STRATIGRAPHIC MODELLING OF MIOCENE COMPOUND CLINOFORMS OF THE NEW JERSEY SHALLOW CONTINENTAL SHELF
Résumé
The New Jersey passive margin with well-developed clinoforms has long been a key location for past
sea-level reconstructions and an end-member of passive margin sedimentary geometries. Recently, IODP
Expedition 313 brings new insights on the facies architecture of Miocene clinothems preserved on the New
Jersey shallow continental shelf feeding discussions on their depositional model and stratigraphic organization.
This paper uses Dionisos, a process-based numerical modelling software, to reproduce the geometry and the
architecture of Miocene clinothems. The objective is two-fold: (1) validate the hypothesis that a depositional
model involving compound clinoform geometry (i.e. paired subaerial and subaqueous delta clinoforms)
explains the facies architecture of Miocene clinothems, (2) evaluate the amplitude of sea-level fluctuations
required to produce such a geometry.
Simulations that consider waves and river discharge as equal contributors in the transport of sediment
succeed in reproducing most key aspects of the Miocene clinothems: the seismic architecture (dip and
geometry of reflectors), the stratigraphic architecture (systems tracts stacking patterns), the facies architecture
(spatial distribution of facies), and the scale and geometry of the clinoforms (dip of the shelf, thickness of the
sequences and height of the clinoforms). The action of waves exerts the prime control on the geometry of the
compound clinoforms, which is characterized by a sandy subaerial delta at the shoreline feeding a muddy
subaqueous delta clinoform that progrades on the shelf below wave base. Our results support recent
depositional models of subaqueous deltas characterized by deepwater mud-dominated material on the topsets
and toesets, and shoreface sand-dominated material on the rollover and foresets. This supposes that topsets
muds were emplaced during highstand times, while the rollover sands mark lowstand times in clinothems,
which are composed of four systems tracts (TST, HST, FSST, LST) building individual T-R sequences. Major
seismic bounding surfaces (i.e. the clinoforms) correspond to Maximum Regressive Surfaces –MRS, which
landward of Expedition 313 coreholes, merge with the sequences boundaries –SB– recognized in onshore
coreholes.
According to this reappraised stratigraphic model and based on Expedition 313 age framework, the New
Jersey clinothems arrange in third-order, ~1.2 Myr-long megasequences. Those megasequences likely
correspond to obliquity cycles that control the growth and decay of large Antartica ice sheets in the Miocene.
A set of simulations was run to test this hypothesis and evaluate the amplitude of sea-level fluctuations. The
best-fit simulation corresponds to a sinusoid-like curve characterized by 1.2 Myr-long cycle with amplitude of
c.60 m. Even though those values are consistent with astronomical and paleoclimate estimates, such sealevel
curve differs from the ones proposed in the literature.