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.

Geologic Cross-Sections, Cenozoic Sediments of the Delmarva Peninsula and Adjacent Area

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 purpose of this report is to characterize Delaware Atlantic Coast beach sand on the basis of sand texture data in order to identify geologic material suitable for beach nourishment.

This map was constructed primarily to indicate the possibilities for artificial recharge into both the surficial sediments of Quaternary age (exclusive of soils) and the older, immediately underlying sediments. However it can also be used to determine where natural recharge might be entering the ground most readily in those areas relatively free from impermeable cover. The surficial sediments include micaceous sands and gravels in the vicinity of the Fall Line derived from underlying crystalline rocks, Holocene marsh deposits, Delaware River sediments, and the Columbia Formation of Pleistocene age. The Columbia Formation is composed of poorly sorted sands with some gravels, silts and occasional clays. The unit is one of the most important ground-water reservoirs in New Castle County.

Columbia sediments in the Middletown-Odessa area are composed of boulders, gravels, sands, silts and clays. These sediments are exposed in four gravel pits where their structures and textures were studied. Subsurface geology was interpreted on the basis of the well-log data from 40 holes drilled in the area of study. Columbia sediments were laid upon a surface made up of the greensands of the Rancocas Formation (Paleocene – Eocene age). The contact between the Rancocas and Columbia Formations is an erosional unconformity.

The Columbia deposits of Delaware form a sheet of sand with a maximum thickness of approximately 150 feet which covers most of the Coastal Plain portion of the State. The dispersal pattern, deduced from foreset dip directions of cross-bedding, indicates that the sediment entered the study area from the northeast, i.e., from the direction of the valley of the Delaware River between Wilmington and Trenton, and spread south and southeast over Delaware.

This investigation was undertaken to locate deposits of rock, sand, gravel, fill and borrow in northern New Castle County which may be potential sources of material for highway construction, and to prepare maps and descriptions of the surficial earth materials relative to their geologic and engineering properties.

The non-marine Cretaceous sediments of northern Delaware older than the Magothy formation cannot be divided accurately into formations or mappable geologic units because their lithologic characteristics are very similar. However, two heavy mineral zones can be distinguished in these deposits: a lower staurolite-kyanite-tourmaline-zircon zone, and an upper tourmaline-zircon-rutile zone with abundant alterites. They have been named the Patuxent zone and the Patapsco-Raritan zone respectively. The Magothy formation is characterized by abundant staurolite and also contains significant amounts of tourmaline. The marine Upper Cretaceous deposits have a greater variety of heavy minerals than the underlying non-marine sediments. They contain abundant epidote; chloritoid, first appearing at the base of the Merchantville formation, is persistently present. Garnet is found in the Merchantville and the Mount Laurel-Navesink formations. The heavy mineral composition of the Cretaceous sediments is shown in table IV.

The complex geologic framework of the Fall Zone in Delaware is primarily caused by diverse structural features present in the crystalline basement rocks that have exerted a considerable influence on the distribution of the overlying sediments of the Coastal Plain.

The feasibility of using geophysical techniques in determining the amount of overburden and the nature of the subsurface along a proposed highway was tested in the Piedmont area of Delaware. The area is underlain by crystalline rocks capped by varying amounts of unconsolidated material or regolith. Seismic refraction and surface resistivity methods were used at selected stations and the interpretations were later compared to results from test holes and to the material exposed in road cuts. In general, interpretation of the seismic refraction results compared quite well with test borings and with field observations made after construction was started. Resistivity data were inconclusive in themselves but provided some additional control points when correlated with seismic refraction data. With proper control, it is concluded that such techniques could be useful in the Piedmont of Delaware for highway planning.

Subsurface geology of the Smyrna-Clayton area, Delaware

The quantitative lithofacies analysis of the Potomac Formation in a small area west of Delaware City revealed that the deposition of these sediments was continuous throughout the time of their formation. The uppermost part of the Potomac sequence appears to have been removed, probably by erosion, prior to the deposition of the younger Upper Cretaceous marine sediments. The sand bodies contained in Potomac deposits have a shoestring channel form and were most probably deposited by unidirectional currents. The direction of the flows, however, cannot be determined on the basis of the available subsurface data.

Pleistocene channels along the margins of the Atlantic Coastal Plain are developed in crystalline and Triassic sediments (Bonini and Hickok, 1958), or into the Cretaceous and Tertiary coastal plain sediments (Widmer, 1965). Deposits in these channels consist of sand and gravel with amounts of silt and clay. For example, the Bear area channel is 50 to 70 feet deep and contains up to 30 feet of sand and gravel overlain by sandy clay. Because they are usually more permeable than the older deposits into which the channels are developed, Pleistocene deposits are important in ground water studies for several reasons: (1) where they are thick enough they may be used as aquifers, as in the case of the Bear channel, and (2) these beds can effectively increase the recharge into the underlying aquifers by absorbing precipitation and transmitting the water to them.

A series of cores was obtained from a boring in the sediments of the Delaware River near the Delaware Memorial Bridge. The mineralogy, texture and palynology of these samples have been studied. The sedimentary and palynological records suggest that the Delaware River, while swollen with Wisconsin meltwaters, deepened its channel and that subsequent flooding of the mouth of the stream by rising sea waters initiated the deposition of estuarine silts in post-Wisconsin time.