Abstract: Nature, Origin, and Classification of Peritidal Tepee Structures and Related Breccias

By

Christopher G. St. C. Kendall

Distinctive peritidal tepee antiform structures, buckled margins of saucerlike megapolygons, are common in marine vadose fenestral and pilolitic limestones and/or dolomites of carbonate platform sequences. They occur in intertidal and supratidal carbonates ranging in age from Silurian to Holocene. These megapolygons commonly form, and are sometimes truncated, before the deposition of the next sedimentary layer. The megapolygons result from the expansion of surface sediments by as much as 15%. The expansion is caused by the following continuously repeated sequence of processes: (1) desiccation and thermal contraction causing small fractures: (2) phases of wetting causing enlargement of fractures: (3) phases of crystallization of calcium carbonate and other minerals causing the enlargement, fill, and cementation of the fractures (precipitation is from brines and meteoric waters); (4) hydration of minerals, thermal expansion, breaking waves, and faulting may add to this disruption.

The development of the tepee fabric can be traced from an initially cemented, subaerial fenestral crust exhibiting expansion and compressional structures to a completely disrupted and brecciated sediment riddled by a labyrinth of fractures and solution cavities. These spaces are filled by numerous phases of internal marine and freshwater cement and sediment, the latter containing penecontemporaneous or younger marine faunas.

Peritidal tepees are useful tools for geologic reconstruction and provide evidence of subaerial exposure, a tropical to subtropical climate, and back-beach or back-barrier deposition. Proper identification of tepees is of economic importance, because they proved good early porosity and permeability for petroleum entrapment and a site for mineralization. Aesthetically, tepee rocks are a fine kaleidoscopic decorative stone.

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