Roller Micrometer Analysis of Grain Size and Shape Sorting Within Sand Laminae from Lacustrine Barrier Islands
James A. Harrell, Robert B. Braun
The dynamics of sand lamination deposition were investigated for two barrier islands in Lake Erie: Cedar Point Spit, Ohio, and Presque Isle, Pennsylvania. A new measurement technique, roller micrometer analysis, was used to describe the grain size and shape distributions of samples. This technique mechanically sizes grains by both their intermediate (I) and smallest (S) principal dimensions and thus divides a sample into fractions containing grains with common I and S dimensions and tabularity (S/I) ratio.
Portions of the two barrier islands are subject to overwashing by wind-driven lake waters. During such events foreshore laminae are eroded and the sand is redeposited in washover fan topset and foreset laminae. At other times, normal wave activity reworks the washover fan deposits into foreshore laminae. In the transport of sand across the barrier islands from the lake margin (foreshore laminae) through the interior (fan topset laminae) and to the lagoon margin (fan foreset laminae), the following trends are observed: mean grain size increases, grain size sorting become poorer, grain size skewness becomes coarser, and, for grains of the same size, the proportion of more tabular grains decreases. These trends indicate, in a lagoonward direction, progressive winnowing from the bed load f the finer and more tabular grains and increased intermixing of the remaining coarser bed load grains.
Roller micrometer analysis is an important new tool for sedimentologists. It provides traditional grain-size distribution data along with the distribution of grain tabularity. Together the two distributions are sensitive indicators of winnowing and selective deposition.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.