Abstract: High Levels of Variation in Size-Frequency Distributions at Laminar and Intralaminar Levels Suggest Revision of Conventional Size-Frequency Data

Brian T. Grothaus, James T. Grace, Robert Ehrlich

Deviations from simple log normality of size-frequency distributions (SFD) increasingly are being interpreted in terms of depositional processes. In doing so, it is tacitly assumed that the sedimentary volume sampled is homogeneous with respect to SFD (i.e., if the sample volume were subdivided and analyzed, truncation points of cumulative SFD would not vary appreciably). However, sandy sediment commonly is laminated and each lamina might have size-frequency data unlike that of the bulk sample. For this reason the variability of SFD on a microlevel is of interest.

Accordingly, soft sediment peels were obtained from several environments, and five to eight adjacent laminae in each were microsampled vertically. Size was determined suitably to produce size data comparable to sieve data.

Significant variation exists in mean size and shape of the SFD at microlevels. Most intralaminar frequency data are coarse-tail truncated and unimodal, although a few coarse-tail truncated bimodal distributions were observed. Boundaries between laminae commonly were marked by sharp grain-size increases. These observations suggest that common characteristics of SFD (truncation points, subpopulations, etc.) cannot have a simple genetic explanation inasmuch as these features vary considerably at microlevels. Truncation points fluctuate in grain size enough to reclassify as much as 20% of the sample from one subpopulation to another. SFD, therefore, must be considered highly composite. The large amount of information expressed at microlevels suggests that this may prove to be the link bet een experimental studies and depositional process.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma