Subaqueous shrinkage cracks in the Devonian of Scotland reinterpreted

Abstract

Abundant lenticular, straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks (syneresis cracks), the two most influential examples being those in the Green River Formation (Picard 1966) and the Devonian Orcadian Basin of Scotland (Donovan and Foster 1972). Cracked surfaces in the Orcadian Basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin. Some lenticular cracks are inferred to have formed during subaerial desiccation. Many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs. The gypsum crystals were often important in providing nuclei for later desiccation cracks. More complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution. Cracks of all types are preserved by sand infills introduced by wind transport across dried-up lake floors. Reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way, partly as gypsum crystals, and partly by limited subaerial desiccation. The remaining examples have either already been reinterpreted in other ways, or are insufficiently documented to rule out a subaerial origin. There is no clear evidence for subaqueously formed cracks being preserved in rocks.