ABSTRACT

The detailed nature of relict sediments resulting from and related to the Holocene transgression has been revealed through a sedimentological study of a densely sampled segment of the Long Island, New York, continental shelf. Bathymetry of the Long Island shelf reflects the relict patterns of subaerial coastal-plain fluvial drainage systems from lower stands of sea level.

The shelf sediments can he divided into an inner (0 to 25 fathoms) and middle (25 to 35 fathoms) shelf clean sand facies and an outer (>35 fathoms) shelf muddy sand facies. Locally on the middle shelf, the outer muddy sand facies is preserved as erosional remnants and also within the interiors of shells that are buried in the clean sand. This evidence supports the view that the outer muddy sediment is relict (Garrison and McMaster, 1966); the sharp "mud line" at about 35 fathoms results not from modern deposition but from the winnowing of the formerly more extensive muddy sediment.

Standard deviation values for the inner shelf sands class them as moderately well sorted for the most part; skewness, in general, is slightly positive. Standard deviation values for the middle shelf sands class them as moderately sorted; both negative and positive skewness values characterize middle shelf sands. The outer shelf sediments are positively skewed and poorly to extremely poorly sorted because of the increasing percentage of fines.

The grain size distributions were plotted on log-phi scale and distinct populations were separated using Hald's technique (1952). In the distributions of the inner and middle shelf sands, three populations (A, B, and C) were recognized which appear to correspond to the three populations described by Moss (1962, 1963) which he supposed resulted from saltation (A), interstitial entrapment and/or suspension (B), and sliding and rolling (C). The interrelationships among skewness, mean grain size, and standard deviation are in light of the dissection of the frequency curves and the influence of flues (< 62 microns).

Using the population concept as well as statistical moment measures of the size distribution, the relict nature of the shelf sediments are interpreted as representing various relict environments. The absence of the B population indicates deposition in the surf zone where intense winnowing occurs. The presence of the C population also suggests deposition in the surf zone. Deposition from currents is indicated by the presence of the B population.

Very few of the shelf sands have the size characteristics of beach swash zone deposits. Most are, however, relict of shallow nearshore environments. Most of the inner shelf sands appear to have been modified by currents, whereas man of the middle shelf sands are relict of deposition in the surf zone.

The outer shelf sands are bimodal and by graphical dissection of the size distribution, a distinct fine sand mode can be traced as a separate sedimentation unit. When the sea was at about 35 fathoms, the fine sand was swept by currents (B population is present) from shoal areas to the northeast of the study area into an embayment area. This relict fine sand deposit spread to the southwest and mixed with the coarser basal sands of the transgression.

Short cores on the inner shelf indicate that fine winnowed sand on the inner shelf probably represents the reworking of a backbarrier facies by the transgressive sea.

The inner shelf sands are mineralogically more mature (orthoquartzose) but more angular (mean roundness (rho) for medium quartz grains) than the middle and outer shelf sands (subarkosic).