ANATOMY OF RAPID MARGIN PROGRADATION - 3-DIMENSIONAL GEOMETRIES OF MIOCENE CLINOFORMS, NEW-JERSEY MARGIN

Documentation of along-strike variations in the morphologies of contin ental-margin clinoforms is essential for understanding mechanisms of p rogradation, one of the fundamental relationships between depositional processes and preserved stratigraphy. Maps based on a grid of commerc ial 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 correlate d 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 sout hward displacement of depocenters over about 5.6 m.y Progradation resp onded 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 cany ons 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 m orphologic analog. By analogy with Pleistocene shelf/slope geometries, an absence of canyons breaching clinoform breakpoints suggests that r ivers did not discharge at paleoshelf edges, indicating that sea level lowstands postulated for the middle-upper Miocene did not expose brea kpoints. Reconstruction of breakpoint paleoelevations supports this co nclusion for three of the four mapped surfaces, suggesting that elevat ions of some Miocene lowstands on the global sea level curve are too h igh by up to 60 m.