THE STRATIGRAPHY OF THE MAGOTHY FORMATION IN THE NORTHERN DELMARVA PENINSULA: NEW DATA FROM THE BOHEMIA RIVER COREHOLE

A continuous wireline corehole was drilled in 2021 at Bohemia River State Park (northeast Maryland Coastal Plain) to study the Magothy Formation, an estuarine Coniaician deposit that includes regionally important aquifer sands. The Magothy Formation is 32 ft thick at this site. The upper part (120-130 ft) is predominantly silty, lignitic clay. The middle part (130-139 ft) is silty clay with plant fragments and becomes sandier downward. The lower part (139-152 ft) is laminated very fine sand with abundant plant debris that transitions downward from muddy to clean sand. The Magothy deposits are overlain by a marine-influenced unassigned interval (107-120 ft) which is overlain by glauconite-rich beds of the Santonian lower Merchantville Formation. This unassigned interval includes dark clay, pebbly sandy mud with siderite nodules, and burrowed muddy sands with scattered granules and pebbles.

Preliminary palynological analyses have so far been made for five samples in and adjacent to the Magothy Formation in the Bohemia River cores. Three samples from the Magothy Formation (125, 130, and 147 ft) contain normapolles-type angiosperm pollen such as Plicapollis sp. F and Santalacites minor that, together with several forms of Momipites, suggest a position in the lower Magothy in the Complexiopollis exigua-Santalacites minor (Ce-Sm) Zone. The rich palynomorph assemblage at 125 ft contains a high relative abundance of reticulate tricolporate types and psilate tricolpate types and a low relative abundance (>1%) of normapolle-type pollen. Pollen at 115 ft in the unassigned interval suggests a position within the younger ?Pseudoplicapollis cuneata-Semioculopollis verrucosa (Pc-Sv) Zone or slightly higher based on the presence of Semioculopollis and Momipites sp. K; the presence of foraminiferal linings indicates marine influence. The distinct change in pollen assemblages between 115 and 125 ft suggests that the marked change at 120 ft from estuarine Magothy deposits to the overlying marine deposits may represent a disconformity. The pollen data from the Bohemia River cores will be compared to additional samples to be analyzed from this core, other coreholes, and outcrops from the Delmarva Peninsula, including outcrop samples recently taken from bluffs at nearby Grove Point, Maryland.