Quaternary Facies Assemblages and Their Bounding Surfaces, Chesapeake Bay Mouth: An Approach to Mesoscale Stratigraphic Analysis

ABSTRACT Shallow marine facies assemblages in the late Quaternary section of Virginia's Eastern Shore Peninsula shed new light on the relationship between mesoscale stratigraphic units (facies assemblages or "depositional systems") and the bounding surfaces from which they have been formed. These units were deposited by the coast-parallel progradation of the Pleistocene barrier spit (Eastern Shore Peninsula) during successive 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 diastem or is capped by it. The assemblages in the spit complex are, in ascending stratigraphic order: (1) several tidal shoal assemblages, each underlain by a channel-base diastem, (2) Two shoreface assemblages separated by an intervening marginal shoal assemblage and its underlying channel-base diastem, and (3) a beach-strandplain assemblage, underlain by a 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, that 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, however, local expressions of a facies "template" that can be generalized to many other settings.