Deep-Water Evaporite Deposition: A Genetic Model

Abstract

Reasoning by analogy with modern salinas, many geologists believe that ancient marine evaporite deposits formed in shallow, slowly subsiding basins in regions of arid or semiarid climate. If this model is considered critically in the light of knowledge of several well-studied ancient deposits, however, inconsistencies arise which suggest the need for an alternative hypothesis. Sedimentation rates inferred from ancient salt deposits appear to be compatible with temperate rather than arid climates. More significantly, the thickness of evaporite salts in many localities requires either extremely rapid subsidence or salt deposition over a period substantially longer than that permitted by present stratigraphic control. In addition, the petrography, stratigraphy, and bromine content of the halite in such basins can be reconciled with the salina model only with great difficulty. The observed inconsistencies may be overcome by postulating that the salts were deposited in basins several hundreds to several thousands of meters deep. A model of evaporite deposition is shown to be both geologically reasonable and oceanographically tenable, and to be consistent with the depositional histories of the Zechstein, Salina, Castile, and at least part of the Elk Point evaporites. Direct stratigraphic evidence of deep-water deposition is available in the Castile Sea. The Elk Point (Prairie or Muskeg) Evaporites of Alberta are examined in the light of the deep-basin model, and implications of the model for the exploration geologist in the region are developed.