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Testing Sequence Stratigraphic Models by Drilling Miocene Foresets on the New Jersey Shallow Shelf

Abstract

We present seismic, core, log, and chronologic data on three lower Miocene sequences (m5.8, m5.4, and m5.2; ca. 20-14.6 Ma) sampled across a transect of seismic clinothems (prograding sigmoidal sequences) in topset, foreset, and bottomset locations beneath the New Jersey shallow continental shelf (Integrated Ocean Drilling Program [IODP] Expedition 313; Sites M27-M29). We recognize stratal surfaces and systems tracts by integrating seismic stratigraphy, lithofacies successions, gamma logs, and foraminiferal paleodepth trends. Our interpretations of systems tracts, particularly in the foresets where the sequences are thickest, allow us to test sequence stratigraphic models. Landward of the clinoform rollover, topsets consist of nearshore deposits above merged transgressive surfaces (TS) and sequence boundaries (SB) overlain by deepening/fining upward transgressive systems tracts (TST) and coarsening/shallowing upward highstand systems tracts (HST). Drilling through the foresets yields thin (<18 m thick) lowstand systems tracts (LST), thin (<18 m) TST, and thick HST (15-90 m). This contrasts with previously published seismic stratigraphic predictions of thick LST and thin to absent TST. Both HST and LST show regressive patterns in the cores. Falling stage systems tracts (FSST) are tentatively recognized by seismic downstepping, though it is possible that these are truncated HST; in either case, these seismic geometries consist of uniform sands in the cores. Parasequence boundaries (flooding surfaces) are recognized in LST, TST, and HST. TS generally are recognized as an upsection change from coarsening to fining upward successions. We find little evidence for correlative conformities. Even in the foresets where sequences are thickest, there is evidence of erosion and hiatuses associated with sequence boundaries. Sequence m5.8 appears to be a single m.y.-scale sequence, but sequence m5.4 is a composite of three ∼100 k.y.-scale sequences; sequence m5.2 may also be a composite sequence, though our resolution is insufficient to demonstrate this. We do not resolve the issue of fractal versus hierarchical order, but our data are consistent with arrangement into orders based on Milankovitch forcing on eccentricity (2.4 m.y. and 100/400 k.y.) and obliquity scales (1.2 m.y. and 41 k.y.).