Modern Deltas (Holocene)
The recognition by earlier researchers of progradational and retrogradational depositional packages in the stratigraphy of the Holocene Mississippi River delta plain resulted in the development of the “delta cycle model”, a conceptual process-response model to explain the genesis and distribution of stacked, laterallyoffset depocenters, and consequently the formation of Mississippi River deltaic- plain headlands. Historically, delta lobes of the Mississippi River delta system have been treated as river-dominated depocenters and sand- rich lithosomes in the headlands an outcome of progradation, and deposition in distributary networks as levee, distributary-mouth-bar, and delta-front sands. However, analysis of headlands along the south-central delta plain reveals, at least locally, sandy coastal lithosomes containing regressive beach ridges that built seaward on the updrift sides of prograding distributaries. Marine reworking of updrift abandoned distributaries and earlier-formed headlands provided a readily available and nearby source of the beach-ridge sand that was captured by youngerprogradational distibutaries. Consequently, the headlands are not simple regressive shorelines solely recording one phase or multiple phases of river-dominated progradation, but rather a facies mosaic containing many environments such as wave- dominated regressive beach-ridge plains and barrier complexes created during subsequent headland transgression. Most of the sand along the south-central shorelines exists in buried beach-ridge plains and the modern barrier-island systems. Understanding the distribution of sand-rich lithosomes in the shallow stratigraphy has important implications for both stratigraphic models of deltas and some coastal Louisiana restoration efforts. Sandy sediment is currently being sought for shoreline renourishment projects, and information regarding subsidence patterns is vital to predicting locations of future shoreline erosion and wetland loss. Because sandy lithosomes are less prone to compaction than the dominantly fine-grained facies of regressive fluvial deposition, composite headlands, constructed by fluvial and marine depositional processes, may be less susceptible to compaction following abandonment.
Figures & Tables
Deltas are amongst the most environmentally and economically important coastal sedimentary environments. Studies of deltas lag behind research in both fluvial and deep-water depositional systems, as well as more geomorphologically oriented land studies. This knowledge lag reflects both a reorientation of the petroleum industry in the last two decades toward deep-water systems, as well as the difficulty of working across the shoreline with the traditional tools used for oceanographic or land-based work. However, deltaic studies are experiencing a renewed focus, because of their global importance in environmental and other societal concerns. This volume stems from a special session: Deltas: Old and New, held at the Annual Geological Society of America conference in October 2002, that was convened to highlight these new directions in deltaic research.