Reports
Petrology and origin of Potomac and Magothy (Cretaceous) sediments, Middle Atlantic Coastal Plain
1969, Glaser, J.D.
Report of Investigations 11
Abstract
The Potomac Group and Magothy Formation (Early-Late Cretaceous), the basal sediments of the Altantic Coastal Plain in Delaware, Maryland, and Virginia, were studied to determine the means of sediment dispersal, provenance, and character of the depositional environments. Thickness variations, cross-bedding, pebble counts, textural analyses, and petrography with special regard to heavy minerals were employed to achieve these objectives.
The Early Cretaceous Potomac Group appears in outcrop as a variable sequence of gravel, sand, silt, and clay, divisible by paleontologic and petrographic criteria into four formations - Patuxent, Arundel, Patapsco, and Raritan - in ascending order. The Potomac Group is unconformably overlain in Maryland and Delaware by sand, silt, clay, and subordinate gravel of the Magothy Formation.
The Cretaceous sediments form a southeasterly-expanding wedge of clastics which are the main fill of the Chesapeake-Delaware Embayment - a broad, shallow basin in the crystalline basement. The lower unit (Patuxent Formation) of the Potomac Group is predominantly sand and gravel, coarsest along the basin margin to the west and fining to the east. Deposition took place in the channels of northeasterly to southeasterly flowing, probably braided rivers. Sand mineralogy, pebble lithologies, and cross-bedding direction define two dispersal systems: one emerging from a southern, mainly granitic source area transporting low staurolite-kyanite-tourmaline, high-feldspar sands and petromict gravels eastward to northeastward into the basin, and the second draining a northern source area of mostly high-grade schists and bringing high staurolite-kyanite-tourmaline, low-feldspar sands and quartzose gravels eastward to southeastward into the basin. Both source areas were located in the adjacent Piedmont region.
The overlying Arundel Formation, restricted to Maryland, consists of thick lenses of dark, massive lignitic clay accumulated as flood basin deposits. The upper Potomac Group (Patapsco and Raritan Formations), distributed mainly across the northern flank of the basin, consists primarily of variegated silt-clay and mostly fine to medium sands deposited on the floodplains of sluggish, southeasterly-flowing rivers. The prevalence of stable minerals (zircon, tourmaline, and rutile) as mixed rounded and angular grains in the sands points to derivation from both a highly weathered Piedmont terrain and from Appalachian sandstones to the west.
The Magothy Formation is a thin clastic sheet, widespread in the northern part of the basin, which grades eastward from fluvial sands and gravels to estuarine-marginal deltaic sands and clays. The fluvial facies was deposited by easterly-flowing streams, and was derived from a high-grade schist terrain in the adjacent Piedmont.
Both the Potomac and Magothy grade basinward into marine sediments which are time transgressive eastward.
The Cretaceous sediments of the Chesapeake-Delaware Embayment constitute, in terms of basin geometry, sediment fill, dispersal pattern, and tectonic setting, a depositional model which may be applicable to other areas of the Atlantic Coastal Plain.
