Bioturbated, dark-greenish-gray silty clay, banded light-gray, white, and red silty clay, and glauconitic, shelly, very fine sandy silt. In the Georgetown Quadrangle, the St. Marys Formation is capped by about 5 to 15 ft of bioturbated, dark-greenish-gray silty clay. A distinct burrowed horizon separates the clay from the underlying banded clay that consists of a 10- to 15-ft thick, compact, color-banded silty clay with scattered white clayey concretions. The banded clay has a sharp contact at its base with underlying glauconitic, very fine, sandy silt. The sandy silt contains shells of the gastropod Turritella. The entire thickness of the St. Marys Formation is less than 100 ft in the Georgetown Quadrangle, thinning from its thickest in the southeast corner to about 50 ft thick in the northwest corner of the map area. Interpreted to be a marine deposit of late Miocene age (McLaughlin et al., 2008).
Yellowish- to reddish-brown, fine to coarse, feldspathic quartz sand with varying amounts of gravel. Typically cross-bedded with cross-sets ranging from a few inches to over three feet in thickness. Scattered beds of tan to reddish-gray clayey silt are common. In places, the upper 5 to 25 feet consists of grayish- to reddish-brown silt to very fine sand overlying medium to coarse sand. Near the base, clasts of cobble to small boulder size have been found in a gravel bed ranging from a few inches to three feet thick. Gravel fraction primarily quartz with lesser amounts of chert. Clasts of sandstone, siltstone and shale from the Valley and Ridge, and pegmatite, micaceous schist, and amphibolite from the Piedmont are also present. Fills a topographically irregular surface, is less than 50 feet thick, and is interpreted to be primarily a body of fluvial glacial outwash sediment (Jordan, 1964; Ramsey, 1997). Pollen indicate deposition in a cold climate during the middle Pleistocene (Groot and Jordan, 1999).
Heterogeneous unit of light-gray to brown to light-yellowish brown, medium to fine sand with discontinuous beds of coarse sand, gravel, silt, fine to very fine sand, and organic-rich clayey silt to silty sand. Upper part of the unit commonly consists of fine, well-sorted sand. Small-scale cross-bedding within the sands is common. Some of the interbedded clayey silts and silty sands are burrowed. Beds of shell are rarely encountered. Sands are quartzose and slightly feldspathic, and typically micaceous where very fine to fine grained. Unit underlies a terrace parallel to the present Delaware Bay that has elevations between 50 and 30 feet. Interpreted to be a fluvial to estuarine unit of fluvial channel, tidal flat, tidal channel, beach, and bay deposits (Ramsey, 1997). Overall thickness ranges up to 50 feet.
A multiple linear regression method was used to estimate water-table elevations under dry, normal, and wet conditions for the Coastal Plain of Delaware. The variables used in the regression are elevation of an initial water table and depth to the initial water table from land surface. The initial water table is computed from a local polynomial regression of elevations of surface-water features. Correlation coefficients from the multiple linear regression estimation account for more than 90 percent of the variability observed in ground-water level data. The estimated water table is presented in raster format as GIS-ready grids with 30-m horizontal (~98 ft) and 0.305-m (1 ft) vertical resolutions. Water-table elevation and depth are key facets in many engineering, hydrogeologic, and environmental management and regulatory decisions. Depth to water is an important factor in risk assessments, site assessments, evaluation of permit compliance data, registration of pesticides, and determining acceptable pesticide application rates. Water-table elevations are used to compute ground-water flow directions and, along with information about aquifer properties (e.g., hydraulic conductivity and porosity), are used to compute ground-water flow velocities. Therefore, obtaining an accurate representation of the water table is also crucial to the success of many hydrologic modeling efforts. Water-table elevations can also be estimated from simple linear regression on elevations of either land surface or initial water table. The goodness-of-fits of elevations estimated from these surfaces are similar to that of multiple linear regression. Visual analysis of the distributions of the differences between observed and estimated water elevations (residuals) shows that the multiple linear regression-derived surfaces better fit observations than do surfaces estimated by simple linear regression.
