hill anD Delaware city, Delaware By John W. Jengo1, Peter P. McLaughlin, Jr.2, and Kelvin W. Ramsey2 University of Delaware Newark, Delaware 2013 1 MWH Americas, Inc. 2 Delaware Geological Survey Use of trade, product, or firm names in this report is for descriptive pur - poses only and does not imply endorsement by the Delaware Geological Survey. table of contentS page ABSTRACT ................................................................................1 INTRODUCTION ...........................................................................1 Purpose and Scope ........................................................................1 Previous Work ...........................................................................2 Acknowledgments ........................................................................3 DATA AND METHODS .......................................................................3 STRATIGRAPHY ...........................................................................4 Potomac Formation .......................................................................5 Definition and Age ....................................................................5 Composition, Textures, and Depositional Environment ...........................................5 Thickness, Distribution, and Bounding Relations ...............................................5 Magothy Formation .......................................................................6 Definition and Age ....................................................................6 Composition, Textures, and Depositional Environment ...........................................6 Thickness, Distribution, and Bounding Relations ...............................................7 Merchantville Formation ...................................................................7 Definition and Age ....................................................................7 Composition, Textures, and Depositional Environment ...........................................7 Thickness, Distribution, and Bounding Relations ...............................................8 Englishtown Formation ....................................................................8 Definition and Age ....................................................................8 Composition, Textures, and Depositional Environment ...........................................9 Thickness, Distribution, and Bounding Relations ...............................................9 Marshalltown Formation ....................................................................9 Definition and Age ....................................................................9 Composition, Textures, and Depositional Environment ..........................................10 Thickness, Distribution, and Bounding Relations ..............................................10 Mount Laurel Formation ...................................................................10 Definition and Age ....................................................................10 Composition, Textures, and Depositional Environment ..........................................10 Thickness, Distribution, and Bounding Relations ..............................................10 Quaternary Geology ......................................................................11 Geomorphology ......................................................................11 Columbia Formation ......................................................................11 Definition and Age ....................................................................11 Composition, Textures, and Depositional Environment ..........................................12 Thickness, Distribution, and Bounding Relations ..............................................12 Delaware Bay Group .....................................................................13 Definition and Age ....................................................................13 Lynch Heights Formation ..................................................................13 Definition and Age ....................................................................13 Composition, Textures, and Depositional Environment ..........................................13 Thickness, Distribution, and Bounding Relations ..............................................14 Scotts Corners Formation ..................................................................14 Definition and Age....................................................................14 Composition, Textures, and Depositional Environment..........................................14 Thickness, Distribution, and Bounding Relations ..............................................15 Marsh and Swamp Deposits ................................................................15 Summary of Lithologic and Stratigraphic Features.............................................15 Radiocarbon Age Dating ...............................................................15 Fill Deposits ...........................................................................16 DISCUSSION .............................................................................17 Origin of the Reybold Paleochannel and Columbia Formation Deposits .................................17 Implications for Hydrogeology ..............................................................19 SUMMARY ...............................................................................20 REFERENCES.............................................................................21 illuStrationS Figure 1. Location map, central New Castle County, Delaware. ......................................2 Figure 2. Generalized stratigraphic column for the area between Wrangle Hill and Delaware City and nearby parts of New Castle County. .....................................3 Figure 3. Three-dimensional relationship of Pleistocene-age Columbia Formation and Cretaceous-age Potomac Formation. ...................................................6 Figure 4. Generalized surficial geologic map of the study area prior to construction of the industrial complex. ...11 Figure 5. Plot of radiocarbon dates versus sample elevation from the study area and the margin of the Delaware River in nearby locations.........................................17 tableS Table 1. Radiocarbon dates collected for this study...............................................4 Table 2. Radiocarbon dates used in sea level curve ..............................................16 plateS Plate 1. Data location map ........................................................Linked File Plate 2. Structural cross sections ....................................................Linked File Plate 3. Structural contour map for the top of the Potomac Formation .........................Linked File Plate 4. Structural contour map for the base of the Columbia Formation .......................Linked File appenDiceS Appendix 1. List of wells used in this study.. .............................................Linked File Appendix 2. Stratigraphic picks used for cross sections. .....................................Linked File Appendix 3. Data used for the map of the top of Potomac Formation. ...........................Linked File Appendix 4. Data used for the map of the base of Columbia Formation.. .........................Linked File SubSurface GeoloGy of the area between wranGle hill anD Delaware city, Delaware abStract 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 hetero - geneous. 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