Stratigraphy presupposes a hierarchy of scales of spatial organization supplemented at the small-scale end by sedimentological concepts (beds, bed sets and bed cosets) and, at larger spatial scales, by sequence-stratigraphic concepts (systems tracts, parasequences, sequences). Between these two endmembers are intermediate-scale bodies described as 'lithofacies', or simply 'facies'. A more restricted concept, granulometric facies, can be described in terms of horizontal grain-size gradients ('facies change') and cyclic vertical grain-size gradients ('stratification'). Assemblages of facies so defined (also called depositional systems) are not random, but occur in a limited suite of patterns. Such assemblages may be linked to two classes of bounding surfaces, a source diastem (the immediate source of the sediment) and a surface of closure (if preserved), between which is sandwiched a transgressive or regressive, basinward-fining facies succession. Systems-bounding surfaces are notably more continuous than internal (gradational) facies boundaries. By thus restricting the definition of a facies assemblage (depositional system), it is possible to describe the Quaternary of the Virginia coast with as few as 12 systems. Depositional systems in the Quaternary of the Virginia coast are allometric, in that any system can be derived from any other by plastic expansion of one or more facies relative to another, or by simple symmetry operations. Self-similarity prevails across this intermediate scale of stratigraphic organization. Facies assemblages (depositional systems) consist of event beds, which themselves have erosional basal boundaries and internal successions of microfacies. At larger spatial scales, depositional systems are repeated, either autocyclic repetitions forced by processes within the basin of deposition or allocyclic repetitions, as 'parasequences' and high-frequency sequences. In the Virginia Quaternary, systems architecture is compatible with sequence architecture and nests conformably within its framework, but analysis of systems architecture reveals rules beyond those governing sequence architecture.
The Upper Floridan aquifer (UFA) beneath the southeastern US Atlantic Coastal Plain and inner continental shelf is a major source of groundwater for coastal Georgia (GA) and South Carolina (SC), where it supplies over 50% of water requirements. Since pumping from the UFA first began at Savannah, GA in the late 1800s, a large ($50 km radius) cone of depression has developed on the aquifer's potentiometric surface. Where a regional Miocene-age aquitard overlying the UFA is thin or absent within this cone, the opportunity exists for seawater to leak downward through the seabed and into the aquifer. Recently completed marine reflection seismic surveys on the GeorgiaSouth Carolina coast identified 11 areas of concern (AOCs) where the UFA is susceptible to seawater intrusion. Results from this project, and from an AOC test-drilling program recently completed, are contributing to a large database of information and models being developed for the UFA by several agencies and academic institutions. The results will provide input necessary for managing groundwater resources and mitigating potential seawater intrusion on the Georgia-South Carolina coast.
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