ABSTRACT

Modern sediments of the James River estuary have been studied to characterize the lithofacies and to relate the facies pattern to estuary morphology and the energy regime. The estuary was formed by Holocene drowning of a river valley incised in Cenozoic coastal plain deposits. Morphologic analysis of sinuosity, width-depth ratios, and convergence characteristics reveals three compartments: 1) bay-mouth, 2) estuary funnel and 3) meander zone. Each compartment exhibits a characteristic lithofacies reflecting different proportions of wave, tidal and fluvial energy. These lithofacies form a longitudinal tripartite pattern, i.e., sand-mud--sand, with coarse-grained sediment at the energetic ends of the system. The seaward facies boundary is transitional as a result of mixing fluvial and ma ine sediment. In contrast, the landward boundary is abrupt as a result of a rapid seaward decrease in the river flood bedload which is partly attenuated by the tide. The tripartite facies develop in a transgressive wedge filling the path of the pre-Holocene fluvial drainage. A model is proposed for recognizing the sequence of ancient transgressive estuarine facies. The sequence fines upward except at the mouth and reflects the seaward evolution of environments from fluvial to fluvial estuarine, estuarine and marine estuarine.