THE STRATIGRAPHY AND STRUCTURE OF THE BURIED
LATE TRIASSIC TAYLORSVILLE BASIN, VIRGINIA AND
MARYLAND: RESULTS OF RECENT SUBSURFACE
LeTOURNEAU, Peter M., OLSEN, Paul E., and KENT,
Dennis V. Lamont-Doherty Earth Observatory of
Columbia University Palisades, NY 10964-8000
The Triassic-Jurassic rift basins of the eastern North American form a
broad zone of linked or isolated half-graben that stretch from Georgia
to the Canadian Maritimes including: onshore, exposed basins;
onshore basins buried beneath coastal plain deposits; and offshore
basins. The exposed basins of the Newark Supergroup (sensu stricto)
are well-known and studies of these have contributed to our
understanding of the tectonics, structure, stratigraphy, and igneous
activity of continental rifts, as well as the depositional environments,
paleogeography and paleontology of the Late Triassic and Early
Jurassic. However, the onshore and offshore buried basins have only
been recognized by geophysical surveys and limited drilling (water
well or geotechnical) since the mid-1970's, and until recently few rock
samples had been obtained from the buried basins.
The buried Taylorsville Basin (TVB) located beneath coastal
plain deposits in Virginia and Maryland was the target of petroleum
exploration from about 1985 to 1992 (Benson, 1992). Six 5,500 ft.
(~1,700 m) stratigraphic test wells produced continuous cores totaling
over 21,000 ft. (~6,100 m) and three deep, 8,000 - 10,000 ft. (~2,100 -
3,000 m), test wells produced chips (mud log) and limited short cores
and sidewall cores. In addition, over 80 mi. (~123 km.) of seismic
reflection profiles were obtained across the NE-SW oriented basin.
This ongoing study has evaluated about 12,000 ft. (3,750 m) of core
from three wells (Payne, Butler, Bowie-Fogg), 7,500 ft. (~2,340 m) of
cuttings from one deep well (Wilkins) that encountered basement, and
available seismic profiles. In addition, paleomagnetic analysis of over
700 samples from the Payne, Butler and Bowie-Fogg wells was
performed to evaluate the paleomagnetic polarity stratigraphy.
The buried TVB is believed to be the subsurface extension of
the exposed Taylorsville Basin centered near Ashland, VA (Benson,
1992; Weems, 1980). While the exposed TVB is one of the smallest
of the Newark basins (circa 19 by 11 km ;140 sq. km), the TVB ranks
as one of the largest at 150 by 50 km or about 4,500 sq. km (Benson,
1992). This relationship is important because the previously studied
exposed portion of the TVB may not be representative of the basin as
a whole, particularly because the exposed portion is in a structurally
complex area near the southern terminus of the basin.
The TVB contains approximately 15,000 ft (~4,690 m) of
fluvial and lacustrine strata, including conglomerate, sandstone,
siltstone and shale, ranging from red-brown to black in color. The
overall stratigraphy of the basin consists of a 1,000 - 2,000 ft. (~312 -
625 m) lower fluvial-lacustrine sequence (I), separated by an
unconformity from a thicker, 13,000 - 14,000 ft. (~4060 - 4375 m)
upper fluvial-lacustrine sequence (II) .
Sequence I forms the base of the rift fill and occupies smaller,
faulted basins separated from the overlying sequence II by a basin-
wide unconformity. The strata of I consist of gray to black lacustrine
siltstone and shale and red-brown fluvial sandstone and siltstone.
Sequence II consists of two assemblages: IIA) gray fluvial-deltaic-
lacustrine sandstone, siltstone and shale; IIB) red-brown fluvial
channel and floodplain sandstone and siltstone. IIA is characterized
by laminated, fossiliferous black shale and gray sandstone and
siltstone. Finer portions of IIB include pedogenic carbonates, root
casts, invertebrate burrows, desiccation cracks, and soil fabrics, while
coarser portions have climbing ripple cross-lamination, basal scour
surfaces, intraformational conglomerate, planar to tangential cross
strata and planar, horizontal lamination. The two assemblages are
indicative of relatively "wet" (IIA) and "dry" (IIB) depositional
sequences. The boreholes also encountered diabase dikes and the
seismic profiles show subvertical dike swarms, although these appear
to occupy a small portion of the basin fill.
The seismic reflection profiles and the borehole dipmeter data
indicate that TVB strata dip both toward and away from the master,
western border fault. The axes of these broad, post-depositional folds
are oriented along the long axis of the basin and may be a result of
NW-SE oriented post-rift compression. One seismic profile that
spans the width of the basin shows a secondary set of faults
paralleling the east-dipping western border fault that likely define a
footwall rider block. The base of the rift basin consists of smaller
half-graben presumably the result of distributed extension during
early rifting. Later rifting was accommodated by the master western
border fault system. A basin-wide unconformity formed during the
transition from early to later phases of rifting. During the later phase
of rifting sequence (II) strata filled the broad half-graben.
This new data from the buried basin suggests that the exposed
TVB represents a portion of the lower basin fill (sequence I).
Additionally, based on lithostratigraphy and paleomagnetic reversal
stratigraphy (Olsen et al., 1996) sequence IIA correlates to the
Lockatong Formation and IIB correlates to the Passaic Formation of
the upper Triassic portion of the Newark Basin.
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