Abstract: Poisson Bed-Thickness Distribution and Turbidite-Pelagite Thickness Correlation, Jurassic Radiolarites, Liguria, Italy
Robert L. Folk, Earle F. McBride
Detailed measurements of latest Jurassic radiolarites at Orrido di Nascio, Liguria, show that beds are made up of ribbonlike alternations of white cherts consisting of closely packed radiolaria, and red shales with scattered radiolaria. Rhythmic interlayering and local graded bedding within the cherts suggest a turbidite origin. Median thickness of chert beds ranges between 16 and 30 mm in different parts of the section, with log ^sgr = 0.25 to 0.30 (meaning approximately that 68% of the bed thicknesses lies between half and double the median thickness). Shale beds have median thickness between 13 and 33 mm.
The section can be divided into two parts. In the lower part, bedding thicknesses of both cherts and shales show a continuous Poisson distribution, i.e., when plotted on cumulative-probability paper with a log-thickness scale, they show a tail to the left (excess number of thin beds) with graphic skewnesses ranging between -0.20 to -0.35; this is indicative of the Poisson distribution, whose log SkG theoretically should always be -0.25. Chert-bed thicknesses show a correlation of +0.82 with the overlying red shales, but only +0.31 with the underlying red shales. The Poisson thickness distribution is produced by the random occurrence of turbidite-triggering events through geologic time (equivalent conceptually to the lengths of line segments resulting when a line is cut at r ndom intervals, a classic Poisson example); the high-positive correlation indicates that the overlying shales are the pelagic tail of the turbidite.
In the upper part of the section, bed thicknesses are midway between log normal and Poisson, with SkG -0.10 to -0.20; and the correlation coefficient between chert-bed thickness and the overlying shales is only +0.28. Thus the upper part of the section was deposited by a different mechanism than the lower part, and is only in part turbiditic; the other depositional mechanism remains uncertain. A causative explanation for the log-normal thickness distribution is unknown.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC