Tepees in peritidal carbonates: origin via earthquake-induced deformation, with example from the Middle Cambrian of western Canada

Tepee structures are common in many ancient carbonate tidal flat successions, and because analogues are known from tropical coastal areas, they are well established as evidence for synsedimentary lithification of subaerially exposed carbonate sediment. Existing explanations involving desiccation, thermal expansion and contraction, fluctuating groundwater pore pressure or diagenetic force of crystallization do not adequately account for the style and degree of deformation seen in some examples, such as those in the Middle Cambrian Cathedral Formation of the southern Rocky Mountains, western Canada. These tepees provide evidence for essentially instantaneous events causing brittle failure of decimetre-thick crusts of ooidal and pisoidal grainstone, followed by their dilation, rupturing, separation, tilting and shingling. They can be explained as breccias generated by sporadic episodes of strong ground motion from earthquakes that emanated from a syndepositionally active fault system nearby. Variably directed tensile and compressive stresses broke crusts, shifted them upwards and shuffled them laterally. Subsequent microbial encrustation and vigorous flushing by seawater in the resulting shelter cavities led to rapid stabilization by cementation. Recognition of the role of seismically induced deformation in explaining some tepee structures furnishes a proxy for synsedimentary tectonism and adds another member to the family of ‘seismites’ which reflect a range of rheological properties of unlithified and lithifying sediments. [Copyright &y& Elsevier]