>Even very slightly acidic solutions, such as most rainwater
>and some groundwater, can react with carbonates such as
>calcite and aragonite.

dilute carbonic acid (rain) can work wonders on releasing calcareous
fossils from calcareous matrix. Dunno how it selects the inorganic
carbonate to dissolve, but it does, and beautiful carbonate (not
silicified) shells can weather undamaged out of a limestone.

>...hydrogen sulfide and organic
>acids produced by decay can dissolve carbonates.

This also happens with iron sulfide (unstable marcasite, not the
more-stable iron pyrite, as it is usually mistaken for, even by
geologists) concretions which result from decomposition and remain in
the rock. I have numerous limestone-derived Cirsotrema, and some
number of these have limonitic infills (decomposed marcasite
concretions); these are often corroded through between the costae by
the sulfuric acid released by the marcasite's breakdown.

>The
>cases you mention where the organic part of the shell is
>preserved but the shell is gone would fit this situation.

Trying to visualize the environment in that deposit. It's a
greensand, implying middle to outer shelf depths (unless the
glauconite is reworked). Hard to imagine anoxia in the water column
there; infaunal elements also argue against this (numerous burrowing
bivalves, one crustacean). Lots of plant debris indicates river
discharge very close by.
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut