The Functioning of Sediment Routing Systems Using a Mass Balance Approach: Example from the Eocene of the Southern Pyrenees

Abstract

Sediment routing systems link source regions undergoing erosion with depositional sinks and involve a volumetric or mass budget. Understanding how these source-to-sink systems function is key to stratigraphic prediction, but estimation of their surface sediment discharges and depositional fluxes on geological time scales is a challenging problem. We recognize a paleosediment routing system from the geological record in the mid-late Eocene Escanilla Formation and time equivalents of the tectonically active wedge-top region of the southern Pyrenees. By mapping the >200-km-long fairway of the Escanilla sediment routing system, we obtain the sediment budget and grain-size fractionation of three time intervals, each of 2.5–2.6-m.yr. duration, over the time period 41.6–33.9 Ma. Four thousand cubic kilometers of sediment, sourced principally from two feeder systems in the high Pyrenees, was deposited in a period of 7.7 m.yr. The positions of moving boundaries, characterized by rapid reduction in the percentage of gravel, sand, and fine grain-size fractions (gravel cline, gravel front, sand front), are sensitive to the dynamics of the sediment routing system. In order to understand these dynamics, total volumes of sediment sequestered as stratigraphy, together with the component volumes of gravel, sand, and fines, are transformed into a mass balance framework. Over time, there was a progressive westward progradation of coarse-grained facies driven by increasing sediment supply from the rapidly eroding Pyrenean orogen. Changes in the rate of downsystem fining of grain size, percentages of grain-size fractions in preserved stratigraphy, position of moving boundaries, and evolution of gross-depositional environments are related to variations in the volume of sediment supplied, the grain-size mix of the supply, and the spatial distribution of tectonic subsidence generating accommodation.