Page 152 - Petrology of Sedimentary Rocks

Mudrocks contain many miscellaneous transported constituents. Fossils including
non-calcareous objects such as spores or spicules, are not infrequent. Glauconite,
Phosphatic pellets, etc., are sometime present.

Structures. Shales have an astonishing variety of structures. Besides such
“normal” structures as thin lamination, graded bedding, slumped bedding, small-scale
cross-bedding, cut-and-fill structures (mostly invisible in hand specimen), shales contain
many peculiar ones. Many are revealed in thin section to be really micro-conglom-
erates, made up of aggregates of soft clay-balls or curdled, vague lumps. Others show
apparent desiccation features and irregular shrinkage (?) cracks. Balls of concentri-
cally-layered silt, slumped, curdled, or otherwise irregular stringers and isolated lumps
of sand are common in some shales. Burrows of animals riddle many specimens of
shale, and some contain clay fecal pellets.

In most clays, the clay minerals are oriented so that the flakes lie parallel with
the bedding. Orientation occurs in clays ranging in age from Cambrian to Pleistocene;
in depth of burial from tens of thousands of feet to a few tens of feet. Therefore it is
not a diagenetic feature that requires great pressures or geological age, rather it forms
upon original deposition of the clay. Some claystones have randomly oriented clay
minerasl; these also may be of any age or depth of burial. The factors that cause
orientation are not now known, but organic burrowing is probably the chief cause of un-
oriented clays.

For a claystone to be fissile requires that all the following conditions be satisfied:
(I) the rock must contain little silt or sand; (2) the rock must show even bedding
undisturbed by slumping or burrowing organisms; (3) it must contain little or no
chemical cement; (4) the clays must be well oriented. If any one of these conditions is
not satisfied, the clay will not be fissile. Yet sometimes one obtains a well-bedded,
“pure” claystone, lacking in cement and with oriented clays that still is not fissile
despite the fact that it fulfills all above requirements. The presence of mica or very
thin laminae of fine silt is an aid to fissility.

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