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Two-Dimensional Paleoslope Modeling: A New Method for Estimating Water Depths of Benthic Foraminiferal Biofacies and Paleoshelf Margins
Stephen F. Pekar (1), Michelle A. Kominz (2)
A new method developed for estimating water depths of benthic foraminifers was implemented for Oligocene foraminiferal biofacies from the New Jersey Coastal Plain. We combined benthic foraminiferal biofacies and two-dimensional flexural backstripping to construct a two-dimensional paleoslope model. The original depositional geometry of New Jersey Oligocene strata was reconstructed by calculating the effects of sediment accumulation and flexural loading. Paleodepth estimates for benthic foraminiferal biofacies were calibrated to inner neritic facies along the resulting nonlinear profile (e.g., with clinoforms). Paleodepth estimates for ten foraminiferal biofacies ranged from 20 ± 10 m to 115 ± 30 m and were consistent with more qualitative estimates based on other methods (e.g., foraminiferal abundances and species diversity).
Applying this method to the New Jersey coastal plain for the Oligocene showed clear distributional patterns of benthic foraminiferal biofacies. This enabled paleobathymetry to be determined for the reconstructed New Jersey margin as well as significant stratal surfaces within sequences (i.e., systems tracts and condensed sections). Paleodepth estimates ranged from nearshore (20 ± 10 m) up to 85 ± 25 m for deposits landward of the clinoform rollover of the underlying sequence boundary and from middle neritic to outer neritic (50 ± 20 m to over 100 ± 30 m) on the slopes of the clinoforms.
During earliest transgressive systems tracts, paleodepths immediately seaward of the clinoform slope ranged from inner neritic (< 40 m) at updip sites to outer neritic (> 100 m) at downdip sites. During early highstands, paleobathymetry ranged from 45 ± 10 to 85 ± 25 m landward of the clinoform rollover to 85 ± 25 m to over 100 ± 30 m on the slope (seaward of the developing prograding sedimentary wedge), while during late HSTs, paleodepths shoaled to as shallow as 25 ± 10 m near the clinoform rollover.
This method can be applied to other continental passive margins to reconstruct the stratal geometry of and estimate water depths on the shelf. Furthermore, by extending this process, relative sea level and eustatic timings and amplitudes could be evaluated.
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