Anatomy of rapid margin progradation; three-dimensional geometries of Miocene clinoforms, New Jersey margin

University of Texas at Austin, Institute for Geophysics, Austin, TX, United States

Documentation of along-strike variations in the morphologies of continental-margin clinoforms is essential for understanding mechanisms of progradation, one of the fundamental relationships between depositional processes and preserved stratigraphy. Maps based on a grid of commercial multichannel seismic data offshore New Jersey, extending more than 70 km along strike and approximately 50 km downdip, reveal the three-dimensional morphology and evolution of four buried surfaces correlated with middle-upper Miocene sequence boundaries calibrated by drilling on the adjacent continental slope. Miocene clinoform breakpoints are not depositional analogs of the modern shelf edge. They are linear to gently arcuate; breakpoint and slope trends indicate a systematic southward displacement of depocenters over about 5.6 m.y. Progradation responded to point (fluvial) sediment sources, but efficient along-strike sediment dispersal muted their influence. Canyons are absent on three of four clinoform slopes; the fourth slope has one v-shaped canyon and a broad erosional area (possible slope failure?). Planar-floored canyons also occur, albeit rarely, seaward of clinoform toes. Apparently, v-shaped and planar-floored canyons, previously ascribed to downslope erosion vs. slope failure/headward erosion, respectively, can coexist. The accretionary northern slope of Little Bahama Bank is a possible morphologic analog. By analogy with Pleistocene shelf/slope geometries, an absence of canyons breaching clinoform breakpoints suggests that rivers did not discharge at paleoshelf edges, indicating that sea level lowstands postulated for the middle-upper Miocene did not expose breakpoints. Reconstruction of breakpoint paleoelevations supports this conclusion for three of the four mapped surfaces, suggesting that elevations of some Miocene lowstands on the global sea level curve are too high by up to 60 m.

This record provided courtesy of AGI/GeoRef.

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