Q:'I have read in a book (arguing against Flood geology and recent creation) that Grand Canyon limestones are so similar to modern lime muds (which today form very slowly) that it is obvious the limestone layers must have taken millions of years to form.'
A: It's true that modern shallow-water lime muds, which are primarily formed by the breakdown of sea-creatures containing calcium carbonate, accumulate at a slow rate, such that it would take around 1,000 years to get a thickness of 30 centimetres (one foot).
However, it is not true that such modern-day lime muds are structurally identical to the ancient Canyon limestone layers. The following div summarizes the main differences.
Dominant crystal type
Canyon limestones: Calcite.
Modern lime muds: Aragonite.
Dominant crystal size (diameter)
Canyon limestones: 4 microns or less.
Modern lime muds: About 20 microns.
Evidence of water-current action
Canyon limestones: Contain fossils with a dominant orientation, showing that a water-current was operating.
Modern lime muds: Lack fossils with a dominant orientation, indicating lack of current action.
Evidence for rapid burial
Canyon limestones: Fossil crinoid heads, which would have deteriorated rapidly if not buried quickly.
Modern lime muds: No such evidence.
In addition, many of these Canyon limestones contain quartz sand grains, indicating they were transported by moving water. If there was enough current to shift the sand, there was enough to transport the lime mud as well.
There is ample evidence to indicate that the thick Canyon limestones were not formed as today's lime muds are, by the 'gentle rain of carbonates' over long time-spans, but instead were formed by the transport of sediments by currents of flowing water.