Passive margin clinothems revisited – The New Jersey margin example
Résumé
Passive margin are for a long time used to reconstruct global
sea-level variations at geological time scale for various reasons.
They usually show a simple and undeform sedimentary record with
a possibility to correlate unconformities regionally and in some
cases worldwide, a gradational evolution of depositional environments
from the continental to the deep marine realms including
the shoreline, and a well-known subsidence history. Sequence
stratigraphy proved to be a wonderful tool to decipher the passive
margin sedimentary record from the earliest outcomes of seismic
stratigraphy (Vail et al., 1977; Posametier et al., 1988) to the
most recent standardization (Catuneanu et al., 2009), applications
(Embry 2008-2009) and simplifications (Neal and Abreu, 2009).
Numerous hypotheses, however, have not been fully tested yet,
like for example, the nature of the sedimentary facies that compose
the prograding clinoform bodies on the shelf – a classic end
member of most passive margin sedimentary record (e.g. Mitchum
et al. 1977; Berg 1982; Alexander et al. 1991; Pirmez et al. 1998
; Hubbard et al., 2010, Helland-Hansen et al., 2012) – the timing
and phases relationship of these sedimentary facies with respect
to relative sea-level changes (Reynolds et al., 1991) or the paleo
water depth of sediment deposited at the toe of the clinoforms
(Greenlee and Moore, 1988).
This paper sought to ground-truth the vertical succession and
lateral facies associations of clinoform deposits in the relatively
simple, passive margin system of the New Jersey shelf by using
the data provided by the IODP-ICDP Expedition 313 (Mountain,
Proust et al., 2010). The arrangement of lithofacies in clinothem
deposits follows complex patterns of prograding shoreface sands
and offshore silts when storm-influenced mud deposits drape the
shelf. Gullies incise the top of the clinothems feeding shelf
slope apron and toe of slope with turbidites and debrites. The
clinothems are interpreted as the subaqueous counterpart of a
subaerial delta detached for more than 60km in the offshore during
transgressive and early highstand times. The subaerial delta
progrades rapidly on the shelf during late highstand, forced regression
and lowstand periods feeding the clinothem with sands
and silts and steps back during early transgressive times leaving
clinothems sealed by offshore muds.
The New Jersey example shows that the model of delta progradation
and reservoir partioning in current passive margin settings
should be strongly revised as well as forcing parameters as relative
sea level never dropped below the clinoform rollover during
lowstand times, as predicted by traditional sea level charts.