Abstract Confined fine-grained depoenters (mudbelts) on continental shelves play an important role as common and major fluviogenic submarine depocenters along the source-to-sink pathway and in global sedimentary and carbon cycles. This study provides a complete high-resolution isopach-based budget analysis using closely-spaced, high-resolution seismic-reflection data of an exemplary mid-shelf mudbelt system located on the open and narrow continental shelf of NW Iberia. The budget analysis reveals that 3.957–4.227km3 of sediments [i.e., 4073 to 4351Mt (dry)] are stored in this depocenter. In conjunction with river-discharge estimates, we calculate that, over the past 5300yr, approximately 34% to 36% of total fluvial sediments supplied to the ocean remain in the shelfal mud depocenter and the balance bypasses the shelf. Total accumulation values for TOC and CaCO3 amount to 40.31 to 43.46t and 174.73 to 186.68Mt, respectively. High-resolution isopach analysis shows high regional morphodynamic variability of the main sediment transit routes, an aspect easily overlooked by core-based or low-resolution profiling studies. The budget analysis reveals persistent low accumulation over the past 5300yr and thus clarifies that a uniformitarian view of applying modern accumulation rates to the late Holocene can significantly underestimate effective sediment off-shelf transport.