Burial of terrestrial organic carbon in marine sediments can draw down atmospheric CO2 levels on Earth over geologic timescales (≥105 yr). The largest sinks of organic carbon burial in present-day oceans lie in deltas, which are composed of three-dimensional sigmoidal sedimentary packages called clinothems, dipping from land to sea. Analysis of modern delta clinothems, however, provides only a snapshot of the temporal and spatial characteristics of these complex systems, making long-term organic carbon burial efficiency difficult to constrain. Here we determine the stratigraphy of an exhumed delta clinothem preserved in Upper Cretaceous (~75 million years ago) deposits in the Magallanes Basin, Chile, using field measurements and aerial photos, which was then combined with measurement of total organic carbon to create a comprehensive organic carbon budget. We show that the clinothem buried 93 ± 19 Mt terrestrial-rich organic carbon over a duration of 0.1–0.9 Myr. When normalized to the clinothem surface area, this represents an annual burial of 2.3–15.7 t km−2 yr−1 organic carbon, which is on the same order of magnitude as modern-day burial rates in clinothems such as the Amazon delta. This study demonstrates that deltas have been and will probably be substantial terrestrial organic carbon sinks over geologic timescales, a long-standing idea that had yet to be quantified.