Sedimentology and paleoceanography of the Early Carboniferous Windsor Group, Nova Scotia: implications for the formation of saline giants

Saline giants are vast marine evaporite successions whose thick economic deposits of gypsum, anhydrite, halite, and potash contain a sensitive record of sea level and climate. Most are interpreted to have formed in restricted... [ view full abstract ]

Saline giants are vast marine evaporite successions whose thick economic deposits of gypsum, anhydrite, halite, and potash contain a sensitive record of sea level and climate. Most are interpreted to have formed in restricted basins where evaporation under arid conditions promoted widespread precipitation of evaporite minerals. Because saline giants lack precise modern analogues, they remain one of the most poorly understood of all chemical sedimentary rocks.

The Early Carboniferous Windsor Group (ca. 345 Ma) of the Maritimes Basin in Atlantic Canada provides a unique window into the processes forming giant evaporites. The Windsor Group is 1 to 2-km-thick and covers an area of 250,000 km2. It consists of two evaporite-rich sedimentary sequences that reflect the eventual freshening of the Maritimes Basin. Subzone B records the transition to open marine conditions and thus, contains an acute signal of the factors controlling the widespread precipitation of evaporites.

Lithofacies stacking patterns indicate Subzone B comprises the lowstand systems tract of the second sequence. Superimposed higher-order fluctuations in relative sea level produced nine-stacked parasequences that are interpreted to reflect high frequency glacioeustatic oscillations during the Late Paleozoic Ice Age. These decameter-scale, aggradational peritidal cycles brine upwards and record increasingly restricted conditions during deposition. Cycles are defined by flooding surfaces overlain by subtidal fossiliferous or evaporite-rich lime mudstone that changes upwards into massive anhydrite or layered halite. Evaporites are in turn overlain by flaser-bedded sandstone, microbialite, and nodular anhydrite. Cycles terminate with terrestrial red beds or a karst diastem that penetrates underlying anhydrite. Each parasequence is interpreted to reflect progradation of intertidal and sabkha environments over subtidal carbonate and evaporite deposits. Dissimilarities in cycle composition between sub-basins imply the development of contrasting brine chemistries resulting from differing recharge rates with the open ocean.

These aerially restricted peritidal evaporites of Sequence 2 are markedly different than the underlying basinwide evaporites of Sequence 1. What Subzone B reveals is that evaporite type is directly linked to the amplitude and frequency of sea level rise and fall during deposition. True saline giants apparently require low amplitude, long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater. Such conditions create extensive, thick economically important evaporites. The high amplitude, short frequency variability in sea level that produced the peritidal evaporites of Subzone B create smaller, subeconomic deposits with more complex facies relationships.