| Abstract: | This article hypothesizes that production and preservation of source-rock type facies in deltaic systems, both landward and seaward of a coastline, is mutually exclusive, time-successive and related to the dynamics of relative sea level. Sedimentologic research in the Holocene Mississippi and Rhône delta complexes suggests that rheotrophic peats and associated organic-rich beds preferably accumulate in the accommodation space created behind landward stepping shorelines in a transgressive systems tract (TST). Such a setting also allows for a sufficient supply of recharging fresh nutrient-rich groundwater into the peat forming mires Kosters, E.C., Suter, J.R., 1993. Facies relationships and systems tracts in the late Holocene Mississippi Delta Plain. Journal of Sedimentary Petrology 63 (4) 727–733.]. Independently carried-out quantitative paleoecological studies in the same delta systems (and in addition in the Orinoco and Po deltas) suggest that in a progradational setting (highstand systems tract/HST), seasonally discharged nutrient- and sediment-laden river waters on the shelf may give rise to anoxia or dysoxia. Subsequent overfertilization of the shelf leads to accumulation of organic-rich mud belts on the shelf VanderZwaan, G.J., Jorissen, F.J., 1991. Biofacial patterns in river-induced anoxia. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society of London, Special Publication no. 58, pp. 65–82.]. Thus, production and preservation of source-rock type facies landward of a shoreline (as peats and related sediments) is preferred in a TST, when accommodation space and nutrient supply are landward of the shoreline. Vice versa, production and preservation of such facies seaward of a shoreline (as organic-rich shelf muds) occurs preferentially in a highstand systems tract. In that situation, accommodation space is on the shelf, where river-fed nutrients are supplied as well. This hypothesis suggests further potential for application of sequence stratigraphic concepts for improved understanding of the occurrence of source-rock type facies. |
