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A study of the regional setting of NABIR's experimental aquifer on Virginia's
Eastern Shore Peninsula of Virginia sheds new light on the relationship between
mesoscale stratigraphic units (facies assemblages or "depositional systems") and the
bounding surfaces from which they have been formed Swift et al. 2003; Parsons et al.,,
2003). These units were deposited by the coast-parallel progradation of the Pleistocene
barrier spit (Eastern Shore peninsula) during successive Quaternary highstands. As a
consequence, each segment of the spit complex has been produced by the coast-parallel
translation of a relatively small growth area at the spit tip. The distal end of the present
(Holocene) barrier system appears to closely resemble the Pleistocene highstand growth
area, and this environment consequently serves as a partial analog for interpreting the
depositional environment of the fossil highstand deposits.
The many facies patterns present in the spit complex can be reduced to four kinds
of facies assemblages (depositional systems), adopting a process-based model (facies
template). In this scheme, definitions of both "facies" and "facies assemblage" are more
limited than is the case in most textbook definitions in that the facies of a given
assemblage are systematically related to each other by grain size and stratal pattern, and
also related to a bounding surface ("source diastem") which is the immediate source of
sediment for the facies assemblage. Vertical transitions between individual facies are
easily identified in outcrop, but the horizontal gradients of facies change are too gentle to
be observed over the short dimensions of the borrow pits, and do not have sufficient
acoustic contrast to appear on ground-penetrating radar records. However, the facies
assemblages, both in the borrow pits and on radar records, stand out by virtue of their
sharply defined bounding surfaces (source diastems). The facies assemblage either buries
its source diastems or is capped by it.
The assemblages in the spit complex are, in ascending stratigraphic order: (1)
several tidal shoal deposits, each underlain by a channel-base diastem, (2) Two shoreface
systems separated by a channel-base diastem, (3) an upper shoreface assemblage capped
by a surf diastem, and (4) a beach-strandplain assemblage, underlain by the same surf
diastem. All of these systems prograded southward as the nose of the spit prograded, and
while they did so, zones of erosion cut the bounding surfaces that separate them. Two
important bounding surfaces are "conjugate" surfaces, which nourished both the facies
assemblage above and the assemblage beneath. As each surface advanced, erosion at
their leading edge spilled sediment forward, down the nose of the spit, while sediment
was also swept backward, aiding in the burial of the surface. A conjugate surface creates
a "sandwich" structure, in which two facies assemblages are separated by the generating
surface. Proximal facies are back-to-back across these sediment-spreading boundaries.
Episodic progradation of the spit tip by development of successive, recurved,
beach ridges has overprinted the horizontal first-order reflectors (separating facies
assemblages) with gently dipping second-order reflectors that separate successive growth
increments of the spit. Although less clearly defined, these growth increments are
essentially high-frequency autocyclic sequences, and constitute the next higher scale of
spatial organization above the depositional systems scale. The manner in which facies
have been organized into depositional systems in the late Pleistocene highstand deposits
of the Eastern Shore is specific to this estuary-mouth setting. These assemblages are,
