Algal banks are widely distributed, are important in late Paleozoic carbonate provinces, and commonly are good reservoirs for accumulation of hydrocarbons. The term "bank" as used herein indicates an unusually thick sequence of carbonate mudstone built by in-place organisms.

Algal banks are formed primarily by phylloid algae; other organisms may also contribute to the development of the banks. Other than algae, fenestrate bryozoans

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and crinoids are the major contributors. Other organisms are commonly present but probably contributed little to the development of the banks.

Phylloid (leaflike) algae aid in formation of banks in 3 major ways: by baffling, trapping sediments, and binding sediments. Phylloid algae may have grown free on the substrate, may have locally attached to the substrate, or may be encrusting.

Algal banks may occur in the shape of a simple mound, a broad lense, or a complex combination of the two forms. Thicknesses of banks or bank complexes range from about 10 to 115 ft; horizontal distances range from less than 0.1 mi to more than 40 mi. The banks stood slightly above the surrounding sea floor. Algal banks occur in shallow, well-illuminated waters along the flanks of structures, along shelf margins, adjacent to deltas, and in shallow epicontinental seas.

There are 4 major types of porosity associated with algal banks: (1) between algal leaves, (2) within algal leaves, (3) beneath algal leaves (umbrella structure), and (4) as a result of dolomitization of algae and/or matrix.

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