Volume change accompanying cleavage development in graptolitic shales from Gisborne, Victoria, Australia

Studies on shape changes in deformed Oncograptus upsilon combined with strain analysis of pressure shadows around framboidal pyrite revealed that a small volume change of the order of 5 percent accompanied the cleavage development in the shales from Gisborne, Victoria. Starting from the proximal end, Oncograptus upsilon exhibits progressive increase in the spacing between successive thecae along the stipes. A new technique utilizing the distance between a specific number of thecae is used for computing the change in length along stipes of Oncograptus upsilon. The low volume change, accompanying a more than 70 percent shortening deformation, indicates that there is a less significant mass loss during cleavage development than has been inferred from some studies on volume change from thecal spacing in deformed graptolites.