Preferred orientation of quartz in a chert breccia.

نویسندگان

  • H R Wenk
  • Y Kolodny
چکیده

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-

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

منابع مشابه

تحلیل ریز ساختی بلورهای کوارتز در آندالوزیت-کردیریت شیست با استفاده از پراش پراکندگی الکترونی

The present study investigates the shape preferred orientation (SPO) and crystal preferred orientation (CPO) of quartz crystals using electron backscatter diffraction (EBSD) in andalusite-cordierite schist from the Ythan Valley, Aberdeenshire, in the Scottish Dalradian block. Quartz crystals in the matrix and andalusite pophyroblasts have inequant to slightly equant shapes. Quartz crystals show...

متن کامل

Crystallographic preferred orientations analysis of quartz crystals in Psammite using electron backscatter diffraction,western Ireland

The present study investigates the crystal preferred orientation (CPO) of quartz crystals in psammitic rocks to ascertain the deformationmechanism using electron backscatter diffraction (EBSD) on quartz crystals from north of the Renvyle-Bofin Slide (RBS) nearLetterfrack in western Ireland. Complete crystallographic orientations were determined for several thousand individual quartz crystalsin ...

متن کامل

Raman Imaging Spectroscopy of a Putative Microfossil from the ∼3.46 Ga Apex Chert: Insights from Quartz Grain Orientation

The utility of nondestructive laser Raman for testing the biogenicity of microfossil-like structures in ancient rocks is promising, yet results from deposits like the ∼3.46 Ga Apex chert remain contentious. The essence of the debate is that associated microstructures, which are not purported to be microfossils, also contain reduced carbon that displays Raman D- and G-band peaks similar to those...

متن کامل

A geochemical view into continental palaeotemperatures of the end-Permian using oxygen and hydrogen isotope composition of secondary silica in chert rubble breccia: Kaibab Formation, Grand Canyon (USA)

The upper carbonate member of the Kaibab Formation in northern Arizona (USA) was subaerially exposed during the end Permian and contains fractured and zoned chert rubble lag deposits typical of karst topography. The karst chert rubble has secondary (authigenic) silica precipitates suitable for estimating continental weathering temperatures during the end Permian karst event. New oxygen and hydr...

متن کامل

Heat-Treating Experiments With Onondaga Chert: Preliminary Results*

Heat-treatment in lithic technology refers to the controlled heating of quartz-rich, brittle, elastic lithic materials in order to reduce fracture toughness and improve knapping qualities. Successful heat-treatment reduces the point-tensile strength of many varieties of micro-crystalline quartz, including chert, without perceptibly altering any of the other properties with regard to fracture. T...

متن کامل

ذخیره در منابع من


  با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید

برای دانلود متن کامل این مقاله و بیش از 32 میلیون مقاله دیگر ابتدا ثبت نام کنید

ثبت نام

اگر عضو سایت هستید لطفا وارد حساب کاربری خود شوید

عنوان ژورنال:
  • Proceedings of the National Academy of Sciences of the United States of America

دوره 59 4  شماره 

صفحات  -

تاریخ انتشار 1968