Reports
Sedimentary facies of the Aquia Formation in the subsurface of the Maryland Coastal Plain
1974, Hansen, H.J.
Report of Investigations 21
Abstract
The marine Aquia Formation of Paleocene age outcrops in Maryland as an irregular band extending from the Potomac River bluffs in western Charles County to the upper reaches of the Sassafras River in southeastern Cecil County. The Formation thickens and becomes more coarsely-textured toward the northeast, traversing at least two first-order facies. The oblique relationship between the outcrop belt and several lithofacies trends suggests that post-Eocene tilting has imparted to the Formation a structural strike that is demonstrably different from its depositional strike.
In the subsurface the Aquia Formation exhibits a tripartite facies pattern: 1, a thick, coarser-textured sandy facies extending southwesterly in outcrop from Kent County to about the Patuxent River valley where it swings south into the subsurface toward southern St. Mary’s County; 2, a finely textured sand to silt-clay facies, occurring chiefly in Charles and southern Prince George’s County; and 3, a thinner, very muddy facies that appears to underlie much of the Eastern Shore seaward of the Choptank River; it does not outcrop.
Facies 1 is interpreted to be an offshore sand bank complex. In addition to being thicker and more coarsely textured than facies 2, it exhibits consistently greater transmissivity values, higher goethite/goethite + glauconite ratios, more intense iron staining of quartz grains, and thick accumulations of calcareously cemented, sand-packed shell beds.
The fine, muddy, glauconitic sands dominating facies 2 were apparently deposited in a lower energy, inner shelf environment occurring landward of the sand bank complex (facies 1). The predominantly silt-clay character of facies 3 is suggestive of outer shelf sediments occurring seaward of the sand bank complex; sand thickness values based on geophysical log data are consistently less than 25 feet. Texturally, the sands are chiefly very fine to fine-grained and exhibit very little iron-staining.
The Aquia Formation, particularly facies 1, contains appreciable amounts of pelletal goethite. The goethite grains differ from associated glauconite grains by being generally irregular to ellipsoidal-shaped rather than polylobate or accordion-shaped; by being generally agglomeratic, rather than microcrystalline; and by being generally more coarsely textured. Both, however, are believed to be derived from fecal pellets, albeit pellets voided by organisms having distinctly different physiological, ecologic, and/or tropic processes.
