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A Fulcrum Approach for Assessing Sediment Source-to-Sink Mass Balance Using Fluvial Paleohydrologic Data Extractable from Cores: An Assessment of Potential and Limitations Using the Cenomanian Bahariya Formation, Egypt

Abstract

Source-to-sink interpretations of genetic equivalency between fluvial feeder systems and basin accumulations infer that the sediment supplied by the rivers is in mass balance with the sediment volume ultimately deposited. This relationship has value for modeling basin fill volumes over time steps and for related projecting of reservoir partitioning and potential. Executing an estimate of this mass-balance assumption is challenging in modern systems but even more difficult in deep-time stratigraphic settings where critical variables from the source and sink system are not preserved or preserved with large uncertainties. Available data sets from ancient systems are likewise often limited to a few localized boreholes. This paper offers a method for estimating mass flux from the source area to the basin sink by calculation of paleohydrologic variables from a cross-section of channel-belts using data extractable from outcrop or core. We use the Cenomanian channels of the Bahariya Formation, Egypt to test this method. Total sediment mass passing through a cross section of channels over a period of time should match with both the total sediment delivered to the channels from the source area and the total volume delivered through these channels to basin. This cross section would constitute a fulcrum across which source and sink sediment mass should balance. Flow transport equations are used to estimate bankfull discharge and sediment concentrations using methods illustrated in Van Rijn (1984) and Parker (2004). These concentrations are projected over longer durations to estimate total channel mass-through flux over basin time spans. These estimates can be tested against known basin accumulation volume and/or estimates of basin denudation. The required calculations can be made from paleohydrologic information routinely extracted from core and/or outcrop data sets by reconstruction of channel-belt architecture and sediment sampling. Calculations of mass flux from Bahariya channels that feed the equivalent fluvial/marine basin show these channels were capable of delivering twice the sediment actually preserved. This method is nascent in its development and limited by large uncertainties in averages and deviations in key parameters used in calculations, particularly in the relationship between bankfull and annual-average discharge. At present the method is accurate only within an order of magnitude.