ABSTRACT

Cores from the red beds constituting the Upper Triassic Passaic Formation, a syn-rift sequence in the Newark Basin, display subhorizontal fractures (veins) filled with satin spar gypsum. Such veins are apparent in the shallow subsurface, 100-300 m below present ground level, but are not observed in outcrops or in quarries because of groundwater dissolution. These fractures are roughly parallel to the present ground surface in the basin, suggesting topographic influence on their formation. Fractures observed in cores are seen to crosscut stratigraphic boundaries, indicating that the veins formed after stratal tilting and faulting and thereby post-date the early Late Jurassic. The gypsum crystals making up the vein fill grew perpendicular to the fracture walls, suggesting that opening and filling of the fractures was contemporaneous and that the fractures are extensional. That gypsum crystallizes from meteoric and connate waters and is derived by dissolution of associated evaporites is reflected in the D, ¹⁸O, ³⁴S, and ⁸⁷Sr/⁸⁶Sr isotopic compositions of the sulfate. Fractures are interpreted to be a result of unloading and exhumation. Fractures were initiated after major depositional and tectonic events took place and post-date the Late Jurassic. Satin spar precipitated in those fractures, carried in by gypsum-saturated brines derived either by dissolution of the near-surface sulfate evaporites or rehydration of more deeply buried anhydrite. Such satin spar veins are not "evaporites", but are composed of chemically remobilized sulfate