ABSTRACT

Eighty gravity cores, twelve piston cores, and 340 dredges for infauna were taken between latitudes 37°30 and 37°50N in water depths of 5 to 38 meters. Sediment types, as indicated by standard hydrometer analyses, ranged between sands and clayey-silts when plotted on the Shepard (1954) textural triangle. Most of the sediments were poorly to very-poorly sorted. Preponderant colors observed were dark greenish gray (5GY4/1) in the reduced zone, olive-gray (5Y4/1) in the oxidized zone (topmost 0.1-5 cm), and black (N1) in patches, streaks, and layers in the zone of reduction. Determinations of bulk chemical composition (percent by weight) in 13 cores showed the following r nges: organic C, 0.18-1.91; inorganic C, 0.02-0.42; total P, 0.12-0.61; total Fe, 1.12-4.52; Na⁺, 0.22-1.97; K⁺, 0.38-1.26; Ca⁺⁺, 0.01-1.30; and Mg⁺⁺, 0.05-0.22. The dominant clay minerals were illite (50%), chlorite (30%), and mixed-layer minerals (20%).

Gravity cores of the surficial 0.9 meter exhibited these ranges of values for the following mass and related physical properties: gross saturated mass unit weight, 1.16-2.00 g/cm³ (in air) and 0.15-0.96 g/cm³ (submerged); average specific gravity of grains, 2.66-2.74; average liquid limit, 18-112; average plastic limit, 16-40; average plasticity index, 2-78; gross water content, 31-335%; gross liquidity index, 12.1-93.2%; gross void ratio at 100% saturation, 0.86-8.98; gross porosity, 46.2-90.0%; and average compression index, 0.31-0.91.

Ninety-four samples from piston cores of the upper 1.02 meters exhibited the following ranges in mass and related physical-property values: cohesion, 7.7-88.4 g/cm² (15.3-181.0 lbs/ft²); sensitivity, 2.2-18.2; water content, 39.3-200.0%; specific gravity of grains, 2.69-2.71; degree of saturation, 29.5-99.3%; void ratio, 1.48-5.61 (determined) and 1.06-5.23 (saturated); and gas content, 0.7-70.5%.

The values of plasticity, gross mass unit weight, and water content compare closely with those found by Richards (1962) for a variety of deep-sea cores. Richards' values for degree of saturation are generally higher, however, owing possibly to difficulties in determination of gas content in cores raised from great depths.

Thirty-nine consolidation-test results are reported for sediments from the extreme lower end of Chesapeake Bay. Miocene sediments are overconsolidated while the overlying Pleistocene and Holocene sediments (<15,000 years in age) are normally consolidated, except in areas of known scour.

Applications of the data include: 1) the use of anomalous strength and void-ratio profiles to delineate areas of spoil deposition; 2) the estimation of amounts of scour of overlying sediments from preconsolidation-pressure values for those beneath; 3) a discussion of the important role of gas content in core shortening during gravity coring; and 4) a brief consideration of design requirements related to the placing of loads on marine sediments. In regard to design it is noted that correlation values between the liquid limit and the compression index, commonly employed in foundation studies, would underestimate the settlements of loads emplaced on these estuarine sediments.

A poor correlation existed between mean grain size and infauna distributions, and no significant correlations were found between any of the mass properties and the animal distributions. Modifications to natural bottom in the form of dredging and related spoil disposal had only a temporary effect on infauna populations. Resettlement of disturbed areas was rapid and appeared to occur as a result of active migration and, more importantly, hydrodynamic distribution of juveniles.