Shell-rich beds are great stratigraphic markers and, despite commonly being fairly thin intervals, can provide a lot of information regarding paleoenvironment and paleohydrology. They represent a pretty subtle linkage between biology, sedimentology, and stratigraphy that serves to elucidate complexly interacting attributes of the rock record, especially in regards to sediment accumulation, substrate consistency, and water quality (to name a few!).
Anyway, I thought I'd show a few pictures of shelly intervals for this Sed Structure Sunday. The picture below is from Egypt, and is a good example of a compound bed. Several distinct horizons of different types and abundances of shelly fossils indicate that, despite the thin-bedded nature of this interval, a lot of time is wrapped up in this horizon.
These next two pictures show horizons dominated by a single type of bivalve, Carolia (not sure if that's spelled right...but it's at least a phonetic spelling of the right genus...), showing it's characteristic thin shell. These are from Egypt as well. These are pretty much in-place, as indicated by their delicately articulated shells, and tell us something about the low-energy, clear water conditions of these deposits.
This is in contrast to the picture below, which is also from Wadi al-Hitan, Egypt. It's a big, thick-shelled Pycnodonte, pretty isolated, in a single interval. Upsection, these guys become more abundant, and are eventually overtaken by (and overgrown with) smaller Gryphaea and Carolia beds.
And this last picture is from the core I was measuring last week; it shows a different kind of shelly interval than the previous pictures. Whereas those pics above show in-situ shell beds, this picture shows an erosional scour and shelly lag. These busted up bivalves were transported as bioclasts, and deposited like any other grain in a siliciclastic system.