Seismic geomorphology of buried channel systems on the New Jersey outer shelf: assessing past environmental conditions |
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| Institution: | 1. Soil Erosion Research Station, Ministry of Agriculture & Rural Development, Emek-Hefer, Israel;2. Geography and Environment, Bar-Ilan University, Ramat-Gan, Israel;3. School of Israel and Jerusalem Studies, Lander Institution, Jerusalem, Israel;1. Department of Earth and Atmospheric Sciences, 312 Science & Research Building #1, University of Houston, Houston, TX 77204-5007, USA;2. School of Geography and Earth Sciences (SGES), McMaster University, 1280 Main Street West, Hamilton, ON L8S 4 L8, Canada;1. University of Antwerp, Department of History, Stadscampus, S.R-A.112, Rodestraat 14, 2000 Antwerp, Belgium;2. University of Antwerp, Ecosystem Management Research Group, Campus Drie Eiken, D.C.223, Universiteitsplein 1, 2610 Wilrijk, Belgium;3. University of Ghent, Department of Geology and Soil Science, S8, Krijgslaan 281, 9000 Ghent, Belgium;4. University of Antwerp, Department of History, Stadscampus, S.D.313, Grote Kauwenberg 18, 2000 Antwerp, Belgium;5. University of Antwerp, Ecosystem Management Research Group, Campus Drie Eiken, D.C.120, Universiteitsplein 1, 2610 Wilrijk, Belgium;1. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy;2. Dipartimento di Scienze Biologiche Geologiche e Ambientali, University of Catania, Italy;3. Rajola S.p.A., Torre del Greco, Italy;4. Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), Rome, Italy;5. Istituto di Scienze Marine (ISMAR), Bologna, Italy;6. Lega Navale Italiana, Sciacca, Italy;7. Formerly at Dipartimento di Scienze Biologiche Geologiche e Ambientali, University of Catania, Italy;1. School of Geography and Environmental Sciences, University of Ulster, Coleraine, Northern Ireland, United Kingdom;2. Geological Sciences School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Westville Campus, Durban, South Africa;3. Laboratory of Coastal Oceanography, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil;4. CB&I — Environmental & Infrastructure, Florianopólis, Brazil;5. Geoscience Institute, Universidade Federal do Rio Grande do Sul CECO/UFRGS, Porto Alegre, RS, Brazil |
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| Abstract: | Quantitative geomorphologic analysis of shallowly buried, dendritic channel systems on the New Jersey shelf provides estimates of paleo-hydrologic parameters needed to link channel morphology to the former hydrodynamic setting. These channels, observed in 1–4 kHz deep-towed chirp seismic data, formed presumably as fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). The presumed fluvial origin of these channels is supported by their incision into underlying Pleistocene strata, a chaotic seismic fill unit at their bases which may be indicative of non-marine gravel lag, and measured stream junction angles that are consistent with a riverine origin. The channels would also have been subjected to estuarine/tidal environments during ensuing sea-level rise. We employ empirically derived hydraulic equations for modern rivers and estuaries to estimate paleo-discharges, velocities and maximum shear stresses, using the preserved and interpolated paleo-channel geometries as a guide. Generally, trunk/main channels have box-like, symmetric cross-sections, with width/depth ratios of >100, whereas smaller, tributary channels have more v-shaped, asymmetric cross-sections with width/depth ratios of ～40–70. The high width/depth ratios, along with low sinuosities (～1.1) and slopes (<0.02°), are consistent with braided streams as specified by a modern river classification system. However, the channels show no evidence of braiding. We hypothesize instead that these channel systems are immature, having had insufficient time to develop high sinuosities that would otherwise be expected before they were drowned by the Holocene transgression. Mean paleo-flow estimates for these systems assuming a tidal environment (1.0–1.5 m/s) are consistent with modern tidal creeks comparable to the sizes of channels observed here (<2 km wide and <25 m deep). Estimated tidal shear stresses would be sufficient to initiate sediment transport of grains 2–8 mm in diameter (coarse sand and fine gravel) as bedload and finer grained material in suspension. However, paleo-flow estimates assuming a fluvial environment (1.1–2.0 m/s) are generally too high for a non-tidal creek, given the presumed low hydraulic gradients in this coastal plain setting. Retrodicted fluvial discharge and boundary shear stresses would have been sufficient to transport particles up to ～15 mm in diameter (gravel) as bedload; these grain sizes are too coarse to be transported by sluggish coastal plain rivers. We conclude that either flows were quite high when this system was first incised fluvially, perhaps due to meltwater pulses following the LGM, or that tidal influences have modified the original fluvial geometry. |
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