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Storm events most commonly produce sheetlike bodies of graded bioclastic material, associated with distinctive sedimentary and biogenic structures. They can also accumulate conspicuous shell mounds if combined with certain biological mechanisms: (1) immobile secondary soft bottom dwellers tend to evolve excessively large and heavy skeletons in order to increase stability, and (2) larval recolonization may selectively occur on sites of earlier settlements, particularly if they are slightly elevated above their surroundings.
Similar to archeological "tells" of Mesopotamia, this combination leads to the self-enhancing, multievent accumulation of monospecific mounds. They can be distinguished from bioherms by (a) a flat base, (b) lack of frame-building, (c) the functional morphology of the constituent species (heavy weight recliners), and (d) the predominance of non-abraded, bivalved specimens not in life position.
Fossil nummulite and oyster tells reach 40 m (131 ft) in thickness and are potentially important as hydrocarbon reservoirs.
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