LATE TRIASSIC LACUSTRINE EVAPORITE MINERALS OF THE NEWARK BASIN, NEW JERSEY AND PENNSYLVANIA

Crystal casts after evaporite minerals are common in the lacustrine deposits of the Late Triassic in the Newark basin. The mineral casts are presently comprised of combinations of calcite, dolomite, analcime, potassium feldspar, gypsum, and anhydrite. In cores through the Lockatong and Passaic Formations in the Newark basin, there are two distinct assemblages of evaporite crystals that have no overlap in occurrence. The older group of evaporite crystal casts are typically filled with silicate minerals or carbonate minerals. The younger group of evaporite casts are currently filled by gypsum and anhydrite.

Evaporite crystals in the older group were formed in deep to shallow saline lakes and in saline mudflats. Equant crystals, now calcite, comprise laminated cumulus deposits, bladed crystals comprise radial bottom-growth, and single crystals or radial crystal aggregates displaced mud both randomly or following cracks and layers. These crystal habits are consistent with evaporative concentration of lake water or a shallow groundwater table. A breccia-like, pelleted mudstone fabric associated with these evaporites is consistent with deposits formed by a puffy efflorescent crust.

Evaporite crystals in the younger group exhibit upward-coarsening sequences of euhedral to anhedral crystals, irregular net-like veins, and crystals filling root casts. These features are consistent with evaporites formed in a soil environment where windblown dust or solutes are recycled downward by rainfall.

Although not diagnostic of any mineral phase, the crystal shapes of the older evaporites are all found in modern occurrences of sodium carbonate salts. Sodium carbonate salts also typically form powdery efflorescent crusts. Furthermore, a highly alkaline brine like that associated with sodium carbonate salts is consistent with the silica mobility needed to produce the early diagenetic cements in this part of the record. The younger crystal casts resemble morphologies of modern gypsum soils, suggesting that the modern fillings represent recrystallization of the original minerals. The older evaporites are interpreted as products of water derived from the surrounding bedrock. The change of chemistry in the younger evaporites is interpreted as the result of recycling of evaporites that formed in proto-Atlantic basins that were developing in the Late Triassic.