biostrome

(geology) A bedded structure or layer (bioclastic stratum) composed of calcite and dolomitized calcarenitic fossil fragments distributed over the sea bottom as fine lentils, independent of or in association with bioherms or other areas of organic growth.
(invertebrate zoology) A flat-bedded, fossil, reeflike structure.

An evenly bedded and generally horizontally layered stratum composed mostly of organic remains, normally considered to be those of sedentary organisms which lived, died, and were buried essentially in place.

The criterion of formation by in-place growth of organisms is subject to some interpretation. Crinoidal limestones obviously resulted from the accumulation of decayed pieces of millions of these stalked echinoderms, often with accompanying detritus of associated fenestrate bryozoans. Usually it is impossible to ascertain whether such debris dropped vertically a few centimeters or meters through the water column as the organisms died and collapsed (an essentially in-place deposit) or whether the layers of crinoidal grainstone were piled mechanically by currents. The same is true of coquinas of many other thin-shelled calcareous tests, such as those of brachiopods, bryozoans, and trilobites.

Biostromal layers need not have been horizontally deposited when they occur as flanking beds around organic buildups. Dips of up to 25 or 30° are possible here. Such biostromes are probably veneers of sessile organisms which lived somewhat above the realm of deposition and were buried as sediment cascaded down the flank of a mound.