I would refer your friend to Faust, 1951 "Seismic velocity as a function of
depth and geologic time" Geophysics v 16, p. 192-206 or A. R. Gregory "Rock
Physics in Seismic Interpretation," inCharles Payton Ed. Seismic
Stratigraphy applications to hydrocrbon Exploration, AAPG Memoir 26, p. 19
While there is a lot of scatter, For any given depth and lithology, the
geologically older rocks have a faster velocity of sound than the younger
rocks. Sandstone surface rocks today have a velocity of sound of around 6000
ft/sec while Ordovician surficial sandstones at the surface today have
velocities around 11,000 feet per second. I have seen them as high as 14,000
ft/sec. But I have never seen a modern surficial sandstone with that high
of a velocity. This is a significant difference in the physical properties
of the rocks. What causes this is that over time the pore space in older
rocks have become more cemented up with limestone and silica. Having
personally worked areas like the Appalachians, the Cretaceous/Jurassic of
Texas and the Tertiary of the Gulf of Mexico, I can testify to the vast
difference in physical properties in those surficial rocks in each place.
Older = faster.
I agree with your friend's last statment entirely.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm