Chert-Shale-Mudstone Chert typs of silica: Microquartz Megaquartz Chalcedonic quartz (From Tucker, 1996)

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1 Chert-Shale-Mudstone Chert is a general term for fine-grained siliceous rock, of chemical, biochemical, biogenetic origin. It is a dense, very hard rock, which splinters with a conchoidal fracture. The modern equivalents of many ancient-bedded cherts - the radiolarian and diatom oozes - cover large areas of deep ocean floors. Bedded cherts consist of three main typs of silica: microquartz, megaquartz, and chalcedonic quartz. Microquartz consists of equant quartz crystals only a few microns across. Megaquartz crystals are larger, reaching 500 um or more in size. It commonly occurs as a pore-filling cement. Chalcedonic quartz is a fibrous variety with crystals varying from a few tens to hundreds of microns in length. They occur in a radiating arrangement. (From Tucker, 1996) 1

2 Scanning electron micrographs of radiolarians Radiolarians are marine zooplankton with a range of Cambrian to Recent. They have duscshaped, elongate and spherical tests with spines and surface ornamentation. They range in size from a few tens to hundreds of microns. (From Tucker, 1996) The radiolarians in this picture are from the Upper Cretaceous of Cyprus. 2

3 Microquartz, megaquartz and chalcedonic quartz Microquartz Chalcedonic quartz Megaquartz Chalcedonic quartz in spherulitic growth structure (From Tucker, 1996) 3

4 Radiolarian chert Plane-polaized light Crossed polars The sample shows the spherical radiolarian tests and a few thin spines set in a matrix masked by red-brown iron oxide. From MacKenzie and Adams,

5 Replacement chert Plaine -polarlized light Crossed polars Many cherts are secondary, usually replacing limestone. Replacement is often partial, silica preferentially picking out certain shell fragments. The pictures show a chert from a layer within a carbonate sequence. The limestone before replacement consists of peloids, ooids and shell fragments. Most of the original grains have been replaced by microquartz. (From MacKenzie and Adams, 1994) 5

6 Mudrock The term Mudock refers to all siliciclastic sedimentary rocks composed of silt-sized and clay sized particles. Mudrock includes two lithologies. siltstone and claystone. Mudstone is indurated mud, which is a mixture of silt with between one-third and two-thirds clays. Shale is any mudrock that exhibits lamination or fissility Argillite is mudrock that has been subjected to low-grade metamorphism. (From Prothero and Schwab, 1996) 6

7 Shale Well-developed fissility in the Amtrim Shale (Mississippian), Michigan, USA. (From Prothero and Schwab, 1996) 7

8 Shale and Mudstone Fissile black, organic-rich shale. Kimmeridge Clay, Jurassic. Yorkshire, England. Massive, non-laminated mudstone with concoidal fracture. Lower Carboniferous. Wales, UK. (From Tucker, 1996) 8

9 Lamination in Mudstone A, Rhythmites consisting of graded silt passing up into clay-grade material. Late Precambrian. B, Rhythmites consisting of alternations of silt-grade quartz and clay-organic matter. Middle Devonian. (From Tucker, 1996) 9

10 Laminated mudstone Laminated mudstone shows subangular silt-grade quartz, muscovite flakes oriented parallel to lamination and organic-clay laminar (dark). Plane-polarized light. (From Tucker, 1996) 10

11 Laminated mudstone and carbonaceous shale A B A, Laminated dark grey to black mudstone consisting silt interstratified with dark shale. B, Shale, showing several beds of carbonaceous shale alternating with graded siltmud couplets From Blatt and Tracy,

12 Photomicrographs of bituminous coal The pictures show vaious macerals. Liptinite Vitrinite Inertinite 1. Liptinite 2. Vitrinite 3. Inertinite (From Tucker, 1996) Liptinite has a low reflectance and so forms the darkest areas. Vitrinite has medium reflectance and so appears medium grey. Inertinite has a high reflectance and so has a very bright, light-grey appearance. 12

13 Photomicrograph of a coal ball The coal balls are early diagenetic nodules which formed before compaction and alteration of the organic matter. These are composed of calcite or dolomite and well - preserved plant material. In this Carboniferous coal ball, fragments of the large trees are abundant. This indicates that the coal was derived from peat of forest-swamp environment (From Tucker, 1996). 13

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