Reports
Paleoecology of the Choptank Formation (Miocene) of Maryland and Virginia
1970, Gernant, R.E.
Report of Investigations 12
Abstract
A rarely used interdisciplinary approach is employed to reconstruct the depositional history and ecology of the Choptank Formation and its fossil organisms. Analysis of microfaunal and macrofaunal assemblages is supplemented by study of biogenic and inorganic sedimentary structures, sedimentary textures, and stratigraphic relationships.
The fossils and sediments of the Middle Miocene Choptank Formation accumulated primarily in inner shelf marine environments. In addition, faunal evidence indicates that this unit originated in the central and southern regions of a cool temperate marine climate.
One of the more important results of this investigation is the hypothesis that the major Choptank shell beds were formed by passing marine swells possibly related to tropical storms. The effect on the substrate was the establishment of a pressure gradient causing flow into the bottom deposits and flow out accompanied by effusion of the sediments. This lifting action and vertical sorting created zones of traction and accumulation providing a mechanism sufficient to explain the significant characteristics of the Choptank shell beds. In fact, this process of shell accumulation may be much more important in modern and ancient seas than indicated by the little attention given to it.
The lowest unit of the formation, the Calvert Beach Member, was deposited in the coldest water of any unit in the Choptank. Its environments were slightly inimical to many types of bottom communities as evidenced by the bivalve dominance of the Lucinacea. The Drumcliff Member represents a shallowing of environments on the inner shelf and possibly slight warming. During the deposition of this unit marine swells passed through the area concentrating shells into great beds.
Maximum shallowing occurred during the St. Leonard Member. Marginal marine environments dominated this unit in outcrop. The best delineated of these was the lower bay environment, which occurred in the area of the modern Calvert Cliffs. The slight warming trend continued into the Boston Cliffs Member but the shallowing trend of the lowest three members was reversed. This member represents a return to open inner shelf environments, where the bottom was again swept by marine swells concentrating great volumes of shells.
Sediments of the Conoy Member were deposited in distinctly deeper water but still probably shallower than 50 meters. The warming trend continued, placing deposition in the southern end of the cool temperature region. The St. Marys Formation generally continues the deepening trend started in the Boston Cliffs Member.
Previously, five subdivisions of the Choptank Fm. were numbered and given biostratigraphic designations. However, they are here considered rock-stratigraphic units and given the following names: Calvert Beach Member, Drumcliff Member, St. Leonard Member, Boston Cliffs Member, and Conoy Member. The “unconformity” with the underlying Calvert Formation, as described by Shattuck, (1904), may exist but is visually much less impressive than he suggested. Although he considered the Choptank conformable with the overlying St. Marys, the present evidence indicates that the contact between the two formations is an erosional-depositional unconformity. To supplement the unrepresentative type section, a principal reference section is designated in the Calvert Cliffs, 0.9 mile southeast of Western Shores, Maryland and 1.0 mile northwest of Calvert Beach, Maryland.