Heterogeneous unit of light-gray to brown to light-yellowish-brown, coarse to fine sand, gravelly sand and pebble gravel with rare discontinuous beds of organic-rich clayey silt, clayey silt, and pebble gravel. Sands are quartzose with some feldspar and muscovite. Commonly capped by one to two feet of silt to fine sandy silt. Laminae of opaque heavy minerals are common. Unit underlies a terrace parallel to the present Delaware River that has elevations less than 25 feet. Interpreted to be a transgressive unit consisting of swamp, marsh, estuarine channel, beach, and bay deposits. Climate during the time of deposition was temperate to warm temperate as interpreted from fossil pollen assemblages (Ramsey, 1997). Overall thickness of the unit rarely exceeds 20 feet.
The Delaware Academy of Science has been instrumental in informing Delaware citizens about science and utilization of local resources. Since 1970 the annual meeting of the Delaware Academy of Science has been used as a time for presentation of ongoing research in various areas of science in the Delaware region. The proceedings of these meetings have resulted in publication of transactions of the Delaware Academy of Science. The 1976 annual meeting focused on aspects of the geology of Delaware. Members of the Delaware Geological Survey and the Geology Department at the University of Delaware contributed papers in their specific disciplines. This volume presents an overview of studies of geological features and processes of evolution of the geology of Delaware. Although this collection of papers does not represent an all-inclusive study of the subject, the selections included in this volume highlight past, present, and future trends in the study of Delaware's geology. It is hoped that the combined bibliographies of all the papers will provide a comprehensive view of the literature for further investigation into the geology of Delaware.
The Generalized Geologic Map of Delaware is a brief summary for general use indicating the major types and locations of rocks present throughout the State, and their interrelationships. The map is preliminary as it is a first step in a continuing program of detailed geologic mapping. It is based upon many existing sources of data; additional detail may be found in the references listed.
The Columbia sediments of Delaware cover almost all of the surface of the Coastal Plain portion of the State. A major unconformity separates these predominantly sandy materials from the underlying rocks of the Coastal Plain. As it includes the materials closest to the surface in most places, the Columbia has great practical importance in Delaware. In addition to the morphology and soils which are largely dictated by the Columbia, it holds about 90 percent of the State's groundwater supplies, is the geologic foundation for most construction, and yields essentially all of the sand and gravel mined here.
The Delaware Geological Survey conducted a review of existing ground-water quality data collected from shallow (less than 100 feet deep) domestic water-supply wells and small public water-supply wells (serving fewer than 100 residents) to determine the extent to which toxic and carcinogenic compounds are present in the shallow ground water serving domestic water supply wells. These data were generated by several agencies including the Delaware Geological Survey, U.S. Geological Survey, Delaware Department of Natural Resources and Environmental Control, Delaware Division of Public Health Office of Drinking Water, and the Delaware Department of Agriculture Pesticide Management Program.
OFR45 Characterization of the Potomac Aquifer, an Extremely Heterogeneous Fluvial System in the Atlantic Coastal Plain of Delaware
Fluvial sands of the subsurface Cretaceous Potomac Formation form a major aquifer system used by a growing population in the northern Coastal Plain of Delaware. The aquifer is extremely heterogeneous on the megascopic scale and connectivity of permeable fluvial units is poorly constrained. The formation is characterized by alluvial plain facies in the updip section where it contains potable water. While over 50 aquifer tests indicate high permeability, the formation is primarily composed of fine-grained silt and clay in overbank and interfluvial facies. Individual fluvial sand bodies are laterally discontinuous and larger-scale sand packages appear to be variable in areal extent resulting in a labyrinth style of heterogeneity. The subsurface distribution of aquifers and aquitards has been interpreted within a new stratigraphic framework based on geophysical logs and on palynological criteria from four cored wells. The strata dip gently to the southeast, with generally sandy fluvial facies at the base of the formation lapping onto a south-dipping basement unconformity. The top of the formation is marked by an erosional unconformity that truncates successively older Potomac strata updip. Younger Cretaceous units overly the formation in its downdip area. In the updip area, the formation crops out or subcrops under Quaternary sands.The fine-grained facies include abundant paleosols that contain siderite nodules and striking mottling that commonly follows ped faces and root traces. These paleosols may serve as regional aquitards. This geologic complexity poses a challenge for determining the magnitudes and directions of ground-water flow within the aquifer that are needed for making informed decisions when managing this resource for water supply and contaminant remediation.
