Abstract: Understanding Sedimentary Response to Relative Sea-Level Change: The Ocean Drilling Program (ODP)'s Mid-Atlantic Transect

Austin, Jr., James A. - Univ. of Texas at Austin, Institute for Geophysics; Nicholas Christie-Blick - Lamont-Doherty Earth Observatory, Columbia Univ.; and the ODP Leg174A Shipboard Scientific Party

An important ODP objective is to study the sedimentary response to various forcing functions, including sea-level change. By "stacking" continental margin transacts globally, tectonic, sediment supply and other local effects can be eliminated to decipher the timing, amplitudes and causes of eustatic changes. ODP's inaugural transect crosses the New Jersey margin. "Nested" geophysical images, and 1970's sampling, have documented prograding, Oligocene and younger sediments beneath this margin whose deposition may be eustatically controlled. Their seismic geometries, lozenge-shaped h cross-section, thin both landward and seaward from rollovers/breakpoints (geographic locations at which each bounding surface steepens seaward into a clinoform). In the vicinity of rollovers, sequence boundaries are characterized by transitions from offlap (landward) to onlap (seaward); offlap at boundaries <12 m.y. old is conspicuous.

In June-July 1997, ODP Leg 174A, the first scientific ocean drilling effort in <200 m water depths, sampled these unconformity-bounded sequences at two shelf locations (Sites 1071 and 1072); a third site (1073) sampled the upper continental slope. Leg 174A augmented Leg 150 on the slope and ongoing coastal plain drilling (Legs 150X/174AX). The goals were to: 1) date Oligocene-Holocene unconformities, and to compare the stratigraphic record with glacial-eustatic changes inferred from deep-sea d¹⁸O variations; 2) constrain amplitudes and rates of sea-level change perhaps responsible for unconformity development; and 3) assess relationships between depositional fades and sequence architecture.

Leg 174A sampled and dated a number of mid-late Miocene to Pleistocene shelf sequence boundaries, by combining seismic characterization (Fig. 1), core analyses, and downhole measurements data (Fig. 2). Current best estimates of ages, incorporating results from the slope as well as the shelf, are m1(s), 11.9-11.4 Ma; m0.5(s), 11.4-8.6 Ma; pp4(s), 1.9-1.4 Ma; and pp3(s), 0.46-0.25 Ma. Surface m1(s) may have been penetrated at the bottom of Hole 1071 F, at a depth of ~405 mbsf (meters below sea floor, depth conversion based on a checkshot survey at Site 1072). Surfaces m0.5(s), pp4(s) and pp3(s) were penetrated at both Site 1071 (water depths, 88-90 m) and Site 1072, 2.6-3.5 km to the southeast (water depths, 98-100 m). These surfaces are located at the following depths: >262 to 269, ~137 and 62 mbsf (1071); and ~302,155 and 58 mbsf (1072). Ages of two additional Pleistocene sequence boundaries were calibrated using an almost completely recovered 664 m-thick sequence at Site 1073: pp2(s), 0.46-0.25 Ma; and pp1(s), <0.25 Ma. Wireline logs, including a checkshot survey at Site 1072 and Schlumberger® Logging-While-Drilling technology, refined correlations between geophysical profiles and sampled stratigraphy.

Lower parts of associated sequences are comparatively sandy and glauconitic, thin seaward over rollovers (using seismic geometry), and accumulated in shallower water than overlying finer grained sediments (50-100 m for the latter; benthic foraminifera). Finely laminated carbonaceous silty clay ~7 m above surface m0.5(s) in Hole 1071F is not bioturbated, is barren of planktonic foraminifera and dinocysts abundant at other levels at this site, and contains pollen and fungal spores. These observations suggest a lagoonal or estuarine environment, implying that the shelf was subaerially exposed to within 3 km of its rollover during development of m0.5(s). In spite of marked offlap, each sequence is transgressive-dominated; sequences above m0.5(s) generally fine upwards and record upward-deepening trends. The shelf did not flood abruptly; it stayed relatively shallow, and sediments relatively sandy, a "keep-up" situation where sediment supply kept pace with sea-level rise. Cyclic highstand deposits of mainly quartz sand several tens of meters thick are localized near rollovers; unfortunately, these sands impeded drilling progress. Lowstand deposits are thin to absent on the deep paleoshelf; previously interpreted lowstand prograding wedges do not exist at or above the level of m0.5(s).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil