ABSTRACT

The Paleocene-Eocene Aquia Formation crops out in the coastal plain of Maryland, Delaware, and Virginia. A field and laboratory study of the exposed Aquia north of Fredericksburg, Virginia, was conducted to determine the petrologic features and the environment of deposition of the sediments, as well as the relationships of this formation with the underlying Brightseat unit.

Sieve and hydrometer analyses of 116 samples show that the Aquia is a well-sorted, very fine to medium-grained sand or silty sand. Median diameter increases and sorting improves both vertically upward through the section and northward along strike. Sediments from the type locality of the Brightseat unit are texturally equivalent to parts of the Aquia at the Aquia type locality.

Glauconite content of the sand grades of the Aquia samples ranges from approximately 20 percent to 70 percent. Lateral and vertical changes in glauconite content are frequent. Both authigenic and detrital glauconite grains are present. The heavy mineral suite of the Aquia is a "full" suite with the exception of hornblende. Clay minerals include illite, glauconite, montmorillonite, and kaolinate. Mineral composition of the Brightseat unit is essentially the same as that of the Aquia, and falls within the range of extremes of Aquia composition.

The Aquia was probably deposited in inner neritic marine waters where generally slow rates of deposition varied from time to time. A change in deposition rate could account for the previously proposed unconformable break between the Brightseat and the Aquia without any withdrawal of the sea. Irregularities in the coastline and changing patterns of current activity resulted in deposition of sediments with varying texture and mineralogy.

Source rocks for the Aquia material included crystalline rocks, sedimentary rocks, and possibly, unconsolidated sediments. The source area was probably to the west in the Appalachian Piedmont, Blue Ridge, and Valley and Ridge provinces. Unconsolidated Cretaceous coastal plain sediments were a minor source of Aquia material. Relief of the source area was low.

Areal variations of lithofacies within the Aquia show that the center of clastic sedimentation shifted somewhat during Paleocene and Early Eocene times. Continuous shoaling of the sea during Aquia deposition culminated in deposition of the pink Marlboro Clay Member of the overlying Nanjemoy Formation in brackish water.