Crystal Casts in Upper Triassic Lockatong and Brunswick Formations

SUMMARY Upper Triassic Lockatong argillite (lacustrine facies) and reddish‐brown Brunswick mudstone (mudflat facies) contain numerous pseudomorphs after glauberite. Those in the Brunswick Formation are large calcite casts of complete crystals both isolated and in bunches in beds a few inches thick, or are slender skeletal crystal casts forming open‐work rosettes in massive mudstone. Those in analcime‐rich dolomitic Lockatong argillite are smaller, more slender, skeletal crystal‐casts filled with dolomite and analcime; most of them are scattered through the rock but locally arranged in vague geometric patterns; less commonly they form conspicuous rosettes and downward‐ diverging conical sprays. Many glauberite crystals (and rarely halite in the Brunswick mudstone) grew in soft mud during Late Triassic time, thus emphasizing dryness and the role of saline water in the later history of the closed carbonate‐sulphate Lockatong lake. Early extraction of calcium and magnesium carbonates and later of sodium and silica during each chemical cycle left the lake enriched in calcium, sodium, and sulphate. These crystallized as glauberite and possibly anhydrite in lake‐bottom mud and in surface mudcracks during the waning of a chemical cycle. Crystals of the soluble salts in the Lockatong mud were then replaced by dolomite and analcime from solutions released during dewatering of the mud.