Limestone clast from conglomerate and limestone unit
This block of limestone (an apparent clast) from the
conglomerate and
limestone unit may look somewhat out of focus, but this effect is caused
by a high degree of recrystallization of the carbonate. Very little of the
original texture and fabric remains. The light-colored patches are "ghosts"
of calcareous algal plates, many of which are oriented at high angles to
the edges of the block, suggesting that original bedding surfaces were
not parallel to present edges. The original "up" direction is also
in
question (i.e., is it right-side up or upside-down?).
This block was slabbed in two directions at right angles.
The left face
continues in the right face, except the view turns a 90-degree corner
at the vertical edge. The crinkly brown lines are "stylolites." These
are
zones where the relatively soluble carbonate has dissolved, leaving an
insoluble residue of iron oxide and clay minerals. The stylolites are most
closely spaced near the top (or bottom?) of the sample, probably
because the original limestone bed was becoming more clayey as it
approached a contact with shale. The dominant stylolite orientation may
reflect the orientation of original bedding. Such highly stylolitic limestones
tend to be altered in other ways such as the extensive recrystallization
seen here, suggesting prolonged exposure to reactive solutions. Most
limestone clasts and beds at this site are much "fresher."
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