ABSTRACT

Many evaporite deposits contain contorted layers or masses of limestone or other rock types that completely envelope nodules of anhydrite or gypsum. The resulting texture has been called mosaic or "chicken wire." Anhydrite in the Detroit River Formation (Middle Devonian) of northern Indiana has a similar texture, but the thin, contorted layers of limestone do not form complete envelopes. The resulting texture is here defined as penemosaic.

Of the numerous theories proposed for the origin of mosaic and penemosaic textures of anhydrite, the two most probably correct are: (1) the interstitial precipitation of anhydrite in unlithified sediments; (2) the mixing of anhydrite and carbonate sediments by wave action and density settling. 'Massive replacement of carbonate rock by anhydrite has also been suggested for deposits closely resembling the Detroit River anhydrite.

A study of insoluble residues of 36 samples of anhydrite and gypsum and 47 samples of interbedded limestone layers and masses from 12 continuous cores through the Detroit River Formation in LaPorte County, Ind., provides convincing evidence that the penemosaic texture of the Detroit River anhydrite is not a replacement phenomenon. Concentration of allogenic quartz in the interbedded limestone layers and masses suggests that they were deposited in a higher energy environment than were the enclosing sulfate rocks, the overlying limestone, or the underlying dolomite. Such an environment could have been produced under sebka conditions or by interruptions of lagoonal sedimentation.