The Seaford area geologic mapping project (Andres and Ramsey, 1995) was conducted by Delaware Geological Survey (DGS) staff and focused on the Seaford East (SEE) and Delaware portion of the Seaford West (SEW) quadrangles (Fig. 1). Data evaluated in support of mapping from these quadrangles and surrounding areas are documented in this report.
RI37 Stratigraphic Nomenclature of Nonmarine Cretaceous Rocks of Inner Margin of Coastal Plain in Delaware and Adjacent States
Rocks of Cretaceous age deposited in continental and marginal environments, and now found along the inner edge of the northern Atlantic Coastal Plain, have historically been classified as the Potomac Group and the Potomac, Patuxent, Arundel, Patapsco, Raritan, and Magothy formations. Subdivisions of the Raritan and Magothy formations have also been recognized. Lithologic characteristics and spatial relationships of the units indicate that only the Potomac Formation and the Magothy Formation can be differentiated in northern Delaware. The complex nonmarine deposits originated on an aggrading coastal plain. Their projections into the deeper subsurface on- and offshore will be important in future studies. No changes in terminology are recommended, but careful use of stratigraphic nomenclature is urged in order to avoid confusion, especially in hydrologic applications.
RI36 History of Oil and Gas Exploration in the Mid-Atlantic Region and Delaware's Involvement in the Federal OCS Leasing Program
There has been sporadic exploration for oil and gas in the Mid-Atlantic region for over 50 years. Non-commercial deposits of oil and gas have recently been discovered in the sedimentary rock section of the Outer Continental Shelf (OCS) 80 miles off the New Jersey-Delaware coast. The oil and gas occurs within entrapment structures in ancient rocks deposited and buried in a deep basin called the Baltimore Canyon trough. This trough forms part of the Coastal Plain and continental shelf geologic provinces on the Atlantic Coast.
This report documents the development of a methodology for mapping ground-water recharge areas in Delaware's Coastal Plain. It is anticipated that the methodology presented herein will evolve as it is applied to other areas in the State and as computerized geographic information systems become more widely available. This report deals with methodology; the recharge area maps generated in the course of the research are available for review at the DGS.
This geologic map shows: (1) distribution of geologic units found at the land surface; (2) updip limit (generally the northern extent) of Miocene and Pliocene geologic units found in the subsurface; and (3) locations of major subsurface faults that affected deposition of the Miocene and Pliocene geologic units. The geologic units shown are defined on their dominant lithologies (i.e., sand, silt, clay) and other characteristics such as presence or absence of shells or other fossils and range of colors.
OFR26 Salinity Distribution and Ground-Water Circulation beneath the Coastal Plain of Delaware and the Adjacent Continental Shelf
The possibility of salt-water encroachment into the aquifers of the Coastal Plain of Delaware from saline-water bodies (Chesapeake and Delaware Canal, Delaware Bay, Atlantic Ocean) has received considerable attention (e.g., Sundstrom et al., 1967, 1971, 1976; Woodruff, 1969). These authors have shown that, so far, little encroachment has taken place. It is also known that a large body of highly saline water occurs at depth beneath the Coastal Plain (Upson, 1966; Back, 1966; Brown and Reid, 1976) and the adjacent continental shelf, but no reports have been published about its origin and shape, and the salinity distribution and flow pattern within it. Yet, this saline-water body has a bearing on the development of fresh-water resources throughout Delaware, the feasibility of constructing injection wells for the disposal of liquid wastes, and radioactive waste disposal in the crystalline rocks beneath the Coastal Plain sediments, and upon the occurrence or migration of hydrocarbons (Bredehoeft and Maini, 1981). It is, therefore, important to study this body of saline water.
This map shows the saturated thickness of the Columbia Formation. The Columbia Formation covers most of the Coastal Plain of Delaware. Because it consists primarily of coarse sand, it is important to the hydrology of the area. It is an important groundwater reservoir and in most places water must pass through it to reach deeper units. The water budget of the Columbia Formation also influences runoff and baseflow components of streamflow. The saturated thickness was determined through interpretation of data in publications and files of the Delaware Geological Survey, United States Geological Survey, and the Water Resources Center of the University of Delaware. The thicknesses shown on the map represent the best judgment of the authors based on available data. Detailed investigations of specific sites will require additional data.
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.