Mudstone is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure and time the platey clay minerals may become aligned and a fisility or parallel layering appears. This finely bedded material that splits readily into thin layers it is called shale. The lack of fisility or layering in mustone may be due to original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mudrocks, mudstone and shale, comprise some 65% of all sedimentary rocks. Looks like clay. Two types of sedimentary rock: limey shale overlaid by limestone. ... For the town in the United States, see Clay, New York. ... Mud is a liquid or semi-liquid mixture of water and soil. ... This article deals with grain size in the context of geology, see crystallite for grain size in materials science. ... A millimetre (American spelling: millimeter, symbol mm) is an SI unit of length that is equal to one thousandth of a metre. ... Shale Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. ...
References
Blatt, Harvey, and Robert J. Tracy, 1996, Petrology, W. H. Freeman, 2nd ed. ISBN 0716724383
The major problem with this is that the pebbly mudstones rest on several hundred metres of andesite [some of it tending towards basalt].
My interpretation is that the pebbly mudstones almost entirely pre-date magmatism and that the andesites underneath the debris flow beds are high-level intrusives.
Overall the pebbly mudstone unit is wedge shaped and was perhaps deposited in a half-graben generated by the extensional teconics that went on to cause the magmatism.
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