Erosion and sediment transfer by debris flows on Miocene intrashelf clinoforms, offshore New Jersey, USA

We integrate legacy 2-D and high-resolution 3-D seismic data with Integrated Ocean Drilling Program Expedition 313 cores and logs to investigate downslope sediment transport on the Miocene New Jersey shelf (offshore eastern United States). Medium- to coarse-grained glauconite and glauconitic quartz sands were recovered at the bases of Lower to Middle Miocene intrashelf clinoforms despite the apparent absence of canyons, channels, or other organized sediment conduits before ca. 13.5−12.7 Ma. 3-D seismic imaging, however, reveals closely spaced U- and V-shaped erosional features at the base of sequence m5.47 (ca. 18 Ma) and within sequence m5.3 (ca. 16.6−16.1 Ma). These morphologies, interpreted as grooves and irregular depressions, record erosion by debris flows transporting sand-rich sediment downslope. Their lengths (≤700 m) and low length-to-width ratios (∼3) suggest incision by substrate blocks locally entrained and rapidly abraded during transport. We propose that basal erosion reflects patchy coupling between the debris flow and the substrate, involving bulk entrainment of loose sands, localized stick-pluck detachment of semi-cohesive blocks that form irregular depressions, and short-lived dragging that produces grooves. Grooved and scarred basal surfaces overlain by debrites are restricted to sequences m5.47 and m5.3. Sequence m5.47 shows deeper incision that disrupts clinoform geometry and partially truncates underlying sequences, whereas m5.3 preserves normal progradation. Both sequences record similar erosional processes with an oblique transport direction (∼50°) relative to clinoform progradation direction, which we attribute to a regional trough to the south. These results demonstrate that debris flows can occur on low-angle intrashelf clinoforms and that both frequent smaller-volume and rare, large-volume mass-wasting events are important in shaping intrashelf stratigraphy.