Terrestrial Palaeoecology and Global Change

Chapter 2. Taphonomy
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Fig. 15. Carbonate cyclicity: hard marl – soft marl alternation in the Cenomanian of northern Crimea. The
hard marl (lower member) is deposited during a humid phase of the cycle with an increased river runoff
diluting surface water and inflicting anoxy down the water column, hence the preservation of plant compressions
and aragonitic shells, enhanced by H 2
S. The soft marl (middle member) corresponds to a drier phase of
the cycle with water circulation restored over the carbonate platform supporting a diverse bottom life
(coprolites); the calcitic aptichi are preserved while the aragonitic shells are dissolved (Krassilov, 1985).
The following model seems accountable for the above taphonomic features:
- Carbonate silt accumulates upon the shelf platform populated by a diverse benthic
community;
- Oxidation of organic material, corrosive to aragonite, prevents preservation of
cephalopod shells, except the calcitic aptichi;
- At the next stage, an influx of terrestrial organic material indicates an increase in
riverine discharge, most probably owing to a more abundant precipitation.
- Delution of surface waters decreases vertical mixing;
- Incorporation of terrestrial organic material in the carbonate sediment generates
anoxy of the boundary layer exterminating benthic life (except inoceramids tolerant of a
mild anoxy level). Pyrite precipitation makes the environment less corrosive to aragonite
providing for preservation of ammonite shells;
- Post-depositional redistribution of lime makes the shelly marls the harder of the
cycle.