Pii: S0375-6742(00)00058-3

Rapid sedimentation by fine-grained sediments on the upper slope of the New Jersey margin has generated nearly lithostatic fluid pressures. We use measured porosity from ODP Site 1073 to predict the in situ fluid pressures. We then simulate the pressure history with one-dimensional and two-dimensional sedimentation-compaction models. Two-dimensional models simulate a compaction-driven flow field that is dominated by lateral flow. This flow field increases overpressure and lowers effective stress on the lower slope. The combination of observation and theory provide a model that illuminates a mechanism through which seeps form and slope failure may occur in any rapidly loaded continental margin. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Overpressure; Compaction; Porosity; New Jersey 1. Introduction and methodology Investigators have evaluated eustatic sea-level variations and the resultant stratigraphic responses along the New Jersey margin (Poag et al., 1987). Others have described the distribution of bathymetric and paleobathymetric features (Veatch and Smith, 1939) and hypothesized on the origins of these features (Twichell and Roberts, 1982). In this paper, we address the compaction behavior of New Jersey margin sediments, its relationship to margin hydrodynamics, and the implications for cold seeps and slope stability. To address the compaction behavior of New Jersey slope sediments, we use core data from ODP Leg 174A, Site 1073 (Fig. 1). First, we characterize the porosity at Site 1073. We use these data to constrain the bulk compressibility of the sediments and to model in situ fluid pressure. We then use both a one-dimensional and a two-dimensional sedimentation–compaction model to simulate the porosity and fluid pressure evolution. In comparison to the onedimensional model, the two-dimensional model predicts higher pressures at Site 1073 as a result of loading geometry and lateral fluid flow. 2. ODP site 1073 description ODP Site 1073 was drilled on the upper continental slope offshore New Jersey to 663 m below sea floor (mbsf). As shown in Fig. 2, silt and clay are the dominant lithologies although a few thin sand beds are present. Core data provided bulk density and porosity. Bulk density data were integrated to determine the overburden stress. We divide the porosity at Site 1073 into three zones: (1) a shallow zone (0–100 mbsf) of decreasing porosity; (2) a thick section (100–550 mbsf) of constant porosity; and (3) a deep zone (550– 660 mbsf) of increased porosity (Fig. 2). A common interpretation of this porosity signature is that the Journal of Geochemical Exploration 69–70 (2000) 477–481 0375-6742/00/$ see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0375-6742(00)00058-3 www.elsevier.nl/locate/jgeoexp * Corresponding author. Tel: 1 1-814-863-9663; fax: 1 1-814863-8724. E-mail address: [email protected] (B. Dugan). shallow sediments are normally compacted and the deeper sediments are undercompacted. Porosity greater than 0.60 below 550 mbsf may indicate that these sediments have not compacted since deposition. Sr and Cl both shift abruptly near the base of the Pleistocene sediments (Fig. 2). From the profiles, we interpret little vertical advection of pore fluids. Sedimentation rates are biostratigraphically constrained to less than 0.1 mm/yr for the Pliocene and Miocene sections. In contrast, Pleistocene sedimentation rates reached a maximum of 1.1 mm/yr (Fig. 2). We interpret that the low Miocene sedimentation rates allowed normal compaction during Miocene deposition. Then, rapid loading by low permeability Pleistocene silts and clays hindered the drainage of Miocene fluids. The lack of drainage has impeded compaction of the Miocene strata and has maintained their high porosity. The low permeability of the Pleistocene sediments has also retarded the expulsion of Pleistocene fluids and created the observed constant porosity signature. 3. Porosity–pressure model We assume that compaction is governed by vertical effective stress …sv ˆ Sv 2 P† (Rubey and Hubbert, 1959).