Preferred orientation of quartz in a chert breccia.

Preferred orientation of minerals hitherto has been studied almost exclusively in metamorphic and igneous rocks, which show the most spectacular effects of deformation or magmatic flow. In sediments, preferred orientation of grains controlled by their shape has been discussed by Sander,' but to our knowledge no one has ever reported preferred orientation produced by deformation under diagenetic conditions. In the present study a strong preferred orientation of quartz is demonstrated in parts of a penecontemporaneous chert breccia from Israel. Modern methods of measurement and analysis are used to determine quantitatively the degree of orientation in the specimen. The results are compared with those obtained in experiments on fine-grained quartz rocks. Geological Background.-The chert comes from the Mishash Formation in southern Israel. This is a marine rock sequence of Campanian (Upper Cretaceous) age. Together with more or less equivalent formations in Jordan, Syria, and Egypt, it forms an extensive belt of chert-phosphorite facies. Chert is the predominant component of the formation; other rock types are porcelanite, siliceous and calcitic phosphorite, chalk, marl, limestone, and dolomite.2'8 The area is structurally a characteristic cratonic terrain. The folding is mild and the typical structures are monoclines. Overfolds are very rare and metamorphism is absent. Rocks of the Mishash formation do, however, show some internal deformation. The chert beds are often crumpled into small intraformational folds, which do not affect overand underlying rocks.' Among the cherty rocks of the Mishash Formation, chert breccias are the most conspicuous rock type. In most cases both the "matrix" and "fragments" in breccias are cherty; sometimes, however, cherty fragments are embedded in a calcitic matrix. The "matrix" is always richer in phosphatic detritus, foraminiferal ghosts, organic matter, and some trace elements (Ti, Fe, Mg, V). The texture of the breccias varies. Sometimes the fragments are irregular in shape, but usually they are laminar, parallel to bedding, and the laminae almost always show variable degrees of rupture and tearing. It can be shown in many cases that the matrix of breccias was not completely solidified while the fragments were hard. This is demonstrated by the following facts. (a) Clastic phosphate grains sank through the matrix and accumulated on top of fragment layers, and (b) microfaults which cut through fragments turn into flexures in the matrix.4' 5 Petrofabric Analysis.-From about 150 thin sections examined by Kolodny4 on a flat stage between crossed polarizers, 12 showed regions of strong extinction parallel to the foliation (which represents either original bedding, or a diagenetic structure, or both). The gypsum plate effect indicates a c-axis maximum perpendicular to the foliation. Preferred orientation was only observed in the siliceous matrix, whereas in the initially more rigid breccia fragments it appeared that preferred orientation was lacking. The 12 thin sections with regions of preferred orientation come from several localities in southern Israel. Most samples were collected in the Ef'e valley close to the Beer Sheva'-Sedom high-