THE SCHOOLEY PENEPLAIN REVISITED: INTEGRATING GEOMORPHOLOGY, STRATIGRAPHY, SEA LEVEL, AND TECTONICS

The Schooley peneplain (SP) in its type area in northern New Jersey is a low-relief (<50 m) upland erosion surface that bevels bedding and structure in gneiss, siltstone, diabase, and basalt. Coastal Plain (CP) marine sediments bordering the SP to the southeast include Paleogene shelf deposits and early to middle Miocene coastal deposits. The Paleogene deposits record paleodepths of 50-120 m, shelf gradients of 1:200 to 1:500, present-day seaward dips of 7-9 m/km, and occur up to 100 m in elevation at the inland edge of the CP, indicating a minimum former extent of 15 to 60 km inland from their present limit, at elevations >100 m above the SP. The Miocene deposits lie unconformably on the older sediments, dip more gently (2-3 m/km), and at their inland limit match in elevation to the seaward edge of the SP. A fluvial gravel caps the Miocene deposits on the highest hills in the CP and occurs as a lag on the seaward edge of the SP. A Pliocene fluvial gravel partly fills a broad valley along the inner edge of the CP that is inset up to 100 m below the older gravel and SP. These relationships indicate that the SP was formed between the Eocene and middle Miocene and was incised and dissected in the middle to late Miocene. Inventories of atmospherically produced ¹⁰Be in clayey regolith at two sites on the SP, adjusted for runoff bypass and shallow surface erosion, indicate a Miocene age is plausible. This history from local stratigraphy matches in time and magnitude to long-term global sea level as reconstructed from temperature-corrected δ¹⁸O records, and from backstrip records in NJ, with peneplanation during a stable to rising sea-level trend between 35-15 Ma, and incision and dissection of the SP during the 50-70 m drop between 15-5 Ma. Flexural uplift since 20 Ma in NJ is modeled at 20 m. Models of mantle dynamic topography (MDT) from passage of the Farallon slab vary between 200 m of uplift to 150 m of subsidence in NJ since 30 Ma. The NJ backstrip records, filtered for a time-moving MDT deformation, indicate 20 m of MDT uplift from 30-15 Ma and 20 m of subsidence from 15-5 Ma (Schmelz and others, 2021). The MDT uplift subtracts from sea-level rise between 30-15 Ma to maintain a stable sea level in NJ during formation of the SP, and the flexural and MDT effects mostly cancel each other from 15-5 Ma, leaving sea-level drop as the main cause of incision of the SP.