ABSTRACT

Considerable, difference of opinion is reflected in geologic literature concerning the environment of deposition of redbeds. Many geologists apparently assume that red coloration (hematite) in sediments can be preserved only under conditions of subaerial deposition, and they therefore assume that all redbeds are continental sediments. These assumptions are not valid. If an adequate source of hematite is available, redbeds can originate in any environment, continental or marine, in which oxidizing conditions exist below the depositional interface, or in which the reducing capacity of the bottom sediments is inadequate to destroy the volume of hematite present.

Sediments deposited in normal marine environments (those having normal salinity and open circulation) rarely are red because if hematite is present it normally is altered to non-red ferrous compounds by the reducing action of decaying organic matter in the bottom sediments. However, if sedimentation and subsidence are comparatively rapid, and if the volume of hematite present is large, red sediments can originate even in normal marine environments. Excellent examples of such redbeds are found in certain red shales, red arkoses, and red limestones in the Minturn formation (Des Moines) along the northeastern flank of the central Colorado basin near McCoy, Colorado. These red sediments contain well preserved marine fossils that reflect deposition under normal marine conditions. Mildly reducing conditions existed in the depositional environment causing some bleaching, but the sediments are predominantly red because they were deposited in an area of relatively rapid subsidence in which the volume of hematite was too great to be completely reduced in the time available.

Marine waters with abnormally high salinity, on the other hand, can be highly favorable for hematite preservation, a fact that has been pointed out by earlier writers. Because organisms are scarce in such waters the bottom sediments commonly lack the reducing influence of decaying organic matter. If good mixing of the overlying water is maintained by wave action or by convection currents, oxidizing conditions prevail below the depositional interface; any organic matter present is destroyed by oxidation and any hematite present is preserved. Redbeds originating under these conditions are nonfossiliferous but possess other characteristics of marine sediments, e.g., well sorted texture, uniform bedding, absence of channel structures, and association with primary dolomite and evaporites. Parts of the Pennsylvanian and Permian strata in the central Colorado basin provide convincing examples of redbeds of this origin.

In petroleum exploration, redbeds should be analyzed carefully for possible marine origin. Although such rocks are unfavorable as source beds of petroleum, they may serve as satisfactory reservoir rocks and may be closely associated with other marine sediments that are highly prospective for oil.

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