MRCSP Phase I Geologic Characterization Report - Midwest ...

APPENDIX A: LOWER CRETACEOUS WASTE GATE FORMATION
135
Much of the stratigraphy and lateral extent of the Saginaw Formation
and other Pennsylvanian-age rocks in Michigan is poorly
known because the majority of the data available on this interval
consists of limited drill-hole logs and records. However, down-hole,
log-based studies of thickness and distribution trends of coals in the
Pennsylvanian may be feasible using available log data, gamma-ray,
resistivity, neutron-porosity, and density logs (Vugrinovich, 1984).
However, these studies are restricted mostly to the deeper portions
of the central Michigan basin (Figure A16-1). Other feasible sequestration
targets within Pennsylvanian rocks could include organicrich,
carbonaceous shale, a significant but, as yet, unquantified
reservoir for CO 2 sequestration.
17. LOWER CRETACEOUS WASTE GATE FORMATION
The Waste Gate Formation of the Potomac Group (Delaware,
Maryland, New Jersey, and Virginia) is a stratigraphic unit that
includes a correlative sequence of subsurface strata underlying
the eastern Delaware-Maryland-Virginia (“Delmarva”) Peninsula
that are significantly different in age, lithology, and petrophysics
from the Patuxent Formation. In the type area, the Potomac Group
is divided into formations but, in many places, the formations are
less distinct lithologically, and one or more formations, or even the
entire group, is treated as an undivided unit. The Waste Gate beds
were previously considered part of the Lower Cretaceous Potomac
Group (undivided) or the lower part of the Patuxent Formation. A
diagram showing the stratigraphy as proposed by Hansen (1984) is
provided as Figure A17-1.
The Potomac Group is a largely non-marine fluvial-deltaic complex
of interbedded sandstones and mudrocks that generally dips
and thickens eastward toward the Atlantic Ocean. While the Waste
Gate Formation is limited to the subsurface primarily under the Delmarva
Peninsula, the younger units of the Potomac Group crop out
updip, west of the Chesapeake Bay and in the northernmost part of
the Delmarva Peninsula (Figure A17-2). The Potomac Group pinches
out altogether at the Fall Line or Fall Zone, which is the boundary
between the Piedmont Province and the Coastal Plain Province,
and across which rivers from the upland (Piedmont) region drop as
rapids or falls to the Coastal Plain (Figure A17-3).
Hansen (1982, 1984) reported that basal beds (i.e., Waste Gate
Formation) decrease in thickness in an up-dip direction, northwestward
toward the Baltimore-Washington corridor, and that the upper
part of this sequence is marked by an unconformity. This places the
Late Cretaceous Magothy Formation on top of the Potomac Group
(Figure A17-1).
The Waste Gate Formation is fairly limited in extent. Hansen
(1984) indicateed that the Waste Gate occurs beneath the southeastern
part of the Delmarva Peninsula, including the southeastern portion
of Maryland’s eastern shore and the northern part of Virginia’s
eastern shore. The unit probably underlies part of southeastern Delaware,
and possibly may extend at least as far north as the southern
tip of New Jersey.
Hansen has also indicated that, offshore of New Jersey, there
are partially coeval units, including some non-marine, feldspathic
sandstones and shales as well as limestones, calcareous shales, and
limy sandstones. The relationship between these units in offshore
wells and the Waste Gate (as identified in onshore wells) is not fully
Figure A17-1.—Stratigraphy of the Potomac Group beneath the eastern Delmarva Peninsula (from Hansen,
1984).